California Code, Health and Safety Code - HSC § 25299.19

“Occurrence” means an accident, including continuous or repeated exposure to conditions, which results in an unauthorized release of petroleum from an underground storage tank.Unauthorized releases at the same site which require only a single site investigation shall be considered as one occurrence. An unauthorized release subsequent to a previous unauthorized release at the same site shall only be considered a separate occurrence if an initial site investigation has been completed for the prior unauthorized release.