The U.S. Geological Survey (USGS) has tested well water in Wayne and Pike counties in northeast Pennsylvania and found that contains low-to-moderate concentrations of naturally occurring methane. The study area has never been developed with oil and gas wells, either by conventional methods or by hydraulic fracturing.
It has long been known that methane gas occurs naturally in groundwater aquifers in many geological sedimentary basins. Methane often gas exists at low concentration dissolved in groundwater underground and will “bubble out” when pumped to the surface. For those on private water well supplies, spurting taps is a typical indication of this phenomenon and can be a hazard without proper venting. . Methane present in groundwater can be a result of biogenic activity or can be from coal gas beds or from deeper shale gas. Biogenic methane is produced by subsurface bacteria and commonly occurs naturally in groundwater aquifers used for water well supplies. The potential presence of methane is why modern sanitary well caps have screened vents.
Scientists have found and investigated methane in drinking water wells near fracked gas wells in the Marcellus Shale, but before now had no baseline study of methane in areas befre they were fracked. Fracking or hydraulic fracturing as it is more properly known involves the pressurized injection of fluids commonly made up of mostly water and chemical additives into a geologic formation. The pressure used exceeds the rock strength and the fluid opens or enlarges fractures in the rock. As the formation is fractured, a “propping agent,” such as sand or ceramic beads, is pumped into the fractures to keep them from closing as the pumping pressure is released. The fracturing fluids (water and chemical additives) are partially recovered and returned to the surface or deep well injected. Natural gas will flow from pores and fractures in the rock into the wells allowing for enhanced access to the methane reserve.
While geologists and engineers believed that in hydraulic fracturing the intervening layers of rock prevent a fissure from extending into the water table, this had not been studied and there were reported instances of contamination of drinking water wells in areas that had been fracked most famously in the less than scientific movie “Gasland.” Only in the past four years has the potential to contaminate drinking water wells been studied. In a small group of studies (see Jackson, et al) that were primarily in the Marcellus region of Pennsylvania, peer-reviewed studies found no evidence of salts, metals, or radioactivity beyond naturally occurring concentrations in drinking water wells near shale gas wells. However, they did find increased levels of methane in groundwater wells.
In Wayne County, about 65% or 22 of the 34 private drinking-water wells tested were found to contain dissolved methane. Most of the methane levels were low, below 100 parts per billion, but levels as high as 3,300 parts per billion were observed.
In the Pike County about 80% or 16 of 20 tested wells contained methane, with two wells having methane concentrations greater than 1,000 parts per billion and as high as 5,800 parts per billion. The concentrations of dissolved methane in about 10 percent of well-water samples in both studies were high enough to allow for isotopic analysis to identify the type of natural gas in the water. In Pike County, the isotopic composition of two methane samples indicated that methane was predominantly microbial in origin. In Wayne County, the isotopic composition of three methane samples indicated a thermogenic origin and a mixture of microbial and thermogenic types of methane.
None of the wells tested in either study were located near currently producing natural gas wells. Both Wayne and Pike counties are within the Delaware River Basin, where a moratorium on shale-gas drilling is in place. These studies show that naturally occurring methane can be found in drinking water wells in areas where no unconventional natural gas development is occurring. The studies also provide background information on other aspects of groundwater quality, such as arsenic, barium, chloride, and radon concentrations so that the impact of shale-gas development on groundwater can be compared to the baseline. Currently, the USGS is continuing to collect data on baseline groundwater quality in areas in Pennsylvania underlain by the Marcellus Shale.