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N aftz and Spangler N .... CO C!..) known to be carcinog~nic and mutagenic (Dipple, 1976; Baumann et aI., 1982). In some cases, oil-field brines applied to road s~rfaces have resulted in chlo- ride concentrations in, shallow ground water that exceeded U.S. Environniental Protection Agency pub- lic drinking-water standards by two to five fold (Bair and Digel, 1990). Surfac~ disposal of oil-field produced brines in Texas has resl.tlted in the extinction or sig- nificant reduction of nt\merous fish species in a 20- kilometer stream reach ~Shipley, 1991). The saline source(s) leading to water-quality degra- dation of fresh ground-water systems is difficult to determine when more'than one potential source exists; however, this type of information is needed by regulatory agencies to d4velop proper plans to prevent future or continued grouhd-water-quality degradation or to remediate existing degradation. An excellent summary of geochemical techniques useful in identi- fYing sources of salinity has been done by Richter and Kreitler (1991). As nonsaline ground water becomes more important for dom4stic supplies in oil-producing regions, the application (jf these techniques to specific study sites will become njore critical. Salinity increases in Iwater in some parts of the Navajo aquifer in south~astern Utah (Figure 1) have been documented in rec~nt years (Avery, 1986; Kim- ball, 1992; Spangler, 191)2). Examples of increasing conoentrations of dissolved solids in water from selected wells in the study area are shown in Fig- ure 2. Much oil is produqed in this part of Utah, and brines associated with oil' production and the injection of the brines have been; suggested as the potential source of the saline water (Kimball, 1992). This area is in the Paradox Basin! where naturally occurring brines contain up to 4QO,000 milligrams per liter (mg/L) of total dissolved; solids (Hanshaw and Hill, 1969) and have mixed with surface water in areas adjacent to the study site (Konikow and Bedinger, 1978' Rosenbauer et al., '1992). The objective of this , . paper is to use halogen ratios and stable isotope data to determine if OFBs are causing the increased salini- ty of ground water in the Navajo aquifer. STUDY AREA DESCRIPTION The study area includ'es about 800 square miles in the southeast corner of Utah (Figure 1). The San Juan River and its largest tributary, McElmo Creek, are the only perennial streams. The Greater Aneth Oil Field covers about 125 square miles in the central part of the study :area (Figure 1) and is sur- rounded by smaller oil arid gas fields. Hydrocarbons are found primarily in carbonate units of the Paradox Formation (Hermosa Group) of Pennsylvanian-age at WATER RESOURCES BULLETIN J an average depth of 5,500 feet below land surface (Clem and Brown, 1984). --, Hydrogeologic Setting The hydrogeolpgy of the study area and adjacent areas has been investigated by Lofgren (1954), Goode (1958), Cooley et al. (1969), Sumsion (1975), Whitfield et aI. (1983), Avery (1986), Weiss (1987), Thomas (1989), and Howells (1990). The hydrogeologic units in the vicinity of the Greater Aneth Oil Field consist of a sequence of Paleozoic and Mesozoic aquifers and interbedded. confining units (Thomas, 1989; Howells, 1990). The approximate depths below land surface of the aquifers and confining units in the vicini~y of the Greater Aneth Oil Field are summarized in Figure 3. The dissolved-solids' concentration in water from each of the aquifers and confining units is summa- rized iriKimball (1992). Median dissolved-solids con- centrations are less than 1,000 mg/L except for the Middle Paleozoic aquifer and Upper Paleozoic confin- ing units, which have median dissolved-solidsconcen- trations exceeding 100,000 mg/L. Howells. (1990) mapped the depth to the base of moderately saline ground waterin the study area. Water with nissolved- solids concentrations larger than 10,000 mg/L may be less than 500 feet below land surface in areas in the Greater Aneth Oil Field. The Middle Mesozoic aquifer is partially in this zone of moderately saline water described by Howells (1990). The Wingate, Navajo, and Entrada Sandstones. of Triassic- to Jurassic-age, along with intervening semi- confining units,are included in the Middle Mesozoic aquifer (Kimball, 1992), The Middle Mesozoic aquifer is the principal aquifer in the study area, generally ranging from 750 to 1,250 feet in thickness. J,'his aquifer correlates with the N aquifer of Cooley et al. (1969) and Avery (1986). In this report, the Middle Mesozoic aquifer will be referred to as the Navajo aquifer. Total depths of wells completed in the Navajo aquifer range from 460 to about 1,500 feet (Table 1). Generally, this aquifer is fully saturated in the study area. Most wells finished in the aquifer flow because the potentiometric surface is above land surface. Dis- solved-solids concentrations in water from the Navajo aquifer ranged from 150 to 17,800 mg/L (Kimball, 1992; Spangler, 1992). Dissolved-solids concentrations generally increase along the ground-water flow path (Kimball, 1992), and increases in dissolved solids at the same well over time have also been documented (Avery, 1986; Spangler, 1992). Potentiometric water levels in the Navajo aquifer indicate that ground water moves toward the San Juan River from highland recharge areas located north, south, and east ofthe study area (Figure 3). 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