The Rocky Mountain Arsenal (RMA) (Operable Unit 21) site comprises part of the 17,000-acre RMA site, which is a former U.S. Army chemical warfare and incendiary munitions manufacturing and assembly plant in Adams County, Colorado. From the 1950s until late 1969, the U.S. Army used the RMA facility to produce the nerve agent GB (isopropylmethyl-phosphonofluoridate). In addition, between 1947 and 1982, private industries leased major portions of the plant's facilities to manufacture various insecticides and herbicides. Operable Unit 21 (OU21), the South Tank Farm Plume (STFP), is one of several areas being addressed as part of the Other Contaminated Sources IRA. From 1947 to 1978, STF tanks 464A, 464B, and others were used intermittently to store bicycloheptadiene (BCHPD) and dicyclopentadiene (DCPD) bottoms generated from pesticide manufacturing. A number of U.S. Army investigations have revealed ground water contamination originating from the areas of light nonaqueous phase liquid (LNAPL) located near tank 464A. Recent investigations have shown that the STFP does not pose significant risk to public health or the environment. The ROD addresses interim management of migration of the STFP and is consistent with the Final Response Action. The primary contaminants of concern affecting the ground water are VOCs including benzene, toluene, and xylenes. The selected interim remedial action for the site is included.