Abstract

Geologic mapping, petrographic analysis, and Acid-Base Accounting testing of rocks along the corridor of a transmission line in the Blue Ridge Province of southwestern North Carolina indicated a potential for acid production in certain areas related to road and tower construction. The potential acid-producing rocks are graphite schist units in the Great Smoky Group (Anakeesta Formation), graphitic metasiltstones of the Murphy Belt (Nantahala Formation), and thin layers of sulfidic rock within the Great Smoky Group (Ammons Formation—Horse Branch Member). Net Neutralization Potential (NNP) for the graphite schist units ranges from −19.27 to 1.81 tons CaCO3 per 1,000 tons of excavated material (an NNP value of −5 tons CaCO3 or less is interpreted as predicting acid drainage). NNP for the graphitic metasiltstones of the Nantahala Formation range from −43.56 to −11.39 tons CaCO3 per 1,000 tons of material. The NNP for a sulfide-rich layer in the Ammons Formation is −26.03 tons CaCO3 per 1,000 tons of material. Treatment for the potential acid-producing material involves stockpiling topsoil, mixing agricultural lime with the excavated material in a ratio based on the lowest NNP value for the particular rock unit, and covering the excavated material with a least 2 (two) ft of stockpiled topsoil to isolate the sulfidic material from the atmosphere, retarding the oxidation of the sulfides. When the treated material is spoiled in a roadbed, a base of limestone aggregate is placed on the existing roadbed, followed by the treated, compacted material with additional lime added after every lift. This is followed by topsoil (compacted) to encapsulate the limestone base and treated material, with limestone aggregate applied as the final road surface. Drainage is provided to divert water from the fills and tower foundation excavations. A monitoring program, in-place before, during, and after construction, was used to identify water-quality variations that might indicate ail effect on stream chemistry from disturbance of potential acid-producing materials. Results of the monitoring program suggest that the methods used to identify and treat the materials have been effective in minimizing the impact of their disturbance on the environment.

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