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This article deals with the measurement of groundwater arsenic (As$$ As $$) contamination. The focus is on using a proper index for the severity of contamination, rather than just using the proportion of observations above a threshold level. We specifically focus on the contamination severity index (CSI) proposed by Sen (2016. Sankhya B, 78B(2), 341–361.). An alternative estimator in contrast to the one given by Sen (2016. Sankhya B, 78B(2), 341–361.) is used here which is useful for a small number of observations. The data used is that collected by the British Geological Society and the Bangladesh Department of Public Health Engineering during 1997–2001. Their analysis was based on the simple proportion of the observations above a threshold level, whereas the CSI measure adequately takes into account the severity of the observations. It is emphasized in this article that the comparison of areas with average arsenic (As$$ As $$) levels to determine arsenic severity is not appropriate in general due to a large variation in the sample values due to the depth of wells. However, an alternative to the CSI proposed in Sen (2016. Sankhya B, 78B(2), 341–361.) has been given in this article that takes into account the depth of wells corresponding to the As$$ As $$ samples. This article also uses the bootstrap methodology in assessing the bias and standard errors of the estimators, and the corresponding bias‐corrected and accelerated confidence intervals. [ABSTRACT FROM AUTHOR]

Ca2+, Mg2+, and HCO3− are extremely common coexisting ions with arsenic (As) in geogenic As-polluted groundwaters. Although extensive research has improved our knowledge of groundwater As removal techniques and mechanisms, there is still a lack of a definite explanation of the distinct influences of Ca2+ and Mg2+ on As immobilization. Furthermore, the question of whether the occurrence of metal-As aqueous complexes has positive or detrimental effects on As adsorption is still open, which hinders our ability to predict the effectiveness of groundwater As removal. The goal of our present work was to investigate the molecular-level interference mechanisms of Ca2+, Mg2+, and HCO3– on arsenic adsorption with batch/column filtration experiments and spectroscopic techniques. The results showed that the co-presence of Ca2+ and As significantly increased As(V) and As(III) adsorption by 22.1 and 12.2% in batch studies and by 20.1 and 16.7% in column adsorptive filtrations, which could be explained by forming a ternary Ca–As–TiO2 complex. Without the surface complex, Mg2+ only had a slightly positive effect on As removal. Co-existence of Ca2+ and HCO3− prevented the generation this surface complex, which significantly decreased the elimination of As(III). Inversely, the As(V) ternary complex and adsorption were not interfered by HCO3−. Moreover, an aqueous Ca–As(V) complex was detected without surface, which facilitated generation of the surface complex and As(V) adsorption. The results of this work clarified the distinct effects of Ca2+ and Mg2+ and As(V) and As(III) adsorption, which was critical in predicting the As adsorption efficiency in column filtration processes. [ABSTRACT FROM AUTHOR]

Arsenic contamination in the groundwater occurs in various parts of the world due to anthropogenic and natural sources, adversely affecting human health and ecosystems. The current study intends to examine the groundwater hydrogeochemistry containing elevated arsenic (As), predict As levels in groundwater, and determine the aptness of groundwater for drinking in the Vehari district, Pakistan. Four hundred groundwater samples from the study region were collected for physiochemical analysis. As levels in groundwater samples ranged from 0.1 to 52 μg/L, with an average of 11.64 μg/L, (43.5%), groundwater samples exceeded the WHO 2022 recommended limit of 10 μg/L for drinking purposes. Ion-exchange processes and the adsorption of ions significantly impacted the concentration of As. The HCO3− and Na+ are the dominant ions in the study area, and the water types of samples were CaHCO3, mixed CaMgCl, and CaCl, demonstrating that rock–water contact significantly impacts hydrochemical behavior. The geochemical modeling indicated negative saturation indices with calcium carbonate and other salt minerals, encompassing aragonite, calcite, dolomite, and halite. The dissolution mechanism suggested that these minerals might have implications for the mobilization of As in groundwater. A combination of human-induced and natural sources of contamination was unveiled through principal component analysis (PCA). Artificial neural networks (ANN), random forest (RF), and logistic regression (LR) were used to predict As in the groundwater. The data have been divided into two parts for statistical analysis: 20% for testing and 80% for training. The most significant input variables for As prediction was determined using Chi-squared analysis. The receiver operating characteristic area under the curve and confusion matrix were used to evaluate the models; the RF, ANN, and LR accuracies were 0.89, 0.85, and 0.76. The permutation feature and mean decrease in impurity determine ten parameters that influence groundwater arsenic in the study region, including F−, Fe2+, K+, Mg2+, Ca2+, Cl−, SO42−, NO3−, HCO3−, and Na+. The present study shows RF is the best model for predicting groundwater As contamination in the research area. The water quality index showed that 161 samples represent poor water, and 121 samples are unsuitable for drinking. Establishing effective strategies and regulatory measures is imperative in Vehari to ensure the sustainability of groundwater resources. [ABSTRACT FROM AUTHOR]

The potential release capacity of arsenic (As) from sediment was evaluated under a high level of exogenous organic matter (EOM) with both bioreactive and chemically reactive organic matters (OMs). The OMs were characterized by FI, HIX, BIX, and SUVA254 fluorescence indices showing the biological activities were kept at a high level during the experimental period. At the genus level, Fe/Mn/As-reducing bacteria (Geobacter, Pseudomonas, Bacillus, and Clostridium) and bacteria (Paenibacillus, Acidovorax, Delftia, and Sphingomonas) that can participate in metabolic transformation using EOM were identified. The reducing condition occurs which promoted As, Fe, and Mn releases at very high concentrations of OM. However, As release increased during the first 15–20 days, followed by a decline contributed by secondary iron precipitation. The degree of As release may be limited by the reactivity of Fe (hydro)oxides. The EOM infiltration enhances As and Mn releases in aqueous conditions causing the risk of groundwater pollution, which could occur in specific sites such as landfills, petrochemical sites, and managed aquifer recharge projects.

