Simulated electrical conductivity response of clogging mechanisms for managed aquifer recharge

Motivated by the need for improved understanding and monitoring of clogging during managed aquifer recharge, we use numerical experiments to evaluate the effect of three different clogging mechanisms on electrical conductivity (EC), porosity, specific surface area, and electrical tortuosity of a simulated sediment pack. The clogging experiments are designed to simulate effect of clogging due to: (a) addition of finer grains, (b) addition of nonconductive films, and (c) addition of conductive films. The simulations involved starting with a random grain pack of 43% porosity, and subsequently reducing the porosity as would occur during clogging. For each of the experiments, we compute the EC response, specific surface area, and electrical tortuosity across the range of porosities. The differences in EC response between (a) and (b) is minor, however, the sediment parameters measuring pore-space configuration show very different responses (i.e., specific surface area and tortuosity), indicating EC is limited in its sensitivity to specific pore configurations. The results from simulations (a) and (b) are well described by Archie's equation. For the conductive film experiments (c), we explore the effect of film growth for four different surface conductivities ranging from 1 Sm-1 to 7 Sm-1. These conductivities correspond to a range of 5-35 times more conductive than the pore fluid conductivity. The bulk EC signal for each of the films results in a distinct manifestation in terms of measured bulk EC. We fit the EC response of the conductive film experiments with a model based on volume fraction occupied by the film; although the model fit the observed results, we required a unique set of fitting parameters for each Film conductivity.