Arsenic migration to deep groundwater in Bangladesh influenced by adsorption and water demand

Vertical distribution of arsenic and bacterial communities in calcareous farmland amending by organic fertilizer and iron-oxidizing bacteria: Field experiment on concomitant remediation

The migration and transformation mechanisms of arsenic (As) in soil environments necessitate an understanding of its influencing processes. Here, we investigate the subsurface biogeochemical transformation of As and iron (Fe) through amended in the top 20 cm with iron oxidizing bacteria (FeOB) and organic fertilizer (OF). Our comprehensive 400-day field study, conducted in a calcareous soil profile sectioned into 20 cm increments, involved analysis by sequential extraction and assessment of microbial properties. The results reveal that the introduction of additional OF increased the release ratio of As/Fe from the non-specific adsorption fraction (136.47 %) at the subsoil depth (40-60 cm), underscoring the importance of sampling at various depths and time points to accurately elucidate the form, instability, and migration of As within the profile. Examination of bacterial interaction networks indicated a disrupted initial niche in the bottom layer, resulting in a novel cooperative symbiosis. While the addition of FeOB did not lead to the dominance of specific bacterial species, it did enhance the relative abundance of As-tolerant Acidobacteria and Gemmatimonadetes in both surface (39.2 % and 38.76 %) and deeper soils (44.29 % and 23.73 %) compared to the control. Consequently, the amendment of FeOB in conjunction with OF facilitated the formation of poorly amorphous Fe (hydr)oxides in the soil, achieved through abiotic and biotic sequestration processes. Throughout the long-term remediation process, the migration coefficient of bioavailable As within the soil profile decreased, indicating that these practices did not exacerbate As mobilization. This study carries significant implications for enhancing biogeochemical cycling in As-contaminated Sierozem soils and exploring potential bioremediation strategies. ENVIRONMENTAL IMPLICATION: The long-term exposure of sewage irrigation has potential adverse effects on the local ecosystem, causing serious environmental problems. Microorganisms play a vital role in the migration and transformation of arsenic in calcareous soil in arid areas, which highlights the necessity of understanding its dynamics. The vertical distribution, microbial community and fate of arsenic in calcareous farmland soil profile in northwest China were studied through field experiments. The results of this work have certain significance for the remediation of arsenic-contaminated soil in arid areas, and provide new insights for the migration, transformation and remediation of arsenic in this kind of soil.

Transferability of Machine Learning Models for Geogenic Contaminated Groundwaters

Machine learning models show promise in identifying geogenic contaminated groundwaters. Modeling in regions with no or limited samples is challenging due to the need for large training sets. One potential solution is transferring existing models to such regions. This study explores the transferability of high fluoride groundwater models between basins in the Shanxi Rift System, considering six factors, including modeling methods, predictor types, data size, sample/predictor ratio (SPR), predictor range, and data informing. Results show that transferability is achieved only when model predictors are based on hydrochemical parameters rather than surface parameters. Data informing, i.e., adding samples from challenging regions to the training set, further enhances the transferability. Stepwise regression shows that hydrochemical predictors and data informing significantly improve transferability, while data size, SPR, and predictor range have no significant effects. Additionally, despite their stronger nonlinear capabilities, random forests and artificial neural networks do not necessarily surpass logistic regression in transferability. Lastly, we utilize the t-SNE algorithm to generate low-dimensional representations of data from different basins and compare these representations to elucidate the critical role of predictor types in transferability.

A Low-Cost Programmable Reversing Flow Column Apparatus for Investigating Mixing Zones

This note describes the development and testing of a novel, programmable reversing flow 1D (R1D) experimental column apparatus designed to investigate reaction, sorption, and transport of solutes in aquifers within dynamic reversing flow zones where waters with different chemistries mix. The motivation for constructing this apparatus was to understand the roles of mixing and reaction on arsenic discharging through a tidally fluctuating riverbank. The apparatus can simulate complex transient flux schedules similar to natural flow regimes The apparatus uses an Arduino microcontroller to control flux magnitude through two peristaltic pumps. Solenoid valves control flow direction from two separate reservoirs. In-line probes continually measure effluent electrical conductance, pH, oxidation-reduction potential, and temperature. To understand how sensitive physical solute transport is to deviations from the real hydrograph of the tidally fluctuating river, two experiments were performed using: (1) a simpler constant magnitude, reversing flux direction schedule (RCF); and (2) a more environmentally relevant variable magnitude, reversing flux direction schedule (RVF). Wherein, flux magnitude was ramped up and down according to a sine wave. Modeled breakthrough curves of chloride yielded nearly identical dispersivities under both flow regimes. For the RVF experiment, Peclet numbers captured the transition between diffusion and dispersion dominated transport in the intertidal interval. Therefore, the apparatus accurately simulated conservative, environmentally relevant mixing under transient, variable flux flow regimes. Accurately generating variable flux reversing flow regimes is important to simulate the interaction between flow velocity and chemical reactions where Brownian diffusion of solutes to solid-phase reaction sites is kinetically limited.

Novel insights into the co-selection of metal-driven antibiotic resistance in bacteria: a study of arsenic and antibiotic co-exposure

The simultaneous development of antibiotic resistance in bacteria due to metal exposure poses a significant threat to the environment and human health. This study explored how exposure to both arsenic and antibiotics affects the ability of an arsenite oxidizer, Achromobacter xylosoxidans CAW4, to transform arsenite and its antibiotic resistance patterns. The bacterium was isolated from arsenic-contaminated groundwater in the Chandpur district of Bangladesh. We determined the minimum inhibitory concentration (MIC) of arsenite, cefotaxime, and tetracycline for A. xylosoxidans CAW4, demonstrating a multidrug resistance (MDR) trait. Following this determination, we aimed to mimic an environment where A. xylosoxidans CAW4 was exposed to both arsenite and antibiotics. We enabled the strain to grow in sub-MIC concentrations of 1 mM arsenite, 40 µg/mL cefotaxime, and 20 µg/mL tetracycline. The expression dynamics of the arsenite oxidase (aioA) gene in the presence or absence of antibiotics were analyzed. The findings indicated that simultaneous exposure to arsenite and antibiotics adversely affected the bacteria's capacity to metabolize arsenic. However, when arsenite was present in antibiotics-containing media, it promoted bacterial growth. The study observed a global downregulation of the aioA gene in arsenic-antibiotic conditions, indicating the possibility of increased susceptibility through co-resistance across the entire bacterial population of the environment. This study interprets that bacterial arsenic-metabolizing ability can rescue the bacteria from antibiotic stress, further disseminating environmental cross-resistance. Therefore, the co-selection of metal-driven antibiotic resistance in bacteria highlights the need for effective measures to address this emerging threat to human health and the environment.

Synergistic inhibition of green rust crystallization by co-existing arsenic and silica

Arsenic and silica are known inhibitors of the crystallization of iron minerals from poorly ordered precursor phases. However, little is known about the effects of co-existing As and Si on the crystallization and long-term stability of mixed-valence Fe minerals such as green rust (GR). GR usually forms in anoxic, Fe2+-rich, near-neutral pH environments, where they influence the speciation and mobility of trace elements, nutrients and contaminants. In this work, the Fe2+-induced transformation of As- and/or Si-bearing ferrihydrite (FHY) was monitored at pH 8 ([As]initial = 100 μM, Si/As = 10) over 720 h. Our results showed that in the presence of As(III) + Si or As(V) + Si, GR sulfate (GRSO4) formation from FHY was up to four times slower compared to single species system containing only As(III), As(V) or Si. Co-existing As(III) + Si and As(V) + Si also inhibited GRSO4 transformation to magnetite, contrary to systems with only Si or As(V). Overall, our findings demonstrate the synergistic inhibitory effect of co-existing Si on the crystallization and solid-phase stability of As-bearing GRSO4, establishing an inhibitory effect ladder: As(III) + Si > As(V) + Si > As(III) > Si > As(V). This further highlights the importance of GR in potentially controlling the fate and mobility of As in ferruginous, Si-rich groundwater and sediments such as those in South and Southeast Asia.

Spatial distribution of groundwater fluoride and arsenic and its related disease in typical drinking endemic regions

c exposure to geogenic fluoride and arsenic iChronic exposure to geogenic fluoride and arsenic in groundwater has a deleterious influence on the health of billions of people globally. The health status of residents impacted by connected diseases is urgently needed. A twelve-year study was carried out to identify the spatial distribution pattern of high fluoride/arsenic groundwater in an arid/semi-arid area and to estimate the population exposed to related disease. A geostatistical interpolation method and a disease inversion model were used. The results indicated that fluoride/arsenic-rich groundwater primarily accumulated in basins of Shanxi Province. Groundwater fluoride exposure provided a health concern to 3.16 million persons (9.08 % of the population), including 2.50 million children at risk of dental caries. Exposure to groundwater arsenic caused a health risk to 4.38 million inhabitants (12.58 % of total), with 1.92 million at risk of lung cancer, 1.87 million at risk of bladder cancer, and 0.29 million at risk of skin cancer, respectively. The pollution and impact of groundwater fluoride and arsenic vary greatly among residents in different environments, and accurate assessment of the affected population is of great significance for residents' health and water quality management. Our research study complements the critical data on the disease risks associated with geogenic-contaminated groundwater and provides scientific basis of water quality management for policy makers.

