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  • Research article
  • Open access
  • Published: 17 July 2020

The quality of drinking and domestic water from the surface water sources (lakes, rivers, irrigation canals and ponds) and springs in cholera prone communities of Uganda: an analysis of vital physicochemical parameters

  • Godfrey Bwire   ORCID: 1 ,
  • David A. Sack 2 ,
  • Atek Kagirita 3 ,
  • Tonny Obala 4 ,
  • Amanda K. Debes 2 ,
  • Malathi Ram 2 ,
  • Henry Komakech 1 ,
  • Christine Marie George 2 &
  • Christopher Garimoi Orach 1  

BMC Public Health volume  20 , Article number:  1128 ( 2020 ) Cite this article

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Water is the most abundant resource on earth, however water scarcity affects more than 40% of people worldwide. Access to safe drinking water is a basic human right and is a United Nations Sustainable Development Goal (SDG) 6. Globally, waterborne diseases such as cholera are responsible for over two million deaths annually. Cholera is a major cause of ill-health in Africa and Uganda. This study aimed to determine the physicochemical characteristics of the surface and spring water in cholera endemic communities of Uganda in order to promote access to safe drinking water.

A longitudinal study was carried out between February 2015 and January 2016 in cholera prone communities of Uganda. Surface and spring water used for domestic purposes including drinking from 27 sites (lakes, rivers, irrigation canal, springs and ponds) were tested monthly to determine the vital physicochemical parameters, namely pH, temperature, dissolved oxygen, conductivity and turbidity.

Overall, 318 water samples were tested. Twenty-six percent (36/135) of the tested samples had mean test results that were outside the World Health Organization (WHO) recommended drinking water range. All sites (100%, 27/27) had mean water turbidity values greater than the WHO drinking water recommended standards and the temperature of above 17 °C. In addition, 27% (3/11) of the lake sites and 2/5 of the ponds had pH and dissolved oxygen respectively outside the WHO recommended range of 6.5–8.5 for pH and less than 5 mg/L for dissolved oxygen. These physicochemical conditions were ideal for survival of Vibrio. cholerae .


This study showed that surface water and springs in the study area were unsafe for drinking and had favourable physicochemical parameters for propagation of waterborne diseases including cholera. Therefore, for Uganda to attain the SDG 6 targets and to eliminate cholera by 2030, more efforts are needed to promote access to safe drinking water. Also, since this study only established the vital water physicochemical parameters, further studies are recommended to determine the other water physicochemical parameters such as the nitrates and copper. Studies are also needed to establish the causal-effect relationship between V. cholerae and the physicochemical parameters.

Peer Review reports

Water is the most abundant resource on the planet earth [ 1 ], however its scarcity affects more than 40% of the people around the world [ 2 ]. Natural water is an important material for the life of both animals and plants on the earth [ 3 ]. Consequently, access to safe drinking water is essential for health and a basic human right that is integral to the United Nations Resolution 64/292 of 2010 [ 4 ]. The United Nations set 2030 as the timeline for all countries and people to have universal access to safe drinking water; this is a Sustainable Development Goal (SDG) 6 of the 17 SDGs [ 5 ]. The availability of and access to safe water is more important to existence in Africa than it is elsewhere in the world [ 6 ]. Least Developed Countries (LDCs) especially in sub-Saharan Africa have the lowest access to safe drinking water [ 7 ]. In Africa, rural residents have far less access to safe drinking water and sanitation than their urban counterparts [ 8 ].

Natural water exists in three forms namely; ground water, rain water and surface water. Of the three forms, surface water is the most accessible. Worldwide, 144 million people depend on surface water for their survival [ 9 ]. In Uganda, 7% of the population depends on surface water (lakes, rivers, irrigation canal, ponds) for drinking water [ 10 ]. The same surface water is a natural habitat for many living organisms [ 11 ] some of which are responsible for transmission of infectious diseases such as cholera, typhoid, dysentery, guinea worm among others [ 12 ]. Surface water sources include lakes, rivers, streams, canals, and ponds. These surface water sources are often vulnerable to contamination by human, animal activities and weather (storms or heavy rain) [ 13 , 14 ]. Globally, waterborne diseases such as diarrheal are responsible for more than two million deaths annually. The majority of these deaths occur among children under-5 years of age [ 15 ].

Cholera, a waterborne disease causes many deaths each year in Africa, Asia and Latin America [ 16 ]. In 2018 alone, a total of 120,652 cholera cases and 2436 deaths were reported from 17 African countries to World Health Organization [ 17 ]. Cholera is a major cause of morbidity and mortality in Uganda [ 18 ]. The fishing communities located along the major lakes and the rivers in the African Great Lakes basin of Uganda constitutes 5% of the Uganda’s population, however these communities were responsible for the majority (58%) of the reported cholera cases during the period 2011–2015 [ 19 ]. Cholera outbreaks affect predominantly communities using the surface water and the springs. There is also high risk of waterborne disease outbreaks in the communities using these types of water [ 20 , 21 ]. Studies of the surface water from water sources located in the lake basins of the five African Great Lakes in Uganda identified Vibrio. cholerae [ 22 , 23 ] though no study isolated the toxigenic V. cholerae O1 or O139 that cause epidemic cholera. Cholera outbreaks in the African Great Lakes basins in Uganda have been shown to be propagated through water contaminated with sewage [ 20 , 24 ]. Cholera is one of the diseases targeted for elimination globally by the WHO by 2030 [ 25 ]. Hence, to prevent and control cholera outbreaks in these communities, promotion of use of safe water (both quantity and quality), improved sanitation and hygiene are the interventions prioritized by the Uganda Ministry of Health [ 26 ]. Most importantly, provision of adequate safe water is a major pillar of an effective cholera prevention program given that water is the main mode of V. cholerae transmission [ 27 , 28 ].

Availability of adequate safe water is essential for prevention of enteric diseases including cholera [ 29 ]. Therefore, access to safe drinking and domestic water in terms of quantity and quality is key to cholera prevention. Water quality is defined in terms of three key quality parameters namely, physical, chemical and microbiological characteristics [ 30 ]. A less common but important parameter is the radiological characteristics [ 31 ]. In regards to the physicochemical parameters, there are five parameters that are essential and impacts life (both flora and or fauna) within the aquatic systems [ 32 ]. These vital physicochemical parameters include pH, temperature, dissolved oxygen, conductivity and turbidity [ 32 ].

pH is a value that is based on logarithm scale of 0–14 [ 33 ]. Aquatic organisms prefer pH range of 6.5–8.5 [ 34 , 35 , 36 ]. Low pH can cause the release of toxic elements or compounds into the water [ 37 ]. The optimal pH for V. cholerae survival is in basic range (above 7). Vibrio cholerae may not survive for long in acidic pH [ 38 ]. A solution of pH below 4.5 will kill V.cholerae bacteria [ 39 ].

Most aquatic organisms are adapted to live in a narrow temperature range and they die when the temperature is too low or too high [ 34 ]. Vibrio cholerae , bacteria proliferate during algae bloom resulting in cholera outbreaks [ 40 , 41 ]. This proliferation could be due favourable warm temperature [ 42 ]. Relatedly, V. cholerae isolation from natural water in endemic settings is strongly correlated with water temperature above 17 °C [ 43 ].

Dissolved oxygen is the oxygen present in water that is available to aquatic organisms [ 34 ]. Dissolved oxygen is measured in parts per million (ppm) or milligrams per litre (mg/L) [ 35 ]. Organisms in water need oxygen in order to survive [ 44 ]. Decomposition of organic materials and sewage are major causes of low dissolved oxygen in water [ 12 ].

Water conductivity is the ability of water to pass an electrical current and is expressed as millisiemens per metre (1 mS m- 1  = 10 μS cm − 1 ) [ 29 ]. Most aquatic organisms can only tolerate a specific conductivity range [ 45 ]. Water conductivity increases with raising temperature [ 46 ]. There is no set standard for water conductivity [ 45 ]. Freshwater sources have conductivity of 100 – 2000μS cm − 1 . High water conductivity may be due to inorganic dissolved solids [ 46 ].

Turbidity is an optical determination of water clarity [ 47 ]. Turbidity can come from suspended sediment such as silt or clay [ 48 ]. High levels of total suspended solids will increase water temperatures and decrease dissolved oxygen (DO) levels [ 12 ]. In addition, some pathogens like V. cholerae, Giardia lambdia and Cryptosporidia exploit the high water turbidity to hide from the effect of water treatment agents and cause waterborne diseases [ 49 ]. Consequently, high water turbidity can promotes the development of harmful algal blooms [ 41 , 50 ].

Given the importance of the water physicochemical parameters, in order to ensure that they are within the acceptable limits, the WHO recommends that they are monitored regularly [ 51 ]. The recommended physicochemical parameters range for raw water are for pH of 6.5–8.5, turbidity of less than 5Nephlometric Units (NTU) and dissolved oxygen of not less than 5 mg/L [ 51 ]. Surface and spring water with turbidity that exceeds 5NTU should be treated to remove suspended matter before disinfection by either sedimentation (coagulation and flocculation) and or filtration [ 52 ].

Water chlorination using chlorine tablets or other chlorine releasing reagent is one of the most common methods employed to disinfect drinking water [ 53 , 54 ]. Chlorination is an important component of cholera prevention and control program [ 55 ]. In addition to disinfection to kill the pathogens, drinking water should also be safe in terms of physicochemical parameters as recommended by WHO [ 51 ]. However, to effectively make the water safe using chlorine tablets and other reagents, knowledge of the physicochemical properties of the surface and spring water being disinfected is important as several of the parameters affect the active component in the chlorine tablets [ 56 ]. For example, chlorine is not effective for water with pH above 8.5 or turbidity of above 5NTU [ 53 ].

Generally, there is scarcity of information about the quality and safety of drinking water in Africa [ 57 ]. Similarly, few studies exist on the physicochemical characteristics of the drinking water and water in general in Uganda. Furthermore, information from such studies is inadequate for use to increase safe water in cholera prone districts of Uganda where the need is greatest. The cholera endemic communities of Uganda [ 19 , 21 , 24 ] have adequate quantities of water that is often collected from the Great lakes, rivers and other surface water sources located within the lake basins. However, the water is of poor quality in terms of physicochemical and microbiological characteristics. Several studies conducted in Uganda have documented microbiological contamination of drinking water [ 20 , 24 , 58 , 59 ]. However, few studies exist on the physicochemical characteristics of these water. Furthermore, these studies focused on few water sources, for example testing the lakes and omitted the rivers, springs and ponds or testing the rivers and omitted the other water types. One such study was carried out on the water from the three lakes in western Rift valley and Lake Victoria in Uganda [ 23 ], This study did not assess the other common water sources such as the rivers, ponds and springs that were used by the communities for drinking and other household purposes. Other studies on water physicochemical characteristics assessed heavy metal water pollution of River Rwizi (Mbarara district, Western Uganda) [ 60 ] and of the drinking water (bottled, ground and tap water) in Kampala City (Central Uganda) [ 61 ] and Bushenyi district (Western Uganda) [ 62 ]. These studies found high heavy metal water pollution in the drinking water tested. The information gathered from such studies is useful in specific study area and is inadequate to address the lack of safe water in the cholera endemic districts of Uganda where the need for safe drinking water is greatest. Several epidemiological studies in Uganda have attributed cholera outbreaks to use of contaminated surface water [ 20 , 21 , 24 , 63 ]. Furthermore, studies conducted on the surface water focus on pathogen identification [ 63 , 64 ] leaving out the water physicochemical parameters which are equally important in the provision of safe drinking water [ 53 ] and are necessary for survival of all living organisms (both animals and plants) [ 44 ].

Therefore, the aim of this study was to determine the physicochemical characteristics of the surface water sources and springs located in African Great Lakes basins in Uganda so as to guide the interventions for provision of safe water to cholera prone populations [ 19 , 20 , 21 , 24 , 58 ] of Uganda. This study in addition has the potential to guide Uganda to attain the United Nations SDG 6 target of universal access to safe drinking water [ 2 ] and the WHO cholera elimination Roadmap [ 25 ] by 2030. Furthermore, these findings may guide future studies including those on causal-effect relationship between physicochemical parameters and infectious agents (pathogens).

