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_, May 15, 2003 <br />Mr. Cazl B. Mount <br />NATURAL SODA, INC. '~ <br />3200 RBC 31 <br />Aifle, Colorado 81650 <br />Senior Environmental Protection Specialist <br />Division of Minerals and Geology / <br />Department of Natural Resources <br />1313 Sherman Street, Room 215 / <br />Denver, CO 80203 f <br />~/t11 g 83- ~ `~ <br />~ECE9VE® <br />P~IAY i 9 2003 <br />Givision ~l Minerals and Geology <br />Phone: (970) 878-3674 <br />Fax: (970) 878-5866 <br />Subject: Status and Options for Subsidence Monitoring We112M-TDR <br />Dear Mr. Mount, <br />As a result of recent 2M-TDR subsidence monitoring well (SMW) completion and discussions <br />with the EPA, this letter presents the options available for continued subsidence monitoring of <br />caverns SH and 6H following the completion of 2M-TDR. <br />The status of 2M-TDR is that only one of the four TDR cables is operating correctly. The other <br />three cables appear to have been damaged during the grouting process. All four cables were <br />operating correctly immediately after installation and after the first grout lift (covered bottom <br />185') was placed. Subsequent grout placement by the tremie line required extensive (six) cement <br />jobs over the B-Groove, and as a result of plugging, tremie line had to be removed and re- <br />inserted. When we checked the cables after the completion (to surface) of the cement jobs, only <br />the third cable or TDR-3 was undamaged. TDR-3 terminates or total depth (TD) at 1783 feet <br />below ground surface or 18 feet below the R-6/ L-5 wntact. Attached is 2M-TDR well diagram. <br />The intent of 2M-TDR was to monitor ground movement below the R-6/ L-5 contact with <br />TDR-1, and TDR-2 to provide early indication of movement 20 to 40 feet below the contact. <br />TDR-3 at (TD) 18 feet below the contact would be used as [he regulatory control for protection <br />of the R-6. If damage was indicated in TDR-3 then mining operations would have to be <br />modified by Natural Soda to limit growth of the caving above the cavern TDR-3 can provide <br />that input to the decision making process, but without TDR-1 and TDR-2 we will have no <br />preliminary indicators that the cave is developing. <br />The following options are under consideration; <br />1. Monitor TDR-3 in 2M-TDR monthly, if damage is detected, notify EPA/BLM and <br />CDMG, proposing alternate subsidence monitoring well and install within 3 months. <br />2. As above but start immediately to design an alternate subsidence monitoring well and <br />install adjacent to 2M-TDR in the fall of 2003 in conjunction with drilling 7H, and be a <br />substitute installation for SMW associated with 7H, until subsidence monitoring is <br />shown to be effective and useful. <br />3. It is possible that measurable subsidence would not be evident in 2M-TDR for 12 <br />months or so, If we do nothing at 2M-TDR now, and install an improved SMW at 7H <br />