Arsenic-rich Groundwater Research Articles

Occurrence and enrichment mechanism of arsenic-rich groundwater from eastern coastal China

Groundwater arsenic (As) contamination represents a noteworthy occurrence within coastal aquifers. This work employs an approach of chemical analysis and geochemical modelling to study the occurrence and enrichment mechanism of As-rich groundwater within coastal aquifers in East China. Recent hydrogeological survey demonstrates that the As contaminated samples are clustered in Rudong Bay and As contamination occurs within both reducing and oxidizing coastal aquifers. The Piper, Gibbs, and Rock-Weathering diagrams signify shallow and deep groundwater are Na–HCO3 and mixed Na·Ca·Mg–HCO3 facies respectively, and the hydrochemistry of contaminated groundwater is governed by weathering and dissolution. Moreover, high salinity in shallow groundwater can be associated with seawater intrusion. The PHREEQC modeling predicts that As (V) (HAsO42−) is the dominant As speciation in shallow groundwater, while As (Ⅲ) (H3AsO3) is the counterpart in deep groundwater. Statistical analysis of chemicals and modeling calculations suggest that alkaline desorption is responsible for As enrichment in shallow groundwater, while reductive dissolution is the primary processes governing As enrichment in deep groundwater. Redox and pH vibrations play an important role in generating the differences of As enrichment mechanism between shallow and deep aquifers in the coastal plain. The findings of this study may enhance understanding the occurrence and enrichment mechanism about As-rich groundwater within coastal area aquifers.

Coupled redox cycling of arsenic and sulfur regulates thioarsenate enrichment in groundwater

High‑arsenic groundwater is influenced by a combination of processes: reductive dissolution of iron minerals and formation of secondary minerals, metal complexation and redox reactions of organic matter (OM), and formation of more migratory thioarsenate, which together can lead to significant increases in arsenic concentration in groundwater. This study was conducted in a typical sulfur- and arsenic-rich groundwater site within the Datong Basin to explore the conditions of thioarsenate formation and its influence on arsenic enrichment in groundwater using HPLC-ICPMS, hydrogeochemical modeling, and fluorescence spectroscopy. The shallow aquifer exhibited a highly reducing environment, marked by elevated sulfide levels, low concentrations of Fe(II), and the highest proportion of thioarsenate. In the middle aquifer, an optimal ∑S/∑As led to the presence of significant quantities of thioarsenate. In contrast, the deep aquifer exhibited low sulfide and high Fe(II) concentration, with arsenic primarily originating from dissolved iron minerals. Redox fluctuations in the sediment driven by sulfur‑iron minerals generated reduced sulfur, thereby facilitating thioarsenate formation. OM played a crucial role as an electron donor for microbial activities, promoting iron and sulfate reduction processes and creating conditions conducive to thioarsenate formation in reduced and high‑sulfur environments. Understanding the process of thioarsenate formation and the influencing factors is of paramount importance for comprehending the migration and redistribution of arsenic in groundwater systems.

Transformation of dissolved organic matter and related arsenic mobility at a surface water-groundwater interface in the Hetao Basin, China

Porewater arsenic mobility above the groundwater table has been recognized as a potential cause of arsenic-rich groundwater, but the processing pathways of dissolved organic matter (DOM) in that hyporheic zone and their effect on porewater arsenic release remain poorly understood. To address these issues, two porewater profiles were sampled in a surface water-groundwater interaction zone from the Hetao Basin, China, to monitor the porewater geochemistry and DOM molecular characteristics. The results show that the porewater arsenic, Fe(II), and DOC concentrations were all significantly higher than those of the intruding pond water, and were located above the conservative mixing model lines. This indicates a net release of these solutes from the sediment. By comparing the porewater with pond water DOM, we found that the carboxyl-rich alicyclic molecules (CRAM) were selectively preserved, carbohydrates and aliphatics/proteins were preferentially consumed, and low O/C-ratio compounds with high bioproduction index (I_bioprod) and terrestrial index (I_terr) were produced. The transformation of CHO to CHOS compounds also represented a pathway of recalcitrant DOM production. The produced recalcitrant organic compounds mostly contributed to the elevated porewater DOC concentrations, but their contribution decreased along the filtration path. The consumption of labile DOM compounds would be responsible for Fe(III) hydroxide reduction and arsenic release. The generated recalcitrant DOM may also be a driver of porewater arsenic mobility by acting as electron shuttles. This study highlights the importance of the hyporheic zone in shaping shallow groundwater DOM composition and the potential contribution to arsenic enrichment.

