Arsenic Enrichment In Groundwater Research Articles

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.

High biodegradability of microbially-derived dissolved organic matter facilitates arsenic enrichment in groundwater: Evidence from molecular compositions and structures

Although biodegradation of organic matter is well-known to trigger enrichment of arsenic (As) in groundwater, the effects of DOM sources and biodegradability on As enrichment remain elusive. In this study, groundwater samples were collected from the Hetao basin to identify DOM source and evaluate biodegradability by using spectral and molecular techniques. Results showed that in the alluvial fan, DOM was mainly sourced from terrestrially derived OM, while DOM in the flat plain was more originated from microbially derived OM. Compared to terrestrially derived DOMs, microbially derived DOMs in groundwater, which had relatively higher H/Cwa ratios, NOSC values and more biodegradable molecules, exhibited higher biodegradability. In the flat plain, microbially derived DOMs with higher biodegradability encountered stronger biodegradation, facilitating the reductive dissolution of Fe(III)/Mn oxides and As enrichment in groundwater. Moreover, the enrichment of As depended on the biodegradable molecules that was preferentially utilized for primary biodegradation. Our study highlights that the enrichment of dissolved As in the aquifers was closely associated with microbially derived DOM with high biodegradability and high ability for primary biodegradation.

Provenance, chemical weathering, and sedimentary environment of the aquifer sediments: Implication for arsenic enrichment in groundwater

High arsenic (As) groundwater has been widely observed in the world, but the provenance, chemical weathering, and sedimentary environment of aquifer sediments hosting high As groundwater remain poorly understood. The geochemical and isotopic characteristics of high-resolution aquifer sediments were investigated to unravel the roles of provenance, chemical weathering, and sedimentary environment of the aquifer sediments. The average values of rare earth elements (REEs) showed the enrichment of light REEs and flat heavy REEs in the aquifer and the Yellow River sediments. The chemical index of alteration reflected incipient and moderate weathering (41 to 71). The δ13CSOM (−27.4‰ ± 4.3‰) and C/N values (8.1 ± 5.8) indicated aquatic environment of sedimentation. Shallow aquifer sediments mostly had relatively high ΣREEs contents and negative Eu anomalies, demonstrating that the aquifer sediments were mainly derived from the Yellow River (up to 94.3% ± 5%). Deep aquifer sediments had relatively low ΣREEs contents and positive Eu anomalies, which originated from the Yinshan Mountains (up to 72.2% ± 20.4%) principally with igneous and metamorphic rocks. Moreover, recycling of the Yellow River sediments from Northeastern Tibetan Plateau enhanced the silicate weathering and increased contents of Fe/Mn oxides and As. The δ13CSOM and C/N values evidenced that organisms of paleo-lake or paleo-Yellow River were responsible for abundant sedimentary organic matter. Concentrations of Fe, Mn, and As were high in shallow groundwater from the plain. The As enrichment in the shallow groundwater was jointly controlled by the provenance and chemical weathering of the aquifer sediments (14%), sedimentary environment (10%), reductive dissolution (35%) of Fe/Mn oxides, As desorption under elevated pH (20%), and competitive adsorption of HCO3- (21%). This study systematically reveals the roles of provenance, chemical weathering, and sedimentary environment of the aquifer sediments and highlights joint contributions of geochemical processes and geochemical conditions to As enrichment in groundwater.

