Ammonium-Enhanced Arsenic Mobilization from Aquifer Sediments

Ammonium-related pathways are important for groundwater arsenic (As) enrichment, especially via microbial Fe(III) reduction coupled with anaerobic ammonium oxidation; however, the key pathways (and microorganisms) underpinning ammonium-induced Fe(III) reduction and their contributions to As mobilization in groundwater are still unknown. To address this gap, aquifer sediments hosting high As groundwater from the western Hetao Basin were incubated with 15N-labeled ammonium and external organic carbon sources (including glucose, lactate, and lactate/acetate). Decreases in ammonium concentrations were positively correlated with increases in the total produced Fe(II) (Fe(II)tot) and released As. The molar ratios of Fe(II)tot to oxidized ammonium ranged from 3.1 to 3.7 for all incubations, and the δ15N values of N2 from the headspace increased in 15N-labeled ammonium-treated series, suggesting N2 as the key end product of ammonium oxidation. The addition of ammonium increased the As release by 16.1% to 49.6%, which was more pronounced when copresented with organic electron donors. Genome-resolved metagenomic analyses (326 good-quality MAGs) suggested that ammonium-induced Fe(III) reduction in this system required syntrophic metabolic interactions between bacterial Fe(III) reduction and archaeal ammonium oxidation. The current results highlight the significance of syntrophic ammonium-stimulated Fe(III) reduction in driving As mobilization, which is underestimated in high As groundwater.

Spatial and seasonal controls on dissolved organic matter composition in shallow aquifers under the rapidly developing city of Patna, India

The distribution and composition of dissolved organic matter (DOM) affects numerous (bio)geochemical processes in environmental matrices including groundwater. This study reports the spatial and seasonal controls on the distribution of groundwater DOM under the rapidly developing city of Patna, Bihar (India). Major DOM constituents were determined from river and groundwater samples taken in both pre- and post-monsoon seasons in 2019, using excitation-emission matrix (EEM) fluorescence spectroscopy. We compared aqueous fluorescent DOM (fDOM) composition to satellite-derived land use data across the field area, testing the hypothesis that the composition of groundwater DOM, and particularly the components associated with surface-derived ingress, may be controlled, in part, by land use. In the pre-monsoon season, the prominence of tryptophan-like components likely generated from recent biological activity overwhelmed the humic-like and tyrosine-like fluorescence signals. Evidence from fluorescence data suggest groundwater in the post-monsoon season is composed of predominantly i) plant-derived matter and ii) anthropogenically influenced DOM (e.g. tryptophan-like components). Organic tracers, as well as Eh and Cl-, suggest monsoonal events mobilise surface-derived material from the unsaturated zone, causing dissolved organic carbon (DOC) of more microbial nature to infiltrate to >100 m depth. A correlation between higher protein:humic-like fluorescence and lower vegetation index (NDVI), determined from satellite-based land use data, in the post-monsoon season, indicates the ingression of wastewater-derived OM in groundwater under the urban area. Attenuated protein:humic-like fluorescence in groundwater close to the river points towards the mixing of groundwater and river water. This ingress of surface-derived OM is plausibly exacerbated by intensive groundwater pumping under these areas. Our approach to link the composition of aqueous organics with land use could easily be adapted for similar hydrogeochemical settings to determine the factors controlling groundwater DOM composition in various contexts.

Emerging organic contaminants in the River Ganga and key tributaries in the middle Gangetic Plain, India: Characterization, distribution & controls

The presence and distribution of emerging organic contaminants (EOCs) in freshwater environments is a key issue in India and globally, particularly due to ecotoxicological and potential antimicrobial resistance concerns. Here we have investigated the composition and spatial distribution of EOCs in surface water along a ∼500 km segment of the iconic River Ganges (Ganga) and key tributaries in the middle Gangetic Plain of Northern India. Using a broad screening approach, in 11 surface water samples, we identified 51 EOCs, comprising of pharmaceuticals, agrochemicals, lifestyle and industrial chemicals. Whilst the majority of EOCs detected were a mixture of pharmaceuticals and agrochemicals, lifestyle chemicals (and particularly sucralose) occurred at the highest concentrations. Ten of the EOCs detected are priority compounds (e.g. sulfamethoxazole, diuron, atrazine, chlorpyrifos, perfluorooctane sulfonate (PFOS), perfluorobutane sulfonate, thiamethoxam, imidacloprid, clothianidin and diclofenac). In almost 50% of water samples, sulfamethoxazole concentrations exceeded predicted no-effect concentrations (PNECs) for ecological toxicity. A significant downstream reduction in EOCs was observed along the River Ganga between Varanasi (Uttar Pradesh) and Begusarai (Bihar), likely reflecting dilution effects associated with three major tributaries, all with considerably lower EOC concentrations than the main Ganga channel. Sorption and/or redox controls were observed for some compounds (e.g. clopidol), as well as a relatively high degree of mixing of EOCs within the river. We discuss the environmental relevance of the persistence of several parent compounds (notably atrazine, carbamazepine, metribuzin and fipronil) and associated transformation products. Associations between EOCs and other hydrochemical parameters including excitation emission matrix (EEM) fluorescence indicated positive, significant, and compound-specific correlations between EOCs and tryptophan-, fulvic- and humic-like fluorescence. This study expands the baseline characterization of EOCs in Indian surface water and contributes to an improved understanding of the potential sources and controls on EOC distribution in the River Ganga and other large river systems.

Quantifying sulfidization and non-sulfidization in long-term in-situ microbial colonized As(V)-ferrihydrite coated sand columns: Insights into As mobility

Sulfide-induced reduction (sulfidization) of arsenic (As)-bearing Fe(III) (oxyhydro)oxides may lead to As mobilization in aquifer systems. However, little is known about the relative contributions of sulfidization and non-sulfidization of Fe(III) (oxyhydro)oxides reduction to As mobilization. To address this issue, high As groundwater with low sulfide (LS) and high sulfide (HS) concentrations were pumped through As(V)-bearing ferrihydrite-coated sand columns (LS-column and HS-column, respectively) being settled within wells in the western Hetao Basin, China. Sulfidization of As(V)-bearing ferrihydrite was evidenced by the increase in dissolved Fe(II) and the presence of solid Fe(II) and elemental sulfur (S⁰) in both the columns. A conceptual model was built using accumulated S⁰ and Fe(II) produced in the columns to calculate the proportions of sulfidization-induced Fe(III) (oxyhydro)oxide reduction and non-sulfidization-induced Fe(III) (oxyhydro)oxide reduction. Fe(III) reduction via sulfidization occurred preferentially in the inlet ends (LS-column, 31 %; HS-column, 86 %), while Fe(III) reduction via non-sulfidization processes predominated in the outlet ends (LS-column, 96 %; HS-column, 86 %), and was attributed to the metabolism of genera associated with Fe(III) reduction (including Shewanella, Ferribacterium, and Desulfuromonas). Arsenic was mobilized in the columns via sulfidization and non-sulfidization processes. More As was released from the solid of the HS-column than that of the LS-column due to the higher intensity of sulfidization in the presence of higher concentrations of dissolved S(-II). Overall, this study highlights the sulfidization of As-bearing Fe(III) (oxyhydro)oxides as an important As-mobilizing pathway in complex As-Fe-S bio-hydrogeochemical networks.

Environmental tracers and groundwater residence time indicators reveal controls of arsenic accumulation rates beneath a rapidly developing urban area in Patna, India

Groundwater security is a pressing environmental and societal issue, particularly due to significantly increasing stressors on water resources, including rapid urbanization and climate change. Groundwater arsenic is a major water security and public health challenge impacting millions of people in the Gangetic Basin of India and elsewhere globally. In the rapidly developing city of Patna (Bihar) in northern India, we have studied the evolution of groundwater chemistry under the city following a three-dimensional sampling framework of multi-depth wells spanning the central urban zone in close proximity to the River Ganges (Ganga) and transition into peri-urban and rural areas outside city boundaries and further away from the river. Using inorganic geochemical tracers (including arsenic, iron, manganese, nitrate, nitrite, ammonium, sulfate, sulfide and others) and residence time indicators (CFCs and SF₆), we have evaluated the dominant hydrogeochemical processes occurring and spatial patterns in redox conditions across the study area. The distribution of arsenic and other redox-sensitive parameters is spatially heterogenous, and elevated arsenic in some locations is consistent with arsenic mobilization via reductive dissolution of iron hydroxides. Residence time indicators evidence modern (<~60-70 years) groundwater and suggest important vertical and lateral flow controls across the study area, including an apparent seasonal reversal in flow regimes near the urban center. An overall arsenic accumulation rate is estimated to be ~0.003 ± 0.003 μM.yr^-1 (equivalent to ~0.3 ± 0.2 μg.yr^-1), based on an average of CFC-11, CFC-12 and SF₆-derived models, with the highest rates of arsenic accumulation observed in shallow, near-river groundwaters also exhibiting elevated concentrations of nutrients including ammonium. Our findings have implications on groundwater management in Patna and other rapidly developing cities, including potential future increased groundwater vulnerability associated with surface-derived ingress from large-scale urban abstraction or in higher permeability zones of river-groundwater connectivity.

