A population time bomb is ticking in Loudoun County, and troubled county officials say there's nothing they can do to defuse it.

"It's frightening, it boggles the mind," says the man in the middle of the growth explosion, Carl Henrickson, chairman of the Loudoun Board of Supervisors.

"I get edgy, I get antsy -- it's staggering," says country administrator Philip A. Bolen as he ponders the financial fallout of the expected gains.

What troubles the county's officials is the $1,400 gap they say is created between the $900 in taxes that the average home in the county pays and the $2,500 it costs the county to service that household. In the past the county has been able to bridge the gap with state and federal funds but the rapid growth projected for the 1980s could make the gap more difficult to close, the officials fear.

"There isn't going to be enough [county money] to go around," Bolen says. "There will have to be cutbacks -- and they will hit the special interests groups." He declines to go into details, saying that's up to his bosses, the supervisors, who will have to decide how and where the ax will fall on their constituents.

While most of the newer suburbs see the 1980s as a time to catch their breath after the explosive growth of the 1970s, Loudoun expects no letup in the new decade. In fact, county officials are worried that the worst is yet to come.

They say the county's population -- about 60,000, according to the newly published census -- could increase about 65 percent by 1990 -- to 100,000 people. "Some residents," says Supervisor Andrew R. Bird Iii, "think, 'I'm the last one, let everything stop from here on.' But it's not going to stop. No one is the last one."

Ground zero for most of Loudoun's population explosion is expected to be the spear-shaped eastern corner of the county -- pointed, perhaps symbolicaly, toward Washington. About 30,000 new residents are likely to settle there in search of the good life for which Loudoun has gained a reputation: a semirural setting, congestion-free roads and the lowest real estate taxes in the region.

But before the decade is over, many fear, Loudoun's current lifestyle may be in critical condition, a victim of a county government that cannot keep its services growing as fast as its population.

Late last week the county was told by its planning department that new growth in the eastern corner alone will require road construction costing $65 million -- most of which apparently will have to be financed by the county. The bill for new scholls will be $40 million to $50 million. The planners haven't yet churned out the cost for extra police, fire and rescue, libraries, parks and recreation and social services, but they, too, will total in the millions.

Developers have agreed to put up $15 million for road costs. but that leaves $50 million of the expected costs unfunded. In the past, the Virginia Department of Highways and Transportation has picked up most of the costs for new and improved roads, but it is so hard-pressed for funds these days it hasn't even put the projects in eastern Loudoun on its plan for 1990.

That leaves one option. If the roads are to be constructed, they will have to be financed in large part by Loudoun taxpayers, who are already restive because of skyrocketing real estate tax assessments and a decision by the supervisors earlier this year to raise the real estate tax rate.

Board chairman Henrickson said that if Loudoun has to pay most of the bill for new roads, the expected growth won't pay for itself unless the houses cost $200,000 each. Most of the houses planned for the county, however, begin at $80,000 and only go up to about $110,000.

Paradoxically, Loudoun has a reputation as an antigrowth jurisdiction. For a while in the 1970s the county, afraid it would become an extension of fast-growing Fairfax to the east or Prince William to the south, did try to place a lid on growth. In one four-year stretch, it rezoned only enough land for four houses to be built.

But the developers didn't let the county's refusal to rezone land stop them. With state court decisions backing them up, they later forced the supervisors to approve rezoning for 10,000 new houses on 20 square miles of land between the Potomac River and Rte. 7 from the Fairfax line west to Broad Run at Rte. 28.

Apart from one present subdivision -- Sugarland Run -- the area is largely virgin territory, which means that expensive roads and services will have to be built there mostly from scratch.

Some members of the Loudoun board are hoping that the county will be spared by outside economic forces -- high mortgage rates and expensive gasoline -- that could put a big dent in the housing market in the 1980s.

Real estate developers don's share that pessimism, noting that most of the housing planned for Loudoun will be priced cheaper than comparable houses in Fairfax and be only 10 miles from Tysons Corner, the rapidly developing white-collar job center for Northern Virginia.

Frank Raflo, one of the supervisors hoping that outside sources will slow the growth, admits his is a faint hope. "If someone accused me of grasping at straws, it would be an accurate observation," he says.

The Loudoun growth projections also have raised questions about whether the county can preserve its agricultural tradition. There are still 1,500 farmers in Loudoun, and despite urbanization, the county still has almost one-third of its land in crop production and another quarter of it devoted to cattle-raising and horse-breeding.

Loudoun planning commission member Ernest M. DeCorte, who lives near Hillsboro in western Loudoun in the heart of farm country, despairs about the county's agricultural future. "Our agricultural heritage is dead," he says.

Troubled by what lies ahead, county officials say one of their next steps will be to appeal to the Virginia legislature for more authority to regulate growth and more money for Loudoun roads.

"We're going to send an SOS, loud and clear, to the next session of the General Assembly," says board chairman Henrickson.

But how sympathetic the legislature will be is, at best, problematic. As county planning director John M. Dugan says, "There are very few jurisdictions in the state with our problems."