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

• Published in: The Science of the total environment 2019-04, Vol.659 (C), p.699-714
• Main Authors: Richards, Laura A., Magnone, Daniel, Sültenfuß, Jürgen, Chambers, Lee, Bryant, Charlotte, Boyce, Adrian J., van Dongen, Bart E., Ballentine, Christopher J., Sovann, Chansopheaktra, Uhlemann, Sebastian, Kuras, Oliver, Gooddy, Daren C., Polya, David A.
• Format: Article
• Language: English
• Subjects: Arsenic; Arsenic - chemistry; Bioavailability; Cambodia; Environmental Monitoring; ENVIRONMENTAL SCIENCES; Geochemical tracers; Groundwater - chemistry; Groundwater monitoring; Groundwater quality; Water Pollutants, Chemical - chemistry
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2018.12.437

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 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. [Display omitted] •Tracers used for monitoring groundwater evolution at a high spatial resolution.•Groundwater arsenic associated with tracers of modern groundwater•Surface-derived organic matter transported in aquifer to depths of >30m.•Groundwater arsenic accumulation rates are depth dependent.•Dual in-aquifer and near surfaces processes drive arsenic mobilization.