Demand for Information on Environmental Health Risk, Mode of Delivery, and Behavioral Change: Evidence from Sonargaon, Bangladesh.

Millions of villagers in Bangladesh are exposed to arsenic by drinking contaminated water from private wells. Testing for arsenic can encourage switching from unsafe wells to safer sources. This study describes results from a cluster randomized controlled trial conducted in 112 villages in Bangladesh to evaluate the effectiveness of different test selling schemes at inducing switching from unsafe wells. At a price of about US0.60, only one in four households purchased a test. Sales were not increased by informal inter-household agreements to share water from wells found to be safe, or by visual reminders of well status in the form of metal placards mounted on the well pump. However, switching away from unsafe wells almost doubled in response to agreements or placards relative to the one in three proportion of households that switched away from an unsafe well with simple individual sales.

Effectiveness of Different Approaches to Arsenic Mitigation over 18 Years in Araihazar, Bangladesh: Implications for National Policy.

About 20 million rural Bangladeshis continue to drink well water containing >50 µg/L arsenic (As). This analysis argues for reprioritizing interventions on the basis of a survey of wells serving a population of 380,000 conducted one decade after a previous round of testing overseen by the government. The available data indicate that testing alone reduced the exposed population in the area in the short term by about 130,000 by identifying the subset of low As wells that could be shared at a total cost of 150 m) wells and a single piped-water supply system by the government reduced exposure of little more than 7000 inhabitants at a cost of US$150 per person whose exposure was reduced. The findings make a strong case for long-term funding of free well testing on a massive scale with piped water or groundwater treatment only as a last resort.

Sand Barriers around Latrine Pits Reduce Fecal Bacterial Leaching into Shallow Groundwater: A Randomized Controlled Trial in Coastal Bangladesh.

We evaluated the effectiveness of a sand barrier around latrine pits in reducing fecal indicator bacteria (FIB) leaching into shallow groundwater. We constructed 68 new offset single pit pour flush latrines in the Galachipa subdistrict of coastal Bangladesh. We randomly assigned 34 latrines to include a 50 cm thick sand barrier under and around the pit and 34 received no sand barrier. Four monitoring wells were constructed around each pit to collect water samples at baseline and subsequent nine follow-up visits over 24 months. Samples were tested using the IDEXX Colilert method to enumerate E. coli and thermotolerant coliforms most probable number (MPN). We determined the difference in mean log10MPN FIB counts/100 mL in monitoring well samples between latrines with and without a sand barrier using multilevel linear models and reported cluster robust standard error. The sand barrier latrine monitoring well samples had 0.38 mean log10MPN fewer E. coli (95% CI: 0.16, 0.59; p = 0.001) and 0.38 mean log10MPN fewer thermotolerant coliforms (95% CI: 0.14, 0.62; p = 0.002), compared to latrines without sand barriers, a reduction of 27% E. coli and 24% thermotolerant coliforms mean counts. A sand barrier can modestly reduce the risk presented by pit leaching.

Advection of surface-derived organic carbon fuels microbial reduction in Bangladesh groundwater.

Chronic exposure to arsenic (As) by drinking shallow groundwater causes widespread disease in Bangladesh and neighboring countries. The release of As naturally present in sediment to groundwater has been linked to the reductive dissolution of iron oxides coupled to the microbial respiration of organic carbon (OC). The source of OC driving this microbial reduction--carbon deposited with the sediments or exogenous carbon transported by groundwater--is still debated despite its importance in regulating aquifer redox status and groundwater As levels. Here, we used the radiocarbon ((14)C) signature of microbial DNA isolated from groundwater samples to determine the relative importance of surface and sediment-derived OC. Three DNA samples collected from the shallow, high-As aquifer and one sample from the underlying, low-As aquifer were consistently younger than the total sediment carbon, by as much as several thousand years. This difference and the dominance of heterotrophic microorganisms implies that younger, surface-derived OC is advected within the aquifer, albeit more slowly than groundwater, and represents a critical pool of OC for aquifer microbial communities. The vertical profile shows that downward transport of dissolved OC is occurring on anthropogenic timescales, but bomb (14)C-labeled dissolved OC has not yet accumulated in DNA and is not fueling reduction. These results indicate that advected OC controls aquifer redox status and confirm that As release is a natural process that predates human perturbations to groundwater flow. Anthropogenic perturbations, however, could affect groundwater redox conditions and As levels in the future.

