Toxicity of metal-contaminated sediments from Keswick Reservoir, California, USA

Keswick Reservoir, California, USA, receives metal-laden acid-mine drainage (AMD) from the abandoned iron Mountain Mine. Mixing of the AMD with reserv oir water causes precipitation and deposition of metal-rich sludge in the r eservoir. Hydroelectric generation activities can scour the sediments and m obilize trace metals cadmium, copper, and zinc into the water column, thus creating potentially toxic conditions to fish and aquatic invertebrates. Se diment samples collected from Keswick Reservoir in 1993 and 1993 were analy zed for acid-volatile sulfides and for simultaneously extractable metals (S EM), and whole sediments and sediment elutriates were tested for toxicity t o rainbow trout (Oncorhynchus mykiss), amphipods (Hyalella azteca), and cla docerans (Ceriodaphnia dubia). Acid-volatile sulfide concentrations in the sediments were low (<10 mu mol/g H2S), indicating that dissolved metals in the sediment pore water were not limited by sulfide. The SEM concentrations were generally high (up to 11 mu g/g Cd, 4,800 mu g/g Cu, and 1,600 mu g/g Zn, dry weight) in the sediments. Whole sediments and 20% w/w sediment elu triates from 16 sites were tested for toxicity. Low survival (as low as 0%) in whole sediments was generally associated with copper and zinc, and to a lesser extent cadmium, concentrations that exceeded probable effect level values for freshwater sediments; survival also may have been influenced by low pH and alkalinity conditions. Low survival (as low as 0%) in sediment e lutriates was also generally associated with higher concentrations of disso lved zinc. Further study is required to formulate sediment cleanup levels t hat are protective of fish and wildlife. Source control in the Iron Mountai n Mine drainage will eventually significantly lessen the production of sedi ments.