All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Comparison of Average and Point Capillary Pressure-Saturation Functions Determined by Steady-State Centrifugation Soil Physics T he capillary pressure-saturation function is an important hydraulic property of variably saturated rocks and soils. Th is function is needed for simulating multiphase fl uid fl ow and chemical transport in porous media in applications such as agricultural crop production, enhanced oil recovery, subsurface C sequestration, and remediation of contaminated soils. In soil physics, the capillary pressure-saturation function is traditionally determined in the laboratory using a hanging water column (Dane and Hopmans, 2002a) or pressure plates (Dane and Hopmans, 2002b). In these methods, a series of capillary pressures, ψ, are imposed at a particular point and the corresponding volumetric water contents for the entire porous medium, 〈θ〉, are determined gravimetrically or manometrically. If the densities of the nonwetting and wetting fl uids are diff erent, ψ will vary with height within the porous medium (Dane et al., 1992; Liu and Dane, 1995a). As a result, the volumetric water content at the point where ψ is controlled, θ, can deviate signifi cantly from the measured 〈θ〉. Th us, use of the capillary pressure-average saturation function, 〈θ〉(ψ), instead of the point capillary pressure-saturation function, θ(ψ), in fl ow and transport models can produce erroneous predictions of important hydraulic properties such as the relative permeability function (Peters and Durner, 2006). Because point measurements of θ are rarely available in hanging water column and pressure plate experiments, computational procedures have been developed to extract the θ(ψ) function from the measured 〈θ〉(ψ) function. Liu and Dane

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 (h c ), where w is the volumetric content of measured air-water and Fluorinert (a nontoxic DNAPL surrogate;

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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, θw(hc), 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 θw(hc) 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 θw(hc) relation and relatively little impact on the air–water θw(hc) relation. Parameters from the air–water relations were used to predict Fluorinert–water θw(hc) relations using the expression: (h0)2 = (σ2/σ1)(h0)1, where (h0)1, (h0)2 and σ1, σ2 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 θw(hc) relation need to be corrected for column height. The corrected Fluorinert–water θw(hc) relation was accurately predicted (R2 ≅ 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 θw(hc) relations can be used to predict DNAPL intrusion into water‐saturated saprolite at a physical point.