This paper describes a modelling study of dissolved heavy metals in riverine systems, undertaken to introduce a new approach of the varied reaction coefficients used in the Advection–Dispersion Equation (ADE). The dissolved heavy metals, i.e. lead and cadmium, were modelled using a 1D hydro-environmental model. It is found that pH and EC play an essential role in adsorption and desorption of heavy metals by the particles in solution. Therefore, in this study we have tried to find the best relationships between pH and EC with the reaction coefficient. Relatively close agreement between predicted results and field-measured dissolved lead and cadmium concentrations were obtained for different varied reaction coefficients. Finally, the best relationships for the reaction coefficients for dissolved lead and cadmium were introduced and the results were successfully compared with the corresponding measured values.