https://orcid.org/0000-0002-6542-6315
Figures
Abstract
Manganese and iron decrease aesthetic acceptability and chlorine disinfection performance in drinking water, and can be toxic at high doses. We present the first characterization of manganese and iron occurrence in drinking water relative to concentration benchmarks for these three outcomes. Manganese and iron concentrations were evaluated in 261 drinking water samples obtained from boreholes and small groundwater-fed piped systems across large rural regions of Ghana, Mali and Niger. One or both metals exceeded aesthetic benchmark concentrations in 30% of samples and reached concentrations likely to interfere with chlorine disinfection in 5% of samples. Manganese exceeded 2011 WHO health-based benchmarks for drinking water (400 µg/L) in 2% of samples, and exceeded updated provisional health-based benchmark concentrations (80 µg/L) in 13% of samples. Iron occurred at levels exceeding health-based benchmarks in 5% of samples. These results suggest that manganese and iron contribute directly and indirectly to public health problems in a substantive proportion of drinking water sources in the study setting. Implementation of rural drinking water systems reliant on groundwater sources should account for the occurrence of these metals during siting, design, construction, operation, and monitoring/surveillance. Strengthening these capacities, particularly with respect to sampling and testing water sources for these metals, may support management and regulatory efforts to manage the occurrence of these metals in drinking water and their potential adverse effects. Finally, generating and synthesizing additional evidence on the occurrence and effects of iron and manganese in drinking water will support national efforts to manage both contaminants, inform discussions regarding the suitability of current health-based Mn guidelines for protecting sensitive life stages, and underscore the value of monitoring Mn as a priority chemical contaminant under Sustainable Development Goal (SDG) target 6.1: “By 2030, achieve universal and equitable access to safe and affordable drinking water for all.”
Citation: Browning EG, Bartram JK, Guo AZ, Tracy JW, Kelly ER, Fisher MB, et al. (2025) Manganese and iron in drinking water in three West-African countries: Implications for health, acceptability, and disinfection. PLOS Water 4(6): e0000234. https://doi.org/10.1371/journal.pwat.0000234
Editor: Guillaume Wright, PLOS: Public Library of Science, UNITED KINGDOM OF GREAT BRITAIN AND NORTHERN IRELAND
Received: February 14, 2024; Accepted: May 9, 2025; Published: June 18, 2025
Copyright: © 2025 Browning et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: All relevant data are within the paper and its Supporting information files.
Funding: This work was supported by World Vision [grant entitled: “Monitoring, Evaluation and Learning for WASH,” as well as World Vision grant #30703 to JKB]. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
1. Introduction
An estimated 2.5 billion people worldwide depend on groundwater sources for drinking water [1]. In rural settings in low- and middle-income countries (LMICs) such as many settings in sub-Saharan Africa, groundwater is often accessed using communal and private boreholes and wells [2]. While groundwater sources are often perceived to be safer than surface water sources, they remain vulnerable to anthropogenic and geogenic contamination [3]. While geogenic contaminants such as srsenic are well-studied [4]; other widespread geogenic groundwater contaminants such as manganese (Mn) and iron (Fe) [5] have been less extensively studied in many settings. In part, this is because iron is not directly toxic at levels present in drinking water [6], while Mn in drinking water was not, until recently, considered to be of particular human health concern at levels commonly encountered in drinking water [7].
Manganese and iron are abundant in the earth’s crust and exhibit chemical similarities. As a result, they frequently co-occur in groundwater and exhibit similar behavior in drinking-water systems, including their tendency to react with dissolved oxygen and free chlorine to form insoluble precipitates [8]. Manganese an iron impair the benefits of drinking water services with respect to aesthetic qualities, direct health effects, and impact on disinfection performance. With respect to aesthetic parameters, iron and manganese affect the color, taste, and in some cases turbidity of drinking water, particularly when these elements are present in oxidized states (oxidation rapidly occurs once groundwater comes into contact with air) [9–11]. These effects tend to reduce the palatability and acceptability of water for both consumption and non-consumption purposes. The benefits of access to improved drinking-water sources are thus diminished if Mn and/or Fe render water aesthetically unacceptable, particularly since consumers may switch to alternative sources that are less microbially safe[12,13].
