Experimental observation of signature changes in bulk soil electrical conductivity in response to engineered surface CO2 leakage

Abstract

Experimental observations of signature changes of bulk soil electrical conductivity (EC) due to CO2 leakage were carried out at a field site at Bozeman, Montana, to investigate the change of soil geophysical properties in response to possible leakage of geologically sequestered CO2. The dynamic evolution of bulk soil EC was measured during an engineered surface leakage of CO2 through in situ continuous monitoring of bulk soil EC, soil moisture, soil temperature, rainfall rate, and soil CO2 concentration to investigate the response of soil bulk EC signature to CO2 leakage. Observations show that: (1) high soil CO2 concentration due to CO2 leakage enhances the dependence of bulk soil EC on soil moisture. The bulk soil EC is a linear multivariate function of soil moisture and soil temperature, the coefficient for soil moisture increased from 2.111 dS for the non-leaking phase to 4.589 dS for the CO2 leaking phase; and the coefficient for temperature increased from 0.003 dS/°C for the non-leaking phase to 0.008 dS/°C for the CO2 leaking phase. The dependence of bulk soil EC on soil temperature is generally weak, but leaked CO2 enhances the dependence,(2)after the CO2 release, the relationship between soil bulk EC and soil CO2 concentration observes three distinct CO2 decay modes. Rainfall events result in sudden changes of soil moisture and are believed to be the driving forcing for these decay modes, and (3) within each mode, increasing soil CO2 concentration results in higher bulk soil EC. Comparing the first 2 decay modes, it is found that the dependence of soil EC on soil CO2 concentration is weaker for the first decay mode than the second decay mode.