Arsenic-contaminated Aquifers Research Articles

A study on coliforms in arsenic-contaminated aquifer sediments of Chakdah, West Bengal, India

A study on coliforms in arsenic-contaminated aquifer sediments of Chakdah, West Bengal, India

Carbon and methane cycling in arsenic-contaminated aquifers

Geogenic arsenic (As) contamination of groundwater is a health threat to millions of people worldwide, particularly in alluvial regions of South and Southeast Asia. Mitigation measures are often hindered by high heterogeneities in As concentrations, the cause(s) of which are elusive. Here we used a comprehensive suite of stable isotope analyses and hydrogeochemical parameters to shed light on the mechanisms in a typical high-As Holocene aquifer near Hanoi where groundwater is advected to a low-As Pleistocene aquifer. Carbon isotope signatures (δ13C-CH4, δ13C-DOC, δ13C-DIC) provided evidence that fermentation, methanogenesis and methanotrophy are actively contributing to the As heterogeneity. Methanogenesis occurred concurrently where As levels are high (>200 µg/L) and DOC-enriched aquitard pore water infiltrates into the aquifer. Along the flowpath to the Holocene/Pleistocene aquifer transition, methane oxidation causes a strong shift in δ13C-CH4 from -87‰ to +47‰, indicating high reactivity. These findings demonstrate a previously overlooked role of methane cycling and DOC infiltration in high-As aquifers.

Arsenic mobilization by anaerobic iron-dependent methane oxidation

Arsenic groundwater contamination threatens the health of millions of people worldwide, particularly in South and Southeast Asia. In most cases, the release of arsenic from sediment was caused by microbial reductive dissolution of arsenic-bearing iron(III) minerals with organic carbon being used as microbial electron donor. Although in many arsenic-contaminated aquifers high concentrations of methane were observed, its role in arsenic mobilization is unknown. Here, using microcosms experiments and hydrogeochemical and microbial community analyses, we demonstrate that methane functions as electron donor for methanotrophs, triggering the reductive dissolution of arsenic-bearing iron(III) minerals, increasing the abundance of genes related to methane oxidation, and ultimately mobilizing arsenic into the water. Our findings provide evidence for a methane-mediated mechanism for arsenic mobilization that is distinct from previously described pathways. Taking this together with the common presence of methane in arsenic-contaminated aquifers, we suggest that this methane-driven arsenic mobilization may contribute to arsenic contamination of groundwater on a global scale.

Diversity of Betaproteobacteria revealed by novel primers suggests their role in arsenic cycling

High arsenic concentration in groundwater is a severe environmental problem affecting human health, particularly in countries of South and South-East Asia. The Bengal Delta Plain (BDP) distributed within India and Bangladesh is a major arsenic-affected region where groundwater is the primary source of drinking water. Previous studies have indicated that members of the bacterial class Betaproteobacteria constitute a major fraction of the microbial community in many of the aquifers within this region. Bacteria belonging to this class are known to be involved in redox cycling of arsenic as well as other metals such iron and manganese, thereby impacting arsenic mobilization and immobilization. While microbial diversity in arsenic-contaminated environments is generally assessed using universal 16S rRNA gene primers, targeted evaluation of Betaproteobacteria diversity remains poorly constrained. In this study, bacterial diversity was investigated in the groundwater from two shallow aquifers (West Bengal, India) based on 16S rRNA gene clone libraries and sequencing using a custom-designed pair of primers specific to Betaproteobacteria. Specificity of the primers was confirmed in silico as well as by the absence of PCR amplification of other bacterial classes. Four major families (Burkholderiaceae, Comamonadaceae, Gallionellaceae and Rhodocyclaceae) were detected among which members of Burkholderiaceae represented 59% and 71% of the total community in each aquifer. The four OTUs (operational taxonomic units; 97% sequence identity) within Burkholderiaceae were close phylogenetic relatives of bacteria within the genus Burkholderia known to solubilize phosphate minerals. Additionally, the OTUs belonging to Gallionellaceae were closely related to the members of the genera Gallionella and Sideroxydans, known to oxidize iron under microaerophilic conditions. These results suggest that members of Betaproteobacteria can potentially influence iron and phosphorus cycling which can influence biogeochemistry in arsenic-contaminated aquifers of the BDP.

