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This past summer was an unusually wet one, and all that rain has led to a record-breaking year for sewage discharges into the Merrimack River.

MRWC is estimated that so far in 2023, about 1.4 to 1.6 billion gallons of Combined Sewer Overflows, or untreated sewage, have been discharged into the river by municipal sewer plants. This figure is based on the year-to-date data that has been submitted by three of the five plants that are permitted to release sewage into the Merrimack, and an estimate of volume discharged by two plants that have not yet made their figures public.

This is a dramatic increase from the previous CSO discharge record of 823 million gallons, which was set in 2021. Our records span the period 2013 to 2023.

Sewer overflows occur in Haverhill, Lawrence, Lowell, Manchester and Nashua. They are caused by archaic underground sewer and street drainpipes installed in the late 1800s and into the mid 1900s. These pipes combine sewage and rainwater that flows off streets, and during heavy rainstorms they produce a huge volume of wastewater that is simply too much for sewage treatment plants to handle. The excess sewage is discharged into the river to prevent it from backing up into homes and businesses.

The Merrimack River is not alone in seeing massive increases in CSO discharges this year. A report by WBUR found similar increases statewide.

The largest discharges on the Merrimack have been recorded in Lowell, where 673 million gallons of untreated sewage have been discharged into the river as of September 30. Lowell has also discharged 610 million gallons of partly treated sewage. Greater Lawrence has discharged 147 million gallons, and Nashua about 9 million gallons. Manchester and Haverhill will not report their 2023 flow data until next year, so MRWC has estimated their 2023 discharges based on historical patterns.

The good news is that significant effort is being made to reduce sewer overflows. Manchester is working on a massive project that will reduce its CSO discharges by 50 percent, It’s expected that within a few months, Lowell will also be embarking on an ambitious project that will reduce its CSO volume substantially. Haverhill and Lawrence are also working on projects to reduce their CSO flow.