Recent drilling for natural gas in the Glaciated Appalachian Plateau area of northwestern Pennsylvania has caused limited, but increasing ground‐water contamination. By evaluating hydrogeologic parameters at a proposed gas well site, such as the ground‐water flow system, permeability of surficial sediments, and the presence of fracture zones, the contamination hazard of the site can be assessed. Three case studies document that the most hazardous sites are generally located on or near valley walls of major drainageways. The relatively steep hydraulic gradient, the frequent presence of highly permeable surficial sediments, and the low to moderate dilution of contaminants along the intermediate‐length flow paths at these sites all contribute to a relatively high pollution hazard. In addition to locating gas wells in high‐hazard hydrogeologic zones, allowing the annulus of gas wells to become pressurized is the other major factor contributing to aquifer contamination.

Medina gas wells and oil wells in northwestern Pennsylvania, northeastern Ohio, and western New York create a potential for contamination of the fresh‐water aquifers that overlie the production zones of these wells. Most of these wells are constructed in a manner which results in an open annulus which is a few hundred to a few thousand feet long below the surface casing of the well. This annulus is a potential avenue of migration of contaminants from strata of higher hydrodynamic pressure into formations of lower hydrodynamic pressure. If gas from the strata exposed to the annulus is not permitted to escape to the atmosphere, the annulus may become pressurized, and a hydraulic gradient may be created between the potential contaminants in the annulus (e.g., brine and/or natural gas) and the overlying fresh‐water aquifers. If a permeability pathway exists between the pressurized annulus and an overlying fresh‐water aquifer, contamination of the aquifer will result. The risk of contaminating fresh ground water with the contents of a gas‐ or oil‐well annulus could be greatly reduced by filling the annulus with cement. An alternative precaution would be to operate the well in a manner that does not allow the annulus pressure to exceed the normal pressure of the formations exposed to the annulus.

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The orientation of streams in the Geneva quadrangle, northwestern Pennsylvania, was studied in order to determine if their orientation has been influenced by fracture traces. Of the 151 visible fracture traces in the study area, 83% are oriented N30°W–N60°W or N30°E–N60°E. Measurement of over 2500 stream segments revealed that 43% of the segments are oriented 30–60° east or west of north, whereas random orientation of streams would result in only 33% of the segments having these trends. This preferred orientation is statistically highly significant (0.001 level). Contrary to expectations, the degree to which stream orientation was influenced by fracture traces does not appear to decrease with increasing drift thickness within the 0–500 ft (0–152 m) range present in the quadrangle.