On or before January 1, 1998, the department shall propose, hold a public hearing, and adopt a finding of the best available technology for each contaminant for which a primary drinking water standard has been adopted. Thereafter, the department shall adopt a finding of the best available technology for each contaminant for which a primary drinking water standard has been adopted at the time the standard is adopted. The finding of the department shall take into consideration the costs and benefits of best available treatment technology that has been proven

effective under full-scale field applications.