Increasing silt and pollution in Springfield's Lake Accotink and Lake Fairfax near Reston, two of the Fairfax County Park Authority's most popular outdoor parks, will be the subject of a $140,000 study under the federal Clean Lakes Program.

The study is to be completed next summer, when public hearings will be held on its findings and recommendations.The report is expected to propose expensive dredging projects to reclaim the two lakes and increased pollution controls on streams feeding into them.

Half of the dredging cost, which county officials expect will exceed $1 million, could be paid through the federal Environmental Protection Agency if it approves the project and has sufficient funds. But Congress cut back this fiscal year's EPA funds from $20 million to $11 million, and only $674,000 of that is allocated for the five-state region that includes Virginia.

The 100-acre Lake Accotink, the second largest lake in Northern Virginia and the most endangered, according to county officials, was closed twice in the early 1970s because of pollution and is quickly disappearing as a lake because of siltation.

An estimated 17 feet of silt -- mud runoff from upstream construction sites and eroding stream beds -- now covers the bottom of Lake Accotink and has made mudflats of much of the upper and most scenic part of the lake, according to Gil Aldridge, conservation superintendent for the park authority.

"While the lake still may appear large, a lot of the water in the upper part is only six to seven inches deep," Aldridge said this week.

Both lakes also are being smothered with algae, caused in part by upstream residents' heavy use of garden fertilizers and "the desire of homeowners to have greener lawns than their neighbors," according to a park authority press release.

Accotink, created as Fort Belvoir's water supply in 1942, was declared surplus and given to the county in 1964, and a 475-acre park was established around it. Much of the silting occurred in the late 1960s, before the county required retaining ponds and other pollution-control measures at construction sites. Boats were getting stuck in the mud in parts of the lake by the mid-1970s.

While most county parks experienced huge increases in visitors last summer, attendance increased only slightly at Accotink -- from 107,000 visitors in the three summer months of 1979 to 135,000 last summer. Boating and fishing activity also declined at the lake.

The 35-acre Lake Fairfax, built in the early 1960s but closed to public swimming several years ago for safety reasons, also is slowly filling with silt. But boating and fishing still continue to be popular at that lake, which the state stocked with bass and bluegills several years ago. Aldridge said he has caught many bass in the lake and "there are even goldfish over two feet long in there, but I didn't fish for them."