Introducing new equations for the reaction coefficient for modelling lead and cadmium heavy metals in riverine systems

Pollutant and sediment transport is one of the main problems in riverine and estuarine systems and has negative effect on life. Heavy metals are one of the main pollutants which modelling of these pollutants near the places that humans are living is very important. Heavy metals are non-conservative in nature and there concentration depends on salinity and pH. The objective of this paper is to introduce new equations for the reaction coefficient of lead and cadmium using pH and EC variation in riverine systems.This paper first describes the governing equations of dissolved heavy metals in riverine systems. Then it introduces the FASTER model for modelling the flow and heavy metal concentration downstream of the river. In the current model the one dimensional advection dispersion equation was solved using the new ULTIMATE QUICKEST and FTCS methods. This method has been verified using a standard example and then used to model the heavy metal concentration in Karoon riverine system. In this regard for calibration and verification of the hydrodynamic model, the model was run in the Mollasani-Farsiat reach of the Karoon River and was calibrated using Ahwaz station observed data. In the end some equations were introduced for the reaction coefficient of lead and cadmium heavy metals and used in the model source code. Later the model was run for five different reaction coefficients for lead and cadmium, which are as follow: 1- zero coefficient reaction, 2- constant coefficient reaction, 3- coefficient reaction as a function of pH, 4- coefficient reaction as a function of EC and 5- coefficient reaction as a function of pH and EC. The results of the model calibration and verification for lead and cadmium heavy metals show that selecting a varied reaction coefficient increases the accuracy of the model. Selecting a varied reaction coefficient with pH and EC will increase the accuracy of the model about %91.71 and %97.71 for lead and cadmium respectively. Therefore this model and the new approach can be used as a useful tool for hydro-environmental modelling in riverine systems.