A Regional Soil and Sediment Geochemical Study in Northern California

Regional-scale variations in soil geochemistry were investigated in a 20,000-km² study area in northern California including the western slope of the Sierra Nevada (SN), Sacramento Valley (SV), and the Coast Ranges. Total element analyses were performed on archived surface soils (n=1300) and stream sediments (n=500) collected for the National Uranium Resource Evaluation program in the 1970's and 1980's. The distribution of four distinct groups of elements demonstrates the interplay of geologic, hydrologic, geomorphologic, and anthropogenic factors. One group of elements (Ca, K, Sr, and rare earth elements), which are abundant in silicic rocks, were elevated in both soils spatially associated with Tertiary volcanic and Mesozoic plutonic rocks in the SN (e.g. median La=28 mg/kg) and in eastern SV soils (median La=20 mg/kg). Chromium, Ni, V, Co, Cu, and Mg constitute the second group of elements, which were elevated in soils overlying UM rocks in the SN foothills (median Cr=3210 mg/kg) and the Coast Range (median Cr=3860 mg/kg). Valley soils showed elevated concentrations of UM-associated elements, particularly in the western SV (median Cr=252 mg/kg). Lower concentrations in the eastern SV (median Cr=150 mg/kg) reflect dilution from silicic rocks at higher elevations in the SN. The segregation of soil geochemical patterns into distinctive groups on either side of the Sacramento River arises from the presence of a natural levee along the river, which provides a barrier to sediment transport. The third group of elements (Zn, Cd, As, and Cu) is characteristic of ores mined in the region. This suite is localized along the Sacramento River levees and adjacent flood basins. Finally, anthropogenic sources of Pb and Zn are localized in major cites and along highways, which is likely due the historic use of leaded gasoline and tire wear, respectively.