Arsenic enrichment in the north Gangetic Plains of Laksar, Uttarakhand, India

This study reports new data on arsenic contamination in groundwater in Laksar, a pristine and semi-rural region of India's Himalayan state of Uttarakhand. Based on a systematic investigation of hydrogeochemical parameters influencing arsenic mobilisation, the most plausible mechanism of arsenic release into groundwater has been proposed. The groundwater in the research area is mostly homogeneous, with the Ca–Mg–HCO3 facies accounting for 87.5 percent of groundwater samples. The combined effect of silicate weathering, and the dissolution of carbonate minerals predominantly influence Laksar groundwater composition, as evident from bivariate plots of Mg/Na vs. Ca/Na and HCO3−/Na vs. Ca/Na. The statistical analysis (PCA and correlation analysis) performed on the geochemical data indicates that natural processes such as carbonate weathering, reductive dissolution of Fe/Mn minerals together with anthropogenic activities such as the utilization of fertilizers in agricultural fields, human and animal waste disposal, exert control on the major ion chemistry, and As enrichment in groundwater. The study area has extremely high arsenic concentrations, ranging from 0.60 to 91.21 μg/L (mean: 15.63 μg/L), with majority of groundwater samples (>50%) having arsenic contents exceeding the WHO permitted limit (10 μg/L). Analysis of the Eh-pH diagram shows that As(III) species (H3AsO3) accounts for 79% of samples, whereas As (V) species (HAsO4−) accounts for the remaining. The observed arsenic enrichment in groundwater can be attributed to As desorption from iron oxide/hydroxide minerals via reductive dissolution mechanism under reducing groundwater conditions, which is supported by negative Eh values and high levels of Fe and comparatively low levels of sulphate and nitrate.

The influence mechanism of hydrogeochemical environment and sulfur and nitrogen cycle on arsenic enrichment in groundwater: A case study of Hasuhai basin, China

Hydro-biogeochemical processes control the formation and evolution of high arsenic (As) groundwater. However, the effects of nitrogen and sulfur cycles in groundwater on As migration and transformation are not well understood. Thus, twenty-one groundwater samples were collected from the Hasuhai basin. Hydrochemistry and geochemical modeling were used to analyze the geochemical processes associated with nitrogen and sulfur cycles. An arsenic speciation model (AM) and a sulfide-As model (SAM) were constructed to verify the existence of As species and the formation mechanism of thioarsenate. A hydrous ferric oxide (Hfo)-As adsorption model (HAM) and a competitive adsorption model (CAM) were used to reveal the adsorption and desorption mechanisms of As. The results showed that high arsenic groundwater (As > 10 μg/L) was mainly distributed under reductive conditions, and the highest concentration was 231.5 μg/L. The modeling results revealed that sulfides were widely involved in the geochemical cycle of As, with H3AsO3 and H2AsO3− accounting for >70 % of the total As, and thioarsenate accounting for 30 %. S/As < 2.5 and S/Fe < l control the formation of thioarsenate. With the high correlation of NH4+, TFe, sulfide, and TAs, the co-mobilization of N and S cycles may facilitate As enrichment in groundwater. A weak alkaline reduction environment triggered by the decomposition of organic matter was the main factor leading to the transfer of As from the aquifer to the groundwater. This research contributes to the development of high-As groundwater, and the findings are of general significance for drinking water in the Hasuhai Basin.

Evolution Mechanism of Arsenic Enrichment in Groundwater and Associated Health Risks in Southern Punjab, Pakistan.

Arsenic (As) contamination in groundwater is a worldwide concern for drinking water safety. Environmental changes and anthropogenic activities are making groundwater vulnerable in Pakistan, especially in Southern Punjab. This study explores the distribution, hydrogeochemical behavior, and pathways of As enrichment in groundwater and discusses the corresponding evolution mechanism, mobilization capability, and health risks. In total, 510 groundwater samples were collected from three tehsils in the Punjab province of Pakistan to analyze As and other physiochemical parameters. Arsenic concentration averaged 14.0 μg/L in Vehari, 11.0 μg/L in Burewala, and 13.0 μg/L in Mailsi. Piper-plots indicated the dominance of Na+, SO42−, Ca2+, and Mg2+ ions in the groundwater and the geochemical modeling showed negative saturation indices with calcium carbonate and salt minerals, including aragonite (CaCO3), calcite (CaCO3), dolomite (CaMg(CO3)2), and halite (NaCl). The dissolution process hinted at their potential roles in As mobilization in groundwater. These results were further validated with an inverse model of the dissolution of calcium-bearing mineral, and the exchange of cations between Ca2+ and Na+ in the studied area. Risk assessment suggested potential carcinogenic risks (CR > 10−4) for both children and adults, whereas children had a significant non-carcinogenic risk hazard quotient (HQ > 1). Accordingly, children had higher overall health risks than adults. Groundwater in Vehari and Mailsi was at higher risk than in Burewala. Our findings provide important and baseline information for groundwater As assessment at a provincial level, which is essential for initiating As health risk reduction. The current study also recommends efficient management strategies for As-contaminated groundwater.

