All property and rights acquired by the department shall be exempt from all taxation, but the department shall continue to make payments in lieu of taxes, on or before December 1 of each year, to each town or city to which the department made such payments as of December 31, 2008. Such sums shall be based on the tax rate then current in each town or city and on the average assessed valuation for a period of 5 years prior to the acquisition by the department; provided that when those towns or cities shall have made a general revaluation of property for tax assessment purposes, then the new values shall be used for the determination of the sums as would have been assessed against the property and rights in such town or city if the same had been included in the tax invoice for such year. Betterments, changes in land use or new structures are exempted and shall not be used for the determination of such sums to be paid in lieu of taxes. Provided, however, that the provisions of this section shall not apply to the dam and any appurtenances thereto owned by the department and located in the towns of Pittsburg and Clarksville, and there shall be paid annually to said towns the sum of $40,000 in lieu of taxes, said sum to be apportioned between the towns in the same ratio that the value of that portion of the dam and any appurtenances thereto located in each town bears to the total value of the dam and any appurtenances thereto as determined by the tax assessors of the towns.