Twenty one New Jersey towns will soon have to tackle long-delayed upgrades to their sewer systems, which send billions of gallons of sewage into the state's waterways every year, according to a new report from Rutgers University and New Jersey Future.

The report puts that price tag at more than $2 billion for towns to move away from combined sewer systems, which commingle rainwater with household and industrial wastewater in the same system of pipes. The federal Environmental Protection Agency has estimated it could be even higher — more than $8 billion, according to The Star-Ledger.

Combined systems are no longer used in new construction because of their tendency to become overburdened during storms and flooding events. When they're overloaded, they send partially-treated or raw sewage through 217 discharge points located along waterways around the state.

Here's a map of state-permitted discharge points in New Jersey:

Federal law mandated cities and towns get off combined sewer systems more than 20 years ago, but the staggering cost of overhauling sewer systems has kept the 21 cities and towns that still have them from making the switch.

Changes to the state Department of Environmental Protection's permitting process for combined sewer outfalls will require permit-holders to start implementing upgrade plans next year.

But the report says that on top of federal penalties, these outdated systems also create a "significant drag" on local economies.

"Upgrading our water infrastructure is not just a federal and state mandate," Pete Kasabach, New Jersey Future's executive director, said in announcing the study. "It's an economic imperative for cities that want to continue attracting businesses and for residents who seek an urban environment where they can enjoy clean, dependable drinking water, and avoid encounters with raw sewage and stormwater."

Many sewer systems haven't been updated in decades, and some in more than a century. The report points out that portions of Hoboken's sewer system date back to the Civil War.

These combined systems have long been the bane of environmental groups that advocate for New Jersey's ailing waterways. Last summer, members of the Hackensack Riverkeeper took a dip in the river to call attention to the need for "swimmable water."

The outfalls pump an estimated 23 billion gallons of sewage into bays and rivers — from the Hudson, Hackensack and Passaic Rivers in the north to the Delaware down south — every year. Most of them sit in some of the most densely populated areas of Northern New Jersey, in cities as large as Newark and Jersey City and towns as small as Guttenberg and Fort Lee. Here's a closer look:

The report, titled "Water Infrastructure in New Jersey's CSO Cities: Elevating the Importance of Upgrading New Jersey's Urban Water Systems," was authored by a team led by Daniel J. Van Abs, a research professor at Rutgers.

It takes a deep dive into the impacts of CSOs and other sources of pollution on the state's drinking water, and looks at four case studies — in Camden, Hoboken, Jersey City, and Paterson — to see how cities are coping with water infrastructure issues. Read the whole thing here »