This study was conducted to inspect the spatial distribution, source identification, and risk assessment of groundwater arsenic (As) in different blocks that lie on the opposite banks of river Bhagirathi (a distributary of river Ganges), Murshidabad, West Bengal, India. It has been observed that the blocks that lie towards the eastern bank of river Bhagirathi have elevated arsenic and comparatively more reducing groundwater (lower oxidation–reduction potential and high iron). About 66% of groundwater samples across the district have arsenic concentration higher than the World Health Organization (WHO) permissible limit. Speciation of groundwater arsenic reveals that about 90% of arsenic species were present as arsenic (III). Further, principal component analysis (PCA) was employed to identify the controlling factors that favor the release of arsenic. PC1 comprises EC, TDS, As, Fe, TOC, and HCO3− with moderate loadings, which suggests microbially mediated degradation of organic matter (OM), helps in reductive dissolution of arsenic-bearing Fe–Mn oxyhydroxides. Results pointed out severe groundwater arsenic poisoning; hence, a health risk assessment was performed for the exposure of arsenic in groundwater, using incremental lifetime cancer risk (ILCR) models coupled with Monte Carlo simulations. On the eastern bank of river Bhagirathi, incremental lifetime cancer risk (ILCR) due to oral exposure (5th to 95th percentile values) ranged from 1.30538E − 04 to 9.31398E − 03 with a mean of 2.84194E − 03 for adults, which is 2841 times higher than the USEPA high safety risk guidelines of one in 1 million. The outcomes of the results will be useful for the policymakers and regulatory boards in defining the actual impact and deciding the pre-remediation goals. [ABSTRACT FROM AUTHOR]

Worldwide, more than 200 million people are estimated to be exposed to unsafe levels of arsenic. Chronic exposure to unsafe levels of groundwater arsenic is responsible for multiple human disorders, including dermal, cardiovascular, neurological, pulmonary, renal, and metabolic conditions. Consumption of rice and seafood (where high levels of arsenic are accumulated) is also responsible for human exposure to arsenic. The toxicity of arsenic compounds varies greatly and may depend on their chemical form, solubility, and concentration. Surprisingly, synthetic organoarsenicals are extremely toxic molecules which created interest in their development as chemical warfare agents (CWAs) during World War I (WWI). Among these CWAs, adamsite, Clark I, Clark II, and lewisite are of critical importance, as stockpiles of these agents still exist worldwide. In addition, unused WWII weaponized arsenicals discarded in water bodies or buried in many parts of the world continue to pose a serious threat to the environment and human health. Metabolic inhibition, oxidative stress, genotoxicity, and epigenetic alterations including micro-RNA-dependent regulation are some of the underlying mechanisms of arsenic toxicity. Mechanistic understanding of the toxicity of organoarsenicals is also critical for the development of effective therapeutic interventions. This review provides comprehensive details and a critical assessment of recently published data on various chemical forms of arsenic, their exposure, and implications on human and environmental health. [ABSTRACT FROM AUTHOR]

Potable groundwater (GW) contamination through arsenic (As) is a commonly reported environmental issue in Pakistan. In order to examine the groundwater quality for As contamination, its geochemical behavior, and other physicochemical parameters, 69 samples from various groundwater sources were collected from the mining area of Pind Dadan Khan, Punjab, Pakistan. The results showed the concentration of elevated As, its source of mobilization, and linked public health risk. Arsenic detected in the groundwater samples varied from 0.5 to 100 µg/L, with an average value of 21.38 µg/L. Forty-two samples were beyond the acceptable limit of 10 µg/L of the WHO for drinking purposes. The statistical summary showed that the groundwater cation concentration was in decreasing order such as Na+ > Ca2+ > Mg2+ > K+, while anions were as follows: HCO3− > SO42− > Cl− > NO3−. Hydrochemical facies results depicted that groundwater samples belong to CaHCO3 type. Rock-water interactions control the hydrochemistry of groundwater. Saturation indices' results indicated the saturation of the groundwater sources for CO3 minerals due to their positive SI values. Such minerals include aragonite, calcite, dolomite, and fluorite. The principal component analysis (PCA) findings possess a total variability of 77.36% suggesting the anthropogenic and geogenic contributing sources of contaminant. The results of the Exposure-health-risk-assessment model for measuring As reveal significant potential carcinogenic risk exceeding the threshold level (value > 10−4) and HQ level (value > 1.0). [ABSTRACT FROM AUTHOR]

