Transport In Wetlands Research Articles

Indicators for contaminant transport in a periodical wind-driven wetland flow

Understanding indicators for contaminant transport in wetlands under wind is essential to agricultural pollution assessment associated with chemical fertilizers and pesticides. Presented in this paper is an analytical study of indicators for contaminant transport in a periodical wind-driven wetland flow. Analytical solutions of velocity profile, total quantity of contaminant in a streamline, position and moving velocity of mass center of contaminant cloud, and environmental dispersivity are rigorously derived and analyzed for a typical wetland flow under the combined action of periodical wind stress and superficial pressure gradient. It is found that the flow direction changes alternatively only for a large oscillatory wind stress. For instantaneous emission of multiple point sources, the total quantity of contaminant in each stream line parallel to the free surface is dependent on the initial distribution of contaminant only in the initial stage and eventually reaches a constant. For long time evolution of contaminant cloud, the mass center of contaminant cloud oscillates periodically around the origin of a coordinate system moving at a speed of the mean value of depth-averaged velocity during one period, and corresponding oscillatory amplitude enhances with the increasing oscillatory wind stress. The amplitude of environmental dispersivity increases with the increase of oscillatory wind stress, and negative environmental dispersivity appears for partial time in an oscillatory period for a large oscillatory wind stress.

Indicators for contaminant transport in a three-layer wetland with wind

Abstract The effect of global climate change is increasing and it can influence agriculture in many ways, especially in that it makes great threaten on agricultural water resources. Since agricultural water supply is a key issue for agricultural production, indicators for contaminant transport in wetlands should be considered in order to solve agricultural water pollution and promote water protection. In this paper, we analyze the environmental dispersion in a typical three-layer wetland flows with the effect of wind taken into account. The analytical solution for velocity distribution is rigorously derived and the dependence of characteristic parameters on averaged dimensionless velocity is recorded. The asymptotic analysis theory of Gill is employed to measure dispersivity for natural wetland flows with a commonly high Peclet number. Both velocity and environmental dispersion are significantly influenced by wind, in terms of its direction and the magnitude of force. The effects of hydraulics and ecological degradation are considered to obtain the analytical solution for the mean concentration and maximal critical length of the region influenced by the process of contaminant transport.

Environmental dispersion in a tidal wetland with sorption by vegetation

Understanding of the solute transport mechanism under the effect of sorption by vegetation in tidal wetland gains its significance for environmental and ecological management. Presented in this paper is a theoretical analysis of effective environmental dispersion in a depth-dominated tidal wetland. Based on the transport in porous media, a linear sorption isotherm model is adopted to account for the sorption by vegetation, and two models for momentum and concentration transport in wetlands are given, respectively. The velocity of flow forced by oscillating pressure is derived, and the effect of dimensionless parameters on velocity pulsation is analyzed. The velocity direction may reverse in the case of pulsation amplitude larger than the mean velocity. Using Aris’s method of concentration moments, we investigate the effective environmental dispersivity and concentration distribution. The effective environmental dispersivity increases over time at the initial stage to attain a steady oscillating status, the growth rate of which depends on the distribution coefficient KD. The variations of concentration distribution with typical dimensionless parameters are determined, which turn out to be consistent with those of dispersivity. The sorption by vegetation leads to lowered concentration and delayed contaminant cloud, contributing to the dispersion.

Indicators for contaminant transport in wetlands

Wetlands provide significant ecological services for urban regions in terms of water supply, wastewater treatment, flood storage, drought resistance, etc. For wetland flows, it is crucial to understand the process of contaminant transport as it provides scientific support for applications associated with various urban services. Two indicators respectively as the critical length and duration are frequently adopted for risk assessment of incidental release of toxic or contaminant cloud. This paper presents a review on recent progresses in the analytical study of contaminant transport in wetland flows by Taylor dispersion at the phase-average scale. The method of concentration moments is introduced. Analytical procedures for determining the key quantity of Taylor dispersivity are given for typical wetlands with free water surface, respectively as the steady flow wetlands, tidal flow wetlands, and the two-layer flow wetlands. As an example of applications, critical length and duration of the contaminant cloud in the steady flow wetlands are analyzed based on the obtained Taylor dispersivity. Results show that in contrast to the temporary, localized influence of COD on water quality, the heavy metal Pb can give rise to more severe damage.

Environmental dispersion in a three-layer wetland flow with free-surface

To further address the characteristic of contaminant transport in wetlands of a multi-layer structure, environmental dispersion of a pulsed contaminant emission into a steady flow is analytically explored in this paper for the typical case of a respectively vegetated or packed three-layer wetland dominated with free-water-surface-effect. The hierarchical structure for the critical length and duration of the contaminant cloud is illustrated for the dispersion of typical contaminant constituents.

