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Whirlwind Mine Groundwater Characterization Report <br /> in thickness. The Brushy Basin aquifer can be a viable aquifer locally but may be inconsistent in <br /> occurrence and with regard to long-term production needs. <br /> The upper Salt Wash sandstone unit, as well as underlying sandstone units receive less <br /> recharge because of the relatively smaller lateral surface area. This is because the Salt Wash <br /> forms steep slopes with narrow exposures of vertical ledges. The amount of recharge received <br /> for this formation is comparatively small compared to the Brushy Basin Member, where outcrop <br /> exposures form a broad slope. <br /> Based on an average annual precipitation at the Gateway ENE climate station of 10.7 inches <br /> (1991 to 2021) and assuming an actual precipitation of 15 inches at the elevation of the project <br /> area, the recharge rate on Beaver Mesa could be as small as 0.8 to 1.5 inches per year. The <br /> total volume of recharge to the area shown in Figure 5 would potentially range from 228 to <br /> 427 acre-ft/yr. <br /> Overall discharge of groundwater in the project area is by springs and evapotranspiration. <br /> Spring discharge has been observed in at least three springs in the study area; DP, Packrat, <br /> and Willow. The combined discharge is estimated to be approximately 15.5 gpm (25 acre- <br /> ft/year). There are likely other springs or seeps that have not been discovered or identified in <br /> previous investigations. Subflow discharge may occur from the Burro Canyon and Brushy Basin <br /> to the Quaternary landslide deposits where shallow groundwater may then undergo <br /> evapotranspiration. <br /> Discharge by evapotranspiration is more difficult to estimate and is often backed out of a water- <br /> balance estimate. Based on the estimates for aerial recharge and spring discharge given, an <br /> over-simplified conclusion is that evapotranspiration is of the order of 200 to 400 acre-ft/yr. This <br /> is a believable evapotranspiration rate, but such a simplified calculation is not particularly useful. <br /> Another form of discharge that occurred in the past was by downward leakage of Burro Canyon <br /> and Brushy Basin groundwater through drill holes and eventual discharge from mines. <br /> Thousands of drill holes were drilled in the area beginning in the 1950s and continuing into the <br /> 2000s. During that time, most holes were abandoned by backfilling with drill cuttings, no grout <br /> was used to seal off and isolate the borehole from groundwater. Drill holes that intercepted mine <br /> workings had the potential to continuously drain shallow groundwater from the upper aquifers to <br /> Western Water S Land, Inc. 52 <br />