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

Abstract
Millions of people worldwide—especially in South and Southeast Asia—are chronically exposed to arsenic in reducing groundwater systems. This arsenic release is widely attributed to the reductive dissolution of arsenic-bearing iron minerals, a process driven by metal-reducing bacteria that utilize bioavailable organic matter as an electron donor. However, the specific characteristics of the organic matter involved, and where within the subsurface these reactions occur, remain under debate.

In a high-resolution study of a largely undisturbed, shallow aquifer in Kandal Province, Cambodia, we employed a suite of geochemical tracers—including ¹⁴C, ³H, ³He, ⁴He, Ne, δ¹⁸O, δD, CFCs, and SF₆—to track the evolution of arsenic-rich, reducing groundwaters along major flow paths. The widespread presence of ³H–³He apparent ages under 55 years challenges earlier models that estimated groundwater residence times to be several hundred years.

Our findings show that surface-derived organic matter can be transported to depths exceeding 30 meters, and correlations between age tracers and arsenic levels suggest this organic matter contributes to in-situ arsenic mobilization. A strong correlation between groundwater age and depth points to a predominantly vertical flow regime, with an average vertical velocity of ~0.4 ± 0.1 m/year across the study area.

We estimate an overall arsenic accumulation rate in groundwater of ~0.08 ± 0.03 μM/year, which aligns with previous studies from similarly reducing aquifers in Bangladesh. While accumulation rates vary by site—such as between sandy and Leukadherin-1 clay-rich sediments—they tend to be highest near the surface. This may reflect proximity to the redox transition zone, variations in organic matter characteristics with depth, or localized influences such as ongoing arsenic mobilization, sorption/desorption, and methanogenesis.