Hydro-geochemical investigations of arsenic rich groundwater using multivariate statistical analysis

Multivariate statistical analysis has been applied to assess the chemical characteristics of high arsenic groundwater from the central-southern part of Bangladesh. A total of 43 shallow ground-water samples were collected and analyzed. The groundwater is almost neutral. The results of cations and anions trends are Na>Ca>Mg>K and HCO3>Cl>SO4>NO3, respectively. Alkalinity has a significant positive correlation with Mg, Ca and K suggesting silicate weathering as the major process of controlling groundwater geochemistry. The significant positive correlation between Na and Cl indicates the seawater intrusion into groundwater. The R-mode cluster al-lows variables into four groups. Alkalinity, Mg, K in the first and Ca in the second, Na and Cl in the third cluster indicate silicate weathering and carbonate dissolution and seawater intrusion, respectively. R-mode factor analysis allows variables into four components having eigen values more than 1 which represent 72.5% of total variances. Component 1 positively loaded with K, Mg, P, and alkalinity suggest silicate weathering. Component 2 positively loaded with Na and Cl suggests seawater influences groundwater. Component 3 positively loaded with Ca suggest the carbonate dissolution. The Q-mode cluster analysis represents the highest Fe concentrations.

Chemical and mineralogical variability of sediment in a Quaternary aquifer from Huaihe River Basin, China: Implications for groundwater arsenic source and its mobilization

Arsenic (As) is a conspicuous contaminant, and exposure to this element through contaminated drinking groundwater poses a significant challenge to public health. Geogenic groundwater arsenic is associated with sedimentary setting. This work concentrates on the investigation of lithology, elemental abundance and mineralogical compositions about the arsenic profile and its effect to the groundwater from Huaihe River Basin, China. There are 90 sediment samples from the borehole at the field monitoring sites were collected and analyzed. The results reveal that sedimentary concentrations of As, Fe, Mn, S, Al, N, organic carbon and mineralogical compositions vary across the Quaternary aquifer. Arsenic abundance of sediments is 10.63 ± 0.56 mg/kg, and peak As concentrations occur between 59.0 m and 64.8 m in fine particle sediments. The specific higher As concentrations in sedimentary aquifer are concordant with arsenic-rich groundwater around the investigated borehole. Fe, Mn, and Al depth profiles follow similar tendency to those of As. Sedimentary As concentrations are significantly correlated to Fe, Al, and Mn concentrations, but are not correlated to organic carbon and S concentrations. Arsenic probably exists in the form of non-crystalline colloids, and Fe, Al minerals are potential host minerals for arsenic. Under alkaline conditions, groundwater arsenic is released and enriched within the Quaternary aquifer by reductive dissolution of As-hosting Fe and Al minerals.

Predictive geospatial model for arsenic accumulation in Holocene aquifers based on interactions of oxbow-lake biogeochemistry and alluvial geomorphology

The identification of arsenic-contamination hotspots in alluvial aquifers is a global-scale challenge. The collection and inventory of arsenic concentration datasets in the shallow-aquifer domain of affected alluvial basins is a tedious and slow process, given the magnitude of the problem. Recent research demonstrates that oxbow-lake biogeochemistry in alluvial plains, mobilization of geogenic arsenic, and accumulation in geomorphologically well-defined areas are interacting processes that determine arsenic-contamination locations. This awareness provides a tool to identify potential arsenic-hotspots based on geomorphological similarity, and thus contribute to a more robust and targeted arsenic mitigation approach. In the present study, a conceptual predictive geospatial model is proposed for the accumulation of dissolved arsenic as a function of interaction of oxbow-lake biogeochemistry and alluvial geomorphology. A comprehensive sampling campaign in and around two oxbow lakes in the Jamuna River Basin, West Bengal (India) provided water samples of the oxbow-lake water column for analysis of dissolved organic matter (DOM) and microbial communities, and groundwater samples from tube wells in point bars and fluvial levees bordering the oxbow lakes for analysis of the geospatial distribution of arsenic in the aquifer. Results show that abundant natural and anthropogenic (faecal-derived) recalcitrant organic matter like coprostanols and sterols in clay-plug sediment favours microbial (heterotrophs, enteric pathogens) metabolism and arsenic mobilization. Arsenic concentrations in the study area are highest (averaging 505 μg/L) in point-bar aquifers geomorphologically enclosed by partially sediment-filled oxbow lakes, and much lower (averaging 121 μg/L) in wells of levee sands beyond the oxbow-lake confinement. The differences reflect variations in groundwater recharge efficiency as result of the porosity and permeability anisotropy in the alluvial geomorphological elements, where arsenic-rich groundwater is trapped in point-bars enclosed by oxbow-lake clays and, by contrast, levee ridges are not confined on all sides, resulting in a more efficient aquifer flushing and decrease of arsenic concentrations.