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.

Surface complexation modeling of arsenic mobilization from goethite: Interpretation of an in-situ experiment

Abstract Sorption has been recognized as a predominant process for the mobility and transport of arsenic (As) in groundwater. However, the model-based description of the chemical and physical mechanisms controlling As interaction with mineral surfaces under natural hydrochemical conditions remains a formidable challenge. In this study we assess and compare the ability of existing surface complexation models (SCMs), the diffuse double layer model (DDL) and the charge distribution multisite complexation model (CD-MUSIC), to simulate As desorption from goethite in groundwater. We consider the outcomes of an in-situ experiment recently performed in an arsenic-contaminated aquifer of Northern China where As-loaded goethite-coated sand was deployed in 7 monitoring wells. Determination and measurements of As-surface species were carried out over a time period of 80 days in all monitoring wells. Simultaneous calibration of the models with the measurements in the 7 wells was performed to obtain single sets of surface complexation parameters for the DDL and the CD-MUSIC models, respectively. Although the general dynamic pattern of the As release at the site is captured by both models, the approach based on the CD-MUSIC agrees best with the field experimental data without modifications of the surface complexation database compiled from previous studies on goethite. Conversely, calibration of the DDL affinity constants led to substantial improvement in the agreement between model simulations and the considered field dataset. The model outcomes and the exploration of the sensitivity of the goethite surface composition to changes in hydrochemical conditions provide insights into the mechanisms controlling arsenic sorption and their different description in the DDL and the CD-MUSIC models. Both SCMs indicate that PO43− acts as the main competitor for As(V) sorption sites, and that Fe(II) does not have a significant effect on the As(V) release despite its strong affinity for the goethite surface. Neither SCM suggests direct binding of As(III) to goethite. However, the CD-MUSIC model predicts significant formation of a goethite-Fe(II)-As(III) complex under the mildly alkaline conditions of the groundwater at the field site and that such complex is insensitive to phosphate competition. The CD-MUSIC implementation also allows capturing the non-linear charge effects of the major ions, including C a 2+ and M g 2+ , which appear to have important implication in the mechanisms of As sorption at the field site.

Extensive arsenic contamination in high-pH unconfined aquifers in the Indus Valley.

Arsenic-contaminated aquifers are currently estimated to affect ~150 million people around the world. However, the full extent of the problem remains elusive. This is also the case in Pakistan, where previous studies focused on isolated areas. Using a new data set of nearly 1200 groundwater quality samples throughout Pakistan, we have created state-of-the-art hazard and risk maps of arsenic-contaminated groundwater for thresholds of 10 and 50 μg/liter. Logistic regression analysis was used with 1000 iterations, where surface slope, geology, and soil parameters were major predictor variables. The hazard model indicates that much of the Indus Plain is likely to have elevated arsenic concentrations, although the rest of the country is mostly safe. Unlike other arsenic-contaminated areas of Asia, the arsenic release process in the arid Indus Plain appears to be dominated by elevated-pH dissolution, resulting from alkaline topsoil and extensive irrigation of unconfined aquifers, although pockets of reductive dissolution are also present. We estimate that approximately 50 million to 60 million people use groundwater within the area at risk, with hot spots around Lahore and Hyderabad. This number is alarmingly high and demonstrates the urgent need for verification and testing of all drinking water wells in the Indus Plain, followed by appropriate mitigation measures.

Fate of Arsenic during Red River Water Infiltration into Aquifers beneath Hanoi, Vietnam.

Recharge of Red River water into arsenic-contaminated aquifers below Hanoi was investigated. The groundwater age at 40 m depth in the aquifer underlying the river was 1.3 ± 0.8 years, determined by tritium–helium dating. This corresponds to a vertical flow rate into the aquifer of 19 m/year. Electrical conductivity and partial pressure of CO2 (PCO2) indicate that water recharged from the river is present in both the sandy Holocene and gravelly Pleistocene aquifers and is also abstracted by the pumping station. Infiltrating river water becomes anoxic in the uppermost aquifer due to the oxidation of dissolved organic carbon. Further downward, sedimentary carbon oxidation causes the reduction of As-containing Fe-oxides. Because the release of arsenic by reduction of Fe-oxides is controlled by the reaction rate, arsenic entering the solution becomes highly diluted in the high water flux and contributes little to the groundwater arsenic concentration. Instead, the As concentration in the groundwater of up to 1 μM is due to equilibrium-controlled desorption of arsenic, adsorbed to the sediment before river water started to infiltrate due to municipal pumping. Calculations indicate that it will take several decades of river water infiltration to leach arsenic from the Holocene aquifer to below the World Health Organization limit of 10 μg/L.

