A regional-scale study of chromium and nickel in soils of northern California, USA Jean M. Morrison a, * , Martin B. Goldhaber a , Lopaka Lee b , JoAnn M. Holloway a , Richard B. Wanty a , Ruth E. Wolf a , James F. Ranville c a US Geological Survey, MS 973, Denver Federal Center, Denver, CO 80225, United States b US Geological Survey, Hawaii Volcano Observatory, MS964, Hawaii National Park, HI 96718, United States c Colorado School of Mines, Golden, CO 80401, United States article info Article history: Available online 3 May 2009 abstract A soil geochemical survey was conducted in a 27,000-km 2 study area of northern California that includes the Sierra Nevada Mountains, the Sacramento Valley, and the northern Coast Range. The results show that soil geochemistry in the Sacramento Valley is controlled primarily by the transport and weathering of parent material from the Coast Range to the west and the Sierra Nevada to the east. Chemically and min- eralogically distinctive ultramafic (UM) rocks (e.g. serpentinite) outcrop extensively in the Coast Range and Sierra Nevada. These rocks and the soils derived from them have elevated concentrations of Cr and Ni. Surface soil samples derived from UM rocks of the Sierra Nevada and Coast Range contain 1700–10,000 mg/kg Cr and 1300–3900 mg/kg Ni. Valley soils west of the Sacramento River contain 80–1420 mg/kg Cr and 65–224 mg/kg Ni, reflecting significant contributions from UM sources in the Coast Range. Valley soils on the east side contain 30–370 mg/kg Cr and 16–110 mg/kg Ni. Lower Cr and Ni concentrations on the east side of the valley are the result of greater dilution by granitic sources of the Sierra Nevada. Chromium occurs naturally in the Cr(III) and Cr(VI) oxidation states. Trivalent Cr is a non-toxic micro- nutrient, but Cr(VI) is a highly soluble toxin and carcinogen. X-ray diffraction and scanning electron microscopy of soils with an UM parent show Cr primarily occurs within chromite and other mixed-com- position spinels (Al, Mg, Fe, Cr). Chromite contains Cr(III) and is highly refractory with respect to weath- ering. Comparison of a 4-acid digestion (HNO 3 , HCl, HF, HClO 4 ), which only partially dissolves chromite, and total digestion by lithium metaborate (LiBO 3 ) fusion, indicates 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 lgL 1 and averaging 16.4 lgL 1 . This suggests redistribution of Cr during weathering and oxidation of Cr(III)-bearing minerals. It is concluded that regional-scale transport and weathering of ultramafic-derived constituents have resulted in enrich- ment of Cr and Ni in the Sacramento Valley and a partial change in the residence of Cr. Published by Elsevier Ltd. 1. Introduction Serpentinites are compositionally ultramafic rocks formed through the subduction and alteration of peridotite and pyroxenite along convergent plate margins (Coleman and Jove, 1992; O’Hand- ley, 1996; Oze et al., 2004b). Although serpentinites (and serpen- tine soils) comprise less than 1% of the earth’s total exposed surface, they are relatively common along the western coast of North America (Oze et al., 2004b). Serpentinites cover approxi- mately 2860 km 2 , 1170 km 2 , and 520 km 2 of the exposed land sur- face in California, Oregon and Washington, respectively (Kruckeberg, 1984) and potentially impact the geochemistry of much greater areas through transport and weathering. This study focuses on two major geological occurrences of serpentinites in California; those associated with the Coast Range Ophiolite (CRO) and those associated with the Western Metamorphic Belt (WMB) located in the western foothills of the Sierra Nevada (Fig. 1). The process of serpentinization occurs as peridotite and pyrox- enite rocks, which contain the Fe- and Mg-rich silicate minerals olivine ((Mg,Fe 2+ ) 2 [Si 2 O 4 ]) and pyroxene (XY(Si,Al) 2 O 6 ), are altered by hydrothermal fluids as they are detached from the subduction block and incorporated into subduction mélanges (Coleman, 1967; Gough et al., 1989; O’Handley, 1996; Oze et al., 2004b). Hydration of pyroxene and olivine forms the serpentine group minerals; lizardite (Mg 3 Si 2 O 5 (OH) 4 ), chrysotile (Mg 3 Si 2 O 5 (OH) 4 ) and antigorite ((Mg,Fe 2+ ) 3 Si 2 O 5 (OH) 4 ). Other minerals commonly associated with serpentinites are magnetite (Fe 2þ Fe 3þ 2 O 4 ), Cr-rich magnetite (Fe 2+ (Fe 3+ ,Cr) 2 O 4 ), chromite (FeCr 2 O 4 ), and other mixed-composition spinels, talc (Mg 3 Si 4 O 10 (OH) 2 ), chlorite ((Mg,Fe) 5 Al[(OH) 8 |AlSi 3 O 10 ]), tremolite ([Ca 2 ][Mg 5 ][(OH) 2 |Si 8 O 22 ]), and brucite (Mg(OH) 2 )(Oze et al., 2004b). 0883-2927/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.apgeochem.2009.04.027 * Corresponding author. E-mail address: jmorrison@usgs.gov (J.M. Morrison). Applied Geochemistry 24 (2009) 1500–1511 Contents lists available at ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem