ISTD Beneath Active Dry-Cleaning Facility:
Knullen, Odense, Denmark[1]
Site Information
A dry-cleaner site in Denmark has been remediated by a combination of ISTD and SEE. This is the world’s first thermal remediation where steam and ISTD were used in combination. The dry-cleaner site posed unique challenges: all treatment was under a building where current dry-cleaning operations had to continue without interruption, with extremely limited access for drilling between existing dry cleaning machines, walls, cables, and pipes. The target DNAPL zone extended to more than 46 ft (14 m) below ground surface (bgs) with more than 7,700 lbs (3,500 kg) of PCE.
Subsurface Geology/
Hydrogeology
The contaminants had migrated from a leaking separation tank through 36 ft (11 m) of till clay down into a high yielding, confined sandy groundwater aquifer.
Project Goals
A critical groundwater production field supplying more than 100,000 citizens is located less than 0.6 mile (1 km) from the site in a down-gradient direction. The PCE plume from the site almost reached this field. The goal of the thermal remediation was removal of the DNAPL hot spot. A decision has not been reached yet on treatment of the plume.
Project Approach
Heat was supplied by 45 ISTD borings and 9 steam injection wells. Mobilized contamination was captured by more than 60 vacuum borings in the fill and clay, and three multi-phase extraction wells in the aquifer. Wells were installed within the active dry cleaner facility without interference with ongoing operations.
Project Results
The heat-up and treatment of the target zone was completed more than 6 weeks earlier than expected (see figure below).
Approximately 7,700 lbs (3,500 kg) of PCE mass was recovered (see figure below).
Soil samples showed that the remediation efficiency in the clay zone and sandy aquifer was better than 99.75% and 95%, respectively. The remaining PCE mass in the target zone after treatment was estimated to be less than 22 lbs (10 kg) and the average soil concentration was less than 0.5 mg/kg.
Impact on Dissolved Plume
The DNAPL source removal has had an immediate impact on the mass discharge from the site. Post treatment PCE concentrations in the source zone were below 0.1 mg/L compared to concentrations above 30-90 mg/L and suspected free phase DNAPL in pre-treatment samples. The mass discharge was reduced by at least 300-fold by the thermal treatment. Since termination of treatment in October 2008 three rounds of water samples have been collected from the sandy aquifer and the effects of the source removal is documented in the closest row of monitoring wells 170 ft downgradient of the site. The groundwater monitoring program includes measurements of temperature and analysis of possible pyrite oxidation which potentially can result in mobilization of nickel and arsenic. Other parameters analysed are NVOC, chloride, and bacteria. The temperature rise in the aquifer downgradient of the site may result in improved conditions for natural dechlorination, which is also being investigated. Furthermore, a groundwater model is currently set up to model the long term effects of the source removal in the dissolved phase plume.
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[1] Nielsen, S.G., H.E. Steffensen, T. Heron, G. Heron, M. Kuhlman, H. Skou, N. Just and L. Dissing. 2008. First Thermal Remediation Using a Combination of Steam and ISTD. Paper P-015, in: Bruce M. Sass (Conference Chair), Remediation of Chlorinated and Recalcitrant Compounds—2008. Proceedings of the Sixth International Conference on Remediation of Chlorinated and Recalcitrant Compounds (Monterey, CA; May 2008). Battelle Press, Columbus, OH.