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<br />Comment 20. Provide available information on depth, shape, and direction and rate of travel for <br />contaminant plumes. Pinnacle Pines, LLC presented Diisopropylmethyl phosphonate (DIMP), <br />methylene chloride, parthion, DDT, malathion, atrazine, ethylbenzene, and xylene and <br />tetrachloroethene as possible contaminants of concern. <br />• Many of the constituents mentioned in the DMG and Pinnacle Pines comments are <br />contaminants of concern (COCs) for the Rocky Mountain Arsenal (RMA) Offpost Study <br />Area remedial activities. Available COC data does not delineate contaminant plumes <br />vertically within the alluvial aquifer. However, RMA data and data presented in the <br />Phase I Environmental Site Assessment Addendum (Attachment I) illustrates that <br />contamination near the proposed Tanabe site is restricted to [he alluvial ground water. <br />DIMP is the most wide spread constituent within the alluvial ground water outside of the <br />RMA boundaries and within the Offsite Study Area. The delineated DIMP plume as it <br />flows to the South Platte River is depicted for 1989, 1993, 1995, and 1997 ih Attachment <br />K, which illustrates the continual decrease in the extent of the plume. <br />Attachment K also illustrates the DIMP plume shape and direction of travel. Most RMA <br />literature suggests that DIMP migrates within the alluvium to the South Platte River at <br />rates slightly below the linear velocity of ground water movement, 1.5 to 10 ft/day near <br />the proposed Tanabe site. The linear ground water velocity at the site is estimated to be <br />6.7 ft/day. <br />DIMP and other constituents listed in the DMG comment and present in the RMA <br />Offpost Study Area COC list are included in the RMA offpost monitoring program. For <br />monitoring wells near the proposed Tanabe site (includes T2S, R67W, Section 10 and <br />surrounding area), results from the 1999 monitoring data are presented on the map <br />included in Attachment L. The approximate shape of the DIMP plume (concentrations <br />greater than or equal to 8 µg/L and less than 100 µg/L) near the proposed Tanabe site is <br />depicted on a separate map, also included in Attachment L. As illustrated in the <br />attachment, several constituents listed in the DMG comment, including malathion and <br />parathion, have not been detected near or upgradient of the proposed Tanabe site. In <br />addition, all non-DIMP detections of constituents are restricted to small areas and <br />concentrations are typically well below applicable Colorado Basic Standards for Ground <br />Water. Methylene. chloride is'the only exception, being, detected at several locations in <br />concentrations slightly above the ground water standard of S µg2. However, <br />representatives of the U.S. Army have confirmed that methylene chloride is a solvent <br />commonly used in analytical laboratories and the possibility of sample contamination at <br />the laboratory could explain the erroneously high methylene chloride results from RMA <br />monitoring samples. <br />As stated, DIMP and non-DIMP constituents are typically restricted to the alluvial <br />ground water. The non-DIMP constituents vary in their rate of migration but generally <br />follow a northwestern flow path. <br />Comment 21. Provide State ground water quality standards and domestic, agricultural or other <br />relevant surface water standards for the contaminants of concern. <br />• Relevant water quality standards for the constituents listed in the DMG comment and <br />other RMA COCs are presented in Attachment M. <br />Page 7 <br />