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<br />including rock mechanics studies and thermomechanical modeling studies, to evaluate potential <br />subsidence and to determine the optimum cavity size, cavity spacing, and mining temperatures for <br />the project. The results of the modeling show that subsidence is unlikely but maybe up to 3 feet <br />at the surface after a significant (geologic) period of time. No subsidence is expected during the <br />solution mining period of approximately 30 yeazs. <br />The EPA has closely reviewed the Subsidence Monitoring Plan and believes that the <br />monitoring is adequate to detect subsurface movement prior to any surface manifestation of <br />movement. The subsurface monitoring plan includes the installation of coaxial cable in solution <br />mining well bores for use of Time Domain Reflectometry (TDR), which is technology used to <br />detect differential movement close to the top of the cavity and above. If subsurface movement <br />occurs, the coaxial cable will break or stretch. The location of the change in coaxial cable <br />properties will be determined by use of cable tester that was developed by the underground cable <br />industry to locate breaks in buried cables. When the cable is cemented into a borehole a baseline <br />signal log will be obtained for the cable installed in each well. The TDR cable will be be <br />monitored on a regular basis using the cable tester, and the results wilt be compared to the <br />baseline log. This testing will allow small changes in the wellbore to be detected.. Borehole <br />geophysical logs will also be run periodically to substantiate the TDR monitoring results, and <br />corrective action implemented if necessary. The shape of the cavity is, of course, an important <br />element that affects subsidence. For that reason, the operator will be required to undertake <br />studies during the initial year to evaluate techniques for assessing cavity shape. Depending on the <br />results of these studies, American Soda may be required to establish additional monitoring wells <br />or institute other techniques to better track cavity influence on the pillazs. EPA concurs with the <br />CDMG decision to requ've that subsidence monuments be established on the first 24 wells to <br />assess the effectiveness of this form of subsidence monitoring. <br />The Permit has been modified to include surface monument monitoring. EPA will <br />require American Soda to submit all subsidence monitoring data in their fourth quarter <br />reports. In order to establish a clear link between the UIC Permit and the activities listed <br />in the Subsidence Monitoring Plan (dated December 23, 1999), EPA has modified the <br />Permit by adopting the Plan as part of the Final Permit (Appendiz d). This modification <br />provides for changes to the Plan based on the results of the review of data collected at the <br />site and the studies to be carried out regarding technologies for determining the shape of <br />the cavity. <br />Comment 7: The applicants mine plan and EPA's result nP discussions in the Statement of Basis <br />on cavity growth and potential im~cts are inadequate. <br />The following seven (7) pazaphrased comments discussed issues and concerns relating to <br />the adequacy of the mining plan as it relates to cavity growth and potential impacts of the mining <br />process: <br />1. The description of cavity development in the EPA record (The Mine Plan) is misleading and <br />does not allow the EPA or public to evaluate the potential impacts of the proposed project. <br />22 <br />