Pursuant to statute, the board of the Hudson River-Black River Regulating District must prepare an estimate of the cost of the reservoirs operated by the regulating district and then apportion such cost, less the amount which may be chargeable to the State, among the public corporations and parcels of real estate benefitted, in proportion to the amount of benefit which will inure to each such public corporation or parcel of real estate by reason of such reservoir. The regulating district board shall certify such apportionment to the Department of Environmental Conservation for approval. Upon department approval, the apportionment shall be served and filed as required by statute.