Dissolved Organic Matter Quality in a Shallow Aquifer of Bangladesh: Implications for Arsenic Mobility.

In some high arsenic (As) groundwater systems, correlations are observed between dissolved organic matter (DOM) and As concentrations, but in other systems, such relationships are absent. The role of labile DOM as the main driver of microbial reductive dissolution is not sufficient to explain the variation in DOM-As relationships. Other processes that may also influence As mobility include complexation of As by dissolved humic substances, and competitive sorption and electron shuttling reactions mediated by humics. To evaluate such humic DOM influences, we characterized the optical properties of filtered surface water (n = 10) and groundwater (n = 24) samples spanning an age gradient in Araihazar, Bangladesh. Further, we analyzed large volume fulvic acid (FA) isolates (n = 6) for optical properties, C and N content, and (13)C NMR spectroscopic distribution. Old groundwater (>30 years old) contained primarily sediment-derived DOM and had significantly higher (p < 0.001) dissolved As concentration than groundwater that was younger than 5 years old. Younger groundwater had DOM spectroscopic signatures similar to surface water DOM and characteristic of a sewage pollution influence. Associations between dissolved As, iron (Fe), and FA concentration and fluorescence properties of isolated FA in this field study suggest that aromatic, terrestrially derived FAs promote As-Fe-FA complexation reactions that may enhance As mobility.

Salinisation of drinking water ponds and groundwater in coastal Bangladesh linked to tropical cyclones.

Salinity is a widespread problem along the Asian coast, mainly in reclaimed lands where most people live. These low-lying areas are vulnerable to impacts from tropical cyclone induced storm surges. The role of such surges on the long-term salinity of water resources, particularly the salinisation of drinking water ponds, a key water resource, requires further investigation. Here we show, using high-resolution measurements of pond hydrology and numerical modelling, that episodic inundation events cause the widespread salinisation of surface water and groundwater bodies in coastal areas. Sudden salt fluxes in ponds cause salinity build-up in the underlying sediments and become a source of salinity. Rapid clean-up of drinking ponds immediately after a surge event can significantly minimize these salinity impacts, which are likely to increase under climate change. Our study has implications for coastal land use and water resources management in tropical deltas.

Transport of E. coli in Aquifer Sediments of Bangladesh: Implications for Widespread Microbial Contamination of Groundwater.

Fecal bacteria are frequently found at much greater distances than would be predicted by laboratory studies, indicating that improved models that incorporate more complexity are might be needed to explain the widespread contamination of many shallow aquifers. In this study, laboratory measurements of breakthrough and retained bacteria in columns of intact and repacked sediment cores from Bangladesh were fit using a two-population model with separate reversible and irreversible attachment sites that also incorporated bacterial decay rates. Separate microcosms indicated an average first order decay rate of 0.03 log10 / day for free bacteria in both the liquid phase and bacteria attached to the solid phase. Although two-thirds of the column results could be well fit with a dual deposition site, single population model, fitting of one third of the results required a two-population model with a high irreversible attachment rate (between 5 and 60 hr-1) for one population of bacteria and a much lower rate (from 5 hr-1 to essentially zero) for the second. Inferred attachment rates for the reversible sites varied inversely with grain size (varying from 1 - 20 hr-1 for grain sizes between 0.1 and 0.3 mm) while reversible detachment rates were found to be nearly constant (approximately 0.5 hr-1). Field simulations based on the fitted two-population model parameters predict only a two-fold reduction in fecal source concentration over a distance of 10 m, determined primarily by the decay rate of the bacteria. The existence of a secondary population of bacteria with a low attachment rate might help explain the observed widespread contamination of tubewell water with E. coli at the field site where the cores were collected, as well as other similar sites.

Sulfate reduction accelerates groundwater arsenic contamination even in aquifers with abundant iron oxides.

