ENHANCED ANAEROBIC DECHLORINATION IN A HIGH GROUNDWATER VELOCITY ENVIRONMENT
Sami A. Fam, Ph.D., P.E. L.S.P., David Falatko, P.E., Jason Blackburn, P.G., Innovative Engineering Solutions, Inc., George Pon, Ph.D., Bioremediation & Treatability Center. Michael Gaudette, Univar USA Inc.
Abstract
We have been conducting a large scale enhanced anaerobic dechlorination (EAD) pilot test since May 2006. The EAD pilot test used a groundwater recirculation system that includes 6 extraction wells and thirteen injection wells spanning an area that is approximately 300 feet long and 100 feet wide. Groundwater (contaminated with 2 to 3 ppm of TCE and its breakdown products) is recirculated within this area at an approximate flow rate of 30 gallons per minute within the sandy aquifer. A sodium lactate substrate was dosed into the aquifer for the first 6 months of the pilot test. We observed limited biodegradation during this period, however, production of vinyl chloride was observed. Ethene production was very limited even after bio-augmenting with our NJ-14 culture. An evaluation of the initial pilot test data indicated the groundwater seepage velocity was greater than originally estimated by others. As a result, the carbon loading required to meet the aquifer electron donor demand was deficient and therefore was increased significantly. Since March 2007, the amendment dose has been increased to approximately 12,000 pounds/month of total organic carbon (TOC) using a whey solution stabilized with molasses. This is the lowest cost electron donor that is available and the whey can provide a longer half-life to take advantage of the groundwater velocity. Since February 2007, we have added more downgradient wells to our groundwater monitoring network. Ethene is being produced in these downgradient wells (60-90 day travel time).
The major benefit of the high groundwater flow velocity is greater areal coverage by the remediation system. The major downside is the high consumption of electron donor. The conceptual remedial plan for the 44 acre site is to install 2 to 3 biobariers (rows of bioremediation wells perpendicular to groundwater flow) to create "clean" groundwater zones that can "flush" out the contamination from the rest of the site over time. The pilot test equipment was built inside of a mobile trailer so that the trailer can be moved from one location to the next. The high groundwater flow velocity at the site offers the potential ability to nearly fully remediate the 44 acres in a 2 to 3 year time frame. The biobarriers will have a downgradient influence of roughly 400 to 600 feet. This distance estimate is based upon observing significantly reduced VOC concentrations in wells downgradient of our pilot testing area.
With EAD, it is most beneficial to install the full remedy as soon as possible so as to influence the entire impacted zone and to minimize recontamination of treated areas and to maintain anaerobic conditions until the treatment is completed. A slow (staged) installation would not be cost effective since the electron donor cost would be additive every year and the electron donor costs will be a significant component of the treatment cost. Electron donor costs are estimated at $250,000/year for this project and an overall estimate of 2.5 million dollars to remediate 44 acres.
