Placement of sand and granular activated carbon in hydraulic fractures for contaminant remediation in low-permeability formations

The injection of granular activated carbon (GAC)-based amendments in artificially-induced fractures provides a promising solution for addressing chlorinated solvent contamination in clay-rich, low-permeability subsurface formations. Achieving high hydraulic conductivity of GAC-filled fractures is critical for long-term treatment efficiency. The evolution of the hydraulic conductivity of GAC-filled fractures in shallow clay formations against various stress and injection conditions has not been fully investigated. To close this knowledge gap, the hydraulic conductivity evolution of GAC-filled fractures created in clay-rich soil was experimentally studied. The hydraulic conductivity and permeability evolutions of sand- and GAC-filled fractures are significantly different. Under low effective stress, the hydraulic conductivity of GAC-filled fractures is more sensitive to variations in effective stress compared to sand-filled fractures, indicating that changes in ground loads might affect the long-term treatment efficiency of GAC-based amendments in shallow formations. The experiments also tested the scenario in which a mixture of sand and GAC particles is placed in the fracture. There exists a critical effective stress at which the benefit of fracture permeability enhancement caused by increased GAC mass fraction is offset by the disadvantage of fracture permeability reduction due to the low mechanical strength of GAC. Our experiments illustrate that this critical effective stress is approximately 500 psi. In practice, the closure stress imposed on an artificially-induced hydraulic fracture is generally lower than 100 psi because in shallow subsurface remediation the formation depth is usually less than 100 ft. Therefore, increasing the GAC mass fraction in particle injections is generally advantageous for enhancing fracture permeability. A double-exponential model was developed to interpret the laboratory data and to predict the minimum hydraulic conductivity of a GAC/sand-filled fracture. This work not only advances the fundamental science in contaminant hydrology, but also supports practical applications of GAC-based amendment injections for subsurface contaminant remediation.