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

Abstract

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, θ 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 θ w ( h c ) relations. The resulting best-fit parameters were 19.54 and 30.10 cm of H 2 O for the displacement capillary pressure head ( h 0 ) 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 H 2 O for h 0 , and 0.026 and 0.034 for 1/ b . The correction procedure had a large effect on the Fluorinert–water θ w ( h c ) relation and relatively little impact on the air–water θ w ( h c ) relation. Parameters from the air–water relations were used to predict Fluorinert–water θ w ( h c ) relations using the expression: ( h 0 ) 2 = (σ 2 /σ 1 )( h 0 ) 1 , where ( h 0 ) 1 , ( h 0 ) 2 and σ 1 , σ 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 θ w ( h c ) relation need to be corrected for column height. The corrected Fluorinert–water θ w ( h c ) relation was accurately predicted ( R 2 ≅ 0.99) by both the fitted and corrected ( h 0 ) 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 θ w ( h c ) relations can be used to predict DNAPL intrusion into water-saturated saprolite at a physical point.