Longitudinal spread of bicomponent contaminant in wetland flow dominated by bank-wall effect

Presented in this paper is a theoretical analysis for longitudinal spread of bicomponent contaminant in a fully developed steady wetland flow dominated by bank-wall effect. Based on the general form of concentration transport equations adopted for wetland flows, an ecological risk assessment model is given for the decay of concentration under the combined action of reversible and irreversible reactions, as well as hydraulic dispersion. Through a combination of the method for solving linear parabolic system and an asymptotic analysis for hydraulic dispersion in the wetland flow, an analytical solution for long time evolution of bicomponent contaminant concentration is rigorously derived and illustrated. The solution is shown to be an extension of known solutions for single component contaminant transport due to an irreversible reaction and hydraulic dispersion, as well as biocomponent contaminant transport due to reversible reactions and hydraulic dispersion. It is found that the concentration ratio of binary components can approach an equilibrium status, with necessary time to obtain the status dependent on transfer and degradation rates of each component. The length and duration of influenced region with concentration of contaminant cloud beyond given environmental standard level are presented for a uniform instantaneous emission into the wetland flow. The result shows that the length increases with time to reach maximum and then decreases to zero, and the duration is sensitive to the variation of a dimensionless parameter to reflect the relative importance of an irreversible action and lateral mass dispersion.