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BMRI -San Luis Pit Bac~ll: Sequentiat Batch Tesfs of Gneiss • <br />The natural and leachability of the Santa Fe Formation were described in the July, <br />1995 Technical memorandum from Geochimica to the Division, and that information <br />will not be repeated here. <br />The sequential batch testing directly addresses the major issues associated with the <br />inundation of the gneissic backfill in the West Pit. The test results show: <br />Although static ABA results might suggest that the gneiss is potentially acid <br />generating (i.e., if subjected to cycles of surface weathering) the gneiss is not acid <br />generating under fully saturated conditions, even when the ground water reacting <br />with the rock contains discernible amounts of dissolved oxygen (1.0 - 5.6 mg/1) and <br />has a positive (oxidizing) redoz potential in the range of +200 - +400 mV, the <br />range commonly measured and inferred in ground water studies. <br />Interaction of the gneiss with ground water produces leachates with alkalinities in <br />excess of 100 mg/1 CaCO3 eq., indicating that the ground water /rock system of a <br />saturated backfill has sufficient bicarbonate alkalinity to maintain the pH of <br />solutions in the alkaline range. <br />The observations of (a) 0.23% to 2.86%"unidentified sulfur" in the static ABA <br />tests and (b) leachability of K and SO4 in the batch tests indicates that the gneiss <br />samples contain discernible concentrations of a jarosite-like phase that has slight <br />solubility when the rock is leached with the low-K local ground water. It is <br />inferred that the Fe that would be expected to leach together with the K and SO4 is <br />solubility controlled under the redoz conditions (> +200 mV) of the batch tests. <br />Thus, the observation of leachable sulfate and very minor decrease in alkalinity of <br />the aqueous phase is associated with the dissolution of a small amount of jarosite <br />(and subsequent hydrolysis of ferric iron), not the oxidation of pyritic sulfides in <br />the rock matrix. This "wash off' of ephemeral sulfate-phases from rack surfaces <br />is commonly observed in many hydrogeologic settings and does not indicate a <br />significant ARD threat, provided the samples will remain fully saturated (as will be <br />the case at San Luis). <br />4. The only metal that appears to be leachable from the gneiss at values that are <br />discernibly above the detection limit is Mn. At low water :rock ratios (i.e., at <br />ratios < 1:1), Mn is leachable from the gneiss at about 1 - 2 mg/1, compared to the <br />ground-water head analysis of 0.71 mg/I. At higher water :rock ratios -equivalent <br />to longer times within the pit and to lateral distance from the pit -, the Mn values <br />are at or below the ground-water head analysis. Ground water monitoring results <br />from the San Luis site have shown a baseline enrichment in Mn, so the observation <br />of Mn leachable at low ppm levels in these tests is both expected and confirmatory <br />of the fundamental reliability of the testing procedures. <br />Geochimica, Inc. 6 95021/26Sep-95 <br />