This was a longitudinal study that was conducted between February 2015 and January 2016 in six districts of Uganda that are located in the African Great Lakes basins of the five lakes (Victoria, Albert, Kyoga, Edward and George). These districts had ongoing cholera outbreaks or history of cholera outbreaks in the previous five to 10 years (2005–2015). In addition, the selected study districts had border access to the following major water bodies (lakes: Victoria, Albert, Edward, George and Kyoga). The study area was purposively selected because the communities residing along these major lakes contributed most (58%) of the reported cholera cases and deaths in Uganda [ 19 , 65 ] and in the sub-Saharan Africa region [ 66 ] in the past 10 years. Water samples were collected monthly from 27 sites used by the communities for household purposes that included drinking. Water samples were then tested to determine the vital physicochemical parameters. The water samples were collected from lakes, rivers, springs, ponds and an irrigation canal that were located in the lake basins of the five African Great Lakes in Uganda. In one site, water was also collected from a nearby drainage channel and tested for V. cholerae [ 22 ] and physicochemical parameters. However, because the channel was not used for drinking the results were omitted in this article. Water samples were analysed to determine the pH, temperature, dissolve oxygen, conductivity and turbidity. The study sites were located in the districts of Kampala and Kayunga in central region of Uganda; Kasese and Buliisa districts in western Uganda; Nebbi and Busia districts in northern and eastern Uganda respectively. The study sites were the same as for the simultaneous bacteriological V. cholerae detection study [ 22 ] and are shown in Fig.  1 .

figure 1

Map showing the location of Uganda, the study districts, major surface water sources and the study sites, February 2015 – January 2016. The blue shades are the African Great Lakes and their basins. (Map generated by ArcGIS version 10.2 [licenced] and assembled using Microsoft Office PowerPoint, Version 2016 [licenced] by the authors)

Rural-urban categorization of the study sites

The study sites were categorized as urban if they were found in Kampala district (the Capital City of Uganda) or rural if they were in the other five remote study districts (Kasese, Kayunga, Busia, Nebbi and Buliisa).

Identification of the study sites and water testing procedures

The sites for water testing were identified with the guidance of the local communities and after direct observation by the study team. Geo-coordinates of the sites were taken at the beginning of the study to ensure that subsequent water collection and measurements were done on water from specific points. Two water collection sites were selected on each of the African Great Lakes in Uganda. The selected sites were in different locations but within the communities with a history of cholera outbreaks in the previous 10 years prior to the study period. For each selected lake point, a site was also selected on a river, a spring and a pond located within the area and being used by the communities for domestic purposes that included drinking and preparation of food. A total of 27 sites, two of which were from each of the five lakes were selected and the water tested. The number of sites on each lake and their locations are shown in Additional file  1 .

Water samples were collected and tested monthly for 12 months by the research assistants who were health workers with background training in microbiology or environmental health. The research assistants received training on water collection and testing from a water engineer. The physicochemical parameters were measured by use of the digital meters namely the Hach meter HQ40d and digital turbidity meter.

Water samples were collected in five-litre containers, three litres were processed for V. cholerae detection by Polymerase Chain Reaction (PCR) test as previously described [ 67 ]. Vibrio cholerae Non O1/Non O139 pathogens were frequently detected in the water samples during the study period [ 22 ]. While the three litres of water were being processed for V. cholerae detection [ 22 ], the rest of the water (2 l), were simultaneously used for the onsite measurement of temperature, pH, conductivity and dissolved oxygen. The Hach meters , HQ40d used in the study, had three electrodes that were calibrated before each monthly testing according to the manufacturers’ manual [ 68 ]. The Hach meter calibrations were done using three specific standard buffer solutions that were for pH, dissolved oxygen and conductivity respectively. Turbidity (total suspended solids or water clarity) was measured using a turbidity meter according to previously published methods [ 49 ]. In addition, the research assistants were provided with Standard Operating Procedures (SOPs) and supervised monthly by the investigators before and during each scheduled monthly measurements.

Data management, analysis and statistical tests

Data were collected, entered, cleaned and stored in the spreadsheet. Errors in the recorded readings were removed using the correct records retrieved from the Hach meters’ HQ40d internal memory. Stata statistical package version 13 was used to analyse the data. Data were analysed to generate means and standard error of the mean for pH, temperature, dissolved oxygen (DO), conductivity (CD) and turbidity. Data were presented in the form of tables and graphs. Comparison for variations between the water samples were carried out using One-Way Analysis of Variance (ANOVA) test. Samples with significant One-Way ANOVA test were subjected to Turkey’s Post Hoc test to establish which of the variables were statistically significant.

The map was created using ArcGIS software, Version 10.2, licenced (ESRI, Redlands, California, USA). The graphs and figures were produced using Microsoft Excel and PowerPoints, Version 2016 (Microsoft, Redmond, Washington, USA). The administrative shapefiles used to create the map were obtained from open access domain, the Humanitarian Data Exchange: . In order to generate the study locations on the map, Global Positioning System (GPS) coordinates for the study sites were converted to shapefiles that were combined with the administrative shapefiles corresponding to the locations.

A total of 318 water samples were tested from 27 sites as follows; lake water 40.9%, (130/318), rivers water 26.4% (84/318), ponds water 17.9% (57/318), spring water 11.0% (35/318) and canal water 3.8% (12/318).

Test results for the lake water collected at the fish landing sites (FLS)

The mean physicochemical test results for pH, temperature, dissolved oxygen, conductivity and turbidity are shown in Table  1 .

The mean physicochemical water characteristics of most of the sites were within the WHO recommended water safety range except for turbidity. Few sites had pH and dissolved oxygen outside the WHO recommended safety range.

Monthly variations of the lake water physicochemical characteristics

There were monthly variations in the physicochemical parameters between the water from the lake sites overtime. Most of the sites had steady pH overtime for the first half of the study period (February – August 2015). Thereafter, the pH reduced slightly during the second half (September, 2015 – January, 2016) of the study period. The highest pH fluctuations were in the months of October – December, 2015. The widest change in pH within the same site was observed at Gaaba Fish landing site, Lake Victoria basin, Kampala district.

There were differences in water temperature on the same lake but at different test sites. These differences were detectable mostly in the months of April, 2015. The lowest and highest water temperatures were both recorded on Lake Edward (Kasese district) at Kayanzi fish landing site of 18.9 °C and at Katwe FLS of 34  ° C in the period between April – August, 2015. Fluctuations in the dissolved oxygen were detectable throughout the study period. Kalolo Fish landing site on Lake Albert, Buliisa district showed the widest fluctuations in dissolved oxygen with the highest value of 10.73 mg/L and the lowest of 2.5 mg/L.

Most test sites had small conductivity fluctuations except for Panyimur and Kalolo both of which were located on Lake Albert in Nebbi and Buliisa districts These districts had high water conductivity fluctuations with arrange of 267.1 μS/cm – 2640 μS/cm at Kalolo (Buliisa district) FLS and 296 μS/cm – 2061 μS/cm at Panyimur (Nebbi district). Water turbidity for the majority of the sites changed overtime. Kahendero fish landing site (Lake George, Kasese district) had the highest turbidity which was most noticeable in the months of October 2015 to January 2016. Majanji fish landing site (Lake Victoria, Busia district) had the lowest and most stable water turbidity. Monthly variations of the lake water physicochemical parameters are shown in Fig.  2 .

figure 2

Monthly variations of lake water physicochemical characteristics (pH, temperature, dissolved oxygen, conductivity and turbidity), February 2015 – January 2016: Part a ) water pH variations; Part b ) water temperature variations; Part c ) water dissolved oxygen; Part d ) water conductivity variations; Part e ) water turbidity variations

River water physicochemical parameter test results

The mean physicochemical characteristics of water from the seven rivers studied are shown in Table  2 .

There were variations in the mean pH, temperature, dissolved oxygen and conductivity between study sites on the rivers. However, these mean parameter variations were in WHO acceptable drinking water safety limit except for River Lubigi, Kampala district which had mean dissolved oxygen below the recommended WHO range. At one time (February, 2015) River Lubigi had dissolved oxygen of 0.45 mg/L. The river water turbidity for all the test sites were above that recommended by WHO of less than 5NTU.

Monthly variations of the river water physicochemical characteristics

Monthly variations in the water physicochemical characteristics of the seven river test sites are shown in Fig.  3 .

figure 3

Monthly variations of the physicochemical characteristics of river water, February 2015 – January 2016: Part a ) water pH variations; Part b ) water temperature variations; Part c ) water dissolved oxygen variations; Part d ) water conductivity variations; Part e ) water turbidity variations

There were variations in the water physicochemical parameters between rivers and within the same river overtime. Most rivers showed fluctuations of water pH and temperature. Some rivers such as R. Nyamugasani and R. Lhubiriha both in Kasese district had wide temperature fluctuations. River Mobuku (Kasese district) had the lowest water temperature recorded over the study period. Fluctuations in dissolved oxygen were highest in R. Lubigi (Kampala district), Lake Victoria basin. Dissolved oxygen for R. Lubigi was below the recommended level of more than 5 mg/L for most of the study period. Seasonal variations of water dissolved oxygen were also more noticeable in R. Lubigi than the rest of the river sites. Relatively more dissolved oxygen was found during the rainy seasons (March – July, 2015, first rainy season and September – December, 2015, second rainy season) than in dry season.

There were small variations in the water conductivity in the majority of the rivers. Wide fluctuations in conductivity were observed for water samples of R, Lubigi (Kampala district). River Nyamugasani (Kasese district, Lake Edward basin) had steady but higher conductivity than all the other rivers. There were variations in turbidity within the same river overtime and between the different rivers. River Sio (Busia district) had the highest and the widest turbidity variations during the study period.

Water test results for the springs and ponds

The mean physicochemical characteristics of spring and pond water are shown in Table  3 .

The mean physicochemical characteristics of water from the springs and ponds showed variations between the sites. The majority of site means values were within the WHO accepted pH range. Two sites, Wanseko pond (Buliisa, district, Lake Albert basin) and Katanga spring (Kampala district, Lake Victoria basin) had mean water pH below the recommended WHO drinking water acceptable range at the acidic level of 5.73 and 6.19 respectively. Forty percent (40%, 2/5) of the ponds and 33% (1/3) of the springs had mean dissolved oxygen below the recommended WHO level. The ponds with the low dissolved oxygen were found within Lake Albert basin. Among the springs, Katanga spring (Kampala district, L. victoria basin) had mean dissolved oxygen that was below the WHO recommended level of 5 mg/L. Conductivities of the spring water were 89.81–3276.36 μS/cm and for ponds 55.99–3280.83 μS/cm. For both the springs and the ponds the differences between the lowest and the highest conductivities were wide.

Monthly variations of the springs and ponds water physicochemical characteristics

The monthly variations of spring and pond water physicochemical characteristics are shown in Fig.  4 .

figure 4

Monthly variations of the physicochemical characteristics of the spring and pond water, February 2015 – January 2016: Part a ) water pH variations; Part b ) water temperature variations; Part c ) dissolved oxygen variations; Part d ) conductivity variations and Part e ) water turbidity variations

There were variations in the water physicochemical characteristics of the spring and the pond water overtime. The variations in water (springs and ponds) were also present between the different sites. The springs had small monthly variations of the water physicochemical parameters while the ponds had wide variations. Mughende pond (Kasese district) had the highest pH for most of the study period. Katanga spring (Kampala district) had the lowest pH compared to other springs during the study period. Kibenge spring (Kasese district) had higher temperature than the rest of the two springs (Katanga spring, Kampala district and Nyakirango spring, Kasese district). Most springs and ponds had slight fluctuations in dissolved oxygen except for Mughende pond (Kasese district). Most springs and ponds except for Panyimur pond (Nebbi district) had small monthly fluctuations in water conductivity. Kibenge spring and pond (both located in Kasese district) had higher conductivity compared to the rest of the springs or ponds. Mughende spring and pond were outliers with higher conductivity than the rest of the water sites. There were variations in water turbidity with months for both the springs and the ponds. Apart from Mughende pond (Kasese district), the rest of the springs and ponds showed variations that had two peaks, the first peak (May – August, 2015) and the second peak (November – January, 2016).

