C150094 Red Mesa Aquifer Groundwater Study 2002

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Red Mesa Aquifer Groundwater Study — Interim Report 2002 4.3 Bedrock Surface A bedrock surface map was drawn from data found in well logs available at the Division of Water Resources and from previous work completed by the Bureau of Reclamation during the A- LP Project (see Figure 8). Sixty-one data points from well logs and 12 data points from Bureau of Reclamation seismograph data were used to draw bedrock surface contours. The impermeable bedrock influences the groundwater hydrology. Bedrock dip, depressions and ridges can modify groundwater flow direction. Traditionally bedrock surface maps contour the top of one particular stratigraphic layer. However, Figure 8 shows contours of the top of various bedrock formations so that the thickness of the unconsolidated glacial outwash, which contains the aquifer; can be determined. 4.4 Aquifer Storage The volume of saturated Red Mesa Aquifer was calculated for the high and low water table surfaces. Each of the water table surfaces were digitally superimposed on the bedrock layer and the volume of material between these surfaces was calculated. Using a porosity of 30 percent, volumetric calculations estimated the water in storage in the Red Mesa terrace gravel aquifer to be: • May 16/17, 2001: 166,500 of • Dec. 18/19, 2001: 165,000 of The maximum storage volume within the Red Mesa Aquifer was estimated to be 414,000 af. Maximum volume was calculated by using a porosity of 30 percent and saturating the entire section of terrace deposits from the bedrock to the ground surface. This storage water includes water bound to the aquifer particles (specific retention) and "free" water that is moving downgradient (specific yield). Stored groundwater fills all the void space (porosity), which is the summation of specific retention plus specific yield. Specific retention is water bound to the soil 991-077/031 Wright Water Engineers, Inc. Page 11 Interim Report