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Metals (Al, Cd, Cu, Fe, Hg, Pb, and Zn) were determined in a resident, invertebrate species to assess the occurrence and distribution of biologically available metal along a 111 km section of the upper Sacramento River affected by acid mine drainage. Samples of the filter-feeding caddisfly Hydropsyche californica (Insecta: Trichoptera) were collected from five stations in the Sacramento River between Redding and Tehama, California. Metal concentrations in these samples were referenced to samples from an uncontaminated tributary, Cottonwood Creek. Aluminum, Cd, Cu, Fe, Pb, and Zn in the body of the insect were separated into cytosolic (soluble) and paniculate components. The cytosol is an intracellular metal fraction and therefore, an indicator of metal bioavailability. The concentrations and the relative proportions of metals in the cytosol identified differences in bioaccumulation among metals and characterized spatial patterns of bioavailable metals. Total Hg was determined in the whole body, but not the cytosol. Results showed that concentrations of Cd, Cu, Pb, and Zn in the whole body, and both the particulate and cytosolic fractions of Hydropsyche from the Sacramento River were significantly greater than in those from the reference site. Enrichment of Cd was greater than the other metals, exceeding the Cd reference concentration by nearly 20 fold at the most upstream station. Concentrations of Hg and Fe in caddisflies were not elevated. Aluminum concentrations were elevated in the cytosol, only. This enrichment was not evident in the whole body because cytosolic Al accounted for less than 1 percent of the total body burden. The vast majority of Al was associated with undigested gut content, the exoskeleton, and other structures associated with the particulate fraction. Concentra*ions in this fraction were not enriched. Likewise, only a minor fraction of the total Fe and Pb were associated with the cytosol. In contrast, a high proportion of the Cd, Cu, and Zn in Hydropsyche sp. were contemned in the cytosol, indicating that these rmtals were biologically available. Maximum concentrations of cytosolic Cd, Cu, Pb, and Al occurred at the three most upstream stations (river km 479-444) and decreased downstream to Tehama (river km 368), although a small increase in bioavailable Cd and Cu was observed at Bend Bridge (river km 415). Except for a small increase in cytosolic Zn at Bend Bridge, Zn concentrations did not attenuate downstream, but instead were uniformly elevated throughout the 111 km section. At Tehama, concentrations of cytosolic Al, Cd, Cu, Pb, and Zn were 1.5 to 5 times greater than reference concentrations from Cottonwood Creek, suggesting that transport of bioavailable forms of these metals extended downstream of Tehama. The results indicate that Hydropsyche in the upper Sacramento River were exposed to elevated concentrations of bioavailable Al, Cd, Cu, Pb, and Zn. Distribution patterns of bioavailable Al, Cd, Cu, and Pb were consistent with documented sourcer of metal from the East Shasta and V^est Shasta mining districts, upstream of Redding.

Metal exposure to a benthic invertebrate, Hydropsyche californica, in the Sacramento River down stream of Keswick Reservoir, California