title: Impact of preferential flow at varying irrigation rates by quantifying mass fluxes.
abstract: Solute concentration and soluble dye studies inferring that preferential flow accelerates field-scale contaminant transport are common but flux measurements quantifying its impact are essentially nonexistent. A tile-drain facility was used to determine the influence of matrix and preferential flow processes on the flux of mobile tracers subjected to different irrigation regimes (4.4 and 0.89 mm h(-1)) in a silt loam soil. After tile outflow reached steady state either bromide (Br; 280 kg ha(-1)) or pentafluorobenzoic acid (PFBA; 121 kg ha(-1)) was applied through the irrigation system inside a shed (3.5 x 24 m). Bromide fluxes were monitored at an irrigation rate of 4.4 mm h(-1) while PFBA fluxes were monitored at an irrigation rate of 0.89 mm h(-1). At 4.4 mm h(-1) nearly one-third of the surface-applied Br was recovered in the tile line after only 124 mm of irrigation and was poorly fit by the one-dimensional convective-dispersive equation (CDE). On the other hand, the one-dimensional CDE fit the main PFBA breakthrough pattern almost perfectly, suggesting the PFBA transport was dominated by matrix flow. Furthermore, after 225 mm of water had been applied, less than 2% of the applied PFBA had been leached through the soil compared with more than 59% of the applied Br. This study demonstrates that the methodology of applying a narrow strip of chemical to a tile drain facility is appropriate for quantifying chemical fluxes at the small-field scale and also suggests that there may be a critical input flux whereby preferential flow is initiated.
authors.affiliation: Hydrology and Remote Sensing Laboratory, USDA-ARS, BARC-West, 10300 Baltimore Boulevard, Beltsville, MD 20705-2350, USA. tgish@hydrolab.arsusda.gov
authors.forename: T J
authors.initials: TJ
authors.lastname: Gish
authors.forename: K J S
authors.initials: KJ
authors.lastname: Kung
authors.forename: D C
authors.initials: DC
authors.lastname: Perry
authors.forename: J
authors.initials: J
authors.lastname: Posner
authors.forename: G
authors.initials: G
authors.lastname: Bubenzer
authors.forename: C S
authors.initials: CS
authors.lastname: Helling
authors.forename: E J
authors.initials: EJ
authors.lastname: Kladivko
authors.forename: T S
authors.initials: TS
authors.lastname: Steenhuis
mesh_terms: D000383:Agriculture; D004396:Coloring Agents / Q000032:analysis; D008962:Models, Theoretical; D014872:Water Movements; D014873:Water Pollutants / Q000032:analysis*; D014881:Water Supply
chemical_list: D004396:Coloring Agents; D014873:Water Pollutants