Co-occurrence of arsenic and iodine in the middle-deep groundwater of the Datong Basin: From the perspective of optical properties and isotopic characteristics

Redox processes can induce arsenic (As) and iodine (I) transformation and thus change As and I co-occurrence, yet there is no evidence that Fe-C-S coupled redox processes have such an impact on the co-occurrence of As and I. To fill this gap, middle-deep groundwater from the Datong Basin were samples for the purpose of exploring how dissolved organic matter (DOM) reactivity affects As and I enrichment and how iron reduction and sulfate reduction processes influence As and I co-occurrence. We identified three DOM components: reduced and oxidized quinone compounds (C1 and C3) and a labile DOM from terrestrial inputs (C2). Two pathways of DOM processing take place in the aquifer, including the degradation of labile DOM to HCO₃^- and the transformation of oxidized quinone compounds to reduced quinone compounds. Electrons transfer drives the reduction of the terminal electron acceptors. The supply of electrons promotes the reduction of iron and sulfate by microbes, enhancing As and I co-enrichment in groundwater. Thus, the reduction processes of iron and sulfate triggered by the dual roles of DOM affect dissolved As and I co-enrichment. As and I biogeochemical cycling interacts with C, Fe, and S cycling. These results provide isotopic and fluorescence evidence that explains the co-occurrence of arsenic and iodine in middle-deep aquifers.

Storm accelerated subsurface Escherichia coli growth and exports to coastal waters

Storm significantly deteriorates coastal water fecal pollution now and beyond. Questions relating to storm exerting on coastal water safety are often intertwined with both surface water and subsurface processes. Stormwater runoff is a vital metric for coastal water fecal pollution under current cognition, while the controls of subsurface system remain unclear. Here, this study leveraged two time-series field data collected in a sandy beach during storm and non-storm periods to probe subsurface Escherichia coli (E. coli) growth and exports to coastal waters under storm events. Results demonstrated that storm events can not only stimulate subsurface E. coli growth, but also accelerate subsurface E. coli exports into the receiving water. Storm-intensified rainfall injected more oxygenous rainwater in the shallow groundwater, subsequently stimulating subsurface E. coli growth. Storm-strengthened wave energy was responsible for accelerating subsurface E. coli exports through enhanced wave-induced recirculated seawater. This study proposes a new insight for the stress of storm events on microbial pollution in coastal waters. The findings are constructive to the prevention of beach ecosystem pollution and can pave the way for coastal safety management to future extreme weather.

Enrichment of High Arsenic Groundwater Controlled by Hydrogeochemical and Physical Processes in the Hetao Basin, China

Based on 447 samples collected from a shallow aquifer (depths from 0 to 150 m) in the Hetao Basin, Northern China, an integrated hydrogeochemical approach was used in this study to conceptualize the enrichment of high arsenic groundwater in the Hetao Basin. An unconventional method of distinguishing hydrogeochemical and physical processes from a dataset was tested by investigating the cumulative frequency distribution of ionic ratios expressed on a probability scale. By applying cumulative frequency distribution curves to characterize the distribution of ionic ratios throughout the Hetao Basin, hydrogeochemical indicators were obtained that distinguish the series of hydrogeochemical processes that govern groundwater composition. All hydrogeochemical processes can basically be classified as recharge intensity of groundwater, evaporation concentration intensity, and reductive degree controlling the spatial distribution of arsenic. By considering the three processes, we found that the concentration of arsenic was more than 10 μg/L when the (HCO₃^-+CO₃^2-)/SO₄^2- ratio was over 4.1 (strong reductive area). As the evaporation concentration intensity increased, the median value of arsenic increased from 10.74 to 382.7 μg/L in the median reductive area and rapidly increased from 89.11 to 461.45 μg/L in the strong reductive area. As the river recharge intensity increased (with the intensity index increasing from 0 to 5), the median value of arsenic dropped from 40.2 to 6.8 μg/L in the median reductive area and decreased more markedly from 219.85 to 23.73 μg/L in the strong reductive area. The results provide a new insight into the mechanism of As enrichment in groundwater.

Biogeochemical properties and potential risk of shallow arsenic-rich sediment layers to groundwater quality in Western Bangladesh

The arsenic-contaminated groundwater has attracted attention in much south and southeast Asian deltas, however, mainly on the deep aquifers. Here, arsenic (As) concentration and its fractionation of the sediment cores in a shallow aquifer in Bangladesh were investigated using ICP-MS, FE-EPMA, XRD and ¹⁴C-AMS chronology techniques. The results of the present study indicated that the peak concentrations of As (54.7-79.1 µg/g) were in peat layers (at a depth of 7.5-8.0 m). Several types of iron (oxyhidr)oxides and framboidal pyrite, which contain As also, were found in the peat samples. The high concentrations of As were in an exchangeable form, As-bearing iron crystalline and As-bearing organic materials. We revealed that the As-rich peat layers were formed from 3170 to 3901 cal yrs before, due to the sea level decrease in this area. The 16S rRNA gene-based phylogenetic analysis revealed that the bacterial strains in the As-rich peats were mainly affiliated with genera Acinetobacter, Enterobacter, Escherichia, Bacillus, Clostridiaceae and Acinetobacter. The geo-accumulation index (I_geo) and ecological risk index assessment were calculated for the sediments, which shows that As-rich sediment layers were in range of moderately to heavily contaminated and considerable classes, respectively. Under the permanent saturated condition, the As-rich peat layers should be considered as an important potential driver of the groundwater As in this area.

Effects of geochemical and hydrodynamic transiency on desorption and transport of As in heterogeneous systems

Spatial and temporal variations in groundwater As concentrations are mainly caused by changes in geochemical and hydrodynamic conditions. In this study, the effects of geochemical and hydrodynamic transiency on As desorption and transport in a layered heterogeneous system with preferential flow paths during continuous or intermittent water extraction were investigated. A flume desorption experiment was performed after an adsorption experiment lasting 99 d with competitive adsorption anions (phosphate) in the influent. The results indicated that although competitive adsorption between As and phosphate at the water/solid interface significantly promoted As desorption from solid materials, marked amounts of As desorbed slowly or were on irreversible sorption sites in the system. As adsorbed by the sand and clay near the preferential flow paths was preferentially released, while the release of As from the interiors of the clay zones was limited by diffusion. Water extraction accelerated As transport between the different layers, and this increased the overall rate of As release from zones limited by diffusion. Desorption rate of As in the layered system was fast initially, followed by a period of slow desorption rate that lasted months. The desorption hysteresis was due to slow desorption controlled by diffusion. The results provide important insights for understanding and modeling As desorption and transport in field systems.

Genome-Resolved Metagenomic Analysis of Groundwater: Insights into Arsenic Mobilization in Biogeochemical Interaction Networks

High-arsenic (As) groundwaters, a worldwide issue, are critically controlled by multiple interconnected biogeochemical processes. However, there is limited information on the complex biogeochemical interaction networks that cause groundwater As enrichment in aquifer systems. The western Hetao basin was selected as a study area to address this knowledge gap, offering an aquifer system where groundwater flows from an oxidizing proximal fan (low dissolved As) to a reducing flat plain (high dissolved As). The key microbial interaction networks underpinning the biogeochemical pathways responsible for As mobilization along the groundwater flow path were characterized by genome-resolved metagenomic analysis. Genes associated with microbial Fe(II) oxidation and dissimilatory nitrate reduction were noted in the proximal fan, suggesting the importance of nitrate-dependent Fe(II) oxidation in immobilizing As. However, genes catalyzing microbial Fe(III) reduction (omcS) and As(V) detoxification (arsC) were highlighted in groundwater samples downgradient flow path, inferring that reductive dissolution of As-bearing Fe(III) (oxyhydr)oxides mobilized As(V), followed by enzymatic reduction to As(III). Genes associated with ammonium oxidation (hzsABC and hdh) were also positively correlated with Fe(III) reduction (omcS), suggesting a role for the Feammox process in driving As mobilization. The current study illustrates how genomic sequencing tools can help dissect complex biogeochemical systems, and strengthen biogeochemical models that capture key aspects of groundwater As enrichment.

