Denis Thoet tests his well every year for bacteria.

But the West Gardiner vegetable farmer said he never thought to check for arsenic or uranium, known carcinogens.

An estimated 11 percent of Maine homes with private wells have levels of arsenic that exceed current health benchmarks, as many as 20 percent have elevated radon levels and an estimated 4 percent have elevated uranium levels, according to state statistics.

And while the state has a robust safety program for public water supplies, there are no regulations for these substances in private wells.

That has some of the more environmentally sensitive gardeners questioning what they’re putting on their plants.

“I suppose it’s a potential area to have tested for irrigation water,” Thoet said. “We have a pond we do part of our irrigation from, and want to use it for more of that. It’s a combination of runoff water and spring water.

“We haven’t tested that for anything yet. There’s usually some kind of bacteria from general runoff that’s fine for irrigation water but not for human consumption.”

In elevated levels, arsenic has been linked to certain types of cancers, childhood learning disabilities, heart disease and low birth rates. Uranium can affect the kidneys, according to the U.S. Centers for Disease Control and Prevention.

Just how much of those contaminants are absorbed by plants we eat? And how much plant-based uranium and arsenic is safe to consume?

Bruce Stanton, director of the Center for the Environmental Health Sciences at Dartmouth Medical School, said one in 10 of Maine’s private wells has levels of arsenic higher than the U.S. Environmental Protection Agency’s maximum contaminant limit, 10 micrograms per liter.

Stanton said there are wells in Maine with arsenic levels as high as 5,000 parts per billion and that it is not uncommon to find wells with levels exceeding 100 parts per billion. He said a good example of a food that absorbs arsenic is rice.

“Rice cereals and biscuits have levels of inorganic arsenic which could easily be consumed in doses well above that found in drinking water,” Stanton said.

Toxin levels in one cup of rice — a grain used as a base in processed foods — are equivalent to that found in 1 liter of water contaminated with 10 parts per billion of arsenic, he said.

He said any “pesticide-free” claim on containers “lulls the consumer into a false sense of security about the product’s safety.”

“Some people on special diets drink rice milk,” he said. “Rice milk has been measured at 50 parts per billion.”

Andrew Smith, state toxicologist, said he relies on a greenhouse study from the mid 1990s that measures the amounts of arsenic a plant takes up from contaminated water.

Smith said John Jemison, a Cooperative Extension water quality and soil specialist, grew vegetables in a greenhouse and used water with different levels of arsenic concentrations. He said Jemison used concentrations of 5, 50, 500 and 5,000 parts per billion.

“We’ve actually seen 5,000 parts per billion in Maine, and clearly at 5,000 he was seeing moderate increases in swiss chard and beet roots,” Smith said. “At 500, he got a little bit in deep roots; and at 5 and 50, he couldn’t detect any increases.

“So if we’re dealing with someone who has 500, we suggest caution when using water for gardens. This just reinforces the importance of testing well water.”

Smith cited a study by researchers at Los Alamos National Laboratory in New Mexico that assessed the risks from exposure to uranium in well water. The study looked at irrigating plants with water containing high uranium concentrations. Over 1,200 micrograms per liter was the highest.

“They used lettuce, radishes, tomatoes and squash and could see, as the concentrations increased, the concentration in the crops increased,” he said.

Jemison said vegetation absorbs larger amounts of uranium than arsenic, with the roots of plants and leafy vegetables absorbing the most.

“The (CDC) folks said the uranium you consume via vegetables out of your garden is small relative to what you might find in drinking water,” he said. “The uptake in greens or radishes is not going to be very much.

“If somebody is still concerned and just don’t want to eat small amounts of uranium, we have other options for them. Collect water in rain barrels and use the water to water your plants. Or they can do drip irrigation off of those barrel.”

Alice Elliott, a master gardener from Richmond, said a rainwater collection system is a safe way to water your garden. It can be as simple as a rain barrel at the end of a downspout.

“If you set up a rain barrel, that catches water off your roof. The quality of that roof water will be pretty good,” Elliott said.