A new USGS study found that 15 percent of groundwater samples from 66 household wells across south-central New York contained methane at levels high enough to warrant monitoring or mitigation. Methane was more common and had higher concentrations in wells located in valleys rather than wells located in uplands. Valley wells that tap confined bedrock were the most likely to contain high levels of methane.
The wells were tested in June and July of 2012 to provide baseline data on methane concentrations in groundwater. The study area includes about 2000 square miles and is considered favorable for Marcellus Shale gas resources. None of the water wells tested was located within a mile of existing natural gas wells.
“This study provides baseline water-quality data that will be useful for decision makers, regulators, industry, and stakeholders concerned about water quality,” said USGS scientist Paul Heisig, who led the study. “It also provides a preliminary framework to help characterize the occurrence of methane in south-central New York that can be refined as additional data are collected,” said Heisig.
“Our testing shows that methane is common in some groundwater wells at the present time. Sampling also shows that methane is not random in groundwater across the landscape,” said Heisig. “The strong pattern of occurrence and the high variability in concentrations that we found shows where we should and shouldn’t expect high methane concentrations in groundwater,” said Heisig.
Methane levels varied by more than five orders of magnitude across the study area covering parts of Broome, Tioga, Chemung, Chenango, and Delaware counties. The samples most likely to contain high methane concentrations were from the valley wells that tap into bedrock groundwater lying beneath glacial clay deposits. More than 50 percent of such samples contained methane at or above the 10-milligrams-per-liter monitoring level. Like a cap on a soda bottle, clay blocks methane gas from escaping into the atmosphere.
Nearly 30 percent of groundwater samples from valleys tested at or above the recommended monitoring level — no samples from wells in uplands exceeded that level. Methane in valley groundwater was mostly thermogenic in origin, derived over millions of years by processes deep within the earth that produce fossil fuels. In contrast, methane found in upland groundwater was mostly generated by bacteria.
Concerns about high methane in household wells focus on the potential of this colorless and odorless gas to accumulate in confined spaces. High concentrations of methane can be flammable or explosive in confined spaces containing oxygen and an ignition source such as an open flame or electrical spark. Mitigation typically consists of enhanced venting of well casings. Private well owners should consider testing their water for methane particularly if their well is in a valley and is not a shallow sand and gravel well. Information on testing and remediation is available online.