The direct human health effects of iron in drinking water are limited, since iron is well tolerated and regulated in healthy individuals, although high levels of iron in drinking water can contribute to adverse health effects in those with iron storage disorders [14]. Manganese is a contaminant of human health concern when present at high levels in drinking water [5,15–19] because it accumulates in the blood and brain and has adverse neurological, reproductive, and respiratory effects [16,18,20,21]. Studies from Canada and Bangladesh suggest that Mn in drinking water may contribute to substantive adverse developmental, behavioral, and/or cognitive outcomes for sensitive life stages (children) [17,22–33] at concentrations occurring in drinking water in these settings. Cognitive and/or neurobehavioral impairment has been reported at levels as low as 100 µg/L [26,27].
With respect to aesthetic effects [9], human health effects [16,17,26–29,31,34–36], and chlorine disinfection performance (important because chlorine is the most commonly used disinfectant for treatment of drinking water [37]. Dissolved iron in drinking-water systems also reduces the aesthetic quality of water and chlorine disinfection performance [10,37,38]; furthermore, health-based standards have been proposed as a precaution against excess cellular iron storage, which can contribute to adverse health effects in those with iron storage disorders [14].
Manganese and iron also interfere with drinking water disinfection. Free chlorine (HOCl) is the most widely used drinking water disinfectant [5,37]. However, when manganese and iron are present in their reduced forms (i.e., Mn2+ and Fe2+, which are often present in reduced groundwaters) they react rapidly to consume free chlorine (Appendix 1), thereby decreasing or eliminating the concentration of chlorine available to inactivate microorganisms. As a result, manganese and iron in groundwater can impair disinfection performance and potentially compromise the microbial safety of drinking water.
By contrast, when present in aerobic waters such as surface waters, which typically contain several milligrams per liter of dissolved oxygen, iron and manganese rapidly oxidize to less soluble forms, which are less problematic with respect to chlorine disinfection. Because of these potential effects of Fe and Mn on drinking water quality, countries like Ghana have identified and mandated testing for Fe and Mn in the National Drinking Water Quality Management Framework (2015); however, capacity for monitoring of these chemicals in water systems serving rural communities can be limited in many LMIC settings [39].
As a result, there is dearth of knowledge on the occurrence of Fe and Mn in drinking water sources in many rural settings in LMICs, including West Africa, where the authors had the opportunity to assess these drinking water constituents as part of water quality monitoring fieldwork conducted in multiple settings, including rural Ghana, Mali, and Niger. While data were sought across a number of countries, usable data were obtained in only these three countries, as described previously [40]. Investigating the occurrence of Fe and Mn in ground water sources in these countries is an important step in characterizing and managing these potential hazards in rural groundwater supplies in these settings and may have relevance to other LMIC settings where groundwater is an important source of rural drinking water. We analyzed water samples collected from 261 small drinking water sources in multiple rural communities spanning large areas of Ghana, Mali, and Niger (Fig 1) to characterize the occurrence of iron and manganese in samples from these sources and compare observed concentrations to health, aesthetic, and disinfection performance benchmarks.
The objectives of this work include: quantifying two understudied geogenic transition metals commonly present in groundwater; assessing the potential aesthetic and health effects of corresponding drinking water exposures; reflecting on the potential role of reduced transition metals in interfering with effective drinking water disinfection using chlorine; and revisiting the legacy assumption that these metals are often of limited public health concern relative to other drinking water constituents. Potential implications include an opportunity to revisit iron and manganese as drinking water constituents worth monitoring, and to explore methods to efficiently characterize their distribution in rural LMIC water systems relying on groundwater sources.
2. Materials and methods
2.1. Ethics statement
This study was approved by University of North Carolina at Chapel Hill IRB (#17–0663), the Ministry of Water Resources in Ghana (reference: TJMSW, Feb 16, 2017), the University of Bamako Medical School in Mali (reference: 2017/105/CE/FMPOS, Aug 10, 2017), and the Ministry of Water Resources in Niger (reference: 000008/MH/A/DGH, Feb 27, 2017). Because data were collected about water systems (and not about any person), these did not comprise human subjects’ data, and there were no human subjects in this work. There were no deviations from approved protocols.
