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John S. Selker

Avoiding groundwater contamination from agricultural activities is possible only if the processes that control deep percolation are understood. The source of contaminant movement to groundwater is typically through preferential flow, processes by which the bulk soil is bypassed by some part of the infiltrating water. Three mechanisms give rise to preferential flow: fingered flow, funnel flow, and macropore flow. Fingered flow occurs in coarse-textured soils and can be minimized by starting with an initially well-wetted profile. Funnel flow is likely in layered soil profiles of silt or coarser-textured soil, in which avoiding slow overirrigation is critical. Macropore flow is observed in all structured soils in which maintaining irrigation rates well below the saturated conductivity of the soil is essential. These prescriptions are quite different than conventional recommendations, which fail to consider groundwater protection.

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Amir M. González-Delgado and Manoj K. Shukla

and water in the soil under field conditions is influenced by macropores that promote their preferential flow ( Beven and Germann, 1982 ; Shipitalo et al., 2000 ). Preferential flow channels in the porous media could enhance herbicide movement from

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Amir M. González-Delgado, Manoj K. Shukla and Brian Schutte

( Beven and Germann, 1982 ; Kramer and Boyer, 1995 ). Several studies have reported deeper than expected herbicide leaching through preferential flow channels ( Flury et al., 1995 ; Rao et al., 1974 ; Shukla, 2014 ). The correct use of agrochemicals

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Tyler C. Hoskins, James S. Owen Jr. and Alex X. Niemiera

(i.e., more evenly) in treatments containing a wheat plant than a fallow column. They postulated that roots grew into large pore spaces and effectively created a homogenous pore size distribution that reduced the preferential flow of water through

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Jeb S. Fields, William C. Fonteno and Brian E. Jackson

of the substrate ( Fonteno et al., 2013 ), and preferential flow ( Dekker and Ritsema, 1994 ). Measurement of substrate wettability has been difficult to assess with the most common method in the literature being the measurement of contact angles

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Ian A. Merwin, John A. Ray, Tammo S. Steenhuis and Jan Boll

Commercial apple (Malus domestica Borkh.) orchards in the northeastern United States receive heavy pesticide inputs and are often located on well-drained soils near surface and groundwater resources. Nonpoint-source water pollution by agrichemicals has been monitored in agronomic crop systems and simulated using computer models and laboratory soil columns, but inadequately studied at field scale in orchards. We monitored the concentrations of agrichemical tracers, nitrate-N, and benomyl fungicide in water samples from two apple orchards under mowed sodgrass (Mowed-Sod), shredded bark mulch (Bark-Mulch), preemergence residual herbicides (Resid-Herb), and postemergence herbicide (Post-Herb) groundcover management systems (GMSs). In one orchard, we evaluated subsurface spatial patterns and flow rates of a weakly adsorbed blue dye (pesticide analog) and potassium bromide (nitrate analog) under trees after six years of Post-Herb and Mowed-Sod treatments. Nitrate and pesticide tracers leached more rapidly and in higher concentrations under Post-Herb treatments, apparently via preferential macropore flowpaths such as root channels, soil cracks, and macrofauna burrows. At another orchard, we monitored subsurface leaching and surface runoff of benomyl and nitrate-N on a whole-field scale. Peak concentrations of benomyl (up to 29 mg·liter-1) and nitrates (up to 20 mg·liter-1) were observed in subsoil leachate under Resid-Herb plots during 1993. In 1994, nitrate concentrations were greater in leachate from all GMSs, with upper ranges from 48 to 66 mg·liter-1, while benomyl concentrations were lower in all GMSs compared with the previous summer. In surface water runoff during 1993, the highest benomyl concentrations (387 mg·liter-1) and most frequent outflows occurred in Resid-Herb plots. During 1994, benomyl runoff was more frequent in both herbicide GMSs, with concentrations up to 61 mg·liter-1 observed in the Post-Herb plots. Weather patterns, irrigation intensity, differing soil conditions under each GMS, and the turfgrass/clover drive lanes affected the relative frequency and concentrations of benomyl and nitrate leaching and runoff. Preferential bypass flow appeared to be a major subsurface leaching pathway, and erosion sediment an important factor in surface movement of these agrichemicals. Our studies suggest that nitrate-N and benomyl fungicide may be more prone to leaching or runoff from orchard soils under some herbicide GMSs in comparison with mowed sodgrass or biomass mulch systems.

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Laurence Gendron, Guillaume Létourneau, Julien Cormier, Claire Depardieu, Carole Boily, Raymond Levallois and Jean Caron

conducted on highly permeable silty soils, we expect a similar behavior in coarse and medium sands and possibly on vertisols, high in clay content and developing preferential flow features ( Coulombe et al., 1996 ), as pulsed irrigation is frequently

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Tyler C. Hoskins, James S. Owen Jr., Jeb S. Fields, James E. Altland, Zachary M. Easton and Alex X. Niemiera

(less than 0.2 μm). With this in mind, the preferential flow of water through macropores (less restricted) over micropores (more restricted) may lead to a portion of the applied tracer solution reaching the base of the column more quickly via transport

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Taryn L. Bauerle, William L. Bauerle, Marc Goebel and David M. Barnard

of measurement in one container of each of the six species. Sensors were inserted 6 inches into the container through a hole drilled in the container wall. Holes were covered with duct tape to prevent preferential flow paths. The sensors were arranged

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Joan R. Davenport, Robert G. Stevens, Kelly M. Whitley and Tanya Winkler

Trans. ASABE. 50 875 883 Magarian, D.M. Russelle, M.P. Lamb, J.F.S. Blumenthal, J.M. 1998 Bromide as a tracer for nitrate-N uptake in alfalfa herbage Agron. J. 90 651 657 McLeod, M. Schipper, L.A. Taylor, M.D. 1998 Preferential flow in a well drained and