Nutrient removal in tropical subsurface flow constructed wetlands under batch and continuous flow conditions

The aim of this investigation was to evaluate the influence of batch versus continuous flow on the removal efficiencies of chemical oxygen demand (COD), nitrogen (N) and total phosphorus (TP) in tropical subsurface flow constructed wetlands (SSF CW). The quantitative role of the higher aquatic plants in nutrient removal in these two operational modes was also investigated. Results indicated no significant difference (p > 0.05) in COD removal between batch and continuous flow modes for either the planted or unplanted treatments. Furthermore, the batch-loaded planted wetlands showed significantly (p < 0.05) higher ammonium removal efficiencies (95.2%) compared with the continuously fed systems (80.4%), most probably because the drain and fill batch mode presented systematically more oxidized environmental conditions. With respect to TP removal, for both planted and unplanted beds, there was significant enhancement (p < 0.05) in batch flow operation (69.6% for planted beds; 39.1% for unplanted beds) as compared to continuous flow operation (46.8% for planted beds; 25.5% for unplanted beds). In addition, at a 4-day hydraulic retention time (HRT), the presence of plants significantly enhanced both ammonia oxidation and TP removal in both batch and continuous modes of operation as compared to that for unplanted beds. An estimation of the quantitative role of aeration from drain and fill operation at a 4-day HRT, as compared to rhizosphere aeration by the higher aquatic plant, indicated that drain and fill operation might account for only less than half of the higher aquatic plant's quantitative contribution of oxygen (1.55 g O2 per m2 per day for batch flow versus 1.13 g O2 per m2 per day for continuous flow).     

Large-scale constructed wetlands for nutrient removal from stormwater runoff: An everglades restoration project

The South Florida Water Management District (SFWMD) constructed a wetland south of Lake Okeechobee to begin the process of removing nutrients (especially phosphorus) from agricultural stormwater runoff entering the Everglades. The project, called the Everglades Nutrient Removal (ENR) project, is a prototype for larger, similarly constructed wetlands that the SFWMD will build as part of the Everglades restoration program. This innovative project is believed to be one of the largest agricultural stormwater cleanup projects in the United States, if not in the world. This publication describes the ENR project's design, construction, and proposed operation, as well as the proposed research program to be implemented over the next few years.     

Multiyear nutrient removal performance of three constructed wetlands intercepting tile drain flows from grazed pastures

Subsurface tile drain flows can be a major s ource of nurient loss from agricultural landscapes. This study quantifies flows and nitrogen and phosphorus yields from tile drains at three intensively grazed dairy pasture sites over 3- to 5-yr periods and evaluates the capacity of constructed wetlands occupying 0.66 to 1.6% of the drained catchments too reduce nutrient loads. Continuous flow records are combined with automated flow-proportional sampling of nutrient concentrations to calculate tile drain nutrient yields and wetland mass removal rates. Annual drainage water yields rangedfrom 193 to 564 mm (16-51% of rainfall) at two rain-fed sites and from 827 to 853 mm (43-51% of rainfall + irrigation) at an irrigated site. Annually, the tile drains exported 14 to 109 kg ha(-1) of total N (TN), of which 58 to 90% was nitrate-N. Constructed wetlands intercepting these flows removed 30 to 369 gTN m(-2) (7-63%) of influent loadings annually. Seasonal percentage nitrate-N and TN removal were negatively associated with wetland N mass loadings. Wetland P removal was poor in all wetlands, with 12 to 115% more total P exported annually overall than received. Annually, the tile drains exported 0.12 to 1.38 kg ha of total P, of which 15 to 93% was dissolved reactive P. Additional measures are required to reduce these losses or provide supplementary P removal. Wetland N removal performance could be improved by modifying drainage systems to release flows more gradually and improving irrigation practices to reduce drainage losses.     