The groundwater arsenic contamination in the Bengal Basin-A review in brief

The presence of arsenic in the groundwater of the densely-populated Bengal Basin evolved as a mass-poisoning agent and is a reason for the misery of millions of people living here. High-level arsenic was detected in the shallow aquifer-tube wells of the basin in the late-20th century. The redox conditions and the biogeochemical activities in the shallow aquifers support the existence of arsenic in its most toxic +3 state. The shallow aquifers are constructed by the Holocene reduced grey sands, having a lesser capacity to hold the arsenic brought from the Himalayas by the Ganga-Brahmaputra-Meghna river system. Among several other hypotheses, the reductive dissolution of arsenic bearing Fe-oxyhydroxides coupled with the microbial activities in the organic-matter-rich Holocene grey sands is believed to be the primary reason for releasing arsenic in groundwater of basinal shallow aquifers. The deep aquifers below the late Pleistocene aquifers and the Palaeo-interfluvial aquifers capped by the last glacial maximum Palaeosol generally contain arsenic-free or low-arsenic water. Ingress of arsenic into the deep aquifers from the shallow aquifers was considered to have been caused by extensive non-domestic pumping. However, studies have found that extensive pumping is unlikely to contaminate the deep aquifer water with higher levels of arsenic within decadal time scales. Irrigation-pumping may produce hydraulic barriers between the shallow and deep aquifer-groundwater and distributes arsenic in the topsoil by flushing. Significant disparities have been observed among the Bengal basinal groundwater arsenic concentrations. However, abrupt spatial variation in groundwater arsenic concentrations has been a key feature of the basin.

Increases in groundwater arsenic concentrations and risk under decadal groundwater withdrawal in the lower reaches of the Yellow River basin, Henan Province, China

The spatiotemporal variability in groundwater arsenic concentrations following extensive groundwater extractions over decades was rarely studied on a large scale. To fill this gap, variations in groundwater arsenic concentrations in the North Henan Plain in China from 2010 to 2020 were investigated. The possibility of high-arsenic groundwater (>10 μg/L) was higher than 40% in aquifers within a distance of 100 m from paleochannels. This may be due to the fact that deposits in paleochannels were rich in organic matter and suitable for arsenic enrichment. Following groundwater withdrawal over ten years from 2010 to 2020, nearly half of groundwater samples (44%) were elevated in groundwater arsenic concentrations, and the proportion of high arsenic groundwater increased from 24% in 2010 to 26% in 2020. These may be related to enhanced Fe(III) oxide reduction under decadal groundwater withdrawal. However, around 56% groundwater samples were decreases in arsenic concentrations because of increased NO₃^- levels in these samples in 2020. Furthermore, extensive groundwater withdrawal decreased groundwater tables averagely by 4.6 m from 2010 to 2020, which induced the intrusion of high-arsenic groundwater from shallow aquifers into deeper ones. More importantly, the long-term groundwater pumping has perturbed groundwater flow dynamics and redistributed high-arsenic groundwater in the plain, leading to 18% more areas and 33.8% more residents being potentially at risk. This study suggests that the threat of groundwater overexploitation may be much more severe than previously expected.

Separation of pollutants from aqueous solution using nanoclay and its nanocomposites: A review

Wastewater is always composed of different pollutants, most of which are toxic to the living being. It is very tough to separate all those diverse groups of contaminants using a single process or single material. Rather a sustainable and environment friendly processes should be adapted to restrict the secondary pollution generation. Nanoclay and its nanocomposites are one of the most used adsorbents that have been modified and used for the separation of almost all types of pollutants, including dyes, heavy metals, fluoride, nitrate, ammonia, emerging pollutants and bacteria. They are relatively inexpensive, easy to exploit and relatively maintenance-free. Thus, recent research bloomed for developing suitable adsorbents, including clay nanocomposites. The advantages and drawbacks of all the clay nanocomposites-based processes have been discussed critically in this article. Nano-clays or other nanoparticles incorporated synthetic and natural polymers-based clay nanocomposites were synthesized, and it was found that they can remove dyes in the range between 48 mg/g and 1994 mg/g. Similarly, they separate a diverse group of heavy metal ions, including As, Cu, Co, Pd, Zn, Cr, Ni, Cd, and Hg, in the range of 0.073-1667 mg/g. The clay nanocomposites also showed fluoride removal efficacy in the range of 0.134-23 mg/g. They are also useful for the separation of emerging pollutants like pesticides, pharmaceuticals, personal care products, trace elements, and particulate matters in the range of 0.1-651 mg/g the clay nanocomposites showed considerable nitrate, ammonia and bacteria removal efficacy too. Though it seems promising, more investigations with real wastewater and pilot-scale studies are recommended to explore large-scale wastewater treatment capabilities.

Arsenic in Petroleum-Contaminated Groundwater near Bemidji, Minnesota Is Predicted to Persist for Centuries

We used a reactive transport model to investigate the cycling of geogenic arsenic (As) in a petroleum-contaminated aquifer. We simulated As mobilization and sequestration using surface complexation reactions with Fe(OH)3 during petroleum biodegradation coupled with Fe-reduction. Model results predict that dissolved As in the plume will exceed the U.S. and EU 10 µg/L drinking water standard for ~400 years. Non-volatile dissolved organic carbon (NVDOC) in the model promotes As mobilization by exerting oxygen demand, which maintains anoxic conditions in the aquifer. After NVDOC degrades, As re-associates with Fe(OH)3 as oxygenated conditions are re-established. Over the 400-year simulation, As transport resembles a “roll front” in which: (1) arsenic sorbed to Fe(OH)3 is released during Fe-reduction coupled to petroleum biodegradation; (2) dissolved As resorbs to Fe(OH)3 at the plume’s leading edge; and (3) over time, the plume expands, and resorbed As is re-released into groundwater. This “roll front” behavior underscores the transience of sorption as an As attenuation mechanism. Over the plume’s lifespan, simulations suggest that As will contaminate more groundwater than benzene from the oil spill. At its maximum, the model simulates that ~5.7× more groundwater will be contaminated by As than benzene, suggesting that As could pose a greater long-term water quality threat than benzene in this petroleum-contaminated aquifer.

Smectite-supported chain of iron nanoparticle beads for efficient clean-up of arsenate contaminated water

Prolonged exposure to inorganic arsenic (As) via drinking water is a major concern as it poses significant human health risks. Removal of As is crucial but requires effective and environment-friendly clean-up technology to avoid any additional risk to the environment. In this study, we developed Australian smectite (smec)-supported nano zero-valent iron (nZVI) composite for arsenate i.e., As(V) sorption. We used a range of tools, including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) and energy dispersion X-ray (EDS) spectroscopy to characterise the material. SEM and TEM images and elemental mapping of the composite reflect that the smectite layer was surrounded by a chain of iron nanobeads evenly distributed on clay particles, which is quite exceptional among currently available nZVIs. The maximum As(V) sorption capacity of this composite was 23.12 mg/g in the ambient conditions. Using X-ray photoelectron spectroscopy we unveiled chemical states of As and Fe before and after the sorption process. Additionally, the release of iron nanoparticles from the composite at various pHs (3-10) were found negligible, which demonstrates the effectiveness of smec-nZVI to remove As(V) from contaminated water without posing any secondary pollutant.

Assessment of potential health risk of major contaminants of groundwater in a densely populated agricultural area

As a key part of Bohai New Area development, Haixing County has been undergoing rapid development. In order to estimate potential risks of chemical parameters to human health of local residents, carcinogenic and non-carcinogenic risks via direct ingestion of drinking water were calculated using human health risk assessment (HHRS) based on triangular fuzzy number. The levels of pH, total dissolved solids, total harness, SO₄ ^2-, Na⁺, Cl^-, SO₄ ^2-, F^-, Fe (total iron), NO₃ ^-, and NO₂ ^- were more or less higher than the permissible limits except parameters As and Mn. The analysis results show that risk level for different crowds in the study area demonstrated an obvious variation, generally in the order of infants > children > adult males > adult females for non-carcinogenic risk values (R ⁿ), while the sequence of the carcinogenic risk values (R ^c) are adult males > adult females > children > infants. When the confidence level was 0.8, the non-carcinogenic risk values (R ⁿ) through drinking water intake were higher than 1, and this implied that potential health impacts on human health for local residents. However, the risks of carcinogenic risk values (R ^c) were lower than 1.0E-4, demonstrating minimal and acceptable health risk. Furthermore, according to the middle values (α = 1) of R ⁿ, the total non-carcinogenic risks for local residents were obtained in the following order: GW (Gaowan Town) > XJ (Xinji-Xiangfang County) > ZM (Zhaomaotao County) > HX (Haixing-Suji Town) > ZH (Zhanghuiting County) > XS (Xiaoshan County), and ZM > XJ > GW > HX > XS > ZH for R ^c. It was also found that the spatial distribution of fluoride level in drinking water is urgently needed to be identified. In conclusion, the potential health risks to residents should cause enough attention both from society and the academic community.