Test results of the other surface water sources: Mobuku irrigation canal water

Mobuku irrigation canal water, water diverted from Mobuku River for irrigation purposes by the Mobuku irrigation scheme was tested because the local communities were using this water for domestic purposes including drinking. Apart from water turbidity which was above the WHO recommended standard of 5NTU, the rest of the water physicochemical parameters (pH, temperature, dissolved oxygen and conductivity) were in the WHO acceptable range as follow: pH of 7.93 ± Standard Error (SE) of 0.23, temperature of 26.57 °C ± SE of 1.25 °C, dissolved oxygen of 6.38 mg/L ± SE of 0.18 mg/L, conductivity of 69.06 ± SE of 2.57) and turbidity of 28.68 ± SE of 9.06NTU.

Monthly variations of physicochemical characteristics of Mobuku irrigation canal water

There were monthly variations in water physicochemical characteristics of Mobuku irrigation canal. The water pH and dissolved oxygen showed two peaks each. The first peak was in March – May, 2015 and the second peak, August – November, 2015. The variations of the Mobuku irrigation canal monthly water physicochemical parameters over the study period is shown in Fig.  5 .

figure 5

Monthly variations of the physicochemical characteristics of Mobuku irrigation canal water, February 2015 – January 2016: Part a ) water pH variations; Part b ) water temperature variations; Part c ) dissolved oxygen variations; Part d ) conductivity variations and Part e ) water turbidity variations

Results of statistical tests for the differences within sites overtime and between sites

One-Way ANOVA test.

There were no statistically significant differences within most of the study sites except for sites on the lakes and the rivers where the pH and temperature differences were statistically significantly within sites overtime. Statistically significant differences in the water physicochemical characteristics were observed between sites (all p -value < 0.05) as indicated in the additional file  2 .

Turkey’s post hoc test

There were statistically significant differences for all water physicochemical parameters for both the lake and river sites. For instance, Lake Edward had both the highest temperature (34 °C, May, 2015) which was registered at Katwe FLS (Kasese district) and the lowest temperature (18.9 °C, April, 2015) which was recorded at Kayanzi FLS (Kasese district). The results of the comparison of the physicochemical parameters of the various lake and river sites are shown in Table  4 .

Similarly, comparison of the springs or pond water showed statistically significant differences for most (80% of the total comparison) of the water parameters (pH, temperature, dissolved oxygen and conductivity) apart from the water turbidity. Turkey’s post Hoc test results for the comparison of springs and pond water physicochemical parameters are shown in Table  5 .

This study showed that water for drinking and domestic purposes from the surface water sources and springs in cholera affected communities/districts of Uganda were not safe for human use in natural form. The water samples from the water sources in the study area did not meet the WHO drinking water quality standards in terms of the important physicochemical parameters. In addition, all the surface water sources and the springs tested had turbidity above the WHO recommended level of 5NTU yet the same water were used for domestic purposes including drinking in the natural form by the households. The study also found variations in the other physicochemical parameters (pH, temperature, dissolved oxygen and conductivity) between study sites on the same lake and between the different water sources.

While the majority of the water sources had mean water physicochemical characteristics (excluding turbidity) in acceptable range, few water sources, mainly the sites on Lake George, including the springs and ponds had pH and dissolved oxygen outside the recommended WHO ranges. These water sources that did not meet the WHO drinking water standards could expose the users to harmful effects of unsafe drinking water including waterborne diseases such as cholera. The present study findings of high water turbidity if due to algae bloom could encourage pathogen persistence and infection spread, including V. cholerae bacteria [ 40 , 41 ] resulting in ill-health and cholera epidemics. In addition, the high water turbidity complicates water disinfection as it gives rise to significant chlorine demand [ 53 ]. The increased chlorine demand can be costly and difficult to ensure constant availability for disinfection of water since Uganda and several other developing countries need and receive supplementary donor support [ 69 ].

In regard to temperature, dissolved oxygen and conductivity, the majority of the surface water sources and springs tested met the recommended WHO drinking water standards. However, a few water sources such as River Lubigi in Kampala district had mean dissolved oxygen below the recommended WHO drinking water standards. Therefore, in order to ensure universal access to safe drinking water, the water sources that had vital physicochemical parameters outside the WHO drinking water range could be targeted for further studies.

There were statistically significant differences in the water physicochemical characteristics between the different sites and sources (lakes, rivers, springs and ponds). Despite these differences, the required approaches to ensure safe water access to the communities may not differ across sites. First and foremost, all sites and water types will need measures that reduce the high water turbidity to WHO acceptable levels. Secondly, in few instances, such as the water sources with pH in acidic range (Katanga spring in Kampala district, Lake Victoria Basin and Wanseko pond in Buliisa district, lake Albert basin) in addition to requiring further studies to identify the causes of the low pH (acidity), such water sources may also require the use of water treatment methods that neutralize the excess acidity [ 54 ]. Furthermore, since acidity is usually associated with increased solubility of toxic heavy metals (lead, arsenic and others) [ 34 ], testing such water for metallic contamination may be required. Heavy metal contamination of water causes ill-health due to chronic exposure which is cumulative and manifest late for correction to be done [ 70 ].

The findings of this study also highlight the differences in water quality between the urban surface water sources and springs (Kampala district) and the rural surface sources and springs (other study districts – Kasese, Kayunga, Busia, Nebbi and Buliisa) The water sources that met the WHO recommended drinking water quality standards [ 53 ] were mostly the rural springs and the rivers. However, these differences between the rural and the urban water sources do not alter the required approaches to ensure access to safe water which is by promoting measures that reduce the high water turbidity in combination with water disinfection to remove the pathogens. The relatively good quality of rural water sources compared to the urban ones could have been due to availability of plenty of vegetation in rural setting that filtered the water along the way downstream and possibly low level of pollution from industrial inputs in rural areas than in urban areas [ 71 , 72 ].

In relation to cholera outbreaks in the study communities, naturally, the physicochemical conditions for survival of V. cholerae O1 occur in an estuarine environment and other brackish waters [ 73 , 74 ]. In such circumstances, the favourable physicochemical conditions for V. cholerae isolation are the high water turbidity [ 49 ] and temperature of above 17 °C [ 43 ]. Interestingly, all the surface water sources and the springs tested had favourable physicochemical characteristics for the survival of V. cholerae in terms of these two parameters (high water turbidity and temperature of above 17 °C). Furthermore, two lakes sites (Kahendero FLS and Hamukungu FLS, Lake George, Kasese district) had also favourable mean pH for the survival of V. cholerae of 9.03 ± 0.17 and 9.13 ± 0.23 respectively. Favourable pH for V. cholerae survival in waters of Lake George was previously documented in the same area [ 23 ]. Hence, the frequent cholera outbreaks [ 19 , 20 , 21 , 24 ] in the study area could be attributed to both the favourable physicochemical water characteristics and use of unsafe water.

There were wide variations in conductivity between water sources and within the same source overtime. High water conductivities were recorded in the months of January to March 2015 (dry season), possibly due to high evaporation which increased the concentration of electrolytes present in water. Likewise, two rivers namely. River Lubigi (Kampala district) and Nyamugasani (Kasese district) had higher mean conductivities of 460.51 ± 57.83 μS/cm and 946.08 ± 3.63 μS/cm respectively than for typically unpolluted river of 350 μS/cm [ 75 ]. Consequently, given that the two rivers flow through areas of heavy metal mining (copper and cobalt mines in Kasese district by Kilembe Mines Limited and Kasese Cobalt Company Limited) and industrial activities (Kampala City), it is possible for the high water conductivity to be due to the heavy metal contamination as previously documented in drinking water in South-western Uganda [ 62 ] and Kampala City [ 61 ]. Thus, specific studies are required on water from the two rivers to determine the true cause of the high conductivity and to guide mitigation measures.

Hence, more efforts are required to promote safe water access in Uganda to attain the WHO cholera elimination target [ 25 ] and SDG 6 by 2030 since 26% (36/135) of mean physicochemical water tests did not meet WHO drinking water quality standards [ 53 ]. These findings together with those of the previous studies which demonstrated the presence of pathogenic V. cholerae in the same water sources [ 22 , 23 , 76 ] should guide stakeholders to improve access to safe water in the Great Lakes basins of Uganda holistically. Thus, measures such as promotion of use of safe water (using water disinfection), health education, sanitation improvement and hygiene promotion that address both the water bacteriological contents and physicochemical parameters should be considered in both the short and medium terms. However, long term plan to increase access to safe water by construction of permanent safe water treatment plants and distribution systems (pipes) should remain a top priority.

In the short and intermediate period, focusing on the measures that reduce water turbidity and disinfection of water (to kill microorganisms) should be prioritized so as to facilitate progress towards attainment of SDGs and cholera elimination in the study area. The basis for such prioritization lies in the fact that high water turbidity raises water temperature and prevents the disinfection effects of chlorine on water. These in return promote survival of the microorganisms and consequently cholera and other waterborne disease outbreaks. Furthermore, though boiling of water is feasible and recommended through technical guidelines [ 26 ] since it addresses both turbidity and kills the micro-organisms, it has issues of poor compliance due to lack of firewood which is the main cooking energy source in these communities [ 70 ]. Therefore, alternative safe water provision targeting reduction of high water turbidity and removal of microorganism by special filters such as decanting and sand filters and flocculation agents which do not need heat energy should be promoted [ 77 , 78 ]. Also, there is a need to explore the use of solar energy (solar water purifiers) [ 79 ] in these communities given their location in the tropics where sunshine is plenty. In the minority of situations, in addition to use of above methods to make water safe, there may be a need to employ different approaches of water purification depending on the water source. For example the water sources with lower or higher than recommended pH [ 53 ] (Wanseko pond, Hamukungu and Kahendero FLS on L. George), use of water treatment reagents that are affected by pH such as chlorine tablets should be reevaluated.

In additional to disinfection and turbidity corrective measures for all the water that were studied, each of the springs in the study area (Katanga in Kampala district and Nyakirango and Kibenge springs in Kasese district) will also need a sanitary survey (a comprehensive inspection of the entire water delivery system from the source to the mouth so as to identify potential problems and changes in the quality of drinking water) [ 80 ]. The findings of the sanitary survey should then guide the medium and long term interventions for water quality improvement in areas served by targeted springs. The following are some of the interventions that could be carried out after a sanitary survey: provision of a screen to prevent the entrance of animals, erecting a warning signs, digging of a diversion ditch located at the uphill end to keep rainwater from flowing over the spring area, establishment of an impervious barrier (a clay or a plastic liner) to prevent potential contaminants from entering into the water or and others measures described in the handbook for spring protection [ 81 ].

Furthermore, as a stopgap measure while access to safe water is scaled up, the communities in the study area should be protected from cholera using Oral Cholera Vaccines [ 82 ]. Protection of these communities is necessary since this study shows that favorable conditions for cholera propagation/transmission are present in the water in the study area. The favorable conditions that were documented in this study included the high water turbidity which makes it difficult to disinfect water [ 53 ] and the water temperature of above 17 °C which speeds up the multiplication of pathogens [ 43 ].

In addition, there were some other important study findings that were not fully understood. For example, some water sources (Kibenge spring and pond (located in Kasese district, western Uganda) had extreme vital physicochemical values for both conductivity and water temperature relative to the rest of above 40 °C and 3000 μS/cm respectively. It is possible that the extreme values were due to geochemical effects documented in water sources around Mount Rwenzori [ 83 ]. However, since there was copper and cobalt mining in Kasese district, high water conductivity could have been due to chemical contamination. Similarly, River Lubigi, Kampala district (central Uganda) had very low dissolved oxygen of less than 1 mg/L during some months (for example in January 2015, dissolved oxygen of 0.45 mg/L) which could have been due to organic pollutants from the communities in Kampala City [ 84 ] that used up the oxygen in the water. Also, Wanseko pond (Lake Albert basin, Buliisa district) had low pH of 4.84 in February 2015. Such water with low pH have the potential to increase the solubility of heavy metals some of which make water harmful when consumed [ 85 ]. Therefore, further studies will be required to better understand such extreme values.

