Quantifying the Pore Size Spectrum of Macropore-Type Preferential Pathways under Transient Flow

It is well known that there is a spectrum of pores in a soil profile. The conventional use of a single lumped value of soil hydraulic conductivity to describe a spectrum of hydraulically active pores may have unintentionally impeded the development of field-scale chemical transport theoryand perhaps indirectly hindered the development ofmanagement protocols for chemical application andwaste disposal. In this study, three sets of four field-scale tracer mass flux breakthrough patterns measured under transient unsaturated flow conditions were used to evaluate the validity of an indirect method to quantify equivalent pore spectra of macropore-type preferential flow pathways. Results indicated that there were distinct trends in how pore spectra of macropore-type preferential flow pathways changed when a soil profile became wetter during a precipitation event. This suggests that the indirectmethod has predictive value and is perhaps a better alternative to the lumped soil hydraulic conductivity approach in accurately determining the impact of macropore-type preferential flow pathways on water movement and solute transport under transient unsaturated flow conditions. THE DISTRIBUTION of soil pore sizes is one of the most basic of all soil physical properties. It is well known that soil pores have a spectrum of equivalent radii. Soil pore spectrum affects infiltration, drainage, soil aeration, surface runoff, soil erosion, and chemical leaching. Arguably, it even influences the ability to determine the impact of various chemical management strategies on surface and subsurface water quality. In soil science, the soil hydraulic conductivity has been traditionally used as a basic soil physical property to describe the impact of soil pores on water movement and solute transport. Darcy (1856) first introduced this parameter when he demonstrated that the water flux through a saturated sand column was proportional to a potential gradient. Buckingham (1907) later extended the experiments and introduced unsaturated soil hydraulic conductivity. Frequently ignored is the fact that soil pores in natural heterogeneous soils are anisotropic. As a result, the soil hydraulic conductivity is a second-rank tensor with nine different hydraulic conductivity vectors (Bear, 1972). This complexity might not influence the overall water movement, but it could have a significant impact on the pore connectivity and hence the mixing of contaminant transport. Since no methods exist for reliably quantifying all of these nine vectors, a single lumped parameter is typically used to describe the soil hydraulic conductivity as a function of soil matric potential or water content. In essence, the method lumps the contributions of all pores across a plane onwatermovement, yielding a single hydraulic conductivity value for a specific condition (e.g., 1 m h for coarse sand or well-aggregated soils vs. 1 cm d for nonstructured clayey soils). Another issue is how large the elementary representative volume should be from which the lumped parameters are determined. If the lumped parameters are determined at a scale that does not represent field-scale flow conditions, the results may have limited application (Khan and Jury, 1990; Jensen and Refsgaard, 1991). Thus, the lumped hydraulic conductivity is a simplified approximation of a complex flow scenario measured at a specific scale that could have additional problems when extended to larger scales of observations. This partly explained why unrealistically large dispersion coefficients were often necessary to obtain a good fit with the chemical transport solutions when field-scale heterogeneities could not be captured by a lumped hydraulic conductivity (Gelhar, 1987). Using a single lumped value of soil hydraulic conductivity to describe the contribution of a spectrum of hydraulically active pores has led to considerable debate over which of the solute transport approaches should be used (van Genuchten and Parker, 1984; Barry and Sposito, 1989; Jardine et al., 1989; Novakowski, 1992). When a soil profile is relatively dry, the matric potential would restrict water movement in the smaller matrix pores. These matrix pores among soil primary particles are self-similar and interconnected. Transport through these matrix pores tends to cause a thorough solute mixing. Hence, there is a well-defined chemical front with a characteristic velocity and spreading. Under this scenario, a lumped soil hydraulic conductivity and dispersivity can very successfully describe the solute transport process (Cassel et al., 1975;Wierenga et al., 1991). However, there are situations (e.g., a relatively wet soil profile or a high intensity precipitation) when water and contaminants can move through macropore-type preferential pathways. Under this scenario, because chemicals in macropores generally would not mix thoroughly with those in the matrix pores, a lumped soil hydraulic conductivity and dispersivity cannot describe the solute transport process (Jaynes et al., 2001). Compensating for transport through macropores eventually evolved into the development of “two-region” K.-J.S. Kung, Dep. Soil Science, Univ. ofWisconsin, Madison,WI 537061299; E.J. Kladivko, Dep. Agronomy, Purdue Univ., West Lafayette, IN 47907; C.S.Helling, Sustainable Perennial Crops Lab., and T.J. Gish,Hydrology and Remote Sensing Lab., USDA-ARS, BARC-W, Beltsville, MD 20705-2350; T.S. Steenhuis, Dep. Biological and Environmental Engineering, Cornell University, Ithaca, NY 14850; D.B. Jaynes, National Soil Tilth Lab., USDA-ARS, Ames, IA 50011. Received 10 Jan. 2006. *Corresponding author ([email protected]). Published in Vadose Zone Journal 5:978–989 (2006). Special Section: Parameter Identification and Uncertainty Assessment in the Unsaturated Zone doi:10.2136/vzj2006.0003 a Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: PFBA, pentafluorobenzoic acid; o-TFMBA, o-trifluoromethyl benzoic acid; 2,6-DFBA, 2,6-difluorobenzoic acid. R e p ro d u c e d fr o m V a d o s e Z o n e J o u rn a l. P u b lis h e d b y S o il S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 978 Published online August 24, 2006