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Salinity Increases in the Navajo Aquifer in Southeastern Utah , use as water wells, primarily for livestock. Windmills pump water from t~e Navajo aquifer and from aquifers in the overlyi~g Morrison Formation and the Dakota Sandstone (Figure 3). In addition, handpumps supply water for outlYing areas from alluvial deposits. Water supplies for tlie communities of Montezuma Creek and Aneth (Fi{ure 1) are derived from wells completed in the Bluff1Sandstone Member of the Mor- rison Formation. Pres~ntly (1993), virtually all water used for oil drilling anll injection operations is derived from numerous shallow wells located in the alluvium along the San Juan ~iver and from brines produced as a by-product of oil-field operations. N ~ CD N History o(Oil-Field Production Substantial driIlin~ activities were begun in the study area during the;early 1950s (Clem and Brown, 1984). Producing zone~ in the Greater Aneth Oil Field and vicinity are prirn~rily in carbonate units of the Paradox Formation of' Pennsylvanian-age, at depths generally ranging froln 5',000 to 6,000 feet. Shortly after the major field <Uscoveries in 1956, water injec- tion was necessary to'maintain field pressures and increase productivit;y. ^ substantial quantity of ground water from th~ alluvium adjacent to the San Juan River has been used as makecup water, for sec- ondary recovery operations in the field. From 1984 to 1991, about 316 million barrels of water were injected back into the Paradox Formation, although yearly injection rates have de~lined substantially (Utah Divi- sion of Oil, Gas, and 14inipg, written communication, 1991). Only one lined ~urface pit in the vicinity of the Greater Aneth Oil FiE!ld currently is being used for salt-water disposal. DATA-BASE COMPILATION The chemical data )lsed in this study were com- piled from both USGS'and non-USGS sources over a period of almost 40 y~ars (Mayhew and Heylmun, 1965; Avery, 1986; Rosenbauer et al., 1992; !Gmball, 1992; and Spangler, 1992). Because the chemical data were not generated un~er uniform sampling and ana- lytical protocols, qultlity-assurance and quality- control information i~ not available to assess the adequacy of the sampl~ collection or chemical analy- ses. With these Iimitat~ons, the data from single wells must be interpreted wlth caution; however, the over- all trends in the data cqllected at multiple sites proba- bly are accurate representations of the ground-water chemistry. The compiled data are shown in Table,.l. Sample sites in the study area are shown in Figure 1. Because of the difficulty in obtaining OFB and non-OFB chem- ical analyses, water-quality data from the Paradox Formation from sites adjacent to the study,area were also included in the data base (Table 1). Off-site OFB data were compiled from Mayhew and'Heylmun (1965) from sites north of the study area near Moab, Utah, and non-OFB data were compiled from Rosen- bauer et al. (1992) from sites northeast of the study area near Paradox Valley, Colorado. Chemical con- stituents included in Table 1 are bromide, iodide, chloride, del oxygen-18 (S180), and del deuterium (SO). Mean concentration or value was used when a site was represented by multiple analyses over time. ",; ;; , SAMPLING AND ANALYTICAL METHODS " -:~ .,', -~ J ., J 'i 'J -I ':-t , ] .} .] ,f~ :1:; ,! '~ '1 " '.~ : '~- :;' .,~ "z. , ,.r. U I 'i i ,'~ '1 .:~ ~ ~ ~ ~ ~ !~ ,~ ,~ ~ " ~ & ~ - ~;; ;.t ~~ }2 "~ il " i$ ,~ - ~~ Sampling and analytical methods used for samples collected, by the U.S. Geological Survey during and prior to this study are described in the following text. Water samples from flowing wells were coltected and processed immediately. Water from wells that had to be pumped or those that had valves to control free flow were allowed to flow for 1-2 hours, or until teJ1l- perature and specific conductance stabiliied, befor,e' samples were taken. Surface-water samples collected from the San Juan River were composited in equal- width increments. Samples collected for bromide, iodide, and chloride were filtered on site through a OA5-micron filter using a peristaltic pUmp and stored in field-rinsed, 250-miIliliter (m!) polyethylene bot- ties. Samples for S180 and SO analysis were unfil- tered and collllcted in 125-ml brown (baked) glass bottles with poly seal caps. Isotope samples were pre- served with mercuric chloride tablets to inhibit growth of microorganisms. Bromide, iodide, and chloride concentrations were determined by ion colorimetry (Fishman and Fried- man, 1989) at the U.S. Geological Survey National Water Quality Laboratory in Arvada, Colorado; The S180 isotopic values were determined 1Jsing the method developed by'Epstein and Mayeda (1953)'. 'The SO isotopic values were determined by analyzing hydrogen quantitatively extracted from the water (Kendall and Coplen, 1985). The S180 and SO isotopic values were determined at U.S. Geological Survey, Water Resources Division laboratories in Reston Vir-' ," , ginia. The S180 and SO results are reportetl relative to Standard Mean Ocean Water (SMOW) in the per- mil notation. Analytical precision is :t 0.1 perroil for S180 and :t 1.0 permil for SO values. 1123 WATER RESOURCES BULLETIN _ _'0," '_ /.