Remediation of contaminated groundwater is an expensive and lengthy process. Permeable reactive barrier of metallic iron (Fe-0 PRB) is one of the leading technologies for groundwater remediation. One of the primary challenges for the Fe-0 PRB technology is to appropriately size the reactive barrier (length, width, Fe-0 proportion and nature of additive materials) to enable sufficient residence time for effective remediation. The size of a given Fe-0 PRB depends mostly on accurate characterization of: (i) reaction mechanisms and (ii) site-specific hydrogeologic parameters. Accordingly, the recent revision of the fundamental mechanisms of contaminant removal in Fe-0/H2O systems requires the revision of the Fe-0 PRB dimensioning strategy. Contaminants are basically removed by adsorption, co-precipitation and size exclusion in the entire Fe-0 bed and not by chemical reduction at a moving reaction front. Principle calculations and analysis of data from all fields using water filtration on Fe-0 bed demonstrated that: (i) mixing Fe-0 and inert additives is a prerequisite for sustainability, (ii) used Fe-0 amounts must represent 30-60 vol.% of the mixture, and (iii) Fe-0 beds are deep-bed filtration systems. The major output of this study is that thicker barriers are needed for long service life. Fe-0 filters for save drinking water production should use several filters in series to achieve the treatment goal. In all cases proper material selection is an essential issue. (C) 2010 Elsevier B.V. All rights reserved.