2.2. Sample collection and analysis
Samples were collected and analyzed as described in Fisher et al. [40]. Briefly: the study setting comprised multiple rural areas spanning large parts of Ghana, Mali and Niger. A cluster-randomized sampling approach was taken; this approach was sub-nationally representative, including all rural census blocks for each country. In each country, clusters comprised census blocks of approximately 200 households. In these, water points were mapped and all improved water sources with metal parts (e.g., taps and standpipes, sources with handpumps) were identified. One such water point from each cluster was randomly selected for water sampling and source inspection. In the study setting, both handpumps and standpipes were generally fed by groundwater, specifically boreholes with either manual lifting (handpump) or mechanical lifting to an elevated storage tank (standpipe). Water quality samples and direct measurements and observations were made. Water samples were collected in 1-L wide-mouthed sample bottles from 261 water points (95 in Ghana, 90 in Mali, and 76 in Niger) included boreholes with handpumps (n = 156) and public taps (n = 105).
At each selected water source, the following water system characteristics were observed and verified by a water committee member or other community member with knowledge of the water system: water system type, age and reported implementer; water source (surface water or groundwater, if known), and handpump type (where applicable). Water system types were classified according to the WHO/UNICEF Joint Monitoring Programme classification of drinking water system types [41]. GPS coordinates, water system functionality, water flow rate, pH, and conductivity were recorded. Flow rate, pH, and conductivity were largely used in other analyses and summaries reported separately [42].
For water sampling, a stagnation period of approximately one hour was observed and documented (for the purposes of quantifying lead and other water-system derived metals, reported separately [40]), after which an unfiltered one liter water sample was collected in a new 1-liter high-density polyethylene bottle (“first-draw” sample) and pH and conductivity were recorded. The samples were preserved (without any other processing or filtration) in the sample container by acidification to a pH of 2.5 or lower with TraceMetal™ grade nitric acid (Thermo Fisher Scientific, Waltham, MA), as is standard practice prior to analysis of total metals in drinking water [43]. Total metals were acidified without filtration and analyzed as a conservative estimate of total Fe and Mn occurrence in drinking water; filtration and speciation were not conducted. Since many users transport and store water for varying durations prior to consumption [42], it is not reasonable to imagine that speciation at the point of collection is representative of the water consumed by populations. Temperature was not recorded- it is likewise not reasonable to equate in-situ groundwater temperature with temperatures occurring when extracted water is treated, stored, and/or consumed. Field blanks and duplicates were collected from a random subset of sources (~5% each for blanks and duplicates).
Acidified samples were delivered to local, accredited commercial laboratories for analysis. Samples collected in Mali were analyzed in an approved laboratory using inductively coupled plasma mass spectrometry (ICP-MS) in accordance with ISO method 17294. Samples collected in Ghana and Niger were analyzed in an approved laboratory in Ghana using inductively coupled plasma optical emission spectrometry (ICP-OES) in accordance with APHA method 3020B [43]. In both laboratories, daily laboratory blanks, duplicates and calibration curves were prepared and analyzed. Lower limits of detection were reported by each laboratory for each element analyzed.
2.3. Data management and statistical analysis
Raw data were cleaned, merged, and analyzed in R (The R Project for Statistical Computing, version 3.5.1). Conductivity was categorized into quartiles and pH was categorized as either below, within, or above the 6.5-8.5 recommended range for drinking water systems [6]. Exceedances for total manganese and total iron were coded as a binary variable (0 = no exceedance, 1 = exceedance) based on benchmarks drawn from guidelines published by the US Environmental Protection Agency [44], the World Health Organization [6], and the Joint FAO/WHO Expert Committee on Food Additives (JECFA) [14], Table 1. A provisional health-based benchmark, potentially relevant to sensitive life stages, was derived from Bouchard et al., and others, who report that Mn concentrations on the order of 100 µg/L in drinking water are associated with cognitive impairment in Canadian and Bangladeshi children [25–27], and from the updated provisional health-based WHO recommendation of 80 µg/L in drinking water for bottle-fed infants [11].
