Using stable isotopes (S, O) of sulfate to track local contamination of the Madison karst aquifer, Montana, from abandoned coal mine drainage

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• 作者：Christopher H. Gammons^a ; ^{cgammons@mtech.edu} ; Allison Brown^a ; Simon R. Poulson^b ; Thomas H. Henderson^c
• 刊名：Applied Geochemistry
• 出版年：2013
• 出版时间：April, 2013
• 年：2013
• 卷：31
• 期：Complete
• 页码：228-238
• 全文大小：1906 K

文摘

The Stockett-Centerville area, south of Great Falls, Montana, was extensively mined for coal in the late 1800s through the early 1950s. After closure, the underground mine portals were sealed and the workings became partially flooded. The coal seams are rich in pyrite, and many of the mines have horizontal drains that are point sources of acid mine drainage (AMD) which flow into dry streambeds and soak into the ground. The Mississippian Madison Limestone is located stratigraphically below the coal and is an important source of drinking water. Twenty-six domestic water wells completed in the Madison near the former coal mines were sampled for water chemistry and stable-isotope analysis of water and dissolved . Madison aquifer groundwater plots close to the meteoric water line and reveals no clear evidence of mixing with water from the mine drains, the latter being slightly enriched in ¹⁸O by evaporation. In contrast, dissolved in the wells has isotopic compositions that suggest mixing from three end-member sources: (1) regional Madison aquifer groundwater with moderate concentrations (110-150 mg/L) that is isotopically enriched (¦Ä³⁴S = 20¡ë; ¦Ä¹⁸O = 14¡ë); (2) AMD water with very high concentrations (>1.9 g/L) that is isotopically depleted (¦Ä³⁴S = ?15¡ë; ¦Ä¹⁸O = ?11¡ë); and (3) local recharge water with low concentrations (<80 mg/L) and intermediate SO₄-isotope characteristics (¦Ä³⁴S = ?10¡ë; ¦Ä¹⁸O = ?6¡ë). The percentage of derived from AMD is estimated to range from 0 % to nearly 80 % . Wells with the highest proportion of AMD have higher concentrations, but otherwise show no mining-associated water quality problems, such as low pH or elevated concentrations of trace metals, underscoring the ability of the Madison aquifer to buffer water quality. Most of the wells with evidence of AMD incursion are older wells that were not cased over their entire depth. It is possible that the open-hole method of completion has allowed some AMD-related water to infiltrate by gravity to the lower Madison aquifer. In this study, stable isotopes of proved to be a more powerful and sensitive tool to track AMD contamination compared to stable isotopes of water or conventional water-quality sampling.