Toxicity and bioaccumulation of DDT in freshwater amphipods in exposures to spiked sediments

The amphipods Hyalella azteca and Diporeia spp. were exposed to sediments d osed with dichlorodiphenyltrichloroethane (DDT), and the toxicity and toxic okinetics were determined. The toxicity was evaluated with the equilibrium partitioning (EqP) and critical body residue approaches. The DDT in the sed iments degraded during the equilibration period prior to organism exposure. Thus, the toxicity using EqP pore-water toxic units (TUs) was evaluated fo r DDT and its degradation product, dichlorodiphenyldichloroethane (DDD), as the ratio of the predicted interstitial water concentration divided by the water-only LC50 values. The sum of TUs (Sigma TU) was assumed to best repr esent the toxicity of the mixture. For H. azteca, the 10-d LC50 was 0.98 an d 0.33 :ETU for two experiments. For Diporeia spp., no toxicity was found i n the first experiment with up to 3 Sigma TU predicted in the interstitial water. However, in the second experiment, the 28-d LC50 was 0.67 ITU. These data suggest that the EqP approach approximately predicts the toxicity for the combination of DDT and DDD in sediment, provided a toxic unit approach is employed. The critical body residue approach also used TU's because DDT is biotransformed by H. azteca and because of the dual exposure to DDT and DDD. Because biotransformation was only determined in the second experimen t, the critical body residue approach could only be evaluated for that case . The TUs were calculated as the ratio of the concentration in the live amp hipods divided by the respective LR50 (residue concentration required to pr oduce 50% mortality) values. The LR50 was 1.1 Sigma TU for H. azteca for th e 10-d exposure and 0.53 for Diporeia spp. after a 28-d exposure. Thus, thi s approach was also quite successful in predicting the toxicity. The accumu lation and loss rates for H. azteca were much greater than for Diporeia spp . Thus, 10-d exposures represent steady-state conditions for H. azteca, whi le even at 28-d, the Diporeia spp. are not at steady state.