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ECOSA Evaluation AdrianBrown <br />4. IMPACT <br />4.1 Storm Water Potential Impact <br />4.1.1 During Construction <br />ECOSA will encroach on part of the catchment of Grassy Valley. Because of the exposed rock surfaces <br />on the ECOSA during construction, all precipitation impinging on the pile surface during construction <br />will be captured by the overburden. Accordingly, there will be no surface storm water runoff to Grassy <br />Valley from the ECOSA footprint during construction. This will result in a maximum reduction of <br />spring snowmelt and stormwater runoff from the footprint of the facility computed as follows: <br />Total basin surface water yield = 0.103 inches per year (Plate 5) <br />Maximum area of ECOSA = 268 acres (Plate 7) <br />Maximum reduction of streamflow = 2.3 acre -feet per year. <br />This water is not lost; it is diverted to groundwater, and ultimately flows in the regional groundwater <br />that flows from the Carlton Tunnel. <br />4.1.2 After Reclamation <br />The surface of ECOSA will be reclaimed after the construction is complete, and the upper surface has <br />ceased being used as an ore storage area. It is expected that stormwater runoff conditions will return to <br />those similar to undisturbed grassland, which should provide approximately the same storm water runoff <br />as the current landform. The storm water runoff exiting the storm water management system at ECOSA <br />is expected to be of approximately the same quality as pre- mining storm water leaving the footprint. <br />Accordingly, no potential incremental flow or water quality impact is expected with respect to ECOSA <br />after reclamation is complete. <br />4.2 Groundwater Potential Impact <br />4.2.1 Potential for Groundwater Impact <br />Groundwater - related potential impact is a result of sulfide oxidation of the overburden within ECOSA. <br />The rock from which the ECOSA will be constructed has an average of approximately 1.36% sulfur, <br />present predominantly in the form of pyrite (Table 4). The Acid Generating Potential ( "AGP ") of the <br />overburden is the equivalent of 42.5 tons calcium carbonate ( "CaCO3 ") per kiloton of overburden, <br />meaning that it has the capacity when fully oxidized of producing an amount of sulfuric acid that would <br />require 42.5 tons of CaCO3 to neutralize. The pyrite in the overburden is subject to oxidation within the <br />ECOSA over time, with atmospheric oxygen acting as the oxidant, storm water infiltration providing <br />hydrogen, and thiobacillus ferrooxidans acting as a biological mediator. The chemical reaction that <br />takes place is in general as follows: <br />FeS + 15/4 0 + 7/2 H -+ Fe(OH) + 2 SO + 4 W <br />1385E.20120224 8 <br />