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the underlying bedrock fomtation. Under the proposed Tanabe pit site, lateral ground water Flow <br />is in anorth-northwest direction. <br />In the Offpost Study Area, several factors are thought to influence the potentiometric surface of <br />the alluvial aquifer. To the northwest, the South Platte River is interpreted as being the regional <br />discharge point for the alluvial Flow system. Recharge to the alluvial aquifer occurs as <br />infiltration of precipitation, seepage from lakes, reservoirs, streams, canals, and buried pipelines, <br />and flow from the underlying confined aquifer system. Locally, First Creek near the RMA north <br />boundary is thought to significantly affect aquifer recharge. Infiltration of surface water from <br />the Burlington Ditch and O'Brian Canal also contributes significant recharge to the alluvial <br />aquifer in the Offpost Study Area. Pumping for irrigation and domestic uses may cause localized <br />changes in the natural ground water elevations and flow directions but, these changes are <br />generally not significant. As part of the RMA remedial activities, drinking water alternatives <br />have been provided for most domestic wells. In addition, most irrigation occurs between the <br />Burlington Ditch and the South Platte River and the majority of irrigation water in this area <br />comes directly from the irrigation canals. <br />Hydraulic gradients vary significantly across the Offpost Study Area. The steepest hydraulic <br />gradients, approximately 0.022 fl1fi, are present in the unsaturated alluvial area located along the <br />northwest boundary of the RMA. Low gradients are commonly present along the paleochannels, <br />which contain thicker deposits of fluvial sand and gravel. Some of the lowest alluvial aquifer <br />gradients occur within and adjacent to the First Creek paleochannel and include the proposed <br />Tanabe site. The average gradient in these areas of the OfTpost Study Area is 0.004 ftJft. <br />Potentiometric surface maps from RMA 1997 monitoring data indicate that the hydraulic <br />gradient across the proposed Tanabe site is approximately 0.006 fUft. <br />The saturated thickness of the alluvial aquifer in the Offpost Study Area varies from 0 to 60 feet <br />and generally varies from 1 to 2 feet seasonally. Below the proposed Tanabe site, the alluvial <br />saturated thickness averages approximately 9 feet. <br />Pumping tests performed during RMA monitoring and remediation have provided a range of <br />hydraulic conductivity values that may be observed along the First Creek paleochannel in the <br />Offpost Study Area. Hydraulic conductivity values range from 235 tllday to 401 tUday and <br />average 334 fl/day. [n addition, pump test results presented in the United States Geological <br />Survey Ground Water Circular l 1 include a well northeast of the proposed Tanabe site with a <br />hydraulic conductivity of 307 ft/day. It would, therefore, seem reasonable that the hydraulic <br />conductivity at the proposed Tanabe site is approximately equal to the Offpost Study Area <br />average of 334 tl/day. <br />The same pumping tests used to estimate Offpost Study Area hydraulic conductivities provide <br />specific yield values ranging from 0.01 to 0.18. Typical documented values of specific yield in <br />unconsolidated material range from 0.16 in sand and gravel to 0.25 in medium to coarse sand or <br />gravel. Considering the aggregate product available within the proposed Tanabe site, realistic <br />estimates of specific yield at the site would likely be at the high end of the range for the OfTpost <br />Study Area, or approximately 0.2. <br />2 <br />