Flow Wetland Research Articles

Concentration distribution of contaminant transport in wetland flows

Summary Study on contaminant transport in wetland flows is of fundamental importance. Recent investigation on scalar transport in laminar tube flows (Wu and Chen, 2014. J. Fluid Mech., 740: 196-213.) indicates that the vertical concentration difference in wetland flows may be remarkable for a very long time, which cannot be captured by the extensively applied one-dimensional Taylor dispersion model. To understand detailed information for the vertical distribution of contaminant in wetland flows, for the first time, the present paper deduces an analytical solution for the multi-dimensional concentration distribution by the method of mean concentration expansion. The solution is verified by both our analytical and numerical results. Representing the effects of vegetation in wetlands, the unique dimensionless parameter α can cause the longitudinal contraction of the contaminant cloud and the change of the shape of the concentration contours. By these complicated effects, it is shown unexpectedly that the maximum vertical concentration difference remains nearly unaffected, although its longitudinal position may change. Thus the slow-decaying transient effect (Wu and Chen, 2014. J. Hydrol., 519: 1974-1984.) is shown also apply to the process of contaminant transport in wetland flows.

Two-scale analysis for environmental dispersion in a two-layer wetland

Studies on environmental dispersion are essential for applications as water management. The two-scale perturbation analysis is applied in this paper to deduce the environmental dispersion model for the typical case of contaminant transport in two-layer wetland flows. The analysis follows the established theoretical framework on the basis of phase average and the concept of Taylor dispersion. By the obtained flow velocity distribution for the two-layer flow, the analytical expression for the environmental dispersivity is deduced and shown to be consistent with previous results by the concentration moment method, while with much simplifications on the expression for ignoring the less concerned time-dependent stage of the dispersivity.

Catchments as heterogeneous and multi-species reactors: An integral approach for identifying biogeochemical hot-spots at the catchment scale

Summary From a biogeochemical perspective, catchments can be regarded as reactors that transform the input of various substances via precipitation or deposition as they pass through soils and aquifers towards draining streams. Understanding and modeling the variability of solute concentrations in catchment waters require the identification of the prevailing processes, determining their respective contribution to the observed transformation of substances, and the localization of “hot spots”, that is, the most reactive areas of catchments. For this study, we applied a non-linear variant of the Principle Component Analysis, the Isometric Feature Mapping (Isomap), to a data set composed of 1686 soil solution, groundwater and stream water samples and 16 variables (Al, Ca, Cl, Fe, K, Mg, Mn, Na, NH4, NO3, SO4, total S, Si, DOC, electric conductivity and pH values) from the Lehstenbach catchment in Germany. The aim was (i) to assess the contribution of the prevailing biogeochemical processes to the variability of solute concentrations in water samples taken from soils, in groundwater and in stream water in a catchment and (ii) to identify hot spots at the catchment scale with respect to 16 solutes along different flow paths. The first three dimensions of the Isomap analysis explained 48%, 30% and 11%, respectively, i.e. 89% of the variance in the data set. Scores of the first three dimensions could be ascribed to three predominating bundles of biogeochemical processes: (i) redox processes, (ii) acid-induced podzolization, and (iii) weathering processes. In general, the upper 1 m topsoil layer could be considered as hot spots along flow paths from upslope soils and in the wetland, although with varying extents for the different prevailing biogeochemical processes. Nearly 67% and 97% of the variance with respect to redox processes and acid induced podzolization could be traced back to hot spots, respectively, representing less than 2% of the total spatial volume of the catchment. In contrast, biogeochemical processes in deep groundwater had only minor effects on the biogeochemical turnover in catchment waters. Hot spots with respect to weathering processes along upslope and wetland flow paths could not be quantified due to effects of preferential sampling in soil solution samples. Predominating flow paths and biogeochemical processes crucial for the variability of stream water chemistry differed substantially for three streams but were consistent with presumed mixing ratios.