Phosphorus retention in small constructed wetlands treating agricultural drainage water

The construction of artificial wetlands has become a measure increasingly applied to reduce nonpoint-source (NPS) pollution and to contribute to the restoration of eutrophic lakes and coastal waters. In a 2-yr study monitoring fluxes of particulate and dissolved phosphorus (P) in a small artificial wetland for the treatment of agricultural drainage water in Central Switzerland, water residence time was identified as the main factor controlling P retention in the system. Since most of the annual P load (62% as dissolved reactive phosphorus, DRP) was related to high discharge events, it was not average but minimum water residence time during flood events that determined the wetland's P retention. In agreement with a continuous stirred tank reactor (CSTR) model, our investigations suggest a minimum water residence time of 7 d to retain at least 50% of the bioavailable P. The investigated wetland retained only 2% of the bioavailable P, since the water residence time was shorter than 7 d during 61% of time in both years. Settling of phytoplankton rather than DRP uptake into phytoplankton limited the retention of bioavailable P. The overall retention efficiency of 23% total phosphorus (TP), corresponding to a surface related retention of 1.1 g P m(-2) yr(-1), was due to the efficient trapping of pedogenic particles.     

Denitrification in constructed wetlands used for treatment of swine wastewater

Constructed wetland treatment of swine wastewater probably involves substantial denitrification. Our objective was to assess denitrification and denitrification enzyme activity (DEA) in such wetlands in relation to plant communities, N loading, carbon or nitrogen limitations, and water depth. Two wetland cells each 3.6 m wide and 33.5 m long were connected in series. One set of cells was planted with rushes and bulrushes, including soft rush (Juncus effusus L.), softstem bulrush [Schoenoplectus tabernaemontani (K.C. Gmel.) Pallal, American bulrush [Schoenoplectus americanus (Pers.) Volkart ex Schinz & R. Keller], and woolgrass bulrush [Scirpus cyperinus (L.) Kunth]. Another set was planted with bur-reeds and cattails, including American bur-reed (Sparganium americanum Nutt.), broadleaf cattail (Typha latifolia L.), and narrowleaf cattail (Typha angustifolia L.). The sets will be referred to herein as bulrush and cattail wetlands, respectively. Denitrification and DEA were measured via the acetylene inhibition method in intact soil cores and disturbed soil samples that were taken during four years (1994-1997). Although DEA in the disturbed samples was greater than denitrification in the core samples, the measurements were highly correlated (r2 > or = 0.82). The DEA was greater in the bulrush wetlands than the cattail wetlands, 0.516 and 0.210 mg N kg(-1) soil h(-1), respectively; and it increased with the cumulative applied N. The DEA mean was equivalent to 9.55 kg N ha(-1) d(-1) in the bulrush wetlands. We hypothesized and confirmed that DEA was generally limited by nitrate rather than carbon. Moreover, we determined that one of the most influential factors in DEA was wetland water depth. In bulrush wetlands, the slope and r2 values of the control treatment were -0.013 mg N kg(-1) soil h(-1) mm(-1) depth and r2 = 0.89, respectively. Results of this investigation indicate that DEA can be very significant in constructed wetlands used to treat swine wastewater.     

Ultraviolet irradiation effects incorporation of nitrate and nitrite nitrogen into aquatic natural organic matter

One of the concerns regarding the safety and efficacy of ultraviolet radiation for treatment of drinking water and wastewater is the fate of nitrate, particularly its photolysis to nitrite. In this study, N NMR was used to establish for the first time that UV irradiation effects the incorporation of nitrate and nitrite nitrogen into aquatic natural organic matter (NOM). Irradiation of (15)N-labeled nitrate in aqueous solution with an unfiltered medium pressure mercury lamp resulted in the incorporation of nitrogen into Suwannee River NOM (SRNOM) via nitrosation and other reactions over a range of pH from approximately 3.2 to 8.0, both in the presence and absence of bicarbonate, confirming photonitrosation of the NOM. The major forms of the incorporated label include nitrosophenol, oxime/nitro, pyridine, nitrile, and amide nitrogens. Natural organic matter also catalyzed the reduction of nitrate to ammonia on irradiation. The nitrosophenol and oxime/nitro nitrogens were found to be susceptible to photodegradation on further irradiation when nitrate was removed from the system. At pH 7.5, unfiltered irradiation resulted in the incorporation of (15)N-labeled nitrite into SRNOM in the form of amide, nitrile, and pyridine nitrogen. In the presence of bicarbonate at pH 7.4, Pyrex filtered (cutoff below 290-300 nm) irradiation also effected incorporation of nitrite into SRNOM as amide nitrogen. We speculate that nitrosation of NOM from the UV irradiation of nitrate also leads to production of nitrogen gas and nitrous oxide, a process that may be termed photo-chemodenitrification. Irradiation of SRNOM alone resulted in transformation or loss of naturally abundant heterocyclic nitrogens.     

