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Ms. Lori Potter November 3, 1999 Page 3 Section 2.4, Page 9, Paragraph 1 Inc Section 1.1 (page 2) states that response item five will consist of the construction of a slurry wall. The stated objective of the slurry wall is to minimize the flow of Rito Seco alluvial groundwater into the West Pit, and to control the outflow of West Pit groundwater into the Rito Seco al uvial aquifer. However, the wording in Section 2.4 is not as definitive with respect to BMRI's commitment to construct a slurry wall; the phrases "further consideration" and "potential use" are employed when discussing the slurry wall. In addition, no schedule is forwarded for the slurry wall construction. Biv1Rl committed to the use of a hydrologic barrier to limit the flow into the West Pit i i a January 25, 1990 public hearing before the MLRB. When asked if there was a "guarantee tha: the Rito Seco water will not... flow into the pi[", Mr. Dean Massey, attorney for BMRI, respond=d: "...there was also a grout curtain that was committed to in the course of that permit to avoid any connection between that pit and the alluvial surface flows from the Rito Seco." A copy of the Reporter's Transcript of Public Hearing, Volume I, Page 266 is attached. The discussion of hydrologic barriers in TR-26 was limited to one paragraph (see TR-26 at page 59), and only considered the installation of a physical barrier that would inhibit the flow of water out of the West Pit. BMRI dismissed the use of a hydrologic barrier, even though earlier discussions (see TR-26 at page 20) indicated that the majority of the water flowing into the West Pit du•ing active mining originated from the Rito Seco alluvium, entering the West Pit through the souti~east corner. The post-mining potentiometric and groundwater chemistry data presented in TR-26 F gures 3-4 and A-30, and Figures 3- 8 through 3-11, respectively, demonstrate that inflow of groundwater from the Rito Seco alluvium into the southeast portion of the West Pit provides a significant portion of the groundwater entering the West Pit. Once in the pit, this water becomes contaminated, then exits the West Pit through the "window" created by the removal of the Sante Fe Formation and the green clay layer, entering the Rito Seco via seeps and underflow. RMC believes that the feasibility of constructing a physical barrier, such as a slurry wall, that both inhibits the flow of water into and out of the West Pit should be quickly examined and established. A properly constructed slurry wall will allow the hydrologic system in the vicinity of the West Pit to return to its pre-mining condition by approximating the function of the "green clay layer". While groundwater extraction from the West Pit and treatment would likely still be required under this scenario, inhibiting the flow of alluvial water into the pit (i.e., keeping clean water cle~in) will reduce the volume of water requiring treatment. This would, in turn, reduce the long germ O & M costs associated with the mine closure. Section 2.4, Page 10, Paragraph 2, Lines 4 and 5 BMRI does not discuss having to continue extracting groundwater from behind the slwry wall to maintain a depressed water table aker installation of the slurry wall. Will continued pump and treatment be required? And, if so, at what rate?