Millions of people globally are exposed to dangerous arsenic concentrations in groundwater-derived drinking water sources. Arsenic mobilization in circum-Himalayan groundwater is widely attributed to the reductive dissolution of iron minerals containing arsenic, although a number of questions remain the fundamental mechanisms of this process. In a high resolution study site in Kandal Province, Cambodia, the evolution of groundwater geochemistry was evaluated along dominant groundwater flowpaths in a heavily arsenic-contaminated aquifer using a suite of geochemical tracers (e.g. As, Fe, SO4, δ 18O, δ 2H, 3H/3He, 14C) to understand the processes which may contribute to arsenic mobilization in shallow, reducing aquifers typical to Southeast Asia.