Water purveyors required to develop a water system plan pursuant to RCW 43.20.230 shall evaluate the feasibility of adopting and implementing water delivery rate structures that encourage water conservation. This information shall be included in water system plans submitted to the department of health for approval after July 1, 1993. The department shall evaluate the following:
(1) Rate structures currently used by public water systems in Washington; and
(2) Economic and institutional constraints to implementing conservation rate structures.