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Sunnyside Mine-Alk- Water Addition <br /> G_F.00HEMICAL EFFECTS <br /> Limitation of Oxygen <br /> The classic stoichiometric equation for the generation of acid drainage by oxidation of <br /> pyrites is: <br /> FeS2(s) + 15/4 02 + 7/2 H2O =__>Fe(OH)3(s) + 2 SO42- (aq) + 4 H+(aq) <br /> Direct oxidation of symmetric base-metal sulfides (e.g., ZnS, PbS) is very slow, and <br /> leaching of base metals from mineralized rocks is due primarily to leaching by acid <br /> derived from the oxidation of pyrites. Thus, the chemical reaction illustrated above is <br /> the critical reaction for evaluation of potential impacts of a mine such as the Sunnyside <br /> on water quality. <br /> It is well established by experimental work (e.g., Singer and Stumm, 1970) that in the <br /> pH range of 4 to 7.5, the kinetics of pyrite oxidation are first order in [0j. That is, <br /> not only is the total amount of pyrite that can be oxidized limited by the stoichiometric <br /> relationship to oxygen, but also the lower the mass of 02 available to react with pyrite, <br /> the lower is the rate of reaction. Thus, if the release of potential acidity can be <br /> controlled by limiting oxygen supply, then the system has an enhanced potential to self- <br /> neutralize mine-pool water through the available neutralization potential of either the <br /> dissolved alkalinity or the alkalinity that is available through heterogeneous reactions <br /> between the water and the solids with which the water is in contact. <br /> These fundamentals of the solution chemistry are the technical basis for the strategy of <br /> flooding mine workings to control acid drainage in the mine pool. Under de-saturated <br /> conditions, the mine workings are open to atmosphere, which contains about 20% 02 <br /> (200,000 ppmv), an effectively infinite reservoir of oxygen compared to available <br /> pyrite. Under saturated conditions, the dissolved oxygen content is limited to the <br /> saturation value of about 10 ppm (STP; lower at 10,000 feet). As this small amount of <br /> oxygen is depleted by the oxygen demand of the system (e.g., by reactions with old <br /> mine timbers and tramp metal, as well as by pyrite on the walls of the workings), both <br /> the rate of reaction and the mass of acidity (and other reaction products) decline. In a <br /> flooded underground mine, the continuing flux of oxygen to the re-saturated portions of <br /> the workings is limited to diffusion from the air-water interface or advective transport <br /> of DO along long flow lines from surface recharge. Therefore, the steady state <br /> condition in the mine pool is expected to be anoxic, which limits acid generation. <br /> The more quickly the mineralized zone of the Sunnyside workings can be inundated, <br /> the shorter the period in which pyrite is exposed to atmospheric oxygen. The shorter <br /> the exposure to oxygen, the lower the amount of acidity and dissolved solutes <br /> (including leachable metals) there will be in the mine-pool water (and therefore in the <br /> surface water to which the mine-pool water may discharge ultimately). Therefore, it is <br /> Geochimica, Inc. 3 ALKALINI17/6-Jun-95 <br />