Pesticides in stream water within an agricultural catchment in southern Sweden, 1990-1996.

Pesticide loss to stream water was studied in a small agricultural catchment in southern Sweden during the period 1990-1996. A total of 38 pesticides were detected in water samples, including 30 herbicides, four fungicides, three insecticides and one metabolite of one of the herbicides. Concentrations of pesticides in stream water were observed throughout the sampling periods. Peak concentrations occurred during the spraying seasons and following runoff events. Daily average concentrations sometimes varied by one order of magnitude from one day to another. Pesticides were also found in water samples as a result of incautious actions during handling and application procedures. Concentrations were lower at the outlet of the catchment area when the water had passed an open part of the stream, compared to concentrations detected in discharge from a culvert system upstream. This was largely a result of dilution from groundwater intrusion during low-flow periods. Sampling at different sites along the culvert demonstrated that the small village situated in the catchment did not contribute to pesticide findings in the culvert discharge. Wind drift had little influence on stream-water quality. Pesticide application for weed control in farmyards resulted in a substantial contribution to the pesticide load in stream water. Pesticide were persistent in the discharge throughout the winter and originated from both autumn and spring applications, as well as from farmyard application. Some autumn applied pesticides prevailed in stream flow during the following summer. Total amounts of pesticides lost in stream flow during May-September each year varied between 0.5 and 2.8 kg during the 7-year period, corresponding to approximately 0.1% of the applied amount. Losses of single pesticides were generally less than 0.3% of the applied amount during individual years. Pesticides from agricultural applications in the catchment constituted, on average, 82% of the total transported amount lost during May-September each year, of which 2% was from autumn application the previous year. There was an overall correlation between amounts used in the catchment and occurrence in the water samples. The total pesticide load in water decreased markedly during the course of the investigation, in accordance with decreased amounts applied during spring and early summer. The results indicate that concentrations of some pesticides entering head-water streams in agricultural areas are close to, and during certain time periods even above those levels demonstrated as having an impact on the aquatic flora and fauna.