Wetlands For Removal Research Articles

Nutrient Removal in Wetlands During Intermittent Artificial Aeration

In a pilot-scale study, the contribution of intermittent artificial aeration to nitrogen and phosphorus removal in three (i.e., aeration in the middle of the substrate, aeration at the bottom of the substrate, nonaerated substrate) subsurface vertical-flow constructed wetlands planted with Typha latifolia L. (cattail) was tested with eutrophic river water in Tianjin (China). In contrast with the nonaerated wetland, aeration enhanced ammonia–nitrogen, total nitrogen, soluble reactive phosphorus, and total phosphorus removal: 10.1, 4.7, 10.2, and 8.8% for aeration in the middle, and 25.1, 10.0, 7.7, and 7.4% for aeration at the bottom of the substrate, respectively. Analysis of above-ground plant biomass indicated that intermittent aeration restrains the increase in biomass, but stimulates assimilation of nitrogen and phosphorus into stems and leaves. Additional total nitrogen removal of 116 kg N/ha and 126 kg N/ha by harvested above-ground T latifolia biomass for intermittent artificial aeration in the middle and at the bottom of the wetland substrate, respectively, was observed. Outflow water quality of aerated subsurface vertical-flow wetlands was better than the one in the nonaerated wetlands during the whole period of the test run. Artificial aeration improved nitrogen and phosphorus removal, especially during the very hot month of August.

Relationships between loading rates and nitrogen removal effectiveness in subsurface flow constructed wetlands

Nitrogen removal of wetlands under 40 different inflow loadings were studied in the field during 15 months. The removal efficiency of four different sets of beds, namely the reed bed, the Zizania caduciflor bed, the mixing planting bed, and the control bed were studied. The outflow loading and total nitrogen (TN) removal rate of these beds under different inflow loadings and pollution loadings were investigated. The inflow loadings of 4 subsurface flow systems (SFS) ranged from 400 to 8000 mg·(m2·d)−1, while outflow loadings were less than 7000 mg·(m2·d)−1. The results showed that the inflow and outflow loading of TN removal rate in SFS presented an obvious linear relationship. The optical inflow loading to run the system was between 2000 to 4000 mg·(m2·d)−1. Average removal rate was between 1062 and 2007 mg·(m2·d)−1. SFS with plant had a better removal rate than the control. TN removal rates of the reed and Zizania caduciflora bed were 63% and 27% higher than the control bed, respectively. The results regarding the TN absorption of plants indicated that the absorption amount was very limited, less than 5% of the total removal. It proved that plants clearly increase TN removal rates by improving the water flow, and increasing the biomass, as well as activities of microorganisms around the roots. The research provided a perspective for understanding the TN removal mechanism and design for SFS.

Impact of photosynthesis and transpiration on nitrogen removal in constructed wetlands

To determine the impact of photosynthesis and transpiration on nitrogen removal in wetlands, an artificial wetland planted with reeds was constructed to treat highly concentrated domestic wastewater. Under different meteorological and hydraulic conditions, the daily changes of photosynthesis and transpiration of reeds, as well as nitrogen removal efficiency were measured. It was found that net photosynthesis rate per unit leaf area was maintained on a high level (average 19.0 μmol CO2/(m2·s)) from 10:00 to 14:00 in July 2004 and reached a peak of 21.1 μmol CO2/(m2·s) when Photon Flux Density was high during the day. Meanwhile, TN and NH4 +-N removal efficiency rose to 79.6% and 89.6%, respectively—the maximum values observed in the test. Correlation coefficient analysis demonstrated a positive correlation among photon flux density, net photosynthetic rate, transpiration rate, and TN and NH4 +-N removal efficiency. In contrast, there was a negative correlation between stomatal conductance and TN and NH4 +-N removal efficiency. Results suggest that the photosynthesis and transpiration of wetland plants have a great impact on nitrogen removal efficiency of wetlands, which can be enhanced by an increase in the photosynthesis and transpiration rate. In addition, the efficiency of water usage by reeds and nitrogen removal efficiency could be affected by the water level in wetlands; a higher level boosts nitrogen removal efficiency.

