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• ~ - Site-Specific Hydrogeologv <br />Introduction. Collection of baseline water level data from the ground water monitor wells <br />at Seneca II-W began in February of 1980. The locations of all present and previously <br />active ground water monitoring sites are presented on Exhibit 7-1, The Hydrologic <br />Monitoring Program, Tab 15, outlines the background, rationale, and history of the use, or <br />discontinued use, of monitoring sites and details the monitoring programs. Li thologic <br />logs and well completion sunmaries for each monitor well can be found in Appendix 7-1. A <br />tabular (including statistics) and graphical presentation of water levels for all monitor <br />wells through their period of record is presented in Appendix 7-2, <br />Ground water monitor well installation began at Seneca II-W in 1974, Three wells, MW-1, <br />MW-2, and MW-3 were completed in portions of the overburden, Wadge coal, and interburden. <br />The annular seals faiied shortly after their completion and no useful information was <br />obtained from the wells prior to their abandonment. The next phase of ground water <br />monitor well installation began at Seneca II-W in 1979. At seven locations monitor wells <br />were installed using the well nest method of completion - more than one well completed in <br />• a common well bore. Table 7-2 presents specific well compietion information concerning <br />each of these well nests. The well nesting method of well completion was commonly used in <br />the past decade. This method is not extensively employed at the present because of <br />problems related to maintaining effective annular seals that truly isolate the hydrologic <br />units to be monitored, Due to annular seal failure for the majority of the well nests at <br />Seneca II-W, many of the wells have been dropped from the monitoring program. <br />In 1980, the USGS, as part of a pre-mining hydrologic study, completed 13 alluvial wells <br />(A1-A13) in the alluvium along upper Sage Creek. In the fall of 1987, Peabody initiated a <br />monitor well drilling program and installed 12 monitor wells (CW-52W-140V, -150V, -160V, <br />-170V, -180V, -14W, -15W, -16W, -17W, -18W, -17WC, and -19A) to replace the monitor wells <br />which were dropped from the monitoring program, and to provide more downgradient monitors <br />that will not be removed throughout the life of the mining operation. Finally, four <br />alluvial wells (CW-52W-20A, -21A, -22A, and -23A) were completed in tributaries draining <br />from Seneca If-W to Dry and Sage Creeks in the summer of 1990. <br /> <br />5 <br />