The leaves drift down, settling on the still, green water of the Mattawoman Creek, sending tiny ripples outward in concentric rings. Butterflies bob between shoreline flowers as the afternoon sunlight angles through the trees.

It is a tranquil scene on this part of the Mattawoman, just past Mason Springs in western Charles County. But the signs of human contact cannot be ignored: the rumbling trucks passing overhead along the Route 225 bridge, the empty beer cans and bags that once contained cheese popcorn and sunflower seeds that litter the ground. And in the water, environmental officials say, the invisible effects of human encroachment are threatening the health of this Potomac River tributary.

The Mattawoman Creek contains levels of nitrogen and phosphorous that exceed state standards, officials from the Maryland Department of the Environment told the Charles County commissioners last week. Reflecting their uses on land, the pollutants act as a fertilizer in the water, spawning blooms of algae that lead to bacterial consumption of dissolved oxygen that can damage habitats of aquatic plants and animals.

"There is an impairment," said Richard Eskin of the technical and regulatory branch of the Department of the Environment. "Basically you're not meeting the water quality standard."

The Mattawoman was first placed on a list of impaired water bodies in 1996. As a result, the federal Clean Water Act required the state to set limits on the amount of pollutants that can enter the creek. A report on these limits, called the total maximum daily load (TMDL), has been sent to the Environmental Protection Agency for approval and was summarized for the commissioners.

The report calls for annual maximums of 218,000 pounds of nitrogen and 18,000 pounds of phosphorous to enter the Mattawoman. Currently, 341,000 pounds of nitrogen and 27,000 pounds of phosphorous are entering the watershed each year, according to a draft of the report. Common sources of nitrogen and phosphorous are fertilizers, septic systems and pet waste. The bulk of the pollutants enter the water as runoff from more developed areas in Prince George's and northern Charles counties, then manifest as algae blooms closer to the Potomac, where the Mattawoman widens and the nutrients cook in slow water, he said.

"We're all aware that the major threat to the Mattawoman in the future is increased development in its watershed, particularly close to the creek," said George Wilmot, an environmentalist from Bryans Road. "I think the county should be a little stronger in limiting that, at least keeping [development] out of the valley itself."

The commissioners said they were committed to improving the water quality of the Mattawoman, but they didn't want to hurt the building industry in the process.

"We're going to have to look at a series of remedies and future development practices," said commissioners President Murray D. Levy (D-At Large). "At the same time, we want economic development, we want better jobs for our community, we don't want to strangle the economy."

Levy said the county would work to pin down the specific sources of the nitrogen and phosphorous and come up with practical solutions that could be implemented easily without too much of a burden on taxpayers.

Once the EPA approves the county's daily nutrient load report, the county will have to show progress toward reducing the pollution, although there is no specific timeline for when the improvement will have to occur, officials said.

"What we're looking for are trends going in the right direction, that evolve over a number of years," Eskin said.

If the decline in water quality continues, Eskin said, a third party could bring a lawsuit against the state or county demanding a halt in development until the creek is cleaned up.

"The other [option] is you or the state will crank down" on building permits, Eskin told the commissioners. "Just as if you didn't have the sewage capacity you may not issue building permits, same thing if you don't have the water quality."

The state agency is conducting a workshop in September to explain to county planning staffs around the state how the TMDL process works and about different techniques to reduce pollutants. Levy said the county would continue to monitor the water quality data from the creek.

"Hopefully, we can make some progress," he said.