Capillary Pressure–Saturation Relations for Saprolite: Scaling With and Without Correction for Column Height

controlled by capillarity, any attempt to predict DNAPL behavior in a porous medium requires determination of Dense nonaqueous phase liquids (DNAPLs) are important subsurthe capillary pressure–saturation relationship, hereafter face contaminants. Information is lacking on DNAPL behavior in heterogeneous porous media such as weathered rock (saprolite). We denoted by w(hc), where w is the volumetric content of measured air–water and Fluorinert (a nontoxic DNAPL surrogate; the wetting fluid and hc is the capillary pressure head. 3M, St. Paul, MN)–water capillary pressure–saturation relations, w(hc), The standard procedure for measuring w(hc) consists close to saturation on an 18-cm-long by 10-cm-diameter undisturbed of introducing a nonwetting fluid at a known pressure column of interbedded sandstone and clayshale saprolite. The Campinto a porous medium, waiting for an equilibrium condibell empirical model was fitted to both w(hc) relations. The resulting tion, and then determining the wetting phase saturation best-fit parameters were 19.54 and 30.10 cm of H2O for the displaceof the sample (Corey, 1994; Lenhard et al., 2002). This ment capillary pressure head (h0) and 0.029 and 0.045 for the poreprocedure produces one pressure–saturation data pair. size distribution index (1/b ), for the air–water and Fluorinert–water Additional data pairs are produced by incrementing the data, respectively. Corresponding model parameters corrected for the nonwetting fluid pressure. The data pairs are then used hydrostatic fluid distribution within the column were 14.08 and 15.96 cm of H2O for h0, and 0.026 and 0.034 for 1/b. The correction procedure to construct a static equilibrium drainage curve that dehad a large effect on the Fluorinert–water w(hc) relation and relatively scribes the capillary behavior of the sample. Such curves little impact on the air–water w(hc) relation. Parameters from the are often parameterized using empirical expressions for air–water relations were used to predict Fluorinert–water w(hc) relathe w(hc) relation such as the Brooks and Corey (1964) tions using the expression: (h0)2 ( 2/ 1)(h0)1, where (h0)1, (h0)2 and equation, the Campbell (1974) equation, and the van 1, 2 are the capillary displacement pressure heads and interfacial Genuchten (1980) equation. tensions with water for air and Fluorinert, respectively. These analyses The majority of pressure cell studies involving DNAPL– showed that direct measurements of the Fluorinert–water w(hc) relawater systems have been performed on homogeneous tion need to be corrected for column height. The corrected Fluorinert– porous media (e.g., Lenhard and Parker, 1987; Demond water w(hc) relation was accurately predicted (R2 0.99) by both and Roberts, 1991). Relatively little is known about the the fitted and corrected (h0)1 values. Thus, the error in prediction introduced by not considering column height or contact angle effects capillary behavior of DNAPLs in heterogeneous porous was relatively small. Our results show that scaled air–water w(hc) media (Kueper et al., 1989; Hinsby et al., 1996; Illangarelations can be used to predict DNAPL intrusion into water-saturated sekare, 1998). The large column sizes required for adesaprolite at a physical point. quate representation of such materials mean that traditional procedures for measuring the w(hc) relation may not be directly applicable. D nonaqueous phase liquids are commonly If the densities of the nonwetting and wetting fluids used as solvents, degreasers, dry cleaning fluids, are different, the capillary pressure will vary with height and pesticide additives. Examples include trichlorowithin the pressure cell (Dane et al., 1992; Liu and Dane, ethylene, perchloroethene, carbon tetrachloride, chloro1995a). Because pressure varies with height, one presform, and polychlorinated biphenyls. As a result of spills, sure value cannot represent pressure conditions everyleaks, or intentional releases, DNAPLs are often preswhere within a tall column, but is only representative ent in the subsurface environment near industrial plants of one elevation in the sample. Because of this limitaand waste disposal or storage facilities (Pankow and tion, the standard procedure for measuring capillary beCherry, 1996). The need to predict the fate and persishavior suggests that samples should be 2 cm tall. The tence of these contaminants has stimulated many studies variation of capillary pressure with height in a sample of flow and entrapment of DNAPLs in porous media of this height is negligible in comparison to the relatively (see reviews by MacKay et al., 1985; Mercer and Cohen, large pressures needed to drain the wetting fluid from 1990; Pankow and Cherry, 1996). small pores (Dane and Hopmans, 2002). Thus, little erBecause DNAPL migration and distribution are largely ror is introduced by assuming that capillary pressure measured at one elevation within the sample is representative of values throughout the sample. E. Perfect, L.D. McKay, and S.G. Driese, Dep. of Earth and Planetary This height constraint presents a problem for samples Sciences, Univ. of Tennessee, Knoxville, TN 37996-1410; S.C. Cropper, 1463 Oxford Place, Cookeville, TN 38506; G. Kammerer, Institute containing fractures or macropores. Large undisturbed for Hydraulics and Rural Water Management, BOKU, Muthgasse 18, columns are often needed to obtain a representative samA-1190 Vienna, Austria; J.H. Dane, Dep. of Agronomy and Soils, ple of the heterogeneity that is present. In these cases, Auburn Univ., Auburn, AL 36849-5412. Received 19 Jan. 2004. Specapillary pressure variations with height may be significial Section: Uncertainty in Vadose Zone Flow and Transport Properties. *Corresponding author ([email protected]). cant in relation to the lower capillary pressures at which Published in Vadose Zone Journal 3:493–501 (2004).  Soil Science Society of America Abbreviations: DNAPL, dense nonaqueous phase liquid; SWSA, Solid Waste Storage Area. 677 S. Segoe Rd., Madison, WI 53711 USA