Longitudinal spread of bicomponent contaminant in wetland flow dominated by bank-wall effect L. Zeng a,⇑ , Y.J. Zhao a , B. Chen b, * , P. Ji a , Y.H. Wu a , L. Feng b a State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin, China Institute of Water Resources and Hydropower Research, Beijing 100038, China b State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing 100875, China article info Article history: Received 17 March 2013 Received in revised form 28 August 2013 Accepted 11 November 2013 Available online 21 November 2013 This manuscript was handled by Laurent Charlet, Editor-in-Chief, with the assistance of Chong-Yu Xu Associate Editor Keywords: Wetland Ecological risk assessment Reversible reaction Irreversible reaction Hydraulic dispersion summary 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 concen- tration 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 irre- versible reaction and hydraulic dispersion, as well as biocomponent contaminant transport due to revers- ible 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 deg- radation 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. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Wetlands have play a very important role in water quality improvement, groundwater recharge, flood storage, drought resis- tance, wildlife conservation, food supply, etc. (Mitsch and Gosse- link, 1993; Costanza et al., 1997). For wastewater treatment engineering associated with constructed wetlands, and ecological risk assessment and ecological restoration related to natural wet- lands (Carvalho et al., 2009; Hammer, 1989; Linder et al., 1994), a fundamental issue is to forecast the transport of contaminant cloud under the combined action of reversible and irreversible reactions, as well as hydraulic dispersion (Chen et al., 2010; Zeng and Chen, 2011). Regarding contaminant transport in wetland flows due to hydraulic dispersion, some theoretical efforts have been made based on Taylor’s analysis on dispersion (Taylor, 1953; Taylor, 1954), method of concentration moments (Aris, 1956; Aris, 1960), and method of multi-scale expansion (Mei et al., 1996). Lightbody and Nepf (2006a) and Lightbody and Nepf (2006b) established an expression of longitudinal dispersion coefficient for flow through a salt marsh controlled by emergent vegetation. Zeng et al. (2011) and Chen et al. (2010) predicted the decay of mean concentration in two-dimensional and three-dimensional wetland channels, respectively. Murphy et al. (2007) and Nepf et al. (2007) explored the characteristics of longitudinal dispersion by turbulence at dif- ferent scales. Zeng et al. (2012a) analyzed the effect of wind on con- taminant dispersion in a free-surface wetland flow. Chen et al. (2011) and Wu et al. (2011a) derived the analytical solution of envi- ronmental dispersivities for a two-zone wetland flow and a two- layer wetland flow, respectively. Zeng et al. (2012b) and Wu et al. (2012) examined the behavior of contaminant dispersion in typical tidal wetland flows with magnitude and direction changing period- ically. However, these studies only examined the effect of pure physical processes associated with advection and mass dispersion at the phase average scale on contaminant transport, and they can- not reflect the effect of reversible or irreversible reactions. Contaminant transport in wetland flows is quite complex, clo- sely associated with physical, chemical and biological processes wherein (US EPA, 1988; US EPA, 1993; US EPA, 1999a; US EPA, 1999b). A reliable prediction for contaminant concentration in the wetland flow requires a reasonable understanding of the behavior of solutes transport under the combined action of reactions and hydraulic dispersion. By use of an exponential transformation, Zeng and Chen (2011) explored the evolution of 0022-1694/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhydrol.2013.11.015 ⇑ Corresponding authors. Tel.: +86 10 68781039; fax: +86 10 68781235 (L. Zeng). Tel.: +86 10 58807368; fax: +86 10 58807368 (B. Chen). E-mail addresses: lizeng0914@163.com (L. Zeng), chenb@bnu.edu.cn (B. Chen). Journal of Hydrology 509 (2014) 179–187 Contents lists available at ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol