2012-02-28_REVISION - M1980244 (100)

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ECOSA Evaluation AdrianBrown cause impact to surface water and shallow groundwater resources in Grassy Valley. The impact assessment therefore addresses the fate of the water from the ECOSA. 4.2.2 Potential for Groundwater Impact during Construction The overburden placed into the ECOSA will be pervious, and will sequester all precipitation that falls on it during construction. Much of the precipitation will rapidly infiltrate into the surface of the facility, and will cause the moisture content in the stored rock to increase from close to zero at emplacement to approach its field capacity of approximately 7.5% by volume of rock (CC &V, 2008). The process is evaluated for ECOSA in Table 5, which indicates that it will take approximately 20 years for ECOSA to reach field capacity, which comprises the construction period of 6 years for the facility plus a further 14 years. Accordingly, it is not expected that there will be significant seepage from ECOSA to the underlying colluvium and bedrock during the construction period of the facility or for a considerable period thereafter. During the period when sequestration is occurring, the prior infiltration to the underlying bedrock from the footprint of ECOSA is being prevented. The amount of infiltration being prevented is computed as follows: Infiltration rate prior to ECOSA construction = Area of ECOSA during construction = Volume of water prevented from infiltrating = 6 inches /year 0 — 268 acres 0 — 134 acre - feet/year (0 — 83 gpm) Over the period of construction of ECOSA the interception of infiltration will result in an average reduction of approximately 41 gpm of flow that was previously within the regional groundwater arriving at the Carlton Tunnel portal. 4.2.3 Potential for Groundwater Impact after Reclamation ECOSA will be reclaimed with soil and colluvium that was obtained from the facility footprint and stockpiled during the operational period of the facility. After reclamation, the rate of infiltration of precipitation through the ECOSA surface will reduce to approximately the same infiltration rate as existed on the natural surface prior to construction. Mass balance analysis on the Carlton Tunnel flow (ABC, 2008) and refined by calibrated groundwater modeling of the District (this document) indicate that this rate is approximately 6 inches per year for locations within the edge of the Diatreme. Once the field capacity of the overburden is reached, which is expected to occur approximately 14 years after the end of the 6 year operational period of ECOSA, seepage flow will exit the base of the facility at approximately 83 gpm (Table 5). Once this water has reached the base of the facility it will enter the underlying colluvium, infiltrate vertically through it into the underlying bedrock, will continue to move downwards in the diatremal rock to the saturated regional groundwater regime at the base of the Diatreme, and will then move laterally within the Diatreme ultimately reaching Carlton Tunnel and flowing to Four Mile Creek. 1385E.20120224 10