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ECOSA Evaluation Adrian Brown <br />• Surface infiltration applied to the entire model at a rate of 6 inches per year. <br />• Carlton Tunnel installed as a drain at approximately 3,500 feet below the topographic surface (at <br />elevation 6,890 feet msl). <br />The model is run using the USGS MODFLOW 96 code (McDonald and Harbaugh, 1988; Harbaugh and <br />McDonald, 1996), operationalized within Visual MODFLOW (WHI, 2000). <br />The model was calibrated using the following information: <br />• Piezometric head information obtained from a total of 124 piezometers in 37 vertical clusters, to <br />depths up to 750 feet, with data collected continuously for a minimum of 1 year. <br />• Flow from the Carlton Tunnel for the period 1992 through the present. <br />Calibration comprised adjustment of infiltration rates and hydraulic conductivity values to provide an <br />acceptable fit between measured and model - calculated values. <br />To verify the groundwater impact assessment for ECOSA, the model was run with ECOSA constructed, <br />and all other MLE -2 facilities implemented. <br />4.3.2 Verification of Passage of ECOSA Seepage into Bedrock <br />The fate of water leaving the base of ECOSA was checked by extracting the results from the model at <br />eight of the locations in the vicinity of the proposed ECOSA location where continuously monitored <br />multiple piezometer strings have been installed. The results are presented in Plate 15. <br />The results are plotted in terms of water pressure as a function of depth at each vertical piezometer <br />string location. This presentation compares the vertical change in water pressure against the hydrostatic <br />pressure increase: <br />• Upward Flow occurs if the water pressure increases with depth more rapidly than the hydrostatic <br />pressure (there are no examples of this artesian flow behavior near the proposed ECOSA). <br />• No Vertical Flow occurs if the water pressure increases at the same rate as the hydrostatic <br />pressure (this is observed at GVPW -01). <br />• Downward Flow occurs if the water pressure increases less rapidly with depth than the <br />hydrostatic pressure (this is observed at GVPW -02, -03, -04, and -05). <br />• Unsaturated Downward Flow occurs if there is no build -up of water pressure with depth (this is <br />approached at GVPW -06 and -07 within the proposed footprint of the ECOSA, and occurs at <br />GVPW -08, in Grassy Valley to the northwest of the proposed ECOSA location). <br />As can be seen in Plate 15, the modeled results generally support the observed results down to the <br />lowest piezometer at 750 feet below ground surface. This is remarkable in an analysis that evaluates the <br />flow in approximately 30 cubic miles of rock, with a large diameter "well" in the middle of it (the <br />Diatreme) exerting more than half a mile of drawdown on the system. <br />1385E.20120224 13 <br />