The risk of trace element contamination of surface waters by wastes generated in a coal combustion power plant utilizing limestone scrubber technology was assessed by evaluating laboratory and site-specific biological and physical-chemical test results. Site-specific results from physical-chemical tests indicated that toxic trace element concentrations were primarily adsorbed onto ash pond suspended and dissolved solids. Treatment (primary settling) of the slurry discharged into the ash pond partially removed toxic fractions of trace elements from the water column. Complexation, competition, and precipitation of remaining soluble toxic elements in the ash pond buffered the final effluent's toxicity to indigenous aquatic fauna.