Residual contamination of chlorinated solvents is commonly found in low-permeability formations. These contaminants serve as continuing sources to contaminate groundwater (i.e., back diffusion), significantly extending the remediation timeframe. Hydraulic fracturing by high-pressure injection has been proposed as a useful alternative for permeability enhancement.

The overarching objective of this project is to advance the fundamental understanding of particulate amendment (granular or powdered activated carbon) transport, compaction, and remedial efficacy in artificially-created hydraulic fractures in low-permeability clay formations contaminated by chlorinated volatile organic compounds (cVOCs). Specific research aims are the following:

1. Determine the transport and distribution of a mixture of granular activated carbon (GAC) and sand particles in a well-controlled laboratory fracture slot system as a function of injection flow rate, fracture dip angle, particle size, and GAC mixing ratio;
2. Quantify the compaction and embedment of GAC/sand mixtures in hydraulic fractures created in a real Department of Defense (DoD) clay/soil matrix and subjected to closure pressures, as well as how such mixtures affect the fracture conductivity under varying particle amount, GAC mass ratio, clay type, and water soaking time;
3. Evaluate changes in cVOC release from clays and silty clays post GAC/sand injection, and the corresponding impacts on back diffusion from clays to adjacent groundwater aquifers.

Previous studies have shown that the form of an artificially-created hydraulic fracture in a shallow low-permeability formation is generally predictable. However, there are still knowledge gaps between advanced understanding of fracture forms and effective delivery and placement of remediation amendments in hydraulic fractures. This project aims to address these fundamental questions using novel laboratory experiments and advanced modeling methods. In Aim R1, a novel laboratory fracture slot system and a fluid/particle-coupled numerical transport model will be developed to investigate the transport and distribution of injected slurry particles in a model hydraulic fracture, where the fracture dip angle, injection flow rate, particle size, and GAC mixing ratio can be precisely controlled. In Aim R2, a custom-built laboratory fracture conductivity cell and a discrete element method/lattice Boltzmann-coupled model will be used to assess the compaction and embedment of a GAC/sand mixture in a hydraulic fracture created in low-permeability aquifer materials collected from a DoD site, as well as how particle amount, GAC mass mixing ratio, clay type, and water soaking time affect the change of fracture conductivity. In Aim R3, two representative flow cell experiments will be used to assess the release of cVOCs from a clay matrix, and from a heterogeneous low conductivity field. A reactive transport model will be modified and used to interpret the results and scale them to larger-scale systems.

This project will develop new insights and empirical correlations from laboratory experiments and scalable numerical modeling tools that allow DoD site managers and remediation practitioners to predict how injection and subsurface lithology conditions affect the transport and distribution of amendments in hydraulic fractures in low-permeability formations, and the corresponding effectiveness in mitigating cVOC back diffusion. Specifically, the new insights and empirical correlations obtained from the laboratory experiments and numerical modeling studies will enable DoD site managers and other stakeholders to better predict GAC amendment transport and distribution in fractures across multiple spatial scales, which will be extremely valuable in optimizing designs of field injection strategies for in situ environmental remediation at DoD sites. Possible product receivers include consulting companies who will use the tools developed from this project, the Environmental Protection Agency or other agencies with whom remediation practitioners interact, and DoD site managers. (Anticipated project completion - 2024)