Some Coloradans can light their water on fire: methane natural gas has penetrated groundwater at 64 percent of sites state regulators tested since 1988 in the northeastern part of the state, University of Colorado researchers have found.

But more than 95 percent of this gas came from naturally occurring microbial processes, often near shallow underground coal seams — not the oil and gas industry.

Methane-tainted groundwater can lead to flammable drinking water trickling from household taps.

The CU study — funded by the National Science Foundation and based on an analysis of Colorado Oil and Gas Conservation Commission records — concluded that the industrial process of hydraulic fracturing, or fracking, is not a primary cause of methane contamination of groundwater.

Methane leaking from oil and gas well bores contaminates groundwater about two times a year on average. And the CU researchers found that rate has stayed about the same since 2001. From 2001 through 2014, dissolved methane linked directly to geological formations holding oil and gas reached 42 water wells in 32 cases, the researchers found.

The findings have been published in the Proceedings of the National Academy of Sciences journal.

“People are concerned about being able to light their tap water on fire. In most cases, this is caused by the presence of methane in groundwater,” said CU scientist Owen Sherwood of the Institute for Arctic and Alpine Research, lead author of the study.

“It’s important to know where the methane is coming from and if it has anything to do with oil and gas wells,” Sherwood said.

“Dissolved methane is quite common in groundwater of the Denver-Julesburg Basin, particularly in a specific aquifer known as the Laramie-Fox Hills aquifer. This aquifer happens to be inter-bedded with coals and the coals are a source of naturally occurring, microbial methane,” he said. “However, accidents do happen in which methane and other gases, such as butane and propane, from faulty oil and gas well bores make their way into groundwater.”

A boom in oil and gas production across northeastern Colorado, using fracking to stimulate production and horizontal drilling, began around 2010 and has raised concern about contamination of groundwater due to leakage from oil and gas well bores. Opponents have circulated videos of tap water that could be ignited.

At 924 water wells where COGCC regulators conducted tests, methane was detected in 593 wells, the study found.

The researchers relied on ratios between carbon and hydrogen stable isotopes and gas molecules to determine that most of the methane in groundwater came from microbes within shallow coal seams.

They said the industrial leaks happened due to inadequate casing, typically hollow steel pipe used to line the inside of the drilled hole. Older gas wells, drilled in the 1970s, typically were cased to a depth of about 300 feet, leaving groundwater aquifers vulnerable to contamination from leaks. Companies today are required to case wells to deeper levels and repair older wells.