Sub-surface Biogeochemical Characteristics and Its Effect on Arsenic Cycling in the Holocene Gray Sand Aquifers of the Lower Bengal Basin

High arsenic (As) content in the fertile delta plains of West Bengal has been widely reported since the 1990s. The shallow gray sand aquifers (GSA) deposited during the Holocene, are more commonly used as potable water sources, but they have high As levels. The release of As into groundwater is influenced by indigenous microbial communities metabolizing different organic carbon sources present in the GSA sediments. After pre-screening the groundwater for assessing their microbial phylogenetic diversity, two 50-m deep boreholes were drilled in the GSAs, and 19 sediment samples were recovered from each core. In each of these samples, grain-size distribution, sequential extraction, and quantification of trace metals and total extractable lipids were analyzed. The aquifer sediments consisted of medium to fine micaceous sand with clay lenses in between them; a thick clay layer occurred on top of both boreholes. Arsenic concentration in these sediments varied from 1.80 to 41.0 mg/kg and was mostly associated with the oxide and silicate-rich crystalline minerals. Arsenic showed a significant correlation with Fe in all fractions, suggesting the presence of Fe-(oxy)-hydroxides bound As minerals. The diagnostic lipid biomarkers showed presence of compounds derived from higher plants (epicuticular waxes) and microbial inputs. The biomarkers were abundant in clay and silt-rich layers. The samples indicated preferential preservation of n-alkanes over other functional compounds (e.g., alcohols and fatty acids), that are more reactive, and hence subject to further degradation. Sediments recovered from the borehole indicated the presence of Eustigmatophytes and vascular plant waxes that are mostly surface-derived. The sedimentary lipids also indicated the presence of complex petroleum-derived hydrocarbons. These compounds provide organic substrates, and support the preferential survival of specific microbial communities in these sediments

Sub-surface Biogeochemical Characteristics and Its Effect on Arsenic Cycling in the Holocene Gray Sand Aquifers of the Lower Bengal Basin

High arsenic (As) content in the fertile delta plains of West Bengal has been widely reported since the 1990s. The shallow grey sand aquifers (GSA) deposited during the Holocene, are more commonly used as potable water sources, but they have high As levels. The release of As into groundwater is influenced by indigenous microbial communities metabolizing different organic carbon sources present in the GSA sediments. After pre-screening the groundwater for assessing their microbial phylogenetic diversity, two50-m deep boreholes were drilled in the GSAs, and 19 sediment samples were recovered from each core. In each of these samples, grain-size distribution, sequential extraction, and quantification of trace metals and total extractable lipids were analyzed. The aquifer sediments consisted of medium to fine micaceous sand with clay lenses in between them; a thick clay layer occurred on top of both boreholes. Arsenic concentration in these sediments varied from 1.80 to 41.0 mg/kg and was mostly associated with the oxide and silicate-rich crystalline minerals. Arsenic showed a significant correlation with Fe in all fractions, suggesting the presence of Fe-(oxy)-hydroxides bound As minerals. The diagnostic lipid biomarkers showed presence of compounds derived from higher plants (epicuticular waxes) and microbial inputs. The biomarkers were abundant in clay and silt-rich layers. The samples indicated preferential preservation of n-alkanes over other functional compounds (e.g. alcohols and fatty acids), that are more reactive, and hence subject to further degradation. Sediments recovered from the borehole indicated the presence of Eustigmatophytes and vascular plant waxes that are mostly surface-derived. The sedimentary lipids also indicated the presence of complex petroleum-derived hydrocarbons. These compounds provide organic substrates, and support the preferential survival of specific microbial communities in these sediments

Temporal dynamics of arsenic uptake and distribution: food and water risks in the Bengal basin

Contaminated food chain is a serious contender for arsenic (As) uptake around the globe. In Nadia, West Bengal, we trace possible means of transfer of As from multiple sources reaching different trophic levels, and associated seasonal variability leading to chronic As uptake. This work considers possible sources-pathways of As transfer through food chain in rural community. Arsenic concentration in groundwater, soil, rice, and vegetable-samples collected detected in different harvest seasons of 2014 and 2016. Arsenic level in shallow groundwater samples ranged from 0.1 to 354 mu g/L, with 75% of the sites above the prescribed limit by WHO (10 mu g/L) during the boro harvest season. High soil As content (similar to 20.6 mg/kg), resulted in accumulation of As in food crops. A positive correlation in As conc. with increase over period in all sites indicating gradual As accumulation in topsoil. Unpolished rice samples showed high As content (similar to 1.75 mg/kg), polishing reduced 80% of As. Among vegetables, the plant family Poaceae with high irrigation requirements and Solanaceae retaining high moisture, have the highest levels of As. Contaminated animal fodder (Poaceae) and turf water for cattle are shown to contaminate milk (0.06 to 0.24 mu g/L) and behoves strategies, practices to minimize As exposure.Funding Agencies|Swedish Research Link-Asia Program [348-20096470]; Department of Science and Technology, Government of IndiaDepartment of Science &amp; Technology (India) [DST/INSPIRE/04/2015/002362]</p

Elemental and biomarker characteristics in a Pleistocene aquifer vulnerable to arsenic contamination in the Bengal Delta Plain, India

