text: The reactions that occur with permeable reactive barriers and ferrous iron are surface based. The surface reactions take three different forms: direct reduction, electron shunting through ferrous iron, and reduction by production and reaction of hydrogen. Pathway A represents direct electron transfer (ET) for Fe to the adsorbed halocarbon (RX) at the metal/water point of contact, resulting in dechlorination and production of Fe. Pathway B shows that Fe (resulting from corrosion of Fe) may also dechlorinate RX, producing Fe. Pathway C shows that H from the anaerobic corrosion of Fe might react with RX if a catalyst is present.