But administration officials insist that figure is misleading. They point to estimates that in certain tributaries, the percentage of nitrogen from septic systems can go much higher -- as much as 19 percent in the lower Western Shore, including Calvert County. In Solomons Harbor, where agriculture is a minor factor, densely packed septic systems could account for more than half of the nitrogen pollution in the watershed, according to the Chesapeake Biological Laboratory in Solomons.
In inland areas of the state, officials are concerned that septic systems can pollute private wells and municipal water systems.
Administration officials say it is important to get a handle on septic systems because they could become an increasing source of nitrogen pollution as more homes sprawl into agricultural areas without sewer service. They note estimates that the state has a capacity for 1.6 million new housing units on septic systems -- four times the number in use today.
Environmentalists say Maryland has taken steps to control nitrogen pollution from sewage treatment plants and farms. Septic systems, they say, are the last major source of nitrogen pollution the state has not addressed.
Advocates also say it is only fair to require septic system users to contribute their share to cleaning up the bay, noting that urban residents pay for nitrogen removal in their sewer bills.
The state estimates that about 30,000 of the state's 400,000 septic systems are failing and in need of replacement. But the administration contends that all conventional septic systems -- not just failing ones -- contribute to pollution because they weren't built to filter out nitrogen.
The administration's bill would force replacement of conventional septic systems with newer technologies that can remove an estimated 60 percent of the nitrogen in the effluent.
Not only are these technologies more expensive, they require more maintenance than conventional systems. Even advocates say the law would have to be backed by regulations requiring homeowners to enter maintenance contracts costing $150 to $250 a year.
The administration and manufacturers of the advanced systems say the cost will decrease as more of the new systems are sold. They say the costs will also be offset because the new technologies will extend the useful life of septic drainage fields.
But Realtors and homebuilders say the measure would have a chilling effect on real estate markets -- especially the 20 percent to 30 percent of Maryland homes that aren't on public sewer systems.
"This is going to affect real estate markets everywhere," Antoun says.
Earl Armiger, a Howard County builder who served on the task force that drafted the bill, says his industry's concerns were ignored. He said his service on the task force confirmed his fears that the true purpose of the initiative was to limit growth in rural areas.
The prospects for the governor's legislation are cloudy. The Senate committee that took up the bill Friday is considered friendly to environmental legislation, but members of the House Environmental Matters Committee indicated that the bill would have to be rewritten extensively to pass.
"I think something's going to come out of here. I can't say what kind of shape it'll be in," says Del. George W. Owings III, a Calvert County Democrat. "I'll tell you, some work needs to be done on the bill."
By the shores of Solomons Harbor, Joy summed up the fundamental difficulty the governor and legislature face in doing that work: "Everybody wants clean water," he says. "Nobody wants to pay."