THE TRANSPORT OF GEOGENIC CHROMIUM AND NICKEL FROM ULTRAMAFIC SOURCES IN NORTHERN, CALIFORNIA: IS THERE A HUMAN HEALTH RISK?

We are investigating the effects of weathering and transport on the bioaccessibility of geologically sourced and potentially toxic Cr and Ni in northern California. Ultramafic (UM) rocks in the Coast Range and western foothills of the Sierra Nevada Mountains host elevated concentrations of Cr (1700 to 10,000 mg kg^-1) and Ni (1300 to 3900 mg kg^-1). Nickel in UM source rocks is associated with serpentine minerals (lizardite, antigorite, and chrysotile) and is generally more easily weathered compared to Cr, which primarily resides in the refractory mineral chromite. Regional-scale transport and weathering of UM source material has resulted in an enrichment of Cr (80 to 1420 mg kg^-1) and Ni (65 to 224 mg kg-1) in the Sacramento Valley relative to U.S. geometric means of 37 and 13 mg kg^-1, respectively. Primary mineral dissolution was observed by scanning electron microscopy. The formation of Cr- and Ni-rich secondary minerals was observed in the <2 µm size fraction by transmission electron microscopy. These secondary minerals include clay minerals (illite/smectite) and nanocrystalline Fe oxides. Clay minerals in UM soils contained Cr and Ni up to 0.7 and 0.9 wt% respectively and the nanocrystalline iron oxides contained 1.2 wt % Cr and 2.8 wt % Ni. Valley soils (<2 µm) had Cr and Ni concentrations as high as 700 and 3300 mg kg^-1 in clays and 1500 and 1800 mg kg^-1 in iron oxides, respectively.

Hexavalent Cr is a respiratory carcinogen and inhalation of Ni may contribute to lung disease. Therefore, exposure to valley dusts through inhalation may be a health concern. Surface soils were sieved to <20 µm and subjected to selective leaching by simulated lung fluid. The results show release of Cr and Ni to the solutions (0.2 to 2.7 µg Cr per g soil and 5 to 71 µg Ni per g soil). Dust samples collected near the cities of Sacramento and Stockton had elevated Cr content suggesting airborne transport of UM source material in the Sacramento Valley. Identifying mineral dust on air filters in urban samples has proven difficult due to filter composition and abundant urban particulates. A causal link between our data and specific health outcomes cannot be inferred, but the elevated incidence of lung cancer in our study area suggests further study is necessary.