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Highlights • A general analytical procedure based on phase averaged formulation is presented. • Concentration moments based long-time asymptotic technique is extended. • High order effects of skewness and kurtosis are covered in the extended model. • Vertical concentration distribution is characterized for layered wetland. Abstract A general analytical procedure based on phase averaged formulation is presented in this work to investigate environmental dispersion in layered wetland. Based on concentration moments, Chatwin's long-time asymptotic technique is applied to describe both the longitudinal distribution of the depth-averaged concentration and the two-dimensional concentration field, including their longitudinal non-Gaussian effects of skewness and Kurtosis. Three wetlands of equal depth-averaged porosity but with different number of layers are studied to illustrate the impact of layered structure on solute transport. The layered structure of wetlands leads to great changes in velocity profiles and evolution of environmental dispersivity, skewness and kurtosis of depth-averaged concentration distribution. In each layer, vertical distribution of concentration is highly nonuniform in the initial stage of transport. Layers with different porosity result in essential difference between layer-averaged concentrations, of which all tend to be normally distributed. [ABSTRACT FROM AUTHOR]

Hydrological processes of contaminant transport in wetland flows are characterized by environmental dispersion. For concentration distribution of scalar dispersion, the preliminary estimate of vertical distribution (Wu et al., 2015) on small time scale is rigorously extended to account for high order effects as skewness and kurtosis. Based on a combination of Aris’ method of concentration moments and Gill’s generalized dispersion model, up to fourth order terms of vertical distribution functions and full-time two-dimensional concentration distribution are derived. The increments of damping factor, mainly representing the density of vegetation in wetlands, is shown to strengthen the mean distribution asymmetry and reduce the longitudinal dispersivity (with respect to skewness and kurtosis), and to increase the non-uniformity of vertical distribution. Therefore, dense vegetation corresponding to a large damping factor will postpone the time when Taylor’s dispersion model holds, and the first order approximation becomes rough on small time scale, demanding higher order modifications. The results provide more detailed and more accurate analytical solution for the typical environmental dispersion in wetland flows. [ABSTRACT FROM AUTHOR]

Highlights • The aspect ratio and damping factor on the dispersion process are characterized. • The lateral-sidewall-effect in a thin wetland channel at large time is dominated. • Skewness of the transverse mean concentration distribution is presented. Abstract Predicting the evolution of concentration distribution of environmental transport in wetland flows has a broad range of applications in ecological engineering practice. By extending a previous study, the spatial concentration distribution of solute transport due to an instantaneous environmental emission in a wetland channel with rectangular cross-section is presented in this work by means of Chatwin's long-time asymptotic technique. To account for the effect of high order moment, skewness of the transverse mean concentration distribution is analyzed. Two important parameters, i.e., aspect ratio (depth to width) and damping factor, on the dispersion process are discussed. Four studied cases illustrated that the transverse concentration distribution is nonuniform even at large time and the aspect ratio can affect the transverse concentration distribution greatly. The cross-sectional mean concentration distribution is asymmetrical in the initial stage and shows an asymptotic Gaussian distribution at large time. The dispersion process in a thick (width equals depth) wetland channel is quicker than that in a thin wetland channel. The spatial concentration distribution is dominated by the lateral-sidewall-effect on a large time scale for environmental transport in a thin wetland channel. [ABSTRACT FROM AUTHOR]

The rapid growth of population in cities places great pressure on urban ecosystem health and management, especially on urban water supply and disposal of wastewater. To cope with urban water degradation, indicators are needed for predicting and evaluating anthropogenic impacts on wetlands. Presented in this paper is a analytical study of the environmental dispersion in a three-layer wetland in terms of the longitudinal evolution of the lateral mean concentration. An environmental dispersion model for the mean concentration in the three-layer wetland is devised as an extension of Taylor's classical formulation. The analytical results illustrate the effect of dimensionless parameters on the environmental dispersivity. Other related indicators for urban water quality assessment in three-layer wetlands, i.e., the critical length and duration of the contaminant cloud of typical contaminant constituents are illustrated and characterized. [ABSTRACT FROM AUTHOR]

Embodied energy as the energy supplied directly and indirectly through input pathways by human society is analyzed for a constructed wetland as a typical ecological engineering. The method of systems accounting as a combination of process and input–output analyses in context of systems ecology is introduced to account the embodied energy as one of the most important ecological indicators of constructed wetland. Based on the latest embodied energy intensity database for the Chinese economy 2007, the embodied energy of a typical constructed wetland in both the construction and operation stages is accounted and compared with those for a cyclic activated sludge plant as a typical conventional wastewater treatment system and a hybrid system of conventional wastewater treatment system and constructed wetland. The indirect energy is revealed to be more important than the on-site direct energy, which accounts for only 30–40% of the total. The energy embodied in the five terms of treating per m3 wastewater and removing per kg BOD, COD, SS, and NH3-N by the constructed wetland is only 6.83%, 17.48%, 13.96%, 22.47% and 9.48% of that by the conventional system, and 9.51%, 38.26%, 27.01%, 39.42% and 13.20% of that by the hybrid system, respectively. The ecological engineering of the constructed wetland is illustrated more favorable than the conventional wastewater treatment due to its considerable ecological benefit. [Copyright &y& Elsevier]

Abstract: Flow and environmental dispersion are essential to ecological risk assessment and ecological restoration associated with wetlands. Presented in this paper is an analytical study for effect of wind on contaminant dispersion in a wetland flow dominated by free-surface effect. Based on the momentum transport equation generally adopted for wetland flow, the solution for velocity distribution in a free-surface wetland flow under wind is rigorously derived, with a limiting case covering the known solution of flow in the absence of wind. The thickness of inverse flow layer, maximum velocity, and its location are obtained. By use of Aris''s method of concentration moments, the expression for environmental dispersivity in the free-surface wetland flow under wind is derived, and that in the absence of wind can be included as a special case. The direction and intensity of wind can influence greatly the environmental dispersivity at a stable stage. The time scale for environmental dispersivity to approach the stable stage is independent of wind. Analytical solution of mean concentration is derived and illustrated for a pulsed contaminant emission into a wetland flow dominated by free-surface effect under wind. For typical pollutant Pb, the length and duration of influence region with contaminant concentration beyond given standard level of water quality are illustrated. [Copyright &y& Elsevier]