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45206-G8
Development of Field-Scale Petrophysical Relations for Imaging Fluid Flow and Transport in Fractured Media with Electrical Resistivity Tomography
The fate and transport of chemicals in groundwater is commonly described by advection and dispersion processes. In many settings, however, observed transport behavior appears inconsistent with the standard advective-dispersive model; rather, concentration histories show long tailing behavior, non-Gaussian breakthrough, and/or rebound after pumping for mass removal has ceased. These phenomena have prompted some to consider dual-domain, rate-limited mass transfer (RLMT) as a controlling process. Determining parameters describing mass-transfer between mobile and immobile domains —or even verifying the occurrence of RLMT—is problematic because geochemical data preferentially sample the mobile component of the pore space. In this work to date, I have shown direct geoelectrical evidence of diffusion between mobile and less-mobile zones in an aquifer (i.e., between fracture and matrix) during an aquifer storage and recovery experiment. I observe a hysteretic relationship between measurements of pore-fluid conductivity and bulk earth conductivity; this hysteresis contradicts advective-dispersive transport and the standard petrophysical model relating pore-fluid and bulk conductivity but can be explained using transport models that include first-order RLMT. Using a simple model, I have demonstrated that electrical geophysical methods can be used to bound estimates of mass transfer rates and immobile porosity. These findings support the hypothesis that RLMT is a fundamental process controlling solute mass transfer and the efficiency of aquifer remediation, and suggests that similar analyses in other geologic settings may help to determine the importance of RLMT on mass transfer processes. This work contributes to an ongoing debate in the hydrologic literature over the validity of bicontinuum mass transfer as an explanation for anomalous transport behavior. The principal contributions of this work to date are (1) evidence of field-scale bicontinuum mass transfer based on geoelectrical data; and (2) development and demonstration of an approach to identify mass transfer rates and immobile porosities—key parameters for predicting transport that are otherwise unattainable in-situ. This research should be of interest as bicontinuum mass transfer impacts contaminant transport in field settings, where monitored contaminant concentrations rebound at the end of pump-and-treat remediation. The costs of remediating the hundreds of thousands of groundwater-contaminated sites in the U.S. alone has been estimated in the hundreds of billions to trillions of dollars for the next 30 years, making improved quantification of the processes controlling tailing and rebound an issue of key importance. This work additionally has application where mass transfer between a mobile and a less-mobile domain could be monitored with electrical methods such as plant venation and animal vascular systems.