Groundwater contamination by geogenic arsenic is a global problem affecting nearly 200 million people. In South and Southeast Asia, a cost-effective mitigation strategy is to use oxidized low-arsenic aquifers rather than reduced high-arsenic aquifers. Aquifers with abundant oxidized iron minerals are presumably safeguarded against immediate arsenic contamination, due to strong sorption of arsenic onto iron minerals. However, preferential pumping of low-arsenic aquifers can destabilize the boundaries between these aquifers, pulling high-arsenic water into low-arsenic aquifers. We investigate this scenario in a hybrid field-column experiment in Bangladesh where naturally high-arsenic groundwater is pumped through sediment cores from a low-arsenic aquifer, and detailed aqueous and solid-phase measurements are used to constrain reactive transport modelling. Here we show that elevated groundwater arsenic concentrations are induced by sulfate reduction and the predicted formation of highly mobile, poorly sorbing thioarsenic species. This process suggests that contamination of currently pristine aquifers with arsenic can occur up to over 1.5 times faster than previously thought, leading to a deterioration of urgently needed water resources.

Sediment delivery to sustain the Ganges-Brahmaputra delta under climate change and anthropogenic impacts.

The principal nature-based solution for offsetting relative sea-level rise in the Ganges-Brahmaputra delta is the unabated delivery, dispersal, and deposition of the rivers' ~1 billion-tonne annual sediment load. Recent hydrological transport modeling suggests that strengthening monsoon precipitation in the 21st century could increase this sediment delivery 34-60%; yet other studies demonstrate that sediment could decline 15-80% if planned dams and river diversions are fully implemented. We validate these modeled ranges by developing a comprehensive field-based sediment budget that quantifies the supply of Ganges-Brahmaputra river sediment under varying Holocene climate conditions. Our data reveal natural responses in sediment supply comparable to previously modeled results and suggest that increased sediment delivery may be capable of offsetting accelerated sea-level rise. This prospect for a naturally sustained Ganges-Brahmaputra delta presents possibilities beyond the dystopian future often posed for this system, but the implementation of currently proposed dams and diversions would preclude such opportunities.

A cluster-based randomized controlled trial promoting community participation in arsenic mitigation efforts in Bangladesh.

OBJECTIVE: To reduce arsenic (As) exposure, we evaluated the effectiveness of training community members to perform water arsenic (WAs) testing and provide As education compared to sending representatives from outside communities to conduct these tasks. METHODS: We conducted a cluster based randomized controlled trial of 20 villages in Singair, Bangladesh. Fifty eligible respondents were randomly selected in each village. In 10 villages, a community member provided As education and WAs testing. In a second set of 10 villages an outside representative performed these tasks. RESULTS: Overall, 53% of respondents using As contaminated wells, relative to the Bangladesh As standard of 50 µg/L, at baseline switched after receiving the intervention. Further, when there was less than 60% arsenic contaminated wells in a village, the classification used by the Bangladeshi and UNICEF, 74% of study households in the community tester villages, and 72% of households in the outside tester villages reported switching to an As safe drinking water source. Switching was more common in the outside-tester (63%) versus community-tester villages (44%). However, after adjusting for the availability of arsenic safe drinking water sources, well switching did not differ significantly by type of As tester (Odds ratio = 0.86[95% confidence interval 0.42-1.77). At follow-up, among those using As contaminated wells who switched to safe wells, average urinary As concentrations significantly decreased. CONCLUSION: The overall intervention was effective in reducing As exposure provided there were As-safe drinking water sources available. However, there was not a significant difference observed in the ability of the community and outside testers to encourage study households to use As-safe water sources. The findings of this study suggest that As education and WAs testing programs provided by As testers, irrespective of their residence, could be used as an effective, low cost approach to reduce As exposure in many As-affected areas of Bangladesh.

Regional-scale hydrogeochemical evolution across the arsenic-enriched transboundary aquifers of the Ganges River Delta system, India and Bangladesh.

