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. Be cause they are often found concurrently, their relationship with respe ct to joint toxicity is of interest, particularly in regard to interpr eting the results of toxicity identification evaluations (TIEs) that p oint to metabolically activated organophosphorous pesticide(s) as caus es of toxicity. Joint toxicity was evaluated using static tests that i ncorporated 48- to 96-h exposure periods using laboratory water, natur al water, and urban storm water. Chemical concentrations were verified analytically. Mortalities were measured at 24-h intervals and joint t oxicity was calculated on the basis of toxic units (TUs) for every tim e interval (1 TU = median lethal concentration [LC50]). Forty-eight-ho ur LC50 values ranged between 0.26 and 0.58 mu g/L for diazinon and be tween 0.058 and 0.079 mu g/L for chlorpyrifos. The 96-h values were ap proximately 65% of their respective 48-h values. Under the assumption of strict additivity, the LC50 (as TUs) of the mixture should be the s um of the respective fractions of diazinon and chlorpyrifos and should total unity. These values were calculated for a total of 12 time inte rvals. 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 na tural and storm water samples were similar to those obtained with labo ratory water. These data confirmed the results of TIEs that suggested that diazinon and chlorpyrifos exhibit additive toxicity when present together.