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

Permeable reactive barriers (PRBs), such as the Waterloo Funnel and Gate Sy stem, first implemented at Canadian Forces Borden facility in 1992, are a p assive remediation technology capable of controlling the migration of, and treating contaminated groundwater in situ. Most of the PRBs installed to da te have been shallow installations created by backfilling sheet-pile shored excavations with iron filing reactive media. More recently continuous tren chers [R. Puls, Installation of permeable reactive barriers using continuou s trenching equipment, Proceedings of the RTDF Permeable Barriers Work Grou p, Virginia Beach, VA, September 1997] and Caissons [J. Vogan, Caisson inst allation of a pilot scale, permeable reactive barrier in situ treatment zon e at the Sommersworth Landfill, NH, Presented to the RTDF Permeable Barrier s Work Group, Alexandria, VA, April 1996], and vertical fracturing emplacem ents [G. Hocking, Vertical hydraulic fracture emplacement of permeable reac tive barriers, Progress Report delivered to the Permeable Reactive Barriers Workgroup of the Remedial Technology Development Forum, Beaverton, OR, Apr il 1998] have been used to create reactive barriers in soil. None of the pr ior 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 eval uate the effectiveness of in situ treatment of chlorinated volatile organic compounds (CVOCs) and metals in that type of aquifer. Three discrete fract ures were identified and treated and were subjected to testing before and a fter treatment. Between 300 and 1700 lb. of 1 mm diameter reactive proppant s were injected into each zone to facilitate treatment. Monitoring data obt ained from adjacent observation wells verified that fracking fluids reached at least 42 ft from the treatment well following hydrofracturing. The conc entrations of many of the CVOCs decreased up to 98% based on the results of pre- and post-RSF treatment analyses. Consistent with other research, conc entrations of CVOCs were noted to decrease including trichloroethene (TCE), tetrachloroethene (PCE), 1,1,1-trichloroethane (1,1,1-TCA), 1,1-dichloroet hane (1,1-DCA), and 1,1-dichloroethene (1,1-DCE) and increases were noted i n concentrations of cis-1,2-dichloroethene (cis-1,2-DCE) and chloroform sug gesting that the rate of transformation of the parent compounds to these da ughter products is higher than the rate of destruction of the daughter prod ucts. The RSF pilot study demonstrated that: (1) zero valent iron foam prop pants have the physical and chemical properties necessary to effectively tr eat CVOCs and metals in groundwater when inserted under high pressures into fractured bedrock. (2) Iron foam reactive media can be placed in bedrock u sing high pressure hydraulic fracturing equipment and polysaccharide viscos ifiers. (3) The extent of the treatment can be monitored in situ using trac ers and pressure transducers. (4) Well capacity is increased by improving h ydraulic conductivity through hydraulic fracturing and proppant injection. The approximate cost of all of the effort expended in the pilot study was a bout US$200,000. Full-scale implementations are projected to cost between U S$100,000 and US$1,000,000 and would depend on site specific conditions suc h as the extent and level of impacted groundwater requiring treatment. This technology can potentially be implemented to create treatment zones for th e passive treatment of CVOC and metal impacted groundwater in fractured roc k aquifers offering a cost-effective alternative to a pump and treat foreve r scenario. (C) 1999 Elsevier Science B.V. All rights reserved.