Integrating Mechanisms that Control the Concentration and Distribution of Groundwater Arsenic in Cambodia

Arsenic contamination in the groundwater of the deltaic regions of southeastern Asia affects as many as one hundred million people. In these areas, the dominant mechanism for arsenic release into solution is the reductive dissolution of arsenic-bearing iron (hydr)oxides. What continues to confound researchers, however, is the heterogeneous nature of arsenic in groundwater. Recent evidence suggests that specific geomorphic environments are linked to regions of elevated groundwater arsenic. Moreover, waters from actively cycling surficial environments represent important sources of groundwater As in many environments. Here, we examine the mechanistic link between the localized expression of iron reduction and the large scale distribution of arsenic in groundwater along the Mekong River in Cambodia in a region impacted by widespread but heterogeneous arsenic contamination. In this region, arsenic levels in groundwater were correlated to sulfate levels, and both were strongly influenced by the extent of local surface water flooding (as quantified by remote sensing). Concentrations of arsenic ranged from 0 to 2100 micrograms/L, and were always found in groundwaters with significant dissolved iron and a sulfate concentration less than 1 mg/L. This indicates that the delivery of As and S was influenced by active redox processes in near-surface environments. In many, but not all, high As regions, groundwater concentrations of conservative halide anions also were similar to those of surficial environments, indicating that they were likely derived from the same locations. Low As regions, however, had widely variable halide concentrations in groundwater that may reflect additional groundwater sources. Thus, the expression of As contamination is strongly influenced by the hydrological connectivity of the aquifer with the surface. The isotopic composition of groundwater (hydrogen and oxygen) of high As areas also is similar to that of surface waters. Dissolved inorganic carbon isotopic composition of waters is more complex, with As-impacted groundwaters having distinct isotopic signatures characteristic of either extensive or relatively limited organic matter decomposition. Regional correlations of As with dissolved organic carbon are also not uniform, suggesting that sedimentary carbon is also important in generating reducing conditions. These data indicate that the distribution of organic carbon helps determine the distribution of arsenic in the environment. Moreover, these data indicate that heterogeneity in arsenic concentrations results from the interplay of variable organic matter content and reactivity within complex hydrological systems that can at least in part be explained regionally based on depositional environment.