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EPA believes the operator's Subsidence Monitoring Plan which includes subsurface monitoring, would detect subsurface movement before there were significant subsidence effects. The subsurface monitoring plan includes the installation of coaxial cable in solution mining well bores for use of Time Domain Reflectometry (TDR), which is technology used to detect differential movement close to the top of the cavity and above. If subsurface movement occurred, the coaxial cable would break or stretch. The location of the change in coaxial cable properties would be determined by use of a cable tester. Borehole geophysical logs will also be run periodically to substantiate the TDR monitoring results, and corrective action could be implemented if necessary. The Permittee is also being required to place surface subsidence monuments to allow detection of any surface subsidence over the life of the project. These monuments will be monitored on a yeazly basis. Section 3.3 of the Subsidence Monitoring Plan, which has been incorporated into the Final Permit (See Appendix I), requires the operator to evaluate technologies to characterize solution cavity size and shape and present their findings to both the EPA and the BLM by June, 2000. EPA will utilize the information collected during the evaluation of cavity shape technologies to determine if the Subsidence Monitoring Plan needs modification. If necessary, EPA could require the drilling of pillar monitoring wells or some other measure, such as additional monitoring wells. As explained in this response, EPA does not believe that American Soda is being allowed to mine "blind" with inadequate regulatory and technical requirements. EPA believes that the Final Permit, as modified establishes adequate control over the project to assure protection of USDWs. In response to concerns, EPA has required modifications in both the Water Monitoring Plan and the Subsidence Monitoring Plan. These plans have been incorporated into the Final Permit. Comment 8: Has the site around water been affected by American Soda's on-site activities The following two pazaphrased comments concern the impact of the experimental test mine on the ground water. 1. Did any excursions of mining solution outside the targeted mining zone occur in the experimental cavities? 2. The experimental test mine operation has affected ground water. Response: EPA is not aware of any data that would indicate there has been excursions of mining solution outside of the mining zone in the test cavities. Claims and concerns that there may have been excursions have not been substantiated with monitoring results. American Soda has operated the Yankee Gulch Experimental Test Mme operation under the approval of several Permits, including a Class V UIC Rule Authorization, a Class I UIC Permit, and additional Federal, State and local Permits that requrre monitoring and reporting of numerous parameters that would indicate a loss of cavity integrity. EPA's Rule Authorization and Class I Permit required American Soda to report any failure and to shut-in any failed well. Monitoring of several 26