POTENTIAL IMPACTS OF WILDFIRES ON SOIL AND WATER CHEMISTRY IN THE SAN GABRIEL MOUNTAINS, CALIFORNIA

Increase in contaminants in soil caused by wildfires is poorly understood, despite it being a significant source of pollutants to receiving waters and reservoirs (Abraham et al., 2017). This study assesses the potential longer-term impacts of a small-scale wildfire that occurred in September 2018 on soil solution chemistry in the San Gabriel Mountains, Los Angeles County, California. Soils were collected from five sites within the burned and unburned areas in March 2019, following the post-fire runoff, for sediment and total element analyses. Batch leaching tests were conducted at different temperatures to determine the extent of size partitioning and mobilization of the major (Na, Mg, Al, K, Ca, and Fe) and minor/trace (Mn, Cu, Zn, Cr, and As) elements in order to understand solute transport. The results showed that burned soils were relatively finer than the unburned soils. For soil chemistry, the burned and unburned soils had similar composition of major and minor elements, mostly consisting of Na 1.7 %, Mg 1.8 %, Al 7.6 %, K 2.0 %, Ca 2.8 %, and Fe 5.4 %, approximately. In addition, the average range of the four interested minor elements of both burned and unburned soils was between 105 and 1,250 ppm, except As, which had a low concentration of 4 ppm. Nevertheless, the batch leaching tests of both soils revealed different results. Both burned and unburned soils exhibited an increasing trend in concentrations with time, however the burned soils showed significantly higher concentrations for some elements. For example, the burned soil solutions had greater concentrations of Mg and Ca by more than six-fold and sevenfold, respectively, compared to that of the unburned soils within four-week period. This possibly results from the fire-induced size partitioning and combustion of organic matter and is in need of further investigation. Experiments are being planned to provide insights into the mechanisms that result in differences between sediments in the burned vs. unburned areas. These results potentially imply that size partitioning and the rate of the elements released into the water, especially contaminants, such as Cr, could be impacted. These results would be valuable for predicting the fate of the contaminants and broader impacts could also include more effective pre- and post-fire mitigation plans.