Household and community systems for groundwater remediation in Bihar, India: Arsenic and inorganic contaminant removal, controls and implications for remediation selection

The presence of arsenic (As) and other inorganic contaminants in groundwater is a key public health issue in India and many other parts of the world. Whilst a broad range of remediation technologies exist, performance can be highly variable, and appropriate selection and management of remediation approaches remains challenging. Here, we have identified and tested the performance of a range of small-scale remediation technologies (e.g. sand filters, multi-stage filtration and reverse osmosis (RO)-based systems; n = 38) which have been implemented in Bihar, India. We have undertaken spot-assessments of system performance under typical operating conditions in household and non-household (e.g. community, hospital, hostel/hotel) settings. The removal of As and other inorganic contaminants varied widely (ranging from ~0-100%), with some solutes generally more challenging to remove than others. We have evaluated the relative importance of technology type (e.g. RO-based versus non-RO systems), implementation setting (e.g. household versus non-household) and source water geochemistry (particularly concentrations and ratios of As, Fe, P, Si and Ca), as potential controls on remediation effectiveness. Source water composition, particularly the ratio ([Fe] - 1.8[P])/[As], is a statistically significant control on As removal (p < 0.01), with higher ratios associated with higher removal, regardless of technology type (under the site-specific conditions observed). This ratio provides a theoretical input which could be used to identify the extent to which natural groundwater composition may be geochemically compatible with higher levels of As removal. In Bihar, we illustrate how this ratio could be used to identify spatial patterns in theoretical geochemical compatibility for As removal, and to identify where additional Fe may theoretically facilitate improved remediation. This geochemical approach could be used to inform optimal selection of groundwater remediation approaches, when considered alongside other important considerations (e.g. technical, managerial and socio-economic) known to impact the effective implementation and sustainability of successful groundwater remediation approaches.

A systematic approach to understand hydrogeochemical dynamics in large river systems: Development and application to the River Ganges (Ganga) in India

Large river systems, such as the River Ganges (Ganga), provide crucial water resources for the environment and society, yet often face significant challenges associated with cumulative impacts arising from upstream environmental and anthropogenic influences. Understanding the complex dynamics of such systems remains a major challenge, especially given accelerating environmental stressors including climate change and urbanization, and due to limitations in data and process understanding across scales. An integrated approach is required which robustly enables the hydrogeochemical dynamics and underpinning processes impacting water quality in large river systems to be explored. Here we develop a systematic approach for improving the understanding of hydrogeochemical dynamics and processes in large river systems, and apply this to a longitudinal survey (> 2500 km) of the River Ganges (Ganga) and key tributaries in the Indo-Gangetic basin. This framework enables us to succinctly interpret downstream water quality trends in response to the underpinning processes controlling major element hydrogeochemistry across the basin, based on conceptual water source signatures and dynamics. Informed by a 2019 post-monsoonal survey of 81 river bank-side sampling locations, the spatial distribution of a suite of selected physico-chemical and inorganic parameters, combined with segmented linear regression, reveals minor and major downstream hydrogeochemical transitions. We use this information to identify five major hydrogeochemical zones, characterized, in part, by the inputs of key tributaries, urban and agricultural areas, and estuarine inputs near the Bay of Bengal. Dominant trends are further explored by investigating geochemical relationships (e.g. Na:Cl, Ca:Na, Mg:Na, Sr:Ca and NO₃:Cl), and how water source signatures and dynamics are modified by key processes, to assess the relative importance of controls such as dilution, evaporation, water-rock interactions (including carbonate and silicate weathering) and anthropogenic inputs. Mixing/dilution between sources and water-rock interactions explain most regional trends in major ion chemistry, although localized controls plausibly linked to anthropogenic activities are also evident in some locations. Temporal and spatial representativeness of river bank-side sampling are considered by supplementary sampling across the river at selected locations and via comparison to historical records. Limitations of such large-scale longitudinal sampling programs are discussed, as well as approaches to address some of these inherent challenges. This approach brings new, systematic insight into the basin-wide controls on the dominant geochemistry of the River Ganga, and provides a framework for characterising dominant hydrogeochemical zones, processes and controls, with utility to be transferable to other large river systems.

Hydrochemical characterization, spatial distribution, and geochemical controls on arsenic and boron in waters from arid Arica and Parinacota, northern Chile

The livelihood of inhabitants from rural agricultural valleys in the arid Arica and Parinacota Region, northernmost Chile, strongly depends on water from high altitude rainfall and runoff to lower elevation areas. However, elevated arsenic, boron, and other potentially harmful elements compromise water quality, especially in rural areas. Samples (n = 90) of surface, underground, cold, geothermal springs, and treated and raw tap water were studied to assess water quality and to determine the main geochemical controls on water composition, origin, and geochemical evolution along dominant flowpaths. Water from major river basins across the region (Lluta, San Jose, Codpa-Chaca, Camarones and Altiplanicas) were collected for hydrogeochemical analysis of a suite of major and trace elements, δD and δ¹⁸O. Our new dataset was supplemented by hydrochemical data (n > 1500 data points) from secondary sources. Results show that 72% of the collected samples had As >10 μg/L (WHO drinking water provisional guideline) and affected 44% of the studied waters used for drinking (n = 32). Based on Chilean irrigation guidelines, elevated salinity (EC > 0.75 mS/cm) affected 80% of sampled waters, which were also impacted by high B (89% > 0.75 mg/L), and As (31% > 50 μg/L). Water composition was strongly controlled by geothermal water and freshwater mixing in high altitude areas. Magnitude and fate of As and B concentration was determined by the geothermal input type. Highest As (~21 mg/L) was associated with circum-neutral Na-Cl waters in Camarones basin, while lower As (~5 mg/L) with acid SO₄ waters in Lluta basin. Additionally, evaporative concentration and sediment-water interactions were shown to control the level of As in surface and groundwaters downstream. This works provides a comprehensive analysis and a conceptual model of geochemical controls on regional water compositions, contributing to better understanding the geochemical processes underpinning the water quality challenges in northern Chile.

Understanding Microbial Arsenic-Mobilization in Multiple Aquifers: Insight from DNA and RNA Analyses

Biogeochemical processes critically control the groundwater arsenic (As) enrichment; however, the key active As-mobilizing biogeochemical processes and associated microbes in high dissolved As and sulfate aquifers are poorly understood. To address this issue, the groundwater-sediment geochemistry, total and active microbial communities, and their potential functions in the groundwater-sediment microbiota from the western Hetao basin were determined using 16S rRNA gene (rDNA) and associated 16S rRNA (rRNA) sequencing. The relative abundances of either sediment or groundwater total and active microbial communities were positively correlated. Interestingly, groundwater active microbial communities were mainly associated with ammonium and sulfide, while sediment active communities were highly related to water-extractable nitrate. Both sediment-sourced and groundwater-sourced active microorganisms (rRNA/rDNA ratios > 1) noted Fe(III)-reducers (induced by ammonium oxidation) and As(V)-reducers, emphasizing the As mobilization via Fe(III) and/or As(V) reduction. Moreover, active cryptic sulfur cycling between groundwater and sediments was implicated in affecting As mobilization. Sediment-sourced active microorganisms were potentially involved in anaerobic pyrite oxidation (driven by denitrification), while groundwater-sourced organisms were associated with sulfur disproportionation and sulfate reduction. This study provides an extended whole-picture concept model of active As-N-S-Fe biogeochemical processes affecting As mobilization in high dissolved As and sulfate aquifers.