Both Phosphorus Fertilizers and Indigenous Bacteria Enhance Arsenic Release into Groundwater in Arsenic-Contaminated Aquifers.

Arsenic (As) is a human carcinogen, and arsenic contamination in groundwater is a worldwide public health concern. Arsenic-affected areas are found in many places but are reported mostly in agricultural farmlands, yet the interaction of fertilizers, microorganisms, and arsenic mobilization in arsenic-contaminated aquifers remains uncharacterized. This study investigates the effects of fertilizers and bacteria on the mobilization of arsenic in two arsenic-contaminated aquifers. We performed microcosm experiments using arsenic-contaminated sediments and amended with inorganic nitrogenous or phosphorus fertilizers for 1 and 4 months under aerobic and anaerobic conditions. The results show that microcosms amended with 100 mg/L phosphorus fertilizers (dipotassium phosphate), but not nitrogenous fertilizers (ammonium sulfate), significantly increase aqueous As(III) release in arsenic-contaminated sediments under anaerobic condition. We also show that concentrations of iron, manganese, potassium, sodium, calcium, and magnesium are increased in the aqueous phase and that the addition of dipotassium phosphate causes a further increase in aqueous iron, potassium, and sodium, suggesting that multiple metal elements may take part in the arsenic release process. Furthermore, microbial analysis indicates that the dominant microbial phylum is shifted from α-proteobacteria to β- and γ-proteobacteria when the As(III) is increased and phosphate is added in the aquifer. Our results provide evidence that both phosphorus fertilizers and microorganisms can mediate the release of arsenic to groundwater in arsenic-contaminated sediments under anaerobic condition. Our study suggests that agricultural activity such as the use of fertilizers and monitoring phosphate concentration in groundwater should be taken into consideration for the management of arsenic in groundwater.

Diverse arsenic- and iron-cycling microbial communities in arsenic-contaminated aquifers used for drinking water in Bangladesh.

Subsurface removal of arsenic by injection with oxygenated groundwater has been proposed as a viable technology for obtaining 'safe' drinking water in Bangladesh. While the oxidation of ferrous iron to solid ferric iron minerals, to which arsenic adsorbs, is assumed to be driven by abiotic reactions, metal-cycling microorganisms may potentially affect arsenic removal. A cultivation-independent survey covering 24 drinking water wells in several geographical regions in Bangladesh was conducted to obtain information on microbial community structure and diversity in general, and on specific functional groups capable of the oxidation or reduction of arsenic or iron. Each functional group, targeted by either group-specific 16S rRNA or functional gene amplification, occurred in at least 79% of investigated samples. Putative arsenate reducers and iron-oxidizing Gallionellaceae were present at low diversity, while more variation in potentially arsenite-oxidizing microorganisms and iron-reducing Desulfuromonadales was revealed within and between samples. Relations between community composition on the one hand and hydrochemistry on the other hand were in general not evident, apart from an impact of salinity on iron-cycling microorganisms. Our data suggest widespread potential for a positive contribution of arsenite and iron oxidizers to arsenic removal upon injection with oxygenated water, but also indicate a potential risk for arsenic re-mobilization by anaerobic arsenate and iron reducers once injection is halted.

