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'W U\. eJt w Member of the Morrison Formation is the confining unit between the "D" and "M" aquifers. The Entrada Sandstone, sandstone members of the Morrison Fonna- tion, and the Dakota Sandstone are included in the D multiple-aquifer system of Cooley and others (1969). Whitfield and others (1983) considered the Bluff Sand- stone to be a separate formation and part of the Meso- zoic Sandstone aquifer. Because the Dakota and Morrison Fonnations crop out in the vicinity of the Greater Aneth Oil Field, the aquifers in these units are generally unconfined (water-table conditions). In the western part of the study area, where the Bluff Sand- stone Member crops out, the aquifer is unconfined, but where the unit is overlain by other shaly members of the Morrison Formation, the aquifer may be confined (arte- sian conditions). Recharge to the unconfined aquifers, including alluvial deposits; the Dakota Sandstone; and the West- water Canyon, Recapture, Salt Wash, and Bluff Sand- stone Members of the Morrison Formation, takes place primarily by direct infiltration of precipitation on out- crop areas and by infiltration of surface water along washes, such as Montezuma Creek. During 1982-83, Avery (1986, table 3, p. 70) measured a discharge of 3.1 ft3/s in Montezuma Creek in T 36 S., R. 24 E., Sec. 35, just north of the study area, but no flow was present in the channel about 6 mi downstream. Discharge from rhe Morrison aquifer is from springs, outflow to streams, evapotranspiration, and from wells. Evapo- transpiration takes place where phreatophytes are present and the water table is relatively close to the sur- face, particularly along washes. On the basis of base- flow data from 1982 to 1983, Avery (1986, p. 40) esti- mated that McElmo Creek gained 1,500 acre-ftlyr (2.1 ft3/s) from the Morrison aquifer between the Colo- rado/Utah State line and the mouth of the creek at the San Juan River. Regional ground-water movement in the Morrison aquifer, below the Brushy Basin Member, is toward the San Juan River. Springs that discharge from Morrison Formarion sandstones where they crop out along the San Juan River valley also indicare that the San Juan River is the primary discharge area for the unconfined Morrison aquifer. Thickness of the Morri- son aquifer is as much as 400 ft in some areas near the Utah/Arizona State line. The Entrada, Navajo, and Wingate Sandstones, along with the intervening semiconfining units, the Carmel Formation and the Kayenta Formation, are included in the Entrada-Navajo aquifer of Thomas (1989) and Howells (1990). Taylor and others (1986) included these formations along with the overlying 14 sandstone members of the Morrison Formation in the middle Mesozoic aquifers (fig. 3). Whitfield and others (1983) included thcse formations along with the Morri- son Formation and the Dakota Sandstone a5 part of the Mesozoic Sandstone aquifer. The Entrada-Navajo aquifer correlates with the "N" aquifer of Avery (1986). The Wanakah Formation is the confining unit between the "M" and "N" aquifers (fig. 3). Cooley and others (1969) defined the N multiple-aquifer system to include the Navajo Sandstone, sandy lithologies of the Kayenta Fonnation, and the Wingate Sandstone. In this report, the Entrada-Navajo aquifer is referred to as the Navajo aquifer. Barnes (unpub. data, 1959, p. 16) also used the term Navajo aquifer to include the Entrada, Navajo, and Wingate Sandstones. The Navajo aquifer is the principal aquifer from which water is withdrawn in the study area both north and south of the San Juan River and is discussed in more detail in the following sec- tions. The "P" aquifer of Avery (1986) includes perme- able units in the Cutler Fonnation, particularly the Cedar Mesa Sandstone Member. The Chinle and Moenkopi Fonnations are the thick confining units between the "N" and "P" aquifers (fig. 3). Whitfield and others (1983) defined the Cutler Formation as the Cutler aquifer. Howells (1990) also included the Rico Formation and part of the Honaker Trail Formation as part of the Cutler aquifer. The "C" aquifer of Avery (1986) is the saturated part of the DeChelly Sandstone Member of the Cutler Formarion and is equivalent to the DeChelly aquifer of Howells (1990). The C multi- ple-aquifer system of Cooley and others (1969) includes the White Rim and DeChelly Sandstone Mem- bers, and the overlying Shinarump Member of the Chinle Formation. In this report, the permeable sand- stones of the Cutler Formation, particularly the DeCh- elly Sandstone, are included in the upper Paleozoic aquifer, partly following the usage of Taylor and others (1986) (fig. 3), along with the lower Mesozoic Shi- narump Member of the Chinle Formation. The Navajo Aquifer The physical boundaries of the Navajo aquifer correspond to the physical limits of the Entrada, Navajo, and Wingate Sandstones in mosr of the study area because the formations that contain the aquifers are generally fully saturated where overlain by the Morrison Formation. Where these formations crop out in the southwestern part of the study area or are at shal- low depths beneath the land surface, the formations are