Assessment of hypertension association with arsenic exposure from food and drinking water in Bihar, India

Epidemiological studies have associated chronic exposure to arsenic (As) from drinking water with increased risk of hypertension. However, evidence of an association between As exposure from food and hypertension risks is sparse. To quantify the association between daily As intake from both food (rice, wheat and potatoes) and drinking water (As_water) along with total exposure (As_total) and hypertension risks in a study population in Bihar, India, we conducted an individual level cross-sectional analysis between 2017 and 2019 involving 150 participants. Arsenic intake variables and three indicators of hypertension risks (general hypertension, low-density lipoprotein (LDL) and high-density lipoprotein (HDL)) were derived, and any relationship was quantified using a series of crude and multivariable log-linear or logistic regression models. The prevalence of general hypertension was 40% for the studied population. The median level of HDL was 45 mg/dL while median value of LDL was 114 mg/dL. Apart from a marginally significant positive relationship between As intake from rice and the changes of LDL (p-value = 0.032), no significant positive association between As intake and hypertension risks could be ascertained. In fact, As_total was found to be associated with lower risks of general hypertension and higher levels of HDL (p-value = 0.020 and 0.010 respectively) whilst general hypertension was marginally associated with lower As_water (p-value = 0.043). Due to limitations regarding study design and residual confounding, all observed marginal associations should be treated with caution.

Geochemical compositional controls on DNA strand breaks induced in in vitro cell-free assays by crushed rock powders from the Panasqueira mine area, Portugal

DNA strand breaks are a common form of DNA damage that can contribute to chromosomal instability or gene mutations. Such strand breaks may be caused by exposure to heavy metals. The aim of this study was to assess the level of DNA strand breaks caused by µm-scale solid particles of known chemical composition with elevated heavy metals/metalloids, notably arsenic, using an in vitro cell-free DNA plasmid scission assay. These samples were incubated with and without H2O2 to see whether damage occurs directly or indirectly through the Fenton reaction. Levels of DNA damage in the absence of H2O2 were < 10%, but in the presence of H2O2, all samples showed higher levels of damage ranging from 10 to 100% suggesting that damage was being incurred through the Fenton reaction. Using bivariate correlation analysis and multiple linear regression, manganese oxide (MnO), sulphur (S), copper (Cu), and zinc (Zn) concentrations in the particulates were found to be the most significant predictors of DNA damage. The mechanism of this DNA damage formation has yet to be thoroughly investigated but is hypothesised to be due to reactive oxygen species formation. Further work is required to assess the extent of contribution of reactive oxygen species to this DNA damage, but this study highlights the potential role of chemistry and/or mineralogy to the extent and/or nature of DNA damage caused by particulates.

Geostatistical model of the spatial distribution of arsenic in groundwaters in Gujarat State, India

Geogenic arsenic contamination in groundwaters poses a severe health risk to hundreds of millions of people globally. Notwithstanding the particular risks to exposed populations in the Indian sub-continent, at the time of writing, there was a paucity of geostatistically based models of the spatial distribution of groundwater hazard in India. In this study, we used logistic regression models of secondary groundwater arsenic data with research-informed secondary soil, climate and topographic variables as principal predictors generate hazard and risk maps of groundwater arsenic at a resolution of 1 km across Gujarat State. By combining models based on different arsenic concentrations, we have generated a pseudo-contour map of groundwater arsenic concentrations, which indicates greater arsenic hazard (> 10 μg/L) in the northwest, northeast and south-east parts of Kachchh District as well as northwest and southwest Banas Kantha District. The total number of people living in areas in Gujarat with groundwater arsenic concentration exceeding 10 μg/L is estimated to be around 122,000, of which we estimate approximately 49,000 people consume groundwater exceeding 10 µg/L. Using simple previously published dose-response relationships, this is estimated to have given rise to 700 (prevalence) cases of skin cancer and around 10 cases of premature avoidable mortality/annum from internal (lung, liver, bladder) cancers-that latter value is on the order of just 0.001% of internal cancers in Gujarat, reflecting the relative low groundwater arsenic hazard in Gujarat State.

Exploratory study of the association in the United Kingdom between hypertension and inorganic arsenic (iAs) intake from rice and rice products

Hypertension risks arising from chronic exposure to inorganic arsenic (iAs) are well documented. Consumption of rice is a major iAs exposure route for over 3 billion people; however, there is a lack of epidemiological evidence demonstrating an association of hypertension risks with iAs intake from rice, especially in areas where there is little exposure from drinking water but a growing demand for rice intake. To address this, we conducted an individual-level cross-sectional analysis to quantify the extent to which daily iAs intake from rice and rice products (E-iAsing,rice) modifies the association between hypertension risks and previously well-established risk factors. The analysis was based on secondary dietary, socio-demographic and health status data of 598 participants recorded in the UK National Diet and Nutrition Survey 2014-2016. E-iAsing,rice and five blood pressure endpoints were derived with potential associations explored through generalized linear models. According to the results, a negative but not significant relationship was found between hypertension risks and E-iAsing,rice after adjusting for major risk factors, notably age, gender, diabetes and obesity, with relatively higher risks being observed for male, middle-aged, overweight, alcohol consumer or Asian or Asian British, Black or Black British and mixed ethnic groups. Though inconclusive and mainly limited by potential incomplete adjustment for major confounders and intrinsic disadvantages of a cross-sectional design, this study was the first quantifying the individual level dose-response relationship between E-iAsing,rice and hypertension risks and is consistent with previous studies on the limited associations of hypertension with low-level arsenic exposure from drinking water. Larger scale cohort studies are indicated to quantify the association but in any event it is likely to be weak.

Arsenic exposure from food exceeds that from drinking water in endemic area of Bihar, India

Extensive evidence of elevated arsenic (As) in the food-chain, mainly rice, wheat and vegetables exists. Nevertheless, the importance of exposure from food towards total As exposure and associated health risks in areas with natural occurring As in drinking water is still often neglected, and accordingly mitigations are largely focused on drinking water only. In this study, the contribution of food over drinking water to overall As exposure was estimated for As exposed populations in Bihar, India. Increased lifetime cancer risk was predicted using probabilistic methods with input parameters based on detailed dietary assessment and estimation of As in drinking water, cooked rice, wheat flour and potato collected from 91 households covering 19 villages. Median total exposure was 0.83 μg/kgBW/day (5th and 95th percentiles were 0.21 and 11.1 μg/kgBW/day) and contribution of food (median = 49%) to overall exposure was almost equal to that from drinking water (median = 51%). More importantly and contrary to previous studies, food was found to contribute more than drinking water to As exposure, even when drinking water As was above the WHO provisional guide value of 10 μg/L. Median and 95th percentile excess lifetime cancer risks from food intake were 1.89 × 10^-4 and 7.32 × 10^-4 respectively when drinking water As was below 10 μg/L and 4.00 × 10^-4 and 1.83 × 10^-3 respectively when drinking water As was above 10 μg/L. Our results emphasise the importance of food related exposure in As-endemic areas, and, perhaps surprisingly, particularly in areas with high As concentrations in drinking water - this being partly ascribed to increases in food As due to cooking in high As water. These findings are timely to stress the importance of removing As from the food chain and not just drinking water in endemic areas.

Emerging organic contaminants in groundwater under a rapidly developing city (Patna) in northern India dominated by high concentrations of lifestyle chemicals

Aquatic pollution from emerging organic contaminants (EOCs) is of key environmental importance in India and globally, particularly due to concerns of antimicrobial resistance, ecotoxicity and drinking water supply vulnerability. Here, using a broad screening approach, we characterize the composition and distribution of EOCs in groundwater in the Gangetic Plain around Patna (Bihar), as an exemplar of a rapidly developing urban area in northern India. A total of 73 EOCs were detected in 51 samples, typically at ng.L^-1 to low μg.L^-1 concentrations, relating to medical and veterinary, agrochemical, industrial and lifestyle usage. Concentrations were often dominated by the lifestyle chemical and artificial sweetener sucralose. Seventeen identified EOCs are flagged as priority compounds by the European Commission, World Health Organisation and/or World Organisation for Animal Health: namely, herbicides diuron and atrazine; insecticides imidacloprid, thiamethoxam, clothianidin and acetamiprid; the surfactant perfluorooctane sulfonate (and related perfluorobutane sulfonate, perfluorohexane sulfonate, perfluorooctanoic acid and perfluoropentane sulfonate); and medical/veterinary compounds sulfamethoxazole, sulfanilamide, dapson, sulfathiazole, sulfamethazine and diclofenac. The spatial distribution of EOCs varies widely, with concentrations declining with depth, consistent with a strong dominant vertical flow control. Groundwater EOC concentrations in Patna were found to peak within ∼10 km distance from the River Ganges, indicating mainly urban inputs with some local pollution hotspots. A heterogeneous relationship between EOCs and population density likely reflects confounding factors including varying input types and controls (e.g. spatial, temporal), wastewater treatment infrastructure and groundwater abstraction. Strong seasonal agreement in EOC concentrations was observed. Co-existence of limited transformation products with associated parent compounds indicate active microbial degradation processes. This study characterizes key controls on the distribution of groundwater EOCs across the urban to rural transition near Patna, as a rapidly developing Indian city, and contributes to the wider understanding of the vulnerability of shallow groundwater to surface-derived contamination in similar environments.