Seasonal influence on sulfate reduction and zinc sequestration in subsurface treatment wetlands

To characterize the effects of season, temperature, plant species, and chemical oxygen demand (COD) loading on sulfate reduction and metals removal in treatment wetlands we measured pore water redox potentials and concentrations of sulfate, sulfide, zinc and COD in subsurface wetland microcosms. Two batch incubations of 20 day duration were conducted in each of four seasons defined by temperature and daylight duration. Four treatments were compared: unplanted controls, Typha latifolia (broadleaf cattail), and Schoenoplectus acutus (hardstem bulrush), all at low COD loading (267 mg/L), plus bulrush at high COD loading (534 mg/L). Initial SO 4-S and zinc concentrations were 67 and 24 mg/L, respectively. For all treatments, sulfate removal was least in winter (4 °C, plant dormancy) greatest in summer (24 °C, active plant growth) and intermediate in spring and fall (14 °C), but seasonal variation was greater in cattail, and especially, bulrush treatments. Redox measurements indicated that, in winter, plant-mediated oxygen transfer inhibited activity of sulfate reducing bacteria, exacerbating the reduction in sulfate removal due to temperature. Doubling the COD load in bulrush treatments increased sulfate removal by only 20–30% when averaged over all seasons and did not alter the basic pattern of seasonal variation, despite tempering the wintertime increase in redox potential. Seasonal and treatment effects on zinc removal were broadly consistent with sulfate removal and presumably reflected zinc-sulfide precipitation. Results strongly suggest that interactive effects of COD loading rate, temperature, season, and plant species control not only sulfate reduction and zinc sequestration, but also the balance of competition between various microbial consortia responsible for water treatment in constructed wetlands.

Molecular characterization of microbial communities in treatment microcosm wetlands as influenced by macrophytes and phosphorus loading

There has been little understanding of soil microbial community patterns in phosphorus (P) processing in treatment wetlands. The description of soil microbial communities has been implicit, often treated as a ‘black box’. Microbes play an important role in P removal by direct P uptake and storage, mineralization of organic P, and direct and indirect control of the redox state of metals important to P removal. We conducted a microcosm wetland study focusing on phosphorus processing in treatment wetlands. The study collected basic data on microbial community structure, attempting to examine the influence of macrophytes and phosphorus loading on microbial communities that is critical in the treatment efficiency of constructed wetlands. Specifically, we fingerprinted soils from microcosms under different conditions of macrophyte and phosphorus loading using length heterogeneity PCR (LH-PCR). We also examined microbial diversity of some of those soils by cloning and sequencing community libraries. Principal coordinate analysis (PCO) and analysis of similarity (ANOSIM) of the fingerprints suggested that high-phosphorus loading has a recognizable impact (global R = 0.6, p = 2.9%) in altering overall soil microbial community structure, seemingly decreasing the microbial diversity (i.e., Shannon–Weiner's diversity index, H′). The clone sequence data showed that different bacterial groups were selected for different phosphorus treatments. No significant effects of the presence of macrophytes on soil microbial communities were detected (global R = −0.15; p = 80%), likely due to low growth of plants and sampling anomalies. Further study is underway to investigate possible linkages between the soil microbial community patterns and major operational components, including vegetation and phosphorus loading in treatment wetlands, which will help us eventually develop microbial community metrics and/or indicators that wetland engineers and managers can utilize as one tool in managing different operational conditions to achieve optimum phosphorus removal in treatment wetlands.