Hydrogeochemical Processes and Potential Exposure Risk of Arsenic-Rich Groundwater from Huaihe River Plain, China

Arsenic poses a danger to environmental health, and arsenic-rich groundwater is a key exposure risk for humans. The distribution, migration, and enrichment of arsenic in groundwater is an important environmental and public health problem. Currently, the Huaihe River Basin is identified as a region of arsenic-rich groundwater in China. This study aims to assess arsenic-rich groundwater potential pollution risk, analyze the hydrogeochemical processes, and trace the ion source based on an analysis of groundwater hydrogeochemical data. The results show that arsenic is the main inorganic chemical substances affecting the water quality in the study area, which presents a high exposure risk for public health. The arsenic concentration of groundwater was f 5.75 ± 5.42 μg/L, and 23% of the considered samples exceeded the drinking water standards of the World Health Organization. The groundwater in the study area underwent evaporation, halite dissolution, and ion exchange processes. The total alkalinity (HCO3−) of the arsenic-rich groundwater mainly ranged between 400–700 mg/L, and the chemical type was mainly of HCO3-Na. In an alkaline environment, the oxidative dissolution and reductive dissolution of arsenic bearing minerals might be the formation mechanism of arsenic-rich groundwater.

Exposure risk of groundwater arsenic contamination from Huaihe River Plain, China

Arsenic poses a danger to public health and drinking arsenic-rich groundwater is the main route for human exposure to this element. The focus of this study is to assess the risk magnitude and likelihood for arsenic-rich groundwater in Huaihe River Plain, China using Risk Magnitude and Indicator Kriging methods. It has been found that 481 in 5515 investigated samples exceed the drinking water standards of the World Health Organization, which present a high exposure risk for public health. Arsenic concentrations range from 0.001 to 356.00 μg/L, with a median of 2.10 μg/L. The proportion of contaminated shallow groundwater samples is 9.77%, and the counterpart from deep layer is 2.85%, respectively. Arsenic concentrations are obviously higher in plain areas than those in hilly areas. High Risk Magnitude and Very High Risk Magnitude samples are sporadically positioned in inland and coastal plain. According to the prediction of Risk Probability maps over shallow and deep groundwater, high arsenic Risk Probability areas is scattered in the inland and coastal portion, and both Risk Probability peaks are similar. Some high arsenic hazard areas have been found to possess high cancer rates, and high Risk Probability peaks are correlated with cancer cluster. The potential high arsenic hazard areas over shallow groundwater encompass more than 4709 km2, while the counterpart over deep groundwater is 1446 km2. 2.88 million people are estimated to be potentially exposed to High Risk Probability of arsenic. This paper carried out research on exposure risk of arsenic contamination from Huaihe River Plain, China, which may provide guidance for regionalization of drinking groundwater safety.