Biodegradation of dissolved organic matter and sedimentary organic matter in high arsenic groundwater system: Evidence from lipid biomarkers and compound-specific carbon isotopes

Organic matter plays important roles in arsenic mobility. Whether sedimentary or dissolved organic matter is the predominant electron donors is still on debate. To fill this gap, both sediments and groundwater were collected from the Hetao Basin, a typical arid-semiarid area hosting high arsenic groundwater, to characterize diagnostic biomarkers and compound-specific carbon isotopes of lipids in dissolved organic matter (DOM) and its depth-matched sedimentary organic matter (SOM). Results showed that SOM in both clay and sand layers were mainly sourced from terrestrial higher plants, but sand SOM had additional sources from microbial and petroleum OM. Compound-specific carbon isotope compositions of DOM and SOM showed that δ13C of low molecular weight (LMW) n-alkanes (δ13CL) were more positive than those of high molecular weight (HMW) n-alkanes (δ13CH). The δ13CH in groundwater (δ13CH-DOM) and sediments (δ13CH-SOM) fall in the range of C3 plants, indicating that DOM and SOM had similar origins. More positive δ13CL than δ13CH suggests preferential degradation of LMW compounds in both sediments and groundwater. In comparison with SOM, more microbial-derived OM was observed in groundwater DOM, especially in high‑arsenic groundwater. High arsenic groundwater was accompanied by the low ratio of n-alkanes to hopanes in depth-matched SOM, and the great difference between δ13CL and δ13CH and low terrestrial/microbial ratios (TAR) in groundwater DOM. This indicates that SOM and DOM should be utilized by indigenous bacteria to fuel the reductive dissolution of Fe oxides for arsenic release into groundwater. Furthermore, more abundant LMW compounds and weaker odd-to-even predominance in DOM than those in SOM support that DOM encountered stronger biodegradation. This study highlights that preferential biodegradation of DOM (especially LMW components and petroleum sourced OM) would provide electrons for the reductive dissolution of arsenic-bearing Fe/Mn oxides and thus induce arsenic enrichment in groundwater.

Abundant Fe(III) Oxide‐Bound Arsenic and Depleted Mn Oxides Facilitate Arsenic Enrichment in Groundwater From a Sand‐Gravel Confined Aquifer

AbstractEnrichment mechanisms of high‐arsenic (&gt;10 μg/L) groundwater in confined aquifers with sand‐gravel sediments were rarely documented. Both groundwater and sediment were sampled to investigate mechanisms of groundwater arsenic mobility in sand‐gravel confined aquifers from the Songnen Basin, China. The results show that, although high arsenic groundwater was present in both shallow sandy (&lt;60 m) and deep sand‐gravel aquifers (60–100 m), relatively higher arsenic groundwater was hosted in the deep confined aquifers. The significantly higher proportion of biologically degradable organic carbon (BDOC) in deep confined groundwater than those in shallow unconfined ones (p = 0.03) suggests that higher arsenic concentrations were associated with higher DOM biodegradability. High‐arsenic groundwater was the result of Fe(III) oxide reduction facilitated by BDOC degradation. Arsenic‐rich pyrite framboids were observed in shallow aquifers, which would lower groundwater arsenic concentrations. Sand‐gravel sediments in deep aquifers had remarkably higher contents of extracted Fe(III) oxides and the bound arsenic, HCl‐extractable Fe(II) minerals, and adsorbed arsenic, but considerably lower contents of Mn oxides and Mn(II) minerals than the shallow sandy sediments. Siderite, which is not as efficient as pyrite in sequestering arsenic, was the dominant Fe(II) mineral in the deep aquifer. In the deep aquifer, Mn oxide reduction was almost completed and Fe(III) oxide reduction was predominated, which resulted in high concentrations of groundwater arsenic due to high arsenic/iron molar ratio in the Fe(III) oxides. This study proposed that geogenic high‐arsenic groundwater can occur in sand‐gravel aquifers being high in Fe(III) oxide‐bound arsenic but poor in Mn oxides under reducing conditions.

