Heavy rains routinely trigger big sewage overflows in Baltimore, but there is growing evidence that chronic leaks from the region's aging, cracked sewer lines are a bigger threat to public health.

Though storm-fed spills can be dramatic, Baltimore's' streams and harbor are also fouled on sunny days as storm drains yield grayish discharges that look and smell like sewage. That is what they are. Even the nearly $2 billion overhaul under way on the 3,100 miles of sewer lines in the city and Baltimore County won't be enough to make those waters safe, experts and activists say.

Leaks allow raw sewage to seep into storm drain pipes, which funnel rain from streets, parking lots and buildings into nearby waterways. In some cases, the waste is being piped directly into storm drains through illegal connections. But mostly it dribbles from fissures and breaks in an underground network that includes century-old brickwork.

"It's incredible to me that in 2011 we have raw sewage leaking," said Michael Hankin, an investment executive who chairs the Waterfront Partnership, a group of Inner Harbor businesses and nonprofit organizations pushing to clean up area waters. "Over in Gwynns Run," he added, referring to a stream flowing through Carroll Park, "kids play in it. How could that be?"

Mayor Stephanie Rawlings-Blake is scheduled to join the partnership Wednesday in releasing a plan to make the harbor swimmable and fishable by 2020. The blueprint is expected to call for increased efforts to curb trash, sewage and other pollutants that foul the Northwest and Middle branches of the Patapsco River, which make up Baltimore's harbor.

But Hankin and others warn that unless the chronic sewage leaks are dealt with, there's little chance to achieve the goal of a "healthy harbor."

"It doesn't take a lot of sewage to contaminate a stream or harbor," said William P. Stack, deputy director of the Center for Watershed Protection, a nonprofit in Ellicott City that works with communities to clean up their waters. Before joining the center last year, Stack spent more than 30 years dealing with the effects of sewage leaks and spills in Baltimore's Department of Public Works.

City and county officials say they would do more about the leaks but are hampered by limited budgets as well as state and federal regulatory requirements to focus on other water quality issues.

"Ideally, if we had all the funding in the world, we could address everything," said Kimberly L. Burgess, chief of surface water management in the city's Department of Public Works. "But we get constrained by having to meet the mandates we have right now."

The city and Baltimore County are bound under consent decrees with state and federal regulators to halt their sewage overflows or face stiff fines. Officials say they are making progress, but the city has reported more than 360 overflows this year, spilling more than 10 million gallons of untreated wastewater into area waters. Baltimore County has tallied 148 overflows that dumped nearly 152 million gallons — two-thirds of it in a spill after a large sewer pipe blew out during Hurricane Irene in August.

The city says it will spend $1 billion by 2016 to repair its sewer system, and the county expects to spend nearly $1 billion by 2020. They also face mandates to curb polluted storm water washing off streets, parking lots and roofs.

Officials at the Center for Watershed Protection and others say the consent decrees are aimed only at halting overflows from the largest sewer lines and contend it is short-sighted not to deal with leaks from the smaller pipes that make up the bulk of the system.

Sewage discharges are a significant source of the nutrient pollution that periodically triggers algae blooms and fish kills in the harbor and the Chesapeake Bay, according to the center. Stack says up to 15 percent of the nitrogen in urban watersheds could stem from failing infrastructure.

"One small leak can deliver hundreds of pounds of nutrients over the course of a year," said Lori Lilly, an ecologist at the Center for Watershed Protection.

The center's samplings of area streams and storm drains have been eye-opening for federal officials directing the bay restoration effort, who have had a hard time accounting for nutrient pollution in some waters.

"There's always been this disconnect with what good science says is coming off the streets compared to what's in our streams," said Richard Batiuk, associate director of the bay program office for the Environmental Protection Agency. "The missing piece could be those illicit discharges."

Consultants with the center teamed up last year with city and county employees and environmental activists to slog 18 miles of streams in the city and Baltimore County, checking for dry-weather discharges from 81 storm drain outlets. They detected sewage and other pollution in four out of five.

"We found a bunch of hot spots," said Lilly, who led the center's sampling effort. At some, they saw broken sewer pipes leaking grayish water and bits of toilet paper into the stream. In other cases, the water coming from storm drain outlets smelled of sewage or a test detected ammonia, a chemical ingredient of urine.

A water hazard in Carroll Park