2012-10-01_REVISION - M1977284 (9)

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Mineral Joe Mine Environmental Protection Plan 38 9.2.2 Regional Aquifer Recharge Recharge to the upper Mesozoic aquifer occurs as infiltration of runoff and direct precipitation. In the Delores River basin, this runoff occurs primarily from spring snowmelt at higher elevations. In the summer and fall, additional runoff occurs from rainstorms that are sometimes intense and usually limited in extent (Weir et al., 1983). Although direct precipitation infiltration may recharge the aquifers on mesas and at higher elevations, little or no recharge to the upper Mesozoic aquifer occurs from direct precipitation or infiltration of snowmelt in the Paradox Valley. As reported by Golder (2009a), studies have shown that diffuse recharge (rain and snowmelt over the valley area) to basin aquifers in arid areas like Paradox Valley is limited or absent due to low precipitation rates, large vadose zones, and the water- scavenging vegetation found in dry areas (Wilson and Guan, 2004; Foster and Smith - Carrington, 1980). Recharge to the lower Paleozoic aquifer occurs outside of the Paradox Basin (Weir, et al., 1983). The Leadville Limestone outcrops north of Durango, and receives recharge at the outcrop from runoff and snowmelt from the San Juan Mountains. 9.2.3 Regional Flow Direction and Aquifer Discharge Regionally, groundwater flows to the west - northwest, discharging to the Dolores River (Weir, et al., 1983, Topper, et al., 2003). Flow direction in the lower Paleozoic carbonate aquifer is shown in Figure 9. The flow regime in the lower aquifer does not appear to be affected by the overlying Mesozoic aquifer, surface topography, or surface water flow, as it is essentially isolated by the confining evaporites (salt layers). Flow directions in the upper Mesozoic aquifer are variable on a local and regional scale, and are affected by geologic structure and areas of recharge and discharge. The overall general flow direction is west toward the Dolores River. As reported by Weir, et al., (1983), "where the Dolores River is deeply incised, water from the Mesozoic sandstone aquifers discharges directly to the Dolores River." However, local flow directions may deviate from this overall regional trend. 9.3 Local Groundwater System The Mineral Joe Mine has been developed in the Salt Wash Member of the Morrison Formation. This unit is unsaturated in the vicinity of the Mineral Joe, although it is known to contain groundwater locally in other areas. Where saturated, the Morrison Formation has been referred to as the "M" aquifer and is groundwater is generally perched on the underlying low - permeability Summerville Formation. 9.3.1 Local Stratigraphic Units The principal stratigraphic units at the Mineral Joe Mine are as follows: • Colluvium - Unsaturated • Dakota Formation — Unsaturated • Mancos Shale Formation — Unsaturated • Brushy Basin Member of the Morrison Formation — Unsaturated • Salt Wash Member of the Morrison Formation — Unsaturated at Mineral Joe, Variably Saturated elsewhere ( "M" Aquifer) • Summerville Formation — Lower Confining Unit 9.3.2 Local Groundwater System near the Mineral Joe Underground Workings A groundwater divide exists partway up the escarpment of Monogram Mesa, whereby any precipitation that might infiltrate into groundwater south of the divide would flow in a south easterly direction along the dip of the bedding. Any precipitation that might infiltrate into groundwater north of the divide would flow 4148B.120927 Whetstone Associates •