Residents of this 8,000-acre development 20 miles east of Washington can boast that their community does its part in the battle against water pollution. Instead of dumping treated wastewater into a river like most towns do, St. Charles uses it to fertilize its trees.

The developers of St. Charles came up with the idea were looking for an economical way to treat sewage until scheduled regional center was built.After sewage is collected and treated in a series of lagoons, it is chlorinated and piped to a 100-acre tract of forestland.

Here it is sprayed onto the wooded fields at a rate of about two inches a week. The idea is for the plants to pick up the chemicals in the water and use them as nutrients.This, in turn, cleanses the water.

It seems to be working. Begun in 1966 as the first spray irrigation system in Maryland, the operation was later expanded to its present size.Now it serves close to 12,000 people, with a capacity of over 1 million gallons a day.

According to Dr. William Sopper, a key figure in spray irrigation research at Pennysylavania State University, the wastewater can be low in toxic metals but still supply three times what a farmer might apply of conventional fertilizer. Many woodlands plants at St. Charles are thriving under irrigation, producing more chlorophyll, larger leaves and greater mass, he said.

But there have also been problems. Dr. J. C. Stevenson of the University of Maryland's botany department, which is studying the project, says that irrigation coupled with a high water table in spring has killed some of the original trees, especially pines.

"The roots die because they can't get enough oxygen," Stevenson said. Other plants grow in their place, causing a whole reorganization of the forest, he said. "Whether that's good or bad we don't know."

One advantage of spray irrigation is that it doesn't empty in to a stream - a big plus in the federal effort to keep pollutants from sewage plants out of navigable waters.

"This system has proved to be more efficient than conventional means of effluent disposal in thta it does not contribute pollution of any kind of existing rivers or stream," said Thomas L. Shater, consulting engineer to the project. The hope is that it will prove both safer and cheaper than the elaborate chemical treatment needed in a convectional plant to meet strict EPA water quality standards.

Far from causing pollution, irrigation has been used in some places to replenish low groundwater. When it was tested at Penn State, the ground water was clean enough after filtering through plants and soil that it could be treated for the public water supply.

At St. Charles, many of the studies are directed at finding out what happens in as area where water tables are often too high rather than too low. Monitors from the University of Maryland keep tabs on standing water during the wet season, checking to make sure it doesn't drain into nearby streamlets, carrying both nutrients and pollutants with it. Other studies on groundwater quality are still underway.

One benefit spray irrigation may offer is economic rather than ecological. The nine lagoons and 15 spray fields at St.Charles accupy a total of 150 acres - land bought years before at roughly $1,500 an acre, according to St. Charles Associates President Charles E. Stuart. With a total cost of about, $350,000, including land, the system was far less ecpensive than the average treatment plant.

Apparently the news that spray irrigation is an economical temporary measure is spreading. In the suburbs of San Antonio, Texas the treatment plant was ordered to stop dumping its overflow into the San Antonio River.