Environmental conditions and phosphorus removal in Florida lakes and wetlands inhabited by Hydrilla verticillata (Royle): implications for invasive species management

Hydrilla verticillata is considered the most problematic aquatic plant in the United States. In south Florida, Hydrilla dominance has also been documented in treatment wetlands. This paper characterizes (1) environmental conditions which favor Hydrilla growth and (2) understand its nutrient removal capability. Despite its occurrence over a wide range of environmental conditions, Hydrilla abundance increased with increasing pH, alkalinity, total P and total N, and decreased with water depth in selected Florida lakes. No relationship was found between color, Secchi depth and Hydrilla abundance. In several Hydrilla-dominated lakes, mean total P concentration (126 μg/l) at inflow was reduced to 106 μg/l at outflow. The maximum inflow total P concentration in a lake with positive nutrient reduction was 148 μg/l. Total P removal efficiency by Hydrilla-dominated lakes and wetlands was comparable to or higher than systems dominated by emergent and other submerged plants. Mean total P settling rates for lakes and a constructed wetland dominated by Hydrilla were estimated at 19 and 34 m/year, respectively, which were higher than or comparable to similar systems dominated by other aquatic plants. Results from this study suggest that reduction of Hydrilla from constructed wetlands will not likely improve nutrient removal performance.

Predicting nitrogen and phosphorus removal in wetlands due to detritus accumulation: A simple mechanistic model

Constructed wetlands have shown a substantial capacity for wastewater treatment under a wide range of conditions, but removal efficiency of nitrogen (N) and phosphorus (P) has been inconsistent and is often low. Models designed to predict N and P removal in wetlands include complex numerical efforts and empirical models, both of which have shortcomings. In this paper we present a simple mechanistic model capable of predicting N and P removal by detritus accumulation in constructed wetlands. Implications of the simplifications made in this model are explored through simulation modeling. Preliminary validation with one system suggests that the model holds promise. The dominant role of refractory organic matter suggests that more research is needed on this topic.

Pharmaceutical Compounds in Wastewater: Wetland Treatment as a Potential Solution

Pharmaceutical compounds are being released into the aquatic environment through wastewater discharge around the globe. While there is limited removal of these compounds within wastewater treatment plants, wetland treatment might prove to be an effective means to reduce the discharge of the compounds into the environment. Wetlands can promote removal of these pharmaceutical compounds through a number of mechanisms including photolysis, plant uptake, microbial degradation, and sorption to the soil. We review relevant laboratory research on these various mechanisms and provide data on the few studies that have examined wetland removal. There is a need to document the degree to which various pharmaceutical compounds are removed in full-scale treatment wetlands, as there is a paucity of data on overall pharmaceutical removal rates.

The role of constructed wetlands in secondary effluent treatment and water reuse in subtropical and arid Australia

Water reclamation and reuse is being actively promoted in Australia. In Queensland, surface-flow constructed wetlands with a diversity of macrophyte types offer the greatest potential for effluent polishing. Constructed wetlands in subtropical climates in coastal regions and arid climates in inland western regions are conducive to high macrophyte growth rates and nutrient removal, in particular nitrogen, producing an effluent suitable for irrigation, restoration of wetlands and/or release into natural waterways. Faecal-coliform removal is also high, producing effluent with <1000 cfu/100 mL and as low as 100 cfu/100 mL, acceptable for agricultural irrigation. Constructed wetlands can be designed to maximise the removal of both nutrients and pathogens by enhancing macrophyte diversity and natural disinfection processes by incorporating lagoons, shallow-water wetlands and subsurface-flow wetlands into the treatment train. Surface-flow wetlands can also be designed to minimise mosquito breeding by increasing macro-invertebrate predators, thereby alleviating community concerns about potential health risks. This paper addresses the role of constructed wetlands in nutrient and pathogen removal in Queensland's wetlands, and presents three case studies with respect to effluent reuse.

