Quantifying the slow release of contamination from immobile porosity (i.e., low-permeability zones and rock matrix around fractures) is of paramount importance to appropriate selection and optimal design of environmental site remedies. Conventional sampling techniques, however, preferentially draw fluid from mobile porosity (i.e., more permeable zones and fractures); hence accurate quantification requires invasive coring and costly laboratory extractions. This project aims to demonstrate a new technology, the Mobile/Immobile-Porosity Exchange Tool (MI-PET) to aid in field-scale, minimally invasive measurement of immobile porosity and the rate coefficients governing exchange between mobile and immobile pore spaces. The technical objectives are to:

• Demonstrate the technology prototype at three DoD sites of varying geological frameworks where back diffusion is documented to occur, providing actionable information to support site remediation;
• Validate the results of the technology using conventional, direct measurements of immobile porosity and contaminant storage from laboratory testing;
• Extend and demonstrate Excel®-based software to analyze data from the field experiments, providing realistic examples for future users of this technology; and
• Transfer the technology to site professionals and regulators through online dissemination, software documentation, webinars/cyber-seminars, and hands-on training courses.

Overview of the key concepts involved in the assessment of dual domain mass transfer controlling back diffusion using a borehole electrical geophysical technology.

The MI-PET is a borehole hydrogeophysical tool to perform focused tracer injections, sampling, and electrical geophysical monitoring. The theory and experimental apparatus underlying the MI-PET resulted from previous multiple research projects funded by SERDP, ESTCP, and DOE, in which electrical geophysical measurements were used to monitor the exchange of ionic tracers between mobile and immobile porosity. The combination of electrical geophysical and conventional hydrologic tracer testing can provide valuable insight into contaminant storage in low-permeability media and subsequent back diffusion. Past work used numerical modeling, pilot-scale field experiments and controlled laboratory experiments conclusively to evaluate the sensitivity of electrical geophysical methods to immobile porosity. The combination of ionic tracer experiments with electrical geophysical measurements allows for estimation of mobile and immobile porosity and inference of exchange rates with easy-to-use graphical methods in a spreadsheet. The hardware comprising the MI-PET includes hydraulic packers, electrodes and sampling ports.

Success for this project will be evaluated based on the strength of correlation between MI-PET results and conventional laboratory analysis of cores (i.e., mercury porosimetry and contaminant extractions) and feedback from professionals who participate in webinars and hands-on training conducted through  technology-transfer efforts.

A borehole tool capable of measuring immobile porosity in situ could reduce costs compared to coring and destructive extractions by 90% or more. Assuming conventional coring/extraction costs on the order of $50K/site and considering that there are over 1000 contaminated sites in the DoD complex, if only 25% of these had problems with back diffusion, the MI-PET could realize cost savings on the order of $11M. Additional savings would result from improved site characterization and remedy design. (Anticipated Project Completion - 2023)