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Groundwater in the Arkansas River alluvium is under unconfined water table conditions. The <br />groundwater in the alluvium adjacent to the river represents a stream - aquifer system in which the <br />groundwater and the surface water are in close hydraulic and hydrologic connection. The static <br />water table fluctuates seasonally and year to year, but is generally no higher than the top of the <br />alluvial sand and gravel deposit. The groundwater is tributary to the Arkansas River and <br />generally flows from west to east through the alluvium (Deere and Ault 2007). <br />The Arkansas River is used extensively for irrigated agriculture in the valley and on the adjacent <br />lands. Water is supplied for irrigation by diversion of surface flow directly from the river and its <br />tributaries through a network of irrigation canals and ditches. Pumping by shallow, relatively <br />high capacity wells (greater than 50 gallons per minute) for irrigation represents a principal <br />source of alluvial groundwater depletion. The primary source of alluvial aquifer recharge is the <br />Arkansas River and its tributaries (USGS 2002). Recharge to the alluvial aquifer also consists of <br />infiltration of applied irrigation water and deep percolation of precipitation. Water levels in the <br />aquifer in close proximity to the river channel can be influenced by changes in stream flow over <br />a short period of time, whereas water levels in the aquifer distant from the river are likely <br />influenced by changes in stream flow over the long term by an overall increase in availability of <br />surface water for irrigation (USGS 2002). <br />2.2 Site Conditions <br />Information on the subsurface conditions for the project site was derived from site boring logs <br />(Deere and Ault 2006), bore logs from adjacent properties (Deere and Ault 2007), and associated <br />sample analyses. Additional subsurface information, groundwater, and alluvial aquifer <br />characteristics, was obtained from published studies and reports prepared by federal and State <br />water resource agencies. Selected reference material is provided in Appendix A. Records were <br />also obtained from the Colorado Division of Water Resources, Office of the State Engineer <br />(SEO) for site and nearby alluvial well information. <br />During the May 2006 geotechnical investigation (Deere and Ault 2006) exploration holes were <br />drilled to define sand and gravel reserves for the mining and reclamation operation (Table 1). <br />The drilling included a total of 16 holes in and around the project site with 8 holes within the <br />project area (Figure 2). Selected exploration borings also included lithologic descriptions and <br />sieve analyses of representative samples (Table 2). <br />Sieve analyses of selected exploration borings were used to evaluate the general grain size <br />distributions, and range of materials within the alluvium below the approximate water table. All <br />of the boring holes extended completely through the overlying overburden soils and alluvium to <br />the underlying bedrock. The overburden soils are primarily silty to sandy clays from 2 to 3 feet <br />thick. The alluvial material, 28 to 34 feet thick, consists of predominantly medium to coarse <br />sand with variable amounts of gravel and fines. The alluvium in the borings contained between <br />4-53 percent gravel, 44-94 percent sand, and 2 -3 percent fines. Overall, the samples indicate <br />that the characteristics of the alluvium are somewhat variable across the site. Most of the <br />analyzed alluvial material is characterized as clayey sand with gravel. The sands are generally <br />clean and well graded, ranging from fine to coarse grained and medium to very dense. The <br />borings extended through the alluvium and bottomed in several feet of moist, grey, <br />siltsone /claystone. This clay layer is interpreted to be the underlying Pierre Shale bedrock. The <br />1Ue earth Page 2 of 11 <br />SOLUTIONS <br />