Temperature and feed strategy effects on sulfate and organic matter removal in an AnSBB

The objective of this work was to analyze the interaction effects between temperature, feed strategy and COD/[SO(4)(2-)] levels, maintaining the same ratio, on sulfate and organic matter removal efficiency from a synthetic wastewater. This work is thus a continuation of Archilha et al. (2010) who studied the effect of feed strategy at 30 °C using different COD/[SO(4)(2-)] ratios and levels. A 3.7-L anaerobic sequencing batch reactor with recirculation of the liquid phase and which contained immobilized biomass on polyurethane foam (AnSBBR) was used to treat 2.0 L synthetic wastewater in 8 h cycles. The temperatures of 15, 22.5 and 30 °C with two feed strategies were assessed: (a) batch and (b) batch followed by fed-batch. In strategy (a) the reactor was fed in 10 min with 2 L wastewater containing sulfate and carbon sources. In strategy (b) 1.2 L wastewater (containing only the sulfate source) was fed during the first 10 min of the cycle and the remaining 0.8 L (containing only the carbon source) in 240 min. Based on COD/[SO(4)(2-)] = 1 and on the organic matter (0.5 and 1.5 gCOD/L) and sulfate (0.5 and 1.5 gSO(4)(2-)/L) concentrations, the sulfate and organic matter loading rates applied were 1.5 and 4.5 g/L.d, i.e., same COD/[SO(4)(2-)] ratio (=1) but different levels (1.5/1.5 and 4.5/4.5 gCOD/gSO(4)(2-)). When reactor feed was 1.5 gCOD/L.d and 1.5 gSO(4)(2-)/L.d, gradual feeding (strategy b) showed to favor sulfate and organic matter removal in the investigated temperature range, indicating improved utilization of the electron donor for sulfate reduction. Sulfate removal efficiencies were 87.9; 86.3 and 84.4%, and organic matter removal efficiencies 95.2; 86.5 and 80.8% at operation temperatures of 30; 22.5 and 15 °C, respectively. On the other hand, when feeding was 4.5 gCOD/L.d and 4.5 gSO(4)(2-)/L.d, gradual feeding did not favor sulfate removal, indicating that gradual feeding of the electron donor did not improve sulfate reduction.     

Oxytetracycline sorption to organic matter by metal-bridging

The sorption of oxytetracycline to metal-loaded ion exchange resin and to natural organic matter by the formation of ternary complexes between polyvalent metal cations and sorbent- and sorbate ligand groups was investigated. Oxytetracycline (OTC) sorption to Ca- and Cu-loaded Chelex-100 resin increased with increasing metal/sorbate ratio at pH 7.6 (OTC speciation: 55% zwitterion, 45% anion). Greater sorption to Cu- than Ca-loaded resin was observed, consistent with the greater stability constants of Cu with both the resin sites and with OTC. Oxytetracycline sorption to organic matter was measured at pH 5.5 (OTC speciation: 1% cation, 98% zwitterion, 1% anion). No detectable sorption was measured for cellulose or lignin sorbents that contain few metal-complexing ligand groups. Sorption to Aldrich humic acid increased from "clean" < "dirty" (no cation exchange pretreatment) < Al-amended < Fe(III)-amended clean humic acid with K(d) values of 5500, 32000, 48000, and 250000 L kg(-1) C, respectively. Calcium amendments of clean humic acid suggested that a portion of the sorbed OTC was interacting by cation exchange. Oxytetracycline sorption coefficients for all humic acid sorbents were well-correlated with the total sorbed Al-plus-Fe(III) concentrations (r(2) = 0.87, log-log plot), suggesting that sorption by ternary complex formation with humic acid is important. Results of this research indicate that organic matter may be an important sorbent phase in soils and sediments for pharmaceutical compounds that can complex metals by the formation of ternary complexes between organic matter ligand groups and pharmaceutical ligand groups.     

