Characterization and mitigation of risks associated with aquatic organism exposures to chlorpyrifos and chlorothalonil

Extensively used in agricultural and urban settings, chlorpyrifos (an insecticide) and chlorothalonil (a fungicide) are transported in stormwater runoff and can pose risks to receiving aquatic system biota. Overall objectives of this dissertation research were: (1) to characterize responses of Ceriodaphnia dubia Richard and Pimephales promelas Rafinesque exposed to individual aqueous solutions of chlorpyrifos and chlorothalonil; (2) to characterize responses of C. dubia and P. promelas in simultaneous and sequential exposures to aqueous mixtures of chlorpyrifos and chlorothalonil; (3) to determine removal rates of chlorpyrifos and chlorothalonil from simulated stormwater runoff in constructed wetland mesocosms; and (4) to measure declines of mortality of C. dubia and P. promelas in exposures to simulated stormwater runoff treated in constructed wetland mesocosms. Exposures of C. dubia and P. promelas to individual solutions of chlorpyrifos and chlorothalonil were used to obtain mortality and sublethal effects data. Those data were then used to predict concentrations of chlorpyrifos and chlorothalonil that would individually elicit partial mortality in simultaneous and sequential exposures to aqueous mixtures of the pesticides. Constructed wetland mesocosm experiments were conducted with simulated stormwater runoff containing risk-based and field-based concentrations of chlorpyrifos and chlorothalonil. In the individual pesticide characterization experiments, C. dubia was more sensitive to chlorpyrifos and P. promelas was more sensitive to chlorothalonil. There were also differences in exposure durations of the pesticides necessary to elicit effects. Less than additive effects were observed in C. dubia exposures to chlorpyrifos and chlorothalonil mixtures, while greater than additive effects were observed in P. promelas exposures. In the constructed wetland mesocosm experiment with field-based concentrations of the pesticide mixture, toxicity was eliminated for P. promelas but not for C. dubia , although a significant decrease in toxicity was observed with C. dubia. First order removal rates were 0.039/h for chlorpyrifos and 0.295/h for chlorothalonil in these experiments. The results from this study indicated that constructed wetlands could be part of an efficient mitigation strategy for stormwater runoff containing these pesticides.