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<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
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