TOXICITY OF CADMIUM IN WATER AND SEDIMENT SLURRIES TO DAPHNIA MAGNA

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Abstract
D. magna were exposed to Cd for 48 h in water and in equilibrated water-sediment slurries using beakers and recirculating test chambers. Differences in toxicity based on dissolved and free Cd2+ between the 2 levels of tested total solids, 100 and 1000 mg/l, were not significant (.alpha. = 0.05). The mean water LC50 of 39 .mu.g/l total Cd was significantly (.alpha. = 0.05) lower than the mean LC50 of 91 .mu.g/l dissolved Cd in the slurries, indicating that Cd adsorbed to the sediment had negligible toxicity. Low LC50 values in 2 sediment bioassays corresponded with low pH. Adsorption isotherms using Cd, well water and freshwater sediment were developed for sediment concentrations approximating those at which the bioassays were performed. Conditional adsorption constants derived from the isotherms predicted the distribution of Cd between well water and sediments. A speciation model predicted that organic ligands released from the sediments strongly complexed the free Cd ion. Based on free Cd ion concentrations, no significant differences (.alpha. = 0.05) could be detected between the mean of the slurry bioassay LC50 (47 .mu.g/l) and aqueous bioassay LC50 (30 .mu.g/l). Speciation results, LC50 values based on free Cd ion concentrations only, and pH results gave indirect evidence that the free Cd ion was the predominant toxic species to C. magna in the bioassays.