Geogenic arsenic is a metabolic hazard to global citizens, due to its presence in most of the rocks. Natural processes such as percolation of rainwater through soil layer and water–rock interaction in weathering process principally lead to the dissolution of arsenic-bearing minerals in the aquifer system. In the present study, arsenic (As)-contaminated groundwater was analyzed covering all blocks (26 blocks) of Murshidabad District, West Bengal, India. Principally, the study focused on the assessment of groundwater quality with respect to arsenic along with other metal ions such as iron, manganese, cadmium and selenium. Tube well water samples (N = 348) were collected during pre- and post-monsoon seasons. The spatial distribution of arsenic levels ranges from 0.086 to 0.513 mg/L in pre-monsoon and 0.059–0.431 mg/L in post-monsoon, which indicates that all groundwater samples of the Murshidabad District exceeds the WHO's permissible limit of arsenic (0.01 mg/L). Water quality index (WQI) data suggested that 5.74% and 10.3% samples are suitable for drinking purpose in the pre-monsoon and post-monsoon season, respectively. Availability of cations are as follows: Ca2+ > Mg2+ > Na+ > K+ , and anions as: SO42− > HCO3− > Cl− > F− > NO3− in both pre-monsoon and post-monsoon seasons. Human health risk due to consumption of groundwater was assessed through USEPA designed methods as follows—hazard quotient (HQ), hazard index (HI), average daily dose of both direct ingestion of groundwater and dermal absorption of groundwater (ADDIngestion, ADDDermal absorption), and exposure frequency (EF). Thus, carcinogenic risk (CR) and non-carcinogenic risk (NCR) were determined. Results revealed that 29% and 37% of inhabitants suffered from carcinogenic and non-carcinogenic risk, respectively. On the basis of occurrence, spatial distribution and health risk assessment results of the targeted area can be marked as a moderate- to high-risk zone. The said zones need special attention for protection of public health. [ABSTRACT FROM AUTHOR]

The spatial and subsurface distribution of groundwater arsenic (As) and iron (Fe) in tubewell water were studied with the help of geospatial technique in Purbasthali Blocks I and II of Purba Bardhaman district in Bengal Delta of West Bengal. About 50.40% of tubewell samples (N = 127) have As above the WHO guidelines' value (> 10 μg/l). Among these contaminated tubewells, about 82.81% tubewells are installed within 60 m depth. All the samples (N = 127) have Fe concentration above 300 μg/l. Maximum concentrations of As and Fe in tubewell samples are 374 μg/l and 17,600 μg/l, respectively. Mean values of As and Fe concentrations are 48 μg/l and 4016 μg/l, respectively. About 29.92% and 33.86% of samples have As and Fe above mean values, respectively. The correlation between As and Fe is significantly positive (R = 0.618) and presence of high concentration (17,600 μg/l) of dissolved Fe indicates strong reducing condition of the aquifers. Maximum number of samples (34.64 %) are from 21 to 40 m depth. Most of the As-contaminated tubewells are located within the depth of 21 to 40 m in the Holocene Newer Alluvium aquifers. Static water level (SWL) depth 10.1 to 15 m below ground surface, occurring in most part of the Katwa Formation, is found to be As-contaminated. Arsenic pollution in the Holocene aquifers is mostly confined in Katwa Formation. Sijua Formation is almost As-safe in groundwater which is in the upper Pleistocene to lower Holocene in age. Most of the As-contaminated tubewells in Katwa Formation are located in the Holocene Newer Alluvium and spatially associated with meander cut-offs, paleo-meander scars, abandoned channels, and back swamp areas. [ABSTRACT FROM AUTHOR]

Groundwater arsenic contamination is a common environmental problem that threatens the health of over 100 million people globally. Apparent seasonal fluctuations in groundwater arsenic concentrations have been reported in various locations worldwide, including the Jianghan Plain, central China. This phenomenon has been attributed to shifts in redox conditions induced by seasonal incursions of surface water. However, it is not clear what processes during the incursion lead to changes in the redox conditions and what is the source of the organic carbon driving these processes. Therefore, we conducted a long-term investigation of stable carbon isotopic compositions in surface water and groundwater, as well as long-term monitoring of hydraulic gradients and geochemical compositions at the Jianghan Plain. Results indicated that a series of biogeochemical processes occurred during surface water incursion, including aerobic microbial respiration, nitrate and sulfate reduction. Groundwater arsenic was removed by adsorption on iron oxyhydroxides produced during oxidation of ferrous iron, resulting in dramatic decreases in arsenic concentrations during surface water recharge seasons. These processes were likely driven by organic carbon vertically transported from surface water and released from the surficial aquitard above 15 m. Groundwater pumping may accelerate the vertical infiltration of oxidizing recharge water and drive exogenous organic carbon to depth. Findings of this study advance the understandings of the mechanisms that cause temporal variations in groundwater As and the importance of exogenous organic carbon that may influence the temporal behavior of arsenic in groundwater. [ABSTRACT FROM AUTHOR]

The quality of groundwater used for human consumption and irrigation in the Hetao Basin of Inner Mongolia, China is affected by elevated salinity as well as high arsenic (As) concentrations. However, the origin of high salinity and its potential impact on As mobility in the Basin remain unclear. This study explores both issues using stable isotopic compositions and Cl/Br ratios of groundwater as well as the major ions of both groundwater and leachable salts in aquifer sediments. Limited variations in δ 18 O and δ 2 H (− 11.13 to − 8.10, − 82.23 to − 65.67) with the wide range of Total Dissolved Solid (TDS, 351–6734 mg/L) suggest less contribution of direct evaporation to major salinity in groundwater. Deuterium excess shows that non-direct evaporation (capillary evaporation, transpiration) and mineral/evaporite dissolution contribute to > 60% salinity in groundwater with TDS > 1000 mg/L. Non-direct evaporation, like capillary evaporation and transpiration, is proposed as important processes contributing to groundwater salinity based on Cl/Br ratio and halite dissolution line. The chemical weathering of Ca, Mg minerals and evaporites (Na 2 SO 4 and CaSO 4 ) input salts into groundwater as well. This is evidenced by the fact that lacustrine environment and the arid climate prevails in Pleistocene period. Dissolution of sulfate salts not only promotes groundwater salinity but affects As mobilization. Due to the dissolution of sulfate salts and non-direct evaporation, groundwater SO 4 2 − prevails and its reduction may enhance As enrichment. The higher As concentrations (300–553 μg/L) are found at the stronger SO 4 2 − reduction stage, indicating that reduction of Fe oxide minerals possibly results from HS − produced by SO 4 2 − reduction. This would have a profound impact on As mobilization since sulfate is abundant in groundwater and sediments. The evolution of groundwater As and salinity in the future should be further studied in order to ensure sustainable utilization of water resource in this water scarce area. [ABSTRACT FROM AUTHOR]