Strength and limitations of this study

This study had several strengths. First, the longitudinal study design that employed repeated measurements of water physicochemical characteristics from the same site and source. This design reduced the likelihood of errors that could arise from one-off measurements seen in cross-sectional study designs resulting in increased validity of the study findings. Second, the inclusion of a variety of the water sources from which drinking and domestic water were collected namely, lakes, rivers, ponds, springs and a canal from different regions of Uganda made the findings representative of the water sources in study districts. Third, use of robust equipment, Hach meters, HQ40d [ 68 ] which automatically compensated for the weather changes (corrected for possible confounders and biases) for the parameters that had effect on each other such as raising water temperature impacting on the water conductivity and dissolved oxygen. Forth, purposive selection of the districts with frequent cholera outbreaks, an important waterborne disease that is targeted for elimination locally within Uganda and globally by WHO [ 25 ]. This meant that the findings had higher potential for used by stakeholders targeting to improve access to safe water and those for cholera prevention.

There were also some study limitations. First, though the study identified the favourable conditions (higher than recommended mean water turbidity and temperature of above 17 °C) for cholera in the study area, we could not report on causal-effect relationship between V. cholerae and the parameters studied. Vibrio cholera e pathogens were detected by use of multiplex Polymerase Chain Reaction (PCR). The results for PCR test were interpreted as positive or negative for V. cholerae O1, O139, non O1, and non O139 [ 22 ]. These data were not appropriate for establishment of causal-effect relationship Therefore, further studies using appropriate methods are recommended to establish such relationships.

Second, during some months of the study, water samples could not be obtained from some sources especially the ponds that had dried up during the dry season. The drying up reduced the number of samples collected from these points. However, since the months without water were few compared to the entire study period, the impact of the missing data could have been minimal.

Third, water samples were only tested for the five key physicochemical water characteristics, Vital Signs [ 32 ] however, there are many other parameters that effect survival and health of living things namely, nitrates, copper, lead, fluoride, phosphates, arsenic and others. Studies are therefore required to provide more information on these other parameters not addressed by the current study.

The study showed that surface and spring water for drinking and other domestic purposes in cholera prone communities in Great Lakes basins of Uganda were unsafe in terms of vital physicochemical water characteristics. These water sources had favourable physicochemical characteristics for transmission/propagation of waterborne diseases, including cholera. All test sites (100%, 27/27) had temperature above 17 °C that is suitable for V. cholerae survival and transmission and higher than the WHO recommended mean water turbidity of 5NTU. In addition, more than a quarter (27%) of lake sites and 40% of the ponds had pH and dissolved oxygen outside the WHO recommended range of 6.5–8.5 and less than 5 mg/L respectively. These findings complement bacteriological findings that were previously reported in the study area which found that use of this water increased their vulnerability to cholera outbreaks [ 22 ]. Therefore, in order for Uganda to attain the WHO cholera elimination and the United Nations SDG 6 target by 2030, stakeholders (the Ministry of Water and Environment, the local governments, Ministry of Health development partners and others) should embrace interventions that holistically improve water quality through addressing both physicochemical and biological characteristics. Furthermore, studies should be conducted to generate more information on the other physicochemical parameters not included in this study such as detection of the heavy metal contamination.

Availability of data and materials

The datasets generated and/or analysed during the current study are available in the Mendeley Data repository, . The cholera incidence data used to identify the study area were from Uganda Ministry of Health and the district (Kasese, Busia, Nebbi, Buliisa and Kayunga) weekly epidemiological reports.


Analysis of Variance


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The authors are grateful to the following: the district teams and the communities in Kasese, Kampala, Nebbi, Buliisa, Kayunga and Busia districts for the cooperation and support; the Ministry of Health, Makerere University School of Public Health, Dr. Asuman Lukwago, Dr. Jane Ruth Aceng and Prof. AK. Mbonye for technical guidance. The authors are grateful to Dunkin Nate from John Hopkins University for training of the field teams on water sampling and testing. The authors also thank Ambrose Buyinza Wabwire and to Damari Atusasiire for the support in creating the map and statistical guidance respectively. Special thanks to the laboratory teams in the district hospitals; CPHL (Kampala) and John Hopkins University (Maryland, USA) for carrying out the water tests.

This study was funded by the Bill and Melinda Gates Foundation, USA, through John Hopkins University under the Delivering Oral Vaccine Effectively (DOVE) project. (OPP1053556). The funders had no role in the implementation of the study and in the decision to publish the study findings.

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GB, DAS, AKD and CGO conceived the idea. GB, CGO, AKD, MR, HK, AK, TO and CMG conducted the investigation. MR, HK and TO carried out data curation. MR, HK, GB, DAS, CMG, AKD and AK analysed data. GB, DAK, AKD, CGO, MR, AK, TO and CMG wrote the first draft. All authors read and approved the final manuscript.

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This study was approved by the Makerere University School of Public Health Institution Review Board (IRB 00011353) and the Uganda National Council of Science and Technology. Cholera data used in selection of the water bodies and study communities were aggregated disease surveillance data from the Ministry of Health with no personal identifiers. The laboratory reports on the water sources found contaminated during the study period were shared immediately with the district team to ensure that preventive measures were instituted to protect the communities. In addition, the communities served by such water sources were educated on water treatment/purification (filtration, boiling, chlorination, use of Waterguard ).

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Additional file 1..

The number and the type of water sources in each of the lake basins in cholera prone communities of Uganda that were enrolled in the study, February 2015 – January 2016.

Additional file 2.

One Way ANOVA test results for the differences within the study sites overtime (February 2015 – January 2016) and between sites.

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Bwire, G., Sack, D.A., Kagirita, A. et al. The quality of drinking and domestic water from the surface water sources (lakes, rivers, irrigation canals and ponds) and springs in cholera prone communities of Uganda: an analysis of vital physicochemical parameters. BMC Public Health 20 , 1128 (2020).

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Groundwater contamination is a global problem that has a significant impact on human health and ecological services. Studies reported in this special issue focus on contaminants in groundwater of geogenic and anthropogenic origin distributed over a wide geographic range, with contributions from researchers studying groundwater contamination in India, China, Pakistan, Turkey, Ethiopia, and Nigeria. Thus, this special issue reports on the latest research conducted in the eastern hemisphere on the sources and scale of groundwater contamination and the consequences for human health and the environment, as well as technologies for removing selected contaminants from groundwater. In this article, the state of the science on groundwater contamination is reviewed, and the papers published in this special issue are summarized in terms of their contributions to the literature. Finally, some key issues for advancing research on groundwater contamination are proposed.

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Groundwater is a major source of fresh water for the global population and is used for domestic, agricultural, and industrial uses. Approximately one third of the global population depends on groundwater for drinking water (International Association of Hydrogeologists 2020 ). Groundwater is a particularly important resource in arid and semi-arid regions where surface water and precipitation are limited (Li et al. 2017a ). Securing a safe and renewable supply of groundwater for drinking is one of the crucial drivers of sustainable development for a nation. However, urbanization, agricultural practices, industrial activities, and climate change all pose significant threats to groundwater quality. Contaminants, such as toxic metals, hydrocarbons, trace organic contaminants, pesticides, nanoparticles, microplastics, and other emerging contaminants, are a threat to human health, ecological services, and sustainable socioeconomic development (Li 2020 ; Li and Wu 2019 ).

Over the past three decades, chemical contamination is a common theme reported in groundwater studies. While groundwater contamination is a great challenge to human populations, this subject also presents a great opportunity for researchers to better understand how our subsurface aquifers have evolved and for decision makers to grasp how we can protect both the quality and quantity of these resources. Fresh water aquifers are one of the most important sections of the Critical Zone (CZ), which extends from the top of the vegetation canopy down to the bottom of the aquifer (Lin 2010 ). As part of the global effort to understand the functions, structures, and processes within the CZ, a range of investigations have been performed that contribute to our knowledge of the circulation and evolution of groundwater (Sawyer et al. 2016 ; Goldhaber et al. 2014 ).

Many of the contaminants in groundwater are of geogenic origin as a result of dissolution of the natural mineral deposits within the Earth’s crust (Basu et al. 2014 ; Pandey et al. 2016 ; Subba Rao et al. 2020 ; He et al. 2020a ). However, due to rapid expansion of the global population, urbanization, industrialization, agricultural production, and the economy, we now are faced with the challenge of the negative impacts of contaminants of anthropogenic origin. The countries most affected by these global changes are those that are going through rapid economic development, with many of them located in the eastern hemisphere (Clement and Meunie 2010 ; Hayashi et al. 2013 ; Lam et al. 2015 ). Thus, it is appropriate that this special issue entitled, “The fate and consequences of groundwater contamination” focuses on studies of the unique challenges related to contaminants of both anthropogenic and geogenic origin in groundwater in several countries in the eastern hemisphere, including China, India, Turkey, Bangladesh, Ethiopia, and Nigeria. Figure  1 illustrates the countries where the research was conducted and the classes of chemical contaminants reported in the articles in this special issue.

figure 1

Eastern hemisphere, showing the countries where the groundwater research was conducted and the classes of contaminants studied in the articles published in this special issue

The range of topics included in articles in this special issue includes: (1) Latest methods for detecting and tracking the movement of groundwater contaminants; (2) Novel techniques for assessing risks to human populations consuming contaminated groundwater; (3) Effects of groundwater contamination on the abiotic environment, such as soil, sediments, and surface water; and (4) Case studies and remedial actions to control groundwater contamination from natural and anthropogenic sources. The co-editors of this special issue anticipate that these articles will facilitate an understanding of the origins and extent of groundwater contamination and its consequences and will provide examples of approaches that can be taken for remediation of groundwater contamination and protection of groundwater quality.

Major Contaminants

Groundwater contamination is defined as the addition of undesirable substances to groundwater caused by human activities (Government of Canada 2017 ). This can be caused by chemicals, road salt, bacteria, viruses, medications, fertilizers, and fuel. However, groundwater contamination differs from contamination of surface water in that it is invisible and recovery of the resource is difficult at the current level of technology (MacDonald and Kavanaugh 1994 ). Contaminants in groundwater are usually colorless and odorless. In addition, the negative impacts of contaminated groundwater on human health are chronic and are very difficult to detect (Chakraborti et al. 2015 ). Once contaminated, remediation is challenging and costly, because groundwater is located in subsurface geological strata and residence times are long (Wang et al. 2020 ; Su et al. 2020 ). The natural purification processes for contaminated groundwater can take decades or even hundreds of years, even if the source of contamination is cut off (Tatti et al. 2019 ).

The numbers of classes of contaminants detected in groundwater are increasing rapidly, but they can be broadly classified into three major types: chemical contaminants, biological contaminants, and radioactive contaminants. These contaminants can come from natural and anthropogenic sources (Elumalai et al. 2020 ). The natural sources of groundwater contamination include seawater, brackish water, surface waters with poor quality, and mineral deposits. These natural sources may become serious sources of contamination if human activities upset the natural environmental balance, such as depletion of aquifers leading to saltwater intrusion, acid mine drainage as a result of exploitation of mineral resources, and leaching of hazardous chemicals as a result of excessive irrigation (Su et al. 2020 ; Wu et al. 2015 ; Li et al. 2016 , 2018 ).

