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w (,.Il ~ o affect water levels; thus, the observed decline probably is the result of withdrawals from only this well. The prominent closed depression in the potentio- metric surface near Montezuma Creek may be the result of pumping from one or more wells in this area (fig. 8). For example, well Nl 0 is capable of discharg- ing an estimated 48 gal/min, and the altitude of the potentiometric surface at this well is about 50 ft lower than the altitude of the potentiometric surface at wells N8 and N9 immediately to the west. Pumping from well N54, at the center of the area of drawdown, also might contribute to rhe lowering of the potentiometric surface in ihis area. The potentiometric low could have been affected by pumping of municipal wells in Monte- zuma Creek that withdraw water from the Bluff and Entrada Sandstones, or by historic pumping of wells for industrial use south of Montezuma Creek (Avery, 1986, p. 32). Water-level declines around other flowing wells in the Aneth area also have likely raken place, but low- ering of the potentiometric surface may not be obvious because of the large distance berween wells (fig. 8). In addition, inaccuracies in the potentiometric surface are inherent because the water-level altitudes determined from hydraulic-head data are from wells of different depths in the Navajo aquifer, and hydraulic head is vari- able with depth. Discharge from wells also has declined with decreasing water levels. Well N I 0 had a water level of about 74 ft above land surface and discharged 87 gal/min in 1983, but the water level had dropped to about 42 ft above land surface and discharge decreased to 48 gal/min when measured in 1992-94 (table I). Ini- tially, new flowing wells in the Montezuma Creek area yielded as much as 500 gal/min and had water levels exceeding 300 ft above land surface; within a month, however, these wells generally discharged less than 100 gal/min and had considerably lower water levels, indi- cating low hydraulic conductivity (low permeability) of the aquifer (Lofgren, 1954, p. 115). Water withdrawals for domestic, irrigation, and industrial use in the Aneth area are probably not the cause of the observed water-level declines in some areas because most water withdrawn for these purposes is derived from shallower aquifers (such as the Bluff Sandstone) that overlie the Navajo aquifer and are sep- arated from the Navajo by confining units. As previ- ously noted, however, many flowing wells in the Aneth area have discharged water from the Navajo aquifer intermittently since the late I 950s and early 1 960s. If these wells are assumed to have been flowing unabated for only half of the time since they were drilled (an average of 15 years), at a rate similar to that measured or estimated during this study (1992-93), about 300 acre-ftlyr, or as much as 4,500 acre-ft, could have been discharged from the Navajo aquifer. Withdrawals from the Navajo aquifer from pumped wells, particularly N8, N9, and N21, are an additional several hundred acre- fllyr, and water-level declines would be expected. Potential for Vertical Movement The potential for upward movement of water from the Navajo aquifer into the overlying Morrison aquifer is a function of the difference in water levels or hydraulic heads between the two aquifers (Thomas, 1989, p. 20). Where hydraulic head in the Navajo aqui- fer is higher than hydraulic head in the Morrison aqui- fer, water from the Navajo aquifer may move upward into the Morrison aquifer. Hydraulic head in the Navajo aquifer is higher than that in overlying aquifers in an area about 10 to 15 mi wide on either side of the San Juan River (Avery, 1986, fig. 14, p. 27). Farther from the river, outside of this area, the hydraulic gradi- ent is downward from the Morrison aquifer to the NavajO aquifer. Depth to the Navajo aquifer increases away from the river, and altitude also increases away from the river; thus, flowing artesian wells are gener- ally present only at the lower altitudes, within about 5 mi north and south of the San Juan River and along Montezuma Creek, where the potentiometric surface is above land surface. The high altitude of the Navajo aquifer in the Abajo Mountains, however, results in flowing wells only 15 mi south of the recharge area or about 30 mi north of the river (Avery, 1986, p. 28). Hydraulic head in the Navajo aquifer is highest near the San Juan River. Water levels determined from pressure heads measured in wells N33 and N34 along the floodplain ofthe river near Aneth average about 245 ft above land surface. Vertical hydraulic gradients in the Navajo aquifer itself also can be present because of the thickness of the aquifer and the difference in hori- zonral and vertical hydraulic conductivity in the aqui- fer. Well N2 is reported to yield water from a depth of about 1,335 ft, with a water level during the study of about I lOft above land surface, but the adjacent flow- ing well (N5) that was drilled to a depth of 600 ft and is open only to the Entrada Sandstone had a water level of about 33 ft above land surface. These intra-aquifer ver- tical head differences provide the potential to move water upward from the Wingate into the Navajo Sand- stone and from the Navajo into the Entrada Sandstone. 21