A dearth of contractors trained to control storm runoff and a shortage of inspectors to oversee hundreds of area building sites mean that tons of earth have washed into streams during recent heavy rains, according to experts.

Streams have filled with silt. The Potomac River often looks like cafe latte, making it more difficult to purify area drinking water. And environmentalists say the soil eventually coats the bottom of the Chesapeake Bay, choking out grasses and killing shellfish and crabs.

All of this happens in spite of Maryland and Virginia laws requiring that construction runoff be held behind miles of black plastic sheeting or in ponds that allow dirt to settle before rainwater is released.

But the laws aren't easy to enforce. In addition to the chronic shortage of building inspectors, laws are a patchwork of rules that differ by jurisdiction as well as by how big or how hilly a building site may be.

Joseph Beben, chief of Fairfax County's site inspections branch, said his staff is so stretched that the 27 inspectors and six supervisors can check only on the largest development sites. They seldom have time to inspect the 600 or so "in-fill" building sites in established areas that can be equally troublesome.

"Yes, it would be nice to have more people. We could do a better job," Beben said. "For instance, these in-fill lots. . . . Once the home builders get in there and they start ripping up the lots, they'll have all this activity going on around the house. They all make a mess."

In Loudoun, the third-fastest-growing county in the country, 400 to 500 building sites are being visited by a staff of four full-time and two part-time inspectors. Only a quarter of their time is allotted to site inspections, said Tony Esse, erosion and sediment control administrator. "They can prioritize and hit those projects that really need it," Esse said. "I could take another four."

Most inspectors readily concede that they can't get to each construction site every fortnight, as state law requires. And state officials know that, especially during today's building boom.

"It's impossible for them to do it," conceded Jacob Porter, who manages erosion and sediment control for the Virginia Department of Conservation and Recreation.

Virginia and Maryland provide courses for contractors on controlling building site runoff, but few take them. Certification is required only for inspectors and, in Virginia, for contractors working for the Virginia Department of Transportation.

Contractors working on privately owned sites don't have to be certified in either state.

"We will not impede your construction if you don't have someone with certification," said Ken Pensyl, who oversees the erosion and sediment control program for the Maryland Department of the Environment. "It's more of an education and outreach program."

While many contractors use the right devices--such as plastic sheeting and settlement ponds--to dam up escaping rainwater, many make mistakes or don't maintain equipment, according to Beben.

"One of the most common mistakes is the silt fence gets run over," Esse said.

To keep up with violations, state and local agencies rely on complaints from residents, a system that generated 611 complaints about erosion and sediment control in Maryland last year, officials said.

In Fairfax, there were 21 violations in the first 20 days of September, about twice the number of violations in a typical September, state officials said.

What washes off construction sites in the area ends up in the Potomac River, where water authorities spend millions of dollars annually getting it out. The Fairfax County Water Authority removes 8,000 to 30,000 tons of sediment from the Potomac every year at a cost of as much as $1.5 million, said Tom Bonacquisti, the authority's director of water quality and production.

The Washington Suburban Sanitary Commission, which serves Maryland suburbs and parts of the District, also takes much of its water from the Potomac. It and the Fairfax County Water Authority are considering adding intake pipes farther from the river shores to avoid some of the silt that concentrates there.

Pumping water from the middle of the river means "a lot less chemicals used, and there's a lot less risk involved," Bonacquisti said.

The Fairfax authority's proposal to install an intake pipe in the middle of the river is tied up in litigation. The Washington Suburban Sanitary Commission has budgeted $581,000 to study a new intake pipe location.

Meanwhile, September's 10-inch rainfall, which was higher than average, swept away loose dirt that had accumulated during the drought, and much of it ended up in the Chesapeake Bay, where it settled in clouds on sea grasses and oyster beds.

"We're worried that a lot of the underwater grass beds are now covered with sediment. We're always worried when rain falls on the watershed because it's going to carry runoff with it," said Estie Thomas, natural resources planner with the Chesapeake Bay Foundation. "Sediment is a large part of the bay's ills."