Monitoring agricultural drainage ditches and the receiving water (NicomeklRiver, Surrey, BC) for toxicity to Ceriodaphnia dubia and probable cause due to organophosphate contamination

The use of organophosphate insecticides on commercial vegetable and blueber ry farmlands bordering the Nicomekl River, Surrey, B.C., creates the potent ial for toxic effects on the biota within the drainage ditches and the rive r itself. To investigate the frequency and magnitude of the toxicity of dra inage ditch water and the river water in the vicinity of ditch discharge po ints, together with the probable cause, water samples were collected from s ix 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 Cerio daphnia dubia were compared to that of Nicomekl River headland waters beyon d the influence of commercial growing operations. Portions of samples provi ng to be lethally toxic to the toxicity-test organism were analyzed for org anophosphate insecticides (OPs) and subjected to treatment with piperonyl b utoxide to ascertain if OPs were the likely contributing cause of the toxic ity. 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 wate r samples tested produced statistically significant mortality. Two (4%) of the ditch water samples were lethally toxic, with 6- and 7-day median letha l concentrations (LC50s) of 39.9 and 36.5%, respectively. Seven (14%) of th e 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 ditc h water samples which proved lethal to C. dubia were likely metabolically a ctive OP insecticide(s). Later chemical analyses on stored portions of the samples revealed trace quantities of chlorpyrifos and/or diazinon. These an d other non-measured OPs are believed to have been responsible for the obse rved lethality. The cause of the inhibited reproduction is for the most par t unknown. Providing the 1997 growing season is a typical growing season in terms of OP insecticide use and rainfall, the C, dubia toxicity test resul ts suggest that during the growing season the Nicomekl River and its draina ge ditches may periodically be contaminated with OP insecticides at concent rations high enough to sublethally or lethally impact sensitive ditch and r iver invertebrate fish-food organisms.