Acute Toxicity of Drainage Ditch Water from a Washington State Cranberry-Growing Region to Daphnia pulex in Laboratory Bioassays

High concentrations of organophosphorous insecticides resulting from cranberry bog applications were detected in the Grayland Drainage Ditch (GDD) system in Grayland, Washington State, during the 1994–1996 Washington State Department of Ecology Pesticide Monitoring Program. This drainage ditch system drains cranberry bogs and enters the Pacific Ocean via the North Cove and Supon Inlet. Concerns about the impact of these pesticides on human and environmental health led to this investigation of the potential impact on an indicator species, Daphnia pulex. To determine the toxic effects of multiple pesticides entering the GDD, standardized laboratory toxicity tests with D. pulex were conducted concurrently with the Washington State Department of Ecology pesticide sampling. Concentrations of three insecticides, diazinon, chlorpyrifos, and azinphos- methyl, were the highest ever detected in state waters. The GDD water was found to cause acute toxicity in 33− of the laboratory bioassays conducted. Regression analysis, however, detected a poor correlation between total insecticide detected and percentage mortality of D. pulex at the two drainage ditch sites studied, Grays Harbor County site and the Pacific County site. However, the relationship between mortality of D. pulex and detected concentrations of diazinon and chlorpyrifos were significant. Sampling schedules for chemical analysis and bioassay testing appear to be the primary reason that statistical analysis failed to correlate mortality with detected OP pesticide concentrations. Grab samples used in toxicity testing may over- or underestimate actual concentrations of contaminants present in the system being studied.