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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

Kamini Singha, Pennsylvania State University

I have been exploring the use of electrical methods for discerning solute transport processes in highly heterogeneous and fractured media. In many settings, observed transport behavior is inconsistent with the classically used 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 (ASR) experiment. Pressure to decrease reliance on surface-water storage has led to increased interest in ASR systems.  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. 

The viability of an ASR scheme is usually measured by recovery efficiency: the ratio of recovered water that meets a predefined standard to total volume of injected fluid.  Recovery efficiency can be degraded by a number of physical and geochemical processes, including RLMT, which describes the exchange of solutes between mobile and immobile pore spaces.  Numerical modeling I’ve conducted shows that RLMT can explain a rebound in salinity during freshwater storage in a brackish aquifer.  Multi-cycle results show low efficiencies over one to three ASR cycles due to RLMT degrading water quality during storage; efficiencies can evolve and improve markedly, however, over ten cycles, exceeding efficiencies generated by advection-dispersion only models.  For an idealized ASR model where RLMT is active, my simulations show a discrete range of diffusive length scales over which storage of freshwater in brackish aquifers may be less-than-viable.

Using these field data as motivation, I have additionally developed a petrophysical basis and experimental methodology for geoelectrical measurement of mass-transfer parameters. 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 is 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.

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