Filter strips are widely prescribed to reduce contaminants in surface runoff from agricultural fields. This study compared performance of different filter strip designs on several contaminants and evaluated the contributing processes. Different vegetation types and widths were investigated using simulated runoff event on large plots (3 m × 7.5 or 15 m) having fine-textured soil and a 6 to 7% slope. Filter strips 7.5 and 15 m wide downslope greatly reduced concentrations of sediment in runoff (76–93%) and contaminants strongly associated with sediment (total P, 55–79%; permethrin, 27–83% [(3-phenoxyphenyl) methyl (±)-cis, trans-3-(2,2-dichloroethenyl)-2,2-dimethyicyclopropanecarboxylate]). They had less effect on concentrations of primarily dissolved contaminants [atrazine, −5–43% (2-chloro-4-ethylamino-6-isopropylamino-s-triazine); alachlor, 10–61% [2-chloro-2′6′-diethyl-N-(methoxymethyl) acetanilide]; nitrate, 24–48%; dissolved P, 19–43%; bromide, 13–31%]. Dilution of runoff by rainfall accounted for most of the reduction of concentration of dissolved contaminants. Infiltration (36–82% of runoff volume) substantially reduced the mass of contaminants exiting the filter strips. Doubling filter strip width from 7.5 to 15 m doubled infiltration and dilution, but did not improve sediment settling. Young trees and shrubs planted in the lower one-half of otherwise grass strips had no impact on filter performance. Compared with cultivated sorghum [Sorghum bicolor (L.) Moench] grass clearly reduced concentrations of sediment and associated contaminants in runoff, but not volume of runoff and concentration of dissolved contaminants. Settling, infiltration, and dilution processes can explain performance differences among pollutant types and filter strip designs.
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