The Applications of Chemical Analyses of Sediments and Soils in ...

could cause uni-directional migration of heavy metals, phosphates and nitrates.B. SEDIMENT - WATER INTERFACEExchanges across the sediment-water interface are of great significance. Freshorganic matter is continually added to the sediment surface and is decomposed leavingresistant humic residues. Oxygen is required for this decomposition at a ratedependent on biological (respiration and metabolism)and chemical reactions(oxidationof ferrous iron, etc.) and diffusion, molecular within the sediment but much more rapiddue to turbulence at the surface (Mortimer, 1971). The depth of the zero oxygen level(between the oxidized surficial layer and reduced sediment) a few millimetres belowthe surface of the sediment is critical in regulating chemical exchange.The depth of the zero oxygen level will change according to the supply ofoxygen, such as during the annual overturn of a lake or stratification. The oxidizedupper layer (microzone) forms an efficient trap for manganese, iron and phosphateboth moving up through the sediment and scavenging metals from the overlyingwater(Gorham, 1958).Iron and manganese concentration in this layer are often an order of magnitudehigher than in the reduced sediments below and are often orange due to 1972; Wolfe,1974).21