The benchmarks for disinfection performance effects were calculated stoichiometrically based on chlorine demand as a function of iron and manganese concentrations, assuming both metals are present in their reduced (2+) forms, assuming free chlorine is present at the 2 mg/L (2000 µg/L) dose recommended by WHO for clear water ((WHO), 2015), and neglecting other sources of chlorine demand. Resulting benchmark concentrations were 1.5 mg/L (Mn) and 3.2 mg/L Fe to completely exhaust 2 mg/L of free chlorine (S2 Text). Benchmark concentrations for Fe and Mn in drinking water are summarized in Table 1, based on evidence from the literature (health and aesthetic effects) and stoichiometric calculations (disinfection performance effects). The proportions of samples exceeding each benchmark were calculated, disaggregated by country, source type, source age, pH, and conductivity. Multivariable linear regressions were conducted to analyze associations between Mn and Fe concentrations controlling for country, source type, source age, pH, conductivity, and stagnation time.
2.4. Geospatial statistics and mapping
Geospatial autocorrelation of sample concentrations was measured using the Moran’s Index value; specifically, global Moran’s I and inverse distance weighting were used to test for autocorrelation. Geospatial analysis was conducted using ArcGIS Pro 3.2 (Esri, Redlands, CA). GIS point files comprising study primary data (in Shapefile format) were overlaid on subdistrict polygon Shapefiles. Open-source shape files for all maps used, including subdistrict boundary files were obtained from GADM (https://gadm.org). Subdistrict boundary files were spatially joined to the point data (using the tool “Spatial Join”) to attach the subdistrict name information to each point. Through this process, each point was assigned the name of the administrative unit that it falls within. Next, summary statistics were calculated on the point file. Using the subdistrict name as the grouping variable, median iron and manganese concentrations were calculated for each subdistrict with one or more observations. The final step was to join the summary statistics table to the subdistrict boundaries, which allowed the median iron concentration to be displayed for each subdistrict as a shading color; manganese occurrence data were displayed as circular icons with size scaled to reflect concentration.
3. Results
3.1. Descriptive statistics: Widespread occurrence of Mn and Fe in drinking water
The reported age of water points ranged from less than 1 year to 61 years. The mean pH was 6.6 (range = 3.27-9.87; S.D. = 0.99). The mean conductivity was 352.9 µS (range = 13–3596 µS; S.D. = 391). Across all three countries, Fe was detected in 76% of samples and Mn in 88% of samples. The arithmetic and geometric mean concentrations were 789.6 and 200.6 µg/L, respectively for Fe and 51.7 and 21.2 µg/L for Mn (Table 2).
3.2. Exceedances of benchmarks for Fe and Mn in drinking water
Across all three countries, exceedances of aesthetic benchmarks by Mn and/or Fe occurred in 30% of samples (n = 79, Table 3). In 23 samples (9%), both Mn and Fe were greater than their respective aesthetic benchmarks. Manganese exceeded prior (2011) WHO health-based benchmarks in 2% of samples (n = 5) and exceeded the current (2020) 80 µg/L WHO provisional health-based benchmark in 13% of samples (n = 34), while Fe exceeded health-based benchmarks in 5% of samples (n = 12). In 5% of samples, the chlorine demand due to the combined concentrations of Mn and Fe (assuming both were present in reduced form) exceeded the disinfection performance benchmark (i.e., would eliminate the recommended free chlorine dose of 2 mg/L for chlorine disinfection) (n = 14). In 10% of samples, the estimated chlorine demand due to Mn and Fe exceeded 1 mg/L free chlorine (n = 25). In 2% of samples (n = 5) the estimated chlorine demand exceeded the 4 mg/L free chlorine dose recommended for turbid water.
3.3. Determinants of iron and manganese occurrence
Multivariable linear regression shows that observed Mn and Fe concentrations were significantly associated with each other. Neither Mn nor Fe concentrations varied significantly with country, source type, source age, or pH. The concentrations of Mn (but not of Fe) were significantly associated with conductivity (Table 4).
The Moran’s Index value, which measures spatial autocorrelation, was 0.009 (p = 0.894) for Mn and -0.01 (p = 0.92) for Fe. There was no evidence of autocorrelation using Global Moran’s I and inverse distance weighting for either Mn or Fe.
4. Discussion
The results reported here represent the first sub-nationally representative multi-country assessment of Mn and Fe occurrence in rural water sources in West Africa relative to aesthetic, health, and disinfection performance benchmarks.
