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<br />of some producing and plugged and abandoned oil
<br />wells. Water also discharges from around the outside of
<br />casings (between the surface casing and borehole
<br />walls) of some water wells and plugged and abandoned
<br />oil wells, and from around a few plugged and aban-
<br />doned oil-well markers. Water that discharges from
<br />and around these wells generally flows onto the ground
<br />and into nearby washes where it infiltrates into the allu-
<br />vial deposits. Some of this water likely infiltrates
<br />through the alluvium and ifllo the underlying Morrison
<br />aquifer. Locations of 56 selected wells in the Navajo
<br />aquifer and other wells, springs, and sites in the study
<br />area are shown in figure 10. Data for springs and
<br />selected wells completed in the Navajo, alluvial,
<br />Dakota, Morrison, and upper Paleozoic aquifers are
<br />presented in table \.
<br />Estimates and measurements of discharge from
<br />individual flowing wells ranged from less than 1 to as
<br />much as 150 gal/min during the study (table 1). Gener-
<br />ally, discharge did not vary for an individual well and
<br />observed variations in discharge from some wells were
<br />the result of controlled free-flow. Differences in dis-
<br />charge from well to well are attributed to differences in
<br />well depth, perforated interval, hydraulic head, and
<br />hydraulic conductivity of the aquifer in the yielding
<br />formation. On the basis of measured and estimated dis-
<br />charge of 30 free-flowing and "leaking" wells, waler
<br />discharged from the Navajo aquifer at the rate of about
<br />375 gal/min (600 acre-ft/yr) in 1992 and 1993. Com-
<br />bined discharge from wells N35 and N46 was almost
<br />100 gal/min and was sufficient to enable water to flow
<br />on the surface directly to the San Juan River. Prior to
<br />1989, when the study and estimates of discharge were
<br />begun, discharge from flowing wells was monitored
<br />only intermittently and discharge from individual wells
<br />may have been variable. In addition, flows have likely
<br />decreased during the time since the wells were con-
<br />structed, particularly in the early years (Lofgren, 1954,
<br />p. 115). Beginning in 1992, selected flowing wells in
<br />the Greater Aneth Oil Field and vicinity were plugged,
<br />reducing the volume of water being discharged by
<br />about 10 to 12 percent.
<br />The contributing formation yielding water to
<br />many flowing wells cannot be determined accurately,
<br />particularly for wells that were not drilled for water
<br />withdrawal purposes or that discharge water by ave-
<br />nues other than through the wellbore. Discussions with
<br />oil-field personnel, "Report of Water Encountered Dur-
<br />ing Drilling" (Utah Division of Water Rights, unpub.
<br />data), and information derived from oil-well logs indi-
<br />cate that water under artesian pressure probably dis-
<br />
<br />18
<br />
<br />charges from stratigraphic units below the Salt Wash
<br />Member of the Morrison Formation (fig. 3) in most
<br />wells. Many of these wells were perforated in multiple
<br />intervals and may yield water from one or all of the for-
<br />mations of the Navajo aquifer. Some wells that yield
<br />water from the upper part of the Navajo aquifer
<br />(Entrada Sandstone) may also yield water from the
<br />overlying Bluff Sandstone Member of the Morrison
<br />Formation. In wells where stratigraphic units below the
<br />base of the Navajo aquifer are perforated or otherwise
<br />open to the wellbore, water also might originate from
<br />other aquifers (James Walker, Navajo Nation Environ-
<br />mental Protection Agency, written commun., 1995).
<br />Water from most Texaco supply wells discharges from
<br />all three sandstones (Entrada, Navajo, and Wingate) of
<br />the Navajo aquifer (Larry Schlotterback, Texaco
<br />Exploration and Production, Inc., written commun.,
<br />1994). Relative percentages of water from individual
<br />formations in the wells, however, generally cannot be
<br />determined.
<br />
<br />Water.Level Declines
<br />
<br />Water levels (hydraulic heads) have declined in
<br />many wells in the Aneth area since the early 1900s.
<br />Lofgren (1954, p. 117) noted that water levels in Bluff
<br />city municipal wells had declined from more than 150
<br />fl above land surface in 1909 to only 80 ft by 1954.
<br />Barnes (unpub. data, 1959, p. 15) also stated that the
<br />altitude of the potentiometric surface in the Navajo
<br />aquifer in T. 40 S., R. 24 E. was 4,830 ft, a minimum of
<br />100 ft higher than rhe altitude of the potentiometric sur-
<br />face determined from water-level measurements in
<br />Navajo aquifer wells in this area during the study.
<br />Water-level declines can be caused by pumping or
<br />long-term discharge from wells that flow freely at land
<br />surface. Water level in a well in T 41 S., R. 24 E., Sec.
<br />18 declined from 200 ft below land surface in 1956, 6
<br />days after the well was drilled, to 378 ft below land sur-
<br />face in 1994, a decline of 178 ft during 38 years.
<br />Because this well had not been pumped for at least 15
<br />years, the decline may have taken place in less than 20
<br />years; however, the apparent decline also might have
<br />been a result of effects from other wells in the vicinity.
<br />Water level in well N6 declined from 113 ft below land
<br />surface in 1963 when the well was drilled, to 205 ft
<br />below land surface in 1981, to 217.5 ft below land sur-
<br />face in 1993, a decline of more than 104 ft during 30
<br />years (table I). This well has been in service since 1963
<br />and no other wells are present in Ihis area that would
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