Research Article: Application of Rapid Environmental Assessment and Co-occurrence Analysis for Identifying, Managing, and Restoring Wetlands

Rapid assessment of land use change at scales ranging from local to global can provide timely, accurate information about land suitability, supporting infrastructure, and regulatory compliance so that developers, resource managers, planners, and decision makers can make critical decisions in the face of uncertainty. One area of concern regarding land suitability is the presence, extent, and quality of wetlands. Federal and state wetland protection legislation and local ordinances have effectively sought to reduce the amount of wetlands converted to other uses or to mitigate for unavoidable removal of wetlands. Identification and delineation of existing or potential wetlands, as well as locations that could serve as mitigation areas, is an important process in environmental planning and development. A method of identifying and delineating wetlands using the technique of rapid assessment at regional scales is presented. This method combines high-resolution remote-sensed and secondary data, including themati...

Temporal export of nitrogen from a constructed wetland: influence of hydrology and senescing submerged plants

Nitrogen export was measured during monthly monitoring in an 18-ha constructed wetland in southeast Sweden 1998–2001. To investigate the influence of increased water flow on wetland nitrogen export, we performed intensive sampling (eight samples per day) of total-N, urea-N, NH 4 +-N, NO 3 −-N in January–March 2001. Investigations of wetland plant distribution 1997–2001 and submerged plant biomass were combined with investigations of plant nitrogen content 1998–2001, to test the hypothesis that nitrogen export was linked to wetland plant decomposition. Nitrogen was exported from the wetland (3.3 kg N ha −1 d −1), during the end of the intensive sampling period (19 February–12 March), coinciding with increased water flow (from 0.2 to 0.6 m 3 s −1). Plant cover expanded rapidly and nitrogen assimilated in the submerged plant community was estimated to be 39 kg ha −1 in July 2001. After senescence in April the following year, plant biomass was reduced by 75%. We concluded that a significant part (40%) of this wetland nitrogen export might be associated with the release of organic and inorganic nitrogen from the senesced submerged plant community. Moreover, we hypothesize that nitrogen removal in wetlands in cold temperate climate may be limited by temporal nitrogen export induced by high water flow velocity when wetland plants are decomposing.

An investigation of the limits of phosphorus removal in wetlands: a mesocosm study of a shallow periphyton-dominated treatment system

The performance of a mesocosm-scale experimental wetland system (9-cm deep raceways with a limerock substrate followed by a limerock filter) that received effluent from a treatment wetland in south Florida was monitored for 19 months. The raceways were operated at a constant hydraulic load of 11 cm per day, resulting in a nominal hydraulic retention time (HRT) of 0.8 days. The vegetation community that developed in these systems varied along the longitudinal phosphorus (P) gradient and became dominated by eutrophic periphyton at the inflow ( Cladophora and Microspora) and oligotrophic taxa along the raceway's mid-regions and outflow ( Schizothrix and Scytonema). The inflow total P (TP) concentration (mean = 18 μg P L −1) was reduced to a mean of 10 μg P L −1 at the raceway outflow and 8 μg P L −1 exiting the limerock filters. Total P removal for the entire system was 0.32 g m −2 per year, or 46% of the inflow TP load, and uptake rates ( k values) were 24 and 32 m per year for the raceway and the raceway/limerock filter system, respectively. Most TP removal occurred within the first 3 m of the raceways at an HRT of 1.3 h. Both soluble reactive and particulate P were reduced to their analytical detection limits, while dissolved organic P was removed less efficiently. There was little evidence for a diel trend in P removal. Following a 1-week dry-out, the raceways exported P for only 24 h after rehydration, after which treatment performance quickly returned to pre-desiccation levels. Nutrient ratios suggest that the raceways were never N-limited and that P limitation increased with distance from the inflow ports. Accrued sediment had a high percentage of non-labile P (>70%). A number of operational and construction issues need to be resolved before this technology could be scaled up for use in conjunction with Everglades restoration.

Nitrate removal in riparian wetlands: interactions between surface flow and soils.