The use of constructed wetlands in the treatment of acid mine drainage

US government regulations require that all effluents from industrial operations, including mining, meet certain water quality standards. Constructed wetlands have proven to be useful in helping to attain those standards. Application of this biotechnology to mine water drainage can reduce water treatment costs and improve water quality in streams and rivers adversely affected by acidic mine water drainage from abandoned mines. Over 400 constructed wetland water treatment systems have been built on mined lands largely as a result of research by the US Bureau of Mines. Wetlands are passive biological treatment systems that are relatively inexpensive to construct and require minimal maintenance. Chemical treatment costs are reduced sufficiently to repay the cost of construction in less than a year. The mine waste water is typically treated in a series of excavated ponds that resemble small marsh areas. The ponds are engineered to facilitate bacterial oxidation of iron. Ideally, the water then flows through a composted organic substrate supporting a population of sulphate-reducing bacteria which raises the pH. Constructed wetlands in the USA are described - their history, functions, construction methodologies, applicabilities, limitations and costs.     

Denitrification potential and organic matter as affected by vegetation community, wetland age, and plant introduction in created wetlands

Denitrification potential (DP) and organic matter (OM) in soils were compared in three different vegetation communities-emergent macrophyte, open water, and forested edge-in two 10-yr-old created riverine wetlands. Organic matter, cold water-extractable organic matter (CWEOM), anaerobic mineralizable carbon (AnMC), and DP varied significantly (P<0.05) among vegetation communities. The surface (0 to 9 cm) soils in the emergent macrophyte community (EMC) showed highest DP (0.07+/-0.01 mg N h-1 kg-1), OM (84.90+/-5.60 g kg-1), CWEOM (1.12+/-0.20 g kg-1), and AnMC (1.50+/-0.10 mg C h-1 kg-1). In the deeper layer (9 to 18 cm), DP and CWEOM (0.04+/-0.01 mg N h-1 kg-1 and 1.13+/-0.20 g kg-1, respectively) were significantly higher in the open water community (OWC) than in the emergent macrophyte and forested edge communities. Plant introduction did not affect DP or OM content and characteristics. After 10 yr of wetland development, mean DP increased 25-fold in the surface layer (from 0.002 to 0.053 mg N h-1 kg-1); OM content more than doubled to 90.80+/-19.22 g kg-1, and CWEOM and HWEOM increased 2.5 and 2.7 times respectively from 1993 (prewetland conditions) to 2004. Humic acids were the most abundant form of OM in 2004 and 1993 samples. Significant (P<0.05) positive relationships between DP and OM, CWEOM, and AnMC were found in the surface layer; in the 9- to 18-cm layer, significant positive relationships were found between DP and CWEOM and AnMC.     

Phosphorus removal in a wetland constructed on former arable land

Phosphorus in surface runoff water may cause eutrophication of recipient water. This study clarifies the mechanisms of P removal in the wetland of Hovi, Finland, constructed on arable land in 1998. Before the construction, the surface soil (removed in the construction) and subsoil (the current wetland bottom) were analyzed for Al and Fe oxides (Al(ox) and Fe(ox)) reactive in P sorption, and for the distribution of P between various pools as well as for P exchange properties. Retention of P from runoff water within the wetland was studied from 1999 to 2001 in situ and factors affecting the P removal (O2 availability and P concentration in water) were investigated in a laboratory microcosm. The processes taking place in the wetland diminished by 68% the total P load and by 49% the dissolved reactive P load. Desorption-sorption tests indicated that without removal of the surface soil, there would have been a risk of the wetland being a source of P, since the equilibrium P concentration of the soil removed was high compared with the mean P concentration of the inflowing water. The subsoil contained less P and high amounts of reactive oxides, which could bind P. Evidently, the P sorption by Al(ox) played an important role in a first phase removal of P, since the wetland retained P efficiently even under anoxic conditions, where Fe tends to be reduced. Fine-textured, mineral soil on the bottom of the wetland (subsoil of the former arable land) seemed to be very efficient in retaining P from agricultural runoff.     

Emission of the greenhouse gases nitrous oxide and methane from constructed wetlands in europe