Impact of Physico-Chemical Heterogeneity on Arsenic Sorption and Reactive Transport under Water Extraction

Heterogeneity in physical and chemical properties is a common characteristic in a subsurface environment. This study investigated the effect of physico-chemical heterogeneity on arsenic (As) sorption and reactive transport under water extraction in a layered system with preferential flow paths. A flume experiment was performed to derive the spatio-temporal data of As reactive transport. The results indicated that the heterogeneous system significantly accelerated downward (vertical direction) As migration as a coupled effect of physical and chemical heterogeneity that led to fast As transport with low As sorption along the preferential flow paths. The results also indicated that such a heterogeneity effect was driven by water extraction that enhanced the downward groundwater flow along the preferential flow paths. Numerical simulations were performed by matching the experimental results to provide insights into the dominant processes controlling the As migration in the heterogeneous systems. The simulation results highlighted the importance of the kinetic oxidation of mineral-bonded Fe(II) to Fe(III) in the clay matrix that dynamically increased As sorption affinity and retarded As reactive transport. A coupled model of reactive transport along the preferential flow paths, sorption-retarded diffusion from the preferential flow paths into the clay matrixes, and reactions that change sorption affinity in the matrix was required to describe the As reactive transport systems with physico-chemical heterogeneities. The results have strong implications for understanding and modeling As downward migration from shallow to deep aquifers under groundwater pumping conditions in field systems with inherent heterogeneity.

Similar retardation of arsenic in gray Holocene and orange Pleistocene sediments: Evidence from field-based column experiments in Bangladesh

Groundwater flow has the potential to introduce arsenic (As) in previously uncontaminated aquifers. The extent to which As transport is retarded by adsorption is particularly relevant in Bangladesh where low-As wells offer the best chance of reducing chronic exposure to As of a large rural population dependent on groundwater. In this study, column experiments were conducted with intact cores in the field to measure As retardation. Freshly collected cores of reduced iron (Fe-II) dominated gray sediment of Holocene age as well as oxidized Fe (III)-coated orange sediment of Pleistocene age were eluted at pore-water velocities of 40-230 cm/day with anoxic groundwater pumped directly from a well and containing 320 μg/L As. Up to 100 μg/L As was immediately released from gray sand but the main As breakthrough for both gray and orange sand occurred between 30 and 70 pore volumes, depending on flow rate. The early release of As from gray sand is attributed to the presence of a weakly bound pool of As. The sorption of As was kinetically limited in both gray and orange sand columns. We used a reversible multi-reaction transport model to simulate As breakthrough curves while keeping the model parameters as constant as possible. Contrary to the notion that dissolved As is sorbed more strongly to orange sands, we show that As was similarly retarded in both gray and orange sands in the field.

Machine Learning Models of Groundwater Arsenic Spatial Distribution in Bangladesh: Influence of Holocene Sediment Depositional History

Recent advances in machine learning methods offer the opportunity to improve risk assessment and to decipher factors influencing the spatial variability of groundwater arsenic ([As]_gw). A systematic comparison reveals that boosted regression trees (BRT) and random forest (RF) outperform logistic regression. The probability of [As]_gw exceeding 5 μg/L (approximate median value of Bangladesh [As]_gw), 10 μg/L (WHO provisional guideline value), and 50 μg/L (Bangladesh drinking water standard) is modeled by BRT and RF methods for Bangladesh and its four subregions demarcated by major rivers. Of the 109 geo-environmental and hydrochemical predictor variables, phosphorus and iron emerge as the most important across spatial scales, consistent with known As mobilization mechanisms. Well depth is significant only when hydrochemical parameters are not considered, consistent with prior studies. A peak of probability of [As]_gw exceedance at ∼30 m depth is evident in the partial dependence plots (PDPs) for spatial-parameter-only models but not in the equivalent all-parameter models, suggesting that sediment depositional history explains interdependent spatial patterns of groundwater As-P-Fe in Holocene aquifers. The South region exhibits a decrease of probability of [As]_gw exceedance below 150 m depth in PDPs for spatial-parameter-only and all-parameter models, supporting that the deeper Pleistocene aquifer is a low-As water resource.

Origin of Groundwater Arsenic in a Rural Pleistocene Aquifer in Bangladesh Depressurized by Distal Municipal Pumping

Across South Asia, millions of villagers have reduced their exposure to high-arsenic (As) groundwater by switching to low-As wells. Isotopic tracers and flow modeling are used in this study to understand the groundwater flow system of a semi-confined aquifer of Pleistocene (>10 kyr) age in Bangladesh that is generally low in As but has been perturbed by massive pumping at a distance of about 25 km for the municipal water supply of Dhaka. A 10- to 15-m-thick clay aquitard caps much of the intermediate aquifer (>40- to 90-m depth) in the 3-km² study area, with some interruptions by younger channel sand deposits indicative of river scouring. Hydraulic heads in the intermediate aquifer below the clay-capped areas are 1-2 m lower than in the high-As shallow aquifer above the clay layer. In contrast, similar heads in the shallow and intermediate aquifer are observed where the clay layer is missing. The head distribution suggests a pattern of downward flow through interruptions in the aquitard and lateral advection from the sandy areas to the confined portion of the aquifer. The interpreted flow system is consistent with ³H-³He ages, stable isotope data, and groundwater flow modeling. Lateral flow could explain an association of elevated As with high methane concentrations within layers of gray sand below certain clay-capped portions of the Pleistocene aquifer. An influx of dissolved organic carbon from the clay layer itself leading to a reduction of initially orange sands has also likely contributed to the rise of As.

Arsenic contamination of Bangladesh aquifers exacerbated by clay layers

Confining clay layers typically protect groundwater aquifers against downward intrusion of contaminants. In the context of groundwater arsenic in Bangladesh, we challenge this notion here by showing that organic carbon drawn from a clay layer into a low-arsenic pre-Holocene (>12 kyr-old) aquifer promotes the reductive dissolution of iron oxides and the release of arsenic. The finding explains a steady rise in arsenic concentrations in a pre-Holocene aquifer below such a clay layer and the repeated failure of a structurally sound community well. Tritium measurements indicate that groundwater from the affected depth interval (40–50 m) was recharged >60 years ago. Deeper (55–65 m) groundwater in the same pre-Holocene aquifer was recharged only 10–50 years ago but is still low in arsenic. Proximity to a confining clay layer that expels organic carbon as an indirect response to groundwater pumping, rather than directly accelerated recharge, caused arsenic contamination of this pre-Holocene aquifer.

Generally it is thought that confining clay layers provide protection to low-arsenic groundwaters against intrusion of shallower, high-arsenic groundwater bodies. Here, the authors show that impermeable clay layers can increase arsenic input to underlying groundwater systems due to reduction of iron oxides coupled to carbon oxidation.

Multilayer arsenic mobilization and multimetal co-enrichment in the alluvium (Brahmaputra) plains of India: A tale of redox domination along the depth

The study attempts to understand arsenic (As) mobilization in a shallow aquifer with depth variation while focusing on the potential co-occurrence of As with priority metals (zinc and lead), using a pilot scale multilevel groundwater monitoring system (MGWS). Groundwater samples (n = 72) were collected bi-weekly (every 15 days) from the multilevel sampler (4.6, 9.2 and 13.8 m depths), installed at Tezpur, Sonitpur district of Brahmaputra floodplain (BFP), Assam, India, for a period of 1 year (August 2013-July 2014). Both geogenic and anthropogenic influences were found to affect the studied unconfined aquifer. At 4.6 m, weathering dominated due to interaction with CO₂ and infiltrating water. Prevalent high pH (7.9-8.6) at all three depths in association with strong oxidizing condition (at 4.6 m) during the drier months seem to play a crucial role in desorption based As release. Multivariate analyses revealed that redox potential (ORP) remains the primary controller of As release at all three depths. With depth, stronger anoxic conditions resulted in the dominance of reductive hydrolysis leading to a co-occurrence scenario of As (max 4.6 μgL^-1) with Zn (max 2514 μgL^-1) and Pb (max 740 μL^-1) with influences of anthropogenic modes of activities like agriculture and dry deposition from a brick kiln. Multi-element enrichment is an emerging concern but the bigger picture would be to understand the peculiarities of individual aquifers, as a generalization can lead to missing a ton of information. In this regard, long-term multilevel monitoring can help in the predictive understanding of the vertical stratification and co-occurrences of multi-metals that can subsequently be applied for water production at the safer depths.

Deep tubewell microbial water quality and access in arsenic mitigation programs in rural Bangladesh

The objective of this paper is to determine whether deep tubewells installed through arsenic mitigation efforts in rural Bangladesh provide better drinking water microbial quality compared to shallow tubewells. We conducted a stratified random cross-sectional survey of 484 households to assess microbial contamination of deep tubewell water at source and at point of use (POU) compared to shallow tubewell water using the Compartment Bag Test. In addition, we measured storage time, distance, travel time and ownership status among both sets of users to assess deep tubewell efficacy and under what conditions they offer poorer or better water quality. Differences in tubewell characteristics were compared using non-parametric Mann-Whitney U tests and two-proportion Z-tests. Prevalence ratios of microbial contamination stratified by water quality, storage time and distance to tubewells and ownership were estimated using unadjusted Mantel-Haenszel tests. There was no significant difference in microbial contamination between shallow and deep tubewells at source. The presence of POU water microbial contamination in storage containers in deep tubewell households was 1.11 times the prevalence in shallow tubewell storage containers (95% CI = 0.97-1.27). Deep tubewell users stored water longer and walked significantly farther to obtain water compared to shallow tubewell users. Among deep tubewell households, those residing farther away from the source were 1.24 times as likely to drink contaminated water from storage containers compared to those located nearby (95% CI = 1.04-1.48). Our findings suggest that deep tubewells have comparable water quality to shallow tubewells at source, but increasing distance from the household exacerbates risk of microbial contamination at POU.

