2012-02-28_REVISION - M1980244 (100)

Home Browse Search

ECOSA Evaluation Adrian Brown piezometric pressures and heads at these well nests have been monitored continuously for more than a year. The results are presented for the ECOSA area in Plate 13. All except one of the strings within the proposed ECOSA footprint exhibit strong vertical downward gradient ( >0.95). One nest (COB -01+ OSABH -02) exhibits a weaker gradient ( -0.07) but even a 7% gradient is significant, and would be considered high in a horizontal groundwater flow environment. This well nest is the only one within the footprint that is located in Precambrian rock; a companion piezometer string 1,500 feet northwest (GVPW -01) is also completed in Precambrian rock and exhibits no vertical gradient. These results indicate that there is sufficient vertical downward gradient to support vertical infiltration under gravity in the Tertiary rocks, but only marginal vertical downward gradient in the Precambrian within the proposed ECOSA footprint. Based on the results of the extensive groundwater investigation beneath the footprint of the proposed redesigned ECOSA, the following conclusions are drawn: 1. Diatremal Locations. Water flow from the base of ECOSA will proceed vertically through the underlying colluvium and into the volcanic bedrock of the Diatreme, ultimately joining the regional groundwater table and exiting the Diatreme via the Carlton Tunnel. In these locations there is no requirement for construction to aid infiltration. 2. Precambrian Locations. Exfiltration from the base of ECOSA at locations underlain by Precambrian rock may be prevented from flowing vertically downward through the colluvium to the bedrock, and then to the regional water table by the lack of a sufficiently high vertical downward hydraulic gradient to allow all the water to flow via the Precambrian rock to the Diatreme. In these locations CC &V is proposing the construction of drainage measures to direct any potential flow to the Diatreme. The selected method is the installation of trenches along contour at the base of the proposed ECOSA, which will prevent downhill flow of seepage along the ground surface to potentially emerge from the toe ECOSA and potentially impact surface water and wetland resources. The design and location of these trenches are presented in Plate 14. 4.3 Impact Evaluation Verification 4.3.1 Cripple Creek Mining District Model The fate of seepage from ECOSA has been verified by the use of the quantitative groundwater flow and transport model of the District'. The quantitative model was created as follows: • Surface topography of a 24 square mile area centered on the District, derived from recent contouring at 500 foot grid spacing. • Twenty -two vertical layers, each up to 250 feet thick, extending from elevation 10,800 feet msl to elevation 5,000 ft msl. • Hydraulic conductivity values applied to all 60,000 model cells based on geology using the results of 74 permeability tests conducted in wells with depths up to 1,000 feet. ' The Cripple Creek Mining District groundwater flow and transport model is presented and described as Attachment XXX of the Hydrogeochemistry Report (ABC, 2011)to the Hydrogeochemistry 1385E.20120224 12