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BMRI -San Luis Pit Bac~IL• Sequential Batch Tests of Gneiss • <br />• Are Pink Gneiss samples acid generating under fully saturated conditions? <br />• What species (especially metals) are leachable under fully saturated conditions, and <br />at what concentrations are those species present? <br />Sequential batch tests allow a rapid and relatively simple evaluation of chemical <br />changes that may occur at source and along a flow path represented by the tested solid <br />samples. Fully dynamic (e.g., column) tests are much more difficult to petfotm <br />reliably and may take many months to produce data equivalent to those of sequential <br />batch tests. Sequential batch tests can be accomplished more quickly, in a manner that <br />controls the effective water:rock ratios, and in a manner that allows a bounding <br />evaluation of leachability. By taking the effluent from one test and reacting it with <br />fresh rock in another test and, at the same time, taking fresh ground water and reacting <br />it with the solid phases that already has been leached in the first test, the sequential test <br />procedure can simulate dynamic processes in a piece-wise manner. <br />A total of six Pink Gneiss samples and fifty gallons of site ground water were shipped <br />from BMRI to Chemac Environmental Services (Englewood) for testing. )Each solid <br />sample was crushed to approximately minus-1/4 inch top size and thoroughly blended. <br />A representative split of the minus-1/4 -inch rock (about 500 grams) was split from <br />each sample, crushed to minus 100 mesh, and set aside (in sealed, plastic bags) for <br />static acid-base accounting and total metals analysis. <br />Figure 1 (taken verbatim from the final Chemac analytical report) illustrates the <br />sequential batch test process. Each batch leaching step was accomplished py <br />combining measured quantities of crushed Pink Gneiss and site ground wader. Liquids <br />and solids recovered after each contact were subsequently contacted with fresh solid <br />and liquid, respectively. The test conditions for each batch contact were as follows: <br />• 24-hour contact time <br />• Continuous mixing using a bottle roll apparatus <br />• Purging of the initial liquid and the initial solid-liquid mixture with nitrogen to <br />avoid oxygenation that would be uncharacteristic of the inundated pit after <br />reflooding; <br />• Eh and dissolved oxygen measured immediately after opening test apparatus <br />following each contact; <br />• Solid-liquid separation using a pressurized filter press. <br />Geochimica, Inc. 2 95021/26-Se~r95 <br />