Nitrogen contaminants, such as nitrate, nitrite, and ammonia nitrogen, are prevalent inorganic contaminants. Nitrate is predominantly from anthropogenic sources, including agriculture (i.e., fertilizers, manure) and domestic wastewater (Hansen et al. 2017 ; He and Wu 2019 ; He et al. 2019 ; Karunanidhi et al. 2019 ; Li et al. 2019a ; Serio et al. 2018 ; Zhang et al. 2018 ). Groundwater nitrate contamination has been widely reported from regions all over the world. Other common inorganic contaminants found in groundwater include anions and oxyanions, such as F − , SO 4 2− , and Cl − , and major cations, such as Ca 2+ and Mg 2+ . Total dissolved solids (TDS), which refers to the total amount of inorganic and organic ligands in water, also may be elevated in groundwater. These contaminants are usually of natural origin, but human activities also can elevate levels in groundwater (Adimalla and Wu 2019 ).

Toxic metals and metalloids are a risk factor for the health of both human populations and for the natural environment. Chemical elements widely detected in groundwater include metals, such as zinc (Zn), lead (Pb), mercury (Hg), chromium (Cr), and cadmium (Cd), and metalloids, such as selenium (Se) and arsenic (As). Exposures at high concentrations can lead to severe poisoning, although some of these elements are essential micronutrients at lower doses (Hashim et al. 2011 ). For example, exposure to hexavalent chromium (Cr 6+ ) can increase the risk of cancer (He and Li 2020 ). Arsenic is ranked as a Group 1 human carcinogen by the US Environmental Protection Agency (EPA) and the International Agency for Research on Cancer (IARC), and As 3+ can react with sulfhydryl (–SH) groups of proteins and enzymes to upset cellular functions and eventually cause cell death (Abbas et al. 2018 ; Rebelo and Caldas 2016 ). Toxic metals in the environment are persistent and subject to moderate bioaccumulation when they enter the food chain (He and Li 2020 ; Hashim et al. 2011 ).

Organic contaminants have been widely detected in drinking water, and many of these compounds are regarded as human carcinogens or endocrine disrupting chemicals. In groundwater, more than 200 organic contaminants have been detected, and this number is still increasing (Lesser et al. 2018 ; Jurado et al. 2012 ; Lapworth et al. 2012 ; Sorensen et al. 2015 ). Some organic contaminants are biodegradable, while some are persistent. The biodegradable organic contaminants originate mainly from domestic sewage and industrial wastewater. Many of these organic substances are naturally produced from carbohydrates, proteins, fats, and oils and can be transformed into stable inorganic substances by microorganisms. They have no direct toxic effects on living beings but can reduce the dissolved oxygen levels in groundwater. Common organic contaminants include hydrocarbons, halogenated compounds, plasticizers, pesticides, pharmaceuticals, and personal care products and natural estrogens, among others (Lapworth et al. 2015 ; Meffe and Bustamante 2014 ). Many of the halogenated compounds (e.g., chlorinated, brominated, fluorinated) are stable in the environment and can be accumulated and enriched in organisms, causing harmful effects in organisms from higher trophic levels, including humans (Gwenzi and Chaukura 2018 ; Schulze et al. 2019 ). The persistent organic contaminants are mainly compounds used for agriculture, industrial processes, and protection of human health (Lapworth et al. 2015 ). Because these compounds degrade very slowly or even not at all, they may permanently threaten the quality of groundwater for drinking purposes (Schulze et al. 2019 ).

Radioactive contaminants in groundwater can originate from geological deposits of radionuclides but also can originate from anthropogenic sources, such as wastes from nuclear power plants, nuclear weapons testing, and improper disposal of medical radioisotopes (Dahlgaard et al. 2004 ; Lytle et al. 2014 ; Huang et al. 2012 ). Radioactive substances can enter the human body through a variety of routes, including drinking water. However, radioactive contaminants have been rarely detected in groundwater at levels that are a threat to human health.

Biological contaminants include algae and microbial organisms, such as bacteria, viruses, and protozoa. For microbial contaminants, more than 400 kinds of bacteria have been identified in human and animal feces, and more than 100 kinds of viruses have been recognized (Shen and Gao 1995 ). Some of these microbial organisms originate from natural sources, but some include microscopic organisms that co-exist with natural algal species and compete for available resources (Flemming and Wuertz 2019 ; Lam et al. 2018 ). Drinking water contaminated by microbial contaminants can result in many human diseases, including serious diarrheal diseases, such as typhoid and cholera. Currently, the COVID-19 virus has resulted in pandemic affecting every corner of the world. This coronavirus is primarily transmitted from person-to-person through respiratory droplets (Centers for Disease Control and Prevention 2020 ). However, water contaminated by this virus also can threaten human health (Bhowmick et al. 2020 ; Lokhandwala and Gautam 2020 ). Algal contamination is very common in surface waters, such as lakes and reservoirs due to eutrophication, but algae are rarely found at a high biomass in groundwater.

Consequences of Groundwater Contamination

Groundwater contamination can impact human health, environmental quality, and socioeconomic development. For example, many studies have shown that high levels of fluoride, nitrate, metals, and persistent organic pollutants are a health risk for human populations (Wu et al. 2020 ). This is especially critical for infants and children who are more susceptible to the effects of these contaminants than adults (He et al. 2020b ; Wu and Sun 2016 ; Karunanidhi et al. 2020 ; Mthembu et al. 2020 ; Ji et al. 2020 ; Subba Rao et al. 2020 ; Zhou et al. 2020 ). For example, “blue baby syndrome,” also known as infant methemoglobinemia, is caused by excessive nitrate concentrations in the drinking water used to make baby formulas. Human health also can be affected by the groundwater contamination through effects on the food production system. Irrigation with groundwater contaminated by heavy metals and wastewater containing persistent contaminants can result in the accumulation of toxic elements in cereals and vegetables, causing health risks to humans (Jenifer and Jha 2018 ; Yuan et al. 2019 ; Njuguna et al. 2019 ).

Groundwater contamination also can negatively affect the quality of lands and forests. Contaminated groundwater can lead to soil contamination and degradation of land quality. For example, in many agricultural areas in arid regions, high groundwater salinity is one of the major factors influencing soil salinization (Wu et al. 2014 ). The soluble salts and other contaminants, such as toxic metals, can accumulate in the root zone, affecting vegetation growth. Groundwater contaminants also can be transported by surface water-groundwater interactions, leading to deterioration of surface water quality (Teng et al. 2018 ).

Sustainable economic development requires a balance between the rate of renewal of natural resources and human demand (Li et al. 2017b ). Freshwater is probably the most valuable of the natural resources. However, chronic groundwater contamination may reduce the availability of freshwater, breaking the balance between water supply and demand and leading to socioeconomic crises and even wars. Water shortages induced by contamination may become a factor causing conflicts among citizens in the future (Schillinger et al. 2020 ), possibly delaying the socioeconomic development of a nation. Groundwater contamination is not only an environmental issue but also a social issue, demanding collaboration between both natural scientists and social scientists.

Articles in the Special Issue

Nineteen papers are included in this special issue. The topics of these papers cover a range of contamination issues, including the sources of geogenic and anthropogenic contamination, seasonal cycles in contamination, human health risks, and remediation technologies. Figure  2 illustrates a word cloud generated using the words in the titles and abstracts of the articles in this special issue, showing the most frequently used terms. The word cloud shows that the most frequently used technical terms in the articles are water, risk, metals, nitrate, fluoride, polycyclic aromatic hydrocarbons (PAHs), health, limits, and values. These terms reflect the main topics of the articles, which cover the assessment of the concentrations of trace metals, fluoride, nitrate, PAHs, and other organic contaminants in groundwater and the associated risks to the health of human populations. Some more minor terms, such as geogenic, source, removal, statistical, EWQI, and mobility, indicate that some articles focus on evaluating the sources of groundwater contamination, approaches to groundwater quality assessment, and contaminant remediation techniques. The main contributions of each article in this special issue are summarized below.

figure 2

Word cloud generated using the words in the titles and abstracts of articles in this special issue

Toxic metals are persistent contaminants and can be bioaccumulated in human tissues via food chain (He and Li 2020 ). In this special issue, six articles focused on the assessing trace metal pollution in groundwater. Çiner et al. ( 2021 ) used multivariate statistical analysis to identify the sources of trace elements in groundwater, including Al, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, and Ba, and assessed the health risks from arsenic contamination in a region of south-central Turkey. Their research results indicate that the carcinogenic risks from exposure to arsenic to both adults and children were higher than the guideline limit, and the geogenic processes are the main cause of trace element contamination in groundwater in this region. Chandrasekar et al. ( 2021 ) also identified geogenic metal contamination in their article focused on the source, geochemical mobility, and health risks from trace metals in groundwater in a Cretaceous-Tertiary (K/T) contact region of India. However, Raja et al. ( 2021 ) concluded that industrial activities and leaching from municipal dumpsites were the main sources of the metal pollution in the groundwater in the industrialized township (Taluk) of Virudhunagar in India.

In addition to contamination of groundwater, trace elements can be transported via groundwater into surface waters and into oceans. In the article by Prakash et al. ( 2021 ), estimates were made of the submarine groundwater discharge and associated trace element fluxes from an urban estuary region to the marine environment in the Bay of Bengal in India. This study revealed that submarine groundwater discharge is an important factor contributing to the fluxes to the sea of dissolved trace elements.

Finding efficient and cost-effective technologies for removal of trace elements from groundwater is crucial for the sustainable management of water resources. Zhao et al. ( 2021 ) studied Cd removal from water using a novel low-temperature roasting technique associated with alkali to synthesize a high-performance adsorbent from coal fly ash. Dutta et al. ( 2021 ) proposed to use electrocoagulation with iron electrodes as a treatment technology for arsenic removal from groundwater, and a pilot scale filtration unit was used to remove ferric hydroxide flocs produced during the process.

Fluoride is of value in trace amounts for promoting dental health, but this anion is toxic when present in high concentrations in water and food (Adimalla and Li 2019 ; Li et al. 2014 , 2019b ; Marghade et al. 2020 ). In this special issue, two articles specifically address fluoride occurrence, distribution, and health risks. The article by Haji et al. ( 2021 ) describes a study of groundwater quality and human health risks from fluoride contamination in a region within the southern Main Ethiopian Rift. Keesari et al. ( 2021 ) used the empirical cumulative density function to estimate the health risks from consuming fluoride contaminated groundwater in northeastern parts of Rajasthan in India. These authors also produced a fluorosis risk map to aid decision makers in taking necessary remedial measures to improve the groundwater quality.

Organic pollutants, including polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs), are common contaminants of anthropogenic origin in groundwater that could cause serious health problems. In this special issue, two articles focused on these organic pollutants. The article by Ololade et al. ( 2021 ) reported an investigation into PAHs and PCBs in groundwater near selected waste dumpsites located in two southwestern states in Nigeria. They found that the more water-soluble, low molecular weight-PAHs accounted for more than 61% of the total PAHs detected across all locations, but surprisingly the more highly chlorinated hexa-PCBs dominated the congener profiles. In another paper in this issue by Ambade et al. ( 2021 ), the occurrence, distribution, health risk, and composition of 16 priority PAHs were investigated in drinking water from southern Jharkhand in the eastern part of India. These authors found that lower and middle molecular weight PAHs were dominant in groundwater from the study area, but the levels are currently below concentrations that are a carcinogenic risk.

Studies of radioactive elements in groundwater often are neglected, but these radionuclides can be a hazard to human health. Adithya et al. ( 2021 ) conducted a study in Tamil Nadu state in southern India to measure the levels of radon (Rn) in groundwater and quantify the health risks. Their study showed that the Rn is released into groundwater from granitic and gneissic rocks within uranium-enriched lithological zones. However, the Rn levels determined in Bequerels per litre were lower than the guideline limit and the groundwater does not pose health risks to consumers.

In this special issue, Adimalla and Qian ( 2021 ) conducted a study on the spatial distribution and potential health risks from nitrate pollution in groundwater in southern India. The article revealed high nitrate levels in groundwater, at concentrations up to 130 mg/L. Both adults and children were judged to face health risks from consumption of nitrate in drinking water, but children were identified as more susceptible to the effects of groundwater nitrate pollution. The paper by Karunanidhi et al. ( 2021 ) describes the improvements in groundwater quality that occurred in an industrialized region of southeastern India between January and June of 2020. These improvements included reduced nitrate contamination, which may have been due to reduced transport of nitrate into groundwater before the monsoon period, but also could have been due to the decline in industrial and agricultural activity in the region during the lockdown in India that began in March 2020 in response to the first wave of the COVID-19 pandemic. In this study, fluoride concentrations of geogenic origin also were lower in groundwater before the monsoon.

