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TECHNICAL REPORTS

Vadose Zone Processes and Chemical Transport

Impact of Preferential Flow at Varying Irrigation Rates by Quantifying Mass Fluxes

^a Hydrology and Remote Sensing Laboratory, USDA-ARS, BARC-West, 10300 Baltimore Boulevard, Beltsville, MD 20705-2350
^b Alternate Crops and Systems Laboratory, USDA-ARS, BARC-West, 10300 Baltimore Boulevard, Beltsville, MD 20705-2350
^c Agronomy Department, University of Wisconsin, Madison, WI 53706-1299
^d Department of Biological System Engineering, University of Wisconsin, Madison, WI 53706-1299
^e Civil Engineering Department, Santa Clara University, 500 Camino Real, Santa Clara, CA 95053
^f Department of Agronomy, Purdue University, West Lafayette, IN 47906
^g Department of Agricultural and Biological Engineering, Cornell University, Ithaca, NY 14853

^* Corresponding author (tgish{at}hydrolab.arsusda.gov).

Received for publication July 1, 2002. 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.

Abbreviations: CDE, convective–dispersive equation • PFBA, penta-fluorobenzoic acid

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