Arsenic Aquifer Sealing Technology in Wells: A Sustainable Mitigation Option

The act of constructing unprotected/unsealed wells to extract water from deep aquifers is a worrisome ongoing practice in most parts of developing countries of the globe. The paper is first of its kind in exploring sealing technology as a potential mitigation measure to prevent arsenic contamination of deep aquifers. The technology has been assessed not only as a safeguard to potential microbiological or mineralogical contamination of aquifers but also as an adaptive option in case of climate-induced disasters like drought or flood where it can serve as emergency safe drinking water source. This paper puts forward comparative findings of mini-arsenic blanket testing of 358 wells (unsealed) performed at an interval of 8 years in Nawalparasi, district of Nepal, along with the performance monitoring of eight different sealed wells ranging from 20 to 80 m deep for over a period of 7 years. The paper focuses on the construction methodology and performance evaluation of four sealed shallow wells constructed in the same district. Mini-arsenic blanket test results show 38, 37, and 25 % of bore wells with respective increasing, decreasing, and constant level of arsenic concentrations whereas the sealed wells exhibit steadiness in arsenic concentration range of particular tapped aquifers within Nepal drinking water quality standard for arsenic of 50 μg/l over a long period, even though the tapped aquifers lie intercepted in between adjacent arsenic elevated aquifers. Sealed shallow wells exhibit good aquifer seal characteristics beyond potential resultant existing positive difference to cause downward aquifer cross-contamination. The presented technology can be used and replicated in deep/multi-aquifer hydrogeology of Nepal and South Asia for extraction of water from deep and safer aquifers in rural and urban water supply systems by escaping overlying arsenic-contaminated aquifers.

Biodegradable Organic Carbon in Sediments of an Arsenic-Contaminated Aquifer in Bangladesh

Laboratory incubations of sediment collected from an arsenic-contaminated aquifer in Bangladesh revealed a hitherto undocumented pool of biodegradable sedimentary organic carbon. Sampling, homogenizing, handling, and/or experimentation with the sediment released organic carbon, causing dissolved organic carbon (DOC) concentrations to reach ∼150–250 mg/L when DOC was mixed with recharge water. The native sedimentary microbial community rapidly consumed the released carbon, producing methane, while no loss of DOC was observed in sterilized sediments. In both sterilized and native incubations, dissolved arsenic equilibrated with the sediment; arsenic concentrations initially dropped and then remained constant over the 180 day experiment. Collectively, these data suggest that in situ perturbations to the aquifer matrix could promote mobilization of bioavailable sedimentary organic carbon. Although this sedimentary organic carbon did not influence arsenic concentrations here, it represents a carbon source that could, in the presence of arsenic-bearing iron (hydr)oxides, fuel microbial reductive release of arsenic.

Depth-resolved abundance and diversity of arsenite-oxidizing bacteria in the groundwater of Beimen, a blackfoot disease endemic area of southwestern Taiwan

The role of arsenite oxidizers in natural attenuation of arsenic pollution necessitates studies on their abundance and diversity in arsenic-contaminated aquifers. In this study, most probable number-polymerase chain reaction (MPN-PCR) and denaturing gradient gel electrophoresis (DGGE) was applied to monitor depth-wise abundance and diversity of aerobic arsenite oxidizers in arsenic-enriched groundwater of Beimen, southwestern Taiwan. The results revealed that the abundance of arsenite oxidizers ranged from 0.04 to 0.22, and the lowest ratio was observed in the most arsenic-enriched and comparatively more reduced groundwater (depth 200 m) of Beimen 1. The highest ratio was observed in the less arsenic-enriched and less reduced groundwater (depth 60 m) of Beimen 2B. DGGE profiles showed a shift in diversity of arsenite oxidizers, consisting of members of the Betaproteobacteria (61%), Alphaproteobacteria (28%) and Gammaproteobacteria (11%), depending on mainly arsenic concentration and redox level in groundwater. Groundwater with the lowest arsenic and highest dissolved oxygen at Beimen 2B harbored 78% of the arsenite oxidizers communities, while groundwater with the highest arsenic and lowest dissolved oxygen at Beimen 1 and Beimen–Jinhu harbored 17 and 22% of arsenite oxidizers communities, respectively. Pseudomonas sp. was found only in groundwater containing high arsenic at Beimen 1 and Beimen–Jinhu, while arsenite oxidizers belonging to Alpha- and Betaproteobacteria were dominated in groundwater containing low arsenic.

