Monitoring Rate-Limited Mass Transfer Using Geophysics

Overview

As part of its applied research initiatives, the USGS Office of Ground Water, Branch of Geophysics (OGW BG) investigated the application of geophysical measurements to help verify and measure rate-limited mass transfer (RLMT), which is thought to be an important control on solute transport in heterogeneous geologic media.

Purpose & Scope

Concentration breakthrough curves from tracer experiments and pump-and-treat contaminant remediation in fractured and heterogeneous porous media commonly show tailing — the progressively slower recovery of concentration through time — and concentration rebound that is not described by advective-dispersive transport. Observations of such anomalous transport behavior have prompted some to consider bicontinuum mass transfer as a controlling process.

In bicontinuum models of geologic media, the pore volume is conceptualized as consisting of (1) a mobile domain of well-connected pore space and (or) connected fracture porosity, and (2) an immobile domain of poorly connected pore space and (or) dead-end fractures (the immobile domain). Advection and dispersion processes occur in the mobile domain, with local rate-limited mass transfer of solute mass between the mobile and immobile domains. (Singha and others, 2007b)

Understanding the processes controlling anomalous tailing and rebound is critical to addressing problems ranging from the design of cost-effective pump-and-treat remediation for groundwater, to the implementation of aquifer-storage recovery (ASR) systems, to the selection of nuclear waste disposal sites.

Experimental field verification of RLMT and measurement of controlling parameters is difficult. In the presence of a bicontinuum, fluid samples would be drawn from the mobile domain, whereas electrical current would flow through both mobile and immobile pore space. Conventional geochemical measurements provide only indirect information about the volume of immobile pore space, and only circumstantial evidence is available to verify the existence of a bicontinuum or to identify values of controlling parameters. This investigation explored the use of a combination of geochemical and geoelectrical measurements to verify the occurrence of bicontinuum transport and measure RLMT.

Analysis of data from a 2005 ASR experiment in a fractured limestone aquifer, Charleston, South Carolina, provided direct evidence of rate-limited mass transfer (RLMT) at the field scale. OGW BG and Pennsylvania State University conducted direct-current electrical-resistance surveys to monitor a push-pull tracer test in fractured rock, in conjunction with groundwater sampling and numerical modeling.

Methods & Activities

The fall 2005 field experiment was conducted at a pilot-scale ASR project in Charleston, South Carolina. The ASR test consisted of the injection, storage, and removal of water in the aquifer at the field site. Freshwater was injected into the high-salinity aquifer over 5 days, stored for 2 days, and then pumped over 4 days. Open-hole hydraulic heads were recorded, and fluid electrical conductivity was measured from small-volume porewater samples. During the ASR experiment, borehole electrical-resistivity data were collected in three sampling wells. Apparent bulk conductivities were measured prior to and during the push-pull experiment. The target zones for ASR and resistivity monitoring were two transmissive, fractured intervals.

Numerical simulations were conducted of fluid flow, transport, and electrical conduction. MODFLOW-2000, a finite-difference model, was used to simulate radial transient flow. Particle-tracking in MT3DMS was used to simulate radial advective transport with first-order, dual-domain RLMT. A finite-volume model written in Matlab was used to simulate three-dimensional electrical conduction.

Results & Conclusions

A hysteretic relationship between measurements of pore-fluid conductivity and bulk electrical conductivity was observed (see fig. 1).
The hysteresis could not be explained by advective-dispersive transport and the standard petrophysical model relating pore-fluid and bulk conductivity.
The hysteresis can be explained using bicontinuum transport models that include first-order RLMT.
Modeling demonstrated that geoelectrical measurements are sensitive to bicontinuum transport and RLMT parameters, which are otherwise difficult to infer from direct hydrologic measurements.

For a detailed discussion of the investigation and results, please refer to Singha and others, 2007b.

[Graphs: Fluid specific conductivity over time; apparent bulk conductivity over time; and Fluid specific conductivity versus apparent bulk conductivity.]

Figure 1. Experimental data at two observation wells. During the storage period, we observed a rebound of salinity (as fluid electrical conductivity) (a, d); concurrently, the bulk conductivity data from electrical measurements at the same locations show little change (b, e). The relation between the bulk electrical conductivity measured by the geophysics and the fluid conductivity measured by the chemical sampling thus appears hysteretic (c, f). Injection was from 0-5 days, storage from 5-7 days, and recovery from 7-10 days. (Singha and others, 2007b)

Support & Collaboration

This research was conducted in collaboration with Dr. Kamini Singha of Pennsylvania State University Department of Geosciences - Hydrogeology Program and was funded by the USGS Groundwater Resources Program, with support from the USGS Toxic Substances Hydrology Program.

Publications

Culkin, S.L., Singha, K., and Day-Lewis, F.D., in press, Implications of rate-limited mass transfer for aquifer storage and recovery efficiency: Ground Water.

Singha, K., Day-Lewis, F.D., Lane, J.W., Jr., 2007a, Geoelectrical evidence of bicontinuum transport in ground water, Eos Trans. AGU, 88(52), Fall Meet. Suppl., Abstract H12C-02.

Singha, K., Day-Lewis, F.D., and Lane, J.W., Jr., 2007b, Geoelectrical evidence of bicontinuum transport in groundwater: Geophysical Research Letters, 34, L12401, doi:10.1029/2007GL030019.

For more information:

For more information on this project, please contact John W. Lane, Jr. (Chief, USGS OGW Branch of Geophysics) or Frederick Day-Lewis (Hydrologist, USGS OGW Branch of Geophysics), or call the Branch of Geophysics at (860)487-7402.

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USGS Programs and Offices can learn more about geophysical equipment available for USGS use and training and support from OGW BG.