GEOCHEMICAL PROCESSES CONTROLLING CONCENTRATIONS OF AL, FE, AND MN IN NOVA-SCOTIA LAKES

Concentrations of Al, Fe, Mn, major ions, and dissolved organic C (DOC) were measured in 37 lakes in Halifax County, Nova Scotia, to determine the factors controlling the behavior of these metals in acidic waters with relatively high concentrations of organic matter. Concentrations of trace metals ranged from < 50 to 700-mu/g liter-1 (Al), < 20 to 960-mu/g liter-1 (Fe), and < 10 to 240-mu-g liter-1 (Mn). Concentrations of Fe and Al were highly correlated with H+ ion and DOC concentration, whereas Mn was correlated most strongly with H+ ion concentration and size of catchment relative to lake surface area. Computation of trace-metal speciation indicated that lakes with pH > 5 were in equilibrium with gibbsite and amorphous iron hydroxide but highly supersaturated with respect to manganite and Mn(IV) oxides. Concentrations of Mn appear to be determined by the magnitude of the source (i.e. the catchment area) relative to the size of the lake. In lakes with pH < 5, concentrations of Al and Fe seem to be regulated by interactions with DOC. Although the metals and organic C enter the lake separately, we propose that precipitation of metal humates is the major process regulating concentrations of Al and Fe and, to a lesser extent, DOC in these lakes.