A brown slick rides the murky pool of water that has replaced the gas pumps and tarmac of the Hungerford Exxon, a 25-year-old service station on a triangle jutting into busy Rockville Pike.

From a corner of the pit, more of the brown gunk gushes out of the red soil and slate rock and into the ground water, which was exposed Monday by workers installing five 8,000-gallon gasoline storage tanks to replace rusted ones.

Montgomery County fire officials issued a stop-work order Monday on remodeling at the station until the source of the pollution can be identified and staunched.

The Rockville excavation illustrates graphically a water contamination problem that federal and state environmental officials acknowledge is on the rise nationally and locally.

The U.S. Environmental Protection Agency estimates that there are "leakers" beneath the ground at a quarter of the nation's 2.3 million operating service stations.

And there are no estimates of the number of potential leakers at thousands of abandoned gas stations across the country or at homes, farms, schools, churches and businesses that store heating oil and other petroleum products in underground tanks.

Regionally, the problem is ballooning in a pattern that is especially pronounced in the older areas of America. Reports of gasoline and other petroleum products seeping from buried storage tanks into ground water have more than tripled in Maryland in the past five years and nearly tripled in Virginia, according to officials.

No statistics were available on the problem in the District of Columbia.

But in Virginia, "Our experience, as well as Maryland's, has been reflective of the problem on the East Coast and across the country," said Dave Chance, coordinator of of the state's Water Control Board Pollution Response Program.

"Everybody agrees that we are seeing more problems with underground leakage by petroleum products," he said.

"Following World War II, there was a proliferation of gas stations." Chance said. "A lot of these stations now are reaching way beyond what" could be considered a normal life span.

But new underground tanks, improperly insulated or installed, are also causing water contamination problems, said William S. Burgess, chief of the Maryland Water Resources Administration spill-response team.

Burgess said he expects a doubling of the number of problems statewide this year, largely because of increased reporting efforts by officials.

"There are lots that go unreported," he said. "We feel strongly that there were more than 117 leaks in the state of Maryland last year."

At the Hungerford Exxon, four gallons of gasoline and 4,000 gallons of ground water were pumped Monday from the water table 12 1/2 feet below the surface, said Sullivan Curran, an Exxon spokesman in Houston. But it takes only one gallon of gasoline to make 1 million gallons of water -- the amount found in four average-size swimming pools -- unsafe to drink, EPA spokesman Truman Temple said.

"Gasoline floats on top of water, but its colorless, odorless, toxic components, such as benzene, xylene and toluene, remain behind," Temple said. Benzene has been linked to cancer in laboratory animals, while xylene and toluene have caused genetic mutations in bacteria in laboratory tests, according to EPA.

"It's really sad. We don't even find out there's a problem until somebody's well is affected, and that means somebody has already lost their drinking water, their household supply of water," Chance said.

Some areas already have rules governing storage tanks, but uniform federal regulations ordered by Congress will not be ready until January 1987.

Maryland officials are drafting new regulations, including mandatory testing and inspection of underground tanks to expand regulatory authority they have had since 1974. The new rules will be discussed at a public meeting today in Annapolis.

Meanwhile, Exxon's Curran said that the corporation has spent $110 million since 1980 on an early detection system of inventory checks and tank integrity testing. He said it has has reduced the number of leaky tanks owned by the firm by two-thirds in four years.

Last year, Exxon found about 65 "leakers" among the 6,300 stations where it owns the tanks, Curran said. Another 1,800 remain to be checked.