Why are arsenic levels in well water so high in a certain part of Vermont?
Middlebury researchers are on the case.
By Joshua Brown
About the time animals first learned to live on land, the eastern edge of our continent crashed into a chain of offshore volcanoes.
As the North American plate sank under the oceanic plate, this arc of volcanic rock scraped onto shore. Rockies-style peaks thrust toward the sky, carrying layers of ocean sediment and strange slivers of the sea floor onto land that would eventually erode and become the gentle Green Mountains.
On an overcast Tuesday, maybe 450 million years later, Peter Ryan (above left ) and Kevin Bright ’06 (above right ) drive along Route 100. They’re headed for a hill near Stowe to see if it might have been made from one of these slivers of sea floor, a slippery green rock called serpentinite. More than geological curiosity moves them. They’re looking for the culprit in an unexpected public health problem.
“Most of the bedrock around here is schist, metamorphosed ocean sediment—like that,” Ryan says, pointing, as the car passes a grey road-cut near the Waterbury highway exit. But outcrops of serpentinite also have been found in this area, which is part of what geologists call Vermont’s Ultramafic Belt. Running the length of the state in a broken line where continents once collided, Ryan says these ultramafics—a family of minerals and rocks that include asbestos, talc, and soapstone—pop out like “bullet holes in the geologic map.”
Turning onto a gravel road, the car winds above Gold Brook and up to the home of Mary and Tom Evslin. Here, near the top of Barnes Hill, the Evslins have a splendid view of the nearby ski runs—and a drinking well with the highest arsenic readings of any water ever tested in Vermont.
Ryan, an associate professor of geology, and Bright, an environmental studies major, have a hypothesis about why. Based on Bright’s senior thesis research, they think the
Evslins’ well might have been drilled into ultramafic rock that is leaching arsenic into their groundwater.
Say “arsenic poisoning” and some may think of 15th- century Italians tipping vials of white powder into each other’s drinks in political assassinations, or the murderous fantasies of Dylan Thomas’s Mr. Pugh, who whispers, “Here’s your arsenic, dear,” as he brings his wife a cup of tea.
Though arsenic is a tasteless, odorless toxin, the Evslins are in no danger. They’ve installed a state-of-the-art filtration system that removes it. But even if they didn’t have the filter, the danger of arsenic in drinking water from
Vermont bedrock is not a sudden death over dinner; it’s cancer in decades.
“If you tell someone their water has elevated arsenic, their eyes bulge out,” says Jonathan Kim, a geologist for the state of Vermont, who has worked closely with Ryan on several environmental contamination issues and served as a partner on Bright’s investigation. “This is what I call a problem of social geology. The science may not translate well to the public. You’ve got to make it clear that the problems are not an acute risk today. It’s an elevated risk over the course of a lifetime.”
Since the 1970s, there has been an epidemic in Bangladesh, where thousands of people have developed diseases from arsenic in wells that, in a grim irony, were dug to avoid bacterial contamination in rivers and ponds. As data from that disaster make clear, risks from arsenic include skin lesions, cancer, and diabetes. But the full range of problems is still under investigation—just like the arsenic discovered in Stowe. Where else in
Vermont’s Ultramafic Belt might it be lurking, and what, exactly, is the source?
“Some schist in New Hampshire is known to have arsenic, so we thought we might find it in our schist,” Ryan says, “but 48 tests conducted by two other students showed no elevated arsenic in all the other bedrock formations around here.”
All the bedrock, that is, except the local Barnes Hill serpentinite. “Based on my study of geochemistry, that’s the last place I’d have thought to look,” Ryan says. “Kevin scoured the journals for links between ultramafic rocks and arsenic, and there’s almost nothing in the published literature about it.”
It seems Bright’s research has turned up something new: his tests of five Barnes Hill serpentinite rocks showed arsenic levels nearly 100 times higher than other rocks in the area; high enough to poison a well.
So why here? “It could be that this one ultramafic rock body was metamorphosed in a strange way, and its geochemical signature ended up being arsenic-rich because of something that happened deep in the crust, during mountain building, that is different elsewhere in the state,” Ryan says with a shrug. Then he laughs. “But that’s just arm waving. We don’t know.”
What’s not arm waving is the data Bright collected from 30 private wells in the Stowe area, including the Evslins’. Tests came back showing that their water had 275 parts of arsenic per billion parts of water. That may sound like a small amount, but it’s more than 25 times higher than the recently lowered EPA “maximum contaminant level” of 10 parts per billion. Two other wells that he tested were also above the limit.
Mary Evslin leads the researchers downstairs to look at an outcrop of rock Bright noticed in the cellar when he was collecting water samples earlier in the year. If it’s serpentinite, it might help tie together his water test results.
Evslin leads the geologists to a small finished room with a large dome of rock sticking through the carpet. “The builder got tired of blasting,” she says with a wry smile.
“This would be an ideal geologist’s office,” Ryan replies with his own smile, sizing up the rock. Then he looks at it closely. “Hmm, it looks like regular schist. I can see a micaceous layer.”
He and Bright look at each other quizzically. “Maybe this is just on the surface,” he says. “It could be your well goes through serpentinite farther down, an old fault or contact.” Other rocks sticking up from the lawn and piled in a nearby stone wall yield the same result: schist.
They wheel out of the driveway and up Barnes Hill Road. Cresting the hill, Ryan points to an outcrop by the side of the road. “What about that rock right there?” he asks.
“We could take a quick whack at it,” Bright says.
Geologists are literal, and soon they’re whacking at the rock with a hammer until a piece comes off. “It looks like schist. But might as well grab a sample,” Ryan says. “We’ll test it.”
They drive down the other side and back toward campus. “Well, we didn’t find ultramafic rock,” Ryan says, looking toward the mountains. “The mystery continues past today,” Bright says.
And the work. Jonathan Kim will be mapping bedrock in the area this summer, and Ryan thinks about next year’s seniors. “Another thesis that focuses on wells in ultramafic rock across the state could confirm Kevin’s hypothesis,” he says, “or it might raise as many questions as it answers.”
Joshua Brown wrote “A Bug’s Life” in the fall 2006 issue of the magazine.