1989-11-22_REVISION - M1988112 - Laserfiche WebLink

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interact by the chosen method of mining. Thus, the prediction of acid mine drainage cannot be based exelus ive ly upon overburden analysis, although to a large extent, the character of the factors mentioned above is controlled by the rock chemistry. At best, overburden analyses provide a first approximation of the expected mine drainage quality and it is only when the strata of various chemistries are allowed to interact in a backf filled mine, which itself is con- stantly readjusting in terms of its hydro- logy, geochemistry, pore space gases and surface re vegetation, that the quality of mine drainage emanating from the backf filled mine can begin to be assessed. Usually these adjustments take place over many years and the true nature of the drainage quality may not be i~mnediately discerned. The assessments of a stratwo's capa- bility to produce alkaline or acid drainage are to be used as guides for the placement of spoil material in order to minimize the environmental impact of the mine of the adjacent streams. lbxic (acid producing) material should be isolated from the near surface environment, whereas alkaline mate- rial should be utilized to enhance revege- tation. '!b extend overburden analyses to the prediction of mine drainage quality without considering the kinetics of acid or alkaline release, the ground water geo- chemistry, the chemistry of recipient streams, the quality of the rain, the arrangement of the spoil material in the backfilled mine (blending versus segrega- tion), the hydrology of the mine, the evo- lution of pore space gases and the chemical loads (as opposed to concentrations) of the acid and alkaline leachates, ignores the multivariant nature of the acid mine drain- age problem and will lead to erroneous con- clusions. The initial objective of this study was to ascertain, through laboratory experi- ments, what effect various leaching solu- tions had upon the leachates of several dazk gray shales. Because each sample was ex- pected to have a different chemical compo- sition the experimental design vas extended to include ancillary study that would test the various rock parameters to determine which parameter best predicts the quality of leachate that would be expected from each sample. Experimental Design Eleven samples of dark gray to black shales were collected Erom newly cut cores taken from several strip mines in Wharton Township in southwestern Pennsylvania. Four samples were collected from above the Brush Creek coal seam, four from above the Mahoning coal seam, and three from above the Lower Kittanning seam. The 11 shale samples were crushed to pass a four milli- meter sieve and subsequently riffled into three representative portions of roughly equal size, giving a total of 3) samples. Earn of these was then further riffled to produce two representative but unequal portions. The larger portion vas used in Table 1. Chemistry of Synthesized Acid Aain (R) and Synthesized Acid Mine Drainage (A). Synthesized Acid Hain (R) Concentration Component _ (mg/1) + H ++ 0.21 Ca++ 1.08 Mg+ 0.15 Na 4.75 K+ 2.25 so4 s.4s C1- 9.39 NO-5 6 .50 Synthetic Acid Mine Drainaaipe (A) concentration Component (mq/1) Fe ~+ 210.0 Al+++ 15.0 Mg++ 51.0 Mn++ 19.0 Ca++ 100.0 Na 2.0 H+ 3.0 SOq 1100.0 Cl- 110.0 the simulated weathering tests, and the smaller portions were used for whole rock analysis. Simulated Weathering Tests. Each of the 11 shale samples vas ri fled into three repre- sentative portions - in essence generating three groups. The 33 samples were placed into individual plastic chambers through which humidified air was constantly passed. At weekly intervals the chambers were opened and one group vas covered with ~deionized water (wl, a second group was covered with synthesized acid rain (r), and the third group was covered with synthet in, acid mine drainage (a). The composition of these solutions is shown in Table 1. The leachate volume was measured and analyzed for pH, specific conduuaance, acidity (hot mineral), alkalinity (cold titratable) and sulfate concentrations. By noting the weight of the rock sample used in the weathering test and the ieacha to volume and quality, all results can be con- verted to milligrams of acidity, alkalinity or sulfate produced for each leaching per 100 grams of sample. These values are plotted versus time to graphically depict acid or alkaline production trends with time. The sulfate ion is produced by pyrite oxidation and is useful in in [e x~pre [inq [he degree of oxidation that has taken place. The specific conductance is a measure of the amount of ions in solution and is useful in interpreting the degree of chemical reactivity. 440