In Situ Bioremediation of Chlorinated Solvents: Evaluation of Intrinsic and Engineered Bioremediation

Problem

Spent chlorinated solvents consisting of 1,1,1-TCA, TCE, 1,1,2-TCA, DCM as well as BTEX and asphaltic compounds were released from two underground storage tanks at the Colorado Department of Transportation materials testing laboratory in Denver. An estimated 5,000 to 15,000 liters of NAPL were released into a highly, fractured bedrock setting resulting in an impact plume that extends one-mile from the point of release.

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Solution

Pelorus Environmental & Biotechnology Corporation was contracted to perform evaluations of bioremedial processes for this site to determine to what extent intrinsic biodegradation processes were occurring and whether or not these processes could be enhanced. A program was implemented to collect; 1) field data to describe the natural attenuation process and 2) laboratory microcosm data to determine relative rates and extents of chlorinated solvent degradation.

Site specific sampling and analysis was performed at the Colorado Department of Transportation Site to collect data for documenting the occurrence of intrinsic biodegradation activity on chlorinated solvent impacts in the groundwater. Along with extensive documentation of daughter products from 1,1,1-TCA and TCE, groundwater indicator parameters indicated that intrinsic biodegradation is an ongoing process occurring at this site. A number of parameters are within the range one would expect for anaerobic activity and reductive dehalogenation, particularly in the source area. Reducing groundwater conditions observed in the plume tend to support the potential for anaerobic biotransformation of the target volatile organic compounds (VOC) impacts. The plume is also constricted in width, exhibiting low transverse dispersion. This coupled with the observation of higher redox conditions outside the plume are an indication that the influx of more oxidizing (e.g., aerobic) waters may be supporting methanotrophic and other co-oxidative activity at the plume fringes.

Bench-scale microcosm studies were directed at identifying the effects of specific treatment amendments on samples representative of impact areas at the site. An offsite sample was evaluated under aerobic conditions, while the source area sample was evaluated under anaerobic conditions. Five treatments including the unamended treatments were evaluated along with a sterile control for each sample location (condition).

Intrinsic Biodegradation Field Analysis. Pelorus developed a sampling and analysis plan for collecting field data to characterize the CDOT plume for groundwater parameters indicative of intrinsic biodegradation potential. The data required to make the preliminary assessment of the intrinsic biodegradative process can also be used to aid in the design of an engineered remedial system should the natural attenuation scenario not be valid for a given site. An on site gas chromatograph was setup to analyze samples for the levels of methane and hydrogen using special well head sampling procedures. A gas-sampling bulb was employed to collect headspace samples via the bubble strip method. In addition a HACH spectrometer was employed for the on site analysis of key inorganic indicator parameters [Fe(II), H2S, NO3/NO2 , SO4]. Sampling transects were established throughout the plume so the trends could be identified down the longitudinal transect and cross gradient. Data analysis was simplified through visualization of plume trends using Surfer surface plots overlaid on the site base map. This process was essential for identifying the plume locations for collection of bench scale microcosm samples, as well as for siting of the remedial treatment system.

Aerobic Microcosms. The aerobically amended series was adjusted for primary substrates (i.e., methane, propane and phenol) and oxygen. Each treatment series was sampled in triplicate for statistical evaluation. For the purpose of measuring the variance within the test and performing a sensitivity analysis on the results generated, microcosms were constructed in triplicate. At pre-determined time intervals samples of the groundwater will be removed from each of the microcosms and analyzed for constituent degradation and production of by-products.

Anaerobic Microcosms. An aquifer sediment and groundwater sample from the source area at the MTL site was collected for use in these tests. Various combinations of treatment amendments (i.e., electron acceptors, electron donors, donor/acceptor and complex nutrient) were investigated during this test to identify the factors that may be limiting the in situ rates of anaerobic biodegradation. Due to the lower levels of anaerobic biomass and generally slower growth rates, these tests were run for a period of 8 months to allow adequate time to quantify degradative responses under natural conditions.

Chemical Oxidation Evaluations. Pelorus has developed a formulation for enhancing the performance of the Fenton’s chemical oxidation process in situ. As part of the evaluation of remedial alternatives, Pelorus performed bench scale testing on a range of sediment samples collected from the within the site plume. The chemical oxidation process was shown to be very effective in degrading all the constituents present in the plume.

The field and laboratory evaluations conducted for the CDOT site indicated that there is an active microbial community that is well acclimated to the biodegradation and biotransformation of the chlorinated solvent impacts. Both anaerobic transformation and aerobic co-oxidation mechanisms operate within the specific areas of the plume possessing the redox conditions necessary to support the respective processes. Source areas of the plume appear to be amenable to enhanced anaerobic treatment processes while the distal plume areas are clearly supportive of an aerobic co-oxidation. In-situ biotreatment and in-situ chemical oxidation with Fenton’s reagent are the remedial alternatives for the site.

Pelorus EnBiotech Corporation has developed the system conceptual design for implementation at this site and is currently supporting Secor International in the detailed engineering design and work plan. The system will consist of an In Situ Module (ISM) designed to deliver oxygen, methane and inorganic nutrients to recovered groundwater and a distribution system for re-injecting amended waters into the treatment area.

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