Faced with heavier rains, cities scramble to control polluted runoff

This story was originally published by Yale Environment 360.

In 2011, Philadelphia's city-owned water utility drew national attention when it began Green City, Clean Waters, a 25-year program to manage an increasing volume of stormwater by using mostly "green infrastructure," such as rain gardens and porous pavements, which allows rain to soak into the ground rather than becoming runoff that pollutes rivers and creeks.

With a plan for green infrastructure to drain some 9,500 acres across the city, the Philadelphia Water Department was considered by many to be at the cutting edge of stormwater management - an increasingly urgent challenge for city governments amid the bigger, more frequent rainstorms now occurring as a result of climate change, especially in the northeastern United States.

The program led Philadelphia and other U.S. cities to install nature-based solutions to absorb and filter rainfall alongside traditional "gray infrastructure," such as pipes, tunnels, and pump stations. Yet while other cities, such as Milwaukee and Boston, saw green measures as complements to gray infrastructure, Philadelphia placed rain gardens and bioswales - vegetated ditches that collect stormwater - at the center of its strategy. But now, critics say, these innovations are proving inadequate at handling the increase in extreme rainfall events. In fact, the amount of overflow from pipes that combine stormwater with raw sewage has actually increased since the Green Cities program began.

Gray infrastructure can take decades to implement and cost billions of dollars, but many cities don't have much of a choice.

An average of 14 billion gallons of polluted stormwater have overflowed from Philadelphia's 164 sewage outfall pipes each year since Green City, Clean Waters began, according to "Unraveling the Facts" a new report from the Restore the River Advocacy Team, a group of water experts and environmentalists that includes the former heads of the interstate Delaware River Basin Commission and the Camden County Municipal Utilities Authority, which manages wastewater in the southern New Jersey county.

"Because of climate change, the city's targets for sewage pollution reduction" - slashing combined sewer overflows by approximately 8 billion gallons a year from the 2006 baseline of 13 billion gallons - "are no longer connected to the reality of the climate induced rainfall conditions Philadelphia is actually experiencing," the report states.

Roughly 700 municipalities in the U.S. rely on combined sewer systems, mostly in the Northeast and around the Great Lakes, according to the Environmental Protection Agency. These cities and towns are taking a hard look at updated climate projections that show an increase in heavy rainfall, and many are doubling down on gray infrastructure projects, which include concrete holding tanks, tunnels, and pipes that can divert and hold onto combined flows until the rain stops and treatment plants recover. Such projects can take decades to implement and cost billions of dollars - green infrastructure is both cheaper and faster to build - but many cities are finding they don't have much of a choice.

In Milwaukee, the Metropolitan Sewerage District, which serves 1.1 million people, relies on both green and gray infrastructure. But its storage tunnels and tanks have done the most to reduce combined sewer overflows into Lake Michigan and local rivers, said Kevin Shafer, the district's executive director. He estimated that the system's gray infrastructure can currently handle two and a half to three inches of rain at a time, while green measures can absorb only about half an inch.

Still, he calls green infrastructure "the icing on the cake" because managing water where it falls protects water bodies from contaminants like bacteria, heavy metals, and trash, and it reduces the risk of basement backups. "You have to have both," he said.

Despite its limited finances and aging infrastructure, Milwaukee's sewerage district has set a goal of eliminating combined sewer overflows (CSOs) by 2035. "With the changing climate we're facing," Shafer has said, "it's like running uphill, and the hill keeps getting steeper and steeper."

Boston, too, has emphasized gray infrastructure in its efforts to upgrade stormwater management. The city is operating under a long-term control plan - a federal mandate to reduce CSO volumes until waterways are in compliance with the Clean Water Act, which set a goal for all rivers and creeks to be "fishable and swimmable." To meet those goals, the city is, little by little, separating stormwater from sewage lines so that rainwater flows into creeks and rivers, leaving only sewage in pipes that lead to treatment plants. The city is also directing stormwater, before it overflows into waterways, into "partial treatment facilities" that screen out trash and other solids, then disinfect water with high doses of chlorine before discharging it through outfall pipes.

Alexandria's new tunnel is designed to reduce the number of overflows per year from 70, during the 2000 to 2016 period, to four.

But according to Max Rome, stormwater program manager for the nonprofit Charles River Watershed Association, the control plan's current goal may be impossible to meet. In 2023, for example, Boston received about 55 inches of rain, somewhat more than in a typical year, but some 70 million gallons of combined sewage and stormwater entered the the Charles River, about five times the amount allowed by the long-term control plan.

