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 the fact that it is a significant source of pollutants to receiving waters and reservoirs. This study assesses the potential longer-term (months/year) impacts of a small-scale wildfire that occurred in September 2018 on soil 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 major-minor 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 show that soils from the burned sites are relatively finer. For soil chemistry, the burned and non-burned soils have similar composition of major and minor elements, mostly consisting of Al 7%, Fe 5%, Ca 3%, Mn 1000 ppm, Ba 600 ppm, and Cu 200 ppm, approximately. However, the batch leaching tests of both soils show different results. Both burned and non-burned soils exhibit an increasing trend in concentrations with time, however the burned soils show significantly higher concentrations. For examples, the concentrations of Mg and Ca of the leached samples from the burned soils are six and seven times, respectively, greater than from the non-burned soils within four weeks. This is possibly due to changes in the chemistry of the soils after the wildfires 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 and As 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.