USE OF CARBON AND HYDROGEN STABLE ISOTOPES FOR SOURCE DETERMINATION OF NATURAL GAS IN GROUND WATER NEAR TIOGA JUNCTION, TIOGA COUNTY, PENNSYLVANIA

In 2001, residents near Tioga Junction, Pennsylvania reported gas bubbles in their well water. Analytical data revealed this gas as methane with minor concentrations of ethane in some instances. Several potential sources of the gas occur in the area including a natural gas storage field for imported natural gas in sandstone of the Oriskany Group (a former native gas production zone) about 3,900 ft below land surface, abandoned natural gas wells, and microbial gas associated with the shallow aquifer system. The U.S. Geological Survey, in cooperation with the Pennsylvania Department of Environmental Protection, investigated the source of the gas in the water wells. Carbon and hydrogen isotopes of methane and carbon isotopes of ethane have been documented as reliable indicators of the origin of different gases, whether thermogenic (such as imported storage-field gas and native Oriskany gas) or microbial (which can be generated by buried and decomposed organic material in sediments). 37 water wells and 8 gas wells were sampled and gases were collected for analysis of isotopes of methane and ethane. The gas-well data indicate the native Oriskany gases and the storage-field gases have significantly different isotopic signatures. The carbon and hydrogen isotopic compositions of methane and carbon isotope compositions of ethane in water from 21 wells indicate thermogenic gas, and they more closely resemble imported storage field gas than the native Oriskany gas. Gas samples from 13 wells appear to have a microbial gas isotopic signature. All of the water samples that indicate a predominately microbial gas isotopic signature are from wells open to fractured-rock aquifers with no mapped glacial overburden present. These wells may have a mixture of thermogenic and microbial gas or may have only microbial gas. Seven water well samples that are drilled into the glacial outwash aquifer display modern carbon in a carbon-14 analysis, indicating the presence of recent microbial gas; however, the overall isotopic composition of methane and ethane clearly indicate the gas in these 7 well samples as primarily thermogenic gas with only a small amount of microbial gas present. There appears to be little or no correlation with the type of hydrogeologic environment or aquifer supplying water to the wells sampled and the presence of thermogenic gas.