PITTSBURGH — New research in Pennsylvania demonstrates that it’s hard to nail down how often natural gas drilling is contaminating drinking water: One study found high levels of methane in some water wells within a half-mile of gas wells, while another found some serious methane pollution occurring naturally, far away from drilling.

The findings represent a middle ground between critics of the drilling technique known as hydraulic fracturing who claim it causes widespread contamination, and an industry that suggests they are rare or nonexistent.

The contamination from drilling is “not an epidemic. It’s a minority of cases,” said Rob Jackson, a Duke University researcher and co-author of the study. But he added the team found serious contamination from bubbly methane is “much more” prevalent in some water wells within 1 kilometer of gas drilling sites.

Methane is an odorless gas that is not known to be toxic, but in high concentrations it can be explosive and deadly.

The Duke paper, published in the Proceedings of the National Academy of Sciences, is an expansion of a 2011 study that attracted widespread attention for its finding that drilling was polluting some water wells with methane. The new study includes results from 141 northeastern Pennsylvania water wells. It found methane levels were an average of six times higher in the water wells closer to drilling sites, compared with those farther away. Ethane, another component of natural gas, was 23 times higher in the homes closer to drilling.

Some of the methane was at dangerous levels. The study found 12 homes with levels above the recommended federal limit of 28 milligrams per liter, and 11 of those water wells were closer to gas drilling sites.