Any person who voluntarily accepts responsibility under section 701 and agrees to pay his or her proportionate share, as determined under section 708, of the response costs and damages listed insection 702 , as ultimately determined, plus an appropriate premium in an amount up to 50% of that person's proportionate share, but not to exceed 15% of the total costs of response and damages, shall not be subject to claims by the department, or by any other responsible person, in excess of that amount.
35 P.S. § 6020.709