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ECOSA Evaluation AdrianBrown <br />The overburden rock also contains approximately 0.21 % carbon, in the form of carbonates (Table 4). <br />The Acid Neutralizing Potential ( "ANP ") of the overburden is the equivalent of 17.9 tons CaCO3 per <br />kiloton of overburden, meaning that it contains neutralizing materials that have the same neutralizing <br />potential as 17.9 tons of calcium carbonate. The carbonates provide some neutralizing capacity for the <br />AGP that is produced by the sulfide oxidation, for example by the following reaction: <br />CaCO + H — H + CO + CaSO <br />Some of the calcium sulfate thus formed will remain in solution, and some may precipitate as gypsum if <br />the concentration in the interstitial water exceeds the solubility limit for gypsum (around 1500 mg/L in <br />this system). <br />The Net Neutralizing Potential ( "NNP ") of the overburden is the ANP (17.9 tons CaCO3 per kiloton) <br />less the AGP (42.5 tons CaCO3 per kiloton) or -24.7 tons CaCO3 per kiloton of overburden, with the <br />negative sign showing that the overburden is potentially net acid generating. <br />After sufficient time has elapsed to allow the acid produced by the oxidation of the sulfide to entirely <br />consume the carbonates in the overburden, further sulfide oxidation may cause the overburden to <br />become acidic, which would mobilize more metals by leaching from the overburden. In addition, the <br />ferric hydroxide formed in the original oxidation reaction can be dissolved, resulting in higher iron <br />concentrations in the liquid seeping from the base of the ECOSA. <br />The result of these processes is that water could potentially flow out of the base of the highly permeable <br />ECOSA will contain the calcium, sulfate, and metals associated with the oxidation of sulfides in the <br />overburden and the neutralization of the products. In general, sufficient calcium and sulfate is produced <br />to saturate the water passing through the ECOSA, resulting in the water initially flowing from the base <br />of the ECOSA maintaining a stable water quality that is very similar to Carlton Tunnel water. In the <br />event that the neutralizing capacity of the overburden materials is exhausted, the water seeping from the <br />base of ECOSA will become acidic, and the concentrations of calcium, sulfate, iron, zinc, and other <br />metals may increase. <br />The process described above is the natural fate of the sulfides in the diatremal rockmass: it will take <br />place over time in the rock that will be placed in the ECOSA no matter whether it is mined or not. It <br />cannot be prevented due to the ubiquitous presence of sulfide, oxygen, and water in the District above <br />Carlton Tunnel level. The difference created by mining is the location of the rock where the reaction <br />takes place (which is moved due to excavation and placement in the ECOSA) and the rate at which it <br />occurs (which is increased by the comminution of the rock by mining). Thus the quantity of sulfuric acid <br />produced and neutralized, and the amount of metals liberated, is not changed by mining; just its location <br />and its timing are changed. <br />The potential for impact arises as a result of the fate of the water after it flows out of the base of the <br />ECOSA. If the water proceeds down through the diatremal volcanic rocks, contacts the abundant <br />carbonate in the Diatreme, and joins the regional groundwater system that flows from Carlton Tunnel, <br />there will be no net impact, as this is the same fate that it would have had naturally. If, however, the <br />water does not follow that path, but emerges at the toe of the ECOSA and joins the surface water system <br />in Grassy Valley, then dissolved constituents in it resulting from sulfide oxidation have the potential to <br />1385E.20120224 9 <br />