Quantifying Pore-Size Spectrum of Macropore-Type Preferential Pathways

1981; Gish and Jury, 1983; Edwards et al., 1993). Both physical and biological processes have temporal and Structural pores associated with macropore-type preferential flow spatial patterns of formation and destruction cycles pathways can accelerate chemical transport in unsaturated soils, thereby potentially causing groundwater contamination. To predict chemical (Gupta et al., 2002). Tillage practices and compaction, transport through these pathways, classical deterministic models defor example, often destroy the continuity of large strucpend on soil hydraulic conductivity, which effectively lumps flow contural pores (Isensee et al., 1990). The coefficient of varitributions from all individual pathways. We contend, however, that quanation of soil hydraulic conductivity, which is often dictifying the pore spectrum of preferential pathways, without lumping the tated by the soil structural pores, ranged from 100 to contributions of individual pores, is the appropriate method for simu400% (Libardi et al., 1980; Warrick and Nielsen, 1980). lating convective chemical transport through macropore-type preferThis suggests that the spatial variability of soil structural ential pathways. In this study, we conducted field-scale experiments pores measured by using coreor block-sized samples is by using an improved tile drain monitoring protocol to measure the very large. As a result, the size spectrum of large strucmass flux breakthrough patterns of conservative tracers. The tails of tural pores measured at several random locations by these patterns suggested that the impact of preferential pathways on contaminant transport can be conceptualized as that occurring through small sample sizes in a field may not represent the speccylindrical capillary tubes. We then proposed a distribution function trum of the entire field. Temporal extrapolation of meabracketed by sharp cut-off points to represent the pore spectrum of surements may be similarly invalid. For these reasons, these tubes. Finally, we used the measured tracer breakthrough curves it is difficult to directly measure the field-scale spectrum (BTCs) as data sources to find the parameters of the proposed funcof the large structural pores, yet this property is among tion. Our results, based on the best fitting, showed that the preferential the most important soil properties when dealing with pathways are naturally clustered into domains; preferential pathways issues related to water quality. with a wide range of pore radii could become active simultaneously Soil characteristic curves and hydraulic conductivity when infiltration rate increases. Because the derived pore spectra sicurves were used in past research from the 1950s and multaneously satisfy both water movement and solute transport, pore 1960s [summarized by Hillel (1980, p. 183–185) and Jury spectra can be used to (i) calculate soil hydraulic conductivity of preferential pathways in deterministic approaches, and (ii) construct multiet al. (1991, p. 89–94)] to quantify the soil pore-size ple probability density functions (PDFs) for the transfer function apdistribution. However, because these curves were typiproach, to accommodate different infiltration patterns. cally measured using homogenized soils where the larger soil structural pores were destroyed, these earlier approaches only addressed the matrix pore spectrum T soil’s capacity to store water and nutrients faamong primary particles after homogenization. Later, incilitates the hydrological and biogeochemical cycles tact soil samples such as soil cores or blocks were used; critical for the existence of terrestrial ecosystems. This however, even intact samples cannot capture the varicapacity hinges on the porous nature of soils. Natural ability and continuity of field-scale, large secondary soils have a spectrum of pores with radii generally rangstructural pores (Shipitalo and Edwards, 1993). ing from 10 3 to 10 7 m. The smaller soil matrix pores Large structural pores can contribute to macroporeare comprised of textural voids among the soil primary type preferential flow, and field experiments confirmed particles, while the larger pores are often made of structhat flow via such pathways bypassed the far-more-prevtural voids among the soil secondary structures. The alent soil matrix pores to cause rapid and deep chemical matrix pores are always interconnected and usually selfleaching (Germann and Beven, 1981; Luxmoore, 1991; similar. Large secondary structural pores are likely to Helling and Gish, 1991). Kladivko et al. (1999) showed exhibit more complexity. Their formation is dictated by that 50% of the total annual pesticide loss generally physical (e.g., shrinking and swelling, wetting and dryoccurred during the first major precipitation event of ing, or freezing and thawing) and biological (e.g., pene25 mm rain after pesticide application. Fast breaktration and movement of living organisms) forces (Bouma, through patterns of adsorbing chemicals through macropore-type preferential flow resembled those of conserK.-J.S. Kung and M. Hanke, Dep. Soil Science, Univ. of Wisconsinvative nonadsorbing chemicals (Kladivko et al., 1999; Madison, Madison, WI 53706-1299; C.S. Helling, Sustainable PerenKung et al., 2000b; Fortin et al., 2002). In analogous cases, nial Crops Lab., and T.J. Gish, Hydrology Lab., USDA-ARS, BARC-W, deep leaching of P through preferential transport was Beltsville, MD 20705-2350; E.J. Kladivko, Dep. Agronomy, Purdue found, even though P loss is assumed to occur mainly by Univ., West Lafayette, IN 47907; T.S. Steenhuis, Dep. Biological and surface runoff (Hergert et al., 1981; Stamm et al., 1998; Environmental Engineering, Cornell Univ., Ithaca, NY 14850; D.B. Jaynes, National Soil Tilth Lab., USDA-ARS, Ames, IA 50011. Reand Beauchemin et al., 1998). Beside agrichemicals, ceived 25 June 2004. *Corresponding author ([email protected]). pathogenic microorganisms such as fecal coliforms and Published in Soil Sci. Soc. Am. J. 69:1196–1208 (2005). Soil Physics Abbreviations: 1-D, one-dimensional; BTC, breakthrough curve; CDE, convection–dispersion equation; PDF, probability density function; doi:10.2136/sssaj2004.0208 © Soil Science Society of America PFBA, pentafluorobenzoic acid; o-TFMBA, o-trifluoromethylbenzoic acid. 677 S. Segoe Rd., Madison, WI 53711 USA 1196 Published online June 28, 2005