Around the world, over 100 million people routinely consume well water with naturally occurring arsenic concentrations that exceed safe drinking-water guidelines. Although the global scale, health impacts, and extreme societal costs of arsenic contamination are apparent, it is less well understood to what extent arsenic may threaten groundwater that is currently arsenic-free. This issue is critical because arsenic contamination of previously uncontaminated aquifers has been observed to occur over timescales of decades. The overall goal of this research is to quantify the potential for future arsenic contamination of groundwater from sources that are both internal and external to aquifers. To achieve this goal, we will conduct a suite of field, laboratory and spectroscopic analyses on sediment samples from a well-characterized aquifer in Cambodia, where groundwater is potentially at risk for future arsenic contamination. In particular, this work will take advantage of natural sediment variability to investigate how the quantities and chemical reactivities of specific minerals govern arsenic concentrations in well water.