The U.S. Department of Energy’s (DOE) Lawrence Livermore National Laboratory (LLNL) has developed and patented an innovative technology called dynamic underground stripping (DUS) with hydrous pyrolysis oxidation (HPO). The DUS/HPO technique relies on both thermal and chemical mechanisms to enhance the remediation and destruction of chlorinated solvents in the subsurface. DUS involves the injection/extraction of steam combined with electrical heating. DUS/HPO involves the injection of air along with the steam. This combination promotes the in situ oxidation of chlorinated solvents to carbon dioxide, chloride ions, and water in the presence of sufficient dissolved oxygen and under high temperatures, which brings about more rapid chemical reactions and higher mass transfer rates.
Previous applications of this technology focused primarily on contaminant removal through steam injection and extraction, along with extensive aboveground treatment of the extracted fluids. This demonstration was conducted at a site without a significant dense nonaqueous phase liquid (DNAPL) source zone present and was concerned primarily with the in situ destruction of trichloroethene (TCE) in the dissolved-phase plume at Solid Waste Management Unit (SWMU) 23 at Beale Air Force Base (AFB) in Marysville, California. The DUS/HPO technique also could be employed as a “polishing” step to reduce elevated groundwater contaminant levels by several orders of magnitude in order to meet acceptable cleanup criteria. Compared to DUS/HPO, other competing chemical oxidation methods (e.g., potassium permanganate injection) may be limited by higher mass transfer limitations and/or poor contact due to displacement of the contaminant during reagent injection.
This demonstration employed a novel mode of DUS/HPO application using a cyclic steam injection and extraction process from a single well, termed the “huff-and-puff” technique. The method involves intermittent operation of the system consisting of active steam/air injection into the subsurface, a passive “soaking” period, which allows the oxygen-laden steam to condense and mix with contaminated groundwater in a heated zone, and then active extraction to recover displaced contaminants and to minimize their migration outside of the target treatment area. The majority of the contaminant is oxidized during the passive “soaking” period. This novel method represents a significant advance over the application of DUS alone, primarily because in situ treatment of the chlorinated solvents results in a reduction in aboveground treatment requirements and costs as follows: (1) contaminants are significantly degraded in situ, which decreases the contaminant levels in the extracted fluids; (2) cyclic steam injection and extraction reduces the volume of extracted fluids; and (3) cyclic operation requires less intensive operation and maintenance of the system. Another potential enhancement to the application of DUS/HPO used in this demonstration was to increase the oxygen delivery rate through the injection of pure oxygen with the steam.
The field demonstration was conducted between May and December 2002. Considerable monitoring was conducted before, during, and after the field demonstration, which included three injection/extraction cycles. Groundwater monitoring was conducted through five wells installed in the target treatment zone surrounding the steam injection/extraction well. In addition to the contaminants of concern (COC), indicator groundwater parameters, such as chloride, alkalinity, dissolved oxygen, etc., also were measured. Thermocouples and electrical resistance tomography (ERT) were used to monitor the size of the heated zone. Bromide was injected with the steam as a tracer to evaluate hydraulic control during the demonstration and system operation.
The monitoring results indicate that the vendor was successful in heating the target treatment zone, despite some challenges, such as the abundance of finer-grained soils at the site. The radius of the heated zone (above ambient temperatures) was estimated to be as large as 20 ft around the steam injection well based on the thermocouple and ERT measurements. The dissolved oxygen distribution coincided approximately with the heated zone, although the oxygen appeared to have distributed in a wider zone; therefore, the vendor was successful in creating conditions conducive to HPO.
COC levels declined considerably in the monitoring wells in the treatment zone, with up to 85% decline in TCE levels and up to 91% decline in tetrachloroethene (PCE) levels observed. cis-1,2-dichloroethene levels in the treatment zone also declined considerably. The analytical results indicated that approximately 64 g of TCE were recovered in the vapor, while the change in groundwater concentrations within the heated radius only, indicate that 52 g of TCE were removed. Therefore, the extraction zone exceeded the thermally heated zone (to >80˚C, approximately 14 ft in radius and 15 ft in thickness).
The extent of the bromide tracer in the aquifer was larger than the influence of the heated zone, indicating that mixing and displacement could have caused some migration of dissolved groundwater constituents. The aborted steam injection Cycle 1a, in which steam was injected but could not be extracted due to a pump failure, could have been one factor in the loss of hydraulic control. Also, chloride levels declined substantially after treatment. Pre-treatment chloride levels in the treatment zone groundwater were unusually high, possibly because of release of vinyl chloride from the grout used in the injection well construction. Chloride served as a conservative tracer, because contributions from any degrading COCs is minimal; pre-treatment TCE and PCE levels at this site were relatively low and were orders of magnitude below the pre-treatment chloride levels. The average 68% decline in chloride levels indicates that the displacement/mixing caused by the injection/extraction cycles could have caused a dilution of dissolved groundwater constituents, including TCE and PCE. Therefore, it is difficult to conclusively attribute the decline in COC levels in the heated zone to degradation processes, such as HPO or microbial activity.