Association of low-level inorganic arsenic exposure from rice with age-standardized mortality risk of cardiovascular disease (CVD) in England and Wales

Adverse health outcomes, including death from cardiovascular disease (CVD), arising from chronic exposure to inorganic arsenic (iAs) are well documented. Consumption of rice is a major iAs exposure route for over 3 billion people, however, there is still a lack of epidemiological evidence demonstrating the association between iAs exposure from rice intake and CVD risks. We explored this potential association through an ecological study using data at local authority level across England and Wales. Local authority level daily per capita iAs exposure from rice (E-iAs_ing,rice) was estimated using ethnicity as a proxy for class of rice consumption. A series of linear and non-linear models were applied to estimate the association between E-iAs_ing,rice and CVD age-standardized mortality rate (ASMR), using Akaike's Information Criterion as the principle model selection criterion. When adjusted for significant confounders, notably smoking prevalence, education level, employment rate, overweight percentage, PM2.5, female percentage and medical and care establishments, the preferred non-linear model indicated that CVD risks increased with iAs exposure from rice at exposures above 0.3 μg/person/day. Also, the best-fitted linear model indicated that CVD ASMR in the highest quartile of iAs exposure (0.375-2.71 μg/person/day) was 1.06 (1.02, 1.11; p-trend <0.001) times higher than that in the lowest quartile (<0.265 μg/person/day). Notwithstanding the well-known limitations of ecological studies, this study further suggests exposure to iAs, including from rice intake, as a potentially important confounder for studies of the factors controlling CVD risks.

Groundwater Arsenic Distribution in India by Machine Learning Geospatial Modeling

Groundwater is a critical resource in India for the supply of drinking water and for irrigation. Its usage is limited not only by its quantity but also by its quality. Among the most important contaminants of groundwater in India is arsenic, which naturally accumulates in some aquifers. In this study we create a random forest model with over 145,000 arsenic concentration measurements and over two dozen predictor variables of surface environmental parameters to produce hazard and exposure maps of the areas and populations potentially exposed to high arsenic concentrations (>10 µg/L) in groundwater. Statistical relationships found between the predictor variables and arsenic measurements are broadly consistent with major geochemical processes known to mobilize arsenic in aquifers. In addition to known high arsenic areas, such as along the Ganges and Brahmaputra rivers, we have identified several other areas around the country that have hitherto not been identified as potential arsenic hotspots. Based on recent reported rates of household groundwater use for rural and urban areas, we estimate that between about 18-30 million people in India are currently at risk of high exposure to arsenic through their drinking water supply. The hazard models here can be used to inform prioritization of groundwater quality testing and environmental public health tracking programs.

Positive Association of Cardiovascular Disease (CVD) with Chronic Exposure to Drinking Water Arsenic (As) at Concentrations below the WHO Provisional Guideline Value: A Systematic Review and Meta-Analysis

To the best of our knowledge, a dose-response meta-analysis of the relationship between cardiovascular disease (CVD) and arsenic (As) exposure at drinking water As concentrations lower than the WHO provisional guideline value (10 µg/L) has not been published yet. We conducted a systematic review and meta-analyses to estimate the pooled association between the relative risk of each CVD endpoint and low-level As concentration in drinking water both linearly and non-linearly using a random effects dose-response model. In this study, a significant positive association was found between the risks of most CVD outcomes and drinking water As concentration for both linear and non-linear models (p-value for trend < 0.05). Using the preferred linear model, we found significant increased risks of coronary heart disease (CHD) mortality and CVD mortality as well as combined fatal and non-fatal CHD, CVD, carotid atherosclerosis disease and hypertension in those exposed to drinking water with an As concentration of 10 µg/L compared to the referent (drinking water As concentration of 1 µg/L) population. Notwithstanding limitations included, the observed significant increased risks of CVD endpoints arising from As concentrations in drinking water between 1 µg/L and the 10 µg/L suggests further lowering of this guideline value should be considered.

A comparative assessment of dilution correction methods for spot urinary analyte concentrations in a UK population exposed to arsenic in drinking water

Spot urinary concentrations of environmental exposure biomarkers require correction for dilution. There is no consensus on the most appropriate method, with creatinine used by default despite lacking theoretical robustness. We comparatively assessed the efficacy of creatinine; specific gravity (SG); osmolality and modifications of all three for dilution correcting urinary arsenic. For 202 participants with urinary arsenic, creatinine, osmolality and SG measurements paired to drinking water As, we compared the performance corrections against two independent criteria: primarily, (A) correlations of corrected urinary As and the dilution measurements used to correct them - weak correlations indicating good performance and (B) correlations of corrected urinary As and drinking water As - strong correlations indicating good performance. More than a third of variation in spot urinary As concentrations was attributable to dilution. Conventional SG and osmolality correction removed significant dilution variation from As concentrations, whereas conventional creatinine over-corrected, and modifications of all three removed measurable dilution variation. Modified creatinine and both methods of SG and osmolality generated stronger correlations of urinary and drinking water As concentrations than conventional creatinine, which gave weaker correlations than uncorrected values. A disparity in optima between performance criteria was observed, with much smaller improvements possible for Criterion B relative to A. Conventional corrections - particularly creatinine - limit the utility spot urine samples, whereas a modified technique outlined here may allow substantial improvement and can be readily retrospectively applied to existing datasets. More studies are needed to optimize urinary dilution correction methods. Covariates of urinary dilution measurements still warrant consideration.

Provenance of drinking water revealed through compliance sampling

Understanding drinking water hydrochemistry is essential for maintaining safe drinking water supplies. Whilst targeted research surveys have characterised drinking water hydrochemistry, vast compliance datasets are routinely collected but are not interrogated amidst concerns regarding the impact of mixed water sources, treatment, the distribution network and customer pipework. In this paper, we examine whether compliance samples retain hydrochemical signatures of their provenance. We first created and subsequently undertook the first hydrochemical analysis of a novel national database of publically available drinking water compliance analyses (n = 3 873 941) reported for 2015 across England and Wales. k-means cluster analysis revealed three spatially coherent clusters. Cluster 1 is dominated by groundwater sources, with high nitrate concentrations and mineralisation, and lower organic carbon, residual chlorine and THM formation. Cluster 2 was dominated by surface water sources and characterised by low mineralisation (low conductivity and major ion concentrations), low nitrate and high organic carbon concentrations (and hence residual chlorine and THM formation). Cluster 3 shows a mixture of groundwater overlain by confining layers and superficial deposits (resulting in higher trace metal concentrations and mineralisation) and surface water sources. These analyses demonstrate that, despite extensive processing of drinking water, at the national scale signatures of the provenance of drinking water remain. Analysis of compliance samples is therefore likely to be a helpful tool in the characterisation of processes that may affect drinking water chemistry. The methodology used is generic and can be applied in any area where drinking water chemistry samples are taken.

Dual in-aquifer and near surface processes drive arsenic mobilization in Cambodian groundwaters

Millions of people globally, and particularly in South and Southeast Asia, face chronic exposure to arsenic from reducing groundwaters in which. Arsenic release to is widely attributed largely to reductive dissolution of arsenic-bearing iron minerals, driven by metal reducing bacteria using bioavailable organic matter as an electron donor. However, the nature of the organic matter implicated in arsenic mobilization, and the location within the subsurface where these processes occur, remains debated. In a high resolution study of a largely pristine, shallow aquifer in Kandal Province, Cambodia, we have used a complementary suite of geochemical tracers (including ¹⁴C, ³H, ³He, ⁴He, Ne, δ¹⁸O, δD, CFCs and SF₆) to study the evolution in arsenic-prone shallow reducing groundwaters along dominant flow paths. The observation of widespread apparent ³H-³He ages of <55years fundamentally challenges some previous models which concluded that groundwater residence times were on the order of hundreds of years. Surface-derived organic matter is transported to depths of >30m, and the relationships between age-related tracers and arsenic suggest that this surface-derived organic matter is likely to contribute to in-aquifer arsenic mobilization. A strong relationship between ³H-³He age and depth suggests the dominance of a vertical hydrological control with an overall vertical flow velocity of ~0.4±0.1m·yr^-1 across the field area. A calculated overall groundwater arsenic accumulation rate of ~0.08±0.03μM·yr^-1 is broadly comparable to previous estimates from other researchers for similar reducing aquifers in Bangladesh. Although apparent arsenic groundwater accumulation rates varied significantly with site (e.g. between sand versus clay dominated sequences), rates are generally highest near the surface, perhaps reflecting the proximity to the redox cline and/or depth-dependent characteristics of the OM pool, and confounded by localized processes such as continued in-aquifer mobilization, sorption/desorption, and methanogenesis.

