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The influence of aquifer sediments provenance and geochemical processes on groundwater solute chemistry, including the fate of groundwater arsenic (As) in aquifers for three distinct tectono-morphic zones (north-western, northern, and southern [NW, N, and S] in the basin of Himalayan Mega Rivers, i.e., Brahmaputra river basin (BRB), and adjoining the Himalayas and Indo-Burmese ranges, have been delineated. The presence of fluvial re-worked quartz, feldspar, and mica along with orogeny-sourced ferromagnesian minerals contained aquifers sediments in the NW and N regions are hypothesized to be derived from the Himalayan system. The S-aquifers mineralogy differs from northern aquifers by a higher proportion of Fe/Mg aluminosilicates, phyllosilicates/clay minerals, derived from the Indo-Burmese ranges. The dissolved As distribution is highly variable among the different alluvial aquifers. The S-region is highly enriched in groundwater As (bdl to 5.53 μM or 415 μg/L, mean 1.77 μM) compared to NW and N regions (bdl to 1.8 μM or 134 μg/L, mean 0.28 μM [60% As-contaminated samples]; bdl to 2.45 μM or 184 μg/L, mean 0.68 μM [65% As-contaminated samples] respectively). More than 92% of groundwater samples in the S-region are enriched with As (>50 μg/L), which draws a distinct difference from the NW and N regions of BRB aquifers. Reaction-path models suggest intense chemical weathering of S-aquifer matrix due to the higher equilibrium (more stable) of secondary mineral phases with the solution (groundwater), as compared to NW and N regions. The present study proposes a model for the geological control and effect of weathering/water–sediment reaction on As-fate and mobilization mechanism in the groundwater system of the barely studied Brahmaputra river basin aquifers. Image 1 • Water-sediment interaction dominates groundwater evolution. • A model of arsenic (As) influx and mobilization in study area is proposed. • Geology is hypothesized to be a major influencing factor for hydrogeochemistry • Tectonics influence the sediment provenance for groundwater arsenic content. [ABSTRACT FROM AUTHOR]

Summary Arsenic (As) is a trace element in the Earth’s crust. However, its presence in elevated concentrations in groundwaters of major aquifers around the world raises concern about its primary source(s). A close look at the global distribution of known As enriched areas reveals an intriguing systematic pattern, where most of the As enriched aquifers are parts of large sedimentary basins adjoining major orogenic belts, suggesting the existence of a large-scale geological process. Many of these sedimentary basins may be tectonically regarded as foreland basins that developed by lithospheric flexure at the time of mountain building processes (orogenesis) along convergent plate boundaries. Arsenic enrichment in the groundwater of these foreland basin aquifers may be ultimately sourced to crustal evolution processes related to plate tectonics, along with transportation of As-enriched magmatic rocks from the depth to surficial deposits, which subsequently release or mobilize the As to groundwater under conducive surficial biogeochemical processes. Circulating As-laden hydrothermal fluids may also be derived from magmas and form part of the discharge in the surficial hot springs in arc environments. These hydrothermal-genic deposits in orogenic belts may also act as the primary provenance for the As-laden sediments that are transported into foreland basins by wind, glacial erosion, and/or streams. Ultimately, the As-laden foreland sediments serve as modern-day aquifers, where the sediments release As into groundwater by water–rock interaction during various biogeochemical processes under conducive hydrogeochemical conditions. The significance of this hypothesis is that it proposes the existence of a common primary source for globally dispersed geogenic As-enriched aquifers, that have been so far mostly studied as individual occurrences. The proposed hypothesis can explain the widespread presence of As in areas as diverse as the Indus–Ganges–Brahmaputra basin (Himalayan orogen), the Chaco-Pampean basin (Andean orogen), Rocky mountain basin (Western Cordilleran orogen), New England and northeastern USA (the Applachian orogen). [ABSTRACT FROM AUTHOR]

Abstract: The deeper groundwater (depending on definition) of the Bengal basin (Ganges–Brahmaputra delta) has long been considered as an alternate, safe drinking-water source in areas with As-enrichment in near-surface groundwater. The present study provides the first collective discussion on extent and controls of elevated As in deeper groundwater of a regional study area in the western part of the Bengal basin. Deeper groundwater is defined here as non-brackish, potable (Cl− ⩽250mg/L) groundwater available at the maximum accessed depth (∼80–300m). The extent of elevated As in deeper groundwater in the study area seems to be largely controlled by the aquifer–aquitard framework. Arsenic-enriched deeper groundwater is mostly encountered north of 22.75°N latitude, where an unconfined to semi-confined aquifer consisting of Holocene- to early Neogene-age gray sand dominates the hydrostratigraphy to 300m depth below land surface. Aquifer sediments are not abnormally enriched in As at any depth, but sediment and water chemistry are conducive to As mobilization in both shallow and deeper parts of the aquifer(s). The biogeochemical triggers are influenced by complex redox disequilibria. Results of numerical modeling and profiles of environmental tracers at a local-scale study site suggest that deeper groundwater abstraction can draw As-enriched water to 150m depth within a few decades, synchronous with the advent of wide-scale irrigational pumping in West Bengal (India). [Copyright &y& Elsevier]

