Few carcinogens in drinking water are allowed at the current level accepted for arsenic, making the House’s vote last week to maintain the level imposed by the Clinton administration but dropped by the Bush administration a reasonable, even encouraging outcome. But Maine has an equal if not more urgent challenge than setting new limits on public water sources – roughly half of the state’s population gets its drinking water from largely unregulated private wells.

The current federal arsenic standard, set in 1942, is 50 parts per billion; the House voted to keep the nation on track to lower it to 10 ppb by 2006. That’s not only World Health Organization’s international standard but Maine’s guideline level adopted last year for private wells. It’s only a guideline, meaning that the state is in the information rather than enforcement business in this case, but the information it has found during the last several years should prompt more and more well owners to have their water sources tested.

The cliche that “the dose makes the poison” must have been coined for arsenic. There is no doubt that the stuff will kill you at high enough levels and that it has been linked to cancer of the bladder, liver, lungs and kidneys. The federal government sets cancer-risk levels at 1 in a million for many water-borne poisons, such as benzene, and the lower level of 1 in 100,000 for others, such as the notorious MTBE gasoline additive. A few carcinogens are allowed as low as 1 in 10,000. If the 10-ppb standard holds up, arsenic will reach that lowest level of 1 in 10,000.

The state’s interest in arsenic began in earnest in the early 1990s, when Bonny Eagle High School completed a routine test of its water and found arsenic levels over the acceptable 50 ppb. Other wells were tested in the Buxton-Hollis area and some of those too showed high levels. More tests around the state turned up a significant number of places that exceeded the standard. State officials have tentatively concluded that distinct geologic belts, often including sulfide-bearing rocks, are a likely source for these higher levels. And there also remains open the question of the contribution made by arsenic-containing pesticides, used regularly in the first half of the 20th century.

Whatever the source, state toxicologist Dr. Andrew Smith urges homeowners to find out whether their wells have high arsenic levels by having them tested, either through a certified private company or by contacting the Bureau of Health.

The National Cancer Institute currently is conducting its own tests in northern New England, which should add significantly to what is known about the geologic sources of arsenic. And a pair of studies from the Environmental Protection Agency is due out before the Senate is expected to consider whether to support the House’s measure on arsenic. Given both the potential danger of arsenic and, in some parts of the country, particularly the Southwest, the high cost of water treatment, the testing is more than justified.

Meanwhile, Congress and the White House have an obligation to support measures that raise arsenic to at least levels that represent the bottom for acceptable risk.