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production field (initially wells 2, 3, 4 in figure 9), as specified below. (i) Two wells for monitoring ground water at the -base of the alluvium up-gradient and down- gradient from the site shall be located in the Yellow Creek drainage. These wells are: 1) the 84 Mesa Well (figure 7) or a new well near this up-gradient location, and 2) a well located down-gradient from the point where drainage from the site enters the Yellow Creek alluvium. (ii) A pillar monitor well shall be located in the center of the triangular area formed by three production wells (see figure 3; for construction details, see MWD-1 Appendix A). The location of this well provides early warning of potential solution mining problems in adjacent cavities through water quality and continuous pressure monitoring of the zone immediately above the Dissolution Surface. Geophysical measurements lower in the hole will monitor mining development in adjacent production wells or caverns, and the structural integrity of the pillar. Two wells may be used to accomplish data acquisition and monitoring, and, unless changed, subsequent solution mining development also will use pillar wells. (iii) A group of process monitor wells for separately monitoring the A-Groove, B- Groove, and the zone above the Dissolution Surface, shall be clustered hydrologically down-gradient from and within 500 feet of the initial well production field, and monitored according to (a) above. The well monitoring the A-Groove also may be used as a water supply well. The operator intends additional daily monitoring of the A-Groove water supply well for TDS and specific conductivity. As new production well fields are completed, additional process monitor well groups may be required and can include appropriately located and converted former production wells. The location of all Page 19 of 120 EPA FZNAL Area Permit No. CO3870-00000