Last year's overflows reflect the increasing intensity of downpours that overwhelm wastewater treatment plants, said Rome. "It's a perfect example of how climate change is changing precipitation. It wasn't that much wetter, but more of the precipitation fell during a smaller [number] of storms."

In cooperation with the EPA, Boston is now revising its long-term control plan for the lower Charles River watershed with an overflow limit of some 38 million gallons a year, based on 2050's projected rainfall, said Rome, citing unpublished data from the Massachusetts Water Resources Authority. That's above the watershed's current 13-million-gallon limit but below the actual overflows of the last few years, he says, suggesting that even the revised target will be hard to meet.

Increased rainfall is also affecting Western cities, including Portland, Oregon, which sharply reduced pollution in the downtown section of the Willamette River and Columbia Slough by building three tunnels to hold stormwater flows. The project, which nearly eliminated CSOs, took 20 years to complete, in 2011, and cost $1.4 billion.

Utilities pay for such upgrades through increased water rates and federal grants. The Biden administration's Bipartisan Infrastructure Law, passed in 2021, is currently providing funds for stormwater control measures.

In Alexandria, Virginia, the city's wastewater authority increased the diameter of a new wastewater tunnel from 10 feet to 12 feet based on a federal projection that the city will see rainfall increasing, from an annual average of 41 inches, between 2000 and 2016, to an annual average of almost 70 inches by 2100.

The 2.2-mile-long tunnel, completed at a cost of $615 million but not due to begin operating until early 2026, will reduce sewage and stormwater overflows into the Potomac River from 140 million gallons a year, between 2000 and 2016, to 17 million gallons a year, said Justin Carl, chief executive of AlexRenew, the city's public wastewater treatment authority. Alexandria's new tunnel is designed to reduce the number of overflows per year from 70, during the 2000 to 2016 period, to four.

The project includes some green infrastructure, like bioretention basins and tree wells that filter and retain stormwater runoff. But the agency decided to limit those measures because most of its CSOs occur in the oldest part of the city, which has little open space and clayey soils, which have a slow infiltration rate when moist.

Only one site in Washington was suitable for nature-based solutions because it had low overflow and wasn't densely developed.

A much larger stormwater project is planned for the nation's capital, where DC Water, an independent local-government authority, is building an 18-mile network of tunnels to hold 249 million gallons of stormwater and sewage until it can be safely pumped to a treatment plant. When completed later this decade, the system will reduce combined sewer overflows by 96 percent, said Moussa Wone, DC Water's vice president for the Clean Rivers Program. The volume of CSO outflows into three urban waterways is expected to decline to an average of 138 million gallons a year from 3.2 billion gallons in 1996, and to cut the number of overflows to four from 82 over the same period.

Green infrastructure is just a small part of the overall project, costing $98 million out of a total budget of $3.29 billion, Wone said. The agency conducted green-infrastructure pilot projects at two sites and concluded that only one, in Rock Creek Park, was suitable for nature-based solutions because it had low overflow volumes and wasn't densely developed.

Acknowledging the climate challenges faced by U.S. cities, the Philadelphia Water Department (PWD) in 2022 issued a 182-page report projecting a 9.5 percent increase in average annual rainfall by 2050, over the 1997-2017 baseline, and stated that stormwater management projects - whether gray or green - must be upgraded to reflect that scenario.

"PWD recognizes the issue; what they're resisting is applying that in any way until the Green Cities plan is completed in 2036," said Nick Pagon, an author of the Unraveling the Facts report who founded Philadelphia Waterborne, a program that taught teens to build boats on the Delaware River. "They don't want anything that smells of reopening the long-term control plan."

The Philadelphia Water Department says it designs stormwater-control measures in light of climate-driven rainfall patterns, but it needs at least 30 years of data - the Green Cities program started in 2006 - to draw conclusions about those patterns.

"Naturally variable precipitation patterns have been a known challenge for centuries, and now climate change impacts are pushing the wide range of possible rainfall amounts upward by some uncertain amount," the agency said in a statement. "Making changes to programmatic targets and assumptions using only the past few years of observed data is unwise."

But John Rumpler, an attorney with the nonprofit Environment America, said cities across the country must accept the need to make large investments to control increasing stormwater flows.

"We are going to need to bite the bullet and make large-scale investments in conventional sewage infrastructure and repairs to stop these billions of gallons of raw sewage from running into rivers," he said. "The increased storms connected to climate change are just making the task all that much more daunting and underscoring the need for greater investments in wastewater infrastructure."