Nearly half a million Mainers are drinking tap water containing amounts of cancer-linked chemicals that, while complying with state and federal limits, still pose a public health danger, according to a new study from a national public health advocacy group.

The Environmental Working Group is calling for stricter regulatory standards for hexavalent chromium, arsenic and nitrate, and urging public water systems to invest in technology that could filter them all out at once.

“The public health benefits are undeniable,” Melanie Benesh, an Environmental Working Group spokeswoman, said Wednesday when the findings were published, adding that the U.S. Environmental Protection Agency “is moving too slowly.”

She said: “Our laws haven’t kept pace with the risks from our tap.”

But the leaders of some of Maine’s public water systems said that they can’t ask cash-strapped ratepayers to pay for expensive filter upgrades when test results are coming back far below all state or federal environmental requirements.

The Sanford Water District had traces of all three chemicals highlighted in the report — higher than Environmental Working Group’s public health goal but well under state and federal standards, said David Parent, the water district’s superintendent.

The Maine Department of Health and Human Services echoed the concern about districts behind held to the Environmental Working Group’s “alternative standards,” instead of what state or federal law requires.

“In some cases, these alternative limits are many times lower than the (limits) set by the EPA under the Safe Drinking Water Act, or are for chemicals not currently regulated under the Safe Drinking Water Act,” DHHS spokeswoman Lindsay Hammes said on Friday.

The Environmental Working Group includes data from 16 of Maine’s largest public water systems in its national report on the dangers of these three chemicals and how the cancer risk could be lower if districts took steps to abide by their health-backed advisories instead of state and federal limits.

Those public water systems serve more than 400,000 residents, according to the Environmental Working Group report and analysis. The group based its findings on publicly available state water quality data. The report did not address private wells, which serve a majority of Mainers.

In the study, which will be published in the November issue of Environmental Research, a peer-reviewed scientific journal that focuses on environmental science and health, the Environmental Working Group estimates that 7,410 Americans could avoid cancer from hexavalent chromium and 43,418 people could avoid cancer from arsenic if the EPA adopted the group’s lower recommended levels.

The federal limit for all chromium, including hexavalent, is 100 parts per billion, or the equivalent of 100 drops of water mixed into a full-sized swimming pool. The federal limit for arsenic in drinking water is 10 parts per billion. The nitrate limit is 10 parts per million, or 10 drops mixed into a standard bathtub.

If the limit for hexavalent chromium was set at 10 parts per billion, the U.S. could prevent about 575 lifetime cancer cases nationwide, according to the study. If the limit was even stricter, at 5 parts per billion, it says about 1,320 lifetime cancer cases could be prevented.

According to an interactive map published at the same time as the report, the Augusta, Brewer, Brunswick-Topsham, Kennebunk-Kennebunkport-Wells, Midcoast Regional Redevelopment Authority, Old Town and Sanford water districts had all three carcinogens show up in their test results.

Some districts questioned the map data, as well as the group’s recommended lower limits.

The general manager of Brunswick’s water district, Craig Douglas, said he has no problem with the group’s numbers, but said it was “pretty unfair and borderline alarmist” to hold public water districts to a standard that is many times higher than a health goal of “zero or near zero.”

The map data and the numbers that the Auburn water district reports to regulators don’t match, said the district’s superintendent Michael Broadbent. He said the water supply has negligible amounts of arsenic and nitrate. The Environmental Working Group report has the district marked as 2.1 times the “public health goal” at 0.041 parts per billion.

Hexavalent chromium, made famous by the movie “Erin Brockovich,” is a form of heavy metal that can damage DNA and increases risks of cancer when ingested. Arsenic, common in Maine’s bedrock, has been tied to bladder, lung and skin cancers, as well as developmental problems.

The EPA sets limits for total chromium and arsenic, but does not have any standards addressing their combined toxicity. Some states, like California, have set their own hexavalent chromium limits, but Maine has not. Maine regulates arsenic in public systems and private wells in the state.

Treating these contaminants in isolation could underestimate the risk for people exposed to both, the study said. 

“Our results demonstrate that co-exposure to arsenic and (chromium) in drinking water can lead to substantially greater lifetime cancer risk than exposure to either contaminant alone,” the study’s authors wrote. “Considering multiple contaminants simultaneously, rather than independently, could improve protection of public health from drinking water exposures.”

The same filtering technology that can be used to reduce one of these three chemicals can be used to treat all three, as well as some forever chemicals and microplastics, according to the study. It says regulators should stop using a chemical-by-chemical approach to evaluate risk and remediation benefits.

“Regulating and treating them simultaneously is a more effective and efficient approach than focusing on each chemical separately,” scientist Tasha Stoiber said about the report. “Updating treatment strategies could improve health while making better use of resources.”