Effects of Calcium on Arsenate Adsorption and Arsenate/Iron Bioreduction of Ferrihydrite in Stimulated Groundwater.

The reduction and transformation of arsenic-bearing ferrihydrite by arsenate-iron reducing bacteria is one of the main sources of arsenic enrichment in groundwater. During this process the coexistence cations may have a considerable effect. However, the ionic radius of calcium is larger than that of iron and shows a low affinity for ferrihydrite, and the effect of coexisting calcium on the migration and release of arsenic in arsenic-bearing ferrihydrite remains unclear. This study mainly explored the influence of adsorbed Ca2+ on strain JH012-1-mediated migration and release of arsenate in a simulated groundwater environment, in which 3 mM ferrihydrite and pH 7.5. Ca2+ were pre-absorbed on As(V)-containing ferrihydrite with a As:Fe ratio of 0.2. Solid samples were analyzed by X-ray diffraction (XRD), scanning electron microscopic (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The results show that calcium and arsenate can synergistically adsorb on ferrihydrite due to the electrostatic interactions, and the adsorbed Ca2+ mainly exists on the surface through the outer-sphere complex. Adsorbed Ca2+ entering the stimulated groundwater was easily disturbed and led to an extra release of 3.5 mg/L arsenic in the early stage. Moreover, adsorbed Ca2+ inhibited biogenic ferrous ions from accumulating on ferrihydrite. As a result, only 12.30% Fe(II) existed in the solid phase, whereas 29.35% existed without Ca2+ adsorption. Thus, the generation of parasymplesite was inhibited, which is not conducive to the immobilization of arsenic in groundwater.

Impacts of active tectonics on geogenic arsenic enrichment in groundwater in the Hetao Plain, Inner Mongolia

High arsenic (As) groundwater has become a serious environmental problem threatening the health of 300 thousand people in the Hetao Plain (HP) of Inner Mongolia, China, where the microscopic mechanisms of As releasing in groundwater have been widely studied. However, the macro-behaviors of As under the large-scale geological conditions, particularly the impact of geogenic arsenic enrichment in groundwater by active tectonics is still uncertain. Here, we combine in-situ measurements of As concentration in groundwater and ore bodies, the locations of ore bodies, the active faults patterns and chronology, and the paleo-earthquake age sequences to explore the transport processes of As from source to groundwater controlled by the geological forces. Results show that the spatial pattern of groundwater As concentration exhibits large heterogeneity in the HP, particularly high in two areas: the Hangjin Rear Banner and Wuyuan area. Langshan, western part of the Western Yinshan Mountains, is more enriched in As-laden ore bodies and black rocks (dark carbonaceous sedimentary rocks and their metamorphic rocks). In the south of HP, reducing groundwater does not contain high As contents because the sediment (i.e., Yellow River alluvial deposit) itself bears little As. The larger vertical uplifting rate made a larger erosion rate of Langshan during the 50 ka B.P to 20 ka B.P, causing higher As background values in sediments. Differential uplifts, piedmont active faults, and earthquakes act as the internal dynamics, whereas the external effects of weathering, transport, and deposition play important roles in driving As-bearing bedrock debris to the foreland basin. The younger with more frequent seismotectonic events renew the As-bearing bedrock debris in sediments and the organic matter reactivity, causing higher As released to groundwater. This study can guide future identification of the geogenic high As groundwater.