Understanding the seasonal and spatial variations in groundwater quality is essential for the protection of human health and to maintain the crop yields. Subba Rao et al. ( 2021 ) used multiple approaches to identify the seasonal variations in groundwater quality and revealed that the groundwater quality for drinking and irrigation purposes was lower in the post-monsoon period relative to the pre-monsoon period. The deterioration of groundwater quality in the post-monsoon period was attributed to contaminant transport occurring through groundwater recharge but also was influenced by topographical factors and human activities.

Understanding the hydrogeochemical processes affecting groundwater chemistry is the basis for effective management of groundwater resources. Ren et al. ( 2021 ) adopted statistical approaches and multivariate statistical analysis techniques to understand the hydrogeochemical processes affecting groundwater in the central part of the Guanzhong Basin, China. The main contribution of this article is that it could help local decision makers to make water management decisions in the densely populated river basin by providing them with useful groundwater management options.

There are four articles in this special issue that focus specifically on methods to assess groundwater quality and humfluoride and associated arsenicosis and fluoan health risks. Shukla and Saxena ( 2021 ) assessed the groundwater quality and health risk in the rural parts of Raebareli district in northern India. Wang et al. ( 2021 ) identified the hydrochemical characteristics of groundwater and assessed health risk to consumers in a part of the Ordos basin in China. Adimalla ( 2021 ) applied two indices: the entropy weighted water quality index (EWQI), and the pollution index of groundwater (PIG) to assess the suitability of groundwater for drinking purpose in the Telangana state in southeastern India. Khan et al. ( 2021 ) assessed the drinking water quality and potential health impacts by considering physicochemical parameters, as well as bacteriological contamination of groundwater in Bajaur, Pakistan.

Collectively, these articles contribute to the literature on scientific developments in the field of groundwater contamination. The case studies presented in these articles are useful for policy makers and the public to understand the current water quality status in these regions. In particular, these articles provide a window into the groundwater contamination issues that are affecting low- and middle-income countries and countries with emerging economies in the eastern hemisphere. Researchers from Europe, North America, and other high-income countries often do not grasp the extent of groundwater contamination from geogenic and anthropogenic sources in these regions and do not realize that many human populations have no choice but to consume the contaminated drinking water.

The Way Ahead

Groundwater contamination is now a global problem and the resolution of these problems requires close collaboration among researchers in universities and government agencies, industries, and decision makers from all levels of government. To solve the groundwater contamination problems, international collaboration is needed. This is particularly true in countries with developing economies where financial resources and access to advanced technologies are not readily available. Special focus should be given to the following aspects of research and training:

Groundwater contamination issues in different countries should be addressed with a range of measures, techniques, and policies. Although groundwater contamination is a global problem, its nature and influencing factors are different between countries, climatic regions, and geological features. It may not be optimal to adopt remediation approaches that are successful in other countries or regions. For example, nitrate pollution is caused by fertilizer and manure applications in some agricultural regions (Zhang et al. 2018 ) but also may be caused by pollution by industrial and domestic wastewater in other areas, or even by explosives used in mineral exploration (Li et al. 2018 ). It may be necessary to use different approaches to mitigate different types of nitrate pollution. Even in instances where fertilizer application is the common cause of nitrate pollution in a tropical and a temperate region, the remediation approaches could be different, as climate factors and soil characteristics will have a great influence on the mechanisms and extent of contaminant transport.

With the rapid technological development, many novel techniques have been developed to study groundwater contamination, including geophysical and geoinformatics techniques. Geographical information systems (GIS) and remote sensing (Ahmed et al. 2020 ; Al-Abadi et al. 2020 ; Alshayef et al. 2019 ; Kannan et al. 2019 ) have accelerated the development of groundwater science. In the future, artificial intelligence, “big data” analysis, drone surveys, and molecular and stable isotope analysis technologies will be more widely available for applications in groundwater research. Groundwater scientists need to adopt and apply these new technologies for the study of groundwater contamination.

Governments, particularly in countries with developing economies need to invest in and encourage research and training in groundwater science. In many regions, human populations have no alternative but to consume groundwater that is contaminated with chemical or biological agents, potentially causing wide ranging health effects. Investment is needed to determine the extent of this contamination and how to remediate the impacts on human health, or to find alternate sources of drinking water.

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Editing a successful special issue is not easy. The Guest Editors must ensure that the topic is of importance and of broad interest so that there are an adequate number of contributors willing to submit their manuscripts. They must also make sure that the peer review process is efficient and effective, while maintaining the high quality of the papers. All of these cannot be fulfilled without the support of the Editor in Chief. So, we are extremely grateful for Prof. Chris Metcalfe’s guidance and support for this special issue. We are also sincerely thankful to the reviewers who provided constructive comments that are essential for maintaining the high quality of the special issue. Last but not the least, the authors whose manuscripts were included and those whose manuscripts were rejected are acknowledged for their interest in contributing to the special issue. The special issue was edited in a situation in which the COVID-19 struck in nearly every corner of the world. We are impressed by the dedication of doctors who fought and/or are fighting against the coronavirus. Prof. Peiyue Li is grateful for the financial support granted by the National Natural Science Foundation of China (41761144059 and 42072286), the Fundamental Research Funds for the Central Universities of CHD (300102299301), the Fok Ying Tong Education Foundation (161098), and the Ten Thousand Talents Program (W03070125), which allow him to carry out various investigations. The year 2021 is the 70th anniversary of Chang’an University. Congratulations!

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Surface water.

Surface water is any body of water found on the Earth’s surface, including both the saltwater in the ocean and the freshwater in rivers, streams, and lakes. A body of surface water can persist all year long or for only part of the year.

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Rivers are a major type of surface water. Surface water is a key component to the hydrologic cycle.

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Rivers are a major type of surface water. Surface water is a key component to the hydrologic cycle.

Surface water is any body of water above ground, including streams, rivers, lakes, wetlands , reservoirs , and creeks. The ocean, despite being saltwater, is also considered surface water. Surface water participates in the hydrologic cycle , or water cycle, which involves the movement of water to and from the Earth’s surface. Precipitation and water runoff feed bodies of surface water. Evaporation and seepage of water into the ground, on the other hand, cause water bodies to lose water. Water that seeps deep into the ground is called groundwater .

Surface water and groundwater are reservoirs that can feed into each other. While surface water can seep underground to become groundwater , groundwater can resurface on land to replenish surface water. Springs are formed in these locations.

There are three types of surface water: perennial , ephemeral , and man-made. Perennial , or permanent, surface water persists throughout the year and is replenished with groundwater when there is little precipitation . Ephemeral , or semi-permanent, surface water exists for only part of the year. Ephemeral surface water includes small creeks, lagoons , and water holes. Man-made surface water is found in artificial structures, such as dams and constructed wetlands .

Since surface water is more easily accessible than groundwater , it is relied on for many human uses. It is an important source of drinking water and is used for the irrigation of farmland. In 2015, almost 80 percent of all water used in the United States came from surface water. Wetlands with surface water are also important habitats for aquatic plants and wildlife.

The planet’s surface water can be monitored using both surface measurements and satellite imagery. The flow rates of streams are measured by calculating the discharge—the amount of water moving down the stream per unit of time—at multiple points along the stream. Monitoring the flow rate of streams is important as it helps determine the impact of human activities and climate change on the availability of surface water. Keeping track of vegetation around bodies of surface water is also important. The removal of vegetation , either through natural means such as fires, or through deforestation, can have a negative impact on surface water. Loss of vegetation can lead to increased surface runoff and erosion , which in turn can increase the risk of flooding.

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  • Published: 15 November 2023

Hydrogen-bearing vesicles in space weathered lunar calcium-phosphates

  • Katherine D. Burgess   ORCID: 1 ,
  • Brittany A. Cymes   ORCID: 1 , 2 &
  • Rhonda M. Stroud   ORCID: 1 , 3  

Communications Earth & Environment volume  4 , Article number:  414 ( 2023 ) Cite this article

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  • Geochemistry
  • Rings and moons

Water on the surface of the Moon is a potentially vital resource for future lunar bases and longer-range space exploration. Effective use of the resource depends on developing an understanding of where and how within the regolith the water is formed and retained. Solar wind hydrogen, which can form molecular hydrogen, water and/or hydroxyl on the lunar surface, reacts and is retained differently depending on regolith mineral content, thermal history, and other variables. Here we present transmission electron microscopy analyses of Apollo lunar soil 79221 that reveal solar-wind hydrogen concentrated in vesicles as molecular hydrogen in the calcium-phosphates apatite and merrillite. The location of the vesicles in the space weathered grain rims offers a clear link between the vesicle contents and solar wind irradiation, as well as individual grain thermal histories. Hydrogen stored in grain rims is a source for volatiles released in the exosphere during impacts.


Spectroscopic observations revealing a widespread hydration signal across substantial portions of the Moon 1 , 2 , 3 have reignited the discussion of the source(s) of lunar water and its mobility on the lunar surface originally raised by the return of Apollo lunar samples 4 . Similarly, although the presence of a tenuous lunar atmosphere, or surface boundary exosphere, was detected and measured by Apollo era experiments 5 , 6 , the first spectroscopic detection of native H 2 in the lunar atmosphere was reported using data collected by the Lyman Alpha Mapping Project (LAMP) on the Lunar Reconnaissance Orbiter (LRO) 7 , 8 . Recent analyses have shown that surface hydration may vary systematically with latitude, temperature, time of day, and presence of magnetic fields 9 , 10 , 11 , 12 , 13 , 14 . Further, telescopic data have shown that molecular water is definitively present in specific locations, potentially trapped within impact glasses or sheltered in voids between grains 15 . H 2 does not stick to the surfaces of silicates at lunar equatorial and mid-latitude temperatures 16 .

Lunar water is thought to originate from multiple sources, including indigenous reservoirs and external sources such as the solar wind, which implants hydrogen ions (H + ). Molecular H 2 , which can form through reaction between two implanted H atoms (or H and hydroxyl) 17 , 18 accounts for 7–54% of solar wind protons 19 . However, the mechanism by which solar wind H becomes trapped in the lunar regolith and its speciation has been challenging to interpret from the remote data available 20 , and thus the solar wind’s contribution to the lunar water budget remains unconstrained.

Laboratory measurements of lunar samples and experimental analogs, as well as modelling, have provided evidence for a relationship between lunar water (including hydroxyl) and the solar wind. Initial results from the Apollo samples demonstrated the release of molecular H 2 from some depth within the sample, similar to the profile of solar wind-derived helium 21 , 22 , but these studies were unable to confirm the presence of water. Some agglutinates, complex glass-welded aggregates found in lunar soil, show elevated H as hydroxyl (–OH) 23 , and pristine regolith grains with spectral features similar to the remote sensing observations from M 3 show a positive correlation between derived H 2 O concentration and maturity based on I s /FeO 24 . However, some degree of terrestrial contamination can be difficult to rule out for surface measurements even in pristine samples 25 . Analyses of Chang’E-5 samples have shown the presence of considerable H in grain rims in all phases measured 26 ; the abundances are much higher than those measured in Apollo samples and do not suffer from potential exposure to the lunar module cabin atmosphere 25 . However, nanoscale secondary ion mass spectrometry (NanoSIMS) used in some measurements cannot discriminate between H species, and thus studies make the assumption all H is present as –OH 26 . In addition, detailed connection between the hydroxyl or hydrogen measurements and other indicators of space weathering beyond bulk maturity index has been elusive, due to the spatial limitations of Fourier transform infrared spectroscopy and SIMS measurements.

