CHROMIUM, NICKEL, AND LEAD IN SOILS OF NORTHERN CALIFORNIA: SOURCES AND BIOACCESSIBILITY

A soil geochemical survey was conducted in northern California in a study area that includes portions of the Sierra Nevada Mountains, the Sacramento Valley, and the Coast Ranges. Approximately 1,300 samples of archived surficial soils were analyzed for Cr, Ni, and Pb by inductively coupled plasma-mass spectrometry (ICP-MS) following a four-acid digestion. In addition, a subset of 17 samples were extracted in a simulated human gastric fluid and analyzed by ICP-MS.

Ultramafic (UM) rocks contain elevated levels of potentially toxic elements Cr and Ni. Soils derived from these rocks are also enriched in these elements. The geometric mean for Cr and Ni in soils of the conterminous U.S. is 37 and 13 ppm, respectively. In contrast, surface soil samples from the foothills of the Sierra Nevada Mountains have elevated Cr (400-6000 ppm) and Ni (500-5000 ppm). These high concentrations are spatially correlated with mapped UM rocks. In addition, Cr (100-300 ppm) and Ni (100-500 ppm) concentrations are also high in the Sacramento Valley, west of the Sacramento River, suggesting the transport of these elements from their source to the west. Data from simulated gastric leaches suggest that Cr and Ni in weathered alluvium soils is nearly two times more leachable than in soils found near the UM belts. We hypothesize that Cr and Ni in soils immediately overlying UM rocks may be transformed to a more bioaccessible form as it is weathered and transported from the UM source rocks to the valley.

In contrast to naturally occurring Cr and Ni, data suggest that Pb is anthropogenic with the largest contribution related to urban areas and along roadways. Data show elevated Pb concentrations, greater than 280 ppm and as high as 2300 ppm, in the soils surrounding urban areas, while non-urban areas are more similar to the U.S. geometric mean (16 ppm). Gastric leach data show that Pb in urban areas is 80% leachable, relative to 40% in the valley and mountainous regions, suggesting that elevated Pb is highly bioavailable. Results of this study show that the processes involved in soil formation and weathering are important in determining the sources, bioaccessibility, and potential toxicity of trace elements in the environment.