A groundwater arsenic (As) distribution in Faizabad, Gonda, and Basti districts of Uttar Pradesh is shown in the entrenched channels and floodplains of the Ghaghara River. Tubewell water samples were analysed for As through flow injection hydride generation atomic absorption spectrometry (FI-HG-AAS) system. About 38, 61, and 42 % of tubewells in Faizabad, Gonda, and Basti districts, respectively, have As[10 μg/l (WHO guideline). Moreover, 15, 45, and 26 % of tubewells in Faizabad, Gonda, and Basti districts, respectively, have As above 50 μg/l. About 86, 69, and 35 % of tubewells in Faizabad, Gonda, and Basti districts, respectively, are from shallow depth (21-45 m), and it is worth noticing that 47 % Ascontaminated (As[10 μg/l) tubewells in these three districts are located within the depth of 10-35 m in Holocene Newer Alluvium aquifers. The high content of As (7.11 mg/kg) is measured in suspended river sediments of the Ghaghara River. Most of the As-contaminated villages in the Ghaghara Basin are located close to abandoned or present meander channels and floodplains of the Ghaghara River. In contrast, tubewells in Faizabad, Ayodhya, and Nawabganj towns are As-safe because of their positions on the Pleistocene Older Alluvium upland surfaces. Quaternary geomorphology plays an important role in groundwater arsenic contamination in the Ghaghara Basin. The sources of groundwater arsenic are geogenic and perennial mountainous rivers in the Ghaghara Basin supplied high sediment loads. The arsenic in groundwater of Ghaghara Basin is getting released from associated sediments which were likely deposited from the Himalayas. The process of release of groundwater arsenic is reductive dissolution of iron hydroxides. [ABSTRACT FROM AUTHOR]

• Carbon cycles in Bangladesh aquifers significantly affect microbial As release. • Sterol biomarkers and Δ14C of PLFA used to examine carbon source cycling. • Δ14C of PLFA confirmed preferential utilization of young carbon sources regardless of sediment age. • Both plant and sewage carbon sources are capable of stimulating arsenic release. • Supply of young plant-derived organic carbon to shallow Pleistocene sediments has yet to result in As release. The presence of dissolved arsenic in shallow aquifers of Bangladesh is widely accepted to require microbial dissimilatory iron-reduction in anoxic aquifers utilizing organic carbon as an electron donor. However, the various potential sources of this carbon, and whether organic carbon sources vary with sediment age (i.e. < 12 kyr-old Holocene vs older Pleistocene sediments) are still poorly understood. To shed light on these questions, natural abundance radiocarbon signatures of in situ microbial phospholipids fatty acids (PLFA), concentrations of sterol biomarkers, and aqueous [Cl-] and [Br-] were compared in two Bangladesh aquifers; a shallow (11–15 m) aquifer low in dissolved arsenic containing oxidized (orange) Pleistocene sands, Dopar Tek (DT), and a shallow (6–21 m) aquifer high in dissolved arsenic containing reduced (grey) Holocene sands, Desert Island (DI). Radiocarbon signatures of PLFA (Δ14C PLFA = −30 to −63 ‰ and +9 to +25 ‰, respectively) indicate microbial utilization of carbon fixed from the atmosphere within the last several decades, the drawdown of which into the shallow portions of both the Pleistocene Dopar Tek and Holocene Desert Island aquifers was likely enhanced by regional pumping activities. Similar results were previously obtained for two other Holocene aquifers in the same region, but to our knowledge this is the first time modern PLFA has been extracted from Pleistocene sediments. At both sites, high proportions of phytosterols, low sewage contamination indices (SCI < 0.7), and generally low Cl/Br ratios (averaging 434 and 544 at Desert Island and Dopar Tek respectively), are consistent with predominantly plant-derived organic carbon inputs. This contrasts with sewage-derived input inferred from higher sewage contamination index values (>0.7) previously observed at the two other shallow Holocene aquifers in the same region. Overall, our observations show that microbial communities within shallow aquifers, including those of Pleistocene age, utilize very recently fixed organic carbon associated with both plant and/or sewage origin. The microbial utilization of organic carbon fixed within the past several decades, likely derived from plants, in the anaerobic Pleistocene, has not, as of yet, led to iron reduction that would be sufficient to increase arsenic concentrations in groundwater. However, the observed microbial utilization of recently fixed carbon within all Bangladesh aquifers studied to date, indicates that pumping enhanced drawdown represents a potential risk to any systems where it might occur. [ABSTRACT FROM AUTHOR]