Experiments on lunar soils and analog materials have demonstrated the formation of –OH due to H + irradiation, and have shown that the –OH signal is stable or slightly decreased at elevated lunar day-like temperatures 27 , 28 . Others have shown that a combination of ion irradiation followed by heating via pulsed laser to simulate micrometeorite bombardment leads to the formation of water-filled vesicles 29 . In temperature programmed desorption experiments that measure both H 2 O and H 2 , H 2 is assumed to be formed from H and –OH combination during the experiment and immediately released 17 , 30 . Simulations suggest that much of the initially implanted H does react with surface material rather than degassing directly to space as H 2 19 , but retention and reaction timing are strongly dependent on parameters that are not well-constrained 18 and could vary greatly based on temperature, composition, and maturity. For example, retention of molecular D 2 in irradiated olivine has been shown to rely heavily on the temperature of the sample during ion irradiation 31 . The potential for long-term trapping of molecular hydrogen has not been considered.

The space weathering features of lunar soil particles at the nanoscale provide detailed context to aid understanding of remotely sensed characteristics of the lunar surface 32 , 33 , 34 , 35 , 36 , 37 , as well as for potential utilization of resources 32 , 38 . To that end, we have examined space weathered soil grains of apatite (Ca 5 (PO 4 ) 3 (F,Cl,OH)) and merrillite (ideal: Ca 9 NaMg(PO 4 ) 7 ), which are the primary reservoir for phosphorous and rare earth elements on the Moon 39 . Apatite is the most common hydrated mineral on the Moon and a common accessory phase on other planetary bodies such as Mercury and multiple asteroids/meteorite types 40 , 41 , 42 , and analyzing how it responds to space weathering will aid in understanding how indigenous water sources interact with the solar wind. Both phases analyzed in this study show evidence of volatile-bearing vesicles in their space weathered rims. Our results demonstrate the presence of hydrogen species in these vesicles, and have important implications for the stability and persistence of molecular H 2 in regions beyond the lunar poles.

The apatite and merrillite grains we studied are from mature Apollo lunar sample 79221. The apatite sample is ~6.5 × 1.5 µm with a large portion of the grain surface on multiple sides being available for study (Fig.  1a, b ). This apatite grain was identified in the scanning electron microscope (SEM) prior to focused ion beam (FIB) preparation, while the merrillite grain was part of a dirt pile and located only after the sample was in the scanning transmission electron microscope (STEM) (Fig.  1c, d ). Space weathering varies between the top and bottom of the apatite, which are defined by how it was mounted in the epoxy and unrelated to its orientation on the lunar surface. The average composition from summed energy dispersive X-ray spectroscopy (EDS) spectra from several maps over the bulk grain indicate the apatite is F-rich (Supplementary Fig.  S1 , S2 ). We assume that F+Cl+OH = 1 on a formula unit basis (i.e., (F+Cl+OH):Ca is 1:5); we calculate an equivalent H 2 O content of ~1.16 ± 0.09 wt%, within the range of measured mare basalt apatites 39 (Table  1 ; Supplementary Table  S1 ). The merrillite grain is much smaller and adhered to an agglutinitic glass grain; it has sizeable REE content, as is common in lunar merrillite 43 .

figure 1

a SEM image of apatite particle mounted in epoxy. The sample surface has a number of adhered grains and apparent melt splashes. b SEM image of thinned FIB section extracted from location of yellow box in ( a ) showing multiple surfaces of grain available for study of space weathering features. c SEM image of dirt pile showing location of extracted slice. d SEM image of FIB section that includes a merrillite grain along with several other soil particles.

The merrillite grain is approximately 500 nm across and has 5–6 nm vesicles along portions of its outer edge, particularly near the interface close to the glass (Fig.  2 ). Compositional analysis with EDS shows minor Y, La, Ce and Nd, which are characteristic of lunar merrillite 43 (Table  1 ; Supplementary Fig.  S2 ). There are nanophase metallic iron particles (npFe 0 ) throughout the glass to which the merrillite grain is attached, including some concentrated near the boundary between the two phases (Supplementary Fig.  S3 ). Electron energy-loss spectroscopy (EELS) analysis of the low-loss energy range show that the plasmon of merrillite is complex, as expected for Ca-phosphates 44 . However, comparison of spectra from several vesicles in the merrillite rim near the glass show the presence of a peak at ~13 eV that is not present in the average bulk merrillite spectrum (Fig.  2d ), indicating they contain H-bearing species. The Ca M-edge in EELS leads to the strong peak in the data at ~35 eV 44 . The low-loss spectra edge shapes and peak height ratios at energies > 20 eV vary across the rim.

figure 2

a , b HAADF image and EDS map of grain adhered to npFe 0 -rich agglutinitic glass. c HAADF image from box marked in ( a ) showing the vesicular space weathered rim of the merrillite. The rim of the glass has abundant 1–5 nm npFe 0 . d Low-loss EELS spectra extracted from vesicles (1-green dashed line; 2-red solid line) and surrounding material (blue solid line). Several vesicles show clear peaks at 13 eV indicative of the presence of H-bearing species in the vesicles. Spectra are normalized to value at 20 eV and offset vertically for clarity. Inset shows spectra without offset. Mer = merrillite.

The bottom of the apatite (as mounted and oriented in Fig.  1b ) shows little evidence of alteration due to space weathering, but portions are coated in vesicular, npFe 0 -rich silicate glass, which is most likely a melt splash and contains both Fe 2+ and Fe 3+ (Supplementary Fig.  S4 ). Most of the top surface of the apatite has a crystalline, vesicular rim with small (2–5 nm) vesicles covered by a thin, poorly crystalline, vesicle-free layer of apatite composition (Fig.  3 ). The vesicles extend to a depth of ~130 nm. There is a slight decrease in P in the poorly crystalline material at the very surface, possibly caused by solar wind irradiation, but no difference exists in composition between the vesicle-rich layer and deeper material. Several small melt blebs containing Si and O with npFe 0 and minor other elements are also present along the surface, coating the poorly crystalline apatite rim (Fig.  3b ). Fast Fourier transform (FFT) images from regions along the surface (Fig.  3c insets) demonstrate the poorly crystalline nature of the surface, while the vesicles are present in crystalline material.

figure 3

a STEM HAADF image mosaic of top surface of apatite grain. b HAADF image at smaller field of view showing vesicles, vesicle-free rim, and npFe 0 -rich bleb. The vesicles extend to ~130 nm. c HAADF image and (inset) FFT patterns showing top ~20 nm is poorly crystalline while region around the vesicles is crystalline apatite. Dashed boxes show locations of FFT patterns.

Within the vesicular rim, there are several larger, elongate (possibly planar) vesicles (Fig.  4a ). Analysis of SAED patterns shows these vesicles lie in the (001) plane when viewed along the [-1 1 0] zone axis (Supplementary Fig.  S5 ). These large vesicles, which are parallel to each other, sit 80-100 nm below the apatite surface, directly beneath a glassy silicate bleb of variable composition that contains npFe 0 and nano-sulfides (Fig.  4b ). Based on both t / λ , where t is the thickness and λ is the inelastic mean free path of the material, and contrast differences in HAADF, the largest vesicle is about 1/3 the total thickness of the sample, or ~25 nm 45 . The EELS signals for spectra from within the vesicles shows the clear presence of a peak at 13.5 eV that is not present in spectra from pixels directly adjacent to the vesicles (Fig.  4c, d ). The sets of spectra, 1 & 2 and 3 & 4, have not been normalized relative to each other. Spectra from spectrum images acquired first with shorter dwell time per pixel and spectrum images collected after other analyses show similar peak intensity (Supplementary Fig.  S6 ), indicating beam damage does not meaningfully affect this measurement at these conditions. Differences between the spectra inside and outside the vesicles at energies less than 12 eV are below the level of noise. Specifically, we see no clear evidence of a peak near 8 eV, which would be associated with molecular water 46 . Differences at higher energies could be due to the thickness differences caused by the vesicle, or structural or compositional variation around the vesicle. Vesicles that lack a ~ 13 eV peak also display decreased intensity at higher energy relative to adjacent pixels.

figure 4

a HAADF image showing region with melt bleb with large vesicles beneath it. b EDS element map shows the bleb is silicate with variable composition of elements including Ti. c HAADF image showing individual pixels in the spectra image. d Low-loss EELS from pixels noted in ( c ). Solid numbered lines are from pixels within vesicles while dotted lines are from pixels directly beneath each vesicle. Solid black line is from unaltered apatite and shows average low-loss signal for the phase. Spectra from each vesicle offset vertically for clarity. e Map of relative intensity of O pre-peak at 531 eV (median filter) showing the regions with smaller pre-peak around the larger vesicles and the surface region with very sharp pre-peak. The silicate and carbon coat have no pre-peak. f Selected oxygen core-loss spectra summed from regions with similar pre-peak intensity, normalized at 540 eV. The oxygen K-edge EELS show large variation around the vesicles in this region beneath the silicate melt bleb. Dotted lines indicate window used for pre-peak map.

Oxygen K-edge EELS shows a pre-peak at ~531 eV that is highly variable in intensity relative to the main edge across the rim (Fig.  4e, f ). It is not limited to the vesicles and is broadly present in many areas of the rim. The pre-peak, suggestive of the presence of O 2 or of excess O and O-O defects 47 , 48 , 49 , 50 , is less pronounced in the region directly encompassing the hydrogen-containing vesicle than the surrounding rim material. In some regions of the rim where large vesicles are not present, such as that shown in Fig.  3b , there is no pre-peak, and the shape closely matches that of pure hydroxyapatite standards 51 . The main O-K edge for the apatite has two peaks, at ~537.5 eV and ~540 eV that vary in intensity relative to each other in the region around the vesicles. In general, the 537.5 eV peak is higher where the pre-peak is lower. There is also a clear difference around 545 eV, with an increase in intensity associated with an increase in the pre-peak. These variations do not directly correlate with pre-peak intensity throughout the apatite grain, but could still indicate differences in the relative F, OH, and Cl abundances. These relative peak intensities have been shown to vary between hydroxyapatite and two forms of tricalcium phosphate 51 , which have differences in oxygen bonding. The glassy silicate bleb shows a very low pre-peak and a broad main peak centered around 539 eV, consistent with its complex oxygen bonding configurations 52 .

The peak at ~13 eV in the low-loss EELS data from vesicles in both the apatite and merrillite grains is a clear indication that they contain hydrogen, most likely as H 2 . Other molecules, including H 2 O, O 2 , and CO 2 have peaks at or near 13 eV as well 53 , 54 . Those molecules, however, include other peaks in both the low-loss and core-loss energy ranges that are not evident in our data. Previous work has shown that hydrated samples will undergo reactions in the electron beam 46 , 55 , 56 , including formation of H 2 . However, in those cases, either other peaks are present, including a peak associated with molecular water at ~8 eV 46 , or the interaction of the electron beam with the material caused the formation of the vesicles themselves over time 55 . In the apatite and merrillite samples, the vesicles are present in initial fast-scan images and maintain their shape and size over the course of these measurements. The intensity of the 13 eV peak is also consistent following repeated measurements, indicating we are not inducing the formation of new H 2 in the vesicles during our analyses. The lack of lower energy peaks (< 10 eV) show that only H 2 is present in the vesicles prior to analysis. Additionally, as noted above, the O-K pre-peak at ~532 eV is broadly present in the rim of the apatite grain rather than being specifically associated with the vesicles, as might be expected if the electron beam was reacting with H 2 O or –OH during the analysis to form H 2 -bearing vesicles.

Based on measurement of the 13 eV peak intensity and the size of the vesicle, we are able to estimate the amount of H 2 in the vesicles 57 . For Spectrum 1 in Fig.  4d , we calculate a concentration on the order 10 27 molecules/m 3 , corresponding to an internal pressure of ~4 MPa at room temperature, assuming ideal gas behavior. A basic estimate of the volume of the vesicle gives a total of 5000–10,000 H 2 molecules in the largest vesicle in Fig.  4 . Vesicle 2 in the merrillite grain (Fig.  2 ) has a diameter of ~7 nm, and we calculate the same H 2 concentration, within a factor of < 2. The small, round vesicles in the apatite (~3 nm) are < 5% of the total thickness of the sample. If they also have similar concentration of H 2 , they would contain only tens of molecules, providing constraints on our detection limits at these microscope conditions.

