WHAT IS A COMBINED SEWER OVERFLOW (CSO)?

The Merrimack River Watershed Council posts alerts on our Facebook page whenever we receive an email from a sewage treatment plant reporting a CSO. Four plants — in Haverhill, Lowell, Lawrence, and Manchester — provide these reports. The Nashua wastewater plant is required to post alerts on the city’s website page, so the MRWC keeps an eye on this page and we post on Facebook and our website as needed.

You can also get notified directly from sewage plants. As of now, there is no centralized system to subscribe in order to receive alerts from all plants, but you can sign up for their notifications individually:

• Haverhill: Email csoalert-sub@haverhillwater.com, view real time information here
• Lawrence: Sign up here
• Lowell: Sign up here
• Manchester: Sign up here
• Nashua: You can monitor the city’s wastewater facility website during and after storm events for information

We should note that the emails from the plants are not always easy to understand, and each plant reports its CSO event differently. For instance, Lowell sends out 3 alerts — one when the CSO begins, one when it ends, and a follow-up a few days later reporting how much sewage was released. Haverhill and Manchester send out one email that links to a map of where discharges have occurred, when they started and ended, and how much volume was released. Greater Lawrence sends emails at the start and end of a CSO release, and several days later reports the volume released.

Each wastewater treatment plant on the mainstem of the Merrimack has a different system for sharing data with the public. In addition to receiving notifications as CSOs are occurring, you can also download data about past CSO occurrences.

• Haverhill: Visit Haverhill’s website and scroll to the bottom of the page to download annual reports these are updated once per year.
• Lawrence: Visit Lawrence’s website and scroll to the middle of the page to download reports. Lawrence updates this information multiple times a year.
• Lowell: Visit Lowell’s Document Center, click the “Public Works” folder, then “Wastewater Utility”, then “Resources” to find quarterly reports.
• Manchester: Manchester does not yet have a system for downloading CSO data.
• Nashua: View Nashua’s report. Volumes are updates multiple times a year.

Rivers are living, moving systems, which means, unfortunately they are a conduit for transporting bacteria from CSO outfalls to other areas of the river downstream Additionally, there are a lot of bacteria contributions to the river from sources other than CSOs, and other pollutants apart from bacteria that contribute to poor water quality in the river

• Other pollutants may include nutrient pollution from farms, legacy industrial pollution in river sediment, storm runoff that doesn’t trigger a CSO but still contributes direct runoff containing bacteria, animal defecation in the river and general trash and other things that are dumped into the river or wash into it when it rains
• This can be particularly exacerbated when it rains after droughts, which is an issue we are seeing more with climate change

While CSOs are a major problem where they occur, natural processes occurring within the river may mitigate the bacteria concentrations as they move downstream- not all CSOs mean the river is unsafe everywhere, and the risks may be delayed in downstream communities. Communities close to the outfall will be affected during and directly after rain events. People downstream may be affected later, less severely affected, or not affected at all.

The connection between when and how large a CSO event is, and when and how serious the health concern in the river is complex, and is impacted by:

• Streamflow: How fast the river is moving determines how fast and how far the bacteria will move downstream. When flows are low, other processes may eliminate the bacteria before it makes its way all the way downstream. When flows are high, the bacteria may move downstream faster than it dies off
• Dispersion: The bacteria plume will be concentrated at the outfall, but will disperse in the river in depth and across the river once it starts mixing.
• Dilution: As soon as the bacteria is contributed into the river, it’s concentration is diluted by the river water.
• Decay/die off: Bacteria are living beings that do not exist and live forever. Their die off rates can be impacted by temperature, sunlight, dissolved oxygen, salinity, existing bacteria concentrations in the river and more.
• Tides push pollutants up and downstream, but also incoming seawater may dilute concentrations or affect die off rates due to high salinity. Tides affect the Merrimack all the way to the Essex Dam in Lawrence.

Sewage can remain in the Merrimack for days after a CSO event—a period during which, in many cases (especially in the nice weather), the skies will have cleared, leaving the river environment deceptively inviting to anglers, boaters, swimmers, and other recreational users of the river. Before you visit the river for recreation, consider how long ago a CSO occurred.

Exposure to raw sewage can cause multiple, serious health problems, particularly for the very young, the elderly, and those with compromised physiology. According to EPA, raw sewage includes disease-causing agents falling into three general categories:

• Bacteria: single-celled organisms that, when pathogenic, can cause cholera, dysentery, shigellosis, and typhoid fever;
• Viruses: parasites that grow on living tissue and, when associated with feces, include hepatitis A, Norwalk-type virus, rotavirus, and adenovirus; and
• Protozoans: about a third of all protozoans are pathogenic, and these can cause gastrointestinal disease, dysentery, and ulceration of the liver and intestines.

A 2015 peer-reviewed health study that included the Lawrence area along the lower Merrimack found a statistically significant association between heavy rains (which trigger CSO events) and an increase in hospital emergency room admissions of people complaining of gastro-intestinal disorders.

Nearly all of the sewage from CSOs is discharged upstream of drinking water plants that provide Merrimack River water to residents in six municipalities: Lowell, Lawrence, Methuen, Andover, North Reading, and Tewksbury. (A sixth, Nashua, receives about a quarter of its drinking water from the river.)  All told, more than 600,000 people get their daily drinking water from the Merrimack River. Within a decade, that number is likely to increase, as Manchester and Haverhill tap into the river to expand local supplies and sell water to other communities.

The Merrimack River is the second-largest surface-based drinking water source in New England. The largest is the Quabbin Reservoir, which, unlike the Merrimack, was built to provide pristine drinking water to Eastern Massachusetts. Drinking water is heavily treated and it is unlikely water coming out of the tap from city supplied water will be contaminated with bacteria. A study by the US Geological Survey did not find contaminants at reportable levels in treated drinking water, despite finding some in source water on the Merrimack, suggesting that water treatment methods are working correctly to eliminate contaminants from CSOs in drinking water. Even still, polluting our drinking water sources is a major concern.

Much of New England’s infrastructure is old and was not designed for the volume of rain runoff that takes place during even short duration storms.  As populations grew, the amount of impermeable surfaces (examples include buildings, paved roads and anything preventing water from infiltrating into the ground) expanded, increasing the amount of stormwater runoff. Even with upgraded wastewater treatment plants, they can easily reach capacity leading to runoff into the Merrimack River. The system is working as intended when it combines raw sewage and stormwater runoff. 

The fix is not an overnight one. It is expensive and would take several years to dig water lines and separate the storm water piping from the sewage piping. Factoring in that water pipes are generally working due to gravity alone, they tend to be some of the deepest utilities underground and are often buried under other utilities. Because of all this, the sewage plants are authorized to dump sewage in the river under these circumstances for the time being.