Seasonal Coastal Groundwater Dynamics at Lahaina Beaches, Hawaiʻi

The August 2023 Lahaina wildfire caused extensive environmental impacts, including the release of untreated wastewater and combustion-derived contaminants such as nutrients and polycyclic aromatic hydrocarbons (PAHs). This study examines seasonal groundwater flow and solute transport dynamics within a post-wildfire beach aquifer. Using field observations and a two-dimensional, density-dependent, variably saturated groundwater model calibrated with year-long data, we simulated groundwater flow and salinity patterns and applied Lagrangian particle tracking to evaluate solute pathways and transit times. Summer conditions are characterized by elevated inland groundwater levels and predominantly seaward flow, resulting in rapid solute discharge to the shoreline. In contrast, enhanced tidal and wave forcing in winter drives greater seawater infiltration, deeper recirculation, and net landward solute transport with longer residence times. Our results highlight the importance of seasonal and tidal variability in controlling post-wildfire contaminant fate and provide insights for time-sensitive coastal management and ecosystem protection.