The hydrogen-bearing vesicles in the Ca-phosphate grains are seen only within the space weathered rim, demonstrating the clear link between solar wind irradiation and their formation. Interestingly, the largest vesicles in the apatite, where the largest amount of hydrogen was seen to accumulate, are observed only near surface features indicating likely heating events. In the region shown in Fig.  4 , these large vesicles are directly below a silicate melt bleb. The conformation of the silicate glass to the apatite surface indicates that it was molten when it made contact, and thus could have provided considerable heat to the apatite to a depth of several hundred nanometers for up to several seconds based on cooling rates of lunar glasses 58 . Flash heating experiments using lunar soil grains have demonstrated the formation of vesicles after heating to 925 °C for < 1 s 59 , while experiments using laser irradiation following ion implantation have linked heating to OH/H 2 O formation and additional release of H 2 beyond that seen in ion implanted-only samples 29 . Our data, together with these experiments, demonstrate the importance of multiple factors, including high temperature, in the trapping and retention of solar wind hydrogen.

In the apatite, flash heating due to the melt bleb could affect indigenous –OH within the crystal structure in addition to the excess H/OH from the solar wind. Dehydroxylation of hydroxyapatite begins around 800 °C, depending on surrounding water vapor content and apatite composition (i.e., OH, F, and Cl content) 60 . In the decomposition reaction, two –OH groups combine to form one molecule of water and leave excess O within the apatite lattice. However, the conditions on the Moon are highly reducing compared to terrestrial conditions, and solar wind irradiation on even short time scales of tens of years is enough to implant substantial H into the space weathered rim beyond that accommodated by the apatite structure 26 . This could influence the reaction of H 2 O, O 2 and H 2 , especially at high temperature 61 . As noted here (Supplementary Fig.  S4 ) and in previous work on space weathered lunar samples 36 , 62 , 63 , oxidized and reduced constituents coexist in these materials.

The variable O-K edge and intense ~532 eV peak can also be related to potential reactions during solar wind irradiation and heating. Simulations of amorphous and crystalline SiO 2 have demonstrated that oxygen-excess defects produce a pre-edge peak at ~532 eV 49 , as seen in this apatite grain. Molecular O 2 has a similar pre-peak and has been identified in vesicles in an interplanetary dust particle 47 , although the O or O 2 here is not confined to the vesicles. The variation in relative heights of the ~537.5 eV and ~540 eV peaks indicates differences in the apatite structure around the vesicles, consistent with potential dehydroxylation or other changes due to heating. Variability in the shape of the low-loss spectra at energies >20 eV in different locations in the grain rims likely also indicates changes in cation bonding or the crystal structure due solar wind irradiation.

Our identification of a hydrogen signal associated with vesicles in a lunar space weathered rim confirms that a solar wind component is trapped within the grain, and persists in vesicles in detectable amounts even after heating to near maximum lunar daytime temperatures (140 °C). Importantly, we have found trapped molecular H 2 , which has a short residence time on the sunlit lunar surface of only a few hours 64 . The H 2 trapped in vesicles in space weathered rims could provide a reservoir that would be released in pulses by small and large impacts and potentially during microcracking due to diurnal thermal cycling 65 . Vesicles are widespread in lunar space weathered rims of many soil grains across all of the main phases (i.e., plagioclase, olivine, pyroxene, ilmenite) 33 , 34 , 35 and could have real consequences for the timing of volatile release into the exosphere as well as availability of volatile resources by crushing.

Until now, only helium has been unequivocally identified in vesicles in the Apollo samples, primarily in oxides 32 and Fe metal 66 . Our results here show H-species as H 2 in phosphate grains, suggesting that diffusion and retention differences between the various lunar phases play an important role in where and how volatiles species are formed, retained, and released on the lunar surface. Similar to helium-bearing ilmenite 32 and space weathered lunar sulfides 67 , some of the vesicles in the apatite appear to be planar and lie in the basal plane of the hexagonal crystal, highlighting the importance of crystal structure in volatile retention. It is also possible that the platelet-shaped vesicles common to hexagonal crystal structures 68 are highly suitable for measurement using STEM when prepared in an advantageous orientation, and thus H or He are more likely to be detected relative to other phases.

Continued measurements of natural samples and experimental simulations across a range of conditions and phases will aid in understanding the variables that affect H 2 , OH, and H 2 O formation and retention related to the solar wind. Recent experiments have shown that the temperature during irradiation can greatly affect the amount of H 2 (or D 2 ) released from olivine even after the sample has been stored at room temperature for several days 31 . Irradiation temperature appears to play a large role in natural samples as well, with Chang’E-5 samples, collected from cooler mid-latitude region compared to Apollo sample sites, retaining substantially more H than the more equatorial samples 26 . Studies following the LCROSS mission hypothesized that some H 2 released by the impact into a permanently shadowed crater could have been stored within defects in grains 69 . Our current results confirm that H 2 can be concentrated in vesicles, similar to He 32 , rather than only spread through the rim in individual defects. Given the need for impact heating for formation of water demonstrated by recent experiments 29 , it is possible that many small vesicles in lunar space weathered rims are in fact filled with H 2 .

The role and importance of composition in the long-term trapping of molecular hydrogen in space weathered rims are not well understood. If Ca-phosphates do in fact retain H 2 in vesicles more reliably compared to silicates, as suggested by our data, this could influence regional differences in remotely sensed signals of hydration or hydrogen-bearing materials. How or when such trapped H 2 could be converted to H 2 O likely relies on a number of factors that are not well constrained. Apatite and merrillite are minor phases on the lunar surface, comprising up to a few percent in specific lithologies, but often present in much lower amounts. Apatite has an overall estimated ~1% modal abundance 39 . However, as the most common naturally OH-bearing phase, it could contribute meaningful amounts to surface water signals and exospheric cycles. The relative contribution by the indigenous hydration would depend strongly on regolith composition and exposure timing. In regions with low agglutinate content, the apatite and merrillite could contribute an outsized proportion of the water present 70 , and some remote data suggest that the KREEP-rich material, which has elevated phosphorous, has a relative increase in surface-bound hydroxyl or molecular water 71 . If the hydrogen content of apatite is increased further due to H 2 retention in vesicular rims, the contribution could be further enhanced. Interestingly, this appears to be in contrast to other hydrated minerals affected by space weathering, such as phyllosilicates from asteroid Ryugu, which are dehydrated by the solar wind 72 .

The retention of some portion of solar wind hydrogen as H 2 in vesicles rather than adsorbed or as part of the structure as either –OH or molecular water has implications for the rates and timing of exospheric cycling of all hydrogen-bearing species. Future work is needed to understand the factors that control the trapping, retention, and speciation, including composition, temperature, and exposure. Additionally, the clear link between space weathering and hydrogen-filled vesicles in apatite and merrillite shows the potential for these minor phases to contribute meaningfully to the total water signal on the lunar surface and its accessibility and mobility. Apatite is a common accessory phase on planetary bodies, including Mercury and several asteroid/meteorite types 40 , 41 , 42 . Therefore, understanding how it is affected by both the solar wind and micrometeorite impacts, and how those processes work together to form water is very important for understanding volatile cycles on not only the Moon, but many bodies throughout the Solar System.

Sample preparation

The apatite and merrillite grains are from mature lunar soil 79221 (I s /FeO = 81). The particles were mounted in epoxy such that one surface of the grain was above the surface of the epoxy and available for imaging in the scanning electron microscope (SEM). Focused ion beam (FIB) samples were prepared using a FEI Helios G3 Dual-Beam FIB-SEM equipped with an Oxford 150 mm 2 SDD energy dispersive X-ray spectrometer (EDS). Protective straps of C were deposited on regions of interest following imaging, first with the electron beam, then with the Ga + ion beam. Sections suitable for STEM analysis were extracted using standard approaches at 30 kV and mounted on a Cu half-grid. The final sample thickness in the regions of interest on the apatite was t / λ  ~ 0.37, where t is the thickness and λ is the inelastic mean free path of the material, calculated using the log-ratio method.

STEM with EELS and EDS

Scanning transmission electron microscopy (STEM) analysis was performed with the Nion UltraSTEM200-X at the U.S. Naval Research Laboratory. Prior to analysis, the sample was held at 140 °C under vacuum for eight hours to drive off adsorbed surface water; a second sample from the apatite grain was loaded after being held under vacuum at room temperature for comparison. The microscope is equipped with a Gatan Enfinium ER electron energy loss spectrometer (EELS) and a windowless, 0.7 sr Bruker SDD-EDS detector. The STEM was operated at 200 kV and ~90 pA, with a 0.1 to 0.2 nm probe. STEM images were collected in bright field (BF) mode and high-angle annular dark field (HAADF) mode, which is sensitive to atomic number and thickness differences. Maximum pixel time during the scan was 16 µs and care was taken to limit the number of imaging scans needed on regions of interest prior to EELS data acquisition.

EELS data were collected as spectrum images, with a full spectrum over a selected energy range collected per pixel. The energy resolution for EELS, based on the full-width at half-maximum (FWHM) of the zero-loss peak (ZLP) is 0.5 eV. Peak alignment to compensate for systematic energy drift during EELS spectrum image acquisition was carried out using Gatan Digital Micrograph software based on shifts in the ZLP. Background removal from core-loss (O-K and Fe-L) spectra used a power-law fit. Dwell times per pixel are 0.1 ms for spectrum images that include the ZLP and up to 50 ms for core-loss Fe and O. Changes to the material and EELS signal was tracked over multiple scans (Supplementary Fig.  S6 ).

EDS data were also collected as spectrum images, allowing for semi-quantitative mapping of each element of interest, as well as summing of regions with uniform composition for quantification. Compositions were calculated using the Cliff-Lorimer method with instrument-specific k-factors. Oxide wt% is calculated from the cation fractions determined using Bruker Esprit 2.0 software. The sample is thin (i.e., t / λ «1), so no absorption correction was required. Errors are calculated based on the counting statistics for each summed region.

Hydrogen concentration

Low-loss EELS data were used to estimate the amount of gas within vesicles following the method of Walsh et al. 57 developed for He. Gas concentration within each bubble is calculated from

where n H2 is the number of H 2 molecules per m 3 , σ is interaction cross-section of H 2 , I H and I ZLP are the intensities of the ~13 eV and zero-loss peaks, respectively, and d is the thickness of the vesicle in the beam direction. The equation has been used for determination of helium content within vesicles in a number of materials 32 , 57 , 73 , 74 , and a similar procedure was used by Leapman and Sun 55 for measurement of H 2 in bubbles in frozen glycerol. For our experimental conditions, σ  = 5 × 10 −23  m 2   75 , and λ  = 225 nm 45 . Sample thickness without vesicles is t/λ  ~ 0.35. The factor of ½ provides the number of hydrogen molecules per unit area rather than hydrogen atoms.

Data availability

Data were collected using Gatan Digital Micrograph (.dm4) and Bruker Esprit (.bcf) formats. Readers for these data types are available through open-access applications such as HyperSpy. Primary data are available through Zenodo, .

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This work was supported by NASA awards 80HQTR19T0057 (ANGSA) and 80HQTR20T0014 (SSERVI RISE2). The samples were made available by the Lunar Sample Curation Office at NASA Johnson Space Center. This research was performed while Brittany Cymes held an NRC Research Associateship award at the U.S. Naval Research Laboratory.

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Katherine D. Burgess, Brittany A. Cymes & Rhonda M. Stroud

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Rhonda M. Stroud

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K.B.: Conceptualization, investigation, writing – original draft; B.C: validation, writing – review & editing; R.S.: Resources, supervision, writing – review & editing.

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Burgess, K.D., Cymes, B.A. & Stroud, R.M. Hydrogen-bearing vesicles in space weathered lunar calcium-phosphates. Commun Earth Environ 4 , 414 (2023).

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Received : 17 April 2023

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Published : 15 November 2023


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