Arsenic (As) dynamics within the extensively contaminated aquifers of the Ganges River delta have been widely studied over the past few decades, but the hydrogeochemical signatures across the delta aquifers remain to be characterized. Here, we characterize the varied geochemical and isotopic (δ18O, δ2H) signatures of groundwater across the delta and interpret the hydrogeochemical evolution pathways and the driving processes on a regional-scale as a function of the delta hydrostratigraphy. Our hydrostratigraphic model identifies three major aquifer sub-systems across the delta from north-west to south-east: a single continuous unconfined aquifer (Type I); a semiconfined vertically-segregated aquifer sub-system (Type II); and a nearly confined multilayered aquifer sub-system (Type III). The Type I aquifer is dominated by Ca-Mg-HCO3-rich waters, while the aquifers to the south (Type II and Type III) exhibits increasing dominance of Na-Cl hydrogeochemical facies at shallow and intermediate depths and Na-HCO3 hydrogeochemical facies in the deep aquifers. The spatial distribution of As is also found to be heavily dictated by hydrostratigraphy, wherein the Type I aquifer sub-system yields similar concentrations across depths, while the Type II and Type III aquifer sub-systems exhibit a sharp increase in As-safe aquifers with depth. Although dominant reducing conditions occur within the delta groundwater, co-occurrence of redox-sensitive solutes from varying redox stability fields indicates to the development of overlapping redox zones. Stable isotopic signatures of groundwater exhibit a progressive depletion away from the Bay of Bengal. The Type I aquifer exhibits relatively homogenous hydrogeochemical signatures, possibly suggesting deeper infiltration of recharge under higher vertical hydraulic gradients, while the Type II and Type III aquifers exhibit variability across depth, which is possibly a reflection of horizontally stratified groundwater flows, dictated by the spatial geometry of the intervening aquitard layers.

The Bengal Water Machine: Quantified freshwater capture in Bangladesh.

Global food security depends on the sustainability of irrigated agriculture. Rising groundwater withdrawals from seasonally humid, alluvial plains across tropical Asia have enabled dry-season rice cultivation. This groundwater pumpage increases available subsurface storage that under favorable conditions amplifies groundwater replenishment during the subsequent monsoon. We empirically quantified this nature-based solution to seasonal freshwater storage capture described as the "Bengal Water Machine," revealing its potential and limitations. On the basis of a million piezometric observations from 465 monitoring wells, we show that the collective operation of ~16 million smallholder farmers in the Bengal Basin of Bangladesh from 1988 to 2018 has induced cumulative freshwater capture that volumetrically (75 to 90 cubic kilometers) is equivalent to twice the reservoir capacity of the Three Gorges Dam.

Fecal contamination of shallow tubewells in Bangladesh inversely related to arsenic.

The health risks of As exposure due to the installation of millions of shallow tubewells in the Bengal Basin are known, but fecal contamination of shallow aquifers has not systematically been examined. This could be a source of concern in densely populated areas with poor sanitation because the hydraulic travel time from surface water bodies to shallow wells that are low in As was previously shown to be considerably shorter than for shallow wells that are high in As. In this study, 125 tubewells 6-36 m deep were sampled in duplicate for 18 months to quantify the presence of the fecal indicator Escherichia coli. On any given month, E. coli was detected at levels exceeding 1 most probable number per 100 mL in 19-64% of all shallow tubewells, with a higher proportion typically following periods of heavy rainfall. The frequency of E. coli detection averaged over a year was found to increase with population surrounding a well and decrease with the As content of a well, most likely because of downward transport of E. coli associated with local recharge. The health implications of higher fecal contamination of shallow tubewells, to which millions of households in Bangladesh have switched in order to reduce their exposure to As, need to be evaluated.

Arsenic-enriched groundwaters of India, Bangladesh and Taiwan--comparison of hydrochemical characteristics and mobility constraints.

Arsenic (As) enrichment in groundwater has become a major global environmental disaster. Groundwater samples were collected from 64 sites located in the districts of 24-Parganas (S), and Nadia in West Bengal, India (Bhagirathi sub-basin), and 51 sites located in the districts of Comilla, Noakhali, Magura, Brahman baria, Laxmipur, Munshiganj, Faridpur and Jhenaida in Bangladesh (Padma-Meghna sub-basin). Groundwater samples were also collected from two As-affected areas (Chianan and Lanyang plains) of Taiwan (n = 26). The concentrations of major solutes in groundwater of the Padma-Meghna sub-basin are more variable than the Bhagirathi sub-basin, suggesting variations in the depositional and hydrological settings. Arsenic concentrations in groundwaters of the studied areas showed large variations, with mean As concentrations of 125 µg/L (range: 0.20 to 1,301 µg/L) in Bhagirathi sub-basin, 145 µg/L (range: 0.20 to 891 µg/L) in Padma-Meghna sub-basin, 209 µg/L (range: 1.3 to 575 µg/L) in Chianan plain, and 102 µg/L (range: 2.5 to 348 µg/L) in Lanyang plain groundwater. The concentrations of Fe, and Mn are also highly variable, and are mostly above the WHO-recommended guideline values and local (Indian and Bangladeshi) drinking water standard. Piper plot shows that groundwaters of both Bhagirathi and Padma-Meghna sub-basins are of Ca-HCO(3) type. The Chianan plain groundwaters are of Na-Cl type, suggesting seawater intrusion, whereas Lanyang plain groundwaters are mostly of Na-HCO(3) type. The study shows that reductive dissolution of Fe(III)-oxyhydroxides is the dominant geochemical process releasing As from sediment to groundwater in all studied areas.

