In some parts of the state, septics have become a major source of water pollution. Overall, they account for only about 5 percent of the nitrogen in the Chesapeake, far less than what comes from farms and development.
But in once-rural areas such as Crownsville and Severna Park, septics are a big part of the story. They deliver more than a quarter of the nitrogen entering the Severn.
The problem isn't the flushed solids. They remain in a holding tank. But the wastewater does not. It flows - untreated - into a drain field, where it is absorbed into the groundwater and then seeps into the river. One gallon of septic waste delivers about 15 times as much nitrogen to a river as a gallon of treated sewage.
The reliance on septics along the Severn stems from the area's history as a resort community. As recently as 30 years ago, families from Baltimore and Washington summered at riverside cottages, crabbing and swimming. But when Route 97 shortened the drive, the area became a suburb. The cottages came down. In their place, newcomers put up fancy homes to live in year-round.
"The new people built a huge new house," Hornor said, "but they kept the septic system."
As whole new developments were built, they too had to rely on septics, because sewer service was never extended to them.
Anne Arundel officials didn't know how many septic systems the county had, or where they were, until about three years ago, when they began charging homeowners $2.50 a month under the state's new flush tax.
What they found, said county public works director Ronald Bowen, was "a real eye-opener." Officials counted more than 40,000 septics. Countywide, those systems delivered an estimated 881,000 pounds of nitrogen to waterways in 2005 - compared to 747,865 pounds from treatment plants.
Bowen is convinced that all septics, whether they work properly or not, ultimately fail the rivers. He wants to extend sewage service to neighborhoods that rely on septic systems. But the county can't force existing communities to accept the service.
"We're not in a position right now to go to a community and tell them, 'We're going to make you pay. We're going to make you connect,' " Bowen said.
"But now that we have a better understanding, I think we should be looking more closely at all of our new growth. We need to recognize that, if we're going to approve new communities on septics, at the very least, they should be nitrogen-removing systems."
