Wash. Rev. Code § 54.04.130
Whenever any municipal corporation acquires by condemnation or otherwise any utility which at the time of acquisition is in private ownership and the employees of such private utility have been for at least two years and are at the time of acquisition covered by any plan for individual annuity contracts, retirement income policies, group annuity contracts, group insurance for the benefit of employees, or any other contract for the benefit of employees, such district shall, when the personnel is retained by the district, assume all of the obligations and liabilities of the private utility acquired with relation to such plan and the employees covered thereby at the time of acquisition; or the municipal corporation may by agreement with a majority of the employees affected substitute a plan or contract of the same or like nature. The municipal corporations acquiring such private utility shall proceed in such manner as is necessary so as not to reduce or impair any benefits or privileges which such employees would have received or be entitled to had such acquisition not been effected. The district may pay all or any part of the premiums or other payments required therefor out of the revenue derived from the operation of its properties.
RCW 54.04.130