EL SEGUNDO, Calif. (AP) _ Police cited two Greenpeace boat operators after divers from the environmental group tried unsuccessfully to plug a Chevron oil refinery pipeline that dumps waste water into the ocean, police said today.

The operators of two motorized Zodiac rafts were cited Thursday after they brought the rubber boats too close to shore, El Segundo police Lt. Jack Wayt said.

Greenpeace sent divers to plug the pipeline because the plant discharges toxic materials ''and this was the only way we could think to stop them,'' Greenpeace spokesman Dave Rapaport said in a statement.

Chevron is licensed to discharge up to 15 million gallons of waste water through the pipe daily, but the average is about 6 million gallons.

A pending, $2.3 million lawsuit filed by the Sierra Club alleges the oil company discharged illegal levels of grease and oil, ammonia, chromium and other petroleum products into the ocean.

Chevron contends such materials are removed from the water by a treatment system before it is discharged, said company spokeswoman Brenda Smyth.

Police were notified Thursday afternoon by lifeguards that nine people had left an anchored sloop in two rubber boats and had taken them illegally within a 300-yard shoreline limit, Wayt said.

Although the divers reportedly located the end of the pipe and marked it with a buoy, turbulent water prevented them from plugging it, police Capt. Tim Grimmond said.

After police issued the boat operators misdemeanor citations, the divers returned to their 40-foot sloop. Wayt said the sloop left the El Segundo area Thursday night.

The pipeline carries treated waste water used in processing petroleum from the plant and dumps it about 500 feet from shore, Ms. Smyth said. If it was plugged, she said, the backed-up water would ''severely impact'' plant operations.

''I don't think they fully understand the systems they are dealing with. There are also submarine lines out there that transport crude (oil) and other products,'' she said.

Police identified the boat operators as Edward L. Martinex, 25, and Diane Jane Desnoyers, 25, both of San Francisco.