Results of the reactant sand-fracking pilot test and implications for the in situ remediation of chlorinated VOCs and metals in deep and fractured bedrock aquifers

Abstract

Permeable reactive barriers (PRBs), such as the Waterloo Funnel and Gate System, first implemented at Canadian Forces Borden facility in 1992, are a passive remediation technology capable of controlling the migration of, and treating contaminated groundwater in situ. Most of the PRBs installed to date have been shallow installations created by backfilling sheet-pile shored excavations with iron filing reactive media. More recently continuous trenchers [R. Puls, Installation of permeable reactive barriers using continuous trenching equipment, Proceedings of the RTDF Permeable Barriers Work Group, Virginia Beach, VA, September 1997] and Caissons [J. Vogan, Caisson installation of a pilot scale, permeable reactive barrier in situ treatment zone at the Sommersworth Landfill, NH, Presented to the RTDF Permeable Barriers Work Group, Alexandria, VA, April 1996], and vertical fracturing emplacements [G. Hocking, Vertical hydraulic fracture emplacement of permeable reactive barriers, Progress Report delivered to the Permeable Reactive Barriers Workgroup of the Remedial Technology Development Forum, Beaverton, OR, April 1998] have been used to create reactive barriers in soil. None of the prior methods are capable of adequately addressing groundwater contamination in deep and fractured bedrock aquifers. The purpose of the RSF pilot study was to install reactive media into an impacted bedrock aquifer, and to evaluate the effectiveness of in situ treatment of chlorinated volatile organic compounds (CVOCs) and metals in that type of aquifer. Three discrete fractures were identified and treated and were subjected to testing before and after treatment. Between 300 and 1700 lb. of 1 mm diameter reactive proppants were injected into each zone to facilitate treatment. Monitoring data obtained from adjacent observation wells verified that fracking fluids reached at least 42 ft from the treatment well following hydrofracturing. The concentrations of many of the CVOCs decreased up to 98% based on the results of pre- and post-RSF treatment analyses. Consistent with other research, concentrations of CVOCs were noted to decrease including trichloroethene (TCE), tetrachloroethene (PCE), 1,1,1-trichloroethane (1,1,1-TCA), 1,1-dichloroethane (1,1-DCA), and 1,1-dichloroethene (1,1-DCE) and increases were noted in concentrations of cis-1,2-dichloroethene ( cis-1,2-DCE) and chloroform suggesting that the rate of transformation of the parent compounds to these daughter products is higher than the rate of destruction of the daughter products. The RSF pilot study demonstrated that: (1) zero valent iron foam proppants have the physical and chemical properties necessary to effectively treat CVOCs and metals in groundwater when inserted under high pressures into fractured bedrock. (2) Iron foam reactive media can be placed in bedrock using high pressure hydraulic fracturing equipment and polysaccharide viscosifiers. (3) The extent of the treatment can be monitored in situ using tracers and pressure transducers. (4) Well capacity is increased by improving hydraulic conductivity through hydraulic fracturing and proppant injection. The approximate cost of all of the effort expended in the pilot study was about US$200,000. Full-scale implementations are projected to cost between US$100,000 and US$1,000,000 and would depend on site specific conditions such as the extent and level of impacted groundwater requiring treatment. This technology can potentially be implemented to create treatment zones for the passive treatment of CVOC and metal impacted groundwater in fractured rock aquifers offering a cost-effective alternative to a pump and treat forever scenario.