George J. Flanjack's reason for buying a house in Pleasant Valley, a community of trim, two-story houses south of Dulles International Airport, was simple enough.

"We came here to get ahead on dollars," says Flanjack, a Northern Virginia utility executive, who figures the three-bedroom home he purchased less than a year ago for $73,500 is now worth at least $13,000 more.

What has helped Flanjack's investment more than he and the other 40 families in Pleasant Valley probably realize was an invisible benefactor who literally funneled money into the project through a pipeline.

That benefactor was Uncle Sam.

By pumping millions of dollars into the Washington area in the early 1970s, the government made Pleasant Valley and numerous other suburban subdivisions possible. The money, used to upgrade and expand sewer capacity in counties like Fairfax, was the government's response to pleas from environmentalists pressing for a cleanup of the Potomac River.

Today -- and $280 million later in Northern Virginia alone -- environmentalists are conceding that their zeal has spawned another form of pollution: suburban sprawl.

Sprawl, say its critics, gobbles up sensitive areas of the landscape, forces continued reliance on the automobile in the middle of a long-term energy crisis, strains the capacity of local government to provide the costly services residents demand and, sometimes, puts houses in the wrong place.

Pleasant Valley seems to be a prime example. There are no schools near Pleasant Valley or the two adjacent subdivisions under construction. There are no stores, libraries or playgrounds.

In addition, all three subdivisions are within a corridor that, according to county planning officials, will experience high levels of noise from Dulles Airport -- whose runways are just two miles away -- when Dulles traffic increases as it is expected to do in the 1980s.

Until expanded and upgraded sewer facilities came to the area in the form of an $82 million regional sewer plant south of the three subdivisions, Pleasant Valley and its neighbors were not possible.

It Pleasant Valley, federal grants total $59.6 million for improved sewer services, amounting to a subsidy of $1,666 per lot.

"The fact that the sewer is here, I'm afraid, pretty much sets the tone," says Fairfax Supervisor Marie B. Travesky, who represents Pleasant Valley and the rest of the rapidly developing Springfield District in Fairfax.

And the sewers will be readily available for sometime to come. Because the Environmental Protection Agency, which approved the sewer grants, was so generous and asked so few questions, Fairfax has enough surplus sewage capacity to build another 100,000 homes.

"The county has sewers coming out its ears," says Norman M. Cole Jr., a Fairfax enviromentalist who championed the Potomac cleanup in the early 1970s.

Cole, an unpaid consultant to Gov. John N. Dalton, describes the excess sewer capacity as a fluke. Federal officials, however, say it was a mistake. Subsidizing Urban Sprawl

"There is no question that in the past, EPA's construction grants program has inadvertently subsidized excess urban sprawl," Thomas C. Jorling, a former assistant administrator with EPA, told a homebuilders' convention last year.

At EPA regional headquarters in Philadelphia, water programs chief Green Jones blames local governments for overbuilding. "The attitude then (in the early 1970s) was, 'Let's get in there and get what we can get. Let's overestimate.'"

Cole agrees that local governments played a key role in the size of the sewer grants, although he notes that other factors helped create the surplus.

"Northern Virginia localities picked the number (for the size of sewer plants) based on more future growth than (has) materialized," he said. "(In addition), water-saving devices came along. There were changes in the pricing of water that encourage conservation."

Still, when Cole was the activist head of the Virginia State Water Control Board, there was no doubt that he and other area officials were bitterly opposed to efforts by Fairfax to slow its growth by limiting sewers.

"Fairfax shouldn't be using sewers as a birth-control pill," Cole said in a stinging commentary then. "Just like any other utility, the county should face up to the responsibility of providing sewage treatment in a timely way so as to control pollution and to handle needed growth and development."

There also was pressure from the Northern Virginia construction industry, which warned five years ago that Fairfax County's sewer plans were so inadequate that demand would go unmet for at least a decade.

So Fairfax "faced up" to its responsibility as defined by Cole. But even as the county board was helping get all the sewer capacity possible, many of its members thought they would be able to help decide how and when it would be used. County Lost Control

"We thought we would have the tools to manage growth and control our own destiny," said Supervisor James M. Scott (D-Providence), one of the board members elected in 1971 with a double mandate to clean up the water and control growth. "But the Virginia courts took them away."

At the same time, Scott said, severe limitations introduced in suburban Maryland pushed development fever from Montgomery County to Fairfax. In addition, he said, "The single-family market (the basic force behind sprawl-type development) exploded and the multi-family market died."

A look at the history of development in Fairfax shows that the supervisors sometimes made decisions that helped encourage sprawl. In 1975, under intense pressure from citizen groups that wanted to preserve their suburban idyll, the supervisors adopted a master plan that, in effect, approved low-density sprawl as a planning policy.

Other decisions by the board -- in combination with the newly available sewer capacity -- brought development to areas that even county officials had not foreseen. One of the most significant examples is what happened in South Run, an area in southern Fairfax. Above the banks of South Run, there was a virtual wilderness that county planners recommended be set aside for park land.

But not far from where beavers play, four builders are one-third finished with a community called Newington Forest. When they are done, there will be 1,708 houses and 200 apartments.

Newington Forest was made possible by expanding and upgrading the lower Potomac Sewer Treatment Plant, a $77 million project to which EPA has contributed almost $58 million. But it also was made possible by the recommendation to acquire the area for park land.

Newington Forest apparently is having a domino effect on the rest of the South Run area, which planners had designated as an "environmental quality corridor."

For example, the county tried to keep sprawl out of the area north of Newington Forest by limiting development to two-to five-acre lots. But a local court ruled last year that the existence of Newington Forest meant that smaller lots -- in effect, a pattern of sprawl -- could be carved out of the wilderness north of the subdivision.

In presenting his successful case to Fairfax County Circuit Court, zoning attorney John T. Hazel Jr. recalled the Newington decision, and noted: "The sanitary sewer . . . was the facility that made all the rest . . . come into place."

Elsewhere, in the northern part of the county, the picturesque valleys and meadows, through which Difficult Run Courses, are beginning to fill up in similar development patterns. The county has made an unusual effort to accomodate that growth. It has decided to pump sewage uphill to the lower Potomac plant and possibly its Alexandria facility, which was expanded and upgraded at a cost (still rising) of $95 million (EPA contribution: $71.3 million).

The rationale for expanding the Alexandria plant to handle 54 million gallons of sewage daily was that Fairfax would continue growing in the already largely developed area south of the Beltway and just west of Arlington and Alexandria. Surplus Sewer Capacity

Although the area is near job centers and services, cheaper land is less developed areas was available, and little of the expected growth has occurred or is on the drawing board. The result is that Fairfax has 9 million gallons of surplus sewer capacity -- or enough to handle 90,000 additional people -- which can serve more distant growth areas. At today's costs, that surplus is worth about $36 million -- far more than the county's bill for the entire 32.4 million gallon-a-day share of the Alexandria plant it purchased in 1973.

Supervisor Martha V. Pennino (D-Centreville), whose district brushes up against Pleasant Valley, questions whether the county will be able to provide the other services -- schools, libraries, police protection -- that development inevitably requires.

"This kind of growth is going to be very costly to future citizens," Pennino said. "We're not going to have the funds to service growth. We'll have sewers, but what about police, fire and rescue and roads?""

Last year, Pennino urged her fellow board members to exclude the Difficult Run area from those zoned for development, but the other supervisors objected. Later, in a stunning reversal, the board decided not consider any areas for exclusion.

Yet Pennino, in other votes, generally went along with sprawl-type development. "That's what the people want," she said.