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

Richards, L. A. et al. (2019) Dual in-aquifer and near surface processes drive arsenic mobilization in Cambodian groundwaters. Science of the Total Environment, 659, pp. 699-714. (doi: 10.1016/j.scitotenv.2018.12.437)

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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 <55 years 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 >30 m, 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 ± 0.1 m·yr−1 across the field area. A calculated overall groundwater arsenic accumulation rate of ~0.08 ± 0.03 μM·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.

Item Type:Articles
Additional Information:This research was funded by a United Kingdom Natural Environmental Research Council (NERC) Standard Research Grant (NE/J023833/1 to DP, BvD and CB) with additional support from a NERC PhD studentship (NE/L501591/1 to DM) and a Leverhulme Trust Early Career Fellowship (ECF2015-657 to LR), both at the University of Manchester, and a NERC Collaborative Awards in Science and Engineering studentship (NE/EEA6549/1 to LC) at Lancaster University under the supervision of Greg Holland, Andrew Binley and DCG. Radiocarbon analysis was supported by the NERC Radiocarbon Facility NRCF010001 (allocation numbers 1814.0414, 1834.0714 and 1906.0415). Stable isotope analysis was conducted at SUERC and was supported by a NERC Isotope Geosciences Facilities Access grant IP-1505-1114.
Glasgow Author(s) Enlighten ID:Boyce, Professor Adrian
Authors: Richards, L. A., Magnone, D., Sültenfuß, J., Chambers, L., Bryant, C., Boyce, A. J., van Dongen, B. E., Ballentine, C. J., Sovann, C., Uhlemann, S., Kuras, O., Gooddy, D. C., and Polya, D. A.
College/School:College of Science and Engineering > Scottish Universities Environmental Research Centre
Journal Name:Science of the Total Environment
ISSN (Online):1879-1026
Published Online:29 December 2018
Copyright Holders:Copyright © 2019 The Authors
First Published:First published in Science of the Total Environment 659:699-714
Publisher Policy:Reproduced under a Creative Commons license

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