Relative importance of hydrogeochemical and hydrogeological processes on arsenic enrichment in groundwater of the Yinchuan Basin, China

Arsenic (As) contamination in groundwater has been widely observed in the world and posed negative impacts on human health. Although factors controlling As concentration in groundwater have been intensively investigated, relative importance of hydrogeological settings and geochemical factors in As enrichment is less known. Groundwater samples from three hydrogeological transects and different aquifer sediment samples from the Yinchuan basin (China) were taken to systematically evaluate roles of hydrogeochemical and hydrogeological factors in As mobility. Dissolved As concentrations ranged between 0.04 and 263 μg/L, and 46% of studied groundwater samples had As concentrations greater than the World Health Organization drinking water guideline (10 μg/L). High As groundwater (As >10 μg/L) generally had ORP values between −175 and −75 mV, while low As groundwater mostly had ORP > −75 mV. Sedimentary Fe and Mn oxides were the major source of groundwater As in aquifers. In each transect studied, As concentrations were positively correlated with concentrations of dissolved Fe and/or Mn or negatively correlated with ORP values. This suggests that reductive dissolution of Fe(III) oxides released the adsorbed As into groundwater. In addition, desorption of As contributed to As enrichment in groundwater, which occurred at high pH and/or high HCO3− conditions. The hydrogeochemical characteristics played a leading role in the control of As concentrations in each transect. However, for all dataset, groundwater obtained in aquifers with higher hydraulic gradients generally had lower As concentrations. It indicates that, at the basin scale, hydraulic gradients and groundwater flow conditions were the dominant factors controlling As distribution. This study emphasizes the relative importance of hydraulic conditions in controlling As behavior at the basin scale yet neglected.

Unraveling influences of nitrogen cycling on arsenic enrichment in groundwater from the Hetao Basin using geochemical and multi-isotopic approaches

Sources and co-cycling of nitrogen species in arsenic-prone groundwater remain poorly understood, which could affect arsenic behavior. Here, geochemical and multi-isotopic characteristics of groundwater from various redox environments were investigated to reveal the effects of nitrogen cycling on arsenic mobility in groundwater systems from the Hetao Basin in China. In deep groundwater along an approximate flow path from the alluvial fan (Zone I) through the transition area (Zone II) and to the flat plain (Zone III), a progressive NO3– depletion with gradually increased δ15NNO3 and δ18ONO3 occurred, accompanied by increases in dissolved NH4+, Fe(II), and arsenic concentrations and a decrease in δ15NNH4. Shallow groundwater in the flat plain (Zone IV) covered wider ranges of δ15NNO3 and δ18ONO3 and relatively lower δ15NNH4. Organic nitrogen mineralization contributed primarily to NH4+ release in all zones, and dissimilatory NO3– reduction to NH4+ (DNRA) was an important NH4+ source in Zone IV. While NH4+ loss mainly occurred via nitrification in Zone I, anaerobic NH4+ oxidation was coupled to Fe(III) oxide reduction (Feammox) in Zones II and III and to NO2– reduction (anammox) in Zones II, III, and IV. Groundwater NO3– was reduced via heterotrophic denitrification in Zones II and III, while the DNRA was favored over denitrification in Zone IV with much higher DOC:NO3– molar ratios. Feammox and heterotrophic DNRA increased the dissolved Fe(II) and arsenic concentrations via the enhanced Fe(III) oxide reduction. However, anammox and Fe(II)-fueled autotrophic DNRA decreased their concentrations due to limited Fe(III) oxide reduction and/or enhanced Fe(II) oxidation. The influences of nitrogen cycling on arsenic behavior are mainly mediated by transformations between Fe(III) oxides and dissolved Fe(II). This study provides the first detailed multi-isotopic picture of nitrogen cycling and the relevant effects on arsenic enrichment processes.

