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<br />t.J-.) <br />UI <br />()l <br />>1.;.. <br /> <br />only partly saturated. The Navajo aquifer generally <br />ranges from 750 to 1,000 ft in thickness in the vicinity <br />of the Greater Aneth Oil Field, with the top of the aqui- <br />fer (top of Entrada Sandstone) averaging 550 ft below <br />land surface (fig. 3). <br /> <br />Recharge <br /> <br />The Navajo aquifer is recharged primarily by <br />infiltration of precipitation on and along the flanks of <br />the Abajo Mountains to the north of the study area (fig. <br />7), Sleeping Ute Mountain and possibly other moun- <br />tains to the east, in Colorado, and the Carrizo Moun- <br />tains to the south, in Arizona (Avery, 1986, p. 28; <br />Thomas, 1989, p. 37; and Howells, 1990, p. 28). <br />Because of the altitude, amount of precipitation, and <br />relation to the surrounding sedimentary rocks, these <br />mountains are likely the source of rechargc for most of <br />the aquifers in the study area and the headwater areas <br />for most tributary streams to the San Juan River. In <br />addition, because the igneous rocks at the core of these <br />mountains were intruded into the overlying sedimen- <br />rary rocks, strata along the flanks generally dip away <br />from the mountains and have been intensely fractured. <br />As a result, Cooley and others ( 1969, pAl) thought that <br />the shanered sedimentary rocks around the margins of <br />the Carrizo Mountains would enhance recharge to the <br />Navajo aquifer through interformational movement of <br />water. Extensive areas of the Wingate Sandstone crop <br />out along the southern part of the Carrizo Mountains <br />and also could provide a substantial source of recharge <br />to the Navajo aquifer (Strobell, 1956). In addition, <br />recharge to the Navajo aquifer may take place by poten- <br />tial upward movement of water through underlying for- <br />mations, as discussed in subsequent sections of this <br />rcport. <br />Most precipitation takes place above 8,000 ft in <br />the AbajoMountains and above 7,000 ft in the Sleeping <br />Ute and Carrizo Mountains. Recharge to the bedrock <br />aquifers from the mountains is estimated to be about 5 <br />to 15 percent of the mean annual precipiration for these <br />areas, compared with an estimated I to 3 percent of the <br />mean annual precipitation in the semi-arid plateau areas <br />below these altitudes (Thomas, ] 989, p. 37); thus, <br />ground-water recharge in these mountains is estimated <br />to range from 8,000 to 25,000 acre-ft/yr or I ] to 34 <br />ft3/s. <br /> <br />On the basis of relative recharge altitudes and <br />sources, projected hydrologic flow paths determined <br />from potentiometric contours, and isotope data pre- <br />senred in subsequent sections of this report, ground- <br /> <br />water movement in the Navajo aquifer was separated <br />into two distinct areas (fig. 8). Primary recharge areas <br />along the flanks of the Abajo Mountains and possibly <br />mountain ranges to the northeast in Colorado are high- <br />altitude sources (greater rhan ] 0,000 ft) and were des- <br />ignated as area]. Primary recharge areas along the <br />flanks of the Sleeping Ute and Carrizo Mountains are <br />relatively low-altitude sources (less than 10,000 ft) and <br />were designated as area 2. Wells completed in the <br />Navajo aquifer north of McElmo Creek and north of the <br />San Juan River downstream from its confluence with <br />McElmo Creek are included in area I. Wells completed <br />in the Navajo aquifer south and east of McElmo Creek <br />and south of the San Juan River are included in area 2. <br />Recharge to the unconfined parts of the Navajo <br />aquifertakes place only in the southwestern part of the <br />study area where the Entrada and Navajo Sandstones <br />crop out, particularly where they are covered by allu- <br />vial and eolian deposits. Recharge to the Navajo aqui- <br />fer also likely takes place over large outcrop areas in <br />upperChin1e Creek south and west of the study area, in <br />Arizona, and moves toward the San Juan River (Avery, <br />1986, p. 28). Although infiltration of precipitation <br />takes place throughout extensive outcrop areas at lower <br />altitudes, annual precipitation rates are very low, aver- <br />age temperatures are considerably warmer, and evapo- <br />transpiration rates are high; hence, recharge to the <br />ground-water system is small compared with recharge <br />that takes place in the mountain areas. <br /> <br />Discharge <br /> <br />Water levels in the Navajo aquifer indicate that <br />ground water moves from mountainous recharge areas <br />in the north, east, and south toward the study area and <br />discharges into the San Juan River (fig. 8). Ground- <br />water movement between recharge and discharge areas <br />appears to be controlled by geologic structure and <br />stratigraphy (Howells, 1990, p. 27) and probably takes <br />place along primary (intergranular, bedding plane, <br />cross-bed) and secondary (fracture) pathways. Ground <br />water probably moves down-dip toward the San Juan <br />River in much of the area because the river flows gen- <br />erally east to west across the Blanding Basin and the <br />strata dip toward the river valley from the north and <br />south. In addition, increasing hydraulic head with <br />depth (resulting in artesian conditions) and measured <br />gains in river discharge indicate that the San Juan River <br />is a discharge area for the Navajo aquifer. Water from <br />the Navajo aquifer moves upward into the Morrison <br />aquifer before discharging into rhe San Juan River <br /> <br />15 <br />