Acid mine waters from the Iron Mountain Superfund Site, Shasta County, California, flow through Spring Creek Reservoir and into Keswick Reservoir on the Sacramento River. In Keswick Reservoir, the acid mine waters have neutralized on mixing with neutral-pH lake water, producing fine-grained, metalrich sediments. Sediment cores were collected during 1997 from both reservoirs for characterization and pore waters were extracted under anoxic conditions. Chemical composition, mineralogical identification, redox chemistry, sorption properties, and toxicity were determined on several samples. Metal concentrations in sediment ranged from 4 to 47 % for Fe, 200 to 4,800 mg/kg (milligrams per kilogram) for Cu, and 85 to 6,600 mg/kg for Zn. Pore waters ranged in pH from 4.7 to 6.7 and their Fe(II) concentration range was 10 to 2,000 mg/L (milligrams per liter). Although pore-water Zn concentrations ranged from 0.1 to 9 mg/L, Cu concentrations were less than 0.01 mg/L. Considerable reductive iron dissolution has occurred in the Keswick Reservoir sediments, but there is little or no indication of sulfate reduction. Adsorption and desorption experiments for Cu, Zn, and Cd on composite sediment samples demonstrated typical sorption behavior for metal ions on iron oxides, except that the adsorption edge is moved about one pH unit lower than expected compared to a hydrous ferric oxide substrate, but similar to that for a schwertmannite (ferric oxyhydroxysulfate) substrate. Schwertmannite was identified in the sediments by x-ray diffraction and Mössbauer spectroscopy. Toxicity tests, using dilutions of Keswick sediment pore waters and Ceriodaphnia dubia as a test animal, demonstrated that iron is the causative agent for both acute and chronic toxicity with a minor contribution to toxicity from zinc
