Site geology includes fine to coarse sand and gravel, with cobbles and boulders from ground surface to a depth of about 10 feet below ground surface (bgs) in the source area. The granular subsurface is underlain by moderately weathered fractured phyllite-muscovite-quartz schist bedrock, with minor limestone inclusions, and areas of saprolite observed in the study area. There are numerous transmissive fractures in the bedrock in the source area vicinity.
Targeted Environmental Media:
 Dense Non-aqueous Phase Liquids (DNAPLs) - Fractured Bedrock
The highest TCE concentrations were detected in fractured phyllite-muscovite-quartz schist bedrock. The plume extended to a primary recharge area for a municipal well.
Major Contaminants and Maximum Concentrations:
- Trichloroethene (120,000 µg/L)
- Polychlorinated biphenyls (PCBs) (var)
No technologies selected.
- Bioremediation (In Situ)
- Reductive Dechlorination (In Situ Bioremediation)
Comments:
Enhanced Reduction Dechlorination (ERD) was used to address chlorinated VOCs in the bedrock and overburden groundwater systems both to reduce the source mass and size of the plume. An injection
program was designed and implemented that consisted of 1,600 pounds of a beta version of ERDENHANCEDý from BioStrykeý Remediation, which was gravity injected into monitoring wells MS-3R, MS-7, and MS-28 in October 2007. A 70:30 mixture, by weight, of lactose and yeast (1,600 pounds total) was blended with approximately 7,500 gallons of potable water. Due to permeability differences, the majority of the slurry (about 98 percent) was introduced into the overburden. A second similar injection was performed in August 2008. About one year after the initial injection, the TCE concentrations in groundwater samples collected from the test wells indicated a decrease of 73 percent in the overburden well and about 98 percent in both fractured bedrock wells. During this time, the breakdown products cis-1,2- dichloroethene (DCE) and vinyl chloride, which had not previously been detected at the site at significant concentrations, were detected in each of the three wells, with cis-1,2-DCE concentrations increasing nearly 2,000 percent in samples collected from bedrock well MS-7, thus confirming TCE dechlorination. The order of magnitude increase in ethane concentrations in samples collected from well MS-7, from less than 3 to 81 ýg/L, verified the reduction of cis-1,2-DCE to vinyl chloride and ultimately to ethene.
The maximum concentration of TCE in groundwater allowable by the MCP in drinking water well recharge areas is 5 ýg/L.
Annual groundwater monitoring in 2010, indicated strongly reducing conditions, showed significant decreases in TCE concentrations (with minimal rebound), and documented that TCE daughter products were still present in groundwater at the two shallower well locations. No chlorinated VOCs were detected in the deeper bedrock well, nearly three years after injections were completed. Groundwater samples collected in July 2011, less than 4 years after the initial injection, indicated the mass of cVOCs (based on mole fraction) had decreased approximately 99 percent to 100 percent in each of the three wells. While the cis-1,2-DCE and vinyl chloride concentrations did increase over this period, the cis-1,2-DCE concentrations subsequently decreased by more than 93 percent from their peak. TCE has not been detected in the three wells since June 2010. The detection of methane in all three wells demonstrates that methanogenic conditions were achieved in bedrock and particularly in the overburden formations, which was also verified by the negative oxidation-reduction potential (ORP) values and minimal dissolved oxygen levels measured in groundwater. These conditions are conducive for robust ERD rates. The beta version of ERDENHANCEDý migrated with groundwater and created groundwater conditions suitable for ERD at least 200 feet downgradient of the injection wells (i.e., at well MW9), where TCE concentrations decreased from a baseline of 640 ýg/L to consistently less than 300 ýg/L after 2008.
The longevity of the additive in bedrock was demonstrated by the elevated total organic carbon (TOC) concentrations that existed in the formation for at least 4 years following the initial injections, which is consistent with the longevity (up to 9 years) observed at other sites. The longevity of the additive in the overburden was limited to about 2 years, likely due to the recharge of oxygenated surface water into the highly permeable overburden.
The results indicated that beta version of ERDENHANCEDý successfully created the desired biogeochemical conditions to destroy residual cVOC source mass and its daughter byproducts in bedrock, as well as in the overburden. Based on a nearly 4-year period with no measurable rebound in TCE concentrations following the initial injection, chemically reducing conditions were maintained in the groundwater system, which resulted in the nearly complete destruction of source mass as well as the downgradient plume.
The treatment was successful in reducing site contaminants.
References:
Reed, Michael. 2011. "Successful Application of Reductive Dechlorination Amendments into Bedrock North Adams, MA". Page 195. 27th Annual International Conference on Soil, Sediments, Water, and Energy. October 17-20. University of Massachusetts Amherst, MA.
MA Department of Environmental Protection. 2007. Brownfields Redevelopment Toolbox a Guide for
Massachusetts Communities. http:www.mass.govdepcleanupbftool.pdf
Schaffner, Richard; Lamb, Steven R; Lindhult, Eric C; Fenelon, Bernard; Reed, Michael. 2012. Green Remediation of CVOC DNAPL in Bedrock. Battelle Conference. Accessed on the GZA website at: www.gza.comnews-eventsgreen-remediation-cvoc-dnapl-bedrock
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