A meta-analysis of the distribution, sources and health risks of arsenic-contaminated groundwater in Pakistan

Globally, millions of people who rely on groundwater for potable purposes and agriculture have been inadvertently exposed to toxic arsenic (As) because of its natural occurrence in groundwater in several countries of Asia, Europe and America. While the presence of As in groundwater and its impacts on human health have been documented in many countries, there is little information on As contamination in Pakistan. This review highlights, for the first time, the extent and severity of As-induced problems in Pakistan based on relevant published papers; discusses possible sources of As contamination of aquifers; and estimates As-induced potential health hazards in the country in relation to global data. Data from 43 studies (>9882 groundwater samples) were used to describe As variability in groundwater of Pakistan and for comparison with global data. The mean groundwater As content reported in these studies was 120 μg/L (range: 0.1-2090 μg/L; SD: ±307). About 73% of the values for mean As contents in the 43 studies were higher than the World Health Organization (WHO) permissible limit (10 μg/L) for drinking water, while 41% were higher than the permissible limit of As in Pakistan (50 μg/L). It was observed that groundwater samples in some areas of Punjab and Sindh provinces contained high As concentrations which were almost equal to concentrations reported in the most contaminated areas of the world. We predicted that the mean values of ADD, HQ and CR were 4.4 μg kg^-1day^-1 (range: 0-77 μg kg^-1day^-1), 14.7 (range: 0-256) and 0.0029 (range: 0-0.0512), respectively, based on mean As concentrations reported in Pakistan. In addition, this article proposes some integrated sustainable solutions and future perspectives keeping in view the regional and global context, as well as the on-ground reality of the population drinking As-contaminated water, planning issues, awareness among civil society and role of the government bodies. Based on available data, it is predicted that almost 47 million people in Pakistan are residing in areas where more than 50% of groundwater wells contain As concentrations above the WHO recommended limit of As in drinking water.

The evaluation of arsenic contamination potential, speciation and hydrogeochemical behaviour in aquifers of Punjab, Pakistan

In this study, we tested 123 groundwater wells from five different areas of Punjab, Pakistan for arsenic (As) contamination level and species, as well as delineated hydrogeochemical behaviour of As in aquifers. Results revealed that 75% and 41% of the groundwater wells exceeded the safe As limit of World Health Organisation (WHO, 10 μg L^-1) and Pakistan-EPA (50 μg L^-1), respectively. Arsenite (As(III)) and arsenate (As(V)) spanned 0-80% and 20-100% of total As (1.2-206 μg L^-1), respectively. The mean As content (5.2 μg L^-1) of shallow wells at 9-40 m depth did not exceed the WHO safe limit, representing a safe aquifer zone for pumping of groundwater compared to deeper wells at 41-90 m (51 μg L^-1) and >90 m (23 μg L^-1) depths. Piper-plot elucidated that the aqueous chemistry was dominated with Na-SO₄, Na-Ca-SO₄, Na-Mg-SO₄ type saline water. Principal component analysis grouped As concentration with well depth, pH, salinity, Fe and CO₃, exhibiting that these hydrogeochemical factors could have potential role in controlling As release/sequestration into the aquifers of study area. Geochemical modeling showed positive saturation indices only for iron (Fe) oxide-phases, indicating Fe oxides as the major carriers of As. Overall, this study provides insights to tackle emerging As threat to the communities in Punjab, Pakistan, as well as help develop suitable management/mitigation strategies - based on the baseline knowledge of As levels/species and factors governing As contamination in the study area.

Insights into arsenic retention dynamics of Pleistocene aquifer sediments by in situ sorption experiments

The migration of arsenic (As) enriched groundwater into Pleistocene aquifers as a consequence of extensive groundwater abstraction represents an increasing threat to the precious water resources in Asian delta regions. Pleistocene aquifer sediments are typically rich in FeIII-(hydr)oxides and are capable to adsorb high amounts of As. This results in a pronounced accumulation of As in Pleistocene aquifers, where high As groundwater infiltrates from adjacent Holocene aquifers. However, As retention by Pleistocene aquifers over long-term time scales remains largely unknown. We studied As sorption in situ by placing natural Pleistocene sediments and pure mineral phases directly inside groundwater monitoring wells at a study site near Hanoi (Vietnam). This in situ exposure allows for constant flushing of the samples with unaltered groundwater and the establishment of undisturbed sorption equilibria similar to those in local aquifer sediments, which is not readily attainable in traditional laboratory sorption experiments. The groundwaters in our experimental wells were characterized by different As concentrations (0.01-6.63 μmol/L) and redox states, reaching from suboxic to anoxic conditions (E_h of +159 to -4 mV). Results show that adsorption is the dominant As retention mechanism, independent from the respective groundwater chemistry (i.e. concentrations of dissolved P, HCO₃^- and Si). Whilst most of the As sorbed within the first week, sorption further increased slowly but consistently by 6-189%, respectively, within six months. Hence, the As sorption behavior of Pleistocene aquifer sediments should be determined over longer periods to avoid an underestimation of the As sorption capacity. Accompanying desorption experiments revealed that about 51% of the sorbed As was remobilized within six months when exposed to low As groundwater. We therefore conclude that a considerable proportion of the As accumulated in the aquifer sediments is prone to remobilization once the As concentrations in migrating groundwater decline. Remobilization of As should be considered in local water management plans to avoid contamination of precious groundwater resources with this As legacy.

Arsenic in groundwater of West Bengal, India: A review of human health risks and assessment of possible intervention options

This paper reviews how active research in West Bengal has unmasked the endemic arsenism that has detrimental effects on the health of millions of people and their offspring. It documents how the pathways of exposure to this toxin/poison have been greatly expanded through intensive application of groundwater in agriculture in the region within the Green Revolution framework. A goal of this paper is to compare and contrast the similarities and differences in arsenic occurrence in West Bengal with those of other parts of the world and assess the unique socio-cultural factors that determine the risks of exposure to arsenic in local groundwater. Successful intervention options are also critically reviewed with emphasis on integrative strategies that ensure safe water to the population, proper nutrition, and effective ways to reduce the transfer of arsenic from soil to crops. While no universal model may be suited for the vast areas of the world affected with by natural contamination of groundwater with arsenic, we have emphasized community-specific sustainable options that can be adapted. Disseminating scientifically correct information among the population coupled with increased community level participation and education are recognized as necessary adjuncts for an engineering intervention to be successful and sustainable.

Sources of groundwater salinity and potential impact on arsenic mobility in the western Hetao Basin, Inner Mongolia

The quality of groundwater used for human consumption and irrigation in the Hetao Basin of Inner Mongolia, China is affected by elevated salinity as well as high arsenic (As) concentrations. However, the origin of high salinity and its potential impact on As mobility in the Basin remain unclear. This study explores both issues using stable isotopic compositions and Cl/Br ratios of groundwater as well as the major ions of both groundwater and leachable salts in aquifer sediments. Limited variations in δ¹⁸O and δ²H (-11.13 to -8.10, -82.23 to -65.67) with the wide range of Total Dissolved Solid (TDS, 351-6734mg/L) suggest less contribution of direct evaporation to major salinity in groundwater. Deuterium excess shows that non-direct evaporation (capillary evaporation, transpiration) and mineral/evaporite dissolution contribute to >60% salinity in groundwater with TDS>1000mg/L. Non-direct evaporation, like capillary evaporation and transpiration, is proposed as important processes contributing to groundwater salinity based on Cl/Br ratio and halite dissolution line. The chemical weathering of Ca, Mg minerals and evaporites (Na₂SO₄ and CaSO₄) input salts into groundwater as well. This is evidenced by the fact that lacustrine environment and the arid climate prevails in Pleistocene period. Dissolution of sulfate salts not only promotes groundwater salinity but affects As mobilization. Due to the dissolution of sulfate salts and non-direct evaporation, groundwater SO₄^2- prevails and its reduction may enhance As enrichment. The higher As concentrations (300-553μg/L) are found at the stronger SO₄^2- reduction stage, indicating that reduction of Fe oxide minerals possibly results from HS^- produced by SO₄^2- reduction. This would have a profound impact on As mobilization since sulfate is abundant in groundwater and sediments. The evolution of groundwater As and salinity in the future should be further studied in order to ensure sustainable utilization of water resource in this water scarce area.

