Characterization of trace metal concentrations and mining legacy in soils, Boulder County, Colorado

Smelting processes contribute to the emission of trace metals through ore deposit burning. Heavy metals are unstable in the atmosphere, and surface soils serve as a major metal sink in the environment. Therefore in regions of smelting activity, soils are thought to be enriched in metals associated with ore processing (Pb, Hg, Cd). The purpose of this investigation is to characterize metal concentrations in soils across a historic mining region of the Colorado Mineral Belt in southwestern Boulder County, Colorado. The approach is to determine (1) whether the geochemical signal thought to be associated with mining is present in surface soils across the region, (2) what the controls on accumulation in the soil profile are, and (3) how the soil geochemistry compares with other soils in the state. This project focused on laboratory analysis to characterize soils in terms of metal concentrations (Al, As, Ba, Cd, Cr, Cu, Fe, Hg, Mn, Pb), acidity (pH), loss on ignition (LOI, approximate % organic content), and grain-size (% fine-grained content). Interpretation of geochemical data included spatial analysis of trace metal soil chemistry (MAS/MILS U.S. Bureau of Mines dataset in ArcGIS) and a comparison of results with statewide soil geochemistry from the 2006 USGS Colorado soil survey (Smith et al., 2010). Trace metal concentrations in southwestern Boulder County soils are enriched in Pb, Cr, Cd, and Hg relative to other Colorado surface soils. Results show that Pb, As, Cd, Mn, and Ba are enriched in surface soils (O and A horizons) with respect to parent material. Mn and Ba enrichment is not likely from anthropogenic sources, and the correlation between these metals in the soil profile suggests that the source of enrichment for Mn and Ba in surface soils may be bioaccumulation. Surface soil enrichment of PB, As, Cd, and Hg suggests a common source: atmospheric deposition from an anthropogenic source. Correlation between concentrations of these elements supports the original hypothesis, that surface soils are enriched in metals associated with mining activity. Additionally, Pb, As, Cd, Hg, Mn, and Ba are correlated with LOI% (approximate organic content); there is no relationship between (1) fine-grained percent content of soils and trace metal concentrations or (2) proximity to former mining or smelting sites and trace metal concentrations. This indicates that the present-day concentrations of surface soil-enriched metals across the region are a function of soil organic content, not of location or of clay contents.