Hydrogeochemical Investigations of Arsenic Rich Groundwater in Munshiganj District of Bangladesh Using Multivariate Statistical Analysis

Hydrogeochemical Investigations of Arsenic Rich Groundwater in Munshiganj District of Bangladesh Using Multivariate Statistical Analysis

Mixing of arsenic-rich groundwater and surface water in drinking water distribution systems: Implications for contaminants, disinfection byproducts and organic components

The utilization of groundwaters containing high levels of arsenic (As) for drinking water purposes presents major health and economic challenges for water utilities. One low-cost approach is to mix arsenic-rich groundwater (GW) with arsenic-free surface waters (SW) to achieve acceptable As levels. In this study we investigated the effect of different mixing ratios on water quality in an eastern Croatian water distribution system (WDS). To investigate the effects of mixing on drinking water quality, we measured the organic matter (OM) composition, disinfection byproduct (DBP) and metal concentrations in differently mixed ratios of GW and SW within the WDS. Fluorescence analysis revealed that the GW and SW had similar OM composition, with an almost equal ratio of humic- and protein-like OM throughout the WDS despite fluorescence indices revealing slightly different OM sources between the two water types. The tyrosine-like OM component was more variable, increasing during warmer months and towards the end of the WDS, most likely due to enhanced biofilm formation. Arsenic concentrations decreased to below 10 μg/L in the second half of the sampling campaign. Acceptable water quality was achieved after a period of destabilization and solubilization of loose deposits within the WDS resulting in their mobilization caused by water quality changes. Principal component and classification analysis, regression models and Spearman correlation coefficients revealed an association between As, OM and DBP concentrations with these correlations suggestive of their role in As mobilization in the WDS. Changing source waters, with different OM content and characteristics, corresponded to variable As release within the WDS.

Hydrogeochemical characteristics of arsenic rich groundwater in Greater Giyani Municipality, Limpopo Province, South Africa

This study evaluated the hydrogeochemical characteristics of groundwater in the Greater Giyani Municipality, Limpopo Province, South Africa. A total of ten (10) groundwater samples were collected from the existing boreholes and analysed for their hydrogeochemical properties using standard laboratory techniques. The data were interpreted using Piper diagram, Gibbs diagram and the scatter plots. The results showed that pH of the samples ranges from neutral to weakly alkaline. The abundance of major anions and cations are in the following order: HCO3− > Cl− > SO42− > NO3− and Na+>Mg2+>Ca2+>K+, respectively. The Piper diagram revealed that the hydrogeochemical facies identified in the study area are Ca–HCO3 and mixed Ca–Na–HCO3 type indicating the predominance of water-rock interaction. The water-rock interaction was further confirmed by the Gibbs diagram. The ionic ratio showed that the major ions originate from weathering of carbonate and silicate minerals. The concentration of arsenic was found to be ranging between 0.1 and 172.53 μg/L with the average of 32.21 μg/L. A total 6 out of 10 samples had arsenic concentration above the WHO recommended limit of 10 μg/L. The study calls for action from public health officials towards evaluating the correlation between arsenic concentration and arsenic related diseases and further development of arsenic remediation techniques.

Exploring Biodiversity and Arsenic Metabolism of Microbiota Inhabiting Arsenic-Rich Groundwaters in Northern Italy.

Arsenic contamination of groundwater aquifers is an issue of global concern. Among the affected sites, in several Italian groundwater aquifers arsenic levels above the WHO limits for drinking water are present, with consequent issues of public concern. In this study, for the first time, the role of microbial communities in metalloid cycling in groundwater samples from Northern Italy lying on Pleistocene sediments deriving from Alps mountains has been investigated combining environmental genomics and cultivation approaches. 16S rRNA gene libraries revealed a high number of yet uncultured species, which in some of the study sites accounted for more of the 50% of the total community. Sequences related to arsenic-resistant bacteria (arsenate-reducing and arsenite-oxidizing) were abundant in most of the sites, while arsenate-respiring bacteria were negligible. In some of the sites, sulfur-oxidizing bacteria of the genus Sulfuricurvum accounted for more than 50% of the microbial community, whereas iron-cycling bacteria were less represented. In some aquifers, arsenotrophy, growth coupled to autotrophic arsenite oxidation, was suggested by detection of arsenite monooxygenase (aioA) and 1,5-ribulose bisphosphate carboxylase (RuBisCO) cbbL genes of microorganisms belonging to Rhizobiales and Burkholderiales. Enrichment cultures established from sampled groundwaters in laboratory conditions with 1.5 mmol L-1 of arsenite as sole electron donor were able to oxidize up to 100% of arsenite, suggesting that this metabolism is active in groundwaters. The presence of heterotrophic arsenic resistant bacteria was confirmed by enrichment cultures in most of the sites. The overall results provided a first overview of the microorganisms inhabiting arsenic-contaminated aquifers in Northern Italy and suggested the importance of sulfur-cycling bacteria in the biogeochemistry of arsenic in these ecosystems. The presence of active arsenite-oxidizing bacteria indicates that biological oxidation of arsenite, in combination with arsenate-adsorbing materials, could be employed for metalloid removal.