Riparian wetlands containing springs are thought to be ineffective at removing nitrate because contact times between the upwelled ground water and the underlying microbially active soils are short. Tracer experiments using lithium bromide (LiBr) and nitrate (NO3-N) injected at the surface were used to quantify residence times and NO3-N removal in a riparian swale characteristic of New Zealand hill-country pasture. An experimental enclosure was used with collecting trays at the downstream end to measure flow and concentration, shallow wells to measure subsurface concentrations, and an array of logging conductivity probes to monitor tracer continuously. The majority of added tracer reached the outlet more slowly than could be explained by surface flow, but more quickly than could be explained by Darcy seepage flow. There was evidence from the wells of tracer diffusing vertically to a depth of at least 5 cm into the surface soil layer, which was permanently saturated and highly porous. During dry weather 24 +/- 9% of added NO3-N was removed over a distance of 1.5 m largely by denitrification. The net uptake length coefficient for this wetland (K = 0.08 +/- 0.03 m(-1)) is slightly higher than the range (K = 0.01-0.07 m(-1)) measured in a small stream channel infested with macrophytes. Nitrate removal is expected to decrease with increasing flow. Seepage flow is estimated to have removed only 7 +/- 4% of the added NO3-N and we hypothesize that vertical diffusion substantially increases NO3-N removal in this type of wetland. Riparian wetlands with springs and surface flows should not be dismissed as having low NO3-N removal potential without checking whether there is significant vertical mixing.

Fate of volatile organic compounds in constructed wastewater treatment wetlands.

The fate of volatile organic compounds was evaluated in a wastewater-dependent constructed wetland near Phoenix, AZ, using field measurements and solute transport modeling. Numerically based volatilization rates were determined using inverse modeling techniques and hydraulic parameters established by sodium bromide tracer experiments. Theoretical volatilization rates were calculated from the two-film method incorporating physicochemical properties and environmental conditions. Additional analyses were conducted using graphically determined volatilization rates based on field measurements. Transport (with first-order removal) simulations were performed using a range of volatilization rates and were evaluated with respect to field concentrations. The inverse and two-film reactive transport simulations demonstrated excellent agreement with measured concentrations for 1,4-dichlorobenzene, tetrachloroethene, dichloromethane, and trichloromethane and fair agreement for dibromochloromethane, bromodichloromethane, and toluene. Wetland removal efficiencies from inlet to outlet ranged from 63% to 87% for target compounds.

Scenario analysis for reduction of effluent load from an agricultural area by recycling the run-off water

We reviewed rates of nitrogen (N) removal in paddy fields and wetlands. Then we developed a numerical model to simulate N flow in an agricultural paddy field area and analyzed scenarios for recycling the agricultural run-off, including field drainage, from an agricultural area with an irrigation/drainage system. In it, we considered N removal in paddy fields, a regulating reservoir, and canals. The review showed that the rate constant for N removal in paddy fields and wetlands was 0.025 m/d under natural conditions and 0.01 m/d under dark conditions. The scenario analysis of recycling paddy field run-off within the model area indicated that a large amount of the effluent load occurred during the transplanting period and that recycling could reduce the effluent N load. The effluent N load would be equal to the inflowing N load at a 48% recycling rate. In this scenario, paddy fields accounted for most of the reduction in the effluent N load, and the mean removal rates per unit area of paddy field and regulating reservoir were 53.6 g ha(-1)d(-1) and 222.6 g ha(-1)d(-1) , respectively.

