The Environmental Protection Agency is calling in the Army.

Finding itself involved in the biggest peacetime public works program in history - the construction of sewage treatment plants - EPA, according to officials of the agency, simply couldn't keep on top of what was being built.

So EPA has invited the Army Corps of Engineers to oversee all major construction projects. There will be plenty to oversee in coming years. During the next five years, EPA expects to dole out a minimum of $24.5 billion for water-cleanup projects.

In a six-state area including the District, Maryland and Virginia, the corps will oversee more than $1.25 million in construction under an agreement still being worked out between the corps and EPA's regional office. The corps will inspect current as well as new construction.

With states and localities providing 25 percent of the money, the overall cost of the projects nationally should reach a staggering $30 billion.

The EPA invitation could not have come at a better time for the corps. With the Carter administration deemphasizing the corps' traditional mission - construction flood-control and water-supply projects - the agency has more people than there is work, corps officials say. About 600 corps engineers and technical people are expected to become involved in sewage-plant construction.

To keep it personnel busy, the corps is also asking on construction assignments abroad. Saudi Arabia, for example, has hired the corps to oversee $2 billion worth of work.

"We welcome the opportunity," said the corps' John Ryan, chief of the civil works construction operations division, about the new agreement with EPA. "We feel we can be of service of the taxpayer and the country."

While the corps is used to being boss, it will be outranked by EPA. "Our agency is in the driver's seat," said Al Pelmoter of EPA's construction operations operations branch.

Though it has been giving out billions of dollars - $18 billion since 1973, with most of it in the past three years - EPA has had virtually no inspection staff.

"Our manpower ceilings have prohibited us from following construction," said James Murphy, chief of the construction operations branch. "We have been trying to get additional people from the Office of Management and Budget. We just didn't have the bodies."

The agency has had from three to four people assigned to metropolitan Washington - the site of almost three-quarter of a billion dollars' worth of construction. But according to Joseph A. Galda, chief of the facilities management branch at regional headquarters in Philadelphia, "they haven't been able to get involved in inspection. We haven't had the time or the manpower."

The team has been involved, he said, in other duties, including trying to resolve the ongoing dispute over the reginal plant Montgomery County wants to build - over EPA opposition - at Dickerson in the northwestern part of the county.

When EPA can concentrate on the planning and designing of facilities, "we might be more responsive to localities," Galda said.

Most of the construction money in the Washington area is being spent at the Blue Plains regional plant in the District, which is in the midst of almost $500 million worth of improvements. When the improvements - designed to better the quality of the effluent - are finished, Blue Plains will be the largest advanced-waste water treatment plant in the country.

Many EPA-funded projects have been marked by cost overruns, some of them considerable. For example, the cost of the Upper Occoquan Sewage Authority plant in Fairfax, one of the most advanced treatment facilities in the country, doubled to about $85 million in less than five years. The reason generally assigned to the overruns has been inflation.

But EPA officials are hoping that with the corps overseeing construction, a tighter rein can be kept on overruns. "I think more contractors are going to have to be on their toes," said Pelmoter. "There will be regular inspection by people who know the construction business."

Eventually, EPA hopes that the states will assume the corps' role.Recent amendments to the Clean-Water Act permit states that oversee the program to receive 2 percent of EPA grant money as reimbursement. So far, California is the only state to run a construction program, although Maryland is moving in that direction.