The discontinuous nature of elevated arsenic (As) in drinking water wells of West Bengal and other regions in the Bengal Basin has led to increased interest in the role that sediment-derived organic matter may play in enhancing reductive dissolution and As mobilization. Higher As concentrations have been observed in groundwater in reduced Holocene (grey) aquifers when compared to oxidized Pleistocene (orange) aquifers. In order to evaluate if the differences in the chemical character of dissolved organic matter (DOM) are present in groundwater in the Holocene and the Pleistocene aquifers that may influence dissolved As concentrations, shallow groundwater and surface water samples were collected from four study sites in Murshidabad district, West Bengal, India, and analyzed for water chemistry parameters and characteristics of DOM. For wells known to typically contain high As concentrations (in Holocene sediments) in Beldanga (10–4622 μg/L, at 35–45 m depth) and Hariharpara (5–695 μg/L, at 6–37, depth) sites, as well as wells characterized by low As concentrations (Pleistocene sediiments) in Nabagram (0–16 μg/L, at 20–45 m depth) and Kandi (5–50 μg/L, at 20–55 m depth), detailed DOM characterization was carried out using fluorescence spectroscopy and parallel factor analysis (PARAFAC). Results from statistical analysis of a variety of optical (absorbance and fluorescence) DOM properties revealed that the DOM in groundwater in the Holocene aquifer had high humification index (HIX) and low freshness index (β:α) values, whereas groundwater in the Pleistocene aquifer comprised more labile and microbial DOM sources. Consistent with the more labile nature of DOM in groundwater in the Pleistocene aquifer, two ratios 1) humic-like to protein-like components (humic:protein) and 2) terrestrially-derived to microbially-derived components (terr:microb) obtained from a four-component PARAFAC model were 1.9 and 2.9 times greater, respectively, in groundwater in the Holocene aquifer than in that of the Pleistocene aquifer, which suggests that the absence of humic-like DOM may be an important limitation to As mobility. [ABSTRACT FROM AUTHOR]

High concentrations of arsenic (As) in groundwater pose a great threat to human health. The motivation of this study was to provide a practical solution for As-safe water in As geogenic areas using granular TiO (GTiO). The kinetics results indicated that the As (III/V) adsorption on GTiO conformed to the Weber-Morris (WM) intraparticle diffusion model. The Langmuir isotherm results suggested that the adsorption capacities for As (III) and As (V) were 106.4 and 38.3 mg/g, respectively. Ion effect study showed that cationic Ca and Mg substantially enhanced As (V) adsorption, whereas no significant impact was observed on As (III). Silicate substantially decreased As (V) adsorption by 57 % and As (III) by 50 %. HCO remarkably inhibited As (V) adsorption by 52 %, whereas it slightly reduced As (III) adsorption by 8 %. Field column results demonstrated that ∼700 μg/L As was removed at an empty bed contact time (EBCT) of 1.08 min for 968 bed volumes before effluent As concentration exceeded 10 μg/L, corresponding to 0.96 mg As/g GTiO. Two household filters loaded with 110 g GTiO in the on-off operational mode can provide 6-L/day As-safe drinking water up to 288 and 600 days from the groundwater containing ∼700 μg/L As and ∼217 μg/L As, respectively. Integration of batch experiments and column tests with systematic variation of EBCTs was successfully achieved using PHREEQC incorporating a charge distribution multisite complexation (CD-MUSIC) model and one-dimensional reactive transport block. [ABSTRACT FROM AUTHOR]

High arsenic (As) groundwater in Xinjiang is mainly found in deep aquifers in the Manas River Basin (MRB) and the Kuitun River Basin (KRB) in the southern Junggar Basin. Groundwater samples (n = 21; including two phreatic water samples and 19 deep confined groundwater samples), were collected along hydrogeological profiles (north-south and west-east) in a high As groundwater area in the MRB in 2021. Inorganic As species, DOC and DIC were determined. As (V) was the major inorganic As species in 95% of the groundwater samples. High As groundwater was mostly found in reducing/sub-oxidizing and alkaline environments. Along the south-north groundwater flow path, a significant positive correlation was found between groundwater As, phosphate and carbonate, suggesting that competitive desorption may have been an important arsenic mobilizing process. Further, a significant negative correlation between groundwater As and the sulfate/chloride ratio and between DOC and the sulfate/chloride were observed - this suggests that sulfate reduction mediated by microorganisms may also have been an important mobilizing process. The lack of correlation between groundwater As and DOC suggests that the composition of organic matter as well as its concentration may have been an important controlling factor in arsenic mobilization. • High As groundwater occurred in alkaline sub-oxidizing condition in low-lying area. • As (V) is the major inorganic arsenic species in 95% groundwater samples in MRB. • Decreased As (V) sorption at high PO 4 3− and pH cause groundwater As enrichment. • Main causes of As release are desorption and desulphidation in alkaline conditions. [ABSTRACT FROM AUTHOR]

A groundwater arsenic (As) distribution in Mirzapur district of Uttar Pradesh is shown in the entrenched channels and floodplains of the Ganga River. A total of 45 tubewell water samples were analysed for As through flow injection hydride generation atomic absorption spectrometry (FI-HG-AAS) system. Iron in tubewell water was analysed by 1,10-phenanthroline method by the use of UV spectrophotometer. About 37.7 % of tubewells in this district have As >10 µg/l (WHO guideline) and 15.5 % of tubewells have As above 50 µg/l. About 84.4 % of tubewells have higher concentrations of iron (Fe) beyond its permissible limit 1 mg/l (BIS guideline). The iron (Fe) content in tubewell water samples varies from 0.1 to 7.6 mg/l. About 75.5 % of tubewells in Mirzapur district are from shallow depth (20-40 m), and it is worth noticing that 37.7 % of As-contaminated (As >10 µg/l) tubewells are located within the depth of 25-40 m in the Holocene Newer Alluvium aquifers. Most of the As-contaminated villages in Mirzapur district are located close to abandoned or present meander channels and floodplains of the Ganga River. In contrast, tubewells in Mirzapur and Chunar towns have low concentrations of As in groundwater (As <10 µg/l) because of their positions on the Pleistocene Older Alluvium upland surfaces. In As-contaminated areas, deeper tubewells (>50 m) in the Pleistocene Older Alluvium aquifers would be better option for As-safe groundwater. The potential sources of As are mainly from the Himalayas and the Himalayas rivers have high content of As in suspended river sediments. [ABSTRACT FROM AUTHOR]

