Abstract: | Acute toxicity tests were conducted in the laboratory with fathead minnows (Pimephales promelas) to determine the 96-h LC50 of cadmium under three conditions: (1) in laboratory water, (2) in water from experimental ponds, and (3) in pond water underlain by sediment. Cadmium was then applied at doses equivalent to the estimated LC50 values to 0.07-ha ponds containing caged fathead minnows. A cadmium ion selective electrode, ultrafiltration, and equilibrium calculations were used to determine cadmium speciation, and several water quality characteristics were measured to correlate differences in mortality between test systems (laboratory and field) with observed differences in water quality. The LC50 estimates (mg l−1) for the bioassays were 4.39 for the laboratory water, 3.52 for the pond water with sediment, and 2.91 for the pond water. Concentrations of Cd2+ decreased and those of cadmium in the particulate (> 1.2 μm) and 300,000 mol. wt (0.018–1.2 μm) fractions increased over the 96-h; cadmium in these fractions was believed to consist of colloidal sized CdCO3 precipitates. Concentrations of Cd2+ decreased at different rates between test systems, regulated by the degree of CdCO3(s) supersaturation which in turn depended on pH and total metal concentrations. Differences in toxicity in the laboratory tests were attributed to differences in water hardness and Cd2+ concentrations. Mortality of fathead minnows was low (0–10%) during the 96-h test period in the ponds due to the higher pH, which produced supersaturated conditions resulting in the rapid formation of nontoxic CdCO3 precipitates and a more rapid decrease in Cd2+ concentrations as compared to the laboratory bioassays. |