TY - JOUR

T1 - Dual in-aquifer and near surface processes drive arsenic mobilization in Cambodian groundwaters

AU - Richards, L.A.

AU - Magnone, D.

AU - Sültenfuß, J.

AU - Chambers, Lee

AU - Bryant, C.

AU - Boyce, A.J.

AU - van Dongen, B.E.

AU - Ballentine, C.J.

AU - Sovann, C.

AU - Uhlemann, S.

AU - Kuras, O.

AU - Gooddy, Daren C.

AU - Polya, D.A.

PY - 2019/4/1

Y1 - 2019/4/1

N2 - Millions of people globally, and particularly in South and Southeast Asia, face chronic exposure to arsenic from reducing groundwater in which arsenic release is widely attributed to the 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. © 2018

AB - Millions of people globally, and particularly in South and Southeast Asia, face chronic exposure to arsenic from reducing groundwater in which arsenic release is widely attributed to the 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. © 2018

KW - Arsenic

KW - Bioavailability

KW - Geochemical tracers

KW - Groundwater monitoring

KW - Groundwater quality

KW - Aquifers

KW - Biogeochemistry

KW - Biological materials

KW - Flow velocity

KW - Groundwater

KW - Groundwater resources

KW - Hydrogeology

KW - Organic minerals

KW - Water quality

KW - Arsenic mobilization

KW - Geochemical tracer

KW - Groundwater residence

KW - Hydrological controls

KW - Metal-reducing bacteria

KW - Reductive dissolution

KW - Hydrochemistry

KW - arsenic

KW - carbon 14

KW - ground water

KW - organic matter

KW - tracer

KW - tritium

KW - accumulation rate

KW - aquifer

KW - bioavailability

KW - desorption

KW - flow velocity

KW - groundwater pollution

KW - iron-reducing bacterium

KW - methanogenesis

KW - mobilization

KW - pollutant transport

KW - pollution monitoring

KW - water quality

KW - analytic method

KW - arsenic mobilization

KW - Article

KW - Cambodia

KW - chemical procedures

KW - dissolution

KW - environmental exposure

KW - priority journal

KW - quality control

KW - sediment

KW - water sampling

KW - Kandal

U2 - 10.1016/j.scitotenv.2018.12.437

DO - 10.1016/j.scitotenv.2018.12.437

M3 - Journal article

VL - 659

SP - 699

EP - 714

JO - Science of the Total Environment

JF - Science of the Total Environment

SN - 0048-9697

ER -