Acid rock drainage (ARD) is a major environmental problem in Pennsylvania. ARD occurs when iron sulfides, most commonly pyrite, oxidize to form sulfuric acid. This weathering reaction typically occurs slowly in undisturbed pyritic rock. When this rock is excavated, however, the greater surface area and availability of oxygen enable iron-oxidizing bacteria to flourish, increasing the reaction rate by many orders of magnitude (e.g., Singer and Stumm, 1970). Although the majority of ARD problems are due to coal mining, ARD can also be caused by highway construction. Typical characterizations can involve drilling test holes and carrying out acid-base accounting on...

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