What officials call “sanitary sewer overflows” are direct discharges from the sewer lines themselves, often caused when pipes are ruptured, clogged with grease or tree roots, or flooded in a rain. This can cause backups that send raw sewage spilling onto streets or spouting from manholes.
In 2004, the EPA estimated 3 billion to 10 billion gallons of untreated sewage is leaked accidentally each year in this manner.
“Most people don’t know much about what goes on underground,” said Denny Dart, chief of Water Enforcement for EPA’s Region 1, covering New England. “When we get big storms and flooding I see people letting their children play in the floodwater. In New England, there’s a very good chance that the floodwaters have sewage in them.”
But the biggest source of pollution — an estimated 850 billion gallons each year — comes as a result of a design shortcut, according to a 2004 report to Congress by the EPA. Many East Coast sewage systems were designed to funnel stormwater runoff into the same treatment plant that handles sewage. In routine weather, this generally works and has the advantage of cleansing both stormwater and sewage before it reaches waterways.
But when there are heavy rains or snowmelt, the systems are overwhelmed and operators divert the deluge directly into the waterways.
To fix the problem, municipalities can build separate systems for sewage and stormwater or build immense underground holding tanks to hold the excess until it can be treated.
But the size of the work yet to be done is daunting. The EPA still lists nearly 3,000 water bodies in New England as “impaired,” meaning they remain too polluted to meet minimum water quality standards.
In 2008 the EPA estimated that another $64 billion is needed to fix the combined sewer problems, $4.2 billion of that in New England.
“The most important thing for people to become more aware of is we are still using rivers and streams as sewage conveyances,” said Khalsa, of the Mystic River Watershed Association. “What’s needed now,” Khalsa said, “ is to look at the great work we’ve done, and redouble our efforts to complete the job.”
The New England Center for Investigative Reporting (www.necir-bu.org) is a nonprofit investigative reporting newsroom based at Boston University and supported in part by media outlets that include The Eagle-Tribune. Emerson College journalism student Vjeran Pavic helped compile the mapping for this project.