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r <br />their demonstration of compliance with 40 Ch'R 262.11 that the waste streams are not <br />hazardous as part of their reporting, and to incorporate these results into their official site <br />file. <br />Comment 11: The cgs can maybe ineffective for controlling Growth of the taut <br />There were two (2) pazaphrased comments relating to the use of a gas blanket in the <br />cavities. <br />1. Draft Area Permit, Page 17, "gas blanket will assist in controlling the vertical growth of the <br />cavity." Parts of the formation contain natural fractures. Those fractures could make it difficult <br />to maintain a gas blanket. What will be done if the blanket fails to give the intended protection? <br />How will excursions be detected and mitigated? <br />2. The gas cap will not be effective in controlling vertical growth of the solution mining cavity. <br />Response: Gas cap technology is widely used in solution mining of salts to control vertical <br />growth of cavities. American Soda successfully utilized a nitrogen gas cap in controlling the <br />vertical growth of test solution mining cavities during their experimental Class V operation since <br />July, 1997. The use of the gas cap technology has proved to be successful under different test <br />solution mining well designs. There were no water level changes in nearby wells monitoring the <br />Dissolution Surface that would indicate a failure of the cavity roof during operation. Additionally, <br />values for process monitoring parameters (injection and production pressure and flow rate) <br />indicate that the cavity roof is intact. <br />In regards to the oral testimony concerns that a gas cap only works in solution mining <br />applications where no insolubles remain in the cavity and that "ledges" of insolubles (such as oil <br />shale) at the cavity roof will trap the gas cap while the mining fluid will be able to continue <br />dissolving nahcolite beyond and above the gas cap, American Soda has indicated that the use of a <br />gas cap was decided upon as a means of preventing the upward growth of the cavern into the <br />halite beds which overlie the nahcalite deposits. American Soda's test project indicates the high <br />temperature and pressure of the injection fluid causes thermal mechanical stresses and spalling <br />with the result that any ail shale "ledge" in the solution mining cavity would become fractured and <br />rubblized as a result of solution mining. The test results indicate that the oil shale ledges should <br />become a permeable zone through which the nitrogen gas could travel and maintain its presence at <br />the top of the cavity. EPA also obtained information from the BLM's Record of Decision (ROD) <br />associated with the Yankee Gulch Project which provides additional details on the utilization of <br />the gas cap. This information was utilized in the evaluation of these comments. For instance, the <br />Shell Oil experiment performed near the Horse Draw Mine (Prrts et al. 1977) indicated that a <br />cylindrical cavity shape was achieved without use of a gas cap. This would seem to indicate that a <br />gas cap may be unnecessary to control vertical growth. <br />More importantly, the solubility of nahcolite istemperature-dependent and the thermal <br />conductivity of the cavity rock is low. Movement of "mining fluids" into zones which are not <br />dvectly connected to the main portion of the cavity, out of control of the gas cap and not in close <br />29 <br />