More Pennsylvanians - some 3.5 million total - rely on well water than residents of 48 other states.
Yet, along with Alaska, Pennsylvania is among the only states that lack regulations on how those wells can be built.
And while water-testing kits are readily available or those concerned about their wells' safety, hundreds of thousands of well owners neglect to perform even basic maintenance.
"That, to me, is very disturbing," said Dana Rizzo, a Westmoreland County-based educator with Penn State Extension. "There are still a lot of people who either don't know or don't want to know."
Without mandated testing, many well users don't realize the prevalence of often-dangerous bacteria in underground wells - 14 percent contained E. coli bacteria, according to a 2009 study commissioned by the Pennsylvania General Assembly.
"I ask people, 'Have you ever had an upset stomach? Have you ever had diarrhea? How do you know it isn't your water?'" Rizzo said.
She compared some cases to vacations in Mexico: In some cases, residents might not feel any effects, but guests can easily become sick from drinking contaminated water.
Less common contaminants, like lead, arsenic and nitrates, can still do serious damage if left unchecked, Penn State Extension water expert Bryan Swistock said.
In many cases, there's almost no way of realizing contaminants might be present without testing, he said.
Among the regulations Pennsylvania lacks are controls on well cap construction, he said. A well's cap, when properly built, can keep insects and rodents from making their way into the water supply.
"But we really lack any of those statewide requirements," Swistock said.
The issue has been raised repeatedly in the state Legislature, he said, but each time it's been shot down or ignored.
The opposition stems in part from a rural mentality that over-regulation is dangerous, Rizzo said.
"It's a fear, it's an uneasiness, it's a concern about the whole Big Brother thing," she said, comparing the opposition to septic-tank users who refuse to accept public sewage, even when their water supply is at risk.
Hope for increased well testing, however, has come from an unusual place: the Marcellus Shale boom.
With health concerns related to natural gas drilling highly publicized across the state, more well owners have begun testing their water, Swistock said.
"It's created a lot of awareness," he said. "There's been a lot of testing, especially in areas where there's drilling in the Marcellus and Utica shale."
With well safety an ongoing issue, the state Department of Environmental Protection is scheduled to hold an online seminar for well users from 7 to 8 p.m. Sept. 19, according to a news release from state Rep. Jerry Stern, R-Martinsburg.
The seminar is set to include lessons on avoiding malfunction, preventing contamination and and conserving water.
"Probably half the residents of Pennsylvania still have private wells," Stern said Friday. "And everyone thinks their well is in perfect operating order, and that their water is the best in the world."