Formatted Title

What a Blast!: Application of Multiple Remedial Strategies in a Low Permeability Aquifer

Background/Objectives

The combination of multiple remedial technologies, utilizing differing oxidative states, biogeochemical environments, and permeability enhancement techniques, have been successfully implemented at a former Department of Defense site with multiple source areas. Remedial activities were implemented to address recalcitrant chlorinated volatile organic compounds (CVOCs) in groundwater, namely trichloroethene (TCE) and its degradation products. The site lithology is comprised of chalk with marl bedding and fractures including hydraulically transmissive weathered zones. Data demonstrated a preference for CVOC absorption into the silty marl beds. Due to increasing real estate transaction pressure, permeability enhancement approaches were implemented that included explosive and hydraulic fracturing to facilitate accelerated remedial timeframes. The remedial action undertaken included in-situ chemical oxidation (ISCO) utilizing sodium permanganate (NaMnO₄) and hydrogen peroxide (H₂O₂), NaMnO₄ recirculation, soil vapor extraction (SVE), and enhanced reductive dechlorination (ERD).  As a result of unique site conditions and permeability enhancement techniques, these multiple remedies successfully operated sequentially and in tandem to address dissolved-phase CVOCs in the fine-grained, low permeability aquifer.

Approach/Activities

Permeability enhancement techniques were pursued following evidence that amendments were not achieving the desired radius of influence (ROI). Both hydraulic and explosive fracturing were used to develop / propagate macro and micro fracture networks, respectively, to facilitate amendment distribution and contaminant contact. Explosive fracturing was carried out by licensed contractors with design specifications to maintain ground vibrations to less than one inch per second (half of the threshold at which concrete may experience cracking). Hydraulic fracturing was also implemented at select wells using a slurry of water, silica sands, and proprietary amendments, applied at pressures up to 300 pound per square inch (PSI) until evidence of fracture propagation was observed via pressure drop and increased flow rate. After permeability enhancement techniques were successfully implemented, an SVE system paired with a recirculation system was installed to maximize destruction capacity of previously applied oxidant and enhance CVOC capture from dewatered marl seams. With source zone groundwater concentrations of TCE significantly reduced, an ERD polish was implemented. Geochemical conditions were flipped using proprietary amendments that enhance natural bacteria populations and facilitate anerobic reductive bioremediation. Utilizing ERD over ISCO aided in managing health and safety risk while providing a low-cost remedial technology targeting remaining mass in the subsurface.

Results/Lessons Learned

 The results of groundwater performance monitoring completed throughout the implementation of the remedial measures confirm the fracturing programs enhanced destruction of CVOC mass in the subsurface using ISCO, recirculation, and ERD remediation techniques. Evidence of successful permeability enhancement was demonstrated through increased SVE effluent concentrations, decreased dewatering / recirculation durations, and increased injection rates with well responses increasing from less than 0.5 gallons per minute (gpm) at 30 PSI to 30 gpm with little backpressure. At some wells, dramatic decreases in TCE concentration were observed shortly after the implementation of permeability enhancement and recirculation with up to a 91% reduction achieved within three months of hydraulic fracturing and a 99% reduction achieved within 12 months of explosive fracturing. Residual dissolved-phase CVOCs are currently being targeted with ERD to reduce or mitigate matrix diffusion / rebound associated with the fine-grained marl seams.