Geoelectrical evidence of bicontinuum transport in groundwater

Abstract

[1] Bicontinuum models and rate-limited mass transfer (RLMT) explain complex transport behavior (e.g., long tailing and rebound) in heterogeneous geologic media, but experimental verification is problematic because geochemical samples represent the mobile component of the pore space. Here, we present geophysical evidence of RLMT at the field scale during an aquifer-storage and recovery experiment in a fractured limestone aquifer in Charleston, South Carolina. We observe a hysteretic relation between measurements of porefluid conductivity and bulk electrical conductivity; this hysteresis contradicts advective-dispersive transport and the standard petrophysical model relating pore-fluid and bulk conductivity, but can be explained by considering bicontinuum transport models that include first-order RLMT. Using a simple numerical model, we demonstrate that geoelectrical measurements are sensitive to bicontinuum transport and RLMT parameters, which are otherwise difficult to infer from direct, hydrologic measurements.

A physical explanation for the development of redox microzones in hyporheic flow

“…However, characterization and sampling of less mobile pore space and associated reactive processes have been historically problematic [Jørgensen, 1977;Dentz et al, 2011]. These issues are primarily attributed to traditional fluid sampling of the saturated subsurface preferentially drawing water from mobile porosity [Harvey, 1993;Harvey and Gorelick, 2000;Singha et al, 2007]. If variability in carbon availability alone was responsible for redox zonation (e.g., microzones have similar hydraulic connectivity to bulk HZ sediments), we would expect stronger signals regarding embedded redox zones when collecting pore water samples.…”

“…Fortunately, emerging microscale and field-scale techniques have potential to better characterize less mobile porosity. For example, measurement of fluid conductivity (σ f ) and co-located bulk conductivity (σ b ) during ionic tracer tests provide information on mobile-immobile exchange [Singha et al, 2007;Briggs et al, 2013b]. Relatively tight pores that connect through pockets of fine-grained or organic material are expected to contribute to the bulk conductivity signal; therefore, electrical data can be collected in situ to directly describe less mobile exchange characteristics.…”

Investigating the impact of advective and diffusive controls in solute transport on geoelectrical data

“…Results from an electrical geophysical dataset obtained during an aquifer-storage and recovery (ASR) test in Charleston, South Carolina indicate that direct-current resistivity methods may be able to help estimate mass transfer behavior in situ (Singha et al, 2007). The authors observed hysteresis between fluid electrical conductivity (EC) and the bulk apparent EC, which contradicts common rock physics relations, and suggested that the hysteresis is an effect of dual-domain mass transfer.…”

“…To evaluate the electrical response to tracer behavior, we simulate a push-pull type test, loosely mimicking the data collection procedure described in Singha et al (2007). The injection/extraction well was represented by a series of 32 points spaced 1 m apart vertically from 9 m to 40 m below the simulated land surface and 46.5 m to the ) 0.075 Dispersivity, primary direction (immobile, α 1 ) 0 m Dispersivity, secondary direction (immobile,α 2 ) 0 m right of the model origin, near the midpoint of the system.…”

“…In the models with dual-domain mass transfer, we follow Singha et al (2007) to calculate the bulk EC:…”