Naturally Occurring versus Anthropogenic Sources of Elevated Molybdenum in Groundwater: Evidence for Geogenic Contamination from Southeast Wisconsin, United States

Molybdenum (Mo) is an essential trace nutrient but has negative health effects at high concentrations. Groundwater typically has low Mo (<2 μg/L), and elevated levels are associated with anthropogenic contamination, although geogenic sources have also been reported. Coal combustion residues (CCRs) are enriched in Mo, and thus present a potential anthropogenic contamination source. Here, we use diagnostic geochemical tracers combined with groundwater residence time indicators to investigate the sources of Mo in drinking-water wells from shallow aquifers in a region of widespread CCR disposal in southeastern Wisconsin. Samples from drinking-water wells were collected in areas near and away from known CCR disposal sites, and analyzed for Mo and inorganic geochemistry indicators, including boron and strontium isotope ratios, along with groundwater tritium-helium and radiogenic ⁴He in-growth age-dating techniques. Mo concentrations ranged from <1 to 149 μg/L. Concentrations exceeding the U.S. Environmental Protection Agency health advisory of 40 μg/L were found in deeper, older groundwater (mean residence time >300 y). The B (δ¹¹B = 22.9 ± 3.5‰) and Sr (⁸⁷Sr/⁸⁶Sr = 0.70923 ± 0.00024) isotope ratios were not consistent with the expected isotope fingerprints of CCRs, but rather mimic the compositions of local lithologies. The isotope signatures combined with mean groundwater residence times of more than 300 years for groundwater with high Mo concentrations support a geogenic source of Mo to the groundwater, rather than CCR-induced contamination. This study demonstrates the utility of a multi-isotope approach to distinguish between fossil fuel-related and natural sources of groundwater contamination.

Lessons Learned from Arsenic Mitigation among Private Well Households

PURPOSE OF REVIEW

Many thousands of research papers have been published on the occurrence, health effects, and mitigation of arsenic in drinking water sourced from groundwater around the world. Here, an attempt is made to summarize this large body of knowledge into a small number of lessons.

RECENT FINDINGS

This is an opinion paper reflecting on why we are far from the goal of eliminating this silent and widespread poison to protect the health of many millions. The lessons are drawn from research in countries representing a range of economic development and cultural contexts. The replacement of household wells with centralized water supplies has reduced population level exposure to moderate (50-100 μg/L) and high (>100 μg/L) levels of arsenic in drinking water in some countries as they become wealthier. However, there remains a very large rural population in all countries where the exposure to low levels (10-50 μg/L) of arsenic continues due to its dispersed occurrence in the environment and frequent reliance on private well. A set of natural (geological and biological), socioeconomic, and behavioral barriers to progress are summarized as lessons. They range from challenges in identifying the exposed households due to spatially heterogeneous arsenic distribution in groundwater, difficulties in quantifying the exposure let alone reducing the exposure, failures in maintaining compliance to arsenic drinking water standards, to misplaced risk perceptions and environmental justice issues. Environmental health professionals have an ethical obligation to help As mitigation among private well water households, along with physicians, hydrogeologists, water treatment specialists, community organizations, and government.

Quantifying Reactive Transport Processes Governing Arsenic Mobility after Injection of Reactive Organic Carbon into a Bengal Delta Aquifer

Over the last few decades, significant progress has been made to characterize the extent, severity, and underlying geochemical processes of groundwater arsenic (As) pollution in S/SE Asia. However, comparably little effort has been made to merge the findings into frameworks that allow for a process-based quantitative analysis of observed As behavior and for predictions of its long-term fate. This study developed field-scale numerical modeling approaches to represent the hydrochemical processes associated with an in situ field injection of reactive organic carbon, including the reductive dissolution and transformation of ferric iron (Fe) oxides and the concomitant release of sorbed As. We employed data from a sucrose injection experiment in the Bengal Delta Plain to guide our model development and to constrain the model parametrization. Our modeling results illustrate that the temporary pH decrease associated with the sucrose transformation and mineralization caused pronounced, temporary shifts in the As partitioning between aqueous and sorbed phases. The results also suggest that while the reductive dissolution of Fe(III) oxides reduced the number of sorption sites, a significant fraction of the released As was rapidly scavenged through coprecipitation with neo-formed magnetite. These secondary reactions can explain the disparity between the observed Fe and As behavior.

Visualizing Arsenate Reactions and Encapsulation in a Single Zero-Valent Iron Nanoparticle

A nanostructure-based mechanism is presented on the enrichment, separation, and immobilization of arsenic with nanoscale zero-valent iron (nZVI). The As-Fe reactions are studied with spherical aberration corrected scanning transmission electron microscopy (Cs-STEM). Near-atomic resolution (<1 nm³) electron tomography discovers a thin continuous layer (23 ± 3 Å) of elemental arsenic sandwiched between the iron oxide shell and the zerovalent iron core. This points to a unique mechanism of nanoencapsulation and proves that the outer layer, especially the Fe(0)-oxide interface, is the edge of the As-Fe reactions. Atomic-resolution imaging on the grain boundary provides strong evidence that arsenic atoms diffuse preferably along the nonequilibrium, high-energy, and defective polycrystalline grain boundary of iron oxides. Results also offer direct evidence on the surface sorption or surface complex formation of arsenate on ferric hydroxide (FeOOH). The core-shell structure and unique properties of nZVI clearly underline rapid separation, large capacity, and stability for the treatment of toxic heavy metals such as cadmium, chromium, arsenic, and uranium.

Microbial communities involved in arsenic mobilization and release from the deep sediments into groundwater in Jianghan plain, Central China

It was shown that groundwater in Jianghan Plain was severely contaminated by arsenic; however, little is known about the mechanism by which the mineral arsenic was mobilized and released into groundwater from the high-arsenic sediments in this area. Here, we collected sediment samples from the depths of 5-230m in Jianghan Plain. Although all of the samples contain high contents of total arsenic, the soluble arsenic was only detectable in few of the shallow sediments, but was readily detectable in all of the deep sediments at the depths of 190-230m. Analysis of the genes of arsenate-respiring reductases indicated that they were not present in all of the shallow sediments from the depths of 5-185m, but were detectable in all of the deep sediments from the depths of 190-230m; all of the identified reductase genes are new or new-type, and they display unique diversity. Microcosm assay indicated that the microbial communities from the deep sediments were able to reduce As(V) into As(III) using lactate, formate, pyruvate or acetate as an electron donor under anaerobic condition. Arsenic release assay demonstrated that these microbial communalities efficiently catalyzed the mobilization and release of the mineral arsenic into aqueous phase. We also isolated a novel cultivable dissimilatory As(V)-respiring bacterium Aeromonas sp. JH155 from the sediments. It is able to completely reduce 2.0mM As(V) into As(III) in 72h, and efficiently promote the reduction and release of the mineral arsenic into aqueous phase. Analysis of the 16S rRNA genes indicated that the deep sediments contain diversities of microbial communities, which were shaped by the environmental factors, such as As, SO₄^2-, NO₃^-, Fe and pH value. These data suggest that the microorganisms in the deep sediments in Jianghan Plain played key roles in the mobilization and release of insoluble arsenic into the groundwater.

Reversible adsorption and flushing of arsenic in a shallow, Holocene aquifer of Bangladesh

The spatial heterogeneity of dissolved arsenic (As) concentrations in shallow groundwater of the Bengal Basin has been attributed to transport of As (and reactive carbon) from external sources or to the release of As from within grey sand formations. We explore the latter scenario in this detailed hydrological and geochemical study along a 300 m transect of a shallow aquifer extending from a groundwater recharge area within a sandy channel bar to its discharge into a nearby stream. Within the 10-20 m depth range, groundwater ages along the transect determined by the ³H-³He method increase from <10 yr in the recharge area to a maximum of 40 yr towards the stream. Concentrations of groundwater As within the same grey sands increase from 10 to 100 to ∼500 µg/L along this transect. Evidence of reversible adsorption of As between the groundwater and sediment was obtained from a series of push-pull experiments, traditional batch adsorption experiments, and the accidental flooding of a shallow monitoring well. Assuming reversible adsorption and a distribution coefficient, K_d, of 0.15-1.5 L/kg inferred from these observations, a simple flushing model shows that the increase in As concentrations with depth and groundwater age at this site, and at other sites in the Bengal and Red River Basins, can be attributed to the evolution of the aquifer over 100-1000 years as aquifer sands are gradually flushed of their initial As content. A wide range of As concentrations can thus be maintained in groundwater with increases with depth governed by the history of flushing and local recharge rates, without external inputs of reactive carbon or As from other sources.

