The Bethelehem Steel Corporation agreed today to pay the state of Maryland $500,000 in fines for polluting a tributary of the Chesapeake Bay with toxic materials at twice the allowable rate.

In exchange, the state gave the sprawling near here an extra three years to reduce drastically the amount of pollutant - nearly 100 tons each day - that it dumps into the Patapsco River. Bethlehem is the industrial polluter of the Chesapeake.

State officials say that the company dumps toxic cyanide, ammonia and chromium into the river from its massive operation in east Baltimore County. Pollution from Bethlehem was said to be the cause of large fish kills in 1971 and 1972.

The three-year extension and the fine - said to be called the largest imposed for a water pollution offense in Maryland - were part of a settlement worked out by company lawyers and state officials and filed today in Baltimore County Circuit Court.

The state sued Bethlehem Steel two days ago charging that the nation's second largest steel company had intentionally failed to meet Maryland standards by July 1, the deadlune for a three-year grace period.

Maryland's attorney general, Francis B. Burch, who unveiled the settlement at a press conference here, called the agreement a "drastic instance" of the state's commitment to clean up its waterways and a warning to industry that "if they don't clean up the mess they'll have to pay the price."

Federal Environmental Protection Agency officials in Philadelphia said the time extension seemed too long and the fine to small for a large industrial polluter that had failed to comply with a deadline originally set by Congress in 1972.

The Bethlehem Steel settlement is the state's first legal action taken against the 280 municipalities and industries in Maryland that have failed to meet water pollution standards by the July 1 cutoff date. Burch said the other polluters had been cited for violations and face administrative action.

According to today's settlement, the steel company will cut discharges from its Sparrows Point plant to the required level of 75,000 pounds a day by September, 1980, after completing installation of about $200 million worth of new equipment and pollution control devices.

The agreement cited three major sources of pollution discharge at the plant - blast furnaces, coke ovens, and a waste pickle liquor facility - and set separate deadlines for bringing each into compliance with state requirements.

The most time consuming improvement will be applied to the waste pickle liquor plant, which cleans steel with acid and spews the used material into the river. Bethlehem Steel is planning to spend $4 million on the first phase of an experimental device to recycle the industrial acid. The company will have completely installed a $15-million corrective device by September, 1980, or have found an alternative remedy, according to the agreement.

The company plans to replace its four polluting blast furnaces with a new $180 million model with low discharges by April, 1979, and reduce pollution from its coke ovens by January, 1979, the settlement said.

If the company fails to comply with the new time table, it faces fines of $1,000 a day for each day of unacceptable pollution level from the waste pickle liquor plant; $3,000 a day from the blast furnaces, and $1,000 a day from the coke ovens.

Half of the $500,000 fine Bethlehem Steel agreed to pay in today's settlement will be used to help finance a state study of pollution in the Baltimore harbor. The remaining $250,000 will go into the state's general fund.

Attorney General Burch, asked whether $500,000 as too small a fine for a nationwide company with total sales last year of $5.2 billion, said the levy was reasonable in light of the firm's recent economic setback.

Bethlehem Steel intentionally delayed construction of the costly new blast furnace, Burch said, because of the economic losses it has experienced as results of foreign competition in the steel industry. "We had one of two alternatives," he told reporters. "Either shut them down or take them to court and get a decree that will force them to comply in the future."

Bethlehem Steel is the largest private employer in the Baltimore area with a work force of 18,000. Last year's payroll exceeded $320 million at the Sparrows Point plant.

Stanley L. Laskowski, chief of the industrial permits branch of EPA's Mid-Atlantic office in Philadelphia, said the Maryland settlement was the largest so far in this region for industries failing to meet the July 1 deadline.

But, he said, the fine would have been twice as large if the state had used EPA's "national penalty policy" of fining the polluter for the equivalent of interest payments it earned on money that should have been used for pollution control.

Laskowski said he questioned whether the state was too lenient in extending the deadlines for curbing pollution from the coke ovens and waster pickle liquor facilities, although he agreed with the new cut off date for the blast furnace.