Even as hulking machines bore water-holding tunnels under the nation’s capital, the city’s water authority is pivoting toward a more nuanced solution to its polluted overflow problem — one that would exchange some underground tunnel boring for greening above the ground.

In May, DC Water and the EPA filed a modification to their 2005 consent decree that would allow the water utility to use green infrastructure as well as “gray” tunnels to absorb excess stormwater in the city.

The consent decree resolved a suit brought by the EPA over violations of the Clean Water Act and DC Water’s discharge permit because its antiquated combined sewer system, which mixes stormwater with raw sewage, routinely overflowed during large storms. The original decree required the water authority and the city to increase its capacity by 2025 so that systems no longer allow raw sewage and pollutants to overflow into the district’s Rivers.

The modification pushes that deadline back to 2030, and instead of additional tunnel miles, the water authority would build rain gardens, water-filtering bioswales and green roofs in two pockets of the city.

The original plan called for DC Water to build three tunnels: the largest along the Anacostia, next largest along the Potomac in Georgetown, and smallest along Rock Creek.

If it performs as planned, the green infrastructure would replace a portion of the Potomac tunnel and eliminate the Rock Creek tunnel. The largest Anacostia River tunnel is already under construction and slated for completion in 2022, a few years ahead of schedule. That tunnel, with its capacity to store 157 million gallons, will reduce the city’s combined sewer overflow to the Anacostia by 98 percent when it is online.

The $2.6 billion price tag for the Clean Rivers Project, as DC Water calls its compliance effort, remains the same under the new plan, which will receive public comments through the end of July. The water authority would have five additional years, until 2030, to complete its final Potomac tunnel, with green infrastructure and portions of the tunnels coming online incrementally.

The green infrastructure is intended to retain the first 1.2 inches of rainwater on nearly 500 acres located in parts of Georgetown and north of Piney Branch Stream. If pervious pavement and bioswales in those areas do their job, the water authority would be able to forsake its smallest Rock Creek tunnel and continue with a longer, narrower tunnel running alongside the Potomac River and connecting with the Anacostia River tunnel.

The shift toward green infrastructure — and greener streetscapes — has received broad public support in hundreds of comments and several forums on the subject this year. And there’s no better spokesman for the change than DC Water’s charismatic general manager, George Hawkins. A lawyer who has held top positions at both the EPA and the District Department of the Environment, Hawkins has espoused the benefits of green infrastructure on local radio shows and at events.

But some environmental groups have raised concerns over whether the green infrastructure can be as reliably effective as gray at keeping polluted water out of the rivers.

While data show that well-constructed rain gardens or green roofs can capture, store and clean stormwater before it enters a combined sewer system, relying on the technology to meet the requirements of a consent decree is new territory.

“I think it’s fair to say that what’s happening here and in other cities is essentially a massive-scale science experiment,” said Jennifer Chavez, staff attorney with the nonprofit law firm, Earthjustice. “They’re studying it as they go.”

If this is an experiment, Hawkins said, it is a “well-calculated” one that he says aims to make the best use of ratepayer funds while meeting the demands of a federal court order. As of October, DC residents will pay $20 per month per equivalent residential unit to fund the Clean River Project, a fee that’s grown incrementally from about $2 since it was first passed by the water authority’s board in 2009.

The latest proposal entails pushing back five years the time line for completion of the project, Hawkins said, to spread out those rate increases and to be on a similar time line with other federal consent decrees.

While the tunnels are relatively easy to maintain, control and measure, they are also massive investments buried up to 10 stories underground, “where nobody sees them, and they only operate in a big storm,” Hawkins said.

“The core question that we have had to face, that any institution seeking to use green infrastructure has to face, is, ‘Can you reach the performance goals of a tunnel using this alternative technique?’ ” Hawkins said from his office at DC Water’s Blue Plains Advanced Wastewater Treatment Plant.

A 1986 map of the city’s sewer and water lines took up a large portion of his office wall, and he referenced it regularly when laying out the water-oriented obstacles that have stymied the city for decades.

The district is among many municipalities in the country whose overtaxed sewer systems have become sources of pollution to local waterways. DC Water’s overflows are the largest point source of nutrient pollution in the Chesapeake Bay watershed, but stemming the tide

of overflows isn’t as easy as turning off the faucet — or even building a tunnel.

When DC Water first waded into the alternative solutions that green infrastructure provides, Hawkins said he asked his engineers to create models based on the best available data, “like you would for any sewer.”

They plugged that data into models and performance charts that helped them construct the right mix of gray and green infrastructure for the system.

Chavez said her organization asked for the comment period on the modification to be extended to have outside engineers review those models and the assumptions behind them. She wants to ensure that the objectives and performance standards for green infrastructure are clear and that the water authority is held accountable for them at regular intervals.

The latest plan also has an “off ramp” built into it. If DC Water isn’t able to install the green infrastructure, or it doesn’t perform as planned, the plan allows for the second tunnel to be enlarged and the third tunnel to be constructed to pick up the slack.

“I’m pretty confident that we’re going to find out it will work,” Hawkins said. “But everybody is in agreement that, if it clearly shows that it doesn’t, we don’t want to continue along that path just because we like it.”

Becky Hammer, a staff attorney with the Natural Resources Defense Council, said the latest proposal has more accountability built into it than a previous one that her organization, “as much as we love green infrastructure,” could not support.

Her organization first asked DC Water to incorporate additional green infrastructure into its long-term control plan five years ago and has been a proponent of doing so, as long as it measures up.

“We want to double check their modeling and make sure that it adds up, that this does achieve the same results as the tunnels would have,” she said.

Doug Siglin, executive director of the Federal City Council’s Anacostia River Initiative, shared the sentiment.

“I think all of us are faced with this trade-off where we really want the green infrastructure solution, but we also want the sense that it’s going to perform well,” he said. “I don’t think it’s realistic to expect the ratepayers to [fund] both.”

While the Anacostia River tunnel is well under way, Hawkins said the Potomac River tunnel had become an engineering nightmare. DC Water’s existing consent decree calls for a

thick tunnel along much of Georgetown’s new waterfront on some lands owned by the National Park Service.

Constructing an underground tunnel there would involve tearing up much of the historic district’s new waterfront and running construction vehicles in and out of an area in which visitors can scarcely find parking. The Park Service required DC Water to conduct an environmental impact study for the construction, which could interrupt cultural, historical and natural elements of the waterfront.

“We’ve been telling everybody, this project in Georgetown is going to be a bear. It is the most challenging thing we’ll ever have to build — something of that size in one of the most expensive and compact parts of DC,” said Hawkins, who is no stranger to harrowing water main projects in the heart of the city.

Before 2014, Hawkins proposed plans that included eliminating the Potomac tunnel, but it became clear that green infrastructure alone couldn’t handle the loads of the highly urbanized region.

In the latest plan, the Potomac tunnel starts east of the Georgetown waterfront and, instead of running to a pumping station near the Lincoln Memorial, uses gravity to convey water to the Anacostia tunnel and then to Blue Plains.

The new tunnel would be half as wide and longer than originally planned, its decreased volume compensated for in part by green infrastructure throughout Georgetown. DC Water already has identified acreage ideal for green infrastructure in those areas, but Hawkins’ next headache might be in making sure the green controls can be constructed as planned.

DC Water does enough street work on its existing pipelines to know the permitting and access issues that can arise when working on city streets.

If navigated well, DC Water could lead the way for other cities considering greener streets as a route to improved water quality.