California Code, Health and Safety Code - HSC § 79760

(a) Except as provided in Section 79765, a party found liable for costs recoverable under this part who establishes by a preponderance of the evidence that only a portion of those costs are attributable to that party's actions shall be required to pay only for that portion.

(b) Except as provided in Section 79765, if the trier of fact finds the evidence insufficient to establish each party's portion of costs under subdivision (a), the court shall apportion those costs, to the extent practicable, according to equitable principles, among the defendants.