Formatted Title
Demonstration of Distribution of Activated Carbon in Fine-Grained Geology by High-Energy, High-Flow Techniques Resulting in BTEX and Naphthalene Concentration Decreases in Soils Leading to Regulatory Closure: No Further Action
Background/Objectives
Contaminants sorbed to low-permeability, fine-grained clays are challenging to remediate due to the initial contaminant entrainment and the subsequent long-term diffusion-controlled release back into transmissive zones. Without appropriate intervention, source depletion may require decades to centuries. A former retail petroleum site in southeastern Kentucky (site) that had proven difficult to remediate and been an open case for over 20 years. The mean initial benzene concentration was 383 μg/Kg, while m/p-Xylene was 20 mg/Kg. Geologically, it is characterized by 15 to 20 feet deep silty and sandy clays with occasional chert layers overlying Upper Mississippian aged Salem and Warsaw limestones. The soils were soft to wet, beginning approximately 6 to 8 feet below the ground surface (bgs). In situ injection of an activated carbon slurry using high-energy, high-flow techniques was selected to enhance permeability, alter the diffusion gradient, and increase the advection-dispersion of site clays and silts.
Approach/Activities
The site was injected using a positive displacement pump at a volumetric flow rate of approximately 35 gallons per minute. Injections were completed at specific depths rather than across intervals. Injections were initiated at about 6 feet and continued in a top-down manner. Vertical injection intervals were spaced every 2 feet. Individual injection points were horizontally spaced 5 feet apart. One month post-injection, 12 continuous soil borings were advanced next to existing site monitor wells, and an additional 28 borings were advanced throughout the injected area for a total of forty soil borings. One hundred twenty soil samples were analyzed for BTEX and naphthalene. The soil cores were inspected macroscopically for activated carbon inclusions by brightfield microscopy and logged lithology. Pictures were taken to document the distribution of carbon in various soil types and along bedding interfaces. A survey was used to accurately define the locations and elevations of all wells and soil bores. Site remedial progress was characterized by post-injection water samples collected from 42 monitoring wells, installed in a grid pattern, distributed within ≈18,000 square feet. ArcGIS and RockWorks17 were employed to present the logged core data. At 40 months, the site was again extensively cored, and soil and aquifer samples were collected and analyzed for site contaminants.
Results/Lessons Learned
Core log descriptions and photographs document the presence of activated carbon in each of the 40 soil borings, indicating carbon distribution throughout the injection field. Laboratory analysis confirmed activated carbon inclusions in the soil cores. Both visual examination and ArcGIS and RockWorks 17 models offer corroborating depictions of carbon distribution. Mass and probability calculations further support distributional models. In this study, no significant balling, clumping, or generalized distribution failures were noted. Groundwater data and soil sampling demonstrated reduced petroleum impact on the site. All examined site contaminants (BTEX and naphthalene) sorbed to the soils and aquifer materials significantly decreased, as shown by the Wilcoxon-Mann-Whitney test. The site has received a regulatory no further action determination. The study indicated that high-energy, high-flow injection of activated carbon enhances remediation of fine-grained media.