Potential Acidic Rock Units in Pennsylvania
Typical geologic weathering of undisturbed rock is a slow process involving the natural stability of minerals on the Earth’s surface. In most cases in Pennsylvania, acidic drainage involves iron sulfide minerals such as pyrite and its exposure to air to create iron oxides and acidic water. Naturally occurring acute acid drainage is uncommon in Pennsylvania. In every significant case, it is the exposure of iron sulfides to air by dropping the water table or excavating rock, the lack of any inherent buffering capability, and the flow of water through the rocks that create acidic drainage.
The chemistry of acid drainage (AD) is understood, and prediction of AD is possible through a combination of several methods. Experience and understanding of specific geologic formations and combinations of geologic units on a regional and local scale are used to assess the potential for acidic drainage. Quantitative onsite prediction of acidic drainage has become possible through the geochemical analysis of the rocks. Calculations of acid-base balance based on neutralization potential and potential acidity can be used to quantitatively predict whether disturbance of a site will cause AD. Such calculations in conjunction with site history and experience, along with judgments with regard to geology and site hydrology, allow accurate predictions to be made.
Although specific geologic units are known to possibly cause problems, numerous factors influence the outcome of mining or excavation of the rock unit. The presence of buffering substances (mainly calcareous minerals) is a chief factor in preventing the development of AD. The occurrence of calcareous minerals is related to the rock type and the depositional environment of rocks at the site. Calcareous minerals can prevent the occurrence of AD in rocks that contain acid-forming minerals. In addition, the kinetics of the reaction may not allow AD to be seen for years.
Coal-bearing rocks of Pennsylvania have been a particular source of AD. However, all coal units and associated formations are not equal sources. Some geologic units are typically AD free; others are commonly sources of AD. Some units will produce AD depending on the overburden composition at the site.
In the Appalachian plateaus of western Pennsylvania, the extent of knowledge is typically based on the mining of coal. Less is known about units that have not traditionally been mined. For specific information about coal mine prediction of these mainly coal containing units, see the report Coal Mine Drainage Prediction and Pollution Prevention in Pennsylvania (Brady and others, 1998). The source of acidity (iron sulfides) is typically depositional, although structural features may have enhanced the occurrence of sulfides.
The abundance and distribution of sulfides in the Appalachian plateaus are often related to depositional environment, whereas the sulfides in the Ridge and Valley physiographic province have been more affected by Alleghanian structural features (faults, fractures, lineaments, etc.) and fluid movement. To the southeast in the Piedmont, structural complexities and metamorphism combine to defy simple explanation of the occurrence of sulfide mineralization. Where sulfides are not areally large or discontinuous, scattered point occurrences of sulfide mineralization are shown on the map.
This map indicates formations that may have acid-forming minerals, primarily pyrite. Although this map is useful for general planning and preliminary site studies, it does not substitute for site-specific subsurface investigations of the rock units that will be disturbed through excavation, mining, or drilling. The occurrence of AD depends on numerous factors, including rock type, mineralogy, geochemistry, geologic structure (e.g., fractures, joints, and faults), changing the water table, surface and sub-surface hydrology, extent of geologic weathering, and depositional environments.
In addition, the extent of Wisconsinan glaciation is indicated on the map. The Wisconsinan glaciation may have removed weathered (oxidized) rocks. Thus the "inert" oxidized zone that typically blankets most rocks in Pennsylvania may be thin or absent in glaciated areas. The effect of older glacial episodes on the oxidized bedrock is not well understood.
The map indicates geologic units and points that may have the potential to generate AD. It of course does not guarantee that AD will occur or conversely, that AD cannot occur in other areas and formations not shown on this map. The occurrence of AD, therefore, is not restricted to the areas delineated on the map.
Site-specific assessment of AD potential is the only reliable way to predict it. Pre-site investigation data is often available from previous studies, including college theses, consultant reports, geologic survey reports, aerial photographs, existing geophysical surveys, and the like. There is, however, no substitute for site specific information including interviews with local residents, geologic logs of borings, analysis of site geochemistry (water and rock), and other sources of information.
General guidelines on recognizing potential acid-producing rock in Pennsylvania
The presence of sulfide-bearing rock formations and isolated occurrences of sulfide deposits in Pennsylvania depends on a wide variety of factors including the rock’s depositional and structural history, its mineralogy and geochemistry, and present surface and subsurface hydrologic and geochemical environment.
