The Environmental Protection Agency termed inadequate a required spill prevention and control plan prepared by Ashland Oil Inc. before its million-gallon fuel spill this month in suburban Pittsburgh.

The EPA said the containment dike around the 4 million-gallon tank in Floreffe, Pa., was large enough to hold its entire contents, but the tank ruptured with such force Jan. 2 that much of the oil gushed over the dike and into a storm sewer emptying into the Monongahela River.

Ashland's spill prevention control and countermeasures plan (SPCC), required under federal regulations, "does not adequately address spill prevention and control specific to the facility; instead, it merely paraphrases the SPCC regulations," said a memo filed by EPA's regional Philadelphia office.

The document said the finding of the plan's "inadequacy" was based on a review of the plan and an inspection of Ashland's Floreffe Terminal near Pittsburgh.

The memo was given to Senate staff members yesterday at an EPA briefing on Capitol Hill. Staff aides were told that about 830,000 people will be affected by the spill, mostly due to depleted water supplies.

The spill's leading edge was eight miles south of Sisterville, W.Va., on the Ohio River yesterday and could reach Huntington by Jan. 27. The tank collapse sent 1 million gallons of diesel fuel spilling into the Monongahela River, which joins the Allegheny River in downtown Pittsburgh to form the Ohio.

Ashland's spill prevention plan was submitted to the EPA in 1974 and updated periodically before the accident, most recently in 1985.

"The problem is that the spill containment plan appears to be more of a general plan useful at any of Ashland's facilities," said Janet Viniski of EPA's Philadelphia office. "It does not contain enough specifics."

She said it is too early to say whether Ashland could be cited for violations and that "our real concern is getting the spill cleaned up and making sure the site gets into compliance."