Formatted Title
DPT Jet Injection for Enhanced Treatment of Chloropicrin in Low-Permeability Soils: A Five-Year Review
Background/Objectives
Historical releases of chloropicrin at a former chemical facility resulted in an area of soil impacts above and below the water table. In situ treatment was the preferred approach due to an active railroad spur located in the treatment area as well as potential health and safety concerns related to chloropicrin emissions to ambient air if the soil were excavated; the compound is a strong irritant to the lungs and eyes. Additional challenges included (1) limited data from other sites supporting the feasibility of treating chloropicrin in situ, (2) complex glacial geology of the treatment area, ranging from highly permeable gravel to low-permeability clay and glacial till, and (3) chloropicrin impacts several orders of magnitude above the cleanup level, including concentrations as high as 930,000 mg/L in groundwater and greater than 10,000 mg/kg in low-permeability soils. We have previously reported the results of bench-scale treatability studies used to develop the treatment approach and amendment dosing in the saturated zone along with the successful field verification of the radius of influence achieved by the direct-push technology jet injection (DPT-JI) method. Here, we present an evaluation of the combined remediation approach supported by groundwater and soil performance monitoring results from five years following the start of treatment.
Approach/Activities
DPT-JI is an injection method that combines high pressure jetting (10,000 psi) and controlled hydraulic fracturing for delivery of amendments into low-permeability formations where remediation is limited by contact between reagents and solvents trapped in the matrix. Using DPT-JI as the primary delivery vehicle, a combined remedy was implemented at the site using fracture-enhanced soil vapor extraction (SVE) for treatment in the vadose zone and in situ chemical reduction with microscale zero-valent iron (mZVI) for the saturated zone. In the vadose zone, the selected full-scale remedy utilized sand-filled fractures with engineered connections to SVE wells to provide flow enhancement in the tight clay/silt layers and facilitate cost-effective treatment. In the saturated zone, treatability studies showed that in situ chemical reduction with ZVI was effective at treating chloropicrin concentrations up to 20,000 mg/kg to below the cleanup standard (0.125 mg/kg). A total of 98 metric tons of mZVI and 24 metric tons of sand were injected into the treatment area during two phases of remedy implementation targeting an area of approximately 1,200 m2.
Results/Lessons Learned
Phase 1 of the remedy implementation involved installation of four fracture-enhanced SVE wells and injection of mZVI at seven locations. After 10 months, concentrations in groundwater immediately downgradient of the treatment area decreased by 96%, demonstrating effective in situ degradation of chloropicrin. The Phase 1 SVE system removed more than 110 kg of chloropicrin from the vadose zone clay/silt layer during the first 18 months of operation. Phase 2 included the installation of 16 additional fracture-enhanced SVE wells and mZVI injection at 27 locations. Five years after the start of treatment, concentrations in groundwater decreased by more than 99% in portions of the treatment area, and the total mass of chloropicrin in the treatment area was shown to decrease by 97% based on soil and groundwater performance monitoring. Additional post-injection soil and groundwater results evaluating further reductions during the sixth year of treatment will be presented.