Perirheic mixing and biogeochemical processing in flow‐through and backwater floodplain wetlands

AbstractInundation hydrology and associated processes control biogeochemical processing in floodplains. To better understand how hydrologic connectivity, residence time, and intrafloodplain mixing vary in floodplain wetlands, we examined how water quality of two contrasting areas in the floodplain of the Atchafalaya River—a flow‐through and a backwater wetland—responded to an annual flood pulse. Large, synoptic sampling campaigns occurred in both wetlands during the rising limb, peak, and falling limb of the hydrograph. Using a combination of conservative and reactive tracers, we inferred three dominant processes that occurred over the course of the flood pulse: flushing (rising limb), advective transport (peak), and organic matter accumulation (falling limb). Biogeochemistry of the two wetlands was similar during the peak while the river overflowed into both. However, during the rising and falling limbs, flow in the backwater wetland experienced much greater residence time. This led to the accumulation of dissolved organic matter and dissolved phosphorus. There were also elevated ratios of dissolved organic carbon to nitrate in the backwater wetland, suggesting nitrogen removal was limited by nitrate transported into the floodplain there. Collectively, our results suggest inclusion of a temporal component into the perirheic concept more fully describes inundation hydrology and biogeochemistry in large river floodplain.

Multitemporal Multitrack Monitoring of Wetland Water Levels in the Florida Everglades Using ALOS PALSAR Data With Interferometric Processing

We present an improved wetland interferometric synthetic aperture radar (InSAR) technique that uses multitrack SAR data and ground-based stage (water level) data to calculate a time series of high spatial resolution water level maps throughout wide wetland areas. The technique was applied to a wetland area in the northern Everglades, Florida, using a four-year-long Advanced Land Observing Satellite (ALOS) Phased Array L-band Synthetic Aperture Rada (PALSAR) data set acquired during 2007-2011. Although the temporal resolution of ALOS PALSAR interferograms is low (multiples of the satellite's 46-day revisit cycle), the multitrack algorithm combines results from the four tracks and significantly improves the observation frequency up to seven days in the best case. A quality control analysis indicates that the average root-mean-square error of the differences between the InSAR- and stage-based water levels is 4.2 cm. The end products of absolute water level time series with improved temporal and high spatial resolutions can be used as excellent constraints for high spatial resolution wetland flow models and other water resource applications.

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.

Transport in a three-zone wetland: Flow velocity profile and environmental dispersion

To achieve better understanding of transport process in real waterways interacted with adjacent aquatic vegetation and riparian buffers, a three-zone model featuring the bank-effect is presented to character the water flow and environmental dispersion. Based on basic formulation in context of porous media flow, the velocity profile of a fully developed flow through the wetland is derived, with that for single zone and two-zone wetland flows recovered as special cases. The environmental dispersivity is determined by the approach of multi-scale analysis, with the effects of dimensionless parameters well illustrated. Application examples are provided to illustrate associated hierarchical structure for the critical length and duration of the contaminant cloud, and a comparison is made between the three-zone wetland and a corresponding three-layer wetland.

Vulnerability of Eco-Hydrological Environment in the Yellow River Delta Wetland

Gao, M.; Liu, S.; Zhao, G.; Yuan, H.; Wei, C.; Wu, Y., and Tang, J., 2014. Vulnerability of eco-hydrological environment in the Yellow River delta wetland.We investigated the relationship between groundwater head and oceanic tidal fluctuations in the Yellow River Delta wetland through on-site hydrological monitoring. Shallow groundwater heads were obviously affected by oceanic tide along the coastal zone. The ranges of the wetland zone can be readily assessed by measuring fluctuation amplitudes or lags. The results show that the influence radius is approximately 12 km to 18 km (when the correlation coefficient is 0.7 to 0.8) under the joint actions of oceanic tide and shallow groundwater seepage flow in clayey silt coastal wetland. A cross-sectional sketch of the coastal wetland model is developed based on monitoring data of groundwater and oceanic tidal fluctuations to study the vulnerability of the eco-hydrological environment in the Yellow River Delta wetland. The coastal wetland consists of three zones (the groundwater seepage zone, the tidal-induced transitional zone, and the tidal zone) with distinctly different hydraulic properties. Analytical solutions are used to estimate the vulnerability of the eco-hydrological environment in the wetland aquifer located in the NE part of the Yellow River Delta wetland, Shandong Province, China. Our results show that changes in the shallow groundwater quality of the wetland are significantly affected by natural factors, such as strong cutoff in the lower reaches, storm tides, and human engineering activities. The northern coastal wetland may be submerged without damp proof when the height of a storm tide reaches 2.4 m. The depth of shallow groundwater and the salinity gradient are key factors that contribute to the vulnerability of the ecological environment. The vulnerability of the eco-hydrological environment is derived from the joint actions of groundwater dynamics, hydrochemistry, and tidal-induced processes under sedimentary stress and water pressure.

