The subsurface regions of treatment marshes are an active ecosystem that interacts with the free water ecosystem above. This subsurface zone consists of three major components: (1) phytomass, consisting of living (biomass) and dead (necromass) plant roots and rhizomes; (2) soils and sediments; and (3) water. As the knowledgeable reader will know, this subsurface region has traditionally been called “wetland soils.” This popular designation conjures an image of an inert nonbiological substrate, and obscures the fact that the subsurface zones are an active ecosystem, in which physical, chemical and biological processes all occur. Those underground processes are all important in the functioning of the total wetland in pollutant removal.