The common iron-sulfide minerals pyrite (FeS2) and less common pyrrhotite (FeS) and marcasite (FeS2) are the dominant sources of acidity. However, specific buffering minerals can offset their presence, and the groundwater and surface water setting can control the development of AD.
In the Appalachian Plateaus physiographic province, the source of acidity (iron sulfides) is typically depositional, although structural features such as faults and lineaments may enhance the occurrence of sulfides or flow of groundwater. Sulfide minerals are associated with coal-bearing rocks, some underclays, and black shales.
Every coal-bearing unit in western Pennsylvania has some potential for AD. Potentially acid-producing rocks include underclays beneath coals, the coal beds themselves, or rocks of the coal overburden. Black shales may contain significant quantities of iron sulfides. The overburden of coals associated with marine or fresh water units are less likely to yield AD than marginal marine (brackish) units due to the association of limestones and other calcareous units with marine and freshwater environments.
Some geologic units may contain only moderate levels of pyrite, but lack buffering calcareous minerals. Sandstones in the Allegheny and Pottsville Formations and the Burgoon Sandstone fall into this category.
A history of AMD, presence of black shales, rocks with low amounts of calcareous minerals, or rocks with pyritic minerals are those that should be especially considered as having the potential to produce AD. However, an unmined area with no acidic drainage does not necessarily mean that there is not a possibility of AD when rocks are excavated or the water table drops and exposes acidic rocks.
Ridge and Valley
Dark shales, sulfide mineralized areas, fractured rocks, and rocks with little calcareous material are zones that should be considered as having the potential to produce AD in the Ridge and Valley physiographic province. This could include formations with thin dark shale layers that are not listed on the map (for example, the Mifflintown Formation in central Pennsylvania).
Dark shales commonly hold primary sulfide minerals derived from its specific depositional environment. The Tuscarora Formation, a sandstone, is the host of some secondary mineralization based on its proximity to a source of sulfide minerals, insignificant calcareous material, and its tendency to fracture. Mobile sulfide-bearing solutions amassed in secondary spaces of the rocks based on complex factors such as intersecting fractures and source fluid movement. Similar conditions may occur in carbonate rocks, but because of their dominant calcareous composition, they pose no known threat of AD.
Concentrated sulfide mineralization may be evident on the surface in the form of oxidized pyrite, leaving behind various iron oxide minerals as gossan. Newly excavated material beneath a gossan cap should be monitored for its ability to produce AD. The presence of sulfide minerals is typically apparent because of their production of iron oxide minerals. However, the potential of AD by rocks such as black shales that can have disseminated microscopic pyrite may be evident only through geochemical analysis.
The Anthracite Region is part of the Ridge and Valley physiographic province. Acid problems can occur when pyritic coal associated rocks are disturbed. The chemistry of the rock between the coals is poorly understood, but the presence of acid mine drainage from more than a century of mining indicates that the potential for acid drainage exists and caution should be exercised when excavating material. Piedmont and Reading Prong Some sulfide mineralization is associated with the Pickering Gneiss and other formations. However, lithologic and structural complexity and regional metamorphism combine to make prediction of sulfide mineralization difficult. It is unlikely that all mineralization has been discovered. Scattered point occurrences of sulfide mineralization attest to the wide distribution of such mineralized areas.
As noted previously, site-specific assessment of AD potential is the only reliable way to predict it.
Brady, K., Smith, M. W., and Schueck, J. (editors), 1998, Coal mine drainage prediction and pollution prevention in Pennsylvania, Pennsylvania Department of Environmental Protection, 375 p.
Miles, C. E., and Whitfield, T. G., compilers, 2001, Bedrock geology of Pennsylvania: Pennsylvania Geological Survey, 4th ser., dataset, scale 1:250,000.
Pennsylvania Geological Survey, 1995, Late Wisconsinan glacial border 1:100,000, Pennsylvania Geological Survey.
Pennsylvania Geological Survey, 1998, Preliminary landform subdivisions of Pennsylvania, Pennsylvania Geological Survey.
For questions regarding the map or the GIS data sets should be directed to Stuart O. Reese, P.G., 717–702–2028, Bureau of Topographic and Geologic Survey, 3240 Schoolhouse Road, Middletown, PA 17057.