Surface wipe and bulk sampling of household dust: arsenic exposure in Cornwall, UK

Dust elemental levels can be expressed as concentrations (bulk samples) or surface loadings (wipe samples). Wipe sampling has not been widely adopted for elements other than lead (Pb). In this study, 433 wipe samples from 130 households in south west England - a region of widespread, natural and anthropogenic arsenic contamination linked with previous mining activities-were analysed to (i) quantify loadings of arsenic (As); (ii) assess the quality of wipe data using QA/QC criteria; (iii) estimate, using published ingestion rates, human exposure to As in dust using loadings and concentrations from 97 bulk samples and (iv) comparatively assess the performance of wipe and bulk sampling using associations with As biomonitoring data (urine, toenails and hair). Good QC performance was observed for wipes: strong agreement between field duplicates, non-detectable contamination of field blank wipes and good reference material recoveries. Arsenic loadings exceeded an existing urban background benchmark in 67 (52%) households. No exceedances of tolerable daily As intake were observed for adult exposure estimates but infant estimates exceeded for 1 household. Infant estimates calculated using bulk concentrations resulted in 4 (3%) exceedances. Neither wipe nor bulk As metrics were sufficiently better predictors of As in biospecimens. Sampling strategies, analytical protocols, exposure metrics and assessment criteria require refinement to validate dust sampling methodologies.

Biomarker-indicated extent of oxidation of plant-derived organic carbon (OC) in relation to geomorphology in an arsenic contaminated Holocene aquifer, Cambodia

The poisoning of rural populations in South and Southeast Asia due to high groundwater arsenic concentrations is one of the world’s largest ongoing natural disasters. It is important to consider environmental processes related to the release of geogenic arsenic, including geomorphological and organic geochemical processes. Arsenic is released from sediments when iron-oxide minerals, onto which arsenic is adsorbed or incorporated, react with organic carbon (OC) and the OC is oxidised. In this study we build a new geomorphological framework for Kandal Province, a highly studied arsenic affected region of Cambodia, and tie this into wider regional environmental change throughout the Holocene. Analyses shows that the concentration of OC in the sediments is strongly inversely correlated to grainsize. Furthermore, the type of OC is also related to grain size with the clay containing mostly (immature) plant derived OC and sand containing mostly thermally mature derived OC. Finally, analyses indicate that within the plant derived OC relative oxidation is strongly grouped by stratigraphy with the older bound OC more oxidised than younger OC.

High resolution profile of inorganic aqueous geochemistry and key redox zones in an arsenic bearing aquifer in Cambodia

Arsenic contamination of groundwaters in South and Southeast Asia is a major threat to public health. In order to better understand the geochemical controls on the mobility of arsenic in a heavily arsenic-affected aquifer in northern Kandal Province, Cambodia, key changes in inorganic aqueous geochemistry have been monitored at high vertical and lateral resolution along dominant groundwater flow paths along two distinct transects. The two transects are characterized by differing geochemical, hydrological and lithological conditions. Arsenic concentrations in groundwater are highly heterogenous, and are broadly positively associated with iron and negatively associated with sulfate and dissolved oxygen. The observed correlations are generally consistent with arsenic mobilization by reductive-dissolution of iron (hydr)oxides. Key redox zones, as identified using groupings of the PHREEQC model equilibrium electron activity of major redox couples (notably ammonium/nitrite; ammonium/nitrate; nitrite/nitrate; dissolved oxygen/water) have been identified and vary with depth, site and season. Mineral saturation is also characterized. Seasonal changes in groundwater chemistry were observed in areas which were (i) sandy and of high permeability; (ii) in close proximity to rivers; and/or (iii) in close proximity to ponds. Such changes are attributed to monsoonal-driven surface-groundwater interactions and are consistent with the separate provenance of recharge sources as identified using stable isotope mixing models.

Arsenic in residential soil and household dust in Cornwall, south west England: potential human exposure and the influence of historical mining

Exposure to arsenic (As) via residential soil and dust is a global concern, in regions affected by mining or with elevated concentrations present in underlying geology. Cornwall in south west England is one such area. Residential soil (n = 127) and household dust (n = 99) samples were collected from across Cornwall as part of a wider study assessing exposure to environmental As. Samples were analysed for total As (soil and dust samples) and human ingestion bioaccessible As (soil samples from properties with home-grown produce). Arsenic concentrations ranged from 12 to 992 mg kg^-1 in soil and 3 to 1079 mg kg^-1 in dust and were significantly higher in areas affected by metalliferous mineralisation. Sixty-nine percent of soils exceeded the 37 mg kg^-1 Category 4 Screening Level (C4SL), a generic assessment criteria for As in residential soils in England, which assumes 100% bioavailability following ingestion. The proportion of exceedance was reduced to 13% when the bioavailability parameter in the CLEA model was changed to generate household specific bioaccessibility adjusted assessment criteria (ACBIO). These criteria were derived using bioaccessibility data for a sub-set of individual household vegetable patch soils (n = 68). Proximity to former As mining locations was found to be a significant predictor of soil As concentration. This study highlights the value of bioaccessibility measurements and their potential for adjusting generic assessment criteria.

Assessing urinary flow rate, creatinine, osmolality and other hydration adjustment methods for urinary biomonitoring using NHANES arsenic, iodine, lead and cadmium data

BACKGROUND

There are numerous methods for adjusting measured concentrations of urinary biomarkers for hydration variation. Few studies use objective criteria to quantify the relative performance of these methods. Our aim was to compare the performance of existing methods for adjusting urinary biomarkers for hydration variation.

METHODS

Creatinine, osmolality, excretion rate (ER), bodyweight adjusted ER (ERBW) and empirical analyte-specific urinary flow rate (UFR) adjustment methods on spot urinary concentrations of lead (Pb), cadmium (Cd), non-arsenobetaine arsenic (As(IMM)) and iodine (I) from the US National Health and Nutrition Examination Survey (NHANES) (2009-2010 and 2011-2012) were evaluated. The data were divided into a training dataset (n = 1,723) from which empirical adjustment coefficients were derived and a testing dataset (n = 428) on which quantification of the performance of the adjustment methods was done by calculating, primarily, the correlation of the adjusted parameter with UFR, with lower correlations indicating better performance and, secondarily, the correlation of the adjusted parameters with blood analyte concentrations (Pb and Cd), with higher correlations indicating better performance.

RESULTS

Overall performance across analytes was better for Osmolality and UFR based methods. Excretion rate and ERBW consistently performed worse, often no better than unadjusted concentrations.

CONCLUSIONS

Osmolality adjustment of urinary biomonitoring data provides for more robust adjustment than either creatinine based or ER or ERBW methods, the latter two of which tend to overcompensate for UFR. Modified UFR methods perform significantly better than all but osmolality in removing hydration variation, but depend on the accuracy of UFR calculations. Hydration adjustment performance is analyte-specific and further research is needed to establish a robust and consistent framework.

Prolonged exposure to arsenic in UK private water supplies: toenail, hair and drinking water concentrations

Chronic exposure to arsenic (As) in drinking water is an established cause of cancer and other adverse health effects. Arsenic concentrations >10 μg L(-1) were previously measured in 5% of private water supplies (PWS) in Cornwall, UK. The present study investigated prolongued exposure to As by measuring biomarkers in hair and toenail samples from 212 volunteers and repeated measurements of As in drinking water from 127 households served by PWS. Strong positive Pearson correlations (rp = 0.95) indicated stability of water As concentrations over the time period investigated (up to 31 months). Drinking water As concentrations were positively correlated with toenail (rp = 0.53) and hair (rp = 0.38) As concentrations - indicative of prolonged exposure. Analysis of washing procedure solutions provided strong evidence of the effective removal of exogenous As from toenail samples. Significantly higher As concentrations were measured in hair samples from males and smokers and As concentrations in toenails were negatively associated with age. A positive association between seafood consumption and toenail As and a negative association between home-grown vegetable consumption and hair As was observed for volunteers exposed to <1 As μg L(-1) in drinking water. These findings have important implications regarding the interpretation of toenail and hair biomarkers. Substantial variation in biomarker As concentrations remained unaccounted for, with soil and dust exposure as possible explanations.