The geological setting of arsenic enrichment in groundwater of the shallow aquifers of the Tista Floodplain, Rangpur, Bangladesh

Arsenic is present in water samples within the studied active floodplain areas of the Tista river, Rangpur Division, Bangladesh. All the water samples contain less arsenic than the WHO prescribed limit of 10 μg/L. 93.33% groundwater samples have higher Mn content than the permissible limit of 0.01 mg/L of WHO. The heavy metal concentrations of water can be expressed as Fe&gt;Mn&gt;Zn&gt;As on the basis of their mean content. The heavy metals are negatively correlated with the well depth which is indicative of the influence of the anthropogenic activities on the concentrations of heavy metals. The arsenic concentration in water samples is higher in the central part of the study area. The coarser grain size, dominance of physical weathering, elevated topography and the effective flushing of groundwater resulted low concentration of arsenic in the groundwater. The EDS study reveals that arsenic occurs as coating materials of the silicate minerals. The river waters also have arsenic content lower than WHOs permissible limit. The factor analysis reveals that the iron and arsenic is released by the chemical weathering of arsenic bearing minerals like pyrite and arsenopyrite. The Fe and Mn derived in the groundwater by the chemical weathering of iron and manganese bearing minerals such as iron rich clay, silicate minerals and iron sulfides. Â

Arsenic enrichment in groundwater and associated health risk in Bari doab region of Indus basin, Punjab, India

Contaminated groundwater is considered as one of the most important pathways of human exposure to the geogenic contaminants. Present study has been conducted in a part of Indus basin to investigate the presence and spatial distribution of arsenic (As) and other trace metals in groundwater. The As concentration varies from bdl-255.6 μg/L and 24.6% of the 73 collected groundwater samples have As above world health organization (WHO) guideline of 10 μg/L. High concentration of As is found along the newer alluvium of Ravi River. As is found with high bicarbonate (HCO3−) and Iron (Fe) and low nitrate (NO3−) indicating reductive dissolution of Fe bearing minerals. However, silicate weathering along with high sulphate (SO42) and positive oxidation-reduction potential (ORP) indicates mixed redox conditions. Weathering of minerals along with other major hydrogeochemical process are responsible for composition of groundwater. With 31.5% of the samples, sodium bicarbonate (Na–HCO3) is the major water facies followed by magnesium bicarbonate (Mg–HCO3) in 30% of samples. As, Fe and other trace metals including copper (Cu), cadmium (Cd), chromium (Cr), zinc (Zn) were used to calculate the health risk for children and adults in the region. Out of 73 samples, 58% has high Fe, 32.8% has high Zn, and 4.1% has high Cd which are above the prescribed limits of WHO guidelines. Health risk of the population has been assessed using chronic dose index (CDI), hazardous quotients (HQ) and hazardous index (HI) for children and adults. The mean CDI values follows the order as Fe > Zn > Cu > As > Cr > Cd, while the HQ values indicates high As hazards for both children and adults. 43.8% of the groundwater samples have high HI for adults, however, 49.3% has high HI for children indicating higher risk for children compared to adults. A large-scale testing should be prioritized to test the wells for As and other trace metals in the study region to reduce health risks.

Roles of different molecular weights of dissolved organic matter in arsenic enrichment in groundwater: Evidences from ultrafiltration and EEM-PARAFAC