A model for the evolution in water chemistry of an arsenic contaminated aquifer over the last 6000 years, Red River floodplain, Vietnam

Aquifers on the Red River flood plain with burial ages ranging from 500 to 6000 years show, with increasing age, the following changes in solute concentrations; a decrease in arsenic, increase in Fe(II) and decreases in both pH, Ca and bicarbonate. These changes were interpreted in terms of a reaction network comprising the kinetics of organic carbon degradation, the reduction kinetics of As containing Fe-oxides, the sorption of arsenic, the kinetics of siderite precipitation and dissolution, as well as of the dissolution of CaCO₃. The arsenic released from the Fe-oxide is preferentially partitioned into the water phase, and partially sorbed, while the released Fe(II) is precipitated as siderite. The reaction network involved in arsenic mobilization was analyzed by 1-D reactive transport modeling. The results reveal complex interactions between the kinetics of organic matter degradation and the kinetics and thermodynamic energy released by Fe-oxide reduction. The energy released by Fe-oxide reduction is strongly pH dependent and both methanogenesis and carbonate precipitation and dissolution have important influences on the pH. Overall it is the rate of organic carbon degradation that determines the total electron flow. However, the kinetics of Fe-oxide reduction determines the distribution of this flow of electrons between methanogenesis, which is by far the main pathway, and Fe-oxide reduction. Modeling the groundwater arsenic content over a 6000 year period in a 20 m thick aquifer shows an increase in As during the first 1200 years where it reaches a maximum of about 600 μg/L. During this initial period the release of arsenic from Fe-oxides actually decreases but the adsorption of arsenic onto the sediment delays the build-up in the groundwater arsenic concentration. After 1200 years the groundwater arsenic content slowly decreases controlled both by desorption and continued further, but diminishing, release from Fe-oxide being reduced. After 6000 years the arsenic content has decreased to 33 μg/L. The modeling enables a quantitative description of how the aquifer properties, the reactivity of organic carbon and Fe-oxides, the number of sorption sites and the buffering mechanisms change over a 6000 year period and how the combined effect of these interacting processes controls the groundwater arsenic content.

Rapid decadal evolution in the groundwater arsenic content of Kolkata, India and its correlation with the practices of her dwellers

Increasing arsenic contamination in the groundwater is one of the biggest environmental challenges that the Bengal delta is facing today. Groundwater is still the main source of water for a large number of population in this region and therefore, significant presence of toxic arsenic has a direct consequence on human lives here. Moreover, arsenic also enters into the food chain through the consumed agricultural products grown in this area. Therefore, acquiring knowledge about the ever-changing map of arsenic contamination and employing adequate protective measures are of utmost importance. Here, we present a comprehensive municipal ward-wise map of the arsenic content of the shallow groundwater table of Kolkata-the most important and highly population dense city of the delta. Comparison with previously available data reveals a rapid change and the grim situation for the city. Our study suggests that it should be an immediate task of the administration to extend treated water service to the whole population of the city for direct consumption, and artificial recharge and maximum rainwater replenishment need to be taken up with utmost urgency to avoid intrusion of toxicity in biological food chains via agricultural products. We hope our study would drive the city planners to reconsider the existing urbanization and development plans of all the cities, placed over arsenic-contaminated groundwater aquifers.

Megacity pumping and preferential flow threaten groundwater quality

Many of the world's megacities depend on groundwater from geologically complex aquifers that are over-exploited and threatened by contamination. Here, using the example of Dhaka, Bangladesh, we illustrate how interactions between aquifer heterogeneity and groundwater exploitation jeopardize groundwater resources regionally. Groundwater pumping in Dhaka has caused large-scale drawdown that extends into outlying areas where arsenic-contaminated shallow groundwater is pervasive and has potential to migrate downward. We evaluate the vulnerability of deep, low-arsenic groundwater with groundwater models that incorporate geostatistical simulations of aquifer heterogeneity. Simulations show that preferential flow through stratigraphy typical of fluvio-deltaic aquifers could contaminate deep (>150 m) groundwater within a decade, nearly a century faster than predicted through homogeneous models calibrated to the same data. The most critical fast flowpaths cannot be predicted by simplified models or identified by standard measurements. Such complex vulnerability beyond city limits could become a limiting factor for megacity groundwater supplies in aquifers worldwide.

Megacities rely on groundwater from aquifers that may be over-exploited and be at risk of contamination. Khan et al. evaluate the complex aquifers supplying Dhaka, Bangladesh and show that extensive groundwater pumping could lead to unpredictable future arsenic contamination in deep aquifers outside the city.

Chemical controls on abiotic and biotic release of geogenic arsenic from Pleistocene aquifer sediments to groundwater

Over 150 million people in South and Southeast Asia consume unsafe drinking water from arsenic-rich Holocene aquifers. Although use of As-free water from Pleistocene aquifers is a potential mitigation strategy, such aquifers are vulnerable to geogenic As pollution, placing millions more people at potential risk. The goal of this research was to define chemical controls on abiotic and biotic release of geogenic As to groundwater. Batch incubations of sediments with natural chemical variability from a Pleistocene aquifer in Cambodia were conducted to evaluate how interactions among arsenic, manganese and iron oxides, and dissolved and sedimentary organic carbon influenced As mobilization from sediments. The addition of labile dissolved organic carbon produced the highest concentrations of dissolved As after >7 months, as compared to sediment samples incubated with sodium azide or without added carbon, and the extent of As release was positively correlated with the percent of initial extractable Mn released from the sediments. The mode of As release was impacted by the source of DOC supplied to the sediments, with biological processes responsible for 81% to 85% of the total As release following incubations with lactate and acetate but only up to 43% to 61% of the total As release following incubations with humic and fulvic acids. Overall, cycling of key redox-active elements and organic-carbon reactivity govern the potential for geogenic As release to groundwater, and results here may be used to formulate better predictions of the arsenic pollution potential of aquifers in South and Southeast Asia.

VULNERABILITY OF LOW-ARSENIC AQUIFERS TO MUNICIPAL PUMPING IN BANGLADESH

Sandy aquifers deposited >12,000 years ago, some as shallow as 30 m, have provided a reliable supply of low-arsenic (As) drinking water in rural Bangladesh. This study concerns the potential risk of contaminating these aquifers in areas surrounding the city of Dhaka where hydraulic heads in aquifers >150 m deep have dropped by 70 m in a few decades due to municipal pumping. Water levels measured continuously from 2012 to 2014 in 12 deep (>150m), 3 intermediate (90-150 m) and 6 shallow (<90 m) community wells, 1 shallow private well, and 1 river piezometer show that the resulting drawdown cone extends 15-35 km east of Dhaka. Water levels in 4 low-As community wells within the 62-147 m depth range closest to Dhaka were inaccessible by suction for up to a third of the year. Lateral hydraulic gradients in the deep aquifer system ranged from 1.7×10^-4 to 3.7×10^-4 indicating flow towards Dhaka throughout 2012-2014. Vertical recharge on the edge of the drawdown cone was estimated at 0.21±0.06 m/yr. The data suggest that continued municipal pumping in Dhaka could eventually contaminate some relatively shallow community wells.

Stimulation of Microbially Mediated Arsenic Release in Bangladesh Aquifers by Young Carbon Indicated by Radiocarbon Analysis of Sedimentary Bacterial Lipids

The sources of reduced carbon driving the microbially mediated release of arsenic to shallow groundwater in Bangladesh remain poorly understood. Using radiocarbon analysis of phospholipid fatty acids (PLFAs) and potential carbon pools, the abundance and carbon sources of the active, sediment-associated, in situ bacterial communities inhabiting shallow aquifers (<30 m) at two sites in Araihazar, Bangladesh, were investigated. At both sites, sedimentary organic carbon (SOC) Δ(14)C signatures of -631 ± 54‰ (n = 12) were significantly depleted relative to dissolved inorganic carbon (DIC) of +24 ± 30‰ and dissolved organic carbon (DOC) of -230 ± 100‰. Sediment-associated PLFA Δ(14)C signatures (n = 10) at Site F (-167‰ to +20‰) and Site B (-163‰ to +21‰) were highly consistent and indicated utilization of carbon sources younger than the SOC, likely from the DOC pool. Sediment-associated PLFA Δ(14)C signatures were consistent with previously determined Δ(14)C signatures of microbial DNA sampled from groundwater at Site F indicating that the carbon source for these two components of the subsurface microbial community is consistent and is temporally stable over the two years between studies. These results demonstrate that the utilization of relatively young carbon sources by the subsurface microbial community occurs at sites with varying hydrology. Further they indicate that these young carbon sources drive the metabolism of the more abundant sediment-associated microbial communities that are presumably more capable of Fe reduction and associated release of As. This implies that an introduction of younger carbon to as of yet unaffected sediments (such as those comprising the deeper Pleistocene aquifer) could stimulate microbial communities and result in arsenic release.

