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It is not certain that the TDR system would be functional even in the more limited post-mining <br />application. Therefore, and to prevent a continual revision of permit conditions, American Soda will <br />develop a plan for future TDR installations only if the test proves that post-mining TDR monitoring is <br />viable. The original Subsidence Monitoring Plan (Appendix J, dated December 23, 1999) has been <br />revised to reflect this approach. EPA proposes to replace the original document with the revised August <br />2004 Subsidence Monitoring Plan as Appendix J. <br />b) Borehole Geophysics -Gamma Logs: Gamma logging is a common technique used to identify <br />geologic formations and the depth at which they occur. Gamma logs essentially read the radioactive <br />signature of the surrounding formation and record the depth at which it occurs. Because each formation <br />has a chazacteristic signature, gamma logging could potentially be used to determine if subsidence <br />occurs by comparing a baseline gamma log (e.g, conducted at the time of well construction) against later <br />gamma logs (during operations) to evaluate whether baseline formation depths have changed. <br />A gamma log is currently required as part of the geophysical logging completed at the time of <br />well drilling. The existing subsidence monitoring plan also requires a gamma log to be run at each well <br />in service after achieving individual well production levels of 85,000 tons, 150,000 tons, and after <br />retirement for the purpose of determining if subsurface subsidence has occurred. The subsidence <br />monitoring plan also requires a gamma log to be run if TDR monitoring indicates that subsidence of 6 <br />inches or more has occurred at the Mahogany Zone. <br />Gamma logs run to date in wells have provided limited value. The well bores are subject to <br />mineral and metals buildup (scaling) during the solution mining process. This scaling affects the ability <br />to run and interpret gamma logs, effectively masking the radioactive gamma signature of the formations. <br />The degree of interference from scaling is somewhat variable, but because the gamma log method does <br />not produce consistent data, it is not considered reliable as a means to identify or evaluate subsurface <br />subsidence. <br />Consideration has been given to continuing the gamma logging gamma logging and install <br />radioactive markers in the well bore (7-inch casing) pipe thread at certain locations in the well (e.g., near <br />the A-groove, B-groove, and crown pillaz). This option could only apply to new well construction and <br />would not resolve the issue with existing wells. This approach would improve the ability to consistently <br />identify the radioactive markers themselves but would not provide complete formation logging for <br />subsidence monitoring. If movement is indicated by this method, it could provide a relative indication <br />of where the movement occurred but would not provide precise depth or travel information for the <br />event. Further, if casing breaks occur as the result of thermal expansion and contraction, a shift in <br />mazker location would not necessarily be related to formation subsidence. This method does not provide <br />a direct measurement of or link to whether subsidence has occurred. Radioactive mazkers have also <br />been known to dislodge from a well bore and this does create some concerns regarding waste <br />management and liability. Additionally, there is a perception issue associated with the potential for one <br />of the mazkeus to come in contact with food-grade product. At this time, American Soda is not <br />proposing to utilize this technique in new well construction. <br />For these reasons, American Soda believes that the requirement for running a gamma log at <br />production levels of 85,000 tons and 150,000 tons should be eliminated. The gamma logs run at the <br />time of well construction and retirement can reasonably be accomplished since scaling is not an issue at <br />construction and is less of an issue after pulling the tubing at well retirement. If it appears that <br />subsidence has occurred, the Director would require additional studies to determine the extent of any <br />Draft Addendum to SOB For Modification No. 9, UIC Area Permit No. C030858-00000 Page 3 of17 <br />