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The request for Major Permit Modification No. 9 included three elements: UIC Permit Modification Request that provided suggested language of the various sections to be modified, including a new section relating to the Temporary Suspension Of Mining Operations; Revised Subsidence Plan, dated August 2004, to replace the original Subsidence Plan and include modification of the requirements for Time Domain Reflectometry (T'DR); and Revised Ground Water, Surface Water, and Process Monitoring Plan dated September 2004, to include a reduction in the number of monitoring parameters. The EPA has reviewed the above mentioned material and is proposing to modify the Permit as described in this Draft addendum to the Statement of Basis. The following three sections I, II and III provide the rationale for modifications to the Permit. These changes include the replacement of Revised Ground Water, Surface Water, and Process Monitoring Plan (Appendix I) and the Subsidence Monitoring Plan (Appendix J). Background information, original Permit language and proposed changes are discussed below. I. Subsidence Monitoring ; Backi?round and Proposed Modifications At the time the UIC permit was issued, American Soda developed a subsidence monitoring plan with input from the BLM, EPA, and CDMG. The subsidence monitoring plan was incorporated into the EPA UIC permit as Appendix J. Several original monitoring measures have not provided the intended information and benefits, and because those measures aze ineffective and costly American Soda has requested modification of those elements. The primary elements of concern aze related to Time Domain Reflectometry (TDR) monitoring and gamma logging at certain production levels from each cavity. a) Time Domain Reflectometry Monitoring: The existing subsidence monitoring plan calls for the installation of TDR cables in the first 23 of 26 wells in the initial mining panel. The subsidence monitoring plan acknowledged uncertainty about the performance of TDR systems in this application and anticipated evaluating its performance before a final monitoring approach was selected for future mining azeas. To date, TDR systems have been installed in all of the 27 existing wells. The TDR systems have failed in every well in which they have been installed. Heat generated in the well bore during mining and water infiltration into the cables from surrounding aquifers are the primary reasons for the failures. At this point, TDR has been demonstrated to be technically infeasible in this application. Two options for modification aze considered: 1) Eliminate TDR monitoring; or 2) Install TDR after mining is complete as part of the Plugging and Abandonment procedure. This would minimize the potential for heat associated with active mining to affect the TDR system. American Soda previously proposed installing TDR in We1120-12 and testing the viability of the TDR technology in apost-mining application. Minor Modification No. 8 of the permit addressed the use of wel] 20-12 as a test installation for TDR. Consistent with the failure of the past installations and the ongoing test of TDR post-mining applicability, American Soda proposes to eliminate the requirement for installation of TDR in each well at the time of construction. Draft Addendum to SOB For Modification No. 9, UIC Area Permit No. CO30858-00000 Page 2 oj27