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Bu![doq Mine Tailings Ponds Evaluation <br />5.2 Oxygen Profile and Oxygen Consumption Measurements in Near Surface Materials <br />The third set of tests which was used to determine if sulfide oxidation is actively occurring <br />within Bulldog tailings involved measuring the variation of oxygen concentrations with depth, <br />and determining the rate of oxygen consumption in neaz-surface tailings at both impoundments. <br />The oxidation of pyrite is typically described by the following reactions: <br />FeS2 + 7/20, + HZO => Fe'`` + 2SOgZ- + 2H~ <br />FeZ~ + 1/40Z + H+ > Fe;` + 1/2Hz0. <br />Each of these reactions consume oxygen; therefore, the concentration of free oxygen in the pore <br />spaces of materials which aze undergoing active sulfide oxidation decrease with depth. An <br />excellent example of oxygen consumption in shallow sulfide-bearing tailings is described by <br />Blowes and Jambor (1990). In those tailings, the percent of oxygen remaining in pore gases at <br />depths of less than one foot below the ground surface were depleted by as much as 90% of <br />atmospheric values. <br />When sulfide mineral waste is the only oxygen consuming material present, the rate of oxidation <br />within the tailings can be estimated by measuring the flux of oxygen through the tailings surface. <br />In order to measure the rate of oxygen consumption at the surface of the Bulldog impoundments <br />a technique developed by Nicholson et al. (1995) was applied as described in Section 3.2. <br />Concurrent measurements of the concentration of oxygen in tailings pore gases at various depths <br />were made adjacent to oxygen flux measurement locations also described in Section 3.2. These <br />oxygen profiles allow the effect of oxygen depletion by organic material, which was used as part <br />of the impoundments reclamation efforts, and the effect of diffusion driven oxygen depletion <br />within the flux cells to be assessed. <br />The results of the oxygen probe profiling aze presented in Figures 5.3a and 5.3b. These results <br />indicate that a maximum of 1.5% of the oxygen present in the atmosphere is consumed in the <br />upper six inches of the tailings. Below six inches, there is no indication of further oxygen <br />consumption within the tailings. The largest decrease of oxygen occurred in the upper two <br />inches of the Upper Pond materials, which were composed primarily of organic materials used in <br />Homeslake hli/ung Company Shepherd 1/iller, lac. <br />y: v'dl+ib„ua y; ./~r 2g aprr(l ~. 1997 <br />