2007-08-31_REVISION - M1977208 - Laserfiche WebLink

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C-PIT HYDROGEOLOGIC INVESTIGATIONS TECHNICAL REVISION NO. 8 MINING PERMIT M-1977-208 4. CONCLUSIONS The wildlife exposure pathway fox C-Pit water was eliminated during Spring 2007 by filing in the pit and covering the small pond remaining in the north end with netting. No adverse impacts on surface or groundwater have been observed to date. The data collected during the TR-8 Hydrogeologic Invesdgadons are consistent with the conceptual model for the Site presented to DRMS at site inspections and meetings held on February 22, 2007 and March 20, 2007. The data confirm that C-Pit is geologically isolated in a sedimentary bedrock basin created by mining the Fort Hays Limestone in this area, and that any subsurface impacts are localized. Whatever the nature of the hydraulic connection between C-Pit and down dip monitoring well CEM-001 (as well as CEM-003) may be, the same connection is not evident at compliance well CEM-005 at the eastern extent of the property. C- Pit and well CEM-001 may be connected by fractures in the unmined limestone to the east of the pit, the shale units above and below the limestone, the contacts between the three units, ox some combination of these possibilities. The absence of a response in compliance well CEM-005 is due to a lack of competent fractures capable of transmitting water at the greater depths of CEM-005. The above conclusions are supported by analysis of [he available geochemical data, which indicates that the groundwater flow system within the C-Pit quarry area appears to be a poorly interconnected flow system. Even though the system has been stressed by raising the C-Pit water elevation over 18 feet during the filling process from March to May 2007, no chemical impacts have been observed down gradient from C-Pit in either the bedrock or the St. Vrain Creek alluvium. Other related conclusions are listed below: • C-Pit is cut into the Smokey Hill Shale unit overlyng the Foxt Hays Limestone, and the limestone has been extracted. The pit floor is the top of the underlying Carlile Shale. C-Pit has minimal hydraulic outlets, and serves as a vessel that accumulates water from multiple sources as its water transmitting capacity to the surrounding bedrock aquifer is significantly less than the volumetric input from upgradient sources. • C-Pit's western boundary is defined by a highwall that is immediately adjacent to the Swede Ditch, which has been enclosed in a CMP culvert (Figure 2). With the exception of the relatively narrow zone running with the Swede Ditch alignment, the area to the west of C-Pit has been mined and backfilled. See cross sections A-A' and B-B' in Figures 3 and 4. • The unmined portion of the Fort Hays Limestone and overlying Smoky Hill Shale underneath the Swede Ditch serves as a paztial bamer to subsurface flow moving down dip with the bedrock structure and along the surface of the Caxlile Shale. However, Field observation indicates that during the imgation season water passes through this barrier via fractures. • Most of the 1,900-foot reach of the Boulder Feeder Canal west of C-Pit from the B-Pit access bridge north is cut into the Carlile Shale that forms the floor of C-Pit. The exception is at the north end of this reach where the vertical alignment of the canal passes through the Fort Hays Limestone, which is visible in the western wall of the canal. 41 Use of contents on this sheet is sublem Io the limitations specified at the end of this dwument. IIDCDEN02\pm~eds\Data~GENICemexlTR-B~Reponinq\CEMEX Repotl OB31-D7 doe