Herbicides in Alberta Rainfall as Affected by Location, Use and Season: 1999 to 2000

A study of herbicides in Alberta rainfall was conducted at 17 locations in 1999 and 18 locations in 2000. Rainfall samples (bulk deposition) were collected using a funnel and glass bottle at weekly to biweekly intervals from April to September. Samples were analyzed for 13 herbicides in 1999 and 19 herbicides in 2000 using a MSD-GC method. Herbicides were detected in the rainfall on most sample dates at every location. The herbicides detected in the highest amounts (in order) were: 2,4-D, MCPA, bromoxynil, dicamba and mecoprop. Detections were quite consistent between years. Herbicide amounts over the season were lowest at the remote locations (21-96 mug/m(2) 2,4-D), intermediate in the City of Lethbridge (82-109 mug/m(2) 2,4-D) and highest in the farming areas (44-315 mug/m(2) 2,4-D). The highest herbicide concentrations (17-53 mug/L 2,4-D) occurred when there was a small rainfall (0.1-0.2 mm) during the agricultural spraying. season. Herbicide concentrations occasionally exceeded the Canadian Drinking Water Guidelines and frequently exceeded the Aquatic Life Guidelines. In general, herbicide levels in rainfall reflected agricultural use (sales) patterns. The highest 2,4-D detections occurred in southern Alberta, the highest MCPA detections were in central Alberta. In a side-by-side test, our funnel method of collecting rainfall yielded herbicide amounts that were nine times higher than a wet-only automated MIC sampler. Most previous Canadian studies used the MIC sampler and therefore may have underestimated levels of herbicides in rainfall. A more expansive study is needed to determine the levels of herbicides in rainfall across the Canadian Prairies.