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A subsidence report shall be submitted to the EPA annually. The report will contain data from surface <br />monument measurements, time domain reflectometry (TDR) data, and borehole geophysical data. The <br />annual subsidence report shall also include a map of surface monument locations, locations of wells <br />where TDR and borehole geophysics were conducted, a list of monument and well coordinates, and a <br />summary of pertinent observations during the reporting period. <br />Proposed language: <br />Cavity development shall be monitored to estimate cavity size. Because the leached solution cavity <br />will be a matrix of oil shale rather than a void, cavity size and shape must be determined by the <br />following indirect methods. Material balances of the volumetric fluid flow rates into and out of the <br />solution mining cavities shall be performed. Continuous monitoring of flow rates, temperature, <br />and fluid densities will moniter ongoing material balance, which will allow the permittee to make <br />calculations of the mass of nahcolite extracted. This information will be submitted quarterly to <br />EPA. A combination of TDR (in test we1120-12) and borehole geophysical techniques shall be <br />employed to evaluate subsurface movement and movement of strata overlying the solution cavities <br />(see Section 3.0, Subsurface Monitoring, Subsidence Monitoring Plan, Appendix .)7. The <br />performance of the TDR monitoring will determine if TDR is incorporated into the long-range <br />monitoring program. If TDR proves to not be an adequate means of monitoring subsurface <br />movement, EPA may require other instrumentation techniques to be evaluated by the permittee, <br />and if any subsidence changes are significant EPA will be notified as specified by Section III, E.10. <br />TDR will be collected and reported quarterly for we1120-12. <br />A subsidence report shall be submitted to the EPA annually. The report will contain data from <br />surface monument measurements, TDR data, and borehole geophysical data. The annual <br />subsidence report shall also include a map of surface monument locations, locations of wells where <br />borehole geophysics were conducted, a list of monument and well coordinates, and a summary of <br />pertinent observations during the reporting period. <br />II. Ini ection Interval: Background and Proposed Modifications <br />The UIC permit currently stipulates that the injection zone must be a minimum of 150 feet below <br />the dissolution surface. The bottom of the injection zone is generally about 800 feet below the <br />dissolution surface for a total injection zone thickness of about 650 feet. The requirement to have 150 <br />feet between the dissolution surface and the injection zone is intended to isolate the lower aquifer from <br />the mining injection since there was considered to be some potential for hydrologic connection between <br />the dissolution aquifer and the overlying B-groove, A-groove, and Uinta aquifers. <br />The lower aquifer generally does not qualify as a USDW within the mining area, however, upper <br />portions of the lower aquifer (upper LS and above) may constitute USDWs at other locations within the <br />basin. The upper aquifer system including the overlying B-groove, A-groove, and Uinta aquifers are <br />generally considered potential USDWs within the Piceance Basin. The R-6 zone below the B-groove has <br />been. shown to be an aquitard, effectively isolating the B-groove from the dissolution surface aquifer at <br />Natural Soda's adjacent sodium minerals lease. Evidence from American Soda's sodium minerals lease <br />FINAL Addendum to SOB For Modification No. 9, UIC Area Permit No. C030858-00000 Page 6 of 29 <br />