LABORATORY SCALE GEOELECTRIC MEASUREMENTS OF MASS TRANSFER

Abstract

Previous numerical modeling and field scale experimental data from saturated porous media suggest that the dual-domain mass transfer (DDMT) of solute tracers has a measureable electrical signature. Here, we present laboratory scale experiments and provide evidence for this electrical signature of DDMT and demonstrate the use of time-lapse electrical measurements with concentration measurements to estimate parameters controlling DDMT, i.e., the mobile and immobile porosity and the rate at which solute exchanges between mobile and immobile domains. We conducted column NaCl tracer tests on unconsolidated quartz sand and the zeolite clinoptilolite, a material with a high secondary porosity. We collected nearly co-located bulk direct-current electrical conductivity (σb) and fluid conductivity (σf) measurements. Our tracer test results for the zeolite show (1) extensive tailing and (2) a hysteretic relation between σf and σb, thus providing evidence of mass transfer. To identify best-fit parameters and evaluate parameter sensitivity, we performed over 2700 simulations of σf, varying the immobile and mobile domain and mass transfer rate coefficient. We emphasized the fit to late-time tailing by minimizing the Box-Cox power transformed root-mean-square error between the observed and simulated σf. Low-field proton nuclear magnetic resonance (NMR) measurements provide an independent, empirical estimate of the volumes of the mobile and immobile domains. The best-fit parameters based from simulated σf are similar to the NMR estimates of the immobile and mobile domain porosities. Our results underscore the potential of using electrical measurements for characterizing DDMT parameters, predicting solute transport in groundwater and optimizing remediation activities.