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t..,) flowing artesian wells in the study area varies substan- c.n tially and is not monitored. Measured discharge from CJl individual flowing wells during the study, however, C ranged from less than I to about 150 gallmin. Most water used for domestic and stock purposes outside of communities on the Navajo Indian Reserva- tion is withdrawn by wells powered by windmills (fig. 5). Windmills were constructed beginning in the 1940s to provide power to pump freshwater for families who lived far from communities where freshwater was avai lable. Depending on location and depth, wells equipped wirh windmills yield water from the Dakota Sandstone, the Morrison Formation (including the Bluff Sandstone Member), and the Entrada, Navajo, and Wingate Sandstones. Generally, windmills pump less than 5 gallmin of water and operate intermittently. The volume of water pumped by windmills from bed- rock aquifers is not monitored and thus is unknown (Michael Johnson, Navajo Water Resources Manage- ment Department, oral commun., 1994). Although most water pumped by windmills is fresh (dissolved- solids concentration less than 1,000 mglL), the water is not approved by Navajo regulatory agencies for drink- ing-water use; hence, families in outlying areas must haul water from approved wells. An additional source of freshwater for outlying areas is derived from wells equipped with hand pumps (fig. 6). These wells gener- ally are located in washes and yield warer from alluvial deposits at shallow depths. The volume of water yielded by hand pumps is also unknown but is rela- tively small compared with yields from other sources in the study area. Drinking-water supplies for the communities of Bluff, Montezuma Creek, and Aneth are obtained from wells. Water from public-supply wells in Bluff is derived from the Wingate and Navajo Sandstones. In 198 I, pumpage from municipal wells in Bluff was about 34.1 acre-ft (Hooper and Schwarting, 1982, p. 55). Water from three public-supply wells in Monte- zuma Creek is pumped from the Bluff and Entrada Sandstones. Average usage from these three wells for the period January 1990 to January 1995 was about 94 acre-ftlYL Drinking water for Aneth is pumped from two deep wells completed in the Bluff Sandstone about 6 mi north of Aneth and then piped to the community for distribution. Average usage from both wells from January 1990 to January 1995 was about 82 acre-ftlYL In addition, water from two wells completed in the Navajo Sandstone between the communities of Bluff and Montezuma Creek is used in natural gas operations by Elkhom Operating Company. During the 1991-92 and 1994 calendar years, withdrawal from these wells averaged about 138 acre-ftlyr (Gene Gruette, Elkhorn Operating Company, written commun., 1995). HYDROLOGIC SETTING The hydrologic system in the vicinity of the Greater Aneth Oil Field consists of Paleozoic and Mesozoic rocks that contain sandstone aquifers and interbedded shaly confining units (fig. 3). The San Juan River is the principal discharge area for the aquifers in rocks of Mesozoic age in the study area. The stratigra- phy and associated hydrologic units of San Juan County, including the Aneth area, have been described by Avery (1986) and Howells (1990). Surface Water The San Juan River is the principal base-level stream in the study area, into which all surface drainage flows either directly or indirectly (fig. I). Discharge of the San Juan River has been regulated at Navajo Reser- voir in northwestern New Mexico since 1962; hence, river discharge is substantially influenced by reservoir releases, which depend on precipitation in the drainage basin. Records of stream discharge from the U.S. Geo- logical Survey streamflow-gaging station at Mexican Hat, Utah, about 20 mi west of the study area, indicate that the mean daily discharge for the 1992-94 water years ranged from 528 ft3/s (August 1994) to 7,123 ft3/s (June 1993) (ReMillard and others, 1993, 1994, and 1995). Stream discharge also is influenced by inflow from McElmo Creek, snowmelt and thunderstorm run- off, withdrawals from the river for irrigation and indus- trial use, and losses from evapotranspiration. The only perennial tributary to the San Juan River in the study area is McElmo Creek (fig. I). McElmo Creek has its headwaters in Colorado but dis- charges into the San Juan River at Aneth. Records of stream discharge from the U.S. Geological Survey streamflow-gaging station at Aneth indicate that mean daily discharge for water year 1982, the last period of record, ranged from 23.4 ft3/s ro 211 ft3/s (ReMillard and others, 1983, p. 230). Discharge of McElmo Creek is influenced by irrigation return flows that contribute to peak flows in late summer (Avery, 1986, p. 17) that likely prevent McElmo Creek from being an ephemeral stream in its lower reaches. Most of the study area is drained by ephemeral streams. The largest ephemeral streams are Monte- 11