PROBING POTENTIAL LINKS BETWEEN GROUNDWATER USE AND GEOCHEMICAL PROCESSES THAT AFFECT WATER QUALITY USING IRON K-EDGE X-RAY ABSORPTION SPECTROSCOPY (Invited Presentation)

Naturally-occurring contamination of groundwater by arsenic (As) affects at least 150 million people globally and leads to both adverse early life and chronic health outcomes for the populations who rely on groundwater as a water resource. In regions such as South and Southeast Asia, groundwater abstraction is increasing rapidly, resulting in changes in groundwater levels. Most research to date has focused on the decline in water supply, but recent evidence suggests that aquifer depletion can also markedly affect groundwater geochemistry and transport or generate groundwater As contamination. The release of As into groundwater is commonly attributed to microbial reductive dissolution of As-bearing iron (Fe) minerals. While extensive measurements of groundwater composition exist, paired observations of groundwater and solid-phase aquifer composition remain lacking or limited to lab-based scales. Here, we examine evidence for widespread alteration of sediments using extensive solid-phase characterization by Fe K-edge X-ray absorption spectroscopy across a redox gradient with known histories. With hierarchical cluster analysis, clusters distinguish between sediments based on active weathering and redox transformations, both of which are diagnostic of depositional history. Notably, the clusters also identify a mineralogical signature and extent of iron reduction potentially leading to future arsenic contamination. These paired observations of aqueous data and broad mineralogical classifications are upscaled to similar deltaic settings through multinomial logistic regression modeling across South and Southeast Asia. Finally, we investigate the intriguing question of whether these aquifers transitioning towards reducing conditions are associated with regions of increased groundwater pumping that have anthropogenically exacerbated As contamination or if these transitioning aquifers are actually more naturally widespread than previously realized.