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tv CJl OJ r...o ences between hydraulic head in the upper Paleozoic and Navajo aquifers for these and several other wells shown in figure 8 are consistent with the head relations determined by Freethey and Cordy (1991); however, hydraulic-head data for some wells in the upper Paleo- zoic aquifer were determined from historical drill-stem test data and compared with water-level altitudes in the Navajo aquifer that were determined during this study. A contemporary comparison of hydraulic-head data from both the upper Paleozoic and Navajo aquifers for these wells would help verify and refine the head rela- tions between these aquifers. Water-level declines in the Navajo aquifer increase the potential for upward movement of water from underlying aquifers, such as the upper Paleozoic aquifer, because the hydraulic gradient between the Navajo and the upper Paleozoic aquifer is substantially increased. In areas where the calculated differences between hydraulic head in the upper Paleozoic and Navajo aquifers are less than 100 ft, as in the Boundary Butte Oil Field area, water-level declines could poten- tially reverse the hydraulic gradient and create a poten- tial for upward movement of water. Implications for upward movement of water from the upper Palcozoic aquifer to the Navajo aquifer are discussed in subse- quent sections of this report. Potential Avenues of Ground.Water Movement Because water in the upper Paleozoic aquifer is geochemically similar to water from many wells in the Navajo aquifer (discussed in subsequent sections of this report), and hydraulic head in the upper Paleozoic aqui- fer is higher than hydraulic head in the Navajo aquifer in a part of the study area (fig. 8), saline water from the upper Paleozoic aquifer potentially could have moved upward into the Navajo aquifer and caused salinization. Possible avenues for upward movement of salinc water from the upper Paleozoic aquifer to the Navajo aquifer include (I) migration upward through fractures and matrix porosity in the overlying confining units of the Chinle and Moenkopi Formations; (2) migration upward through the long string casing of plugged and abandoned wells; (3) migration upward through annuli between the well casing and borehole walls of produc- ing oil wells and injection wells, and plugged and aban- doned wells; or (4) a combination of these avenues. Fractures enhance vertical movement of water between aquifers, particularly in confining units where hydraulic-conductivity values are small; but fractures likely are not vertically continuous in thick sedimentary sections or open at depth because of overburden pres- sure and secondary cementation. The Chinle and Moenkopi Formations between the Cutler Formation (upper Paleozoic aquifer) and Navajo aquifer (fig. 3) are about 1,200 ft thick in the vicinity of the Greater Aneth Oil Field and are estimated to have an average vertical hydraulic-conductivity value of only about 0.0011 fUd (Jobin, 1962, pI. I). Assuming an effective matrix porosity of about 15 percent for shales and inter- mixed siltstones and sandstones (Freeze and Cherry, 1979, p. 37) and a hydraulic-head difference of about 450 ft, on the basis of formation pressure data from the Mobi118-43B well and from potentiometric-contour data in this area (fig. 8), an average linear velocity of about 0.0027 fUd is obtaincd (Freethey and others, 1994, p. 7). Thus, about 1,200 years would be required for water to move from the Cutler Formation up to the base of the Navajo aquifer (Wingate Sandstone) and mix with water in this formation. If open fractures are locally present to enhance upward movement of water, travel time could be substantially less. Because the hydraulic-head value used in this example is probably larger than average when compared with other head data for wells in this area, average linear velocity would be less and travel time would be considerably greater. Nonetheless, anomalously high levels of salinity docu- mented in water from some wells (N 17 and N35) prior to discovery and development of the Greater Aneth Oil Field indicate natural pathways for the movement of saline water into the Navajo aquifer or an in situ source of salinity (discussed in subsequent section of report). Borehole casings of plugged and abandoned oil wells and dry holes are a potential avenue through which water could move from the upper Paleozoic aquifer to the Navajo aquifer in some areas. Casing corrosion from saline water in the Cutler Formation has been documented in modem oil-field operations, but upward movement of water from these strata through the wellbore is generally inhibited by the weighted mud used in completion of the well (Larry Schlotterback, Texaco Exploration and Production, Inc., written com- mun., 1994). The Navajo aquifer may have been more susceptible to salinization in older oil wells and explor- atory drill holes that were not plugged and abandoned according to current standards and procedures, particu- larly those plugged and abandoned prior to discovery of the Aneth Field; those in which the integrity of the cement and mud used to plug the wells has been com- promised throughout time; and those in which surface casing was set to a depth insufficient to provide protec- tion of all freshwater zones. In these wells, saline water 29