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Description
Semi-arid urban coastal streams are at an increased risk of future fire disturbances due to climate change and anthropogenic activity. These disturbances can impact ecosystem services such as rainfall-runoff processes and the transportation of soil-bound contaminants. Soil moisture content and infiltration are important antecedent hydrologic conditions for rainfall-runoff processes that can influence the transport and storage of toxic heavy metals like mercury (Hg). Inorganic mercury can be converted into a potent neurotoxin, methylmercury (MeHg), and mobilized to coastal waterways through runoff and erosion. This thesis analyzed soil moisture content, infiltration, and soil Hg concentration (total mercury (THg) and MeHg) in a perennial semi-arid urban stream channel in Southern California. In June 2018, a 38-acre fire burned a portion of the riparian zone in Alvarado Creek, a tributary of the San Diego River in California, United States. Three transects (two unburned and one burned) were monitored monthly to the evaluate the complex spatial and temporal dynamics of soil moisture and infiltration patterns. Multiple soil samples across all three transects were obtained to investigate the consequences of fire for the transport and storage of THg and MeHg concentrations. Average dry season soil moisture content was less than 5 percent volume water content (%VWC) for all transects, and the burned transect exhibited the lowest %VWC during the wet season. Infiltration rates displayed a high degree of spatial and temporal variability, however the location with the highest burn severity had the lowest average infiltration rate. THg concentrations ranged from 10.63 ng/g to 182.38 ng/g, and MeHg concentrations ranged from 0.03 ng/g to 1.58 ng/g. THg and MeHg concentrations increased at all transects after the first significant post-fire precipitation event of the wet season, and the largest increase was displayed at the downstream locations of the burned area. This thesis demonstrated the differences of hydrologic properties and terrestrial Hg contamination among burned and unburned transects in a fire disturbed semi-arid urban stream channel. The research improves the characterization of post-fire hydrologic conditions for rainfall-runoff processes and provides the first high resolution mercury soil analysis of an urban fire disturbed stream in Southern California.
