Seasonal variation of herbicide concentrations in prairie farm dugouts.

Prairie farm dugouts are frequently constructed for use as potable water sources. Consequently, cumulative pesticide inputs via atmospheric deposition and surface runoff may constitute a risk to human health. Since, relative to other pesticides, herbicides are used in greatest amount on the Canadian prairies, herbicide concentrations were intensively monitored in three dugouts over three growing seasons. Herbicides were detected in the water of all three dugouts each growing season which may reflect cumulative inputs from atmospheric and surface processes over the lifetimes of the dugouts, which varied from 11 to 22 yr. Detections, which were not continuous, tended to be seasonal in nature. During the 3-yr study, detections were most frequent during the spring application period and late fall following dugout turnover. Between these periods, herbicide concentrations generally decreased to below detection limits. The reappearance of herbicides in the dugout water during fall turnover and in concentrations generally greater than those present during the spring application period suggest that, under appropriate environmental conditions, the bottom sediments may act as a source of herbicides to the water column. In general, herbicide inputs due to deposition of application drift did not result in detectable concentrations of herbicides in the dugouts. In the only year that winter samples were monitored, herbicides were also detected during ice cover. On the basis of monthly sampling over each growing season, median concentrations of 9 of the 10 herbicides monitored were less than 0.05 microg L(-1). The exception, 2,4-D, which has been used extensively on the Canadian prairies for more than 50 yr and in greatest amounts, was the most frequently detected herbicide. In no case did herbicide concentrations exceed Canadian drinking water guidelines; however, on occasion maximum herbicide concentrations did exceed aquatic life and irrigation water guidelines.