THE ORIGIN AND BIOACCESSIBILITY OF CHROMIUM AND NICKEL IN SOILS OF THE SACRAMENTO VALLEY, CALIFORNIA: POTENTIAL LINKS TO HUMAN HEALTH

A study in north-central California is investigating the regional transport and mobility of geogenic trace elements and their potential risk to human health. Historic and modern land use patterns (e.g., mining, agriculture) and the Mediterranean climate with cool, wet winters and hot, dry summers exacerbate the potential for human exposure to airborne trace elements and minerals. Ultramafic (UM) rocks, primarily serpentinite, in the Coast Range and Sierra Nevada foothills contain high levels of Cr (1700 to 10,000 mg kg^-1), Ni (1300 to 3900 mg kg^-1), and asbestiform minerals, which all have human health implications. Hexavalent Cr and asbestiform minerals are respiratory carcinogens, and inhalation of Ni may contribute to lung disease. Regional-scale transport and weathering of UM rocks 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 the U.S. geometric means of 37 and 13 mg kg^-1, respectively. Nickel in UM source rocks and soils is associated with serpentine minerals (lizardite, antigorite, and chrysotile) and is more labile compared to Cr, which primarily resides in the refractory mineral chromite ([Mg,Fe²⁺][Cr³⁺,Al,Fe³⁺]₂O₄).

In Sacramento Valley soils, much of the Cr remains as chromite. However, some Cr and much of the Ni are associated with more labile (bioaccessible) phases. Results from transmission electron microscopy analysis of the <2 µm fraction of source and valley soils reveal Cr and Ni associated with nanocrystalline iron oxides and clay minerals (smectite/illite). Asbestiform minerals are also present in this fraction. Dust samples collected near the cities of Sacramento and Stockton have elevated Cr content suggesting airborne transport of UM source material in the Sacramento Valley. We are investigating potential links between soil geochemistry and human-health issues as Cr and Ni-rich minerals undergo weathering. 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 2.72 µg Cr per g soil and 5 to 71 µg Ni per g soil). Although a causal link between our data and specific health outcomes cannot be inferred, the elevated incidence of lung cancer in our study area suggests further study is necessary.