The potential atmospheric impact of constructed wetlands (CWs) should be examined as there is a worldwide increase in the development of these systems. Fluxes of N(2)O, CH(4), and CO(2) have been measured from CWs in Estonia, Finland, Norway, and Poland during winter and summer in horizontal and vertical subsurface flow (HSSF and VSSF), free surface water (FSW), and overland and groundwater flow (OGF) wetlands. The fluxes of N(2)O-N, CH(4)-C, and CO(2)-C ranged from -2.1 to 1000, -32 to 38 000, and -840 to 93 000 mg m(-2) d(-1), respectively. Emissions of N(2)O and CH(4) were significantly higher during summer than during winter. The VSSF wetlands had the highest fluxes of N(2)O during both summer and winter. Methane emissions were highest from the FSW wetlands during wintertime. In the HSSF wetlands, the emissions of N(2)O and CH(4) were in general highest in the inlet section. The vegetated ponds in the FSW wetlands released more N(2)O than the nonvegetated ponds. The global warming potential (GWP), summarizing the mean N(2)O and CH(4) emissions, ranged from 5700 to 26000 and 830 to 5100 mg CO(2) equivalents m(-2) d(-1) for the four CW types in summer and winter, respectively. The wintertime GWP was 8.5 to 89.5% of the corresponding summertime GWP, which highlights the importance of the cold season in the annual greenhouse gas release from north temperate and boreal CWs. However, due to their generally small area North European CWs were suggested to represent only a minor source for atmospheric N(2)O and CH(4).     

Pesticide removal from container nursery runoff in constructed wetland cells

The increased use of pesticides by container nurseries demands that practices for removal of these potential contaminants from runoff water be examined. Constructed wetlands may be designed to clean runoff water from agricultural production sites, including container nurseries. This study evaluated 14 constructed wetlands cells (1.2 by 4.9 m or 2.4 by 4.9 m, and 30 or 45 cm deep) that collected pesticide runoff from a 465-m2 gravel bed containerized nursery in Baxter, TN. One-half of the cells were vegetated with bulrush, Scirpus validus. The cells were loaded at three rates or flows of 0.240, 0.120, and 0.060 m3 d(-1). Herbicides-simazine (Princep) [2-chloro-4,6-bis(ethylamino)-s-triazine] and metolachlor (Pennant) [2-chloro-N-(2-ethyl-6-methylphenyl)-N-2-methoxy-1-methylethyl-acetamide] -were applied to the gravel portion of the container nursery at rates of 4.78 and 239 kg ha(-1), respectively, 9 July 1998, and at rates of 2.39 and 1.19 kg ha(-1), respectively, 17 May 1999. Pesticides entering the wetland and wetland cell water samples were analyzed daily to determine pesticide removal. At the slower flow rate, which corresponds to lower mass loading and greater hydraulic retention times (HRTs), a greater percentage of pesticides was removed. During the 2-yr period, cells with plants removed 82.4% metolachlor and 77.1% simazine compared with cells without plants, which removed 63.2% metolachlor and 64.3% simazine. At the lowest flow rate and mass loading, wetland cells removed 90.2% metolachlor and 83% simazine. Gravel subsurface flow constructed wetlands removed most of the pesticides in runoff water with the greatest removal occurring at lower flow rates in vegetated cells.     

A proposed methodology for measuring incremental environmental benefits from using constructed wetlands to control agricultural non-point-source pollution

A binary response estimation procedure using contingent valuation data is proposed for estimating the incremental environmental benefits associated with the use of constructed wetlands to control agricultural waste-water runoff. The model uses two binary responses per individual while circumventing the non-independence problems associated with follow-up valuation questions. Case study results suggest that the methodology provides a potentially effective means for measuring the non-market benefits of constructed wetlands in addition to water-quality maintenance.1998 Academic Press     

Operation strategy of a sequencing batch reactor for simultaneous removal of wastewater organic matter and nutrients

The Sequencing Batch Reactor (SBR) system employing activated sludge process is an alternative wastewater treatment technology. A cycle of the conventional SBR system generally consists of five periods, with complete aeration during the React period to oxidize the organic matter and nitrify the ammonium-nitrogen of wastewater. Laboratory-scale reactors were used to evaluate the feasibility of incorporating alternative aerobic-anoxic-aerobic stages within the React period for simultaneous removal of organic matter, N and P. Two cycles of SBR process per day were maintained.Under the operation strategy of 0.75-h fill, 8-h react (with continuous aeration), 3.25-h settle, draw and idle periods, the treatment performance became consistent after running the system for two to four cycles (1–2 days). The percentages of both BOD₅ and COD removal were around 94% from Cycle 2 onwards, the BOD₅ content dropped from initial 251 mg L⁻¹ to less than 14 mg L⁻¹ in the final effluent. A steady nitrification (about 97%) was obtained from Cycle 4 onwards, with 1 mg NH₄⁺-N L⁻¹ and 25 mg NO₃⁻-N L⁻¹ present in the final effluent. This suggested that the time required for SBR system to acclimate and reach an equilibrium state was relatively short when compared with the time needed for continuous flow activated sludge system. The findings also show that 4-h aeration during the react period was long enough to achieve more than 90% nitrification. With the incorporation of a 3-h anoxic stage after the initial 4-h aeration of the react period, a satisfactory denitrification process was observed, with nitrate level dropped from 27 to around 8 mg L⁻¹ within 3 h. The second aeration stage did not cause significant change in wastewater nitrogen content. The wastewater phosphate content declined rapidly during the initial 4-h aeration and P-release was not observed during the anoxic stage. A slight reduction of P was found in the second aeration stage suggesting that more P-uptake occurred in this stage. A 12-h cyclic SBR system with the incorporation of 4-h aerobic, 3-h anoxic and final 1-h aerobic stages into the 8-h react period was demonstrated to be able to remove C, N and P simultaneously.     

