An 'undercover' problem: N.E. groundwater pollution.

For the inhabitants of East Middlebury, Vt., it was a ''jolting'' exposure to a vexing problem. That, at least, is how one resident describes what happened after Ronald Dayton first found out that one of the underground gas tanks in front of his general store was leaking. Shortly afterwards, some residents noticed an oily smell wafting about. Then came the state Health and Hazardous Waste Department officials, who early this summer detected an underground plume of gasoline seeping away from the station and toward the town's main water supply wells.

''The station had been there for ages,'' recalls Middlebury town planner Fred Dunnington. ''A few years ago, no one ever would have thought anything about a little gasoline getting dumped in the ground.''

Fortunately for East Middlebury, only a small, unused well was affected. But the town's inhabitants seem to have gotten the message. ''We're certainly going to be more sensitive for the potential of this in the future,'' asserts Mr. Dunnington.

Thus, yet another community was rudely introduced to what many are dubbing the ''ground-water decade.'' Once officials were almost exclusively concerned with the more obvious environmental offenses - pollution of the air, lakes, and streams, and most anything else easily seen. Now they are directing much of their attention to contamination of ground water, which slowly courses through underground pores and fissures and provides the United States with nearly half of its drinking water.

''The problem is acute, because 90 percent of the rural population in New England depends solely on ground water for its supply,'' says Michael Deland, administrator for the New England branch of the US Environmental Protection Agency. While the EPA estimates that only about 3 percent of the nation's 15 quadrillion-gallon ground-water supply may be contaminated at levels above officially safe drinking-water standards, much of that is concentrated in areas where dependence on ground water is greatest.

In recent years there have been increasing incidents of contamination to ground water from numerous sources - runoff from landfills, road salt, pesticides, fertilizers, hazardous chemicals, as well as petroleum.

While there is no nationwide or New England-wide monitoring system to determine the magnitude of the problem, the consensus in the environmental community is that it is serious and getting worse. According to one EPA official , there have been ''about 100'' incidents of groundwater contamination, each affecting from one to fifty wells, in New England over the last two years. In Massachusetts alone, says John Terry, director of the state Department of Enviromental Quality Engineering (DEQE), 34 million gallons of groundwater are lost daily to contamination.

''We don't yet know what the exact trends are,'' says Marian Mlay, director the EPA's Office of Groundwater Management, ''but we're hearing about many more incidents.''

Scientists say that once ground water is contaminated, it can be difficult or virtually impossible to repurify it. The ground-water environment is dark, cool, and nearly devoid of biological activity. Because of that, ground water can remain tainted for many decades.

Seven years after Provincetown, Mass., had to shut down its largest well system when 3,000 gallons of gasoline from a local service station's underground tanks inched to within a few hundred feet of the water supply, the town is still importing water from neighboring communities. Hydrologists say some pockets of groundwater there could remain unusable for centuries.

''There has been an 'out of sight, out of mind' attitude toward pollution for so long,'' says Velma Smith of the Environmental Policy Institute (EPI) in Washington, D.C. ''The truth is that there really is no 'away' to throw something. It all goes somewhere, and now we're going to have to start doing something about it.''

Government at the national, state, and local level is grappling with the issue of how to do just that.

At least a dozen bills to help stiffen various existing laws affecting ground-water protection were floated in Congress this year. Last month, the EPA, which administers six federal laws that provide various means of ground-water protection, attempted to tighten up the patchwork by announcing its first ''ground-water protection strategy''.

The EPA's plan includes the introduction of an assortment of federal programs , such as an effort to curb the number of storage tanks leaking petroleum and other chemical compounds. But the strategy would place primary responsibility for safeguarding of drinking water with the states.

For their part, many states are introducing land-use regulations that classify areas for levels of protection depending on how close they lie to major underground water systems. They are also commencing ambitious projects to map the locations of aquifers - underground layers of porous rock that contain water.

In New England, Massachusetts and Connecticut sometimes buy the land above the aquifers.

''The days are ending when we put a landfill over an aquifer,'' says Mr. Terry of the Massachusetts DEQE.

Some experts say landfills are a good example of how well- intentioned efforts to deal with one type of environmental problem can end up spawning others. ''When we focused so much on surface water and air pollution, the ground became more and more the repository for pollution,'' observes the EPI's Miss Smith. ''The sludge that goes into a landfill doesn't just disappear.

Landfills scattered about the country, holding by some estimates as much as 80 percent of the nation's hazardous waste, eventually leak some of their contents. Sooner or later, experts say, much of this will ooze into underground waterways.

There are some 1,400 landfill sites in New England alone. Several methods to shrink their number are in hand, among them the stepping up of recycling efforts to transform waste materials into other products as well as altering industrial processes to reduce the amount of waste being generated.

But until such changes can be made, dump sites continue to pose a hazard to ground-water supplies. ''There are about 500 landfills in Massachusetts, and each one is a ground-water threat,'' says Mr. Terry of the DEQE. Most underground gasoline tanks have an average life expectancy of about 20 years. But nationally, their average age is about 30 years. Federal estimates say that as much as 25 percent of the nation's storage tanks - the contents of which range from gasoline to dry-cleaning fluid - are quietly leaking into the ground.

No one knows how many of the tanks there are in the country, but the number runs into the millions. According to the EPA, one gallon of gasoline can contaminate 750,000 gallons of water to the extent that its presence can be smelled and tasted. Marcel Moreau, a geologist working for the Maine Department of Environmental Protection, estimates that more than 30,000 gallons of petroleum enter the environment daily in his state alone.

Large oil corporations such as Exxon and Mobil have embarked on programs to replace the tanks at all their service stations, nationwide.

The EPA is formulating guidelines for the construction of hazardous material storage tanks - something that has never before been done. And while Connecticut and Maine are the only New England state that now have underground storage tank regulations on the books, all the other states are considering various steps to control them.

Even then, the largest part of the problem will remain to be dealt with. ''Legislation just isn't the whole answer,'' insists the EPA's Ms. Mlay. Many incidents of ground-water contamination are caused by the actions of unsuspecting individuals.

''We're not talking about the big multinational corporations here,'' says the EPA's Michael Deland. ''It's ordinary people, the ones who run the mom-and-pop stores and don't know the laws, that are doing some of the damage.''

Ronald Dayton of East Middlebury wasn't aware of Vermont regulations requiring that his abandoned tanks be removed or filled with sand or that he must take daily readings and keep an inventory. Such precautions, state officials say, might have averted such an incident. Recent attention paid to such subjects may be helping to correct the situation somewhat, says geologist Moreau. ''The word is getting out that people should be watching for potential ground-water hazards.''

Maine has embarked on a major effort to alert the population about such once-ignored hazards as leaking underground petroleum tanks. The program apparently has had some success: last year, 25 cases were reported; this year's number has already hit 58.

Some experts think that education about the problem may not be enough. They say the ''ground-water decade'' holds an indication of the type of environmental problem that will arise with increasing frequency: where cause-and-effect is difficult to pinpoint and remedial action yields results with less certainty than it did in the days of ''simple'' environmental problems like air pollution.

''No doubt about it,'' says Michael Deland of the EPA, ''this is the wave of the future in environmental problems.''