A groundwater arsenic (As) survey in Mirzapur, Varanasi, Ghazipur, Ballia, Buxar, Ara, Patna, and Vaishali districts of UP and Bihar shows that people from these districts are drinking As-contaminated groundwater (max. 1,300 μg/l). About 66 % of tubewells from Buxar to Mirzapur areas and 89 % of tubewells from Patna to Ballia areas have As > 10 μg/l (WHO guideline). Moreover, 36 % of tubewells from Buxar to Mirzapur areas and 50 % of tubewells from Patna to Ballia areas have As above 50 μg/l. Most of the As-affected villages are located close to abandoned or present meander channels of the Ganga River. In contrast, tubewells located in Mirzapur, Chunar, Varanasi, Saidpur, Ghazipur, Muhammadabad, Ballia, Buxar, Ara, Chhapra, Patna, and Hazipur towns are As-safe in groundwater because of their positions on the Pleistocene Older Alluvium upland surfaces. The iron (Fe) content in tubewell water samples varies from 0.1 to 12.93 mg/l. About 77 % As-contaminated tubewells are located within the depth of 21 to 40 m in the Holocene Newer Alluvium aquifers. The potential source of As in sediments carried through the rivers from the Himalayas. Maximum As concentrations in the Older and Newer Alluvium sediments are 13.73 and 30.91 mg/kg, respectively. The Himalayas rivers, i.e. Yamuna, Ganga, Gomati, Ghaghara, Gondak, Buri Gandak, and Kosi rivers carrying suspended sediments have high content of As (max. 10.59 mg/kg). [ABSTRACT FROM AUTHOR]

• High As levels in the Pleistocene aquifer imposed by natural hydrogeological processes. • Preferential flow paths created due to the low subsidence rate in a river delta. • Flow of high As groundwater from Holocene to Pleistocene aquifers. • Surface derived organic matter seeping through thick, permeable clay layers. • Groundwater As levels in Holocene aquifers related to the flushing extent. The relation between arsenic groundwater concentrations and hydrogeological processes was investigated in the proximal part of the Red River delta, Vietnam, west of the depression cone formed by the exploitation of groundwater in Hanoi. Flow paths in the Quaternary aquifers were modeled based on previously interpreted geological structure and hydrogeological data gathered during field work in 2014–2017. Sedimentary structures and simulated flow patterns were compared with the spatial distribution of the groundwater arsenic concentration. The regression of the sea in the area started 4 ka BP in the Holocene. The low tectonic subsidence rate of the Red River delta led to intensive erosion and replacement of fine grained sediments of the sea level high stand by sandy channel belts, resulting in hydraulic connections between the Pleistocene and Holocene aquifers. The Pleistocene aquifer is recharged by both regional flow paths and naturally occurring vertical recharge through Holocene sand and clay layers. Young groundwater (<40 a) in the shallow Holocene aquifer generally discharges to surface water bodies. The shallow flow system is also seasonally recharged with surface water, as indicated by δ18O enrichment of groundwater and oscillating groundwater ages in wells in the vicinity of water channels. The deeper flow system discharges into the Red River and Day River or flows parallel to the rivers, toward the sea. The overall pattern of arsenic groundwater concentrations (decreasing with increasing sediment age) is modified by groundwater flow. The arsenic contamination of the Pleistocene aquifer of the Red River delta is not only caused by the intensive groundwater abstraction in Hanoi, as reported previously, but also by the natural flow of high arsenic groundwater from Holocene to Pleistocene aquifers in areas located outside of the depression cone. Groundwater with < 50 µg L-1 arsenic is found in the Pleistocene aquifer close to the recharge zone in the mountains bordering the Red River delta and in the Holocene and Pleistocene aquifers where clay deposits were eroded. Close to the recent Red River channel, recharge of older Holocene and Pleistocene sediments occurs partially by arsenic-contaminated groundwater from the youngest Holocene aquifers, and here arsenic concentrations exceed 50 µg L-1. A high arsenic concentration is also present in the early Holocene-Pleistocene aquifer, beneath thick clay layers, indicating a limited extent of flushing and the inflow of fresh organic matter. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

The present study investigates the bioavailability, soil to plant transfer and health risks of arsenic (As) in the coastal part of Chianan Plain in southwestern Taiwan. Groundwater used for irrigation, surface soils from agricultural lands and locally grown foodstuffs were collected from eight locations and analyzed for As to assess the risks associated with consuming these items. The concentration of As in groundwater ranged from 13.8 to 881μg/L, whereas surface soil showed total As content in the range of 7.92–12.7mg/kg. The available As content in surface soil accounted for 0.06–6.71% of the total As content, and was significantly correlated with it (R 2 =0.65, p <0.05). Among the leachable fraction, the organic matter (3.23–54.8%) and exchangeable portions of oxides (6.03–38.4%) appear to be the major binding phases of As. The average As content in fourteen studied crops and vegetables varied from 10.3 to 151μg/kg with maximum in mustard and minimum in radish. All the plants showed considerably higher As content (21.5±3.64–262±36.2μg/kg) in their roots compared to the edible parts (9.15±1.44–75.8±22.9μg/kg). The bioaccumulation factor (BAF) based on total As (ranging from 0.0009 to 0.144) and available As in soil (ranging from 0.039 to 0.571) indicate that mustard, rice, amaranth and spinach are the highest accumulators of As. Although the health risk index (HRI) of the studied crops and vegetables ranged from only 0.0068–0.454, with the maximum in rice, the combined HRI indicates an alarming value of 0.88. Therefore, the possible health risks due to long-term consumption of rice and other As-rich foodstuffs could be overcome by controlling the contamination pathways in the water–soil–plant system. [ABSTRACT FROM AUTHOR]

