A system failure in the pierside transfer system was determined to be the cause of the overflow, said Navy spokesman Tom Danaher.
Crews stopped pumping and set up a containment boom when the spill was discovered. Port perations personnel began a cleanup using vacuum trucks and absorbent pads until sunset.
An investigation into the system failure and the environmental impact was started. Cleanup resumed Tuesday morning at sunrise, Danaher said.
The effects of this spill to wildlife or property are unknown at this time, he said.
Federal and state environmental agencies were notified of the spill.