Technology alternatives for decontamination of arsenic-rich groundwater—A critical review

Abstract Arsenic (As) contamination of groundwater is being reckoned as a global problem, as over 296 million people residing in more than 100 countries have already been reported to be affected by arsenic-rich groundwater. Developing countries like Bangladesh, India, Thailand, Taiwan and Vietnam are the worst victims of this terrible event. Arsenic naturally exists in more than 320 mineral forms mainly as arsenates, sulfides, sulfo-salts, arsenides, arsenites, oxides, silicates and elemental arsenic. The lifetime cancer risk of arsenic at its Maximum Contaminant Level (MCL of 10 μ g/L) is estimated at 0.7 in 100, while for other carcinogens, it ranges from ( ∼ 0.001 to 0.012) in 100. Water, besides food, is the main source of arsenic ingress into the human system. Both groundwater and surface water are being found contaminated with arsenic due to natural and anthropogenic activities respectively Over the years, various physico-chemical and biological methods viz. oxidation, coagulation–flocculation, adsorption, biological sorption, ion-exchange, membrane processes, treatment with bio-organism and electrocoagulation have been profusely experimented and reported by different researchers for arsenic remediation from the groundwater. In this paper, various technologies available for arsenic remediation, their removal mechanisms, cost-effectiveness and sustainability have been reviewed in a critical and rigorous manner. Attempts have also been made to point out the advantages and drawbacks of different methodologies and future research needs in the sphere of arsenic remediation from groundwater.

Arsenic retention in cooked rice: Effects of rice type, cooking water, and indigenous cooking methods in West Bengal, India

This study evaluated the concentration of arsenic in paired raw and cooked rice prepared by individual households in arsenic-endemic rural area of West Bengal. The aim was to investigate how the cooking habits of rural villagers of West Bengal might influence the arsenic content of rice meals. It was found that the use of arsenic-rich groundwater for cooking could elevate the arsenic concentration in cooked rice (up to 129% above the raw sample), thereby enhancing the vulnerability of the rural population of West Bengal to arsenic exposure through rice consumption. The risk is heightened by the habit of drinking the stewed rice water (gruel) in the local communities. The cooking method employed, rice variety, background arsenic concentration in raw rice and cooking water arsenic concentration were found to be important predisposing factors that could affect the accumulation of arsenic in cooked form. The fundamental indigenous cooking practice followed by the villagers requires use of low-arsenic water for cooking as a necessary strategy to alleviate arsenic exposure in their staple food.

Arsenic Level and Risk Assessment of Groundwater in Vehari, Punjab Province, Pakistan

Arsenic (As) contamination in different areas of the world has been reported, which poses serious health threats to humans by its different routes of exposure. There exists comparatively rare data regarding arsenic level in groundwater of Punjab, Pakistan, which is normally used for drinking purpose. Therefore, the present study was planned to assess the contamination of arsenic in the drinking water of district Vehari in Punjab, Pakistan. A total of 156 drinking water samples were taken from different previously unexplored (rural and urban) areas. Various groundwater sources (electric pump, hand pump, and tube well) were targeted at different depths (50–350 ft) of the three tehsils (Burewala, Vehari, Mailsi) of district Vehari. Drinking water samples were subjected to measurement of the arsenic level, as well as other parameters of water were also checked including pH, electrical conductivity, chloride, cations, carbonates, and bicarbonates. It was found that 95% of groundwater samples were unfit for drinking purpose, because some areas of district Vehari were found with arsenic levels higher than World Health Organization (WHO) safe limit of As (10 μg/L) in drinking water. Arsenic-induced health risks were also estimated with respect to average daily dose, hazard quotient, and carcinogenic risk for the people who were relying on arsenic-rich groundwater for drinking purpose. It is found that the people of district Vehari are at the serious carcinogenic health risk. This study highlights that urgent management and monitoring measures are imperative for the people in the study area, in order to minimize the arsenic-mediated health effects, as well as to develop effective remediation strategies for arsenic-contaminated drinking water.

