Due to the high costs of remediating dense non-aqueous phase liquid (DNAPL) sources, technologies that can effectively treat the saturated zone resulting in destruction and containment, reduced treatment times, and lower costs are critically in demand. A significant number of Department of Defense (DoD) facilities have used chlorinated solvents as degreasing agents in the past. The estimated capital and operation and maintenance (O&M) cost of cleanup at each site is $3.6 and $3.5 million, respectively (present worth).
Bioaugmentation is an in-situ remediation approach where complete dechlorination of chlorinated ethenes is stimulated by supplying microorganisms that have demonstrated the ability to completely dechlorinate chlorinated ethenes in the presence of the appropriate electron donors and nutrients. Using either naturally occurring microorganisms or those added through bioaugmentation, enhanced rates of biodegradation at the DNAPL:water interface will increase the concentration gradient driving DNAPL dissolution. Increasing the concentration gradient will result in more rapid DNAPL dissolution and a reduction in the time required for cleanup. In the event that the increase in degradation rates is insufficient to significantly enhance DNAPL removal, rapid biodegradation of the high volatile organic compound (VOC) concentrations typically encountered in DNAPL source zones (e.g., tens to hundreds of mg/L) will provide biological containment of the groundwater plume, thereby reducing cleanup times and/or the O&M cost of the conventional containment approach of pump and treat systems.
The objective of this demonstration was to evaluate the performance of bioaugmentation at field scale to enhance rates of biodegradation at the DNAPL:water interface, thereby increasing the concentration gradient driving DNAPL dissolution. This demonstration used tetrachloroethene (PCE) as the primary DNAPL in a porous media groundwater system. The combination of field and laboratory investigations sought to determine if bioaugmentation could stimulate complete dechlorination to non-toxic end products, as well as increase the mass flux from a source zone when biological dehalorespiration activity is enhanced through nutrient addition and/or bioaugmentation.
The demonstration proved that biological systems can be applied and can promote enhanced dissolution of a PCE DNAPL source zone. To assess enhancement of mass discharge, two types of analysis were evaluated—the production of chloride and the production of daughter products converted to PCE equivalents. These two approaches produced a range in mass discharge increases ranging from 2.2 to 18.6. The most conservative value of 2.2 is calculated based on the increase in chloride ion observed at the extraction wells between the baseline and bioaugmentation phases of the experiment. When using the predicted PCE and the actual PCE equivalents (the sum of PCE and all of the degradation products produced from the PCE), the increase in mass discharge ranged from 4.4 to 18.6. Conservatively, this project demonstrated an average increase in mass discharge ranging from 2.2 to 4.5 during the bioaugmentation phase relative to baseline (groundwater extraction only) conditions.
Through operation of this bioaugmentation system, an appreciation was gained for the level of monitoring, parameters to monitor, sampling frequency, distribution or mixing of nutrients/microorganisms, and loading of nutrients that are necessary to apply bioaugmentation at other sites. The enhancement in mass flux and the corresponding decrease in treatment time that ultimately justifies selection of this technology as a source remediation alternative were estimated, and rigorous operational and performance data that will encourage regulatory acceptance of the technology were developed.