NEWTON - Call it the clean little secret of the sewer business: Most of the wastewater flowing through Greater Boston's pipes to the new $3.7 billion sewage treatment plant on Deer Island was clean before it got there.
More than 60 percent of the wastewater treated at Deer Island last year was ordinary water that sneaked into pipes through leaks and breaks. Backwash from the tides or runoff from storm drains represents an enormous waste of treatment dollars, hundreds of sewer backups into homes, and a diversion of billions of gallons that could have stayed in wells and rivers.
''A large amount of the ground water that should ... recharge streams is actually leaking into sewer pipes and being carried out to Deer Island for treatment,'' said Michele Barden of the Neponset River Watershed Association. ''Here, everyone is talking about how expensive sewers are. Yet in many communities, 70 percent of their flow is'' clean water.
This so-called ''infiltration and inflow'' into the sewers, which is common in older sewer systems, has become the latest obstacle in the costly campaign to modernize the region's plumbing. Already, the issue is bogged down in a bitter dispute over who is in charge and who should pay the potentially huge bill.
Yesterday, a task force set up by the Massachusetts Water Resources Authority to address the problem had to be coaxed into continuing its role. Members are frustrated by federal and state regulators trying to force expensive new construction despite the billions the MWRA has already spent on the system, driving local sewer rates sky-high.
''What you're hearing from us is a great fear of costs, of open-ended cost increases,'' said Michael Hornbrook, chief of the MWRA's sanitary flow division, to regulators at the meeting in Newton. He estimates that reducing water leaks into the sewers by even one-third would cost at least $3 billion.
The MWRA, and most of the 43 communities on the MWRA sewer system, strenuously object to a new requirement from the US Environmental Protection Agency and the state Department of Environmental Protection that the authority reduce leaks into the pipes, even if it means forcing communities to do so.
Community officials say they do the best they can with limited resources, and they do not want the MWRA or EPA ordering them to do more. ''We think we're doing a pretty good job,'' said Joseph Celano, superintendent of water and sewer in Ashland.
That's small consolation for the hundreds of households where stormwater causes sewer pipes to overfill, backing up into their homes.
But environmental groups say that many communities have gotten so accustomed to astronomical leak rates, they do not take the issue seriously. For instance, the town of Canton wants to build a new public well, but it is losing 2.8 million gallons of fresh water each day to the sewers.
''I find it hard to stomach that towns are looking for new water supplies, but yet not managing their existing water supplies well,'' said Barden, whose organization has sued to stop Canton's new well.
Boston has been investing in leak reduction for more than 20 years, diverting an estimated 181 million gallons of clean water from the sewers during a typical severe storm.
''Boston has done such a terrific job they should be held up as a model, and the other [sewer] commissioners don't look at it that way,'' said Barden.
MWRA officials have known for years that the 5,400 miles of municipally owned pipes that connect communities to the system are riddled with leaks. There are many reasons water gets into the system: ancient clay pipes have broken open, joints have failed, tide gates have rusted open, and gutters on urban houses drain to the sewer.
Eastern Massachusetts' high water table makes it even harder to design a tight sewer system. ''It would be like trying to build something in a swimming pool that doesn't leak,'' said Carl Leone, the MWRA's authority on infiltration and inflow.
The actual rate of infiltration and inflow varies with the amount of precipitation - in the wet year of 1998, the leak rate was 62.1 percent - and it varies widely between communities as well. Dedham, which has some of the oldest sewer pipes, has an 80 percent leak rate. Ashland, with one of the newest systems, has a leak rate of just 42 percent.
Leone is convinced that the MWRA's leak problems are about the same as other New England urban areas.. But he cannot know for certain, since sewer officials apparently do not like to talk about infiltration and inflow.
''We have had a hard time getting numbers out of'' other sewer districts, Leone said. ''People generally don't like to talk about it that much.''
But the cost of fixing these leaks was so difficult to determine that the MWRA concluded in the mid-1980s that it was more cost-effective to build a new sewage treatment plant on Deer Island big enough to handle vast amounts of fresh water rather than trying to reduce leaks.
Today, more than $4 billion in sewer treatment system improvements later, residents of the MWRA water and sewer service area pay average annual bills of nearly $700 per household, and local officials are wary of spending even more.
The MWRA started a campaign to reduce infiltration and inflow five years ago, doling out $54 million to municipalities to reduce leakage. It admits it is a modest step. But coupled with improvements in sewage treatment, the MWRA is making progress in reducing instances of sewer system overflow into homes.
Still, regulators from both the state and the EPA say more action is needed. In the permit for the MWRA to discharge treated sewage from the Deer Island plant outside of Boston Harbor, the agencies called on the authority to develop a plan to reduce infiltration and inflow, without specifying how much the authority had to do.
''There's no way for any of us to know in advance what the cost of all this is going to be,'' said EPA attorney Ken Moraff, who promised that the agency will be reasonable.
Moraff, along with state regulators, managed to persuade the MWRA's task force not to suspend work altogether as members had threatened. But members made it clear they will fight the new permit.
''I'm telling you now, the MWRA's role will never be as a regulator,'' said Joseph Favaloro of the MWRA Advisory Board, which represents the municipalities.