Modeling of a pulse-like release of Cr(VI) in a contaminated site during an extreme rain event based on HHMM

Legacy industrial waste has caused persistent and chronic contamination of groundwater. However, in the context of global climate change, increasingly frequent extreme precipitation events may trigger the instantaneous and high-intensity release of pollutants. Despite this risk, few studies have examined contaminant migration during extreme precipitation events at contaminated sites. To investigate this overlooked issue, this study employed the Hydrology & Hydraulics Module Model (HHMM)-a three-dimensional (3D) hydro-hydrodynamic model-to simulate and quantify the short-term migration patterns of Cr(VI) within the vadose zone and aquifer system under a typical extreme precipitation event. The results demonstrate that the migration response of Cr(VI) shows significant vertical differentiation and depth-dependency. Particularly, in the shallow groundwater, the heavy rainfall induced a "pulse-like" contamination event. This resulted in Cr(VI) concentrations rapidly reaching 1.41 mg L^-1 and exceeding the drinking water safety limit by over 28 times shortly after the event. The response of Cr(VI) concentration at a depth of 4 m exhibited a significant lag, reflecting the complex process of contaminant transport lagging behind the water flow. In contrast, the deep groundwater system exhibited a strong buffering effect. Its concentration dynamics were dominated by long-term, slow transport processes. In fact, its inherent advection-dispersion migration pattern remained unaltered by this transient event. This study thus argues that, in an era of climate change, the paradigm for assessing and managing contaminated sites must evolve from static monitoring to event-driven dynamic warning. Such a shift is critical to effectively mitigate the potential of sudden environmental health crises.