인공습지를 이용한 하구담수호 유입하천수 수질개선 현장실험결과 분석

Wetland system is widely accepted as one of natural water purification systems around the world for nonpoint sources pollution control. Constructed wetlands have become a popular technology for treating contaminated surface and waste water. In this study, the field experiment to reduce nonpoint source pollution loadings from polluted stream waters using wetland system was performed from June 2002 to March 2004. Four wetlands were used and the size of each one was 0.8ha. Water of Dangjin stream flowing into Seokmun estuarine reservoir was pumped into wetlands. Inflow and hydraulic residence time of the system was 500 <TEX>$m^3$</TEX>/day∼1,500 <TEX>$m^3$</TEX>/day, 2∼5 days, respectively. After 2 year operation, plant-coverage of the wetlauds was about 70% from bare soil surface at initial stage . Average water quality of the influent was <TEX>$BOD_5$</TEX> 4.17 mg/L, TSS 18.45 mg/L, T-N 4.32 mg/L, and T-P 0.30 mg/L. The average removal rate of <TEX>$BOD_5$</TEX>, TSS, T-N and T-P during the study period was 5.6%, 46.6%, 45.7%, and 54.8%, respectively. Organic (<TEX>$BOD_5$</TEX>) removal rate was low and the reason might be low influent concentration. Wetland removal rate of T-P was about 10% higher than T-N. Performance of the experimental system was compared with existing data base (NADB), and it was within the range of general system performance. Overall, the wetland system was found to be adequate for treating polluted water stream with stable removal efficiency even during the winter period. Most of the nonpoint source pollutions from watershed are transported by streams or ditches, and they could be controled by constructed wetland system before entering the lake or reservoir.

제올라이트 칼럼에 의한 인공생활하수처리

Constructed wetlands typically cost less to build and operate, and require less energy than standard mechanical treatment technology but they have similar performance to centralized wastewater treatment plants. Therefore, they were constructed especially many in rural areas, where are small villages but not industries. Plantless column tests were performed to investigate the possibility on using zeolite as a filter medium of constructed wetland for the wastewater treatment. Removal efficiency was 94.63% T-P 41.41% and 99.75% at hydraulic load 314 and filtering height 100 cm filled with a zeolite mixture. This zeolite mixture consisted of 1 : 1 by volume of zeolites in the diameter range of 0.5 to 1 mm and 1 to 3 mm. Accordingly, hydraulic load 314 was considered as optimal. Three zeolite mixtures were used to determine the optimal mixing ratio by volume of a zeolite (A) in the diameter range of 0.5 to 1 mm to a zeolite (B) in the diameter range of 1 to 3 mm 1 : 3, 1 : 1 and only B in A to B by volume were tested at hydraulic load 314 and filtering height 100 cm removal efficiency was more than 89% at mixing ratios of 1 : 3 and 1 : 1 in A to B. That of T-P ranged 56.42 to 58.72% and, that of T-N and was 87% and 99% regardless of mixing ratios of two zeolites. Removal efficiency was lower at the column filled with only B. Removal efficiency was better at Inter medium filled with mixing ratio 1 : 1 in A to B than with the other mixing ratios. Thus, it was found that the mixture of mixing ratio 1 : 1 in A to B was appropriate far Inter medium of constructed wetland Removal efficiency was higher in down-flow than in up-flow, and all contaminants were removed most in 20 cm filter height near feeding area.

Damköhler number distributions and constituent removal in treatment wetlands

Using hypothetical wetland simulations and data from the literature, Kadlec [Eco. Eng. 15 (2000) 105] recently demonstrated that plug-flow models commonly used to quantify treatment wetland performance fail to describe conditions other than those under which calibration data are collected. Parameters of these models (removal rate constants ( k) and background concentrations ( C*)) demonstrate apparent dependence on inlet concentration and hydraulic loading rate which is not alleviated by including dispersion to address non-ideal flow. The phenomenon can be understood as resulting from an interdependence between k and local flow velocity, due to the functional dependence of each on drag-inducing surfaces (and attached biofilms) associated with submerged vegetation and litter. This paper presents a simple method, based on theoretical considerations, for determining C* using inlet–outlet data, independent of the degree of mixing or the nature of the removal processes. This paper also expands upon the hypothetical multi-channel example to suggest a modeling approach in which a wetland is treated conceptually as an ensemble of parallel, non-interacting stream tubes in plug-flow, characterized by a continuous distribution of Damköhler numbers. The Damköhler number distribution (DND) can be estimated from the residence time distribution (RTD) under the assumption of uniform flow path length. For such a wetland, under steady state conditions and with constant inlet concentration, the fraction of a removable pollutant remaining as a function of normalized distance from inlet to outlet is given by the Laplace transform of the DND. Similarly, the DND can be derived from the inverse Laplace transform of the normalized concentration versus normalized distance curve. Given both an RTD and a DND, it is possible to investigate the relationship between k and residence time, and the mechanistic nature of the removal process. Employing these concepts makes it possible to generate an expression for normalized concentration as a function of fractional distance that is unaffected by changes in inlet concentration, and inherently takes into account changes in hydraulic loading rate.

