Formatted Title
Manners Piece: Getting to the Last Bite of Contaminant Mass through Fracture Emplacement
Background/Objectives
Initial site investigation activities at a pharmaceutical manufacturing facility (active from early 1950s to late 2000s) concluded in 1997. The manufacturing processes and related support activities included the use of chlorinated and non-chlorinated volatile organic compounds. The primary contaminant of concern (COC) within the area of interest is toluene where historical concentrations have exceeded 1000 ppm in groundwater (along with the presence of free product) with a regulatory cleanup level of 1 ppm. Since the mid-1990s various remedial activities were performed to reduce source mass (e.g., free product recovery via manual bailing and dual-phase extraction as well as a pump and treat system installed and operated from 2007 until early 2011) but were relatively ineffective in advancing remediation at the site. In 2011 a biosparge system was installed (and subsequently expanded in 2016 and 2017) with the objective of converting the aquifer to aerobic conditions thereby resulting in faster degradation of toluene. A passive venting and SVE system were added to the biosparge system in 2015 and 2017/2018, respectively. While the biosparge, passive venting, and SVE systems were more successful than previous strategies in removing toluene mass from the site, the site hydrogeology were determined to be a significant impediment in achieving site cleanup. A reasonably thick (10 to 15 feet) fat clay unit lies just below source area and within the vertical range of seasonal water table fluctuations. Supplemental site investigations showed that this clay unit contains significant residual toluene mass and contributes to observed seasonal concentration fluctuations (i.e., the seasonal-high water table intersects the clay unit allowing residual toluene mass to desorb into groundwater).
Approach/Activities
While the existing biosparge, passive venting, and SVE systems are reducing toluene concentrations, the rate of progress was determined to be insufficient to meet project objectives. To accelerate the site to closure/reduce the overall remedial timeframe, it was determined that the remaining toluene mass residing in the clay unit required more direct treatment. After assessing multiple alternatives, additional biosparge wells with focused hydraulic fracture emplacement was determined to be the most efficient strategy. Cased borings in five locations along the center plume were installed to allow for fracture emplacement of sand lenses in the clay unit at specific intervals previously identified during a vertical aquifer profiling investigation. Each location included two to three PVC-cased borings that were hydraulically fractured and filled with high permeability sand lenses within the targeted vertical intervals. A total of 11 new biosparge wells were installed within the PVC-cased borings to better access the residual toluene mass and distribute oxygen within the clay unit via the fracture-emplaced high-permeability sand lenses.
Results/Lessons Learned
The fracture emplacement process served to increase the effective permeability of the soil matrix thereby allowing for better access to contaminant mass that was previously inaccessible to the existing remedies at the site. This was particularly effective when dealing with shallow, low permeability clay, as the fracture emplacement significantly increased the effective radius of influence and overall effectiveness of the biosparge/SVE systems (i.e., increased volumes and flow rates and better oxygen distribution within the clay unit). By increasing the ability of the remedial systems to access and recover toluene from the unsaturated/partially saturated low permeability sediments, the pace of cleanup is now on target.