It shall be the duty of such commissioners to examine the lands in the district and to apportion and assess the amount of the judgment, interest and costs as hereinbefore defined, of the condemnation proceeding, and of the estimated cost of the proposed improvement, and of the price of any fill made with material dug or dredged from such waterway, upon the several lots, blocks, tracts and parcels of land in said district, in the proportion in which they will be severally benefited; which assessment shall be a proportionate charge upon each square foot of land contained in each separate lot, block, tract or parcel of land.