Abstract
To improve contaminant detection and monitoring, it is important to understand the role of the bulk pore fluid chemistry on the induced polarization (IP) response of rocks and soils. IP methods can be more effective than traditional resistivity surveys because they are more sensitive to changes in electrochemistry at the mineral grain-pore fluid interface caused by small amounts of dissolved contaminant. Clean and contaminated materials have been shown to have comparatively different IP responses in both laboratory and field experiments. However, electrochemical surface properties are also dependent on the bulk pore fluid chemistry and grain microgeometry. The effect of contaminants on the IP response is superimposed upon these other bulk fluid and microgeometry effects. To document the influence of pore fluid chemistry on the IP response of earth materials, experiments were performed to measure the complex conductivity and time domain IP of Berea sandstone cores as a function of pore fluid pH, ionic strength, and cation type. A minimum in surface conductivity and IP response is observed at pH 3, the approximate point of zero net surface charge for quartz. While surface conductivity increases with ionic strength, the net IP response, or phase angle, decreases with ionic strength. A suite of cores saturated with different salt solutions of equal conductivity demonstrates significant variability in the IP response as a function of cation type. These experiments are applicable to other quartz-dominated systems and help to link field IP measurements to geochemical parameters, thus improving the IP characterization of geochemical environments.
Published Version
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