Modelling nitrogen removal in potential wetlands at the catchment scale

The reduction of nitrogen fluxes from land to sea is an important task in areas with estuarine or marine eutrophication. Wetland creation has been proposed as one method to reduce nitrogen from streams draining agricultural areas. In this study, a scenario of nitrogen removal in created wetlands was evaluated by mathematical modelling of nitrogen fluxes in a catchment (224 km 2) in southern Sweden. The scenario was based on topographically realistic siting of 40 potential wetlands with a total area of 0.92 km 2 (0.4% of the catchment area). Nitrogen removal in the wetlands was described with a simple and robust first-order model, which was modified and evaluated against data from eight monitored surface-flow wetlands. However, the modifications gave no substantial support for changing the basic model. For catchment-scale modelling this wetland model was incorporated into a dynamic process-based catchment model (HBV-N). The catchment was then divided to several coupled subbasins, so that the wetland influence on nitrogen load could be estimated separately for each potential wetland. The modelling showed that the 40 potential wetlands would reduce the nitrogen transport to the coast with approximately 6%. Specific removal rates ranged between 57 and 466 kg ha −1 yr −1 for the different wetlands, depending on residence time (size and hydraulic loading) and nitrogen concentration in inflow. Due to temperature dependence and seasonal variation in water discharge, significant decrease in nitrogen concentrations mainly occurred during summer periods with low loading. The study illustrates that catchment modelling is a useful method for analysing wetland creation plans, and that wetland creation must cover fairly large areas and be combined with other measures in order to achieve substantial reduction of nitrogen fluxes to coastal waters. Further monitoring of existing wetlands will improve the removal expression and decrease uncertainty. For instance, at present it could not be deducted whether wetlands with low average residence times (<2 days) have net removal or net resuspension on an annual basis.

Fundamental processes within natural and constructed wetland ecosystems: short-term versus long-term objectives

Use of wetland ecosystems for water pollution control consists essentially of sustained induced disturbances as pollutants are loaded to complex biological communities. Objectives are to maximize pollutant loading, incorporation, and retention while maintaining highest levels of community metabolism and minimal alteration of community structure. Several basic processes are emphasized: (a) macrophyte productivity in relation to shoot:root ratios, and nutrient availability; (b) macrophyte life history strategies, succession, and biodiversity under constant pollutant stress; (c) importance of standing dead and particulate detritus; (d) functions and controlling mechanisms of heterotrophic and autotrophic periphyton in pollutant retention and recycling; (e) coupling of microbial metabolism to macrophyte retention of pollutants; (f) gaseous losses to the atmosphere; (g) losses of dissolved organic matter and its utilization; and (h) water losses by evapotranspiration and effects on wetland efficacy. Short-term wetland removal efficiencies are confounded by massive variations in retentive capacities diurnally, seasonally, and spatially, in exceeding physiological tolerance levels, and in species succession. Problems of channelization, altered microhydrology, and assimilation/retention are major in natural and non-engineered ecosystems. Wetlands are highly ephemeral and variable in their capabilities for sequestering and retention of nutrients and other pollutants.