Seasonal coastal groundwater dynamics in Lahaina beaches, Hawai'i: Implications for contaminant transport in a...

The August 2023 wildfire in Lahaina, Maui, resulted in extensive environmental damage, including the release of untreated wastewater and combustion-derived pollutants such as nutrients and polycyclic aromatic hydrocarbons (PAHs) into the environment. Motivated by this event, this study investigates the seasonal dynamics of groundwater flow and solute transport within a beach in a post-wildfire setting. Utilizing field observations and a two-dimensional, density-dependent, variably saturated groundwater model calibrated with year-long data, we simulated groundwater flow and salinity distributions. Lagrangian particle tracking was subsequently employed to assess solute pathways and transit times under contrasting seasonal conditions. Results indicate that during the summer, the inland groundwater table in the beach is relatively higher, likely due to a delayed inland recharge signal, leading to predominantly seaward groundwater flow with rapid solute transport to the shoreline. Conversely, winter conditions, marked by enhanced tidal effects and wave activity, induce greater seawater infiltration, deeper recirculation cells, and a net inland-directed flow, leading to landward movement and retention of solutes within the aquifer. Extended transit times under these conditions enhance the potential for in-situ transformations, including sorption, microbial redox reactions, and mineral precipitation/dissolution, particularly affecting redox-sensitive species like phosphate, ammonium, and iron-bound contaminants. Spatial variability in nutrient concentrations across the beach subsurface underscores the role of tidal modulation in influencing solute export. These findings highlight the necessity of incorporating seasonal and tidal variability into coastal contaminant fate assessment and inform time-sensitive management strategies for post-wildfire recovery and the protection of nearshore ecosystems in Hawai'i and similar coastal regions.