Significant progress has been made to minimize the pollution.
A series of upgrades costing $17 million increased the capacity of the Greater Lawrence treatment plant more than 40 percent, to 135 million gallons of day, allowing the district to reduce its overflows by more than half.
A second phase of improvements that will cost up to $20 million is planned.
Haverhill has also upgraded its treatment plant to increase capacity, at a cost of $20 million.
The hundreds of millions of gallons of sewage and runoff that still runs into the Merrimack in a year is serious matter and a potential health threat.
But it’s worth noting that before the sewage treatment plants began to open along the river in the 1970s, the river itself was the sewer system.
A 1963 engineering study concluded the Merrimack was polluted from the New Hampshire border to the sea.
“The pollution resulting from excessive bacterial counts makes any type of recreational use of the river hazardous to the public health,” said the report by Camp, Dresser & McKee.
Today recreational boaters, fishermen and even swimmers along stretches of the river enjoy the reclaimed waterway — though swimming is prohibited in the days after a quarter inch or more of rain falls.
But the report by NECIR is a reminder that more work needs to be done.
“The most important thing for people to become more aware of is we are still using rivers and streams as sewage conveyances,” the executive director of the Mystic River Watershed Association, Ekongkar Singh Khalsa, told NECIR.
“What’s needed now is to look at the great work we’ve done, and redouble our efforts to complete the job.”