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 released Monday. But he added that the team found serious contamination from bubbly methane is "much more" prevalent in some water wells within one 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 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. Jackson said the researchers believe that faulty drilling can cause methane pollution, but that natural causes can, too. Eighty percent of all the water wells they tested contained some level of methane, including many with no nearby drilling.

In 2011, Pennsylvania strengthened rules for the steel casing and cement around the top of a gas well that are meant to protect water supplies from contamination, but some older wells weren't drilled to those standards.

There was some good news, Jackson said: The Duke researchers haven't found any evidence that chemicals from hydraulic fracturing, or fracking, have contaminated water wells.