Contractors for Alexandria scrape and flush a sewer at the corner of King and Union streets in 2012. The city will speed up by 14 years its study of what to do about the 11.3 million gallons of sewage spilled into the Potomac River each year. (Tracy A. Woodward)

Alexandria will speed up by 14 years its study of what to do about the 11.3 million gallons of sewage spilled into the Potomac River each year from a single pipe in Old Town, the City Council unanimously agreed Wednesday night.

The city is under pressure from the state and federal governments — and local environmentalists — to stop allowing overflows from its combined stormwater and sewer system, which happen every time it rains more than a trace.

The council earlier this year approved a $188 million plan to catch and hold the overflows from three of its four outfalls, which drain south into Hunting Creek and then into the Potomac. But the council said it would wait until 2032 to study how to address the biggest overflow, the one that dumps sewage directly into Oronoco Bay at the foot of Pendleton Street.

On Wednesday, the council voted to have the city environmental services office launch its study of how to address the Oronoco outfall in 2018. Planning would start in 2026, followed by construction in the 2030s.

The council voted in May to build a 1.6-million-gallon storage tunnel and a 3-million-gallon holding tank for the three outfalls that empty south of the Capital Beltway.

The state Department of Environmental Quality, which must approve the sewer repair plan, sent the city a letter in mid-October questioning its assumptions on rainfall.

That tunnel and tank project would be paid for with bonds, potentially adding $10 to $15 per month to the average residential sewer bill, which now ranges from $45 to $60. City officials say a similar fix at the Oronoco site would probably add another $12.50 a month to sewer bills.

Mayor Allison Silberberg (D) and council member Paul Smedberg (D) have asked state lawmakers who represent Alexandria to seek state money to defray some of those costs.

A spokesman for the Potomac Riverkeeper Network on Thursday called the city’s decision to alter its time frame “spinning to a ridiculous degree,”and argued that the years-long studies are merely foot-dragging because the technology to address the overflows is well established.

“That is unacceptable to us and should be unacceptable to the citizens of Alexandria who want to recreate in the river safely,” said Phillip Musegaas, vice president of programs and litigation for the environmental group. “To us, it makes no sense to deal with half the problem now and push the other half of the problem far down the road.”

Silberberg said city officials are moving as fast as they can.

“This issue has been festering in the city not just for years, but decades,” she said. “Given the magnitude of the construction that will be necessary, the city’s timeline is a reasonable and responsible approach.”

More than 800 cities nationwide are grappling with sewer-overflow issues, including Chicago, Philadelphia and the District, which is building a $2.6 billion, 13-mile tunnel to store sewage and rainwater en route to the Blue Plains wastewater-treatment plant, just across the river from Alexandria.

Until this week, Alexandria planned to address the North Old Town sewage overflow in the short term by requiring separate sewage and stormwater systems in new projects and encouraging installation of permeable pavement, green roofs and more trees.

After pushback from some residents, environmentalists and state Sen. Scott Surovell (D-Fairfax), whose legislative district lies just south of Alexandria, the council decided to also speed up its plan to study of what else might be needed.

At their meeting, council members credited local activist Jack Sullivan, who wrote a report to a citizens advisory committee recommendation, for pushing them to come up with a more proactive plan for Oronoco Bay.

Silberberg said his effort was “a great reminder that one citizen can make a difference.”

Sullivan called the council’s action “a step in the right direction” Thursday, and said he was glad the council had “acknowledged that we have an obligation to clean up” the Oronoco Bay sewage overflow as well as others.