Dual in-aquifer and near surface processes drive arsenic mobilization in Cambodian groundwaters

Laura A Richards,Daniel Magnone,Jürgen Sültenfuß,Lee Chambers,Charlotte Bryant,Adrian J Boyce,Bart E Van Dongen,Christopher J Ballentine,Chansopheaktra Sovann,Sebastian Uhlemann,Oliver Kuras,Daren C Gooddy,David A Polya

doi:10.1016/j.scitotenv.2018.12.437

Abstract

Millions of people globally, and particularly in South and Southeast Asia, face chronic exposure to arsenic from reducing groundwaters in which. Arsenic release to is widely attributed largely to reductive dissolution of arsenic-bearing iron minerals, driven by metal reducing bacteria using bioavailable organic matter as an electron donor. However, the nature of the organic matter implicated in arsenic mobilization, and the location within the subsurface where these processes occur, remains debated. In a high resolution study of a largely pristine, shallow aquifer in Kandal Province, Cambodia, we have used a complementary suite of geochemical tracers (including 14C, 3H, 3He, 4He, Ne, δ18O, δD, CFCs and SF6) to study the evolution in arsenic-prone shallow reducing groundwaters along dominant flow paths. The observation of widespread apparent 3H-3He ages of <55years fundamentally challenges some previous models which concluded that groundwater residence times were on the order of hundreds of years. Surface-derived organic matter is transported to depths of >30m, and the relationships between age-related tracers and arsenic suggest that this surface-derived organic matter is likely to contribute to in-aquifer arsenic mobilization. A strong relationship between 3H-3He age and depth suggests the dominance of a vertical hydrological control with an overall vertical flow velocity of ~0.4±0.1m·yr−1 across the field area. A calculated overall groundwater arsenic accumulation rate of ~0.08±0.03μM·yr−1 is broadly comparable to previous estimates from other researchers for similar reducing aquifers in Bangladesh. Although apparent arsenic groundwater accumulation rates varied significantly with site (e.g. between sand versus clay dominated sequences), rates are generally highest near the surface, perhaps reflecting the proximity to the redox cline and/or depth-dependent characteristics of the OM pool, and confounded by localized processes such as continued in-aquifer mobilization, sorption/desorption, and methanogenesis.

Highlights

Millions of people in South and Southeast Asia face chronic exposure to groundwater containing dangerous concentrations of naturallyoccurring arsenic (Smedley and Kinniburgh, 2002; Charlet and Polya, 2006; Ravenscroft et al, 2009; World Health Organization, 2011)
The study region is in the Kien Svay district, northern Kandal Province, Cambodia, in the Lower Mekong Basin (Fig. 1), an area heavily affected by groundwater arsenic (Charlet and Polya, 2006; van Dongen et al, 2008; Lawson et al, 2013; Lawson et al, 2016; Polizzotto et al, 2008; Kocar et al, 2008; Polya and Charlet, 2009; Polya et al, 2005; Benner et al, 2008; Gillispie et al, 2016)
Groundwater chemistry is dominated by calcium, magnesium and bicarbonate, typical of most arsenic-bearing groundwaters in S/SE Asia (Smedley and Kinniburgh, 2002; Lawson et al, 2013; Polizzotto et al, 2008; Richards et al, 2017a)

Summary

Introduction

Millions of people in South and Southeast Asia face chronic exposure to groundwater containing dangerous concentrations of naturallyoccurring arsenic (Smedley and Kinniburgh, 2002; Charlet and Polya, 2006; Ravenscroft et al, 2009; World Health Organization, 2011). Arsenic release in shallow aquifers typical to this region is widely attributed to the reductive dissolution of arsenic-bearing Fe(III) minerals (Islam et al, 2004) This process is driven by metal reducing bacteria and fuelled by electron donors provided by bioavailable organic matter (OM) (Charlet and Polya, 2006; Islam et al, 2004; Bhattacharya et al, 1997; van Geen et al, 2004; Postma et al, 2007; Rowland et al, 2009). Isotopic data, including 3H, He, Ne, δ18O, δD, δ13C and 14C, have been used to probe the relative importance of various types of OM in driving such arsenic mobilization in a number of studies (Aggarwal et al, 2000; Harvey et al, 2002; van Geen et al, 2003; Lawson et al, 2008; Sengupta et al, 2008; van Dongen et al, 2008; McArthur et al, 2011; Neumann et al, 2011; Lawson et al, 2013; Lawson et al, 2016; Datta et al, 2011). Determining the relative importance of these various inputs and clarifying these controls in dynamic alluvial systems are essential in determining how groundwater arsenic hazard may change in the future (Harvey et al, 2002; Lawson et al, 2016; Polya and Charlet, 2009)

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