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r cavity. For that reason the Draft Permit established the maximum injection pressure (Section C. 3.) for the Yankee Gulch project at 700 pstg. Review of the information supplied with the application indicates that the operating pressures planned aze below that required to hydraulically fracture the saline zone strata. Hydraulic fracturing of the strata did not occur during the test mining of two cavities by American Soda and apparently did not occur, under similar pressures, during the Shell field test conducted in the eazly 1970s. Although minor fracturing induced by thermal stresses, or spalling, will occur in the immediate cavity wall, such fractures are desirable to allow injection fluids to come into contact with nahcolite enclosed in the oil shale matrix. Thermal stress, however, diminishes within a short distance of the cavity wall due to the relatively low thermal conductivity of the rock. Available information indicates that the cavities should be stable. Rock quality data from a saline zone core has indicated the strata to be of excellent quality and substantially better than the oil shale strata above the Dissolution Surface. Because of planar bedding in the saline zone materials, a flat roof was relatively easily developed during underground mining at previously operated oil shale mines. Therefore. it is anticipated that the cavity roof would be more or less level and possibly slightly concave upward in shape. Thermal stress-induced fracturing would generally cause spalling of the bedding planes in response to undercutting by dissolution beneath the gas cap. EPA agrees with commentors that subsidence is an important issue. With an extraction ratio as low as 12 percent, however, the Yankee Gulch Project is expected to have faz fewer subsidence effects than other room-and-pillar mining operations. The applicant has performed substantial site operational simulations, including rock mechanics studies and therntomechanical modeling studies, to evaluate potential subsidence and to determine the optimum cavity size, cavity spacing, and mining temperatures for the project. The results of the modeling show that subsidence is unlikely but may be up to 3 feet at the surface after a significant (orders of magnitude greater than the period of mining) period of time. No subsidence is expected during the solution mining period of approximately 30 years. The information obtained from the applicant covers three possible subsidence scenarios that were evaluated. Two of these scenarios were considered to be highly unlikely. All three analyses resulted in less than 1 foot of surface subsidence over several hundreds of years. All three scenarios had 200-foot-diameter cavities spaced on 300-foot centers. The first scenario, which included 30 feet of yielded (reduced strength, but not failed) rock azound the perimeter of each cavity, resulted in 0.54 foot of subsidence. The 30 feet of yielded rock was estimated from a thermomechanical cavity analysis. This first scenario was considered the most likely scenario of the three evaluated, although development of full-size cavities with diameters of 200 feet is not yet demonstrated and actual cavity size is likely to be less than 200 feet in diameter. The second scenario assumed complete failure of the 30 feet of perimeter rock although, based on the results of the thermomechanical study, this level of failure is not predicted to occur. This analysis resulted in 0.70 foot of subsidence. The third scenario, which is considered the least likely scenario, assumed the 30 feet of perimeter rock yielded (reduced strength, but not failed) and that a fault zone traversed the mining panel. The resulting subsidence under this scenario was 0.74 foot. EPA believes that effects of this small amount of subsidence, occurring over an extended time, would not be damaging to the aquifer systems overlying the solution mining zone. 25