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BMRI -San Luis Pit Bac~HCT Tests • Reactions (1), (3) and (4) are written here as irreversible reactions to recognize that heterogeneous reactions in a dynamic (effectively open) system are not expected to be characterized by reversible equilibrium. The homogeneous redox reaction for iron is written as reversible to recognize the ephemeral nature of redox conditions in a waste- rock pile. These reactions illustrate the chemical rational for tracking acid-rock drainage (ARD) is kinetic tests using pH, sulfate, and total iron concentrations; the acid-base capacity measurements -alkalinity and acidityl -- are also tracked as gross measures of the progress of reaction and to evaluate the degree to which a solution may be buffered. Sulfate is the dominant ion by mass in all the HCT solutions. Because the solutions must be electrically neutral, the sulfate must be balanced by counter ions. As a rule of thumb, the sulfate concentration can be estimated as ca. 70% of the total dissolved solids. All three samples show an initial surface reactivity when the sample is first tensed. This is characteristic of most HCT testing, and it represents the dissolution of very fine surface coatings on the sample particles. The release of iron and sulfate in Sample A together with the relatively low pH of the Week 1 sample (4.36) indicates that there was a small amount of jarosite [KFe3(SO4)3.(OH)6] on the waste rock surface. All three of the samples become less leachable with time, and after Week 5 they are essentially inert. In particular, it is considered that the rapid decline toward non- detectible sulfate in even Sample A indicates that the reduced sulfur available for oxidation under cyclical weathering has been depleted in all the samples. Although all three of the samples have final pH values below 7, Samples B and C are still producing alkalinity after 28 weeks, and Sample A is producing only a very low , (14 mg/1 CaCO3 eq.} amount of acidity. None of these samples produces a ~ve11 ~ ,,t~-~ buffered leachate solution, and none of the water would be aggressive at dissolving any ~ other phases. Thus, metals would not be released from other waste rock. Vi{'ere a ' solution such as the leachate from Sample A to be released from the waste-rack mass (which is unlikely given the net alkalinities of the leachates from the other materials), it would have only a very ephemeral effect on the pH of ground water because the acidity of the solution is very low. Finally, the Division will recall that under fully saturated test conditions, none of the Pink Gneiss samples generated any acidity or released metals. As BMRI intend to dispose of the Pink Gneiss waste rock entirely below the water table, the sequential ~ ~- . ~ For convenience in tabulation, we have chosen to present the alkalinity and acidity values in terms o(a "net alkalinity", defined to be the ditrerence of alkalinity minus acidity. Thus effluents with net acidity have neea[ive "net alkalinity" values in the tables. elk - a~td Geochimica, Inc. S~~~~ 95021/1-Feb-96