2000-05-22_PERMIT FILE - M2000002 (17)

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Hollc Belbnund • hlnhile Prenu.c Concrete. lrrr. • pn~,~, ~ Atm~ ?2. 2000 Commerce City, Colorado. The RMA was established in 1942 as a chemical weapons manufacturing site. Most of the RMA was placed on [he NPL as a Superfund site in 1987, with the Basin F Area added in 1989. As such, RMA is subject to compliance with CERLCA, also known as the federal "Superfimd" law. RMA's current focus is environmental cleanup, with the transfer of RMA land to the U.S. Department of the Interior upon completion of the cleanup for use as a National Wildlife Refuge. The Rocky Mountain Arsenal's major subordinate command is [he Chemical and Biological Defense Command at Aberdeen Proving Ground. Maryland. The major command for the RMA is the Army Material Command in Alexandria. Virginia. A11 remedial environmental activities at the RMA are governed by a December I5, 1998 Final Record of Decision issued under authority of CERCLA and subject to ongoing study by various federal, state, local, and private entities, in addition to the Anny. These include the United State's Environmental Protection Agency (EPA), the United States Fish and Wildlife Service (FWS), the Colorado Department of Public Health and Environment (CDPHE), [he Tri-County Health Department (THCD), the South Adams County Water and Sanitation District, the Agency for Toxic Substance and Disease Registry (ATSDR), and Shell Oil Company (Shell). Location of the Tanabe Property Relative to the RMA The southeastern boundary of the Tanabe property is located approximately ].66 miles northwest of the RMA within [he Offpost Study Area for ground water contamination that has migrated off RMA property (Figure I ). The Offpos[ Operable Unit is defined as the media (ground water in the alluvial aquifer) requiring remediation within the Offpos[ Study Area. This area is subject to the "Rocky Mountain Arsenal Offpost Operable Unit Final Record of Decision, Commerce City. Colorado," dated December 19. 1995. To control the migration of contaminants off the RMA, the U.S. Army and Shell installed three major containment/treatment systems: I) the Northern Boundary Containment System (NBCS); 2) [he Northwest Boundary Containment System (NWBCS) completed in 1982 and 1984; and 3) the Interim Response Action - Groundwater Intercept and Treatment System North of the Arsenal (IRA-A) completed in 1993. These systems are designed to contain and treat contaminated ground water before it passes the O'Brian and Burlington Ditches, northwest of the RMA. Ground water quality monitoring conducted by the TCHD has identified areas of detectable DIMP concentrations (equal to or above 0.35 µg/L) within the Offpost Study Area. The "northern plume" is the only area that contains ground water DIMP concentrations that exceed the Colorado Ground Water Standard (CGWS) of 8.0 us/L (Figure 2). Areas containing detectable ground water DIMP concentrations that do not exceed the CGWS are referred to as "fringe areas." The northern DIMP plume and fringe areas are confined primarily to ground water within the alluvial aquifer. Depth to ground water, according to records maintained by the State Engineer, is between 20 and 24 feet below ground surface. Depth to bedrock was not available. The northern DIMP plume flows off the RMA north of the NCBS and the IRA-A. Both the size of the plume and DIMP concentrations decrease at the ERO Resources Corporation