Urinary arsenic profiles reveal exposures to inorganic arsenic from private drinking water supplies in Cornwall, UK

Private water supplies (PWS) in Cornwall, South West England exceeded the current WHO guidance value and UK prescribed concentration or value (PCV) for arsenic of 10 μg/L in 5% of properties surveyed (n = 497). In this follow-up study, the first of its kind in the UK, volunteers (n = 207) from 127 households who used their PWS for drinking, provided urine and drinking water samples for total As determination by inductively coupled plasma mass spectrometry (ICP-MS) and urinary As speciation by high performance liquid chromatography ICP-MS (HPLC-ICP-MS). Arsenic concentrations exceeding 10 μg/L were found in the PWS of 10% of the volunteers. Unadjusted total urinary As concentrations were poorly correlated (Spearman's ρ = 0.36 (P < 0.001)) with PWS As largely due to the use of spot urine samples and the dominance of arsenobetaine (AB) from seafood sources. However, the osmolality adjusted sum, U-As(IMM), of urinary inorganic As species, arsenite (As(III)) and arsenate (As(V)), and their metabolites, methylarsonate (MA) and dimethylarsinate (DMA), was found to strongly correlate (Spearman's ρ: 0.62 (P < 0.001)) with PWS As, indicating private water supplies as the dominant source of inorganic As exposure in the study population of PWS users.

Arsenic hazard in Cambodian rice from a market-based survey with a case study of Preak Russey village, Kandal Province

This study comprises a market-based survey to assess the arsenic (As) hazard of Cambodian rice, encompassing rice from seven Cambodian provinces, comparisons with rice imported from China, Vietnam and Thailand, and assessments of 15 rice varieties. Rice samples (n = 157) were collected from four large markets in Kandal Province and analysed for As using inductively coupled mass spectrometry. The mean As concentration for Cambodian rice (0.185 µg g(-1), range 0.047-0.771 µg g(-1)) was higher than that for imported rice from Vietnam and Thailand (0.162 and 0.157 µg g(-1), respectively) with mean As concentrations highest in rice from Prey Veng Province resulting in a daily dose of 1.77 µg kg(-1) b.w. (body weight) d(-1). Between unmilled rice varieties, Cambodian-grown White Sticky Rice had the highest mean As concentration (0.234 µg g(-1)), whilst White Sticky Rice produced in Thailand had the lowest (0.125 µg g(-1)), suggesting that localised conditions have greater bearing over rice As concentrations than differences in As uptake between individual varieties themselves. A rice and water consumption survey for 15 respondents in the village of Preak Russey revealed mean consumption rates of 522 g d(-1) of rice and 1.9 L d(-1) of water. At water As concentrations >1000 µg L(-1), the relative contribution to the daily dose from rice is low. When water As concentrations are lowered to 50 µg L(-1), daily doses from rice and water are both generally below the 3.0 µg kg(-1) b.w. d(-1) benchmark daily limit for a 0.5% increase in lung cancer, yet when combined they exceeded this value in all but three respondents.

Microbial ecology of arsenic-mobilizing Cambodian sediments: lithological controls uncovered by stable-isotope probing

Microbially mediated arsenic release from Holocene and Pleistocene Cambodian aquifer sediments was investigated using microcosm experiments and substrate amendments. In the Holocene sediment, the metabolically active bacteria, including arsenate-respiring bacteria, were determined by DNA stable-isotope probing. After incubation with (13) C-acetate and (13) C-lactate, active bacterial community in the Holocene sediment was dominated by different Geobacter spp.-related 16S rRNA sequences. Substrate addition also resulted in the enrichment of sequences related to the arsenate-respiring Sulfurospirillum spp. (13) C-acetate selected for ArrA related to Geobacter spp. whereas (13) C-lactate selected for ArrA which were not closely related to any cultivated organism. Incubation of the Pleistocene sediment with lactate favoured a 16S rRNA-phylotype related to the sulphate-reducing Desulfovibrio oxamicus DSM1925, whereas the ArrA sequences clustered with environmental sequences distinct from those identified in the Holocene sediment. Whereas limited As(III) release was observed in Pleistocene sediment after lactate addition, no arsenic mobilization occurred from Holocene sediments, probably because of the initial reduced state of As, as determined by X-ray Absorption Near Edge Structure. Our findings demonstrate that in the presence of reactive organic carbon, As(III) mobilization can occur in Pleistocene sediments, having implications for future strategies that aim to reduce arsenic contamination in drinking waters by using aquifers containing Pleistocene sediments.

High arsenic in rice is associated with elevated genotoxic effects in humans

Arsenic in drinking water may cause major deleterious health impacts including death. Although arsenic in rice has recently been demonstrated to be a potential exposure route for humans, there has been to date no direct evidence for the impact of such exposure on human health. Here we show for the first time, through a cohort study in West Bengal, India, involving over 400 human subjects not otherwise significantly exposed to arsenic through drinking water, elevated genotoxic effects, as measured by micronuclei (MN) in urothelial cells, associated with the staple consumption of cooked rice with >200 μg/kg arsenic. Further work is required to determine the applicability to populations with different dietary and genetic characteristics, but with over 3 billion people in the world consuming rice as a staple food and several percent of this rice containing such elevated arsenic concentrations, this study raises considerable concerns over the threat to human health.

Characterisation of organic matter associated with groundwater arsenic in reducing aquifers of southwestern Taiwan

Arsenic (As) in groundwaters extensively used by people across the world constitutes a serious public health threat. The importance of organic matter (OM) as an electron donor in microbially-mediated reduction of As(V) or Fe(III)-bearing As-host minerals leading to mobilisation of solid-phase arsenic is widely recognised. Notwithstanding this, there are few studies characterising OM in such aquifers and, in particular, there is a dearth of data from the classic arsenic bearing aquifers in southwestern Taiwan. Organic geochemical analyses of sediments from a known groundwater arsenic hot-spot in southwestern Taiwan revealed contributions of thermally mature and plant derived origin, consistent with OM sources in all other Asian groundwater aquifer sediments analysed to date, indicating comparable sources and routes of OM transfer. The combined results of amended As(V) reduction assays with the organic geochemical analysis revealed that the microbiological process of dissimilatory As(V) reduction is active in this aquifer, but it is not controlled by a specific source of analysed OM. These indicate that (i) part of the OM that was considered to be less bio-available could still be used as an electron donor or (ii) other electron donors, not analysed in present study, could be controlling the rate of As release.

Pond-derived organic carbon driving changes in arsenic hazard found in Asian groundwaters

Microbially mediated reductive processes involving the oxidation of labile organic carbon are widely considered to be critical to the release of arsenic into shallow groundwaters in South and Southeast Asia. In areas where there is significant pumping of groundwater for irrigation the involvement of surface-derived organic carbon drawn down from ponds into the underlying aquifers has been proposed but remains highly controversial. Here we present isotopic data from two sites with contrasting groundwater pumping histories that unequivocally demonstrate the ingress of surface pond-derived organic carbon into arsenic-containing groundwaters. We show that pond-derived organic carbon is transported to depths of up to 50 m even in an arsenic-contaminated aquifer in Cambodia thought to be minimally disturbed by groundwater pumping. In contrast, in the extensively exploited groundwaters of West Bengal, we show that pond-derived organic carbon is transported in shallow groundwater to greater depths, in excess of 100 m in the aquifer. Intensive pumping of groundwaters may potentially drive secular increases in the groundwater arsenic hazard in this region by increasing the contribution of bioavailable pond-derived dissolved organic carbon drawn into these aquifer systems and transporting it to greater depths than would operate under natural flow conditions.

