PNNL

Abstract

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 scale. Concentrations of sediment As (0.47– 7 µg/g) correlated more strongly with Fe (r2>0.81) than with Mn (r2>0.37) 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 Fe 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 1.88 mg/L (± 0.88 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. Geogenic As release is strongly controlled by the reductive dissolution of Fe oxides, which could be influenced by the abundance of Mn oxides present due to their order before Fe in redox reduction reactions. Furthermore, the main mechanism preventing allogenic As from contaminating Pleistocene aquifers is its sorption onto Fe oxides, with oxidation via Mn oxides enhancing sorption potential. 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.