PITTSBURGH – In at least four states that have nurtured the nation's energy boom, hundreds of complaints have been made about well-water contamination from oil or gas drilling, and pollution was confirmed in a number of them, according to a review that casts doubt on industry suggestions that such problems rarely happen.

Data on drilling-related complaints in Pennsylvania, Ohio, West Virginia and Texas indicate major differences in how the states report such problems.

Texas provided the most detail, while the other states provided only general outlines. And while the confirmed problems represent only a tiny portion of the thousands of oil and gas wells drilled each year in the United States, the lack of detail in some state reports could help fuel public confusion and mistrust.

Pennsylvania received 398 complaints in 2013 alleging that oil or natural gas drilling polluted or otherwise affected private water wells, compared with 499 in 2012. The Pennsylvania complaints include allegations of short-term diminished water flow, as well as pollution from stray gas or other substances. More than 100 cases of pollution were confirmed over five years.

Just hearing the number of complaints shocked Heather McMicken, an eastern Pennsylvania homeowner who complained about water-well contamination that state officials eventually confirmed.

"Wow, I'm very surprised," said McMicken, recalling that she and her husband never knew how many other people made similar complaints, since the main source of information "was just through the grapevine."

The McMickens were one of three families that eventually reached a $1.6 million settlement with a drilling company.

Over the past decade, hydraulic fracturing, or fracking, has led to a boom in oil and natural gas production. It has reduced imports and led to hundreds of billions of dollars in revenue for companies and landowners, but also created pollution fears.

Extracting fuel from shale formations requires pumping water, sand and chemicals into the ground to break apart rock and free the gas.

Some of that water, along with large quantities of existing underground water, returns to the surface, and it can contain high levels of salt, chemicals, heavy metals and naturally occurring, low-level radiation.

But some conventional oil and gas wells are still drilled, so the complaints about water contamination can come from them, too. Experts say the most common pollution involves methane, not chemicals from the process.

Some people who rely on well water near drilling operations have complained about pollution, but there's been considerable confusion over how widespread such problems are. For example, starting in 2011, the Pennsylvania Department of Environmental Protection aggressively fought efforts by news organizations to obtain information about complaints related to drilling.

The department has argued in court filings that it does not count how many contamination "determination letters" it issues or track where they are kept.

Steve Forde, a spokesman for the Marcellus Shale Coalition, the leading industry group in Pennsylvania, said in a statement that "transparency and making data available to the public is critical to getting this historic opportunity right and maintaining the public's trust."

When the state Environmental Department concludes natural gas development causes problems, Forde said, "our member companies work collaboratively with the homeowner and regulators to find a speedy resolution."

In Pennsylvania, the number of confirmed instances of water pollution in the eastern part of the state "dropped quite substantially" in 2013, compared with previous years, Department of Environmental Protection spokeswoman Lisa Kasianowitz said. Two instances of drilling affecting water wells were confirmed there last year, she said, and a final decision hasn't been made in three other cases.

Experts and regulators agree that investigating complaints of water-well contamination is difficult, in part because some regions have natural methane gas pollution. A 2011 Penn State study found that about 40 percent of water wells tested before gas drilling failed at least one federal drinking water standard.