Bulk Deposition of Pesticides in a Canadian City: Part 1. Glyphosate and Other Agricultural Pesticides

Winnipeg is a city in the Canadian Prairies with a population of about 600,000. Like many other cities and towns in this region of Canada, the city is surrounded by agriculture. Weekly bulk deposition samples were collected from May to September in 2010 and 2011 and analyzed for 43 pesticides used in Prairie agriculture. Fourteen herbicides, five herbicide metabolites, two insecticides, and two fungicides were detected with 98.5 % of the samples containing chemical mixtures. Glyphosate is the most widely used pesticide in Prairie agriculture and accounted for 65 % of the total pesticide deposition over the 2 years. Seasonal glyphosate deposition was more than five times larger in 2011 (182 mm rain) than 2010 (487 mm rain), suggesting increased glyphosate particulate transport in the atmosphere during the drier year. The seasonal deposition of ten other frequently herbicides was significantly positively correlated with the amount of herbicides applied both in and around Winnipeg (r=0.90, P<0.001) and with agricultural herbicide use around Winnipeg (r=0.63, P=0.05), but not with agricultural herbicide use province wide (P=0.23). Herbicides 2,4-D (2,4-dichlorophenoxyacetic acid), dicamba, and mecoprop had known urban applications and were more consistently detected in samples relative to bromoxynil and 2-methyl-4-chlorophenoxyacetic acid (MCPA) whose frequency of detections decreased throughout August and September. The Canadian Water Quality Guidelines for irrigation water were frequently exceeded for both dicamba (75 %) and MCPA (49 %) concentrations in rain. None of glyphosate concentrations in rain exceeded any of the Canadian Water Quality Guidelines established for this herbicide.