Transport of a volatile contaminant in a free-surface wetland flow

Presented in this paper is an analytical study of a pulsed volatile contaminant emission into a free-surface wetland flow. A simplified model is given for contaminant transport under the combined action of advection, mass dispersion, apparent reaction, and volatilization at the free water surface. The effect of periodic apparent reaction on contaminant transport is separated from the hydraulic effect via an extended transformation, with a limiting case covering the known transformation for constant apparent reaction rate. The analytical solutions of zeroth and first order concentration moments are rigorously derived and illustrated. It was found that the amount of contaminant decreases from the bottom bed to the free-surface under volatilization, and the total amount of contaminant decays with time. It was also found that the moving speed of the mass center of the whole contaminant cloud increases, as the ratio of volatilization coefficient to vertical effective mass dispersivity increases.

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.

Methylmercury production in sediment from agricultural and non-agricultural wetlands in the Yolo Bypass, California, USA

As part of a larger study of mercury (Hg) biogeochemistry and bioaccumulation in agricultural (rice growing) and non-agricultural wetlands in California's Central Valley, USA, seasonal and spatial controls on methylmercury (MeHg) production were examined in surface sediment. Three types of shallowly-flooded agricultural wetlands (white rice, wild rice, and fallow fields) and two types of managed (non-agricultural) wetlands (permanently and seasonally flooded) were sampled monthly-to-seasonally. Dynamic seasonal changes in readily reducible ‘reactive’ mercury (Hg(II)R), Hg(II)-methylation rate constants (kmeth), and concentrations of electron acceptors (sulfate and ferric iron) and donors (acetate), were all observed in response to field management hydrology, whereas seasonal changes in these parameters were more muted in non-agricultural managed wetlands. Agricultural wetlands exhibited higher sediment MeHg concentrations than did non-agricultural wetlands, particularly during the fall through late-winter (post-harvest) period. Both sulfate- and iron-reducing bacteria have been implicated in MeHg production, and both were demonstrably active in all wetlands studied. Stoichiometric calculations suggest that iron-reducing bacteria dominated carbon flow in agricultural wetlands during the growing season. Sulfate-reducing bacteria were not stimulated by the addition of sulfate-based fertilizer to agricultural wetlands during the growing season, suggesting that labile organic matter, rather than sulfate, limited their activity in these wetlands. Along the continuum of sediment geochemical conditions observed, values of kmeth increased approximately 10,000-fold, whereas Hg(II)R decreased 100-fold. This suggests that, with respect to the often opposing trends of Hg(II)-methylating microbial activity and Hg(II) availability for methylation, microbial activity dominated the Hg(II)-methylation process, and that along this biogeochemical continuum, conditions that favored microbial sulfate reduction resulted in the highest calculated MeHg production potential rates. Rice straw management options aimed at limiting labile carbon supplies to surface sediment during the post-harvest fall–winter period may be effective in limiting MeHg production within agricultural wetlands.

Relative Significance of Microtopography and Vegetation as Controls on Surface Water Flow on a Low-Gradient Floodplain

Surface water flow controls water velocities, water depths, and residence times, and influences sediment and nutrient transport and other ecological processes in shallow aquatic systems. Flow through wetlands is substantially influenced by drag on vegetation stems but is also affected by microtopography. Our goal was to use microtopography data directly in a widely used wetland model while retaining the advantages of the model’s one-dimensional structure. The base simulation with no explicit treatment of microtopography only performed well for a period of high water when vegetation dominated flow resistance. Extended simulations using microtopography can improve the fit to low-water conditions substantially. The best fit simulation had a flow conductance parameter that decreased in value by 70 % during dry season such that mcrotopographic features blocked 40 % of the cross sectional width for flow. Modeled surface water became ponded and flow ceased when 85 % of the cross sectional width became blocked by microtopographic features. We conclude that vegetation drag dominates wetland flow resistance at higher water levels and microtopography dominates at low water levels with the threshold delineated by the top of microtopographic features. Our results support the practicality of predicting flow on floodplains using relatively easily measured physical and biological variables.

