Report on Estimated Cost for Expansion Tamarack Managed Groundwater recharge Project

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Basis for Estimates The costs for project construction are based on the capital costs associated with the project elements listed above, as well as anticipated appurtenant facilities required for a fully functional project. Cost data were obtained from a number of sources, including recent as -built costs provided by Northern Colorado Water Conservancy District (NCWCD), quotations supplied directly by vendors, and costs cited in Means Construction Cost index. In some cases, unit prices were estimated based on proportioning from known costs. The Environmental Assessment (Final Environmental Assessment, 1999) contemplated a total of 35 wells, divided between the TRSWA and the PESWA (up to 31 wells at the TRSWA and up to 6 wells at the PESWA). The project as described herein would result in a total of 31 wells at the TRSWA and a single infiltration gallery at the PESWA. Additional withdrawal facilities may be constructed at the PESWA as needed to fulfill the purposes and needs of the project, while remaining consistent with the scope of the environmental assessment. The need for such facilities will depend on the performance and yield of the infiltration gallery to be constructed as part of the current project. Figure 1 shows a typical well construction detail, while Figure 2 shows a typical surface completion at a well. The wells are screened within alluvial sediments overlying shale bedrock. Well depths were estimated to be 60 feet in the western area of the project, and 120 feet in the central'and eastern area of the project, based on experienced drilling depths as reported by NCWCD. Wells are typically completed with 18 -inch diameter steel casing, 18 -inch diameter wire -wrap and /or perforated casing, and gravel packed within a 36 -inch diameter hole. Following development, the wells are tested for yield and typically produce about 1,500 gallons per minute (gpm). The depth to bedrock at the PESWA is about 24 feet near the proposed pumping location. Water levels at this same location are about 7 feet below ground, providing a saturated thickness of 17 feet. For a saturated thickness of 17 feet, and an assumed hydraulic conductivity of about 10,000 gallons per day per square foot (as described below), the yield of a well completed at this location is estimated to be about 300 gallons per minute, significantly lower than yields of wells at the TRSWA. Therefore, an infiltration gallery is proposed in lieu of the approximately 8 wells that might be required to obtain a target flow rate of 2,250 gallons per minute (about 5 cubic feet per second (cfs)). A test infiltration gallery containing a 40 -foot intake lateral installed to a depth of 11 feet yielded about 670 gallons per minute (1.5 cfs). The gallery proposed as part of the expansion project would rely on 100 feet of intake laterals installed to a depth of about 24 feet. The required length of intake laterals was estimated using the following equation: L = 2880 ro Q /[K (D2 -d2)], Where L = required length of lateral in feet, ro = the distance to the point of no drawdown, Q = flow rate in gallons per minute, K = hydraulic conductivity in gallons per day per square foot, D = initial saturated thickness in feet, and d = saturated thickness at the centerline of the gallery during operation in feet (Groundwater and Wells, 1986, Fletcher G. Driscoll, Ed). Values in the above formula were estimated, and subsequently calibrated against the results of testing of the test infiltration gallery installed on site. A calibrated value of about 10,000 gallons per day per square foot for hydraulic conductivity falls within the anticipated range of hydraulic conductivity values for gravel (Groundwater -3- f304'L6