Landscape Position and Burn Intensity Influence Heat-Induced Soil Chromium Contamination

Wildfires are increasing in extent and severity, posing a worldwide threat to human and environmental health. Fire-induced oxidation of Cr in soils can generate hexavalent chromium (Cr(VI)), a mobile and toxic Class A carcinogen, but the controls on Cr(VI) generation across landscapes are poorly constrained. Here, we use natural soil samples, laboratory experimentation, and spectroscopic analyses to simulate and quantify how burn intensity and landscape position drive Cr(VI) generation and mobility across an unburned serpentine soil toposequence in southwestern Oregon. Maximum Cr(VI) was generated in summit soils burned at 400 °C, and Cr(VI) concentrations decreased downslope. Although reactive secondary minerals generally transformed toward more ordered phases with increased burn intensity, Cr(VI) was mainly associated with poorly ordered (oxy)hydroxides that formed in soils between 200-600 °C. The availability of Cr and Cr(VI) was negligible in soils burned at 800 °C due to Cr incorporation into newly crystallized minerals. Column experiments revealed that Cr(VI) efflux from burned soils could persist at levels exceeding drinking water screening levels for up to two years after a wildfire event, depending on hillslope position. Overall, these findings demonstrate that both fire intensity and geomorphic context (e.g., degree of soil weathering) codetermine Cr(VI) risks to soil and water quality in postfire landscapes.