Groundwater arsenic survey in Cachar and Karimganj districts of Barak Valley, Assam shows that people in these two districts are drinking arsenic-contaminated (max. 350 μg/l) groundwater. 66% of tubewells in these two districts have arsenic concentration above the WHO guideline value of 10 μg/l and 26% tubewells have arsenic above 50 μg/l, the Indian standards for arsenic in drinking water. 90% of installed tubewells in these two districts are shallow depth (14-40 m). Shallow tubewells were installed in Holocene Newer Alluvium aquifers are characterised by grey to black coloured fine grained organic rich argillaceous sediments and are mostly arsenic contamination in groundwater. Plio-Pleistocene Older Alluvium aquifers composed of shale, ferruginous sandstone, mottle clay, pebble and boulder beds, which at higher location or with thin cover of Newer Alluvium sediments are safe in arsenic contamination in groundwater. 91% of tubewell water samples show significantly higher concentrations of iron beyond its permissible limit of 1 mg/l. The iron content in these two districts varies from 0.5 to as much as 48 mg/l. Most of the arsenic contaminated villages of Cachar and Karimganj districts are located in entrenched channels and flood plains of Newer Alluvium sediments in Barak-Surma-Langai Rivers system. However, deeper tubewells (>60 m) in Plio-Pleistocene Older Alluvium aquifers would be a better option for arsenic-safe groundwater. The arsenic in groundwater is getting released from associated Holocene sediments which were likely deposited from the surrounding Tertiary Barail hill range. [ABSTRACT FROM AUTHOR]

Abstract: The distribution of As-pollution in groundwater of the deltaic aquifers of south-eastern Asia may be controlled by the subsurface distribution of palaeo-channel sediments (As-polluted groundwaters) and palaeo-interfluvial sediments (As-free groundwaters). To test this idea, termed the palaeosol model of As-pollution, we drilled 10 sites, analysed groundwater from 249 shallow wells (screened <107mbgl), field-tested another 149 for As, and used colour as a guide to the presence or absence of As-pollution in a further 531 wells. Our work was conducted along a 32-km traverse running W to E across southern West Bengal, India. At seven drill sites we logged a palaeo-interfluvial sequence, which occurs as three distinct units that together occupy 20km of the traverse. These palaeo-interfluvial sequences yield As-free groundwaters from brown sands at depth<100m. The palaeo-interfluvial sequences are separated by two deep palaeo-channels, which were logged at 3 sites. The palaeo-channel deposits host As-polluted groundwater in grey sands. Our findings confirm the predictions of the palaeosol model of As-pollution. We show again that well-colour can be used both to successfully predict the degree of As-pollution in groundwater, and to locate regions of buried palaeo-interfluve that will yield As-free groundwater for the foreseeable future. [Copyright &y& Elsevier]

Abstract: One borehole was installed in the Blackfoot Disease (BFD) area to collect organic-carbon and metal-rich sediments from various depths. Carbon- and metal-rich groundwaters were also collected to quantify arsenic and humic substances from different parts of the BFD area in Chianan Plain. UV–visible spectroscopy was performed to detect the presence of iron in humic substances. Fourier Transform Infrared (FTIR) spectra showed that the functional groups such as COO−, Cd Cic ring) were present in humic substances of southwestern Taiwan, and they act as chelating agents to bind with trace elements in sediments. Nuclear magnetic resonance (NMR) spectroscopy also suggests that signals peaking at 30, 58, 68, 101.22, 168.23 and 210ppm were assigned to the presence of aliphatic carbon, N-alkayl C, O-alkayl C, anomeric carbon, carboxyl carbon, and ketonic carbon of functional groups, respectively. The binding nature of arsenic with humic substances was also characterized using FTIR microscopy. FTIR spectra of laboratory-synthesized As–Fe–humic substance (HS) complexes were compared to those of Fe–HS complex and humic substances in order to demonstrate the presence of binary Fe–HS and ternary As–Fe–HS complexes in the aquifer. Organometallic complex (humic substances in combination with arsenic) in groundwater of southwestern Taiwan may cause BFD. [Copyright &y& Elsevier]

This study was conducted to investigate the relationships between arsenic () levels in human hair and demographic factors on accumulation in humans. Hair samples of 619 subjects (383 females and 236 males), including 390 confirmed arsenicosis patients, 11 fish and six vegetable samples were analyzed using the Proton Induced X-ray Emission (PIXE) method. The results revealed that in hair accumulates in different patterns for males and females in accordance with age. Within patient groups, male patients accumulated higher amounts of than female patients. For all individuals, middle aged females accumulated higher amounts in hair than children, adolescents and females over 50 years old, while children, adolescents and males over 50 accumulated higher amounts of than middle aged males in both villages. Surprisingly, high levels of concentrations were found in hair samples of those living near safe drinking water in both villages, irrespective of age and sex. There were weak positive correlations between hair levels and groundwater levels in both villages. Furthermore, toxic levels of were found in different kinds of food products. Taken together, these findings demonstrate that accumulation in hair is dependent not only on water contamination, but also on food contamination. [ABSTRACT FROM AUTHOR]