Recharge of low-arsenic aquifers tapped by community wells in Araihazar, Bangladesh, inferred from environmental isotopes

More than 100,000 community wells have been installed in the 150-300 m depth range throughout Bangladesh over the past decade to provide low-arsenic drinking water (<10 μg/L As), but little is known about how aquifers tapped by these wells are recharged. Within a 25 km² area of Bangladesh east of Dhaka, groundwater from 65 low-As wells in the 35-240 m depth range was sampled for tritium (³H), oxygen and hydrogen isotopes of water (¹⁸O/¹⁶O and ²H/¹H), carbon isotope ratios in dissolved inorganic carbon (DIC, ¹⁴C/¹²C and ¹³C/¹²C), noble gases, and a suite of dissolved constituents, including major cations, anions, and trace elements. At shallow depths (<90 m), 24 out of 42 wells contain detectable ³H of up to 6 TU, indicating the presence of groundwater recharged within 60 years. Radiocarbon (¹⁴C) ages in DIC range from modern to 10 kyr. In the 90-240 m depth range, however, only 5 wells shallower than 150 m contain detectable ³H (<0.3 TU) and ¹⁴C ages of DIC cluster around 10 kyr. The radiogenic helium (⁴He) content in groundwater increases linearly across the entire range of ¹⁴C ages at a rate of 2.5×10^-12 ccSTP ⁴He g^-1 yr^-1. Within the samples from depths >90 m, systematic relationships between ¹⁸O/¹⁶O, ²H/¹H, ¹³C/¹²C and ¹⁴C/¹²C, and variations in noble gas temperatures, suggest that changes in monsoon intensity and vegetation cover occurred at the onset of the Holocene, when the sampled water was recharged. Thus, the deeper low-As aquifers remain relatively isolated from the shallow, high-As aquifer.

Arsenic in Groundwater: The Deep Late Pleistocene Aquifers of the Western Bengal Basin

in groundwaters from 145 wells across central West Bengal, India, those from Pleistocene aquifers at depths >70 m beneath paleo-interfluves contain <10 μg/L As. Pleistocene aquifers beneath deep paleo-channels typically host groundwaters containing 10-100 μg/L As at depths between 120 and 180 m. The depth profiles of As and SO4 and the conservative tracers Cl/Br, δ(18)O, and δ(2)H show that the As in Pleistocene groundwater beneath deep paleo-channels is relict and does not arise from migration downward of As-polluted groundwater in overlying aquifers. We postulate that the As was liberated in situ by reduction of minimal iron oxyhydroxides in the gray Pleistocene sands by organic matter infiltrating from riverbeds during late Pleistocene or earliest Holocene times. Mitigation of the widespread As-pollution in shallow aquifers through exploitation of deep Pleistocene aquifers would improve if guided by an understanding of the distribution of buried paleo-channels and paleo-interfluves and the knowledge that As may be present naturally in groundwater at depths >150 m beneath deep paleo-channels.

A quarter century of the Pacific Basin Consortium: looking back to move forward

The Pacific Basin Consortium (PBC) was formed 25 years ago to address significant public health challenges to vulnerable populations imposed by environmental threats in the region, including areas surrounding the rim of and in the Pacific Ocean. Originally focused on toxic waste pollution, the PBC has broadened its efforts over the years, embracing a health focus and more of a balance between engineering and public health. This move was informed by the PBC's close relationship with the National Institutes of Health (NIH) National Institute of Environmental Health Sciences (NIEHS) Superfund Hazardous Substances Basic Research and Training Program (Superfund Research Program, or SRP), which played a dynamic role in the PBC from its early days. In addition, a sub-focus on children's environmental health emerged, which helped set the agenda for children's environmental health research in the region. Progress has also been made in reducing harm from some threats, particularly via extensive interventions to remediate arsenic in drinking water in Bangladesh, western Thailand, and Vietnam. However, many of the environmental health problems in the Pacific Basin region persist, including air pollution, inadequate safe drinking water, undernutrition, and a growing electronic waste problem. In the Pacific Basin and elsewhere, people with the lowest incomes often live in areas with the worst pollution. Although it is difficult to implement, dynamic strategic networking efforts are vital to understanding and correcting the inequities that persist in global environmental health. The PBC can help accomplish this by continuing and expanding its work to foster and enhance collaborations and communications between environmental health and engineering investigators and to integrate investigator-initiated research. As the PBC looks forward, there is also a need to exert increased effort to establish and maintain partnerships, to develop community-based primary-care and health services for vulnerable populations, as well as to connect with researchers in the eastern side of the Pacific basin and those in smaller island states.

Association of Arsenic and Phosphorus with Iron Nanoparticles between Streams and Aquifers: Implications for Arsenic Mobility

The microbial oxidation of organic matter coupled to reductive iron oxide dissolution is widely recognized as the dominant mechanism driving elevated arsenic (As) concentrations in aquifers. This paper considers the potential of nanoparticles to increase the mobility of As in aquifers, thereby accounting for discrepancies between predicted and observed As transport reported elsewhere. Arsenic, phosphorus, and iron size distributions and natural organic matter association were examined along a flow path from surface water via the hyporheic zone to shallow groundwater. Our analysis demonstrates that the colloidal Fe concentration (>1 kDa) correlates with both colloidal P and colloidal As concentrations. Importantly, increases in the concentration of colloidal P (>1 kDa) were positively correlated with increases in the concentration of nominally dissolved As (<1 kDa), but no correlation was observed between colloidal As and nominally dissolved P. This suggests that P actively competes for adsorption sites on Fe nanoparticles, displacing adsorbed As, thus mirroring their interaction with Fe oxides in the aquifer matrix. Dynamic redox fronts at the interface between streams and aquifers may therefore provide globally widespread conditions for the generation of Fe nanoparticles, a mobile phase for As adsorption currently not a part of reactive transport models.

Redox Zonation and Oscillation in the Hyporheic Zone of the Ganges-Brahmaputra-Meghna Delta: Implications for the Fate of Groundwater Arsenic during Discharge

Riverbank sediment cores and pore waters, shallow well waters, seepage waters and river waters were collected along the Meghna Riverbank in Gazaria Upazila, Bangladesh in Jan. 2006 and Oct.-Nov. 2007 to investigate hydrogeochemical processes controlling the fate of groundwater As during discharge. Redox transition zones from suboxic (0-2 m depth) to reducing (2-5 m depth) then suboxic conditions (5-7 m depth) exist at sites with sandy surficial deposits, as evidenced by depth profiles of pore water (n=7) and sediment (n=11; diffuse reflectance, Fe(III)/Fe ratios and Fe(III) concentrations). The sediment As enrichment zone (up to ~700 mg kg^-1) is associated with the suboxic zones mostly between 0-2 m depth and less frequently between 5-7 m depth. The As enriched zones consist of several 5 to 10 cm-thick dispersed layers and span a length of ~5-15 m horizontally from the river shore. Depth profiles of riverbank pore water deployed along a 32 m transect perpendicular to the river shore show elevated levels of dissolved Fe (11.6±11.7 mg L^-1) and As (118±91 μg L^-1, mostly as arsenite) between 2-5 m depth, but lower concentrations between 0-2 m depth (0.13±0.19 mg L^-1 Fe, 1±1 μg L^-1 As) and between 5-6 m depth (1.14±0.45 mg L^-1 Fe, 28±17 μg L^-1 As). Because it would take more than a few hundred years of steady groundwater discharge (~10 m yr^-1) to accumulate hundreds of mg kg^-1 of As in the riverbank sediment, it is concluded that groundwater As must have been naturally elevated prior to anthropogenic pumping of the aquifer since the 1970s. Not only does this lend unequivocal support to the argument that As occurrence in the Ganges-Brahmaputra-Meghna Delta groundwater is of geogenic origin, it also calls attention to the fate of this As enriched sediment as it may recycle As into the aquifer.

Flow and sorption controls of groundwater arsenic in individual boreholes from bedrock aquifers in central Maine, USA

To understand the hydrogeochemical processes regulating well water arsenic (As) evolution in fractured bedrock aquifers, three domestic wells with [As] up to 478 μg/L are investigated in central Maine. Geophysical logging reveals that fractures near the borehole bottom contribute 70-100% of flow. Borehole and fracture water samples from various depths show significant proportions of As (up to 69%) and Fe (93-99%) in particulates (>0.45 μm). These particulates and those settled after a 16-day batch experiment contain 560-13,000 mg/kg of As and 14-35% weight/weight of Fe. As/Fe ratios (2.5-20 mmol/mol) and As partitioning ratios (adsorbed/dissolved [As], 20,000-100,000 L/kg) suggest that As is sorbed onto amorphous hydrous ferric oxides. Newly drilled cores also show enrichment of As (up to 1300 mg/kg) sorbed onto secondary iron minerals on the fracture surfaces. Pumping at high flow rates induces large decreases in particulate As and Fe, a moderate increase in dissolved [As] and As(III)/As ratio, while little change in major ion chemistry. The δD and δ(18)O are similar for the borehole and fracture waters, suggesting a same source of recharge from atmospheric precipitation. Results support a conceptual model invoking flow and sorption controls on groundwater [As] in fractured bedrock aquifers whereby oxygen infiltration promotes the oxidation of As-bearing sulfides at shallower depths in the oxic portion of the flow path releasing As and Fe; followed by Fe oxidation to form Fe oxyhydroxide particulates, which are transported in fractures and sorb As along the flow path until intercepted by boreholes. In the anoxic portions of the flow path, reductive dissolution of As-sorbed iron particulates could re-mobilize As. For exposure assessment, we recommend sampling of groundwater without filtration to obtain total As concentration in groundwater.