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May 15, 2003 -' t/ ~ 4 °" Mr. Paul S. Osborne `~ ~/ ~ ~C C' -tv~q; ~ [G ~ 1 ~oz.a /'L~ a i'ce` rQ Ground Water Program U.S. EPA Region VIII, Mail Code 8P-W-GW ~~ ` ~'~ ~ 999 18`s Street, Suite 300 Denver, CO 80202-2466 /w-- is ~ ~ -/~~ Se ~4~ yh4 Subject: Status and Options for Subsidence Monitoring Wel12M-TDR f' Dear Mr. Osborne, As a result of recent 2M-TDR subsidence monitoring well (SMW) completion and discussions with the EPA, this letter presents the options available for continued subsidence monitoring of caverns SH and 6H following the completion of 2M-TDR. The status of 2M-TDR is that only one of the four TDR cables is operating correctly. The other three cables appear to have been damaged during the grouting process. All four cables were operating correctly immediately after installation and after the first grout lift (covered bottom 185') was placed. Subsequent grout placement by the tremie line required extensive (six) cement jobs over the B-Groove, and as a result of plugging, tremie line had to be removed and re- inserted. When we checked the cables after the completion (to surface) of the cement jobs, only the third cable or TDR-3 was undamaged. TDR-3 terminates or total depth (TD) at 1783 feet below ground surface or 18 feet below the R-6/ L-5 contact. Attached is 2M-TDR well diagram. The intent of 2M-TDR was to monitor ground movement below the R-6/ L-5 contact with TDR-1, and TDR-2 to provide early indication of movement 20 to 40 feet below the contact. TDR-3 at (TD) 18 feet below the contact would be used as the regulatory control for protection of the R-6. If damage was indicated in TDR-3 then mining operations would have to be modified by Natural Soda to limit growth of the caving above the cavern. TDR-3 can provide that input to the decision making process, but without TDR-1 and TDR-2 we will have no preliminary indicator's that the cave is developing. The following options are under consideration; 1. Monitor TDR-3 in 2M-TDR monthly, if damage is detected, notify EPABLM and CDMG, proposing alternate subsidence monitoring well and install within 3 months. 2. As above but start immediately to design an alternate subsidence monitoring well and install adjacent to 2M-TDR in the fall of 2003 in conjunction with drilling 7H, and be a substitute installation for SMW associated with 7H, until subsidence monitoring is shown to be effective and useful.