Comparison of drinking water, raw rice and cooking of rice as arsenic exposure routes in three contrasting areas of West Bengal, India

Remediation aimed at reducing human exposure to groundwater arsenic in West Bengal, one of the regions most impacted by this environmental hazard, are currently largely focussed on reducing arsenic in drinking water. Rice and cooking of rice, however, have also been identified as important or potentially important exposure routes. Quantifying the relative importance of these exposure routes is critically required to inform the prioritisation and selection of remediation strategies. The aim of our study, therefore, was to determine the relative contributions of drinking water, rice and cooking of rice to human exposure in three contrasting areas of West Bengal with different overall levels of exposure to arsenic, viz. high (Bhawangola-I Block, Murshidibad District), moderate (Chakdha Block, Nadia District) and low (Khejuri-I Block, Midnapur District). Arsenic exposure from water was highly variable, median exposures being 0.02 μg/kg/d (Midnapur), 0.77 μg/kg/d (Nadia) and 2.03 μg/kg/d (Murshidabad). In contrast arsenic exposure from cooked rice was relatively uniform, with median exposures being 0.30 μg/kg/d (Midnapur), 0.50 μg/kg/d (Nadia) and 0.84 μg/kg/d (Murshidabad). Cooking rice typically resulted in arsenic exposures of lower magnitude, indeed in Midnapur, median exposure from cooking was slightly negative. Water was the dominant route of exposure in Murshidabad, both water and rice were major exposure routes in Nadia, whereas rice was the dominant exposure route in Midnapur. Notwithstanding the differences in balance of exposure routes, median excess lifetime cancer risk for all the blocks were found to exceed the USEPA regulatory threshold target cancer risk level of 10(-4)-10(-6). The difference in balance of exposure routes indicate a difference in balance of remediation approaches in the three districts.

Arsenic release and attenuation in low organic carbon aquifer sediments from West Bengal

High arsenic concentrations in groundwater are causing a humanitarian disaster in Southeast Asia. It is generally accepted that microbial activities play a critical role in the mobilization of arsenic from the sediments, with metal-reducing bacteria stimulated by organic carbon implicated. However, the detailed mechanisms underpinning these processes remain poorly understood. Of particular importance is the nature of the organic carbon driving the reduction of sorbed As(V) to the more mobile As(III), and the interplay between iron and sulphide minerals that can potentially immobilize both oxidation states of arsenic. Using a multidisciplinary approach, we identified the critical factors leading to arsenic release from West Bengal sediments. The results show that a cascade of redox processes was supported in the absence of high loadings of labile organic matter. Arsenic release was associated with As(V) and Fe(III) reduction, while the removal of arsenic was concomitant with sulphate reduction. The microbial populations potentially catalysing arsenic and sulphate reduction were identified by targeting the genes arrA and dsrB, and the total bacterial and archaeal communities by 16S rRNA gene analysis. Results suggest that very low concentrations of organic matter are able to support microbial arsenic mobilization via metal reduction, and subsequent arsenic mitigation through sulphate reduction. It may therefore be possible to enhance sulphate reduction through subtle manipulations to the carbon loading in such aquifers, to minimize the concentrations of arsenic in groundwaters.

The role of indigenous microorganisms in the biodegradation of naturally occurring petroleum, the reduction of iron, and the mobilization of arsenite from west bengal aquifer sediments

High levels of naturally occurring arsenic are found in the shallow reducing aquifers of West Bengal, Bangladesh, and other areas of Southeast Asia. These aquifers are used extensively for drinking water and irrigation by the local population. Mechanisms for its release are unclear, although increasing evidence points to a microbial control. The type of organic matter present is of vital importance because it has a direct impact on the rate of microbial activity and on the amount of arsenic released into the ground water. The discovery of naturally occurring hydrocarbons in an arsenic-rich aquifer from West Bengal provides a source of potential electron donors for this process. Using microcosm-based techniques, seven sediments from a site containing naturally occurring hydrocarbons in West Bengal were incubated with synthetic ground water for 28 d under anaerobic conditions without the addition of an external electron donor. Arsenic release and Fe(III) reduction appeared to be microbially mediated, with variable rates of arsenic mobilization in comparison to Fe(III) reduction, suggesting that multiple processes are involved. All sediments showed a preferential loss of petroleum-sourced n-alkanes over terrestrially sourced sedimentary hydrocarbons n-alkanes during the incubation, implying that the use of petroleum-sourced n-alkanes could support, directly or indirectly, microbial Fe(III) reduction. Samples undergoing maximal release of As(III) contained a significant population of Sulfurospirillum sp., a known As(V)-reducing bacterium, providing the first evidence that such organisms may mediate arsenic release from West Bengali aquifers.

Hydrogeochemical behavior of arsenic-enriched groundwater in the deltaic environment: comparison between two study sites in West Bengal, India

Groundwaters have been collected from deltaic areas of West Bengal (Chakdaha and Baruipur blocks) to record their hydrogeochemical characteristics, and to verify the mechanism of arsenic (As) release. The data reveals that shallow (<70 m) groundwaters in both areas are of Ca-Mg-HCO(3) type; however deeper (>70 m) groundwaters in Baruipur areas are slightly enriched with Na, Cl and SO(4), indicating possible saline water intrusion. The groundwater is anoxic (mean Eh: -124 and -131 mV) with high levels of As (mean: 116 and 293 microg/L), Fe (mean: 4.74 and 3.83 mg/L), PO(4) (mean: 3.73 and 3.21 mg/L) and Mn (mean: 0.37 and 0.49 mg/L), respectively for Chakdaha and Baruipur areas. The observed values of As and bicarbonate (mean: 409 and 499 mg/L) in the shallow aquifer are indicative of redox processes (e.g., oxidation of organic matter) favouring the release of As. Moreover, the presence of DOC in the shallow aquifer suggests that organic matter is young and reactive, and may actively engage in redox driven processes. Our study further confirms that both Fe- and Mn-reduction processes are the dominant mechanisms for As release in these groundwaters.

Arsenic in hair and nails of individuals exposed to arsenic-rich groundwaters in Kandal province, Cambodia

The health implications of the consumption of high arsenic groundwater in Bangladesh and West Bengal are well-documented, however, little is known about the level of arsenic exposure elsewhere in Southeast Asia, where widespread exploitation of groundwater resources is less well established. We measured the arsenic concentrations of nail and hair samples collected from residents of Kandal province, Cambodia, an area recently identified to host arsenic-rich groundwaters, in order to evaluate the extent of arsenic exposure. Nail and hair arsenic concentrations ranged from 0.20 to 6.50 microg g(-1) (n=70) and 0.10 to 7.95 microg g(-1) (n=40), respectively, in many cases exceeding typical baseline levels. The arsenic content of the groundwater used for drinking water purposes (0.21-943 microg L(-1) (n=31)) was positively correlated with both nail (r=0.74, p<0.0001) and hair (r=0.86, p<0.0001) arsenic concentrations. In addition, the nail and hair samples collected from inhabitants using groundwater that exceeded the Cambodian drinking water legal limit of 50 microg L(-1) arsenic contained significantly more arsenic than those of individuals using groundwater containing <50 microg L(-1) arsenic. X-ray absorption near edge structure (XANES) spectroscopy suggested that sulfur-coordinated arsenic was the dominant species in the bulk of the samples analysed, with additional varying degrees of As(III)-O character. Tentative linear least squares fitting of the XANES data pointed towards differences in the pattern of arsenic speciation between the nail and hair samples analysed, however, mismatches in sample and standard absorption peak intensity prevented us from unambiguously determining the arsenic species distribution. The good correlation with the groundwater arsenic concentration, allied with the relative ease of sampling such tissues, indicate that the arsenic content of hair and nail samples may be used as an effective biomarker of arsenic intake in this relatively recently exposed population.

Probing the biogeochemistry of arsenic: response of two contrasting aquifer sediments from Cambodia to stimulation by arsenate and ferric iron

Many millions of people worldwide are at risk of severe poisoning through exposure to groundwater contaminated with sediment-derived arsenic. An ever-increasing body of work is reinforcing the link between microbially-mediated redox cycling in aquifer sediments and the mobilisation of sorbed As(V) into groundwaters as the potentially more mobile and toxic As(III) anion. However, to date, few studies have examined the biogeochemical cycling of Fe and As species by microbes indigenous to Cambodian sediments. In this study two contrasting sediments, taken from a shallow As-rich reducing aquifer in the Kien Svay district of Cambodia, were used in a laboratory microcosm study. We present evidence to show that microbes present in these sediments are able to reduce Fe(III) and As(V) when provided with an electron donor, and that the two sediments respond differently to stimulation with Fe(III) and As(V). Shifts in the community composition of the two sediments after stimulation with As(V) suggest a potential role for members of the beta-Proteobacteria in As(V) reduction, a phylogenetic grouping known to contain microorganisms capable of As(III) oxidation, but not previously implicated in As(V) reduction. PCR-based analysis of the sediment microbial DNA using primers specific to the arrA gene, (a gene essential for microbial As(V) respiration), indicates the presence of microorganisms capable of dissimilatory As(V) reduction.