Three approaches to define desired soil organic matter contents

Soil organic C is often suggested as an indicator of soil quality, but desirable targets are rarely specified. We tested three approaches to define maximum and lowest desirable soil C contents for four New Zealand soil orders. Approach 1 used the New Zealand National Soils Database (NSD). The maximum C content was defined as the median value of long-term pastures, and the lower quartile defined the lowest desirable soil C content. Approach 2 used the CENTURY model to predict maximum C contents of long-term pasture. Lowest desirable content was defined by the level that still allowed recovery to 80% of the maximum C content over 25 yr. Approach 3 used an expert panel to define desirable C contents based on production and environmental criteria. Median C contents (0-20 cm) for the Recent, Granular, Melanic, and Allophanic orders were 72, 88, 98, 132 Mg ha(-1), and similar to contents predicted by the CENTURY model (78, 93, 102, and 134 Mg ha(-1), respectively). Lower quartile values (54, 78, 73, and 103 Mg ha(-1), respectively) were similar to the lowest desirable C contents calculated by CENTURY (55, 54, 67, and 104 Mg ha(-1), respectively). Expert opinion was that C contents could be depleted below these values with tolerable effects on production but less so for the environment. The CENTURY model is our preferred approach for setting soil organic C targets, but the model needs calibrating for other soils and land uses. The statistical and expert opinion approaches are less defensible in setting lower limits for desirable C contents.     

Maximum rates of nitrate removal in a denitrification wall

Denitrification walls are constructed by mixing a carbon source such as sawdust into soils through which ground water passes. These systems can reduce nitrate inputs to receiving waters by enhancing denitrification. Maximum rates of nitrate removal by denitrification need to be determined for design purposes. To determine maximum rates of nitrate removal we added excess nitrate (50 mg N L(-1)) to a trench up-gradient of a denitrification wall during a 9-d trial. Bromide (100 g L(-1)) was also added as a conservative tracer. Movement of nitrate and bromide was measured from shallow wells and soil samples were removed for measurements of denitrification, carbon availability, nitrate, and other microbial parameters. Rates of nitrate removal, determined from the ratio of NO3-N to Br and ground water flow, averaged 1.4 g N m(-3) of wall d(-1) and were markedly greater than denitrification rates determined using the acetylene block technique (average: 0.11 g N m(-3) of wall d(-1)). These nitrate removal rates were generally lower than reported in other denitrification walls. Denitrification rates increased when nitrate was added to the laboratory incubations, indicating that despite large nitrate inputs in the field, denitrification remained limited by nitrate. This limitation was partially attributed to nitrate predominantly moving through zones of greater hydraulic conductivity or in the mobile fraction of the ground water and slow diffusion to the immobile fraction where denitrifiers were active.     

