2008-02-04_PERMIT FILE - C1982056A

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As previously noted, the Wadge overburden aquifer exists under confined aquifer conditions in the center of the • ground water basin. Although the confining shale units limit vertical movement of Bound water, the artesian pressure in the center of the basin resulting from recharge at relatively high elevations at the basin rim is sufficient to cause upward migration of ground water through existing joints and fissures. This upward movement of ground water results in ground water discharge to the Fish Creek watershed. Ground water discharge for the Wadge overburden aquifer to the surface and alluvial ground water systems within the permit and adjacent areas is limited to the Fish Creek drainage. No significant ground water discharge from the Wadge overburden occurs in Foidel Creek as indicated by the lack of base flows during dry periods. Assuming that the Wadge overburden aquifer is in dynamic balance with recharge and discharge being approximately equivalent, the discharge rate would be about 11,000 Bpd or 12.5 acre-feet/year. Most of this discharge would be via the Fish Creek alluvial deposits and would tend to be concenttated in the area of subcrop under the Fish Creek valley. The calculated discharge rate represents a small proportion of the total flow in Fish Creek even during low flow periods. Baseflow for Fish Creek is typically about 3 cfs equivalent to about 2,000,000 Bpd so that discharge from the Wadge overburden aquifer constitutes about 0.5 percent of the low flow discharge. It has been estimated that approximately 25 percent of the total streamflow of Fish Creek upstream of the Twentymile Park Basin results from ground water discharge (Boettcher, 1972). Most of the ground water discharge to Fish Creek occurring within the Twentymile Park Basin is derived from regionally extensive bedrock aquifers such as Twentymile Sandstone. A potentiometric map for the Twentymile Sandstone aquifer has been constructed based on 1983 water level data from wells completed in this unit (Map 33, Twentymile Sandstone Potentiometric Surface). The configuration of the potentiometric surface reflects the general trend of ground water movement in bedrock units of Twentymile Park. Recharge to the aquifer occurs primarily in the vicinity of the unit outcrop in the southern and western margins of the basin. Ground water flow tends to be in a northerly and easterly direction from the recharge areas to the discharge areas in the northeast margins of the basin. Discharge from the aquifer appears to be primarily to the Fish Creek drainage, probably via alluvial deposits. A small amount of discharge occurs in the Foidel Creek drainage, but this is not sufficient to sustain any baseflow in the creek. Flow net calculations for the Twentymile Sandstone aquifer, using the same equations described above, were used to estimate recharge and discharge for this unit. Hydraulic gradients in the aquifer are in the order of 1 percent near the discharge area and the width of the flow path in this area is about 8000 feet An average permeability value of about 3 fUday was assumed to be representative of the aquifer. The average thickness of the unit in this part of the Twentymile Pazk Basin is about 120 feet. Using these values yields a flow rate through the aquifer of about 300,000 Bpd (0.5 cfs). This constitutes about 16 percent of the average low flow in Fish Creek. The Twentymile Sandstone is believed to contribute the major proportion of bedrock discharge to Fish Creek as it Traverses the Twentymile Park Basin. The calculated contribution from this unit is consistent with observed flows in the creek at upstream and downstream gauging stations as discussed in the surface water section. The storage characteristics of the Twentymile Sandstone aquifer may be evaluated using the equation developed by Lohman (1972) for storage capacity under typical hydrostatic pressure conditions. For this calculation an average thickness for the aquifer of about 150 feet, a fracture porosity of 0.1, and a confined storage coefficient of 0.0001 was assumed. The total storage capacity of the Twentymile Sandstone aquifer over the area of the Twentymile Park ground water basin underlain by this aquifer, about 35 square miles, is calculated to be about 336,000 acre-feet. There are insufficient wells completed in the Trout Creek Sandstone aquifer within the Twentymile Pazk Basin to construct a potentiometric surface for this unit. Consequently, ground water recharge and discharge cannot be calculated using the flow net analysis technique. An approximate method of calculating recharge to the unit may be made by examining the probably situation under which recharge occurs. It is believed that most of the recharge to the Trout Creek Sandstone probably occurs in the vicinity of the unit outcrop in the southern margins of the Twentymile • Park Basin. In these areas the unit is overlain by about 300 feet of bedrock including the Wadge overburden. The quantity of recharge passing through to the Trout Creek Sandstone in these areas is governed primarily by the vertical permeability of the overlying bedrock units and the extent of the potential recharge area. This area is limited to the dip slopes where the Twentymile Sandstone has been removed by uplift and erosion. Permit Renewal No. 3 2.04-27 l2/l0/98