Characterization of arsenic-contaminated aquifer sediments from eastern Croatia by ion microbeam, PIXE and ICP-OES techniques

Groundwater arsenic contamination has been evidenced in eastern Croatia and hydrochemical results suggest that the occurrence of arsenic in the groundwater depends on the local geology, hydrogeology, and geochemical characteristics of the aquifer. In order to perform the sediment characterization and to investigate arsenic association with the other elements in the sediments, 10 samples from two boreholes (PVc-3 and Gundinci 1) in eastern Croatia were analyzed using two techniques: PIXE (without sample pre-treatment) and ICP-OES (after digestion), as well by ion microbeam analyses.The results of the PIXE and ICP-OES techniques showed quite good agreement; however, greater discrepancies were observed at the higher arsenic and manganese mass ratios. According to both techniques, higher As mass ratios were observed in the sediments from the PVc-3 core (up to 651mg/kg and 491mg/kg using PIXE and ICP-OES analyses respectively) than from the Gundinci 1 core (up to 60mg/kg using both techniques). Although arsenic association with Fe is expected, no correlation was observed. The microbeam analyses demonstrated that arsenic is associated with sulphides and iron in the most As-contaminated sample from the PVc-3 core, while this relationship was not evident in the most As-contaminated sample from the Gundinci 1 borehole.

Pond-Derived Organic Carbon Driving Changes in Arsenic Hazard Found in Asian Groundwaters

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

Groundwater arsenic concentrations in Vietnam controlled by sediment age

Arsenic contamination of groundwater continues to threaten the health of millions of people in southeast Asia. The oxidation of organic carbon, coupled to the reductive dissolution of arsenic-bearing iron oxides, is thought to control the release of sediment-bound arsenic into groundwater. However, the cause of the high spatial variability in groundwater arsenic concentrations—which can range from 5 to 500 μg l−1 within distances of a few kilometres—has been uncertain. Here, we combine measurements of sediment age, organic-matter reactivity and water chemistry at four locations along a cross-section of the arsenic-contaminated Red River floodplain in Vietnam to determine the origin of variations in groundwater arsenic concentrations. The burial age of the aquifer sediments, determined using optical stimulated luminescence, ranged from 460 years near the course of the present-day river to 5,900 years at the margin of the floodplain. The groundwater arsenic content and the reactivity of sedimentary organic carbon, determined using radiotracer measurements of the rate of methanogenesis, declined with sediment age. The sedimentary pools of both iron and arsenic also declined with the burial age of the sediments. We suggest that the age of aquifer sediments is a key determinant of groundwater arsenic concentrations. Arsenic contamination of groundwater threatens the health of millions of people in southeast Asia. Measurements in an arsenic-contaminated aquifer in Vietnam point to sediment age as a key determinant of groundwater arsenic concentrations.

Multidimensional spectrofluorometry characterization of dissolved organic matter in arsenic-contaminated shallow groundwater

Multidimensional spectrofluorometry was employed to characterize dissolved organic matter (DOM) in arsenic-contaminated shallow aquifers at Jianghan Plain (JHP), central China, to better understand the effect of biogeochemical processes on arsenic mobilization. The microbial humic quinone and amino acid components identified indicate the importance of microbially mediated processes in the reduction of iron oxyhydroxides. The relationship of quinone and amino acid-like components with Fe2+ and S2− helps us understand the sequential redox transformation (SRT) in the high arsenic aquifer system and the origin of bicarbonate in groundwater. Correlation between DOM components and dissolved arsenic and Fe suggests that arsenic mobilization could be linked to the microbial reduction of iron oxyhydroxides with liable DOM as electron donors as well as to the electron shuttling function served by humic quinones.

Using multivariate statistical methods to assess the groundwater quality in an arsenic-contaminated area of Southwestern Taiwan

