A toxicological and chemical evaluation of agricultural runoff discharged into the Nicomekl River, throughout one growing season

Agricultural pesticide and manure use 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 receiving waters. To investigate this possibility, water samples were collected from six drainage ditches and four river locations every three weeks between May and November, 1997. For each of the 85 water samples collected, chronic (7 ± 1 day) survival and reproduction of the cladoceran test organism Ceriodaphnia dubia was determined and compared to that for samples of river water collected upstream of the area of concern. Throughout the 6-month monitoring period, none of the 35 riverwater samples tested exhibited statistically significant mortality, and only two of the 50 ditchwater samples obtained were lethally toxic, with 6-day LC50's of 39.9% and 36.5%. For the remaining 83 water samples, C. dubia reproduction in 5 ditchwater and 5 riverwater samples, from the region of concern, was statistically lower than that in samples of upstream river water. One of the sublethally toxic riverwater samples exhibited toxic responses (paralysis) characteristic of organophosphorous (OP) pesticide contamination, and was collected immediately downstream of a lethally toxic ditch which had discharged within the previous 24 hours. Another sublethally toxic ditch sample had a total ammonia concentration of 10.8 mg/L NH3-N which was believed to be responsible its observed C. dubia reproduction inhibition. This ditch discharged for minimal durations (1-2 hours/day) in the days prior to and following its confirmed NH₃ contamination. In the immediate vicinity of these discharges, ammonia concentrations within the river may have exceeded acute Canadian water quality guidelines for ammonia intended to protect sensitive fish and invertebrate species from acute toxic effects. Further downstream, fully-mixed ammonia concentrations should not have exceeded these acute guidelines, but may have exceeded safe chronic exposure levels. The remaining 24 ditchwater and 18 riverwater samples which were measured for total ammonia all had NH3 concentrations well below the water quality guidelines for chronic exposure. A biological toxicity identification evaluation (TIE) using piperonyl butoxide (PBO) determined that the toxicant(s) in each of the two ditchwater samples which proved lethal to C. dubia were metabolically active OP insecticide(s). Solid phase extraction full-ion-scan gas chromatography mass spectroscopy (GC/MS) analyses performed on the lethal and some of the sublethal samples immediately following observed toxicity were unable to detect the presence of OPs. Liquid-liquid extraction select-ion-scan GC/MS of the lethal samples detected 0.02 - 0.03 µg/L diazinon (OP) in each of the two acutely lethal samples, and 0.03 µg/L chlorpyrifos (OP) and 3 µg/L prometryn (herbicide) in one of the lethal samples; even though there was evidence of OP insecticide losses` during the frozen storage of these samples before their chemical analyses. C. dubia bioassays using portions of the thawed samples used for these later chemical analyses exhibited lesser toxicity relative to that for the fresh samples. Consideration of the analytical values for diazinon and chlorpyrifos together with the toxicity values for these pesticides, determined as part of this investigation and by other researchers, led to the tentative conclusion that diazinon and/or chlorpyrifos were responsible (or at least partly so) for the observed toxic effects. Prometryn is appreciably (i.e., four orders of magnitude) less toxic than either of these two OP pesticides. Diazinon's American suggested acute water quality criteria of 0.08 /^g/L was possibly exceeded in the Nicomekl River during the recorded discharge of one of the two OP contaminated ditches, in the days prior to its confirmed lethal toxicity. Five ditch sediments and three river sediments were collected from the study site in October, 1997, in order to appraise their toxicity to benthic invertebrates. Chronic (14-day) survival and growth inhibition to the amphipod test organism Hyallela azteca for each sample was compared to that for sediment collected from the upstream Nicomekl site. None of sediments collected within the region of agriculture under investigation showed statistically-lower survival relative to that for river sediment collected upstream of the region of concern. One drainage ditch sediment statistically inhibited H. azteca growth. Using a standardized (Environment Canada) laboratory test method and upstream river water as the control water, the majority of ditchwater and riverwater samples collected at 3-week intervals throughout this 6-month monitoring study were not acutely or chronically toxic to Ceriodaphnia dubia. This study did identify that there can be occasional municipal pumping of toxic agricultural runoff waters into the Nicomekl River in the summer months. However, overall, the study site's 1997 agricultural activities and drainage ditch discharges should not have had a significant toxic effect on the biota of the Nicomekl River.

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