Temporal evolution of atrazine, metolachlor, and deethylatrazine concentrations in runoff from grass filter strips in a four-season study

Abstract

The optimization of grass filter strip dimensions is important to strike a balance between environmental benefits and loss of agricultural land. The objective of this study was to investigate the temporal evolution of dissolved atrazine, metolachlor, and deethylatrazine (DEA) concentrations as well as the DEA to atrazine ratio (DAR) values in runoff from grass filter strips for the first (four to five) rain events occurring after annual herbicide application over four seasons (1998 to 2001) under natural rain conditions. The 16 plots (65 m long by 3 m wide [213 ft long by 10 ft wide]) were laid in a completely randomized block design with four treatments: 0 m filter strip and grass filter strips of 3, 6, or 9 m (10, 20, or 30 ft). The soil texture was 42% silt, 38% sand, and 20% clay with 3.1% of organic matter; the pH was 6.3; and the slope was 2% to 3%. Runoff-generating plots were left bare in order to maximize runoff. The strips were composed of 45% creeping red fescue (Festuca rubra L.), 45% bent grass (Agrostis alba L.), and 10% perennial ryegrass (Lolium perenne L.). Grass filter strips reduced exported dissolved herbicide concentrations at least by half for the first two rainfalls of each study year. By the end of the sampling period for each season, dissolved concentrations for grass filter strips were most of the time, except for metolachlor in the 6 m grass strip in 1999, less than the criteria for the protection of aquatic life (chronic) (7.8 μg L⁻¹ [7.8 ppb] for metolachlor (provisory) and 1.8 μg L⁻¹ [1.8 ppb] for atrazine), which was not the case with the 0 m strip. Grass filter strips reduced the frequency at which these criteria were exceeded. Generally, exported dissolved herbicide concentrations decreased with time except when a rain event of high intensity occurred. DAR values never exceeded 0.66 and generally showed an increasing trend with time during the sampling periods of 1998 to 2000. Grass filter strips are an efficient mitigation measure against the risk of contamination of surface waters by atrazine and metolachlor.