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Proposed Tanabe Pit Site <br /> Hydrostratigraphic Report <br /> The Rocky Mountain Arsenal (RMA) is located approximately 2 miles southeast of the proposed <br /> Tanabe pit site. Previous industrial and waste disposal practices at RMA resulted in the on-site <br /> contamination of soil, surface water, sediment, and ground water. As a result, in 1987, RMA <br /> was added to the National Priorities List, a list of sites requiring environmental cleanup. For <br /> cleanup activities, the RMA is divided into two Operable Units (OU): Onpost and Offpost. The <br /> Offpost OU is defined as the media (ground water in the alluvial aquifer) requiring remediation <br /> within the Offpost Study Area. This Offpost Study Area covers approximately 27 square miles <br /> and is defined as the area southeast of the South Platte River, north of g0'h Avenue, southwest of <br /> Second Creek, and north of the north and northwest boundaries of RMA (Figure 1). As seen on <br /> Figure 1, the proposed Tanabe pit site is within the Offpost Study Area. <br /> Geologic Setting <br /> Alluvium in the Offpost Study Area consists of unconsolidated deposits of Quaternary age. The <br /> material is composed primarily of terrace gravel, colluvium, eolian sand, loess, and fluvial <br /> sediments. The maximum alluvial thickness in the Offpost Study Area is approximately 100 feet <br /> with an average thickness of about 40 feet. Average thickness of the gravel deposit at the <br /> proposed Tanabe site is approximately 25 feet. The alluvium is comprised of two dominant <br /> lithostratigraphic units: an upper unit of fine-grained eolian silt and clay depostis which mantle <br /> lower units of well-sorted fluvial sand and gravel. The base of these alluvial deposits ties on the <br /> irregular erosional surface of the underlying bedrock formation, the Denver Formation. The <br /> bedrock surface configuration is largely a result of the paleodrainage system and is characterized <br /> by numerous paleochannels. Lower alluvial units of fluvial sand and gravel are typically thickest <br /> in these paleochannel drainage systems. <br /> Three main paleochannels have been identified in the Offpost Study Area. The approximate <br /> orientation of these paleochannels is identified on the bedrock surface elevation map in Figure 2. <br /> The most significant of these, the Northwest paleochannel, originates from the northwest <br /> boundary of the RMA and trends northwest. The other dominant paleochannels extend from the <br /> RMA northern boundary. One paleochannel trends north and is referred to as the Northern <br /> paleochannel while the other trends to the northwest. The paleochannel trending northwest from <br /> the northern RMA boundary runs approximately 1000 feet northeast of the proposed Tanabe site <br /> and is referred to as the First Creek paleochannel. <br /> Alluvial Aquifer Characterization <br /> The alluvial aquifer in the Offpost Study Area is generally classified as unconfined, <br /> heterogeneous, and transversely isotropic. Despite external stresses that have caused significant <br /> localized water table gradient fluctuations over time, the water table configuration in the alluvial <br /> aquifer has remained relatively unchanged for decades. Potentiometric surface maps from RMA <br /> 1997 monitoring data indicate the primary lateral flow direction in the Offpost Study Area is to <br /> the north and northwest toward the South Platte River, reflecting the general northwest slope of <br /> I <br />