At a glance
Location: Virginia
Geology: Sandy clay
Contaminant(s): DCE, PCE, VC
ETEC Solution(s): CarBstrate, ISD system
Unique challenge(s): Low-permeability aquifer
Site Summary
A former dry-cleaner property that had been redeveloped into retail shopping faced persistent chlorinated ethene contamination in groundwater after soil excavation and multiple oxidant injections. The plume impacted a downgradient drainage ditch and had minimal natural attenuation. Enhanced Reductive Dechlorination (ERD) via groundwater recirculation with CarBstrate™ delivered by an ISD™ system achieved a 99% reduction in total solvent mass over 25 months, complete dechlorination (evidenced by ethene), and placement of the site into the Long-Term Stewardship Program.
Site conditions and remedial goal
Chlorinated solvents were released through a grease trap during dry-cleaner operations. The PCE cleanup area covered approximately 2,000 square feet. Depth to water ranged from 5 to 10 feet bgs, with the treatment interval from 5 to 25 feet bgs. Baseline concentrations included PCE up to 862 µg/L, TCE up to 129 µg/L, cis-1,2-DCE up to 26 µg/L, and low vinyl chloride.
Prior activities included excavation of 320 tons of contaminated unsaturated soil (achieving unsaturated-soil closure), three injections of sodium permanganate over 6 years, and installation of a vapor barrier during redevelopment. PCE remained in groundwater after 10 years. Sulfate concentrations ranged from 100 to 450 mg/L. Negligible natural attenuation was occurring, and the plume was impacting a downgradient drainage ditch. The remedial approach needed to control migration while distributing electron donors throughout the low-permeability volume.
ETEC Solution
A 10-gpm In-Situ Delivery (ISD™) system (no carbon pre-treatment; integrated extraction/transfer pump controls; automated soluble-substrate metering; programmable for multiple injection locations; telemetry; low power requirements) was used for continuous groundwater recirculation and CarBstrate™ delivery. Four 4-inch injection wells (screened 5–25 ft bgs with bentonite/grout seals and drop tubes) and two 4-inch extraction wells (screened 5–30 ft bgs with Grundfos Redi-Flo3 submersible pumps) were installed. CarBstrate™ - a dry, carbohydrate-based, food-grade soluble substrate containing nitrogen, phosphorus, micronutrients, and B vitamins, and pH-buffered - was applied (total 3,500 lb.: 250 lb. initially followed by 30 lb. per week). No bioaugmentation was performed. Groundwater was recirculated at 0.14 gpm (~6,000 gallons per month; 150,000 gallons total over 25 months, equivalent to ~2.5 pore volumes). The system enabled slow, consistent movement of substrate through the entire impacted volume while providing hydraulic control of the migrating plume.
Remedial outcome
Negative oxidation-reduction potential (ORP) was achieved in all onsite wells. Soluble iron and manganese increased, indicating reductive conditions; sulfate was utilized; and total organic carbon was present across the treatment area. Complete ERD was confirmed by ethene production. Total solvent mass was reduced by 99% in 25 months. Individual monitoring wells showed substantial declines (e.g., MW-4 essentially 100% PCE reduction; overall large reductions in PCE, TCE, and cis-DCE across the treatment area). The approach overcame residual effects of prior oxidant applications and elevated sulfate, protected the downgradient drainage feature, and advanced the site into the Long-Term Stewardship Program.
