The assignment of volunteers under the Washington serves program shall not result in the displacement of currently employed workers, including partial displacement such as would result from a reduction in hours of nonovertime work, wages, or other employment benefits. Participating agencies, programs, or activities may not terminate, lay off, or reduce the working hours of any employee for the purpose of using volunteers under the Washington serves program. In circumstances where substantial efficiencies or a public purpose may result, participating agencies may use volunteers to carry out essential agency work or contractual functions without displacing current employees.