Molecular weight (MW) is a fundamental property of dissolved organic matter (DOM) that may affect the interaction between DOM and metals/metalloids. However, understanding of MW property of groundwater DOM and its roles in arsenic (As) mobilization needs to be improved. In this study, two surface water samples and 12 groundwater samples with different concentrations of As and dissolved organic carbon (DOC) were taken to evaluate the effects of DOM MW on As mobilization by using sequential ultrafiltration technique under a N2 atmosphere. Concentrations of As, Fe, DOC, and spectroscopic properties of OM in each ultrafiltered sample were analyzed. Variations of As, Fe and DOC concentrations in different ultrafiltration fractions show that Fe colloids mainly exist in large size particles (10 kDa-0.45 μm fractions), while organic colloids are mainly present in middle size particles (5–10 kDa fractions). The positive correlations between As and Fe in 10 kDa-0.45 μm fractions (R2 = 0.76, p < 0.01) and between As and DOC in 5–10 kDa fractions (R2 = 0.61, p < 0.05) indicate that the complexation of As with large-size Fe colloids and middle-size DOM promotes As mobilization. Parallel factor analysis (PARAFAC) of the three-dimensional fluorescence spectra reveals that DOM in all size fractions comprises three major components, namely C1 (terrestrial humic-like component), C2 (microbial humic-like component), and C3 (protein-like component). The positive correlation between C1 (%) and As concentrations (R2 = 0.73, p < 0.05) possibly confirms that terrestrial-derived humic-like substance (HS) should be conducive to As enrichment. The spectroscopic indices of various fractions show that low MW DOM is characterized by more microbe-associated OM. Some samples have more than 90% of As in the ‘truly dissolved’ fractions, suggesting that microbial utilization of labile low MW DOM should lead to the enrichment of As, in addition to complexation reactions. Therefore, both As-Fe-HS complexation and microbial degradation of low MW DOM being related to the reductive dissolution of As-bearing Fe oxide minerals enhance As mobilization and enrichment in groundwater.

Vertical mixing with return irrigation water the cause of arsenic enrichment in groundwater of district Larkana Sindh, Pakistan

Stable isotopes ratios (‰) of Hydrogen (δ2H) and Oxygen (δ1⁸O) were used to trace the groundwater recharge mechanism and geochemistry of arsenic (As) contamination in groundwater from four selected sites (Larkana, Naudero, Ghari Khuda Buksh and Dokri) of Larkana district. The stable isotope values of δ2H and δ1⁸O range from 70.78‰ to −56.01‰ and from −10.92‰ to −7.35‰, relative to Vienna Standard for Mean Ocean Water (VSMOW) respectively, in all groundwater samples, thus indicating the recharge source of groundwater from high-salinity older water. The concentrations of As in all groundwater samples were ranged from 2 μg/L to 318 μg/L, with 67% of samples exhibited As levels exceeding than that of World Health Organization (WHO) permissible limit 10 μg/L and 42% of samples expressed the As level exceeding than that of the National Environmental Quality Standard (NEQS) 50 μg/L. The leaching and vertical mixing with return irrigation water are probably the main processes controlling the enrichment of As in groundwater of Larkana, Naudero, Ghari Khuda Buksh and Dokri. The weathering of minerals mostly controlled the overall groundwater chemistry; rock-water interactions and silicate weathering generated yielded solutions that were saturated in calcite and dolomite in two areas while halite dissolution is prominent with high As area.

Arsenic enrichment in groundwater in southern flood plain of Ganga-Son interfluves

Arsenic contamination is extensively present in the Holocene alluvial aquifer of middle plain of Ganga. Arsenic concentration in most of the public supply networks exceeds the maximum allowable limit of 50 μg/l (BIS 2003) for drinking water. The primary purpose of this work is to find out the hydrochemical characteristic of groundwater and identify the process of controlling the arsenic contamination and possible relationship among arsenic and other elements through factor analysis. High concentrations of As, Fe and NO3 are responsible due to the redox process and anthropogenic activity. The depth profile of As shows that As depleted with depth and concentration of NO3 is low associated with higher As. Piper Trilinear diagram shows hydrochemical facies categorize into three groups: (a) Ca–HCO3, (b) Na–Ca–HCO3 and (c) Na–HCO3. Dominance of Ca–HCO3 over SO4 + Cl indicates that high recharge phenomena is operated in groundwater sample of study area. According to the Eh-pH diagram, Arsenate (V) and Arsenite (III) are both type of species present in groundwater sample. The poor relationship between As and Fe indicates that, process of releasing As in groundwater are reduction of arseniferous iron hydroxide coupled to precipitation of Fe as FeCO3 when anoxic conditions develop during microbial oxidation of sedimentary organic matter. Multivariate statistical analysis reveals the relationship between different elements. Correlation matrix suggested that strong positive pair association between As–pH, NO3–Cl and Na–Cl etc. are responsible due to carbonate and silicate weathering with some anthropogenic activity. The present study shows that the As concentration in groundwater sample in the study area does not only make alarm but also calls for the immediate action.