Abstract: Elevated As concentrations in groundwater of the Huhhot basin (HB), Inner Mongolia, China, and the western Bengal basin (WBB), India, have been known for decades. However, few studies have been performed to comprehend the processes controlling overall groundwater chemistry in the HB. In this study, the controls on solute chemistry in the HB have been interpreted and compared with the well-studied WBB, which has a very different climate, physiography, lithology, and aquifer characteristics than the HB. In general, there are marked differences in solute chemistry between HB and WBB groundwaters. Stable isotopic signatures indicate meteoric recharge in the HB in a colder climate, distant from the source of moisture, in comparison to the warm, humid WBB. The major-ion composition of the moderately reducing HB groundwater is dominated by a mixed-ion (Ca–Na–HCO3–Cl) hydrochemical facies with an evolutionary trend along the regional hydraulic gradient. Molar ratios and thermodynamic calculations show that HB groundwater has not been affected by cation exchange, but is dominated by weathering of feldspars (allitization) and equilibrium with gibbsite and anorthite. Mineral weathering and mobilization of As could occur as recharging water flows through fractured, argillaceous, metamorphic or volcanic rocks in the adjoining mountain-front areas, and deposits solutes near the center of the basin. In contrast, WBB groundwater is Ca–HCO3-dominated, indicative of calcite weathering, with some cation exchange and silicate weathering (monosiallitization). [Copyright &y& Elsevier]

Regularmonitoring of arsenic (As) in groundwater is crucial frompublic health perspectives as millions of people are suffering due to use of contaminated aquifer water for drinking purposes. The routine analyses, especially in developing nations, are mostly done in localised government/non-government laboratories with limited resources, having the target of analysing large number of samples in each run. Thus apart from analytical sensitivity, cost-effectiveness of the method and eco-friendliness of the experimental operation are key surreptitious factors. This demands optimisation of total As measurement methods and finding a method that gives 'optimumbenefit' considering all these factors together. The present study therefore evaluates four common As (total) measurement methods [iodometric-colorimetric method, silver diethyl dithiocarbamate method, molybdenum blue method and hydride generation atomic absorption spectrophotometric (HG-AAS) method] practised in the Bengal Delta Plain, in view of their analytical sensitivity, related environmental hazard and experimental costs. It was found that the HG-AAS method is analytically more sensitive, whereas the iodometric-colorimetricmethod and the molybdenum blue method are better choices in terms of eco-friendliness and cost-effectiveness, respectively. However, when all three factors (analytical reliability, environmental hazard and cost) are considered simultaneously, themolybdenumbluemethodwas found to be placed first in the 'optimum performance rank' list. It was also found that both environmental hazard and cost play a more crucial role than analytical reliability, although this is case specific andwould differ from place to place around the globe. Finally based on the results, we have hypothesised a water testing strategy for developing countries such as India where the molybdenum blue method can be adapted as a screening method and later the HG-AASmethod can be used to precisely identify the groundwater samples with As concentration below the WHO drinking water guideline value of 10 μg/L. [ABSTRACT FROM AUTHOR]

A low-cost rapid screening tool for arsenic (As) and manganese (Mn) in groundwater is urgently needed to formulate mitigation policies for sustainable drinking water supply. This study attempts to make statistical comparison between tubewell (TW) platform color and the level of As and Mn concentration in groundwater extracted from the respective TW (n = 423), to validate platform color as a screening tool for As and Mn in groundwater. The result shows that a black colored platform with 73% certainty indicates that well water is safe from As, while with 84% certainty a red colored platform indicates that well water is enriched with As, compared to WHO drinking water guideline of 10 μg/L. With this guideline the efficiency, sensitivity, and specificity of the tool are 79%, 77%, and 81%, respectively. However, the certainty values become 93% and 38%, respectively, for black and red colored platforms at 50 μg/L, the drinking water standards for India and Bangladesh. The respective efficiency, sensitivity, and specificity are 65%, 85%, and 59%. Similarly for Mn, black and red colored platform with 78% and 64% certainty, respectively, indicates that well water is either enriched or free from Mn at the Indian national drinking water standard of 300 μg/L. With this guideline the efficiency, sensitivity, and specificity of the tool are 71%, 67%, and 76%, respectively. Thus, this study demonstrates that TW platform color can be potentially used as an initial screening tool for identifying TWs with elevated dissolved As and Mn, to make further rigorous groundwater testing more intensive and implement mitigation options for safe drinking water supplies. [ABSTRACT FROM AUTHOR]

Abstract: Arsenic (As) induced identifiable health outcomes are now spreading across Indian subcontinent with continuous discovery of high As concentrations in groundwater. This study deals with groundwater hydrochemistry vis-à-vis As exposure assessment among rural population in Chakdaha block, West Bengal, India. The water quality survey reveals that 96% of the tubewells exceed WHO guideline value (10 μg/L of As). The groundwaters are generally anoxic (−283 to −22 mV) with circum-neutral pH (6.3 to 7.8). The hydrochemistry is dominated by HCO3 − (208 to 440mg/L), Ca2+ (79 to 178mg/L) and Mg2+ (17 to 45mg/L) ions along with high concentrations of AsT (As total, below detection limit to 0.29mg/L), FeT (Fe total, 1.2 to 16mg/L), and Fe(II) (0.74 to 16mg/L). The result demonstrates that Fe(II)–Fe(III) cycling is the dominant process for the release of As from aquifer sediments to groundwater (and vice versa), which is mainly controlled by the local biogeochemical conditions. The exposure scenario reveals that the consumption of groundwater and rice are the major pathways of As accumulation in human body, which is explained by the dietary habit of the surveyed population. Finally, regular awareness campaign is essential as part of the management and prevention of health outcomes. [ABSTRACT FROM AUTHOR]