Filter strip performance and processes for different vegetation, widths, and contaminants

Filter strips are widely prescribed to reduce contaminants in surface runof f from agricultural fields. This study compared performance of different fi lter strip designs on several contaminants and evaluated the contributing p rocesses. Different vegetation types and widths were investigated using sim ulated runoff event on large plots (3 m x 7.5 or 15 m) having fine-textured soil and a 6 to 7% slope. Filter strips 7.5 and 15 m,vide downslope greatl y reduced concentrations of sediment in runoff (76-93%) and contaminants st rongly associated,with sediment (total P, 55-79%; permethrin, 27-83% [(3-ph enoxyphenyl) methyl (+/-)-cis, trans-3-(2,2-dichloroethenyl)-2,2-dimethylcy clopropanecarboxylate]). They had less effect on concentrations of primaril y dissolved contaminants [atrazine, -5-43% (2-chloro-4-ethylamino-6-isoprop ylamino-s-triazine); alachlor, 10-61% [2-chloro-2'6'-diethyl-(methoxymethyl ) acetanilide]; nitrate, 24-48%; dissolved P, 19-43%; bromide, 13-31%], Dil ution of runoff by rainfall accounted for most of the reduction of concentr ation of dissolved contaminants, Infiltration (36-82% of runoff volume) sub stantially reduced the mass of contaminants exiting the filter strips. Doub ling 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 t he lower one-half of otherwise grass strips had no impact on filter perform ance. Compared with cultivated sorghum [Sorghum bicolor (L.) Moench] grass clearly reduced concentrations of sediment and associated contaminants in r unoff, but not volume of runoff and concentration of dissolved contaminants . Settling, infiltration, and dilution processes can explain performance di fferences among pollutant types and biter strip designs.