Differentiating between Geogenic and Anthropogenic Chromium in Groundwater at a Site in the Sacramento Valley, California

摘要

Hexavalent chromium (Cr(VI)) contamination of groundwater is often attributed to anthropogenic sources, since it is used in many industrial applications. However, the general occurrence of Cr(VI) in groundwater throughout California, as recently reported by the United States Geological Survey, suggests that Cr(VI) can also be of geogenic origin. One Site where the origin of Cr(VI) is not fully understood is a pair of small landfills in the Sacramento Valley, which accepted wastewater sludge and animal waste of unknown chromium content from Site operations. Cr(VI) groundwater concentrations as high as 700 micrograms per liter (μg/L) measured downgradient suggest the landfills are an anthropogenic source of chromium contamination. However, elevated background Cr(VI) concentrations (up to 180 μg/L) in nearby water supply wells, and the presence of chromium-bearing serpentinite soils at the Site suggest Cr(VI) might be naturally occurring, or perhaps derived from naturally occurring chromium via changes to the Site’s geochemistry caused by the landfills. Soil at the Site is partly derived from ultramafic rocks of the Coast Range. These Site soils are naturally high in Cr (up to