Hydrogeochemical Characteristics of Areas with Arsenic Poisoning from Drinking Water and Arsenic Enrichment in Groundwater in Songnen Plain of China

Hydrogeochemical Characteristics of Areas with Arsenic Poisoning from Drinking Water and Arsenic Enrichment in Groundwater in Songnen Plain of China

Coupled iron, sulfur and carbon isotope evidences for arsenic enrichment in groundwater

Summary It is generally accepted that microbial processes play a key role in the mobilization and enrichment of arsenic (As) in groundwater. However, the detailed mechanism of the metabolic processes remain poorly understand. We apply isotopic measurements of iron (δ56Fe vs. IRMM-14), sulfur (δ34SSO4 vs. V-CDT) and carbon (δ13CDIC vs. V-PDB) to an experimental field plot in the Datong Basin, northern China. An array of monitoring wells was installed in a ≈1700-m2 plot in which high concentrations of As, ranging from 4.76 to 469.5 μg/L, were detected in the groundwater. The measured range of δ34SSO4 values from 10.0‰ to 24.7‰ indicates the prevalence of microbial sulfate reduction within aquifers. The range of δ56Fe values measured in the groundwater suggests microbial Fe(III) reduction and the occurrence of isotopic exchange between Fe(II)aq and FeS precipitation. The low δ13CDIC values (up to −33.6‰) measured in groundwater are evidences for the microbial oxidation of organic matter, which is interpreted as the light carbon pool within the aquifer sediments. The high As (As > 50 μg/L) groundwater, which has higher δ34SSO4 and δ56Fe values and lower δ13C values, indicates the following: (1) microbial reduction of sulfate causes the mobilization of As through HS− abiotic reduction of Fe(III) minerals and/or formation of As-sulfur components; and (2) direct microbial reduction of Fe(III) oxides, hydroxides and oxyhydroxides cannot increase As concentrations to greater than 50 μg/L. Re-oxidation of Fe-sufide explains how sample C1-2 can have a high As concentration and low δ34SSO4 and high δ56Fe values. The results provide new insight into the mechanism of As enrichment in groundwater.

Spatial and temporal patterns of groundwater arsenic in shallow and deep groundwater of Yinchuan Plain, China

Groundwater is a strategic resource in Yinchuan Plain, with deep groundwater increasingly used for drinking water supply to reduce arsenic exposure of rural villagers who have relied on shallow tube wells for drinking. To understand the sources and mobilization processes for arsenic enrichment in groundwater, a well water and sediment geochemistry study was carried out. Considerable spatial variability of groundwater arsenic exists. Arsenic concentration in deeper groundwater (40–250m) was generally less than 10μg/L (n=26) except for seven wells, with a mean of 7.0μg/L (n=33). For shallower depths (4–40m), As concentration increased from <1μg/L in the alluvial plain at higher elevation to 177μg/L near the center of alluvial lacustrine plain at lower elevation. Two clusters of high arsenic, shallow groundwater run nearly parallel to the Yellow River present day and ancient courses as two narrow strips where groundwater flow is sluggish. Arsenic content in sediments ranged from 3.7μg/g to 49.8μg/g with an average of 9.2μg/g, and was positively correlated with concentrations of sediment Cu, Fe2O3, Mn, Ba, Zn, F and organic C. Concentrations of arsenic between Dec. 2007 to Aug. 2011 in groundwater samples collected monthly vary more for the shallower depths (8m: 76±29μg/L; 12m: 116±27μg/L; 15m: 88±26μg/L; 20m: 164±14μg/L) than for the deeper depths (30m: 35±4μg/L; 80m: 33±5μg/L). Temporal variability of As concentrations in shallow groundwater correlates with water levels, driven by irrigation to a large extent.