Single and combined effects of phosphate, silicate, and natural organic matter on arsenic removal from soft and hard groundwater using ferric chloride

In order to assess the effects of phosphate, silicate and natural organic matter (NOM) on arsenic removal by ferric chloride, batch coprecipitation experiments were conducted over a wide pH range using synthetic hard and soft groundwaters, similar to those found in northern Vietnam. The efficiency of arsenic removal from synthetic groundwater by coprecipitation with FeCl3 was remarkably decreased by the effects of PO4 3−, SiO4 4− and NOM. The negative effects of SiO4 4− and NOM on arsenic removal were not as strong as that of PO4 3−. Combining PO4 3− and SiO4 4− increased the negative effects on both arsenite (As3+) and arsenate (As5+) removal. The introduction of NOM into the synthetic groundwater containing both PO4 3− and SiO4 4− markedly magnified the negative effects on arsenic removal. In contrast, both Ca2+ and Mg2+ substantially increased the removal of As3+ at pH 8–12 and the removal of As5+ over the entire pH range. In the presence of Ca2+ and Mg2+, the interaction of NOM with Fe was either removed or the arsenic binding to Fe−NOM colloidal associations and/or dissolved complexes were flocculated. Removal of arsenic using coprecipitation by FeCl3 could not sufficiently reduce arsenic contents in the groundwater (350 μg/L) to meet the WHO guideline for drinking water (10 μg/L), especially when the arsenic-rich groundwater also contains co-occurring solutes such as PO4 3−, SiO4 4− and NOM; therefore, other remediation processes, such as membrane technology, should be introduced or additionally applied after this coprecipitation process, to ensure the safety of drinking water.

An arsenate-reducing and alkane-metabolizing novel bacterium, Rhizobium arsenicireducens sp. nov., isolated from arsenic-rich groundwater.

A novel arsenic (As)-resistant, arsenate-respiring, alkane-metabolizing bacterium KAs 5-22T, isolated from As-rich groundwater of West Bengal was characterized by physiological and genomic properties. Cells of strain KAs 5-22T were Gram-stain-negative, rod-shaped, motile, and facultative anaerobic. Growth occurred at optimum of pH 6.0-7.0, temperature 30°C. 16S rRNA gene affiliated the strain KAs 5-22T to the genus Rhizobium showing maximum similarity (98.4%) with the type strain of Rhizobium naphthalenivorans TSY03bT followed by (98.0% similarity) Rhizobium selenitireducens B1T. The genomic G+C content was 59.4mol%, and DNA-DNA relatedness with its closest phylogenetic neighbors was 50.2%. Chemotaxonomy indicated UQ-10 as the major quinone; phosphatidylethanolamine, phosphatidylglycerol, and diphosphatidylglycerol as major polar lipids; C16:0, C17:0, 2-OH C10:0, 3-OH C16:0, and unresolved C18:1 ɷ7C/ɷ9C as predominant fatty acids. The cells were found to reduce O2, As5+, NO3-, SO42- and Fe3+ as alternate electron acceptors. The strain's ability to metabolize dodecane or other alkanes as sole carbon source using As5+ as terminal electron acceptor was supported by the presence of genes encoding benzyl succinate synthase (bssA like) and molybdopterin-binding site (mopB) of As5+ respiratory reductase (arrA). Differential phenotypic, chemotaxonomic, genotypic as well as physiological properties revealed that the strain KAs 5-22T is separated from its nearest recognized Rhizobium species. On the basis of the data presented, strain KAs 5-22T is considered to represent a novel species of the genus Rhizobium, for which the name Rhizobium arsenicireducens sp. nov. is proposed as type strain (=LMG 28795T=MTCC 12115T).

