Abstract
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 the cores. Pennsylvania guidelines for highway construction require a drillhole every 500 ft (152 m). However, in areas of near-vertical beds or steeply dipping veins, sizeable pyritic zones can easily be missed. Induced polarization (IP) is a geophysical method that can be used to detect pyritic zones before excavation. The purpose of this article is to report the results of IP surveys of hydrothermal pyrite veins in the sandstones and shales of the Appalachians. IP is an electrical geophysical method developed by the mining industry to prospect for disseminated sulfide deposits. IP measures chargeability, the ability of a material to retain electrical charge. In general, rocks with pyrite contents of approximately one percent or greater have elevated chargeabilities. Although the potential value of IP in screening highway corridors for potential ARD problems has been known for some time (Jones et al., 1982), the technique is not widely used. To our knowledge, all of the published case studies (Jones et al., 1982; Anderson et al., 1990; Byerly, 1996; Butler et al., 2004) discuss the detection of pyritic zones in meta-sedimentary rocks. Although these studies were successful in locating some pyritic zones, they were unable to quantify the relationship between IP response …
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