Sediment Manganese Oxide Content As an Indicator of Groundwater Arsenic Pollution Potential.

Arsenic is a contaminant of global concern that is often found in well water at concentrations above human health standards. Geogenic arsenic release from aquifer solids to groundwater is frequently a consequence of arsenic or iron reduction, but manganese oxides, which have higher reduction potentials than those of oxidized arsenic or iron, can serve as redox buffers that limit reductive As solubilization. The primary objective of this study was to quantify how Mn oxides influence the rates and extent of geogenic As release from aquifer solids. A suite of sediments, with naturally varying ratios of Mn to Fe oxides from a low-As-groundwater Pleistocene aquifer in Cambodia, were utilized in year-long laboratory batch experiments. Anaerobic incubation of the sediments with labile dissolved organic carbon (DOC) increased the release of As, Mn, and Fe over parallel incubations without carbon addition or with a carbon and sodium azide treatment. Aqueous As speciation and sediment X-ray absorption spectroscopy analyses throughout the experiments demonstrated that As, Mn, and Fe were reductively mobilized within the DOC treatments. Across the suite of experiments, Mn release occurred before As and Fe release, and sediments with higher initial Mn concentrations had slower initial As and Fe release to groundwater. However, throughout the year-long experiments, after a certain threshold of Mn was released, As was rapidly mobilized from sediments. These collective results demonstrate that Mn oxides may buffer Fe reduction and concomitant As release, and accordingly, the abundance and reactivity of Mn oxides in aquifers may serve as possible predictors for timescales of future As contamination of groundwater due to influx of labile DOC. Additionally, simple field tests of reducible Mn in sediments could be of use for assessing the As pollution potential of Pleistocene aquifers across South and Southeast Asia.