4.1. Exceedances of concentration benchmarks
The finding that Mn and/or Fe exceed aesthetic benchmarks in 30% of sources is of concern if these exceedances prompt households to use potentially less safe alternative sources [5]. In addition to direct effects of Fe and Mn on taste, these metals form complexes and precipitates that contribute to turbidity and discoloration of food prepared using the water, further reducing acceptability, especially where populations are accustomed to using surface water. Field reports from the late 1980s–early 1990s suggest that, in parts of southern Ghana and Niger, up to 30% of handpumps were abandoned due to corrosion, which can contribute to aesthetic issues associated with dissolved Fe and other corrosion-related metals in the water [45], and more recent evidence suggests that corrosion and breakdown remain important issues for handpump operation in many settings [46–48].
The finding that Mn exceeded WHO benchmarks in 2% of samples suggests that some populations in the study setting are at risk of adverse health effects from Mn in drinking water. While Fe concentrations exceeded health-based benchmarks in 5% of samples, Mn exceedances are of direct health concern for the general population because excessive exposure to Mn is associated with adverse neurological, reproductive and respiratory effects [11,16–18,20–22,24,25,27,28,31,34,35,49]; however, for sensitive sub-populations with iron metabolism disorders, Fe exceedances are problematic [10]. We also found that 11% of samples exceeded the secondary health benchmark concentrations of Mn previously reported to adversely impact cognitive and/or neurobehavioral outcomes among children in Canada and Bangladesh, and on-par with WHO’s provisional health-based guideline values [11,17,23–27,50]. These findings suggest that if these concentrations (e.g., 80–100 µg/L) do in fact represent conservative benchmarks for adverse health effects in sensitive life stages (children and developing fetuses) across different settings and populations, millions in West Africa and other populations relying on groundwater with high Mn concentrations) may be at risk for cognitive and/or neurological impairment.
The finding that 10% and 5% of water samples contained Mn and Fe in amounts together capable of consuming 1 mg/L and 2 mg/L of free chlorine, respectively, suggests that the recommended 2-mg/L chlorine dose would fail to treat drinking water at the household or community level in these settings [37]. In 2% of samples, even a 4-mg/L dose would be eliminated. These are conservative estimates of the proportion of water samples for which chlorine disinfection is compromised because organic matter, ammonia, and other reduced species increase chlorine demand further; because chlorine dosing is not always consistent from day-to-day in a given system; and because free chlorine may be intentionally dosed at less than 2 mg/L in some systems. Furthermore, the benchmark used for disinfection performance is complete elimination of free chlorine (assuming all Fe and Mn in reduced state), while even partial reduction in free available chlorine impairs disinfection performance relative to performance at the target disinfectant dose. While there was no evidence that water from these sources was disinfected with free chlorine at the source, approximately 5% of households in the study setting report practicing household water disinfection at the point of use [42].
4.2. Determinants of Fe and Mn occurrence
The strong correlation between Fe and Mn occurrence is consistent with previous studies which also report correlations between the two metals in groundwater [8,38]. This result is consistent with the similar chemistries of the two metals, and may also reflect the importance of common geological origins in the study settings. Iron is ubiquitous in the earth’s crust [51], and manganese deposits are plentiful in many parts of the study setting [52]. While some occurrence could also be attributable to corrosion of materials present in water systems, such as steel (which generally contains both manganese and iron), iron is typically present in corrosion layers on steel at a mass fraction ~2 orders of magnitude higher than manganese [53], while Mn occurred in water samples at levels well above this in our study (Table C in S1 Text). The lack of a significant association of the occurrence of either element with water source type, age, or pH contrasts with the findings of Langenegger et al. in West Africa [45]. Our findings suggest that geogenic influences (mobilization from subsurface mineral deposits) may dominate as the major source of Fe and Mn in the present work, whereas other studies have suggested that corrosion may dominate in some cases [45,46]. This change may reflect improvements in handpump siting and/or construction, or differences in the subnational sites captured by the two studies.
Our finding of an association of Mn, but not Fe, with conductivity may indicate geogenic sources of Mn with higher salinity, due to underlying geology; or may indicate some secondary role of galvanic water system corrosion in Mn occurrence in water samples. The lack of significant association of Fe or Mn concentrations with country suggests that occurrence is widespread across the study setting. Furthermore, the fact that Moran’s I was not statistically significant for either Fe or Mn indicates that their concentrations are not geographically clustered. Together, these findings suggest a phenomenon that is largely geogenic and widespread in the study setting, with potential secondary influences of corrosion.
