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Two newly released reports are shedding new light on the impact that Combined Sewer Overflows are having on the Merrimack River.

One report provides a deeper look at how CSOs impact the health of people living in the Merrimack Valley, while the other gives insight into the scope of sewage releases into the Merrimack, and how it compares to other rivers in Massachusetts where sewage discharge events occur.

Health Study

Researchers at Boston University just released a study that shows a link between large-scale CSO discharge events and significant increases in Merrimack Valley hospital emergency room visits for gastro-intestinal disorders. These kinds of disorders — usually resulting in vomiting and dehydration — are commonly caused by bacteria that is found in sewage.

The researchers found that when “large” CSO events occur, local hospitals see a 22 percent increase in emergency room visits. When “very large” events occur, there’s a dramatic increase in emergency room visits — a 62 percent increase. A “large” event is a release of 27 million to 79 million gallons, and a “very large” release is more than 79 million gallons. Last year there were 20 “large” releases and 5 “very large” releases on the Merrimack.

The BU research team focused its work solely on the Merrimack River. The results are similar to a report that was completed in 2015 that found a link between CSO events on the Merrimack River and increased hospital visits for gastro-intestinal issues.

One key piece of information — exactly how people are getting sick — is not fully understood. The BU researchers looked at whether there were higher incidents among people who get their drinking water directly from the Merrimack (for example, Lawrence, Lowell, and Methuen), and found no clear evidence. They also examined communities that pump water from the Merrimack to augment their supply (for example, Andover), and again found no connection. And, they looked at communities that border the river but don’t use it as a drinking water source at all. Again, they found no correlation.

A likely source is people who come in direct contact with contaminated river water, such as from swimming or perhaps boating or fishing. It’s also speculated that contaminated water could be inhaled from vapor, but this is not definitive.

Mass. CSO Annual Report

The state Department of Environmental Protection released its annual report on sewage discharges statewide, delving into a wide variety of data for 2023. It was a particularly wet year, breaking many records, and that resulted in some unusually high volumes. The report indicates that the Merrimack is clearly the most impacted waterway in the state in several key categories.

Here are the key takeaways:

Total volume: Statewide, about 7.2 billion gallons of partially treated and untreated sewage was released into state waterways. Two waterway areas had the largest volume — Merrimack River (30% of the total) and Boston Harbor (also 30%). The next highest volume was in the Fall River/New Bedford area (22%), followed by the Connecticut River (15%)

Untreated sewage: The Merrimack was hands-down the most impacted by untreated sewage releases, with 40% of the total statewide. Next highest was the Fall River/New Bedford area (27%), followed by the Connecticut River (25%). Boston Harbor was just 4% of the total. Untreated sewage has a much higher concentration of bacteria pathogens than partially treated sewage.

You might wonder how its possible that the Merrimack and Boston Harbor tied for most overall volume but were at opposite ends of the spectrum for untreated sewage volume. The answer is that much of the volume released into Boston Harbor is partially treated in what’s called a “high flow” process that aims to remove as much bacteria as is possible in a very short period of time — it passes through a filter that screens out larger material, and then is chlorinated to kill off most bacteria, then de-chlorinated and discharged. This “high flow” process also happens at treatment plants on the Merrimack, but not nearly at the scale that it occurs at Boston’s Deer Island plant. The “high flow” treatment is the last line of defense for sewage treatment; the next step, if the volume gets too high, is releasing untreated sewage directly into a waterway.

Plants with largest discharges: There are 19 sewer plants statewide that are permitted to release CSO discharges, but two stand out due to their volume — the Massachusetts Water Resources Authority’s Deer Island plant, which manages all of the sewage for Boston and 42 surrounding communities, followed closely by Lowell’s Duck Island plant, which treats sewage generated by Lowell and 4 surrounding towns. The MRWA plant accounts for 27% of the state’s total volume; Lowell for 25%. The third highest volume plant in the state is Fall River (15%), followed by Springfield (7%).

Though they release similar amounts of CSO effluent, the Lowell and MRWA plants are vastly different in size. MWRA plant has a peak capacity of 1.2 billion gallons per day, while Lowell’s peak capacity is 62 million gallons per day.

Number of days: The river/waterway that had the most number of days in which sewage discharge events happened was the Connecticut River, with 103 days. Next highest were waterways that are tied into the New Bedford/Fall River CSO discharge network — Clark Cove (84), Acushnet River (74), Taunton River (71), New Bedford Inner Harbor (71), and Mount Hope Bay (66). Next up was the Merrimack River, at 62 days. In the metro Boston/MWRA area, the river with the most number of incident days was the Mystic, at 31 days.

What is a CSO?

Combined sewer overflows, or CSOs for short, often occur in older industrial cities during rainstorms. In the Merrimack Valley, they occur in Manchester and Nashua, NH, and in Haverhill, Lawrence and Lowell, Mass. These are cities with street drainage and sewer systems that were built many decades ago, long before modern clean water standards were developed…. and before there were sewer treatment plants. The concept was to have all wastewater combined into a single pipe and discharged into the river for “disposal.” It was a common practice worldwide.

From the mid 19th century up until just a few decades ago, the Merrimack had a reputation for being one of the most polluted rivers in the Northeast. There were no sewage treatment plants, and many factories dumped waste directly into the river. It was dangerous to swim in the Merrimack — records from that period show that bacteria levels exceeded the safe standards nearly every day. On a few particularly bad occasions, it was almost 3,000 times higher than the safe standard.

In the early 1970s, the passage of the Clean Water Act radically changed this. Cities were required to build sewer treatment plans to handle their wastewater. And on most days — dry days when it doesn’t rain — no sewage is released into the river, and the Merrimack is clean enough to swim in on most days. But the old combined pipes are still lingering under the streets in many places, and that causes problems when it rains. Some sewer and street drains are still combined, and so when it rains the sewer pipes are overwhelmed with an excessive amount of water.

To release pressure, some of the backup is diverted into the river. This saves the sewer plant from suffering damage, and also minimizes backups into homes and out of manhole covers and drains. But it can cause high levels of bacteria and other contaminants to spill into the river.

The problem is hard to fix. The more common approach is called “separation” — streets must be dug up, pipes removed, and sewer lines separated from street drains. The street drains are diverted directly to the river, which still impacts the cleanliness of the river but is far less noxious than having sewage discharged into the river. Another approach is “storage,” in which massive underground storage tanks and pipes are installed specifically to collect CSO discharge. This contaminated water is then pumped to a sewer plant after the storm has passed for treatment.

Both fixes are extremely expensive. For example, the city of Manchester is spending over $300 million to separate its pipes, and Lowell expects to spend $200 million in its first stage of a major pipe separation project.

Another approach is “green infrastructure” — creating natural areas where excess water is absorbed into the ground. Trees planted along streets are an example of green infrastructure, as trees can absorb significant amounts of water. It’s much less expensive, but it’s also difficult to do it on a scale that provides significant benefit.

The Merrimack River isn’t alone in its CSO problems. Nationwide there are over 800 places where CSO discharges occur.