2015-08-18_PERMIT FILE - C1982056A (4)

Home Browse Search

The storage characteristics of the Wadge overburden aquifer were evaluated in Permit 79 -177 (Pages 779 -51 and 779- 52). Using an equation developed by Lohman (1972) for storage capacity under typical hydrostatic pressure conditions, the total storage capacity of the Wadge overburden aquifer over the entire area of the Twentymile Park ground water basin was calculated to be 708,000 acre -feet. 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 ground 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 gpd or 12.5 acre- feet/year. Most of this discharge would be via the Fish Creek alluvial deposits and would tend to be concentrated 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 gpd, 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. Groundwater recharge and discharge in the Wadge/Wolf Creek Interburden and Wolf Creek Seam can be estimated in a manner similar to that described above for the Wadge Overburden flow net analysis. It is reasonable to assume that the aquifer thickness and hydraulic gradient for the Wadge/Wolf Creek Interburden and Wolf Creek Seam are equivalent to those of the Wadge Overburden. The average hydraulic conductivity of the Wadge /Wolf Creek Interburden and Wolf Creek Seam is 0.012 gpd /sq. ft (0.0016 ft/day). Calculation using these values yields an average recharge rate of 450 gpd (0.0007 cfs). If dynamic equilibrium between recharge and discharge is assumed, the discharge rate would also be 450 gpd (0.0007 cfs). 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 ft/day was assumed to be representative of the aquifer. The average thickness of the unit in this part of the Twentymile Park Basin is about 120 feet. Using these values yields a flow rate through the aquifer of about 300,000 gpd (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 TR13 -83 2.04 -27 11/03/14