With deer and groundhogs scampering over its well kept grounds, and the waters of Chesapeake Bay lapping at its edges. Aberdeen Proving Ground in northeastern Maryland doesn't fit the image of a major polluter. There are no obvious smokestacks belching smoke, no obtrusive pipes spewing gunk into the bay.

But the Environmental Protection Agency (EPA) says that Aberdeen, one of the Army's major weapons testing facilities, is among the nation's 18 worst federal violators of antipollution laws. And last week, as part of a crackdown on federal polluters across the country. EPA announced it might even sue Aberdeen unless it cleans up its act.

As a visit to Aberdeen made clear this week, though, it's not so easy to sort out the good guys and the bad guys in a confrontation like this.

Aberdeen officials say they are dumbfounded. They knew they had some problems, they say, but were working on them. What kind of game is EPA playing?

EPA officials, in turn, scoff at Aberdeen's surprise. They knew what was coming, they say. Who's kidding whom?

The main source of the dispute is Aberdeen's sewage treatment plants - three big ones, seven small ones. It's their job to clean the water that carries Aberdeen's sewage, most of which is human waste, and then to discharge the cleaned water into several small creeks and rivers that feed into the upper Chesapeake Bay.

The plants themselves are innocuous-looking structures - much less innocuous looking than the Army weapons dotting the grounds. For the most part they consist of low lying buildings with outdoor vats that give off that unmistakable sewage smell - mostly methane gas.

Through a combination of techniques - filtering out solid wastes, decomposition of the wastes with the aid of air, sunlight, and bacteria, and chlorination - the plants remove about 85 per cent of the pollutants in the wastewater.

But the standards set forth under the 1972 Water Pollution Control Act now require the use of more advanced chemical treatment, and the removal of about 90 per cent of the pollutants.

That advanced treatment was supposed to be in place in all industrial plants and sewage treatment facilities by July 1, 1977, and Aberdeen does not dispute that it has missed that deadline.

What it disputes is the EPA's charge that Aberdeen has not been moving along as speedily as it could. As EPA officials wrote in a letter to Aberdeen this month, they believe the facility is "proceeding on such a casual schedule that present and future bad faith can be assumed."

Aberdeen officials say they don't understand that charge. Since the EPA notified Aberdeen in mid-1975 what its final pollution limits would be, says Byron Glovier, an environmental engineer with Aberdeen, employees there have been working as quickly as possible on the problem.

Because of a variety of Army regulations on hiring and purchasing procedures and the preparation of construction plans, he says, it took until the end of 1976 before Aberdeen could even submit its proposed construction plan to its Army high command.

"It would have taken some sort of regulatory miracle to move more quickly," says Glovier. He says Aberdeen has to wait for Congress to approve the Army's request for $6 million in construction funds for the needed improvements.

The earliest that Aberdeen could even get the money, he says, is mid-1978, which means that Aberdeen could not possibly be in compliance with the antipollution law before 1980.

Aberdeen authorities say they warned EPA all along that they would not be able to meet the deadline. "But we kept running up against the same thing," says William Russell Jr., an engineering technician. "'Don't give us your excuses,' they'd say. 'Just do it.'"

The one other thing that throws them back on their ears, say Aberdeen officials, is the EPA's designation of Aberdeen as a "major" polluter.

"We discharge less than 2.5 million gallons (of wastewater) a day," says Russell. "That's nothing compared to some of the other plants in this region."

The EPA does not lend a sympathetic ear to such defenses. "We told them a year-and-a-half ago that their schedule was not acceptable," says Jim Harper, a chemist with EPA's regional office in Philadelphia. "Other federal facilities have acted in a timely fashion. To us it appears that Aberdeen has been dragging its feet."

Indeed says Don Knott, chief of the EPA's federal facilities section in regional headquarters, it's because of Aberdeen's "nonresponsiveness" - not because of its output of pollutants - that EPA named it a major polluter.

Aberdeen could have eliminated pollution problems at its seven smaller plants fairly cheaply by connecting them directly to the larger plants, Knott says - but Aberdeen hasn't done that. Aberdeen could have received construction money through emergency funding - but it chose not to seek it.

"It's not the amount of waste that made us determine they're one of the worst," says EPA's Harper. "It's the lact of responsiveness. You can go about doing projects in a lot of ways. There are mechanisms for doing things quickly. They took about the longest route they could take to solve their problem. "Thank goodness we didn't fight World War II in that way."

While EPA is obviously displeased with Aberdeen, there's some question whether the antipollution agency can or will actually carry out its threat to sue the Army facility. It's not clear. EPA officials concede, whether as a federal agency. EPA has the right to sue another part of the federal government.

But "whether or not we can sue is relatively unimportant," says EPA spokesman. "We hope to use all sorts of administrative procedures first. Only if all else fails will we try to go to court."