HR: 1340h
AN: H43C-1503    [Abstracts]
TI: Naturally Occurring Cr and Ni in the Sacramento Valley: II. Mn Oxides and the Mobility of Cr(VI) and Ni
AU: * Mills, C T
EM: cmills@usgs.gov
AF: U.S. Geological Survey, Denver Federal Center, MS 964, Denver, CO 80225, United States
AU: Morrison, J M
EM: jmorrison@usgs.gov
AF: U.S. Geological Survey, Denver Federal Center, MS 964, Denver, CO 80225, United States
AU: Goldhaber, M B
EM: mgold@usgs.gov
AF: U.S. Geological Survey, Denver Federal Center, MS 964, Denver, CO 80225, United States
AU: Foster, A L
EM: afoster@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 901, Menlo Park, CA 94025, United States
AU: Wolf, R E
EM: rwolf@usgs.gov
AF: U.S. Geological Survey, Denver Federal Center, MS 964, Denver, CO 80225, United States
AU: Wanty, R B
EM: rwanty@usgs.gov
AF: U.S. Geological Survey, Denver Federal Center, MS 964, Denver, CO 80225, United States
AB: Soil manganese oxides can strongly affect the mobility and redox state of several toxic trace metals. We are studying the biogeochemical origin of Mn oxides and their association with Cr and Ni in soils of the Sacramento Valley, California. Both Cr and Ni are likely derived from ultramafic rocks that underlie Coast Range drainages to the west of the study area. The impact of weathering and erosion of these rocks is evident in the high levels of total Cr (80 to 1420 μg g^-1) and nickel (65 to 224 μg g^-1) that occur broadly in western Sacramento Valley soils. Although much of the Cr is bound in refractory spinels as Cr(III), some mobilization of Cr is apparent in the coincidence of enriched soils with high contents of Cr(VI) in ground water. Data from the National Water Information System (NWIS) shows 7 of 12 sampled wells within a 600 km² area in the Sacramento Valley having Cr(VI) concentrations between 60 and 100% of the CA maximum contaminant level for drinking water (50 μg l^-1). A 3-meter depth soil profile collected within the lower Putah Creek watershed was examined to investigate processes contributing to the oxidation and mobilization of natural Cr(III). Hydroxylamine hydrochloride-reducible Mn was determined for 8 depth intervals as a measure of manganese oxide occurrence. Concentrations of reducible Mn varied between 360 and 690 μg g^-1 with depth and peaked at 2.7 m below the surface. Concentrations of anion exchangeable Cr(VI) were as high as 6 ng g^-1 and were positively correlated (r²=0.59; p=0.07) with reducible Mn. Scanning electron microscopy of soil minerals from the 2.9 to 3.0 m interval showed Cr-bearing spinel grains enclosed within Mn oxide micro concretions suggesting a potential mechanism for the oxidation of natural Cr(III) to mobile Cr(VI). Consistent with the known tendency of Ni to sorb on Mn oxides, substantial Ni (13 to 45 μg g^-1) was released in the reducible Mn fraction and it strongly correlates (r²=0.76; p=0.005) with reducible Mn concentration. Further work is exploring potential biogeochemical origins of the Mn oxides within these soils using phospholipid fatty acid and rDNA analyses and assessing the effects of seasonal redox cycling events on Mn oxides and the availability of Cr(VI) and Ni within the vadose zone of valley soils.
DE: 0330 Geochemical cycles (1030)
DE: 0409 Bioavailability: chemical speciation and complexation
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0486 Soils/pedology (1865)
SC: Hydrology [H]
MN: 2007 Fall Meeting