The spill was apparently caused by a car striking a pump at the gas station, and the emergency shutoff not working properly. The incident happened around 5 p.m. at a 76 station near Glen Arm and Arroyo.
The drain empties into the Alhambra Wash. Local residents reported a strong odor from the spill.
A county hazmat team responded to supervise cleanup operations. In the meantime, residents were advised to stay indoors and keep their windows closed.
The city of San Marino tweeted out a map of the affected area.
Residents were told the odor from the spill could last at least six hours.