The Environmental Protection Agency yesterday proposed regulations that would require municipalities to filter out cancer-causing chemicals that have been found in the water systems of dozens of U.S. communities, including the District [WORD ILLEGIBLE] Fairfax County.

EPA Administrator Douglas Costle said the cost of the proposed regulations to the communities could reach $450 million in the next 3 to 5 years alone, which might add $6 to $10 to the yearly water bills of residents affected.

At a news conference Costle said a recent EPA survey turned up high levels of carcinogenic chemicals in more than one-quarter of the 113 communities samples showed the presence of 21 known or suspected cancer-causing chemicals, Costle said.

With more than 70,000 synthetic chemicals now being produced in the country. Costle said, the problem of harmful chemicals making their way into drinking water is growing. The 113-city sample, he said, provides "a glimpse through the window of what the future holds."

"It is time," Costle said, "for Americans to take out an insurance policy before the problems gets out of hand."

The proposed regulations would limit the presence of a chemical group known as trihalomethanes to under 100 parts per billion in water systems. Trihalomethanes, or THMS, are formed when chlorine is added to drinking water to kill bacteria, a process most municipalities use.

The chemical group is an undesirable product of the reaction between chlorine and organic substances in untreated water, and contains chloroform, a carcinogen.

Under the proposed regulations, all communities of more than 10,000 would have to monitor their water systems for the presence of THMS. Those with a population of more than 75,000 would have to install activated charcoal filters if the THM level rises above the proposed ceiling.

The major cities that EPA says have high levels of THM in their drinking water include Miami, Washington, Cincinnati, Philadelphia and Louisville.

A spokesman estimated yesterday that it could cost $3.5 million to install such a system and as much as $7.5 million annually in operation and maintenance costs.

Fairfax county, which has also had high THM levels in the past, said the county would comply with conversion requirements although it could cost more than $1 million.

EPA officials yesterday estimated that as may as municipalities could end up having to build carbon filtration systems into their water supplies. Filtration through granular activated carbon is the most effective method known for removing organic chemicals such as THMs from water, according to the EPA.

The agency has estimated the cost of a new filtration plant for a city of 80,000 at between $2 million and $5 million. For a city of over 1 million the estimated cost could run to $27 million.

Costle said the most likely cities to require the new filtration systems are those largely ones situated along heavily industralized waterways. In addition to human-caused spills and chlorine treatment of drinking water, he said other sources of cancer-causing chemicals are pesticide runoffs in agricultural areas.

The proposed drinking water regulations were drawn up after the EPA was sued by the Environmental Defense Fund in 1975 for failing to include organic chemicals in its list of drinking water standards enacted after the passage of the 1974 Safe Drinking Water Act.

EDF spokesman Jacqueline Warren yesterday calles the proposed regulations "the first major advance in drinking water treatment in the United States in 50 years."

Costle said the regulations are expected to be enacted in about six months, after a 90-day public comment period. Construction of filtration equipment is not likely to be finished for 4 to 5 years, he said. CAPTION: Picture, DOUGLAS COSTLE . . . $450 million cost