Groundwater is a major water resource in Southwestern Taiwan; hence, long-term monitoring of water quality is essential. The study aims to assess the hydrochemical characteristics of water in the arsenic-contaminated aquifers of Choushui River alluvial fan and Chianan Plain, Taiwan using multivariate statistical methods, namely, factor analysis (FA), cluster analysis (CA), and discriminant analysis (DA). Factor analysis is applied to reveal the processes controlling the hydrochemistry of groundwater. Cluster analysis is applied to spatially categorize the collected water samples based on the water quality. Discriminant analysis is then applied to elucidate key parameters associated with the occurrence of elevated As concentration (>10 μg L(-1)) in groundwater. Major water types are characterized as Na-Ca-Cl and Na-Mg-Cl in the Choushui River alluvial fan and Chianan Plain, respectively. Inorganic species of arsenic (As), particularly As(III), prevail in these two groundwater catchments, and their levels are higher in the Chianan Plain than in the Choushui River alluvial fan. Through FA, three factors, namely, the degree of salination, As reduction, and iron (Fe) reduction, are determined and denoted irrespective of some differences between the factorial compositions. Spatial distribution patterns of factors As reduction and Fe reduction imply that the redox zonation is delineated by As- and Fe-dominance zones separately. The results of CA demonstrate that three main groups can be properly explained by the factors extracted via FA. Three- (Fe(2+), Fe(3+), and NH (4) (+) ) and four-parameters (Fe(2+), Fe(3+), NH (4) (+) , and Ca(2+)) derived from discriminant analysis for Choushui River alluvial fan and Chianan Plain are elucidated as key parameters affecting the distribution of As-contained groundwater. The analytical results indicate that the reductive dissolution of Fe minerals is prerequisite for the mobilization of As, whereas the shift of redox condition from Fe- to As-reducing leads to the accumulation of dissolved As in this area.

Hydrochemical and Sediment Biomarker Evidence of the Impact of Organic Matter Biodegradation on Arsenic Mobilization in Shallow Aquifers of Datong Basin, China

Biomarker and hydrochemical characteristics of geogenic arsenic-contaminated aquifers at Datong Basin, northern China, were analyzed to better understand the impact of organic matter (OM) biodegradation on arsenic enrichment in groundwater. The hydrochemical characteristics of high arsenic groundwater from the Datong Basin indicate that arsenic mobilization and iron and manganese oxide/hydroxide reduction were controlled by biodegradation of OM. The elevated value of alkalinity produced by microbial oxidation of OM is another important factor for arsenic mobilization via competitive sorption. Bulk geochemistry of the sediments shows that arsenic has close correlation with iron and manganese, indicating iron- and manganese-bearing minerals could be the major pools for arsenic. Results of biomarker analysis reveal that all the sediments contained natural petroleum-sourced hydrocarbons which may have undergone biodegradation, as suggested by the carbon preference index, C29 sterane, and the distribution pattern of hopanes. The presence of unresolved complex mixtures in all samples also indicates the natural petroleum origin of hydrocarbons and the effect of biodegradation. At some depths (5.4–11.8, 31–33.2, and 40–48.4 m below the land surface), the samples have low n-alkane content and no odd-over-even predominance, suggesting that indigenous microbes within the aquifer can preferentially remove the petroleum-sourced n-alkanes. The bioavailability of organic carbon is very important to promote the microbially mediated reductive dissolution of iron oxides/hydroxides and subsequent arsenic release from aquifer sediment into groundwater.

Groundwater flow in an arsenic-contaminated aquifer, Mekong Delta, Cambodia

To advance understanding of hydrological influences on As concentrations within groundwaters of Southeast Asia, the flow system of an As-rich aquifer on the Mekong Delta in Cambodia where flow patterns have not been disturbed by irrigation well pumping was examined. Monitoring of water levels in a network of installed wells, extending over a 50 km 2 area, indicates that groundwater flow is dominated by seasonally-variable gradients developed between the river and the inland wetland basins. While the gradient inverts annually, net groundwater flow is from the wetlands to the river. Hydraulic parameters of the aquifer ( K ≈ 10 −4 ms −1 ) and overlying clay aquitard ( K ≈ 10 −8 ms −1 ) were determined using grain size, permeameter and slug test analyses; when coupled with observed gradients, they indicate a net groundwater flow velocity of 0.04–0.4 ma −1 downward through the clay and 1–13 ma −1 horizontally within the sand aquifer, producing aquifer residence times on the order 100–1000 a. The results of numerical modeling support this conceptual model of the flow system and, when integrated with observed spatial trends in dissolved As concentrations, reveal that the shallow sediments (upper 2–10 m of fine-grained material) are an important source of As to the underlying aquifer.