Flashy hydrology, flashier contaminants: redox-driven geogenic contaminant mobilization under increasing hydrologic variability

As precipitation regimes intensify and engineered recharge expands, the vadose zone faces increasingly dynamic wetting–drying cycles that generate redox oscillations and alter geogenic contaminant mobility. Oxygen depletion and microbial respiration can dissolve Fe and Mn (oxyhydr)oxides, key sorption hosts for arsenic (As), chromium (Cr), uranium (U), and related trace elements, producing transient, element-specific, and history-dependent responses. Synthesizing field observations, laboratory experiments, and large-scale monitoring datasets, we identify three management-relevant insights. First, contaminant mobilization under flashy hydrology can occur in short-lived pulses lasting hours to days, faster than typical groundwater-monitoring intervals. Second, single-contaminant frameworks can misrepresent overall geogenic risk because the same wetting event may mobilize arsenic through reductive dissolution while attenuating chromium and uranium through reduction to less mobile forms. Third, repeated wetting–drying cycles may alter or deplete reactive Fe and Mn mineral phases that buffer contaminant release, increasing vulnerability under sustained hydrologic variability. Changing geogenic contaminant concentrations in US municipal groundwater over the past three decades indicate that these processes are already detectable at network scales. Addressing them requires event-scale, multi-element monitoring and process-based models that represent redox hysteresis and mineral transformations from the vadose zone to the aquifer.