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' BMW <br />S <br />L <br />is 1998 B <br />k <br />C <br /> - <br />an <br />u <br />ac <br />frBi <br />eochemi.~ry <br />' " ... <br /> 2. Interaction of the gneiss with ground water produces leachates with alkalinities in <br />' excess of 100 mg/L CaCO3eq., indicating that the ground water !rock system of a <br /> saturate backfill has sufficient bicarbonate alkalinity to maintain pH of solutions in <br /> the alkaline range. <br />1 " <br />` <br />" <br /> 3. The observation of (a) 0.23% to 2.86 <br />% <br />unidentified sulfur <br />in the static ABA tests <br /> and (b) leachability of K and SO4 in the batch tests indicates that the gneiss samples <br />' contain discernible concenvations of a jarosite-like phase that has slight solubility <br /> when the rock is leached with the low-K local ground water. It is inferred that the <br /> Fe tha[ would be expected to leach together with the K and SOa is solubility <br />' controlled under the rexlox conditions (> +200 m~ of the batch tests..,. This <br /> "wash off'" of ephemeral sulfate-phases from rock surfaces is commonly observed <br />' in many hydrogeologic settings and does not indicate a significant ARD threat, <br />provided the samples will remain fully saturated (as will be the case at San Luis) <br />" <br /> . <br /> The role of formation and dissolution of secondary sulfate minerals in controlling <br />' solution chemistry has been widely documented in the past few years (e.g., All et al., <br /> 1994; Alpers et al., 1994; see Morin and Hutt,1997 for additional case studios). The <br /> primary acid-generating reaction (oxidation of pyrite) cannot be sustained at significant <br /> rates once the solids are inundated (e.g., Posey, 1996; Blowes et al., 1994; Morin and <br /> Hutt, 1997; Pedersen et al., 1993, 1994). Therefore, the wash-off" process will not <br /> continue to generate SOa (and therefore TDS) at the observed, early-time rates. As <br /> fresh ground water moves through the backfill, the ground-water conditions within the <br /> backfill will return toward pre-mining conditions. Because of the mining and <br /> backfilling activities, the materials now in the pit differ from those that were present <br /> before mining and nnr chould not expect that the ground water chemistr~wQu_Id be_ <br /> e~sted i____ n that wne before mining. <br />identical to that which <br />' _ <br />' DLSCUSSION <br />~' <br /> .u. <br /> The observed grotmd-water conditions in a location such as that of well BF-2.~C;,t rLbg_. <br />' evaluated using the kinetic-test data developed for TR-18 in 1995-1496. Table 1 <br />" <br /> fo~tfre three sets of hutnidity- <br />presents the sulfate concentrations as a function ofiim~ <br /> cell experiments conducted to evaluate the acid-generating potential of the pinlt gneiss. <br />' The results also are summarized on Figure 1. The pattern of all three experiments seen <br /> in Figure 1 shows the early generation and subsequent washing out of soluble sulfate. <br /> These results are typical of a rock that is not expected to be acid generating (e:,g., <br />' Main and Hutt, 1997). For each of the three tests, there comes a tune in the leaching <br /> history (ranging from Week 8 for composite 3 to Week 21 for composite 2), after <br /> which sulfate no longer is released at detectible levels. <br /> <br /> Table 2 calculates the total mass of sulfate released from each of the three experiments. <br />' Over the course of 28 cycles of leaching under fully oxidizing conditions, the total <br /> Geochimica, Inc. 16-Now98 <br /> <br />~~ ? <br />