Joint acute toxicity of diazinon and ammonia to Ceriodaphnia dubia

Dazinon is an organophosphorous pesticide widely found in municipal effluents as well as in agricultural and urban storm‐water discharges. Ammonia is frequently also present in such effluents as a consequence of bacterial degradation of organic material. Because these two contaminants may occur concurrently, their relationship with respect to joint toxicity is of interest, particularly in regard to interpreting the results of effluent tests and subsequent toxicity identification evaluations (TIEs). In this particular case, we obtained an effluent sample that exhibited toxicity to Ceriodaphnia dubia . Toxicity identification evaluations manipulations suggested that ammonia (40 mg/L as total NH3) and diazinon (0.75 μg/L) both contributed to toxicity. As part of the Phase 3 confirmation studies, an independent investigation was conducted to evaluate the interactions between these two toxicants using static tests that incorporated a 48‐h exposure period. Chemical concentrations were verified analytically. Mortalities were measured at 24‐h intervals and joint toxicity calculated on the basis of toxic units (TUs) for each toxicant. The 48‐h LC50 values (lethal concentrations for 50% of the population) for the individual chemicals averaged 0.36 μg/L and 1.11 mg/L for diazinon and un‐ionized ammonia, respectively. Under the assumption of strict additivity, the sum of TUs contributed by each toxicant at the LC50 of the mixture should total unity. The TUs associated with the mixtures in laboratory water averaged 1.37 and 1.47, respectively, for 24‐ and 48‐h exposure intervals. These results were similar to those obtained with the actual effluent sample and suggest that diazinon and ammonia exhibit less than additive toxicity when present together.