A MAP OF GEOLOGIC UNITS IN PENNSYLVANIA WITH THE POTENTIAL TO PRODUCE ACIDIC DRAINAGE

Acidic drainage in Pennsylvania has historically been associated with coal mining. Recent highway construction has made it clear that acidic drainage is not limited to areas of coal-bearing rocks. The Pennsylvania Departments of Conservation and Natural Resources, Environmental Protection, and Transportation have cooperated in producing a map that can alert developers to areas where rocks may pose a threat of acidic drainage. The map also shows acid mine drainage (AMD) impacted streams. These streams are restricted to areas with Pennsylvanian Period strata in western PA and the Anthracite Region. In western Pennsylvania the association is primarily with the Allegheny and Monongahela Formations, which historically have been the main coal-producing formations. Other formations with acid-producing potential include the Marcellus (central and eastern PA), the Pickering Gneiss (southeastern PA), the Hardyston (in Lehigh County), and hydrothermal sulfide deposits (central and southeastern PA). The Wisconsinan glacial margin is shown because glaciation influenced the chemistry of geologic materials in two ways. It has long been known that the alkaline tills of northwestern PA, if present, can prevent acidic drainage. This is evidenced by the lack of AMD impacted streams even in the presence of the Allegheny Formation in northwestern PA. The glacial margin is also significant because south of the margin there is a nearly ubiquitous oxidized zone that ranges from ~7 m to 30 m thick. Because of oxidation, sulfides are absent and disturbance of this zone will not cause acidic drainage. Within the glacial margin, however, the oxidized zone can be absent due to glacial erosion and sulfides may be encountered at shallow depth. The map was developed with geographic information system (GIS) software. Geologic data layers of the selected potentially acidic units were derived from the 1:250,000 scale digital data sets of bedrock geologic units of the Pennsylvania Geological Survey. This digital data set is based on the 1980 Geologic Map of Pennsylvania (Berg and others, 1980), which delineated 194 bedrock geologic units. The digital data set included 195 geologic units, which closely correspond to those of the 1980 map. Selection of the layers of the potentially acidic units was based on experience of the three cooperating agencies.