Authors:
Ralph Baker | TerraTherm, Inc. | United States
PhD Gorm Heron | TerraTherm, Inc. | United States
Steffen Griepke | TerraTherm, Inc. | United States
Niels Ploug | Krüger A/S | Denmark
Dense Non-Aqueous Phase Liquid (DNAPL) source areas containing halogenated volatile organic compounds such as trichloroethene (TCE) and tetrachloroethene (PCE) often have given rise to significant dissolved plumes in groundwater, threatening or even leading to closure of downgradient private and municipal water supply wells. At many such sites, hydraulic containments via pump-and-treat have been implemented to limit migration and protect well fields, but their long-term operation is costly and in most cases must continue indefinitely. Removal of the DNAPL source area by excavation or other means such as in situ thermal remediation (ISTR) offers the potential to diminish or end the need for hydraulic containment if the associated dissolved plume attenuates sufficiently following source removal. A question often raised is whether this occurs, or whether back-diffusion of contaminants from secondary sources such as low permeability lenses in the dissolved plume precludes it.
TerraTherm has been charged with conducting DNAPL source removal using ISTR at dozens of sites, but until recently groundwater data for the associated dissolved plumes have not been readily available. This paper presents a compilation of cases where such data are now becoming available. The data indicate that implementation of a thorough ISTR in a DNAPL source area can result in rapid and complete attenuation of the associated dissolved plume, such that long-standing pump-and-treat systems are no longer needed and can finally be turned off.
At a site in Upstate New York, USA, dissolved plumes of PCE and daughter products TCE, cis-1,2-Dichloroethene and Vinyl Chloride emanated from the DNAPL source area and extended ~1 km downgradient beneath a residential neighborhood. Two years following completion of source treatment using ISTR, the dissolved plume concentrations had diminished so dramatically that the regulatory agency gave permission for two out of three of the pumping wells that had operated there for decades to be shut off, and the final pumping well is expected to be able to be shut off soon.
At a site in Odense, Denmark, a very similar story unfolded following ISTR treatment of a PCE source area. There the source area resided beneath an operating dry cleaner facility, which had to remain in operation throughout implementation of ISTR. This paper will summarize these and several other cases with similar results, showing that application of thermal remediation can result in not only unobtrusive and thorough removal of the DNAPL source, but also effective diminution of dissolved plume groundwater concentrations and achievement of drinking water standards. In each case, information concerning the subsurface lithology and hydrology will be presented so that generalizations concerning the settings and applicability of the results can be drawn.