A Regional-Scale Study of Chromium and Nickel in Soils of Northern California

Regional influences on soil geochemistry were investigated in northern California across a 22,000 km² area including the Sierra Nevada, the Sacramento Valley, and the northern Coast Range. Results show that soil geochemistry in the valley is controlled primarily by the transport and weathering of parent material from the Coast Range and the Sierra Nevada. Ultramafic (UM) rocks (e.g. serpentinite) crop out extensively in the Coast Range and Sierra Nevada. These rocks and their associated soils have elevated concentrations of Cr and Ni (surface soil samples of the Sierra Nevada and Coast Range contain 1700 to 10,000 mg/kg Cr and 1300 to 3900 mg/kg Ni). Valley soils west of the Sacramento River contain 80 to 1420 mg/kg Cr and 65 to 224 mg/kg Ni, reflecting significant contributions from UM sources in the Coast Range. Valley soils east of the river contain lower Cr and Ni concentrations (30 to 370 and 16 to 110 mg/kg, respectively), which is likely the result of dilution by granitic sources of the Sierra Nevada.

Chromium occurs naturally as Cr(III), a non-toxic micronutrient, and Cr(VI), a soluble toxin and carcinogen. X-ray diffraction and SEM of UM-derived soils show the primary residence being Cr(III) in the refractory minerals chromite and other Cr-rich spinels (Al, Mg, Fe, Cr). Chemical extractions indicate a lower proportion of chromite-bound Cr in valley soils relative to UM source soils. Groundwater on the west side of the Sacramento Valley has particularly high concentrations of dissolved Cr ranging up to 50 µg L^-1 and averaging 16 µg L^-1 suggesting that a redistribution of Cr during weathering and oxidation of Cr(III)-bearing minerals occurs. Regional-scale transport of ultramafic-derived constituents result in enrichment of Cr and Ni in the Sacramento Valley and weathering within these soils partially changes the residence of Cr to more soluble phases.