Selenium removal and mass balance in a constructed flow-through wetland system

A field study on the removal of Se from agricultural subsurface drainage was conducted from May 1997 to February 2001 in the Tulare Lake Drainage District (TLDD) of San Joaquin Valley, California. A flow-through wetland system was constructed consisting of ten 15- x 76-m unlined cells that were continuously flooded and planted with either a monotype or combination of plants, including sturdy bulrush [Schoenoplectus robustus (Pursh) M.T. Strong], baltic rush (Juncus balticus Willd.), smooth cordgrass (Spartina alterniflora Loisel.), rabbitsfoot grass [Polypogon monspeliensis (L.) Desf.], salt-grass lDistichlis spicata (L.) Greene], cattail (Typha latifolia L.), tule [Schoenoplectus acutus (Muhl. ex Bigelow) A. L?ve & D. L?ve], and widgeon grass (Ruppia maritima L.). One cell had no vegetation planted. The objectives of this research were to evaluate Se removal efficiency of each wetland cell and to carry out a mass balance on Se. The inflow drainage water to the cells had average annual Se concentrations of 19 to 22 microg L(-1) dominated by selenate [Se(VI), 95%]. Average weekly water residence time varied from about 3 to 15 d for Cells 1 through 7 (target 7 d), 19 to 33 d for Cells 8 and 9 (target 21 d), and 13 to 18 d for Cell 10 (target 14 d). Average weekly Se concentration ratios of outflow to inflow ranged from 0.45 to 0.79 and mass ratio (concentration x water volume) from 0.24 to 0.52 for year 2000, that is, 21 to 55% reduction in Se concentration and 48 to 76% Se removal in mass by the wetland, respectively. The nonvegetated cell showed the least Se removal both in concentration and in mass. The global mass balance showed that on the average about 59% of the total inflow Se was retained within the cells and Se outputs were outflow (35%), seepage (4%), and volatilization (2%). Independent measurements of the Se retained in the cells totaled 53% of the total Se inflow: 33% in the surface (0-20 cm) sediment, 18% in the organic detrital layer above the sediment, 2% in the fallen litter, < 1% in the standing plants, and < 1% in the surface water. Thus, about 6% of the total Se inflow was unaccounted for in the internal compartments.     

The influence of vegetation on sedimentation and resuspension of soil particles in small constructed wetlands

When initiatives to mitigate soil erosion are insufficient or fail, constructed surface flow wetlands (CWs) could be a final buffer to reduce pollution before reaching recipients. The objective of this study was to determine the influence of CW vegetation on the retention of soil particles from arable land. Retention was measured with water flow-proportional sampling systems in the inlet and outlet, sedimentation traps, and sedimentation plates in four small CWs over a period of 5 yr. The surface area of the CWs was 265 to 900 m2, and the average hydraulic loads were 1.2 to 3.4 m d(-1). Watershed areas were 0.5 to 1.5 km2. Annual soil particle retention was 30 to 80% or 14 to 121 kg m(-2). Results show that macrophytes stimulate sediment retention by mitigating resuspension of CW sediment. Five years after construction, resuspension had decreased approximately 40% and was negligible. As vegetation cover increases, the influence of macrophytes on soil particle retention reaches a level where other factors, such as hydraulic load and sediment load, were more important. Macrophytes increased the hydraulic efficiency by reducing short-circuit or preferential flow. However, vegetation did not have any influence on the clay concentration in the sediment. Hence, a possible stimulation of particle flocculation was not detected. Vegetation makes it possible to use the positive effect of a short particle settling distance in shallow ponds by hindering resuspension.     

Rapid removal of nitrate and sulfate in freshwater wetland sediments

Anaerobic microbial processes play particularly important roles in the biogeochemical functions of wetlands, affecting water quality, nutrient transport, and greenhouse gas fluxes. This study simultaneously examined nitrate and sulfate removal rates in sediments of five southwestern Michigan wetlands varying in their predominant water sources from ground water to precipitation. Rates were estimated using in situ push-pull experiments, in which 500 mL of anoxic local ground water containing ambient nitrate and sulfate and amended with bromide was injected into the near-surface sediments and subsequently withdrawn over time. All wetlands rapidly depleted nitrate added at ambient ground water concentrations within 5 to 20 h, with the rate dependent on concentration. Sulfate, which was variably present in porewaters, was also removed from injected ground water in all wetlands, but only after nitrate was depleted. The sulfate removal rate in ground water-fed wetlands was independent of concentration, in contrast to rates in precipitation-fed wetlands. Sulfate production was observed in some sites during the period of nitrate removal, suggesting that the added nitrate either stimulated sulfur oxidation, possibly by bacteria that can utilize nitrate as an oxidant, or inhibited sulfate reduction by stimulating denitrification. All wetland sediments examined were consistently capable of removing nitrate and sulfate at concentrations found in ground water and precipitation inputs, over short time and space scales. These results demonstrate how a remarkably small area of wetland sediment can strongly influence water quality, such as in the cases of narrow riparian zones or small isolated wetlands, which may be excluded from legal protection.