4.3. Comparison to evidence from other settings
The concentrations of Mn detected in this study are broadly consistent with those reported elsewhere in sub-Saharan Africa, including Uganda and Nigeria [54,55], and other LMIC contexts such as India, Bangladesh, and Turkey [56–58].
4.4. Limitations
4.4.1. Internal validity.
Due to logistical constraints, two commercial laboratories were used for analysis of water samples (one in Ghana and one in Mali). Differences in analytical methods and equipment between these labs may contribute to variation in the concentrations of Mn and/or Fe reported across the countries, although this was minimized through calibration and quality control.
We report total concentrations of Fe and Mn in drinking water, and methods used did not allow for speciation of these elements. For the purposes of determining aesthetic, health, and disinfection effects, Mn and/or Fe were assumed to be in their reduced forms as a conservative “worst-case” assumption. If these elements were present in oxidized states, the aesthetic effects would be greater and the disinfection performance effects less than our estimates. Given the tendency of water from many of the sources studied to be transported and stored at the household level, it is perhaps unreasonable to assume that the speciation of samples taken at the point of collection would be representative of the distribution of species present in water consumed by individuals in their homes. Thus, the use of total metal concentrations without further speciation is potentially a reasonable and conservative approach in this setting. However, it is also worth noting that if Mn or Fe were present in groundwater in oxidized states, these metals can be expected to remain oxidized during extraction, transport, and storage, and may therefore contribute little to chlorine demand. Measurements of oxidation reduction potential (Eh) and dissolved oxygen concentrations (DO) can be made in-situ when water samples are collected, and can be useful in updating the conservative assumptions made in this work with actual data to better understand the speciation of Mn, Fe, and other metals. Furthermore, it is worth noting that Eh may interact with pH, conductivity, and other factors to control the speciation of metals such as Mn and Fe in groundwater and subsurface environments, and having a more detailed understanding of these parameters may therefore be helpful in understanding patterns in the occurrence, fate, and transport of these metals in groundwater and drinking water more generally, as well as their interactions with other species. For example, oxidized Mn and Fe can reduce the mobilization of arsenic in subsurface environments [59]. Further work should explore the contemporaneous in-situ measurement of Eh, DO, temperature, and other groundwater characteristics at the time of sample collection to enable such considerations to be reflected in further studies. In addition, collection of both filtered and unfiltered samples, as well as collection of additional system characteristics such as depth, etc. could be useful in better understanding the presence of dissolved vs particulate species and enhancing the ability to control for depth in analyses, etc.. Finally, scaled-up monitoring of such groundwater and drinking water samples to larger scales and larger numbers of observations could enable more precise investigation of potential spatial and temporal trends that may not be apparent at the scale and sample size of the current study. Furthermore, it is worth noting that Fe and Mn concentrations can vary with weather and precipitation, and future studies may seek to better incorporate these variables during data collection, analysis, and interpretation.
4.4.2. External validity.
The concentration benchmarks for aesthetic, health, and disinfection performance effects are based on published values and stoichiometry. However, the range of concentrations over which aesthetic, health, and disinfection effects occur is broad, particularly when sensitive subpopulations are considered, and cannot be fully captured by a single benchmark value. Furthermore, reduced forms of Mn or Fe interfere with disinfection performance at any concentration, making any benchmark for disinfection performance a simplification. Likewise, the concentrations of Mn and Fe at which health and aesthetic effects occur vary by individual and subpopulation, setting, life stage, and other factors; studies have suggested that Mn may contribute to adverse health effects on sensitive life stages at low concentrations on the order of those indicated in the new provisional health-based WHO guideline values [11,17,19,25,27,36,49].
4.5. Recommendations
Drinking water sources that contain Fe and Mn exceeding WHO guideline values should be avoided or treated to decrease concentrations below these levels wherever possible.
Attention should be paid to Fe and Mn concentrations in siting, construction, operation, and disinfection of small groundwater systems in settings reliant on groundwater. Specifically, pre-comissioning tests should include Fe and Mn.
