It’s all about the drinking water.

Hundreds of millions of gallons of untreated sewage that gush into the Merrimack River — 800 million last year, according to a recent estimate — surely pose a health risk to everyone downstream. Think of all the boaters and swimmers and fishermen. But the much greater danger seems to be that the Merrimack is a source of drinking water, including right here in Andover.

That was the conclusion of a study of five years of data from sewage outflows during heavy rains, and the reports of local emergency rooms. A half-dozen researchers poring over the data pointed to a connection between the overflows, with their bacteria including many varieties of coliform, and ER visits by people with gastrointestinal illness.

Their study looked at three areas, including the Merrimack Valley, where they found “significantly increased cumulative risk for rate of ER visits … for all ages eight days following” a major rain event. They drew data from 11 cities and towns along the river. The same connection was not apparent, however, in two-dozen communities around Boston Harbor, which also takes on untreated effluent from combined sewer outfalls.

Writing four years ago in the journal Environmental Health Perspectives, the researchers concluded “drinking water quality may be adversely impacted by the presence of combined sewer overflows that discharge into drinking water sources after heavy rainfall.” It’s not a definitive link, to be sure. But it is an important and alarming relationship.

That these overflows are still allowed to happen 47 years after the passage of the Clean Water Act is astonishing. To be sure, these systems are the design of a century or more ago, when the health dangers of allowing sewage to run in the streets was plaintive but of less concern than pouring it all into the river. Sewage at first was steered straight into the nearest body of water. That was until treatment plants were built. (The Greater Lawrence Sanitary District was created by the Legislature in 1968 and its system went online in 1977.)

But in the Northeast, Upper Midwest and Pacific Northwest, architects of these improvements built in the spillover systems that haunt our public health today.

For 25 years, the U.S. Environmental Protection Agency has made a mission of cajoling communities into either rebuilding sewer and stormwater systems, or vastly expanding treatment capacity to avoid the possibility of overflow. Both prospects are extremely expensive. Progress has been slow.

A decade-old report estimated that a public works project to address five of six such overflows in the country would cost nearly $51 billion. A more recent count found 772 communities in the United States, home to 40 million people, are served by combined sewers.

The problem is as urgent as it is expansive. A changing climate means more and bigger storms that cause the overflows, as the researchers noted four years ago. It also poses an immediate threat, as their study concluded.

Capping nearly 50 combined sewer outfalls along the Merrimack River will take a lot of money and vision, and it requires the absolute focus of our state and local leaders.

In the meantime, it's essential that our lawmakers quickly improve the notice, now scant, given to the public when these overflows happen.

Sewage treatment districts along the river detect these events in real time, and they are required to report to environmental regulators within 24 hours of an overflow. In this connected age, there is absolutely no conceivable reason that information cannot be immediately delivered almost instantly to those who boat, swim, fish — and drink from — the river.

Our lawmakers should act post haste to require such notice from the sewage treatment districts on the river, passing legislation that already has the support of more than 80 lawmakers, according to Statehouse reporter Christian M. Wade.

Then they can turn to the essential work of ensuring the quality of our drinking water and the overall health of the Merrimack River.

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