An elevated level of arsenic (As) in the Indo-Gangetic delta plain aquifers has been reported since the 1990s. Organic matter (OM) present in groundwater and aquifer sediments supports the microbial communities in these aquifers. During installation of a drinking water well, 26 sediment intervals of 6 m each were retrieved up till 156 m from a Pleistocene brown sand aquifer (BSA). Grain size distribution, sequential extraction of metals and total extractable lipids were analyzed in each sample. These parameters were statistically correlated in order to establish relationship between the physical vs. inorganic and organic characteristics, and how these properties affected the distribution of As in BSAs. The aquifer sediments consisted of medium to coarse sand except the surface sediments and those at the bottom of the well, which had high clay and slit content. Arsenic (As) concentration in sediments ranged from 2 to 21 mg/kg and indicated a strong correlation with grain size. Arsenic was mostly associated with crystalline oxides and silicate-rich minerals. Arsenic showed significant correlation with Fe in all fractions, and suggests presence of pyrite bound As-bearing minerals in these sediments. The diagnostic sedimentary lipid biomarkers indicated presence of compounds derived from vascular plants and microbial cell wall. This inference was supported by various diagnostic lipid ratios. The biomarkers were abundant in surface and deeper layers, which had high clay and silt content. The BSA sediments indicated preferential preservation of n-alkanes over other functional compounds, which were more reactive and subject to degradation. The thick clay layer at 132-156 m contained visible plant fragments, and OM in this layer indicated preferential preservation of organic carbon most likely due to the absence of specific microbial communities that degraded these compounds and mobilized As. Statistical analyses indicated the influence of selective inorganic and organic components (As, Fe and fatty acids) controlling the co-distribution of various inorganic and organic components in the aquifer.Funding Agencies|Department of Science and Technology, Government of India; Swedish Research Link-Asia Program [2009-6470]; Linkoping University, Sweden</p

Organic Carbon transport model of abandoned river channels - A motif for floodplain geomorphology influencing biogeochemical swaying of arsenic

Meandering-river geomorphology, forming abandoned channels/lakes with organic carbon-burial and microbial reductive dissolution, play many crucial roles in controlling arsenic (As) fluxes in sinks such as contaminated aquifers of riverine alluvial plains across the world. Suhiya oxbow-lake in the middle alluvial plain of the River Ganga, was selected as the natural laboratory. A top-down multidisciplinary approach was chosen employing satellite imagery to analyse the annual oxbow-lake surface vegetation dynamics (Eichhornia and Hydrilla). Side-scan sonar profiles across two oxbow lakes along with River Ganga core data and vintage topographical maps, estimated the lake-sedimentation rate of 9.6 cm/yr. Organic carbon [amino acids, aromatics, lingo-phenols and lipids hydrocarbons] infiltration-based on hydrophobicity and molecular-mass was detected at different depths along the water and sedimentary column. Elemental analysis showed lake surface to groundwater the As conc. varied from (0.37 to 185 μg/l). A microbial diversity based study showed that large sized photoautotrophs Nostoc, Anabaena are replaced by Fe-oxido-reducing As-metabolizing bacteria e.g. Acidovorax, Dechloromonas and enteric organisms e.g. Enterobacter, Salmonella at bottom of water column. Based on these inferences, a conceptual organic carbon transport model was constructed to understand the preferential preservation and microbial diagenesis resulting in mobilization of As and other geogenic elements

Clay-plug sediment as the locus of arsenic pollution in Holocene alluvial-plain aquifers

Shallow aquifers in many Holocene alluvial basins around the world have in the last three decades been identified as arsenic pollution hotspots, in which the spatial variation of natural (or: geogenic) arsenic concentration is conditioned by the meandering-river geomorphology and the fluvial lithofacies distribution. Despite the large amount of publications on the specifics of the pollution, still many uncertainties remain as to the provenance and processes that lead to arsenic enrichment in aquifers. In this paper, arsenic in abandoned and sediment-filled meandering-river bends (or: clay-plugs) is highlighted as a primary source of aquifer pollution. The combination of high organic-carbon deposition rates and the presence of chemically-bound natural arsenic in sediment of this specific geomorphological setting creates the potential for microbially-steered reductive dissolution of arsenic in an anoxic environment, and subsequent migration of the desorbed arsenic to, and stratigraphic entrapment in, adjacent sandy point-bar aquifers. To assess the magnitude of the arsenic source in clay-plug, bulk sediment volume calculations were made of twenty clay plugs on the Middle Ganges Plain of Bihar (India), by combining clay-plug surface area analysis of Sentinel-2 satellite data, side-scan sonar depth profiling of oxbow lakes and the Ganges River, and sedimentological data from five cored shallow wells. ICP-MS based elemental analysis of 36 core sub-samples, complemented with published concentration data in a similar geomorphological setting in West Bengal, India, yielded an average arsenic content of 28.75 mg/kg sediment in the 12-m-thick clay plugs, which amounts to a total arsenic volume of 0.07 – 3.13 . 106 kg per clay plug. A scenario is presented for the release of arsenic from the clay-plug sediment by microbial metabolism, followed by migration of the desorbed arsenic to the bordering point-bar sands.Applied Geolog

Microbial mitigation of hazardous compounds in agro-ecosystems

Currently, many agro-ecosystems are contaminated with toxic, hazardous compounds which can be broadly categorized as (a) anthropocentrically introduced xenobiotic agrochemicals (e.g., pesticides, insecticides, and fungicides), (b) secondary metabolites produced as a result of plant-microbe interactions, and (c) heavy metals which are mainly introduced through natural and anthropogenic processes. Organophosphates (OP), carbamates, pyrethroids, and neonicotinoids are among the most dominant groups of xenobiotic agrochemicals reported globally. These are highly toxic to non-target organisms including humans and many have been banned by the US Environmental Protection Agency or the European Union. Among these, neonicotinoids were introduced recently (in the 1990s), being comparatively less toxic to non-target higher vertebrates and most extensively used (149 crops in 120 countries). An example of a naturally produced hazardous compound is Deoxynivalenol (vomitoxin, DON), which is a highly toxic secondary metabolite (mycotoxin) produced by the mold Fusarium while infecting staple crops. Finally, Pb, Cr, Cd, As, Zn, Cu, and Fe are commonly encountered heavy metals in agroecosystems globally