Filter Strip Performance and Processes for Different Vegetation , Widths , and Contaminants

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-6isopropylamino-s-triazine); alachlor, 10-61% [2-chloro-2’6’-diethylN-(methoxymethyl) acetanilide]; nitrate, 24-48%; dissolved P, 1943%; 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 onehalf 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. F ILTER STRIPS are narrow strips of permanent vegetation widely prescribed to reduce contaminants in surface runoff from adjacent agricultural fields. Nonpoint source pollution by contaminants in agricultural runoff is a major cause of poor surface water quality in the USA (National Research Council, 1993). Accurate prediction of water quality improvement resulting from widespread installation of filter strips is hindered by limited quantitative information on design factors that determine performance. Chief among these factors are vegetation composition, width, and the types of contaminants filter strips are employed to control. Grass vegetation is recommended for filter strips (U.S. Natural Resources Conservation Service, 1997). Extensive research and modeling has been conducted on grass strips (e.g., Arora et al., 1996; Barfield et al., 1979; Dillaha et al., 1989; Hayes et al., 1984; Magette et al., 1989; Williams and Nicks, 1988; Young et al., 1980). Few experimental studies, however, have comSchool of Natural Resource Sciences, Univ. of Nebraska, Lincoln, NE 68583-0814, and USDA-FS Natl. Agroforestry Ctr., East Campus, Univ. of Nebraska, Lincoln, NE 68583-0822. Journal Series no. 12533 of the Agric. Res. Div., Univ. of Nebraska. Received 28 Sept. 1998. *Corresponding author (mdosskey/[email protected]). Published in J. Environ. Qual. 28:1479-1489 (1999). pared grass with other vegetation types. Forest vegetation has been shown to remove substantial amounts of sediment and nutrients from agricultural runoff (e.g., Cooper and Gilliam, 1987; Lowrance et al., 1984; Peterjohn and Correll, 1984; Vought et al., 1991). Indirect comparisons of forest vegetation and grass reveal no clear differences for retaining sediment and nutrients from surface runoff (Daniels and Gilliam, 1996; Doyle et al., 1977). Contour-planted corn (Zea mays L.) was reported to be more effective than grass at retaining feedlot runoff and its contents of solids and nutrients (Young et al., 1980). Comparison of filter strips to agricultural vegetation, such as cultivated row crops, is necessary to properly evaluate the water quality impact of land conversion to permanent vegetation. Wider filter strips generally obtain greater retention of contaminants in surface runoff than narrower strips. Decreasing concentrations of sediment and nutrients in surface runoff have been observed along forested transects (Peterj ohn and Correll, 1984). Rigorous experiments on grass filter strips show a clear nonlinear relationship for sediment, where most sediment (53-86% of suspended mass) is retained within 4.6 m of the uphill edge, with much smaller additional amounts (4-17%) retained by doubling this width (Dillaha et al., 1989; Magette et al., 1989; Mickelson and Baker, 1993). Width relationships for nutrients and pesticides have received less study and the results appear more variable than for sediment (Dillaha et al., 1989; Mickelson and Baker, 1993). The impact of filter strips on runoff water quality depends on the type of contaminant. Several direct comparisons show sediment in runoff is consistently retained by grass to a greater degree than soluble nutrients and herbicides (Arora et al., 1996; Asmussen et al., 1977; Dillaha et al., 1989; Mickelson and Baker, 1993). Differences in impact among contaminants may be attributed to mechanisms in filter strips that affect each type, such as settling, infiltration, and dilution. Few experimental studies have been conducted that separate the effects of these various processes (Arora et al., 1996). The objectives of this study were to (i) quantify the impact of filter strips on agricultural surface runoff and several contaminants commonly found in it; (ii) quantify the benefit of doubling filter strip width; (iii) compare the effectiveness of different vegetation compositions; (iv) evaluate and compare settling, infiltration, and dilution processes in filter strips; and (v) estimate the water Abbreviations: PVC, polyvinyl chloride; LDPE, low density polyethylene; TSS, total suspended solids; TN, total nitrogen; N+N, nitrate + nitrite nitrogen; TP, total phosphorus; BAP, bioavailable phosphorus; TDP, total dissolved phosphorus; BR, bromide; ATR, atrazine; ALA, alachlor; PER, permethrin.