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3. TEXT CHANGES It is important [o note that this analysis is based on the assumption that the dissolution surface is a dynamic feature and that the solution of nahcolite is an ;tctive natural process in the Piceance Basin. The alternate assumption is that the dissolution surface is a stable feature and only minimal nahcolite dissolution is presently taking place. If this is cortect, there is the low possibility that the stability of the dissolution could be upset and a new epoch of nahcolite dissolution would be initiated. If this were to happen, the long-term impacts to the quality of the lower aquifer could be much greater than those projected here. There is a low probability that this scenario would occur, and monitoring will detect changes in water quality so remedial actions can be applied. The following discussion is provided to better define possible effects to the water quantity and quality of the hydrologic system on lease from well completion and abandonment. WRC proposes [o drill and complete 1 l production well pairs (22 wells) per year over the life of the project. Each well would be completed by cementing the bottom 100 feet and by placing a chemical gel-type mud (casing pack) behind the outside 8 5/8-inch casing to surface. The 100- foot cement column would extend from the Boies Bed upward into the lower aquifer. Upon abandonment of the borehole, the casing above the cemented area would be removed and used for subsequent wells. A cast iron bridge plug would be set just above the casing shoe and 100 to 150 feet of cement placed upon the plug sealing off the Saline Zone from the lower aquifer. Additional plugging would be accomplished by placing chemical gel type mud (casing pack) from the top of the cement to within 65 feet of the surface where a cement plug would be set. Cementing the bottom 100-foot interval of the production casing is to provide a hydraulic seal bt:twcen the production cavity and the lower aquifer, ther<:by guarding against upward movement of brines into the lower aquifer, and also to provide stability to the well bore and casing string. Taking into account the technical concerns of placing the 100-foot cement column and the cement to formation bond, coupled with the stress placed on the well strings from horizontal drilling within the cavities and other operational procedures (Appendix B), there is probable concern that effects might occur to the lower aquifer during production of the solution cavities. Should the a:ment column remain intact and provide an adequate hydrtulic seal between the cavity and the base of the lower aquifer, then no impacts would be expected. However, if We reverse were true, increased dissolved solids would communicate into the base of the lower aquifer degrading the aquifer quality. As stated previously, as the mine progressed and predicted caving occurred, direct communication between the base of the lower aquifer and the cavities would be established, thereby increasing the likelihood of lower aquifer degradation. Cavity collapse and/or subsidence in itself would not exclude concern for proper isolation between the Saline Zone and the lower aquifer. Therefore, adequate monitoring of the dissolution surface will be required so that changes in total dissolved solids of the lower aquifer can be detected during production phases over the life of the project. For production well completion, casing pack would be placed between the outer casing and the formation rock for the proposed 22 wells per year under the Proposed Action as proposed by WRC. This casing pack would be subject to dynamic well bore conditions that might compromise its intended function to isolate and insure aquifer integrity. Production wells would be online from 90 to 120 days. Although all wells would be drilled and completed at approximately [he same time, they would not necessarily be put into production concurrently. Therefore, many wells may be in nonuse for several months after completion. The use of casing pack is proposed by the applicant in place of cement to allow the removal of well casing for reuse and economic considerations. Assuming the casing pack functions as WRC predicts and Bow gradients are in the downward direction, no impacts to the aquifer systems would be expected from this action. Should the casing pack dissipate or not seal the zones as predicted, then any increased movement of upper aquifer water to lower aquifer water would not adversely effect water quality under any of the altematives. Although upper aquifer water would flow in the downward direction to the lower aquifer resulting in small volume changes, no net loss to the hydrologic system is expected. However, if the Bow gradient were reversed, lower to upper, because of stress on the system from subsidence and/or from pumping of the production water supply well(s) under any of the commerical-scale alternatives, then upper aquifer water would be impacted by slightly poorer water of the lower aquifer. This degradation could exceed [he State of Colorado groundwater quality standards (Colorado Water Quality Control Commission 1987). Therefore, BLM will require [hat the A-groove above the Mahogany Zone and the B-groove below the Mahogany Zone be monitored to ensure aquifer integrity. In addition, BLM shall require that a cement plug be set across the Mahogany Zone upon well abandonment. Frequency of monitoring will be dependent and variant upon production and subsidence effects to the aquifer systems. 4.4.5.1 No Action Alternative Up to three cavities will be constructed under the No Action Alternative. The groundwater monitoring system required for [he pilot project should detect any significant increase in dissolved solids entering into the groundwater system in sufficient time For remedial action to be taken. 3-19