Evaporative concentration of arsenic in groundwater: health and environmental implications, La Laguna Region, Mexico

High arsenic concentrations in groundwater have been documented in La Laguna Region (LLR) in arid northern Mexico, where arsenic poisoning is both chronic and endemic. A heated debate has continued for decades on its origin. LLR consisted of a series of ancient connected lakes that developed at the end of a topographic depression under closed basin conditions. This study addresses the isotopic, chemical composition of the groundwater and geochemical modeling in the southeasternmost part of the LLR to determine the origin of arsenic. Groundwater samples were obtained from a carbonate and granular aquifers and from a clayey aquitard at terminal Viesca Lake. Results show that groundwater originated as meteoric water that reached the lakes mainly via abundant springs in the carbonate aquifer and perennial flooding of the Nazas-Aguanaval Rivers. Paleo-lake water underwent progressive evaporation as demonstrated by the enrichment of δ18O, δ2H and characteristic geochemical patterns in the granular aquifer and aquitard that resulted in highly saline (>90,000mS/cm), arsenic-rich (up to 5000μg/L) paleo-groundwater (>30,000years BP). However, adsorption or co-precipitation on iron oxides, clay-mineral surfaces and organic carbon limited arsenic concentration in the groundwater. Arsenic-rich groundwater and other solutes are advancing progressively from the lacustrine margins toward the main granular aquifer, due to reversal of hydraulic gradients caused by intensive groundwater exploitation and the reduction in freshwater runoff provoked by dam construction on the main rivers. Desorption of arsenic will incorporate additional concentrations of arsenic into the groundwater and continue to have significant negative effects on human health and the environment.

A Review and Evaluation of the Impacts of Climate Change on Geogenic Arsenic in Groundwater from Fractured Bedrock Aquifers

Climate change is expected to affect the groundwater quality by altering recharge, water table elevation, groundwater flow, and land use. In fractured bedrock aquifers, the quality of groundwater is a sensitive issue, particularly in areas affected by geogenic arsenic contamination. Understanding how climate change will affect the geochemistry of naturally occurring arsenic in groundwater is crucial to ensure sustainable use of this resource, particularly as a source of drinking water. This paper presents a review of the potential impacts of climate change on arsenic concentration in bedrock aquifers and identifies issues that remain unresolved. During intense and prolonged low flow, the decline in the water table is expected to increase the oxidation of arsenic-bearing sulfides in the unsaturated zone. In addition, reduced groundwater flow may increase the occurrence of geochemically evolved arsenic-rich groundwater and enhance arsenic mobilization by reductive dissolution and alkali desorption. In contrast, the occurrence of extreme recharge events is expected to further decrease arsenic concentrations because of the greater dilution by oxygenated, low-pH water. In some cases, arsenic mobilization could be indirectly induced by climate change through changes in land use, particularly those causing increased groundwater withdrawals and pollution. The overall impact of climate change on dissolved arsenic will vary greatly according to the bedrock aquifer properties that influence the sensitivity of the groundwater system to climate change. To date, the scarcity of data related to the temporal variability of arsenic in fractured bedrock groundwater is a major obstacle in evaluating the future evolution of the resource quality.

Molecular analysis of microbial community in arsenic-rich groundwater of Kolsor, West Bengal

ABSTRACTBacterial community composition within the highly arsenic (As) contaminated groundwater from Kolsur, West Bengal was analyzed over a period of 3 years using 16S rRNA gene clone library and Denaturing Gradient Gel Electrophoresis (DGGE). Molecular phylogenetic study revealed abundance of α-Proteobacteria (56%) and Firmicutes (29%) along with members of β-Proteobacteria, Verrucomicrobia and Sphingobacteria as relatively minor groups. Along with consistent physicochemical environment, a stable microbial community structure comprising of bacterial genera Agrobacterium-Rhizobium, Ochrobactrum, Pseudomonas, Anoxybacillus and Penibacillus was recorded over the three years study period. Presence of cytosolic arsenate reductase (arsC) gene was observed within the microbial community. Phylogenetic analyses revealed that all the arsC sequences were closely related to the same gene from γ-proteobacterial members while the community was consisted of mainly α-proteobacterial groups. Such phylogenetic incongruence between 16S rRNA and arsC genes possibly indicated horizontal gene transfer (HGT) of the ars genes within the groundwater community. Overall, the study reported a nearly stable geomicrobial environment and genetic determinant related to As homeostasis gene, and provided a better insight on biogeochemistry of As contaminated aquifer of West Bengal.