IDENTIFYING GEOLOGIC AND ANTHROPOGENIC TRACE METAL SOURCES TO THE CRITICAL ZONE ALONG AN URBAN-RURAL TRANSECT IN SOUTHERN CALIFORNIA

Human activities have altered the chemical nature of soils in the Critical Zone, not just through direct land-use changes, but also through long-range transport of pollutants. The combustion of fuels and manufacturing can release trace metals, however, distinguishing added trace metals can be masked by variations in lithology. We studied trace metals in residuum-derived soils in the transverse mountain ranges in Southern California, focusing on the effect of lithology and distance from urban epicenters on trace metal concentrations in soil. We collected surface soils, weathered and unweathered rock samples from exposures and outcrops, and four endemic coastal sage scrub plants over 50 sample sites. The samples were analyzed for plant available elements and physicochemical properties pH, % organic matter, and soil texture. XRF and strong acid extraction of soil and rock samples showed that trace metal concentrations were within the range of typical unpolluted soils and did not exhibit a strong urban-rural gradient. However, lithology significantly affected trace metal concentrations. Soils derived from sedimentary marine rocks showed the highest concentrations of As (4 mg/kg), Cd (0.9 mg/kg), Cu (11 mg/kg), and Pb (20 mg/kg), which decreased with depth. Tau values (normalized to Ti) were calculated using unweathered rock samples and show net accumulation of trace metals in the soil; average tau value is equal to 1.18 across sample sites. However, Tau values differed across rock types, soils derived from volcanic basalts had a Tau value of only 0.12, while soils derived from Mesozoic granite and Precambrian schists had an average Tau value of 1.56 and 1.64, respectively. These findings signify that despite trace metal concentrations being low, soils across Southern California are enriched in trace metals in their upper horizons, rather than being inherited during weathered from the underlying bedrock.