Temporal and seasonal variability of arsenic in drinking water wells in Matlab, southeastern Bangladesh: a preliminary evaluation on the basis of a 4 year study.

Temporal and seasonal variability of As concentrations in groundwater were evaluated in As-affected areas of Matlab, southeastern Bangladesh. Groundwater samples from 61 randomly selected tubewells were analyzed for As concentrations over a period of three years and four months (from July 2002 to November 2005) and monitored seasonally (three times a year). The mean As concentrations in the sampled tubewells decreased from 153 to 123 µg/L during July 2002 to November 2005. Such changes were pronounced in tubewells with As concentration >50 µg/L than those with As concentrations <50 µg/L. Similarly, individual wells revealed temporal variability, for example some wells indicated a decreasing trend, while some other wells indicated stable As concentration during the monitoring period. The mean As concentrations were significantly higher in Matlab North compared with Matlab South. The spatial variations in the mean As concentrations may be due to the differences in local geological conditions and groundwater flow patterns. The variations in mean As concentrations were also observed in shallow (<40 m) and deep (>40 m) wells. However, to adequately evaluate temporal and seasonal variability of As concentration, it is imperative to monitor As concentrations in tubewells over a longer period of time. Such long-term monitoring will provide important information for the assessment of human health risk and the sustainability of safe drinking water supplies.

Spatial distribution of manganese in groundwater and associated human health risk in the southern part of the Bengal Basin.

The scarcity of arsenic and iron-free safe drinking water is an alarming issue in the southern part of the Bengal Basin. The objectives of the present study were to investigate the spatial distribution of manganese (Mn) concentration in the shallow and deep groundwater and its associated health risks for the children and adults of entire southern Bengal Basin. The Mn concentration in the groundwater varied from 0 to 5.4 mg/L with an average value of 0.47 mg/L that exceeded the WHO's and Bangladesh drinking water guideline values of 0.4 and 0.1 mg/L, respectively. Mn concentration in the shallow wells overrode the deep ones. About 23% of the shallow wells and 11% of deep wells exceeded the WHO's safety limit of Mn concentration for human health. The human health risk related to Mn contamination was estimated by computing the average daily dosage (ADD) and hazard quotient (HQ) values for children and adults. The average computed HQ values found 0.108 and 0.099 for children and adults, respectively. The HQ values delimitated that children are posing a higher risk compared to the adults for the shallow wells. Deep wells were found risk-free for both children and adults. The areal coverage of shallow wells with HQ values > 1 was minimal compared to the total study area and covered only a small portion of Patuakhali and Barguna districts. The rest of the site does not pose any health risk due to Mn contamination for children and adults.

Spatiotemporal distribution of boron in the groundwater and human health risk assessment from the coastal region of Bangladesh.

High concentrations of naturally-occurring and man-accentuated boron in groundwater possess a potential threat to the health and well-being of humans worldwide. In Bangladesh, only a few studies focused on the health risks of boron-containing groundwater. Therefore, the present study investigated the spatiotemporal distribution of boron concentrations in groundwater and its associated health risks in the coastal districts of Bangladesh. A total of 268 samples from the shallow and deep wells during the wet and dry season (137 and 131 samples, respectively) were collected to determine the level of boron concentrations and health risk status. The groundwater boron concentrations ranged from 0 to 4.10 mg/L with an average concentration of 0.68 mg/L, which was much lower than the WHO and the values of Bangladesh drinking water standard guideline. The boron concentrations in the shallow wells override the deep ones with insignificant seasonal variation. Boron contamination affected 10% of the shallow wells in the wet season and only 6% of them in the dry season, whereas only 1% of deep wells exceeds the WHO guideline values of 2.40 mg/L during the wet and dry season, respectively. The human health risk of boron was determined by computing estimated daily intake (EDI) and hazard quotient (HQ) values for infants, children, teenagers, and adults. The average HQ value delimitated that children have high risk followed by teenagers, adults, and infants. About 90-95% of the studied samples were free from boron contamination because of having HQ values < 1 and the rest of the samples possess a high risk for children. For the overall study area, the deep wells were found safer than the shallow wells, which were more susceptible to boron contamination aided by localized freshwater inputs.