A multiobjective ant colony optimization approach for scheduling environmental flow management alternatives with application to the River Murray, Australia

[1] In regulated river systems, such as the River Murray in Australia, the efficient use of water to preserve and restore biota in the river, wetlands, and floodplains is of concern for water managers. Available management options include the timing of river flow releases and operation of wetland flow control structures. However, the optimal scheduling of these environmental flow management alternatives is a difficult task, since there are generally multiple wetlands and floodplains with a range of species, as well as a large number of management options that need to be considered. Consequently, this problem is a multiobjective optimization problem aimed at maximizing ecological benefit while minimizing water allocations within the infrastructure constraints of the system under consideration. This paper presents a multiobjective optimization framework, which is based on a multiobjective ant colony optimization approach, for developing optimal trade-offs between water allocation and ecological benefit. The framework is applied to a reach of the River Murray in South Australia. Two studies are formulated to assess the impact of (i) upstream system flow constraints and (ii) additional regulators on this trade-off. The results indicate that unless the system flow constraints are relaxed, there is limited additional ecological benefit as allocation increases. Furthermore the use of regulators can increase ecological benefits while using less water. The results illustrate the utility of the framework since the impact of flow control infrastructure on the trade-offs between water allocation and ecological benefit can be investigated, thereby providing valuable insight to managers.

The Influence of Hydrologic Restoration on Groundwater-Surface Water Interactions in a Karst Wetland, the Everglades (FL, USA)

Efforts to rehydrate and restore surface water flow in karst wetlands can have unintended consequences, as these highly conductive and heterogeneous aquifers create a close connection between groundwater and surface water. Recently, hydrologic restoration efforts in the karstic Taylor Slough portion of the Everglades has changed from point source delivery of canal water (direct restoration),to the useofa series of surface water recharge retention basins (diffuse restoration). To determine the influence of restoration on groundwater- surface water interactions in the Taylor Slough headwaters, a water budget was constructed for 1997-2011 using 70 hydro- meteorological stations. With diffuse restoration, groundwater seepage from the Everglades toward the urban boundary in- creased, while the downstream delivery of surface water to the main portion of the slough declined. The combined influence of diffuse restoration and climate led to increased intra-annual variability in the volume of groundwater and surface water in storage but supported a more seasonally hydrated wetland compared to the earlier direct tactics.The data further indicated that hydrologic engineering in karst wetland landscapes en- hances groundwater-surface water interactions, even those designed for restoration purposes.

Seed germination from deposited sediments during high winter flow in riparian areas

In this study we examine how seed richness, diversity and composition from winter deposited sediment vary at increasing distances from a river channel. The objectives were to (1) describe the abundance, diversity, species composition and indicator species of seed deposited in a riparian wetland at different distances from the stream, (2) determine the range of variability in seed deposition at two spatial scales namely within distances and between distances from the river, and (3) determine the temporal patterns in seed germination at different distances from the river. A transect was established perpendicular to the river channel extending 101m into the river valley. Samples of winter deposited sediment were collected in early spring in 20cm×20cm plots with three replicates positioned 2m, 16m, 23m, 41m, 70m and 101m from the river. Germination from the sediment seed pool was followed for six weeks under, respectively, moist and wet conditions in a greenhouse with a natural light regime and a mean temperature of 20°C. Overall, we found that winter high flow deposited a substantial amount of viable seeds in the inundated area. Germination was most successful under moist conditions where the number of seedlings ranged from 1050 to 3817m−2. Species richness (10.7±1.5 species), Shannon diversity (2.13±0.13) and evenness (0.90±0.03) peaked in samples taken 16m from the river channel. Variation in vegetation parameters was very high between samples, and 36% of the observations showed higher variation within replicates from the same distance than between distances reflecting high variability on a small spatial scale. The higher diversity of seedlings 16m from the river coincided with a confluence zone emerging within the floodplain due to the complex flow arising during inundations, causing higher sediment deposition. Most seeds and most species germinated within the first three weeks, indicating that species commonly deposited with winter high flow in riparian wetlands show early germination, giving them a competitive advantage when vegetation establishes in early spring.

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.

