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3. TEXT CHANGES <br />The base of the lower aquifer is directly overlying the <br />saline mineral and oil shale intervals, which make up the <br />saline or high resistivity zone of the Parachute Creek Member <br />of the Green River Formation. The contact between the <br />lower aquifer and the Saline Zone is ~~Iled the dissolution <br />surface. Dissolution of the saline minerals by the lower <br />aquifer a an active natural geologic process. The rate and <br />extent of this process is not completely known and may <br />be quite variable, depending om permeability throughout <br />the basin. Basin wide, there may be as many as 20,000 <br />acres of dissolution surface which are un direM contact with <br />saline minerals. <br />The upper and lower aquifers alre separated by the <br />Mahogany Zone of the Parachute Creek Member of the <br />Green River Formation (Figure 3-I A). Regionally, the <br />Mahogany Zone is considered a semiconfining layer between <br />the two aquifers, which allows some communication <br />between the two aquifers through secondary porosity <br />developed in fractures in the Mahogany Zome. Because of <br />its high kerogen content, the rock is less brittle; therefore, <br />the Mahogany Zone a less susceptible to fracturing than <br />the strata containing the upper or lower aquifers. Im some <br />places in the basin the Mahogany Zone is not fractured <br />and allows very little communication between the aquifers. <br />In other areas substantial communication takes place (Weeks <br />et al. 1974; Robson and Saulnier 1988; Daub, Weston and <br />Rosar 1985; Wright Water Engineers and Daub & <br />Associates 1985; Weston 1984; Industrial Resources, Inc. <br />1984). <br />The two bedrock aquifers are generally confined or <br />artesian. This means that the hydraulic head (water level) <br />of a well in the water bearing zones will be higher them <br />the top of Ote aquifer. The relative degree of communication <br />between the aquifer systems cam be inferred by the difference <br />in the hydraulic head of the two aquifers. If there is a great <br />deal of communication between the two aquifers, one would <br />expect little difierence in the hydraulic head between the <br />aquifers. There are few places in the basin where the head <br />difference exceeds 200 feet; generally, the difference io head <br />does not exceed 1t7p feet (Weeks et al. 1974). <br />The aquifer systems are estimated to contain between <br />6.5 and 22 million acre-feet of water in storage at any one <br />ume (Robson and Saulnier 1981). Ttie saturated thickness <br />of the upper and lower aquifers range'. from 1,300 to 1,5110 <br />feet. The aquifers discharge approximttely 18,000 acre-fcet <br />per year to the surface water, directly through springs or <br />indirectly through discharge to [he aluvial aquifer. <br />3.4.2.1 Sire Specific <br />Hydrogeologic characteristics in the sodium lease area <br />vary slightly from the regional description based upon limited <br />site-specific dale. Alluvial aquifer material occurs on lease <br />in small sections of Stake Springs Draw and Yellow Creek. <br />Collectively, the areal extent of these areas is approximately <br />80 acres. A perched aquifer does exist in the lease area <br />and is approximately 400 feet below the surface. Water <br />yield within this zone is estimated to be about 10 gallons <br />per minute. <br />The hydraulic head difference between the upper and <br />lower aquifers ranges from zero to tens of feet within the <br />sodium leases. The difference in hydraulic head at the <br />proposed well field is less them 10 feet. Limited data indicates <br />more communication between the upper and lower aquifers <br />within the sodium leases than regionally with the hydraulic <br />gradient thought to be in the downward direction. <br />Approximately the lower half of the Mahogany Zone within <br />the lease area has dissolution features that now contain and <br />yield water. At least 1,000 acres of the dissolution surface <br />within the sodium leases is in contact with saline minerals. <br />This is a minimum amount because it only represents the <br />acreage of the truncation zone of the Boies Bed; it does <br />not measure the contact of the saline minerals of the Cr <br />SE Bed with the dissolution surface. <br />The aquifer pump tests conducted by the Multi Mineral <br />Corporation and the USGS in 1981 in the eastern portion <br />of the leases indicate that the direction of greatest <br />permeability of the upper aquifer is to the northeast and <br />the lower aquifer is to the northwest (Weston 1984). The <br />hydraulic gradient in the aquifers is down dip to the north. <br />Very little data are available that clearly describe the rate <br />of flow of the aquifer system within the sodium leases. <br />Preliminary indications, based upon the pump test, are that <br />the upper aquifer flows at least 150 feet per year and that <br />the lower aquifer flows at about 90 feel or more per year. <br />3.4.3 Grotutdwater Quality <br />3.4.3. / Regional <br />Groundwater quality in the Piceance Basin varies widely <br />both between and within the aquifers and by geographic <br />location. The alluvial aquifer is classified as a sodium <br />bicarbonate type, with concentrations of dissolved solids <br />ranging from 470 mg/I to a high of 6,720 mg/I. Average <br />IeveLs of dissolved solids in the alluvium are nearer to 1,750 <br />mg/1 over the entire basin (Weeks and Welder 1974). Higher <br />TDS levels occur downstream toward the White River; they <br />are attributed mainly to irrigation water returns, groundwater <br />inflow from bedrock aquifers, and the concentrating effect <br />of evapotranspiration (Weeks et al. 1974). <br />3-7 <br />