Critical pathway analysis to determine key uncertainties in net impacts on disease burden in Bangladesh of arsenic mitigation involving the substitution of arsenic bearing for groundwater drinking water supplies

It has been increasingly recognised that calculation of the disease burden due to populations, such as in Bangladesh, extensively using hazardous arsenic bearing well waters, must explicitly account for the trade-off between diarrhoeal disease incidence and that of arsenic-related diseases. This is because it is likely that moves to alternative drinking water sources, be they surface waters or even more distant groundwaters, without further mitigation would result in a concurrent increase in diarrhoeal disease. In this paper, we update the model of Lokuge of the effects of such arsenic mitigation on disease burden in Bangladesh, using updated population data and background disease estimates. We run a critical pathway analysis on the model using Standardised Mortality Ratios (SMRs) for diabetes mellitus and ischemic heart disease from different epidemiological studies recently reviewed by Navas-Acien. Our analysis agrees with that of Lokuge that mitigation simply involving the substitution of a range of surface waters for well water sources with As > 50 microg/L would have a net positive impact on disease burden, as determined by deaths and Disability Life Adjusted Years (DALYs). In contrast, however, there is considerable ambiguity in the analogous results for mitigation for all the population exposed to well water with As > 10 microg/L. Depending upon the data source chosen for diabetes mellitus and ischaemic heart disease SMRs, such mitigation is modelled to have either a positive or a negative net impact on overall disease burden. The modelled negative impacts are entirely commensurate with the rationale for seeking groundwater as an alternative to surface waters as a drinking water supply, and highlight the practical requirement for multiple mitigation strategies, including those directed at ensuring the microbiological safety and continued protection of any alternative water supplies. Our study highlights the need for (i) adequate epidemiological studies involving multiple exposure categories, ideally resulting in an accurate dose-response relationship for arsenic uptake and the non-malignant high incidence conditions diabetes mellitus and ischemic heart disease for individuals with the socioeconomic and nutritional status of the Bangladeshi populations, and (ii) refined estimates of the diarrhoel disease burden arising from usage of surface waters.

A Fourier transform infrared spectroscopic study of Mg-rich, Mg-poor and acid leached palygorskites

The FTIR spectra of pure magnesium-rich (Mg-rich) and magnesium-poor (Mg-poor) palygorskites, before and after short-term (<7 h) and long-term (360 h) acid leaching are presented here. Comparison of decomposition spectra of Mg-rich and Mg-poor palygorskites clearly shows that the absorption peaks related to pairs of octahedral cation differ depending on the octahedral site occupancy. Short-term acid leaching of palygorskites results in significant changes to FTIR absorption bands near 1200 and 790 cm-1. As the acid attack progresses, the band at 1200 cm-1 shifts to lower wavenumbers, whilst the band at 790 cm-1, which here is assigned to SiU-O-SiD symmetrical stretching vibration, shifts to higher wavelengths. Longer-term leaching of palygorskites results in the disappearance of 900-1200 cm-1 absorption bands, showing that the palygorskite has largely decomposed to amorphous silica. Assignments of several other bands have been made as follows: several vibrations relate to OH, i.e. 847 cm-1, hygroscopic water (1635 cm-1), Si-O vibrations 1100, 611-621, 470-481 cm-1, etc. appear in the FTIR spectra of 360 h acid leached palygorskite. Three bands near 1100, 611-621 and 470-481 cm-1 relate to Si-O vibration of an ideal hexagonal (Si2O5)n sheet.

XAS and XMCD evidence for species-dependent partitioning of arsenic during microbial reduction of ferrihydrite to magnetite

Poorly crystalline Fe(III) oxyhydroxides, ubiquitously distributed as mineral coatings and discrete particles in aquifer sediments, are well-known hosts of sedimentary As. Microbial reduction of these phases is widely thought to be responsible for the genesis of As-rich reducing groundwaters found in many parts of the world, most notably in Bangladesh and West Bengal, India. As such, it is important to understand the behavior of As associated with ferric oxyhydroxides during the early stages of Fe(lll) reduction. We have used X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) to elucidate the changes in the bonding mechanism of As(III) and As(V) as their host Fe(III) oxyhydroxide undergoes bacterially induced reductive transformation to magnetite. Two-line ferrihydrite, with adsorbed As(III) or As(V), was incubated under anaerobic conditions in the presence of acetate as an electron donor, and Geobacter sulfurreducens, a subsurface bacterium capable of respiring on Fe(lll), but not As(V). In both experiments, no increase in dissolved As was observed during reduction to magnetite (complete upon 5 days incubation), consistent with our earlier observation of As sequestration by the formation of biogenic Fe(III)-bearing minerals. XAS data suggested that the As bonding environment of the As(III)-magnetite product is indistinguishable from that obtained from simple adsorption of As(lll) on the surface of biogenic magnetite. In contrast, reduction of As(V)-sorbed ferrihydrite to magnetite caused incorporation of As5+ within the magnetite structure. XMCD analysis provided further evidence of structural partitioning of As5+ as the small size of the As5+ cation caused a distortion of the spinel structure compared to standard biogenic magnetite. These results may have implications regarding the species-dependent mobility of As undergoing anoxic biogeochemical transformations, e.g., during early sedimentary diagenesis.

Molecular analysis of arsenate-reducing bacteria within Cambodian sediments following amendment with acetate

The health of millions is threatened by the use of groundwater contaminated with sediment-derived arsenic for drinking water and irrigation purposes in Southeast Asia. The microbial reduction of sorbed As(V) to the potentially more mobile As(III) has been implicated in release of arsenic into groundwater, but to date there have been few studies of the microorganisms that can mediate this transformation in aquifers. With the use of stable isotope probing of nucleic acids, we present evidence that the introduction of a proxy for organic matter ((13)C-labeled acetate) stimulated As(V) reduction in sediments collected from a Cambodian aquifer that hosts arsenic-rich groundwater. This was accompanied by an increase in the proportion of prokaryotes closely related to the dissimilatory As(V)-reducing bacteria Sulfurospirillum strain NP-4 and Desulfotomaculum auripigmentum. As(V) respiratory reductase genes (arrA) closely associated with those found in Sulfurospirillum barnesii and Geobacter uraniumreducens were also detected in active bacterial communities utilizing (13)C-labeled acetate in microcosms. This study suggests a direct link between inputs of organic matter and the increased prevalence and activity of organisms which transform As(V) to the potentially more mobile and thus hazardous As(III) via dissimilatory As(V) reduction.

Interactions between the Fe(III)-reducing bacterium Geobacter sulfurreducens and arsenate, and capture of the metalloid by biogenic Fe(II)

Previous work has shown that microbial communities in As-mobilizing sediments from West Bengal were dominated by Geobacter species. Thus, the potential of Geobacter sulfurreducens to mobilize arsenic via direct enzymatic reduction and indirect mechanisms linked to Fe(III) reduction was analyzed. G. sulfurreducens was unable to conserve energy for growth via the dissimilatory reduction of As(V), although it was able to grow in medium containing fumarate as the terminal electron acceptor in the presence of 500 muM As(V). There was also no evidence of As(III) in culture supernatants, suggesting that resistance to 500 muM As(V) was not mediated by a classical arsenic resistance operon, which would rely on the intracellular reduction of As(V) and the efflux of As(III). When the cells were grown using soluble Fe(III) as an electron acceptor in the presence of As(V), the Fe(II)-bearing mineral vivianite was formed. This was accompanied by the removal of As, predominantly as As(V), from solution. Biogenic siderite (ferrous carbonate) was also able to remove As from solution. When the organism was grown using insoluble ferrihydrite as an electron acceptor, Fe(III) reduction resulted in the formation of magnetite, again accompanied by the nearly quantitative sorption of As(V). These results demonstrate that G. sulfurreducens, a model Fe(III)-reducing bacterium, did not reduce As(V) enzymatically, despite the apparent genetic potential to mediate this transformation. However, the reduction of Fe(III) led to the formation of Fe(II)-bearing phases that are able to capture arsenic species and could act as sinks for arsenic in sediments.