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

S Kumar,D Ghosh,M.E Donselaar,F Burgers,A.K Ghosh

doi:10.1016/j.catena.2021.105255

S Kumar, D Ghosh + Show 3 more

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https://doi.org/10.1016/j.catena.2021.105255

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Abstract

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.

Highlights

Arsenic-bearing aquifers pose a worldwide, severe health hazard for many millions of people who lack access to controlled piped water systems for consumption and irrigation (Smith et al, 2000)
Shallow aquifers in many Holocene alluvial basins around the world have in the last three decades been iden tified as arsenic pollution hotspots, in which the spatial variation of natural arsenic concentration is conditioned by the meandering-river geomorphology and the fluvial lithofacies distribution
Common denominator is the large spatial variation in concentrations of naturally occurring arsenic in the groundwater, which is controlled by the geomorphology of the river landscape and by the spatial distri bution of sedimentary facies and inherent porosity and permeability anisotropy

Summary

Introduction

Arsenic-bearing aquifers pose a worldwide, severe health hazard for many millions of people who lack access to controlled piped water systems for consumption and irrigation (Smith et al, 2000). In terms of geomor phological setting, many of the polluted aquifers occur in Holocene meandering-river landscapes (Zheng et al, 2005; Shah, 2008; Weinman et al, 2008; Sahu and Saha, 2015; Donselaar et al, 2017). Common denominator is the large spatial variation in concentrations of naturally occurring (or: geogenic) arsenic in the groundwater, which is controlled by the geomorphology of the river landscape and by the spatial distri bution of sedimentary facies and inherent porosity and permeability anisotropy (Weinman et al, 2008; Donselaar et al, 2017; Jakobsen et al, 2018). In addition to the Ganges-Brahmaputra-Meghna Delta, examples of similar geomorphological setting are the Mekong River, Cambodia (Richards et al, 2018); Red River, Vietnam (Berg et al, 2007; Postma et al, 2012, 2016; Stopelli et al, 2020; Trung et al, 2020; Wallis et al, 2020), Pearl River, China (Huang et al, 2011); Hetao Basin, Inner Mongolia (Guo et al, 2008; Cao et al, 2018), Río Dulce, Argentina (Bundschuh et al, 2004; Bhattacharya et al, 2006), Pazna River, Bolivia (Ramos et al, 2014), Lower Katari Basin, Bolivian Altiplano (Quino Lima et al, 2021)

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