Contaminant transport in a two-zone wetland: Dispersion and ecological degradation

Understanding the fate of contaminant in wetland flows is essential in applications such as ecological risk assessment and environmental hydraulic design. Presented in this paper is an analytical study on the dispersion of contaminant in a two-zone wetland, with the effect of ecological degradation taken into consideration. Environmental dispersion is discussed separately via an exponential transformation for the general formulation of contaminant transport. Taylor’s classical analysis for solute dispersion in a long and thin tube flow is rigorously generalized for the dispersion of the lateral mean contaminant concentration in the longitudinal direction. A method of asymptotic analysis is adopted instead of the concentration moment method in order to simplify the process of deduction and the expression of the analytical solution. Gill’s method of mean concentration expansion is applied to model the concentration deviation terms produced in an averaging operation. With the velocity profile obtained previously, the enhancement of the environmental dispersivity under long time evolution is determined and shown to be consistent with that obtained by the method of concentration moment. Analytical solutions for the evolution of the contaminant concentration and the influenced region of the contaminant cloud are obtained by combining both the hydraulic and the ecological effects. For typical pollutant as the heavy metal Hg, the evolution of contaminant cloud is illustrated by critical length and duration in an application with concentration beyond some given environmental standard level. Results show that for wetland flows with a two-zone structure, the influenced region is reduced evidently while the duration of the contaminant cloud remains nearly unchanged, compared with that for the single-zone wetland flow.

Boundary Effects on Benthic Microbial Phosphorus Concentrations and Diatom Beta Diversity in a Hydrologically-modified, Nutrient-limited Wetland

Water flow and flooding duration in wetlands influence the structure and productivity of microbial communities partly through their influence on nutrient loading. The effect of flow-regulated nutrient loads is especially relevant for microbial communities in nutrient-poor settings, where delivery controls nutrient uptake rates and the intensity of microbial interactions. We examined the effect of hydrologic history and proximity to water sources on nutrient enrichment of benthic microbial assemblages (periphyton) and on their diatom species composition, along the artificial boundaries of Taylor Slough, a historically phosphorus-depleted drainage of the Florida Everglades. Concentrations of phosphorus in periphyton declined from the wetland boundary near inflow structures to 100-m interior, with spatial and temporal variability in rates dependent on proximity to and magnitude of water flow. Phosphorus availability influenced the beta diversity of diatom assemblages, with higher values near inflow structures where resources were greatest, while interior sites and reference transects contained assemblages with constant composition of taxa considered endemic to the Everglades. This research shows how hydrologic restoration may have unintended consequences when incoming water quality is not regulated, including a replacement of distinctive microbial assemblages by ubiquitous, cosmopolitan ones.

Performance of a vertical subsurface flow (VSF) wetland treatment system using woodchips to treat livestock stormwater

This study was conducted to develop a vertical subsurface flow (VSF) wetland remediation system packed with woodchips to control stormwater pollution arising from livestock agriculture. Three lab-scale VSF wetlands were operated with recirculation during the interval (Δ) between storms as 2, 4 and 8 days, respectively. The fed water was 100% recirculated one time per 24 h; the recirculation frequency was 1, 3 and 7 times at Δ of 2, 4 and 8 days, respectively. The constructed wetland systems proved to be effective in reducing total suspended solid (TSS), but also had potential for increasing TSS in the effluent due to the properties of the woodchips. The release of organic matter, especially in the dissolved form, occurred during the initial 60 days. The removal efficiencies of total nitrogen (TN) were 26.2%, 34.1% and 50.0% at Δ of 2, 4 and 8 days, respectively. Nitrification was promoted by the abundant oxygen supplied when the water in wetland was recirculated and fed into the wetland. Denitrification was stable and effective due to the availability of carbon sources. The influent total phosphorus (TP) was reduced from an average of 2.05 mg L(-1) to 1.79 mg L(-1), 1.36 mg L(-1) and 0.86 mg L(-1) at Δ as 2, 4 and 8 days, respectively. The result shows that woodchips can be used as substrate material for VSF wetland treatment systems to control nutrient influx from livestock stormwater.

Transport of bicomponent contaminant in free-surface wetland flow

This paper presents a theoretical analysis of a pulsed bicomponent contaminant emission into a free-surface wetland flow. The basic equations are for the bicomponent contaminant transport in the wetland flow under the combined action of advection, mass dispersion, and ecological reaction at the phase averaged scale. The effect of the ecological reaction is separated from the hydrodynamic effect via a set of widely used transforms. The analytical solution for the evolution of the depth-averaged concentration is rigorously derived, with a limiting case covering the known solution for the single component contaminant transport. It is found that the depth-averaged species concentration of the bicomponent contaminant can approach an equilibrium state determined by the distribution coefficient.