Flooding and monsoonal recharge generate distinct contaminant mobilization pathways in a shallow floodplain aquifer

Extreme hydroclimatic events, such as hurricanes and monsoon-season rainfall, can dramatically alter groundwater recharge patterns and mobilize contaminants, but the mechanisms that distinguish short-term from sustained impacts on aquifer chemistry remain poorly understood. This study investigated how flooding and prolonged wet-season recharge generate distinct contaminant-mobilization regimes in a sandy floodplain aquifer in South Texas. Groundwater and surface-water quality monitoring occurred across three hydroclimatic periods: post-hurricane flooding, dry season, and the summer monsoon. Field measurements were integrated with geochemical and statistical modeling to evaluate hydrologic and geochemical controls on latent controls on contaminant behavior under contrasting recharge conditions. Results show that flooding triggered rapid, transport-dominated spikes in ammonium, Zn 2+ , and fecal indicators (total coliform, E. coli ), whereas the monsoon period was characterized by elevated Pb 2+ and phosphate, consistent with redox-driven internal processes and sustained hydrologic connectivity. Modeling results further indicated that contaminant dynamics were governed more strongly by recharge persistence, redox evolution, and nutrient interactions than by total rainfall alone. Notably, surface-water–groundwater exchange likely continued well beyond the flood event, challenging the assumption that flood-driven contamination is a short-lived phenomenon. Finally, the co-occurrence of heavy metals, fecal markers, and phosphate in drinking-water flood plain aquifers indicates that exposure risks persist across seasons in infrastructure-limited communities. These findings highlight the need for long-term groundwater monitoring frameworks that account for both rapid event-scale contamination and prolonged wet-season geochemical evolution.