PM12 Metals in the Environment
Exhibit Hall
8:00 AM - Monday
(PM206) Hydrobiogeochemical cycling of Cu and Zn near an AMD-affected stream.
Bambic, D1, Silk, W2, Alpers, C3, Green, P4, 1 Larry Walker Associates, Davis, CA, USA2 University of California, Davis - Dept. of Land, Air, and Water Resources, Davis, CA, USA3 U.S. Geological Survey, Sacramento, CA, USA4 University of California, Davis - Dept. of Civil and Environmental Engineering, Davis, CA, USA
ABSTRACT- In the western U.S., the Forest Service estimates that acid mine drainage (AMD) from 20,000 to 50,000 mines is impacting between 8,000 and 16,000 kilometers of streams (U.S. Forest Service 1993). This study examines the effect of such drainage on the local environment. At two week intervals throughout a water year, surface water, soil, and plants were sampled to characterize the spatio-temporal patterns of Cu and Zn cycling in the arid Hinckley Run watershed. Hinckley Run receives water from deep mine-workings. The herbaceous species sampled were Bromus carinatus (California Brome), Lolium multiflorum (Italian Ryegrass), and Vicia tetrasperma (Slender Vetch). Analysis of the Zn-Cu relationship in ground water, surface water, soil, and plant tissue was very useful. The groundwater exhibited an explicit Zn-Cu ratio. The Zn-Cu ratio suggested distinct sources of Zn and Cu in the watershed, significant seasonal differences in Zn and Cu loading, the presence of evaporated salts in soils, and the suitability of 0.01M CaCl2 for prediction of plant uptake. The spatial extent of the influence of Hinckley Run was investigated by sampling soils and vegetation near the stream bed. The effect of the acidic, metalliferous stream on soil metal bioavailability was dramatic but very local (constrained to within a few meters). However, the total soil metals were unaffected by the stream. The seasonal variation in Cu and Zn cycling was also examined. The results suggest temporally-invariant field studies may be inadequate in arid environments. The tissue Cu concentration of the grasses, but not the leguminous vetch, dramatically decreased during the winter. At the same time Zn uptake fluctuated, an indication of plant developmental stages and metal mobility differences. Temporal analysis of the stream-soil interaction reflected conservation of solute mass under reactive transport with negligible advective-dispersive flux. The results will be useful in future modeling of AMD-impacted watersheds.
Key words: hydrobiogeochemistry, acid mine drainage, plant uptake, Zn-Cu ratio