A comparison of toxicity tests conducted in the laboratory and in experimental ponds using cadmium and the fathead minnow (Pimephales promelas)

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.