A general algorithm (NONEQUI) was developed to simulate the kinetics of sorption and cation exchange processes among heavy metals and substrates in acid (pH < 6.5) lake or stream environments. Because the model is formulated in a kinetic framework, both fast and slow (relative to both the time frames of biological metal uptake and adsorption as well as hydrological residence time) metal substrate reactions can be simulated without incurring computational errors due to the assumption of chemical equilibrium at all times. The model is useful in that it predicts temporal and spatial concentrations of both bound and unbound metals. This is particulary important because it is commonly the unbound species of a metal (e.g., a free divalent cation of cadmium, Cd 2+) that is its most toxic form. The model was developed for predicting the fate and transport of any divalent cation including mercury. Important transformations of mercury were included that other cations do not significantly experience: methylation, demethylation, volatilization, and humic acid reduction of Hg 2+ to Hg 0. The latter reaction has potential for significantly affecting the fate, transport, and effects of heavy metals in waters of the southeastern U.S.