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Proposed languase: Cavity development shall be monitored to estimate cavity size. Because the leached solution cavity will be a matrix of oil shale rather than a void, cavity size and shape must be determined by the following indirect methods. Material balances of the volumetric fluid flow rates into and out of the solution mining cavities shall be performed. Continuous monitoring of flow rates, temperature, and fluid densities will moniter ongoing material balance, which will allow the permittee to make calculations of the mass of nahcolite extracted. This information will be submitted quarterly to EPA. A combination of TDR (in test well 20-12) and borehole geophysical techniques shall be employed to evaluate subsurface movement and movement of strata overlying the solution cavities (see Section 3.0, Subsurface Monitoring, Subsidence Monitorine Plan. Appendix .n. The performance of the TDR monitoring will determine if TDR is incorporated into the long-range monitoring program. If TDR proves to not be an adequate means of monitoring subsurface movement, EPA may require other instrumentation techniques to be evaluated by the permittee, and if any subsidence changes are significant EPA will be notified as specified by Section III, E.10. TDR will be collected and reported quarterly for we1120-12. A subsidence report shall be submitted to the EPA annually. The report will contain data from surface monument measurements, TDR data, and borehole geophysical data. The annual subsidence report shall also include a map of surface monument locations, locations of wells where borehole geophysics were conducted, a list of monument and well coordinates, and a summary of pertinent observations during the reporting period. II. Infection Interval: BackEround and Proposed Modifications The UIC permit currently stipulates that the injection zone must be a minimum of 150 feet below the dissolution surface. The bottom of the injection zone is generally about 800 feet below the dissolution surface for a total injection zone thickness of about 650 feet. The requirement to have 150 feet between the dissolution surface and the injection zone is intended to isolate the lower aquifer from the mining injection since there was considered to be some potential for hydrologic connection between the dissolution aquifer and the overlying B-groove, A-groove, and Uinta aquifers. The lower aquifer generally does not qualify as a USDW within the mining azea, however, upper portions of the lower aquifer (upper LS and above) may constitute USDWs at other locations within the basin. The upper aquifer system including the overlying B-groove, A-groove, and Uinta aquifers are generally considered potential USDWs within the Piceance Basin. The R-6 zone below the B-groove has been shown to be an aquitazd, effectively isolating the B-groove from the dissolution surface aquifer at Natural Soda's adjacent sodium minerals lease. Evidence from American Soda's sodium minerals lease also supports that the R-6 zone is an effective aquitazd. Previous pressurization of the dissolution aquifer (increase of over 150 feet of head) did not result in pressurization at the B-groove. There aze essentially two components relating to isolation of mining fluid from the overlying aquifer systems. The first component is the mechanical integrity of the well bore and injection system. The second component is geological integrity of the receiving formation. While American Soda tests for mechanical integrity and can generally confirm the mechanical integrity of the injection system through operational data, it cannot control the geologic integrity of the mining zone. Natural fractures in the host rock have the potential to provide a pathway for mining fluid to reach the dissolution surface. Fractures may self-heal as a function of the mining process but there is a potential for mining fluid to Draft Addendum to SOB For Modification No. 9, UIC Area Permit No. CO30858-00000 Page 6 of17