Stochastic modeling of reactive transport in wetlands

This study describes the development of a general model for reaction in and performance of spatially heterogeneous bioreactors such as treatment wetlands. The modeled domain possesses local-scale velocities, reaction rates and transverse dispersion coefficients that are functions of an underlying heterogeneity variate representing one or more controlling biophysical attributes, for example, reactive surface area (submerged plant) density. Reaction rate coefficients are treated as related to local velocities in an inverse square fashion via their mutual dependence upon the variate. The study focuses on the solution for the steady-state case with constant inlet concentration. Results compare well with exact solutions developed for laterally-bounded systems in which the heterogeneity is represented explicitly. Employing the bicontinuum analogue of a second-order model, an expression for an effective longitudinal dispersion coefficient as a function of travel distance is developed using the method of moments. The result provides insights into the behavior of concentration as transverse mixing drives the system asymptotically toward Fickian longitudinal dispersion. The model may represent an improvement over other approaches for characterizing treatment wetland performance because it accounts for evolving shear flow dispersion, and because parameters are few in number, physically based, and invariant with mean velocity.

Influence of Common Reed (Phragmites australis) on CH4 Production and Transport in Wetlands: Results from Single-Plant Laboratory Experiments

Laboratory culture experiments were conducted with common reed (Phragmites australis) to elucidate the role of root exudates on CH4 production in wetland soils as well as the importance of different plant organs as routes of CH4 to the atmosphere. In the 50 d experiment period, root exudates ranged from 0.03 to 1.53 µmolg−1d−1, which increased with reed growth. CH4 production rate of soil was stimulated as root exudates collected was added. CH4 transport capacity rate also increased with plant growth and influenced by light intensity. Root tips were the most important part of controlling diffusion of CH4 into reed shoots, and leave transport accounted for 45.34% of total emissions into the atmosphere.

Notes on modelling of environmental transport in wetland

This paper presents an effort towards a basic model for environmental transport of momentum, heat and mass transfer in the wetland. To smear out the discontinuity between the two phases of water and solid in the wetland, the continuum models distinctively applying for the water body and solid frame are transformed via the technique of phase average to give equations for a virtual single-phase flow in the entire domain of the wetland. Then to filter out the vortex and fluctuation common in the wetland flow, the operation of large eddy simulation (LES) is applied to yield a basic model for practical simulation. With reference to the modelling of flows in porous media and turbulent flows, closure relations are presented for the correlation terms due to the phase average and large eddy simulation.

Instability of the viscous sublayer for ripple inception

Understanding of the solute transport mechanism under the effect of sorption by vegetation in tidal wetland gains its significance for environmental and ecological management. Presented in this paper is a theoretical analysis of effective environmental dispersion in a depth-dominated tidal wetland. Based on the transport in porous media, a linear sorption isotherm model is adopted to account for the sorption by vegetation, and two models for momentum and concentration transport in wetlands are given, respectively. The velocity of flow forced by oscillating pressure is derived, and the effect of dimensionless parameters on velocity pulsation is analyzed. The velocity direction may reverse in the case of pulsation amplitude larger than the mean velocity. Using Aris’s method of concentration moments, we investigate the effective environmental dispersivity and concentration distribution. The effective environmental dispersivity increases over time at the initial stage to attain a steady oscillating status, the growth rate of which depends on the distribution coefficient KD. The variations of concentration distribution with typical dimensionless parameters are determined, which turn out to be consistent with those of dispersivity. The sorption by vegetation leads to lowered concentration and delayed contaminant cloud, contributing to the dispersion.

Pollutant transport in wetlands. Marshes, bogs, or other types of wetland prove to be excellent receivers for the disposition of treated wastewaters containing small amounts of dissolved nitrogen and phosphorus

The factors which control the rate and extent of ion sorption on peat and other wetland soils have not been fully elucidated. Rates are fast in comparison to typical biological processes, thus suggesting a rapid capture mechanism such as adsorption, which can make nutrients more readily available to the organisms present. The presence of these fast sorption mechanisms results in the observed first-order removal rates in wetland systems. Information is lacking on the temperature dependence of adsorption rates, and it is conceivable that during winter operation the sorption step may provide a significant resistance to nutrient removal. Operating experience with wetland treatment systems has revealed an apparent nutrient saturation phenomenon in the vicinity of wastewater introduction; this behavior is consistent with an approach to an adsorption equilibrium. The rate of nutrient removal in unsaturated zones may be limited only by mass transfer. 32 references, 11 figures.