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2. Surface Water from the Permit Area <br />As described in the mine plan, the combination of earthen berms along the natural berm on the east <br />side of Pit will keep all runoff from the 100 yeaz 24 hour event on site. A National Pollution <br />Elimination Discharge Permit (NPDES) will not be required since the pit will be able to contain the <br />100 year event. <br />3. General Ground Water Hydrology and Geoloav <br />The proposed Haldorson Sand and Gravel Mine is located on a thick and extensive terrace ofpost- <br />Durango gravels of Pleistocene age on what is known as Spring Creek Mesa. These gravels, as well as <br />the entire area, are underlain by extremely thick impermeable layers of Mancos Shale. 1n the <br />immediate floodplain of the Uncompahgre River, alluvial gravels of recent age aze found overlying <br />the shale and the post-Durango gravels do not exist in this area. <br />Spring Creek Mesa has an elaborate network of irrigation canals and has been irrigated for over a <br />century. During irrigation season, water from the ditches as well as the irrigated land itself enters the <br />gravels and due to their high porosity and permeability, the water travels lower until it encounters the <br />shale layers through which it cannot travel, such as the Mancos Shale. It will then travel along the dip <br />of the shale to the lower terrace floodplain of the river and eventually enter the river system. After <br />irrigation is shut off and the gravels are allowed to drain, it is likely that the entire Mesa becomes dry <br />to the Mancos Shale. This is based on observations at other similar sites throughout western <br />Colorado. The levels that the water reaches above the shale depends on the porosity and permeability <br />of the gravel, the dip of the shale, the amount of water that is allowed to filter into the ground, the <br />proximity of this infiltration to the site and other factors. Map B-2 -Surrounding Area shows the base <br />of Spring Creek Mesa in relation to the Haldorson Sand and Gravel Mine, as well as the other nearby <br />sand and gravel pits. <br />Haldorson Sand and Gravel Mine 28 <br />