Joint acute toxicity of diazinon and chlorpyrifos to Ceriodaphnia dubia

Diazinon and chlorpyrifos are two organophosphorous pesticides widely found in municipal, agricultural, and urban storm water discharges. Because they are often found concurrently, their relationship with respect to joint toxicity is of interest, particularly in regard to interpreting the results of toxicity identification evaluations (TIEs) that point to metabolically activated organophosphorous pesticide(s) as causes of toxicity. Joint toxicity was evaluated using static tests that incorporated 48- to 96-h exposure periods using laboratory water, natural water, and urban storm water. Chemical concentrations were verified analytically. Mortalities were measured at 24-h intervals and joint toxicity was calculated on the basis of toxic units (TUs) for every time interval (1 TU = median lethal concentration [LC50]). Forty-eight-hour LC50 values ranged between 0.26 and 0.58 mu g/L for diazinon and between 0.058 and 0.079 mu g/L for chlorpyrifos. The 96-h values were approximately 65% of their respective 48-h values. Under the assumption of strict additivity, the LC50 (as TUs) of the mixture should be the sum of the respective fractions of diazinon and chlorpyrifos and should total unity. These values were calculated for a total of 12 time intervals. The TUs associated with the mixtures in laboratory water ranged between 0.89 and 1.46, with an average of 1.13. The values for the natural and storm water samples were similar to those obtained with laboratory water. These data confirmed the results of TIEs that suggested that diazinon and chlorpyrifos exhibit additive toxicity when present together.