Aquifer-Scale Observations of Iron Redox Transformations in Arsenic-Impacted Environments to Predict Future Contamination

Iron oxides control the mobility
of a host of contaminants in aquifer
systems, and the microbial reduction of iron oxides in the subsurface
is linked to high levels of arsenic in groundwater that affects greater
than 150 million people globally. Paired observations of groundwater
and solid-phase aquifer composition are critical to understand spatial
and temporal trends in contamination and effectively manage changing
water resources, yet field-representative mineralogical data are sparse
across redox gradients relevant to arsenic contamination. We characterize
iron mineralogy using X-ray absorption spectroscopy across a natural
gradient of groundwater arsenic contamination in Vietnam. Hierarchical
cluster analysis classifies sediments into meaningful groups delineating
weathering and redox changes, diagnostic of depositional history,
in this first direct characterization of redox transformations in
the field. Notably, these groupings reveal a signature of iron minerals
undergoing active reduction before the onset of arsenic contamination
in groundwater. Pleistocene sediments undergoing postdepositional
reduction may be more extensive than previously recognized due to
previous misclassification. By upscaling to similar environments in
South and Southeast Asia via multinomial logistic regression modeling,
we show that active iron reduction, and therefore susceptibility to
future arsenic contamination, is more widely distributed in presumably
pristine aquifers than anticipated.