Where iron or manganese occur at levels of concern, source substitution and/or pretreatment of groundwater may be warranted. Treatment by oxidation (e.g., with chlorine, permanganate, or by aeration) followed by clarification/sedimentation or filtration is relatively feasible and cost-effective. Biological filtration is an alternative [60–62]. In both cases, sustaining treatment may be challenging in small, community-managed water systems.
Disinfection protocols should be updated as needed to test and account for chlorine demand from Fe and Mn in groundwater, as well as other sources, to ensure effective disinfection and adequate free chlorine residuals.
Many suitable iron and manganese testing methods are available, including the laboratory methods used in this work (more accurate) as well as test strips and colorimetric methods (somewhat less accurate but easy to implement in the field for rapid diagnostic purposes).
Research is needed to determine whether adverse outcomes, direct or indirect, are occurring in the study setting (and elsewhere) as a result of Fe and/or Mn in groundwater. Potential adverse outcomes to be assessed could include: 1) avoidance of improved drinking water sources containing excessive Fe and/or Mn; 2) extent of source switching to safely- versus unsafely-managed water sources when improved sources have elevated Fe/Mn; 3) exposure to microbial hazards as a result of inadequate disinfection in the presence of elevated Fe/Mn, and/or 4) adverse health outcomes associated with manganese in drinking water at concentrations exceeding provisional WHO guideline values [11,16–18,24–28,31,36,49,50,58,63].
Such evidence would be helpful in reviewing the suitability of current WHO guidelines and national standards for protecting sensitive life stages.
Finally, more work is needed to characterize exposure to Mn and its consequences. Intake from water and other sources such as food; and study of the associated disease burdens could inform decisions on whether Mn should be considered a priority chemical contaminant of concern in drinking water, along with arsenic, fluoride, and lead, in monitoring safely managed drinking water under SDG target 6.1.
5. Conclusions
This is the first multi-country study evaluating Mn and Fe concentrations in rural West African settings with respect to aesthetic, health, and disinfection effects.
Our results suggest that a substantive proportion of the population in the study setting is exposed to Fe and/or Mn in drinking water at concentrations exceeding benchmarks for health, aesthetic acceptability, and/or disinfection performance. Specifically, 2% of samples exceeded the WHO health-based Guideline Value for manganese in drinking water, and 11% exceeded levels associated with lower IQ among children in previous studies. A substantive proportion of water sources exceeded aesthetic (30%) and chlorination disinfection performance (5%) benchmarks, with the potential for adverse consequences related to microbial safety.
Improved management and monitoring, by implementers and regulators, especially at the local level supported by appropriate regulation, are needed. Recommendations include suitable testing of groundwater for Fe and Mn (using methods capable of detecting these elements with adequate sensitivity) before new water system commissioning; surveillance of these elements in existing systems; substitution of sources exceeding applicable Fe and/or Mn benchmarks with sources conforming to these benchmarks where feasible; and adapting chlorination and/or other treatment processes to address the presence and effects of Fe and/or Mn. Strengthening regulations and surveillance agencies; and improving water sampling, testing, and potentially water treatment capacities may be necessary for some systems and settings. Location- and source-specific recommendations may be needed to ensure adequate chlorine disinfection, especially in settings where geospatial variability in the occurrence of these metals is substantive.
Supporting information
S1 Text.
Additional regressions, calculations, and details. Table A. Multivariable linear regression of manganese and iron concentrations as a function of country, pH, and conductivity. Table B. Multivariable linear regression of manganese and iron concentrations as a function of water system age, type, country, pH, and conductivity. Table C. Mean, median, and maximum concentrations of iron (Fe) and manganese (Mn) in drinking-water samples after a 1-h stagnation period. (Source: Fisher et al., Manuscript in Submission).
https://doi.org/10.1371/journal.pwat.0000234.s001
(DOCX)
S2 Text. Stoichiometric calculations for chlorine disinfection performance benchmark concentrations of (reduced) iron and manganese in groundwater.
https://doi.org/10.1371/journal.pwat.0000234.s002
(DOCX)
S1 Checklist. Waterpoint survey used for data collection at each sampled water source.
https://doi.org/10.1371/journal.pwat.0000234.s003
(DOCX)
Acknowledgments
The authors gratefully acknowledge World Vision for its support of this work, as well as the World Vision Ghana, Mali, and Niger country offices for their invaluable assistance in accessing field sites to collect samples and data.
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