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Whirlwind Mine Groundwater Characterization Report <br /> Aerial recharge to aquifers occurs in the highlands of Beaver Mesa and Dolores Point to the <br /> west as well as in the mine vicinity by infiltration of precipitation. This recharge rate is not <br /> expected to exceed 5 or 10% of total annual precipitation. Recharge on Beaver Mesa is <br /> expected to be more effective as the area is higher in elevation and likely receives greater <br /> overall precipitation. In addition, the mesa is relatively flat which decreases runoff and increases <br /> overall infiltration. The Dakota Sandstone and Burro Canyon Formation form bedrock on the top <br /> of Beaver Mesa and Dolores Point. The underlying Brushy Basin and Salt Wash Members of <br /> the Morrison Formation are exposed on moderate and steep west-facing slopes, respectively, <br /> that define the west edge of Dolores Point. These exposures, where soils are thin or <br /> nonexistent, provide surface area for aerial recharge to the formations as well as the surface <br /> area for competing evapotranspiration. <br /> Precipitation that recharges these formations percolates downward and migrates laterally to the <br /> east in the direction of geologic dip, a gentle 1 to 3 degrees east. The low dip angle results in a <br /> low hydraulic gradient in saturated units; groundwater is expected to migrate eastward at a slow <br /> rate. <br /> Recharge to the Burro Canyon percolates downward where it merges with the saturated portion <br /> of the aquifer, which forms an unconfined aquifer above the less permeable mudstones of the <br /> Brushy Basin. In places, if the underlying mudstone is thin or absent, the saturated intervals <br /> extend into an upper sandstone unit of the Brushy Basin. The mudstone itself may form a <br /> portion of the aquifer where it is weathered, or brittle and fractured. <br /> Groundwater within the middle and lower sandstones of the Brushy Basin as well as within <br /> underlying mudstones where conditions allow, also forms saturated intervals at depth, away <br /> from the exposed outcrop on the west and east sides of Dolores Point. It is likely these Brushy <br /> Basin aquifers form mostly from aerial recharge where exposed. The vertical migration of <br /> groundwater from the Burro Canyon and from one Brushy Basin sandstone to an underlying <br /> sandstone is expected to be small as these individual saturated zones may be perched, <br /> unconfined, and separated by dry to occasionally variably saturated interstitial mudstone units. <br /> These individual sandstone "aquifers" are further limited by their lenticular geometry—some of <br /> these units may only extend a few hundreds of feet in the lateral direction and substantially vary <br /> Western Water& Land, Inc. 51 <br />