Chemical variability of sediment and groundwater in a Pleistocene aquifer of Cambodia: Implications for arsenic pollution potential

Low-arsenic (As) groundwater from Pleistocene aquifers is vulnerable to future geogenic and allogenic arsenic pollution in South and Southeast Asia, threatening the millions who use it as a "safe source" of drinking and irrigation water. The abundance and chemical reactivity of iron and manganese oxides within these aquifer sediments control the occurrence and mobility of arsenic. In the present study, sediment samples varying in As, Fe, and Mn content were obtained from a Pleistocene aquifer in the Kandal Province of Cambodia. Laboratory and spectroscopic characterization of the sediment combined with groundwater analyses revealed that the availability and abundance of sedimentary As varied across a Pleistocene aquifer from the pore to field scales. Concentrations of sediment As (0.47-7 μg/g) correlated more strongly with Fe (R² > 0.66) than with Mn (R² > 0.35) concentrations in sediment well cuttings and tended to peak between 10 and 15 m. Chemical extractions and X-ray adsorption spectroscopy indicated the majority of As was strongly adsorbed to aquifer sediments or coprecipitated in oxides in the form of As(V) but that As(III) could be found in sediment microenvironments across the aquifer. Groundwater chemistry and Mn mineralogy indicated that the Pleistocene aquifer was suboxic, with average dissolved oxygen of 1.9 mg/L (±0.9 mg/L), redox potential of 0.155 V (±0.097 V), and abundant Mn(III/IV) oxide minerals. According to our results, allogenic As transport and geogenic As release will likely be dictated by localized geochemical processes that vary over a range of scales. Collectively, the specific Fe and Mn mineralogy and content within aquifers will ultimately govern As pollution potential, so understanding their multi-scale distributions and variability is essential for better predicting future risks to well-water quality in currently low-As aquifers.