Any volunteer or any person who provides assistance or advice in attempting to mitigate or in mitigating the effects of an actual or threatened discharge of hazardous materials or who attempts to prevent, dispose of, or clean up or prevents, disposes of, or cleans up any such discharge shall not be subject to any civil liability or penalty, except that the person whose act or omission, in whole or in part, caused such actual or threatened discharge shall be liable for any negligence of such volunteer or other person.