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ECOSA Evaluation AdrianBrown <br />2. The creek in Grassy Valley flows briefly during spring snowmelt, which occurred in 2010 in <br />mid - April. Flows were modest and short- lived, totaling 2.6 acre feet, the equivalent of 0.028 <br />inches of precipitation over the 1,124 acre catchment area. <br />3. During the balance of the year, where the average daily temperature is above freezing, surface <br />water flow is dependent on precipitation, generally occurring when precipitation exceeds 0.24 <br />inches in any day (i.e. stormwater). In general the flow lasts for less than a day after the storm. <br />Less than 1% of the storm precipitation that falls on the catchment in excess of 0.24 inches per <br />day reports to the stream, representing a very small basin yield. <br />4. During all of 2010 there were a total of 9.6 acre feet of surface water flow at GV -03, <br />representing the equivalent of 0.10 inches of precipitation over the GV -03 catchment area of <br />1,124 acres. This represents a basinal yield of 0.69% of the 14.97 inches of precipitation <br />measured in Grassy Valley in 2010. This is a very small basin yield; normal montane yields are <br />in the order of 25% - 40 %. <br />In summary, there is almost no surface water flow in Grassy Valley adjacent to ECOSA. Most of the <br />precipitation that falls on the valley evapotranspires, with the balance infiltrating to the basin soil and <br />bedrock. No water re- emerges from the ground as potential springs within the footprint of the Diatreme <br />in Grassy Valley. <br />2.8 Groundwater <br />The regional groundwater system associated with the District is dominated by the Diatreme, a volcanic <br />caldera approximately 7 square miles in area (Plate 3). The Diatreme is located within the host <br />Precambrian -age granite, granodiorite, gneiss, and schist. The diatremal rocks are moderately <br />permeable, while the surrounding Pikes Peak granite is of low permeability. As a result, the Diatreme is <br />an enclosed regional groundwater regime, with limited hydrologic interaction with the surrounding <br />rockmass. <br />Historic drainage tunnels driven into the Diatreme in the 20 Century have drained groundwater flow <br />from the Diatreme and the immediately adjacent granite. The Carlton Tunnel is the deepest (elevation <br />7,000 ft msl) and most recent (1941), and has created a reduction in the groundwater table in the <br />diatremal rockmass to approximately 3,500 feet below the original ground surface. This results in the <br />Diatreme acting like a very large diameter "well" in the District, drawing groundwater vertically <br />downward within the wellbore (the diatremal rockmass) and drawing water laterally from the <br />surrounding Precambrian rock. This collection covers any water that infiltrates at any surface mine, <br />facility, or OSA in the entire District, including ECOSA. <br />The conceptual hydrologic flow model of the District is presented schematically in Plate 6. <br />1385E.20120224 4 <br />