2005-10-24_REVISION - M2000016 (2)

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TETRATECH RMC Mr. Carl Mount October 24, 2005 Page 2 -Riverbend Mine Groundwater Monitoring and Mitigation Plan in which Figure 1 has been updated to show the recharge basins on the north side of Cell 1, Phase 7 and the east side of Cell 1, Phase 8. 2. In a letter from Tetra Tech RMC addressed to Carl Mount (DMG) titled "Sixth Adequacy Review Response, Lafarge West, Inc. Riverbend Operation 112(c) Permit Amendment 01, Permit no. M-2000-016, "Tetra Tech states under numerous circumstances that agreements between owners of man-made structures and Lafarge West, Inc. will be submitted to the Division as they are received. Specifically: item #3 refers to an agreement with the owner of well 13697; item #4 refers to an agreement with the owner of well 12790; item #10 refers to 600 foot well spacing agreements, and; item #I2 refers to signed agreements with the Gupton Bottom Ditch Company and the Brighton Ditch Company. The Division, at the recommendation of the Attorney general's Office, requires that these documents be submitted prior to approval of this amendment. Please submit the required documents. Initially, Lafarge submitted a groundwater model to determine the potential disturbance to the prevailing hydrologic balance. The initial approach was to prepare the model and rely on the information provided by the model. The Division's comments made in the previous review of the groundwater model required Lafarge to obtain agreements with adjacent well owners. With input from Division staff regarding the groundwater model, Lafarge shifted the focus from modeling efforts to the development of a comprehensive groundwater monitoring and mitigation plan. We believe that this plan adequately protects existing wells and vegetation near the proposed mine. Agreements with adjacent landowners should not be required prior to granting of the permit because Lafarge will base any future groundwater mitigation actions on the comprehensive groundwater monitoring and mitigation plan they have developed. This plan will protect the surrounding well owners. Highlights of the groundwater monitoring and mitigation plan include: • The addition of rechazge basins adjacent to cottonwood galleries and potentially impacted wells; A monitoring well network of 64 wells, of which 34 are proposed as part of this monitoring and mitigation plan; and The following trigger points to initiate mitigation: i. A complaint from a well owner within 1,500 feet of the site boundary; ii. The relative seasonal groundwater level at any of the domestic wells or monitoring wells differs from the baseline conditions by more than two feet; or iii. The qualified individual inspecting adjacent off-site vegetation denotes stressed vegetation. Consequently we offer the following commitment. Lafarge will obtain the 600-foot well spacing agreements per the State Engineer's Office requirements six months prior to the