Recommended Sampling Intervals for Arsenic in Private Wells.

Geogenic arsenic in drinking water is a worldwide problem. For private well owners, testing (e.g., private or government laboratory) is the main method to determine arsenic concentration. However, the temporal variability of arsenic concentrations is not well characterized and it is not clear how often private wells should be tested. To answer this question, three datasets, two new and one publicly available, with temporal arsenic data were utilized: 6370 private wells from New Jersey tested at least twice since 2002, 2174 wells from the USGS NAWQA database, and 391 private wells sampled 14 years apart from Bangladesh. Two arsenic drinking water standards are used for the analysis: 10 µg/L, the WHO guideline and EPA standard or maximum contaminant level (MCL) and 5 µg/L, the New Jersey MCL. A rate of change was determined for each well and these rates were used to predict the temporal change in arsenic for a range of initial arsenic concentrations below an MCL. For each MCL and initial concentration, the probability of exceeding an MCL over time was predicted. Results show that to limit a person to below a 5% chance of drinking water above an MCL, wells that are ½ an MCL and above should be tested every year and wells below ½ an MCL should be tested every 5 years. These results indicate that one test result below an MCL is inadequate to ensure long-term compliance. Future recommendations should account for temporal variability when creating drinking water standards and guidance for private well owners.

Prevalence of SARS-CoV-2 in Communities Through Wastewater Surveillance-a Potential Approach for Estimation of Disease Burden.

The episodic outbreak of COVID-19 due to SARS-CoV-2 is severely affecting the economy, and the global count of infected patients is increasing. The actual number of patients had been underestimated due to limited facilities for testing as well as asymptomatic nature of the expression of COVID-19 on individual basis. Tragically, for emerging economies with high population density, the situation has been more complex due to insufficient testing facilities for diagnosis of the disease. However, the recent reports about persistent shedding of viral RNA of SARS-CoV-2 in the human feces have created a possibility to track the prevalence and trends of the disease in communities, known as wastewater-based epidemiology (WBE). In this article, we highlight the current limitations and future prospects for WBE to manage pandemics.

Modeling regional-scale groundwater arsenic hazard in the transboundary Ganges River Delta, India and Bangladesh: Infusing physically-based model with machine learning.

For the last few decades, toxic levels of arsenic (As) in groundwater from the aquifers of the Ganges River delta, India and Bangladesh, have been known to cause serious public health concerns. Innumerable studies have advocated the control of geomorphologic, geologic, hydrogeologic, biogeochemical, and anthropogenic factors on arsenic mobilization, flow, and distribution patterns within the Ganges River delta. We have developed transboundary regional-scale models for computing the probability of groundwater As concentrations to exceed the WHO permissible thresholds for drinking water of 10 µg/L within the Ganges River delta as a function of the various geomorphologic-(hydro)geologic-hydrostratigraphic-anthropogenic controlling factors, using statistical methods and artificial intelligence (AI) [i.e., machine learning] techniques namely, Random Forest (RF), Boosted Regression Trees (BRT) and Logistic Regression (LR) algorithms, followed by probabilistic delineation the high As-hazard zones within the delta. A "hybrid multi-modeling approach" was adapted for this study, which involved the introduction of hydrostratigraphic parameters (aquifer connectivity and surficial aquitard thickness) derived from a high-resolution transboundary hydrostratigraphic model developed for the Ganges River delta aquifer system, as predictors for modeling groundwater As probabilities within the delta. The RF model outperforms the BRT and LR model in terms of model performance. Model outputs suggest the dominant influence of surficial aquitard thickness and groundwater-fed irrigated area (%) on groundwater As. While, the north-central and southern regions of the Ganges River delta show low As-hazard (<10 µg/L), the western and north-eastern regions demonstrate elevated hazard level (>10 µg/L). An estimated 30.3 million people are found to be exposed to elevated groundwater As within the study area. Thus, our study demonstrates that such hybrid, predictive models are not only helpful in delineating the regional-scale distribution of groundwater As-hazard zones in the areas with limited As data but is also useful in identifying the possible exogenous forcing that may have led to the worst, natural pollution in human history.