The use of organophosphate insecticides on commercial vegetable and blueberry farmlands bordering the Nicomekl River, Surrey, B.C., creates the potential for toxic effects on the biota within the drainage ditches and the river itself. To investigate the frequency and magnitude of the toxicity of drainage ditch water and the river water in the vicinity of ditch discharge points, together with the probable cause, water samples were collected from six drainage ditches, and four river locations, at 3-week intervals between June and November 1997. For each of the water samples collected, chronic (7 ± 1 day) survival and reproduction of the cladoceran test organism Ceriodaphnia dubia were compared to that of Nicomekl River headland waters beyond the influence of commercial growing operations. Portions of samples proving to be lethally toxic to the toxicity-test organism were analyzed for organophosphate insecticides (OPs) and subjected to treatment with piperonyl butoxide to ascertain if OPs were the likely contributing cause of the toxicity.

Throughout the 6-month monitoring period, a total of 50 ditch water samples and 35 Nicomekl river water samples were collected. None of the river water samples tested produced statistically significant mortality. Two (4%) of the ditch water samples were lethally toxic, with 6- and 7-day median lethal concentrations (LC50s) of 39.9 and 36.5%, respectively. Seven (14%) of the ditch water samples and three (9%) of the river water samples inhibited C. dubia reproduction. A biological toxicity identification evaluation using piperonyl butoxide determined that the toxicant(s) in each of the two ditch water samples which proved lethal to C. dubia were likely metabolically active OP insecti-cide(s). Later chemical analyses on stored portions of the samples revealed trace quantities of chlorpyrifos and/or diazinon. These and other non-measured OPs are believed to have been responsible for the observed lethality. The cause of the inhibited reproduction is for the most part unknown. Providing the 1997 growing season is a typical growing season in terms of OP insecticide use and rainfall, the C. dubia toxicity test results suggest that during the growing season the Nicomekl River and its drainage ditches may periodically be contaminated with OP insecticides at concentrations high enough to sublethally or lethally impact sensitive ditch and river invertebrate fish-food organisms.

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