Environmental Protection Agency officials overrode the warnings of EPA scientists earlier this year and agreed to the planned reopening of the Savannah River nuclear plant in South Carolina, which the scientists said would pose serious health and environmental threats.

On the recommendation of Paul Cahill, director of the EPA's office of federal activities, Anne M. Burford, then EPA administrator, told the Energy Department that it could proceed with the project without preparing an environmental impact statement.

The Reagan administration wants to reopen the nuclear plant near Aiken, S.C., closed since 1968, to produce plutonium for new nuclear weapons.

The National Security Council instructed the Energy Department to restart the reactor by Oct. 1.

According to internal EPA memos released by Sen. Ernest F. Hollings (D-S.C.), EPA scientists warned that reactivating the Savannah River plant could contaminate drinking water wells in nearby communities with mercury and other dangerous chemicals.

In addition, scalding water discharged from the plant would flow into the Savannah River, which flows through Georgia and South Carolina, carrying radioactive wastes and destroying 1,000 acres of wetlands, according to a memo to Cahill from an EPA radiation expert.

Cahill's office examines whether federal government projects pose health or environmental hazards.

EPA scientists recommended in memos that the plant not be reopened unless the government conducted a major environmental impact study.

EPA sources said that Atlanta regional Administrator Charles R. Jeter also recommended such a study, but was overruled. Saying that he never put a recommendation in writing, Jeter said, "I had my input. I have to leave it at that."

Jeter said he believes that EPA can control the environmental impact of the reactor through existing laws on air, water and hazardous wastes.

Hollings wrote President Reagan to say that he was "astounded and deeply disturbed" by the EPA decision "since it appears to be a deliberate attempt to prevent the Congress and the citizens of my state from learning about the potential dangers to health and safety."

A spokesman for Sen. Mack Mattingly (R-Ga.) said he favors a strong defense but "believes it will be small solace to the people of Georgia and South Carolina whose water is poisoned if they are told they'll have a brand-spanking-new nuclear arsenal to make up for it."