Abstract：
Coal mine drainage (CMD) in Appalachia is a widespread source of dissolved metals, SO4, and acidity that can degrade aquatic habitats and water supplies for decades following mine closure and flooding. In the bituminous coalfield of Pennsylvania, the Irwin Coal Basin (ICB) contains a series of partly to completely flooded, abandoned underground mines separated by leaky barriers within the Pittsburgh coal seam. CMD originated throughout the basin from minepool aquifers that formed after mine closures dating from 1910 to 1957. Historical and recent water quality data for eight CMD sites across the ICB, plus mineralogy and cation-exchange capacity of overburden lithologies, were analyzed to quantify important reactants and evaluate spatial and temporal water-quality trends. As overburden thickness and residence time increase along a ~ 50-km flowpath northeast to southwest in the basin, CMD becomes more alkaline, and Na concentrations increase. Since the 1970s, all eight ICB discharges have become less acidic, with exponential decreases in acidity, SO4, and Fe concentrations; only two CMD remain net-acidic (acidic pH at equilibrium). Exponential decay models that include a steady-state asymptote consistent with background groundwater chemistry and siderite equilibrium describe the early-stage, rapid contaminant concentration decay immediately after the \"first flush\" (initial flooding) and the progressive evolution toward late-stage background conditions. A geochemical evolution PHREEQC model indicates that spatial and temporal trends in pH, net-acidity, SO4, Fe, and major cations could be explained by the continuous dilution of first flush water by ambient groundwater combined with sustained water-mineral reactions involving pyrite and carbonates (calcite, dolomite, siderite) plus cation-exchange by clays (illite, chlorite, mixed-layer illite/smectite). These data and model results indicate that 1) cation-exchange reactions enhance calcite dissolution and alkalinity production, resulting in the evolution of CMD to Na-SO4-HCO3 type waters, and 2) siderite equilibrium could maintain dissolved Fe >16 mg/L over the next 40 years.
摘要：
阿巴拉契亚州的煤矿排水（CMD）是溶解金属的广泛来源，SO4和酸度会在矿山关闭和洪水后数十年内降低水生栖息地和供水。在宾夕法尼亚州的沥青煤田，欧文煤盆地(ICB)包含一系列部分到完全淹没的，匹兹堡煤层内被泄漏的屏障隔开的废弃地下矿井。CMD起源于整个盆地的雷池含水层，该含水层在1910年至1957年的矿山关闭后形成。整个ICB八个CMD站点的历史和近期水质数据，加上上覆岩性的矿物学和阳离子交换能力，进行了分析，以量化重要的反应物，并评估时空水质趋势。随着覆盖层厚度和停留时间沿着盆地东北到西南约50公里的流路增加，CMD变得更碱性，和Na浓度增加。自1970年代以来,所有八个ICB放电都变得酸性较低，随着酸度的指数下降，SO4和Fe浓度；只有两个CMD保持净酸性（平衡时的酸性pH）。指数衰减模型包括与背景地下水化学和菱铁矿平衡一致的稳态渐近线，描述了早期阶段，在“第一次冲洗”（初始洪水）和向后期背景条件的逐步演变之后，污染物浓度立即迅速衰减。地球化学演化PHREEQC模型表明，pH值的时空趋势，净酸度，SO4,Fe,和主要阳离子可以通过环境地下水对第一次冲洗水的连续稀释以及涉及黄铁矿和碳酸盐（方解石，白云石,菱铁矿)加粘土的阳离子交换(伊利石，绿泥石,伊利石/蒙脱石混合层)。这些数据和模型结果表明，1）阳离子交换反应增强了方解石的溶解和碱度的产生，导致CMD演变为Na-SO4-HCO3型水域，和2）菱铁矿平衡可以在未来40年内保持溶解的Fe>16mg/L。
