Capillary pressure-saturation relations for saprolite: scaling with and without correction for column height

Dense nonaqueous phase liquids (DNAPLs) are important subsurface contaminants. Information is lacking on DNAPL behavior in heterogeneous porous media such as weathered rock (saprolite). We measured air-water and Fluorinert (a nontoxic DNAPL surrogate; 3M, St. Paul, MN)-water capillary pressure-saturation relations, theta(w)(h(c)), close to saturation on an 18-cm-long by 10-cm-diameter undisturbed column of interbedded sandstone and clayshale saprolite. The Campbell empirical model was fitted to both theta(w)(h(c)) relations. The resulting best-fit parameters were 19.54 and 30.10 cm of H2O for the displacement capillary pressure head (h0) and 0.029 and 0.045 for the pore-size distribution index (1/b), for the air-water and Fluorinert-water data, respectively. Corresponding model parameters corrected for the 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 had a large effect on the Fluorinert-water theta(w)(h(c)) relation and relatively little impact on the air-water theta(w)(h(c)) relation. Parameters from the air-water relations were used to predict Fluorinert-water theta(w)(h(c)) relations using the expression: (h0)2 = (sigma 2/sigma 1)(h0)1, where (h0)1, (h0)2 and sigma 1, sigma 2 are the capillary displacement pressure heads and interfacial tensions with water for air and Fluorinert, respectively. These analyses showed that direct measurements of the Fluorinert-water theta(w)(h(c)) relation need to be corrected for column height. The corrected Fluorinert-water theta(w)(h(c)) relation was accurately predicted (R2 approximately equal to 0.99) by both the fitted and corrected (h0)1 values. Thus, the error in prediction introduced by not considering column height or contact angle effects was relatively small. Our results show that scaled air-water theta(w)(h(c)) relations can be used to predict DNAPL intrusion into water-saturated saprolite at a physical point.