MOUNT VERNON — Living in the lush, wooded countryside with fresh Maine air, Wendy Brennan never imagined her family might be consuming poison every day.

But when she signed up for a research study offering a free T-shirt and a water-quality test, she was stunned to discover that her private well contained arsenic.

“My eldest daughter said … ‘You’re feeding us rat poison.’ I said, ‘Not really.’ But I guess essentially … that is what you’re doing. You’re poisoning your kids,” Brennan said. “I felt bad for not knowing it.”

Recent research suggests even small levels of arsenic may be harmful. In fact, the U.S. Environmental Protection Agency has been prepared to say since 2008 that arsenic is 17 times more toxic as a carcinogen than the agency now reports. Women are especially vulnerable. EPA scientists have concluded that if 100,000 women consumed the legal limit of arsenic each day, 730 of them eventually would get lung or bladder cancer.

The EPA, however, hasn’t been able to make its findings official, an action that could trigger stricter drinking water standards. The roadblock: a single paragraph inserted into a committee report by a member of Congress, an investigation by the Center for Public Integrity found. The paragraph essentially ordered the EPA to halt its evaluation of arsenic and hand over its work to the National Academy of Sciences.

The congressman, Mike Simpson, an Idaho Republican, said he was concerned that small communities couldn’t meet tougher drinking water standards, and he questioned the EPA’s ability to do science. But a lobbyist for two pesticide companies acknowledged to the Center for Public Integrity that he was among those who asked for the delay. As a direct result of the delay, a weed killer that the EPA was going to ban at the end of 2013 remains on the market.

The tactic is among an arsenal of tricks used by industry and lawmakers to virtually paralyze EPA scientists who evaluate toxic chemicals. In 2009, President Obama signed an executive order seeking to stop political interference with science. The EPA unveiled a plan to evaluate far more chemicals each year than had been done during the George W. Bush or Bill Clinton administrations. In the past two years, however, it’s completed only six.

It’s now unclear when the agency’s arsenic review will be finished, even though scores of studies have linked arsenic not just to cancer, but also to heart disease, diabetes and strokes.

Meanwhile, people like Brennan, who lives with her two daughters and two grandchildren, are left to worry about all the arsenic-tainted water they’ve consumed. Brennan participated in a study by Columbia University researchers, who found levels of arsenic in her well water that were more than five times the federal standard. The researchers also reported that children who drank water containing arsenic – even at levels that met the federal standard – scored six points lower on IQ tests than children who drank clean water.

“Your job as a mother is to give your kids the best,” said Brennan, who installed an $800 filter that removes arsenic from her water. “Just by giving my kids juice … giving them cups of water, which you are supposed to do, I was actually giving them a sediment that’s settling in their body, and I may not know for 10 years if it’s affected them.”