The quality of well water depends on geological events from 400 million years ago, according to a new U.S. Geological Survey study of more than 4,800 wells.

The study identifies bedrock formations throughout New England that are most likely to hold certain contaminants.

The study documented 14 contaminants of concern, including harmful levels of not just arsenic, but uranium, radium, radon and manganese in groundwater. The contaminants are associated with cancer, reproductive and developmental problems, kidney and blood diseases, diabetes and a weakened immune system.

Sarah Flanagan, the study’s author, said it underscores the need to have well water tested to see whether a filtration system is needed to avoid health concerns.

“The underlying geology is the biggest impact,” Flanagan said.

For area residents, the presence of three rock types in the region known as the Central Maine Basin are important: shale,granite and the sedimentary rocks that make up the basin.

“We’re smack in the middle of the Central Maine Basin,” said Doug Reusch, geology professor at University of Maine’s Farmington campus.

The Central Maine Basin is about 430 million years old. It extends into Aroostook County. It is known for light-colored rocks called calcareous metasedimentary materials. They are, as the name implies, formed from sediments at the bottom of the ocean.

“If you were to drop some hydrochloric acid on them, that would fizz a little bit,” Reusch said.

The calcium carbonate that provides the fizz comes from ancient reefs that grew in the ocean south of the equator hundreds of millions of years ago. Over time, they have traveled from those exotic climes all the way to Maine.

Because calcareous sedimentary rocks are associated with arsenic, they have traditionally been thought of as something to avoid when drilling wells.

However, Reusch said the report has shaken that assumption.

“Before reading this report, I would have said stay away from central Maine, but here they are saying that other common rocks are bad, too,” Reusch said.

Shale is another rock that can be an indicator of contaminated water. Within the Central Maine Basin, an expanse of shale called the Temple Stream formation extends from Farmington to Rumford to Phillips.

Tom Eastler, professor of environmental geology at the University of Maine at Farmington, said different shades of black can be a telltale sign.

“Some of these layers are black,” Eastler said. “Those are the ones that you hope you don’t put a well into. Those are black shale turned into slate.”

As chemicals within the slate form rocky crystals, they leave open spaces behind. When water flows through those pores, it can deposit sulfur and iron, leaving rocks rusty.

“Some of them make the water so bad, the sulfur and iron are off the scale,” Eastler said.

Eastler said that it can be impossible to know whether a particular deposit of shale has undergone this process. His own water, he said, is of a high quality, while that of a neighbor’s is undrinkable.

“It’s pure luck,” he said, “absolute luck.”

Granite, which can be found in areas of New Sharon and Jay, is associated with radon, Eastler said.

Radon is generally considered to be a harmful, radioactive material that can cause cancer; but Eastler said the levels of radiation found in the rock aren’t harmful. In fact, he noted one study suggesting that a minor background level helps lower levels of cancer, morbidity and mortality.

The important thing, Flanagan said, is that homeowners have their wells tested. Many health-threatening contaminants can’t be detected by taste alone, and it can be impossible to draw conclusions from the environment.

“Even two wells side by side can have completely different water chemistry,” Flanagan said. “It’s just a complicated geologic history.”

In many cases, wells with contaminants can be equipped with filtration systems that will make them safe for human consumption. All 14 of the contaminants identified in the study can be reduced or eliminated through a variety of treatments.

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