Aquacultural tilapia (Oreochromis mossambicus L.) and shrimp (Penaeus monodon L.) from groundwater-cultured ponds in southwestern Taiwan were analyzed to estimate arsenic (As) bioaccumulation and the potential health risk to human intake. Most of aquacultural ponds exhibited higher arsenic than maximum allowed concentrations (50 μg L−1) in pond water of Taiwan. Arsenic levels in tilapia in Budai, Yichu and Beimen were 0.92 ± 0.52 μg g−1, 0.93 ± 0.19 μg g−1 and 0.76 ± 0.03 μg g−1, respectively and in shrimp was 0.36 ± 0.01 μg g−1 in Beimen. Total arsenic in tilapia is highly correlated (R2 = 0.80) with total arsenic concentration of pond water. Total arsenic in fish showed high correlation with that in bone (R2 = 0.98), head (R2 = 0.97) and tissue (R2 = 0.96). Organic arsenic species (DMA) was found higher relative to inorganic species of As(III) and As(V). The average percent contribution of inorganic arsenic to total arsenic in fish samples was 12.5% and ranged between 11.7 to 14.2%. Bioaccumulation factors (BAFs) for total arsenic in fish ranged from 10.3 to 22.1, whereas BAF for inorganic arsenic ranged from 1.33 to 2.82. The mean human health cancer risk associated with the ingestion of inorganic arsenic in the fish was estimated at 2.36 × 10−4 ± 0.99 × 10−4, which is over 200 times greater than a de Minimus cancer risk of 1 × 10−6. The mean human health hazard quotient associated with ingesting inorganic arsenic in the fish was 1.22 ± 0.52, indicating that expected human exposure exceeds the reference dose for non-cancer health effects by 22%. These results suggest that the inhabitants in this region are being subjected to moderately elevated arsenic exposure through the consumption of tilapia and shrimp raised in aquaculture ponds. [ABSTRACT FROM AUTHOR]

This study was carried out to investigate biological and statistical nature of arsenicosis risks on a severely arsenic (As) affected population in the Banglish village in Comilla district of Bangladesh. Subject's hair As and vegetables As levels were estimated by PIXE method, and arsenicosis status of the subject was examined by a physician. Arsenicosis was found in 45% female and 61% male subjects. Arsenic levels > 0.50 ppm showed greater arsenicosis risks that significantly varied with subject's sex and age. Unlike children, young (≥10 years) to middle aged (~49 years) population were significantly affected by arsenicosis. Logistic regression model analysis showed that arsenicosis risks are not only the devastating toxicity of As accumulation in human body through ingestion of contaminated drinking water, but the summation of all sources of As accumulation. Human hair As levels were found as the biomarker ranging from 0.03-8.3 μg/g in stage I arsenicosis patients, and from 1.1-56.9 ppm in the stage II patients up to 49 age. Hair As level was declined to 2.2-25.5 ppm at > 49 aged population. The findings inferred that groundwater As affecting the public health through contaminating total environment, that in turn posing an alarming situation in the study area. [ABSTRACT FROM AUTHOR]

Sediments from shallow aquifers in Bengal Delta, India have been found to contain arsenic. Rivers of Ganga-Brahmaputra system, responsible for depositing these sediments in the delta, have created a store of arsenic. Geomorphological domains with different depositional styles regulate the pattern of distribution of zones with widely different content of groundwater arsenic. The high arsenic zones occur as narrow sinuous strips confined to channel deposits. A few iron-bearing clastic minerals and two post-depositional secondary products are arsenic carriers. Secondary siderite concretions have grown on the surface of the clastic carriers in variable intensity. The quantity of arsenic in all clastic carriers is in excess of what is generally expected. Excess arsenic is contributed by the element adsorbed on the concretion grown on the surface of the carriers, which adds up to the arsenic in the structure of the minerals. Variable abundance of concretions is responsible for the variable quantity of arsenic in the carriers and the sediment samples. Fe2+ for the growth of siderite concretions is obtained from the iron-bearing clastic carriers. The reaction involves reduction of trivalent iron to bivalent and the required electron is obtained by transformation of As3+ to As5+. It is suggested that oxidation of As3+ to As5+ is microbially mediated. In the Safe zone arsenic is retained in the carriers and groundwater arsenic is maintained below 0.05 mg/l. In the Unsafe zone sorbed arsenic is released from the carriers in the water through desorption and dissolution of concretion, thereby elevating the groundwater arsenic level to above 0.05 mg/l. [ABSTRACT FROM AUTHOR]

Many sedimentary aquifers across the globe are contaminated with As, a known toxicant and carcinogen, thereby making millions of people vulnerable to a health hazard. Although the source of As enrichment is being linked to the provenance of the sediments (i.e. hard rocks), there are limited studies of groundwater As enrichment in hard rock areas. In this study, we collected the groundwater samples from complex hard rock aquifer system across the Himalayan collision zone in Upper Indus River Basin (UIRB) to understand the source and to evaluate the hydrogeochemical processes responsible for groundwater As mobilization. The study suggests that groundwater As enrichment comes from volcanic rocks and ophiolitic mélange. Such, As contaminated aquifers are widely used for groundwater exploitation for drinking and other purposes in Ladakh (UIRB). The results indicated that As enrichment occurs with an increase in temperature and depth. The results also indicated that the groundwater in Ladakh is described by multiple hydrochemical facies with pH circum-neutral to alkaline in nature. Higher pH, Fe2+ and Mn and lower ORP, NO 3 −, and SO 4 2− in groundwater are found to be associated with high As groundwater indicating metal oxides/hydroxides under reducing conditions are predominantly controlling the groundwater As mobilization. • 70% of groundwater samples have As concentration above the WHO prescribed limit. • Groundwater As contamination is geogenic in nature, mostly found in volcanics and ophiolitic melange aquifers. • Arsenic contamination in groundwater increased with increasing depth and temperature. • Arsenic release was mainly caused by reduction of iron oxides followed by desorption. [ABSTRACT FROM AUTHOR]