Assessing the leaching of nutrients and trace metals from wildfire ashes and heated soils through soil column tests: Potential impacts on groundwater

Wildfires are a significant diffuse source of pollution to water systems. Ash deposited after forest fires contains hazardous contaminants that can leach into groundwater, posing risks to water quality. While wildfire effects on surface water are well-documented, the processes and risks of contaminant leaching into groundwater remain poorly understood. This study evaluated the leaching behavior of nutrients (N and P) and trace metals (V, Mn, Co, Ni, Cu, Zn, As, Cd, and Pb) in ash-soil systems using soil column tests with two soil types (granite [SG] vs. schist [SX]) and two heating temperatures (150 °C vs. 500 °C). Significant differences in leaching were observed based on soil type, fire severity, and ash deposition. Ash addition increased leachate concentrations of N, P, Mn, Zn, Co, and V, with Mn often exceeding drinking water standards. In general, SG soils exhibited greater contaminant mobilization, except for N, Cd, V, and Zn, which were found in higher concentrations in SX soil leachates. Higher temperatures decreased soil organic matter and altered pH and conductivity. These findings highlight the critical role of soil texture, heating, and ash deposition in post-fire contaminant mobility and underscore the urgent need for targeted management strategies to protect groundwater in wildfire-prone regions.