Final Explanation of Significant Differences for T-25 Area Groundwater (Operable Unit 1) US Army SSC.pdf

This is an Explanation of Significant Differences (ESD) document for environmental remediation services at the U.S. Army Natick Soldier Systems Center in Natick, Massachusetts, covering modifications to the T-25 Area Ground Water (Operable Unit 1) remedial action under Contract Number W912CG-05-D-0007.

The document describes a modification to the previously selected remedy for groundwater contamination in the T-25 Area, which is contaminated with volatile organic compounds (VOCs)—primarily tetrachloroethene (PCE) and trichloroethene (TCE)—located 30 to 65 feet below ground surface. The significant change consists of discharging an additional 15 gallons per minute of extracted groundwater from the Buildings 22 and 36 Area and Buildings 63, 2, and 45 Area to the T-25 Area Treatment Facility for treatment. In 2007, the average flow from the T-25 Area was 65 gallons per minute; adding 15 gallons per minute from the other building areas results in a combined flow of approximately 80 gallons per minute. The selected remedy employs Groundwater Extraction with Air Stripping/Carbon Adsorption and Long-Term Monitoring, Institutional Controls, and Monitored Natural Attenuation. The groundwater extraction system has been operating since November 1997 and treats contaminated groundwater through air stripping and activated carbon adsorption at a treatment facility. The treatment facility has a capacity of approximately 160 gallons per minute, so the increased volume remains within operational capacity with no significant cost impact. The U.S. Environmental Protection Agency, the U.S. Department of the Army, and the Massachusetts Department of Environmental Protection all concurred with and authorized this modification. Public participation compliance was achieved through notification in the MetroWest Daily News and presentation at a Restoration Advisory Board meeting on April 20, 2006.