Surface Flow Wetland Research Articles

English

  The aim of this study was to evaluate the removal of faecal bacteria and nutrients from domestic wastewater, in surface flow wetlands vegetated with Echinochloa pyramidalis. Horizontal surface flow (HSF) wetlands were fed with primarily treated domestic wastewater at organic loading rates varying from 20.74 to 27.15 g BOD/m2/day for two consecutive years. Water samples were collected at two weeks interval from the inflow and the outflow of the treatment wetlands for physicochemical and faecal bacteria analyses. Physicochemical parameters with highest removal rates were total suspended solids (TSS, 62-80%), Nitrates (57-73 %) and PO4-3 (57-84 %). Faecal indicators reductions of 1.4 and 0.3 log units (1st year) and 1.15 and 0.65 log units (2nd year) were respectively recorded in the vegetated and the control wetlands. Reductions of total coliforms, faecal coliforms and faecal streptococci were significantly higher in the vegetated wetland than in the non-vegetated control in both seasons of the first year. In the contrary, there was no significant difference in the removal rates of faecal indicators between vegetated wetland and the non-vegetated control during the dry season of the second year.  Wetlands vegetated with E. pyramidalis was proven to be continuously efficient in the removal of faecal indicators for the two years of study, with no significant differences between the seasons of the year and between the years.   Key words: Wastewater, wetlands, Echinochloa pyramidalis, faecal bacteria removal, nutrients removal

English

In late 2008, a passive treatment system was completed to treat mine drainage in the Tar Creek watershed, located in the Tri State Lead-Zinc Mining District of the Central United States. Untreated mine drainage at this site was characterized by elevated levels of Fe, Zn, Pb, Cd, As and sulfate. Mine water quality improvement at this and many other passive treatment sites has been well-documented, but these systems may have additional water quality impacts on the surrounding environment that were not considered in the design phase and are not typically monitored. An example is the potential production of nuisance constituents (i.e., potentially harmful concentrations of non-target contaminants). This research focused on export of two potential nuisance constituents: sulfide and nutrients. Research objectives were to determine nuisance constituent inputs and outputs for each unit of the ten-cell passive treatment system. The passive treatment system was sampled for typical mine drainage and potential nuisance constituents from Fall 2010 through Summer 2011. Phosphorus and N were found in the mine water at levels in excess of U.S. Environmental Protection Agency recommended ambient criteria for lakes or streams for Nutrient Ecoregion IX. Phosphorus and N were removed in the iron oxidation ponds and surface flow wetlands. Sulfide and nutrients were exported from the vertical flow bioreactors. All nuisance constituents were exported from the system at least seasonally. Although re-aeration decreased sulfide levels, it was still exported from the system at levels that exceeded aquatic toxicity and nuisance odor levels. Export of P, sulfide, and possibly N is expected to decrease as the system ages and readily digestible compounds in the vertical flow bioreactor compost layer are consumed.

Comparison of the removal of chlorpyrifos and dissolved organic carbon in horizontal sub-surface and surface flow wetlands

Chlorpyrifos is an organophosphorus pesticide widely used in Colombia for agricultural and domestic pest control. It is also commonly found in water sources in rural areas. Constructed wetlands are being used as viable alternatives for the treatment of domestic wastewater with large organic loads. For this research, three pilot-scale subsurface horizontal flow constructed wetlands and three horizontal surface flow wetlands were used for the treatment of synthetic wastewater with different concentrations of chlorpyrifos (0.0μgL−1, 478μgL−1, 589μgL−1 and 788μgL−1), 100mgL−1 of dissolved organic carbon and nutrients. The wetlands were constructed in equal dimensions and in the same size range as the gravel bed (3.18–6.35mm) and planted with Phragmites australis. The efficiencies of the removal of the pesticide and dissolved organic carbon were then determined. Additionally, other physicochemical parameters, as well as 3,5,6-trichloro-2-pyridinol, the main breakdown product of chlorpyrifos, were measured. The average removals of this agrochemical and dissolved organic carbon in the subsurface horizontal flow constructed wetlands were 93% and 92%, respectively, and in the horizontal surface flow wetlands, the average removal was 95% for both compounds. The removal is the result of the joint action of microorganisms and the adsorption and absorption of roots and rhizomes of plants found in wetlands. Both types of wetlands are very efficient at treating the domestic wastewater contaminated with pesticides and dissolved organic carbon, although the results were slightly better in the surface flow wetlands.

Constructed Wetlands With Floating Emergent Macrophytes: An Innovative Stormwater Treatment Technology

The treatment of urban stormwater poses numerous technical and operational challenges, particularly due to the intermittent and highly variable nature of hydrologic and pollutant inputs. Floating emergent macrophyte treatment wetlands (FTWs) are a hybridization of ponds and wetlands that offer potential advantages for treatment of these highly variable flows. FTWs utilize rooted, emergent macrophytes growing on a mat or raft floating on the surface of the water rather than rooted in the sediments. Thus, they can tolerate the widely fluctuating water depths typical of stormwater systems, without the risk of the plants drowning. The roots hang beneath the floating mat and provide a large surface area for biofilm attachment. The authors provide a review of the FTW concept, structure, function, and treatment efficiency reported to date and discuss the potential advantages of this emerging technology for stormwater applications. Although still limited, the available data from mesocosm and pilot studies on removal of key pollutants such as organic matter, suspended solids, nutrients, and metals shows that they can significantly enhance performance of pond systems, and provide similar or better performance than surface flow wetlands for a range of polluted waters. Further studies are needed to verify the apparent potential of FTWs treating stormwater at full scale.

Study on Different Substrates in Stable Surface Flow Wetland

This paper discussed the feasibility and advantage of the removal efficiency of low concentration sewage using three Stable Surface Flow Wetlands (SSFW) with bamboo splint and palm silk as substrates. Under the same hydraulic loading, the result of experiment indicated that the removal rate of COD,TP, TN were 73.97%, 61.42%, 28.98% respectively in SSFW with a bamboo splint substrate, and the corresponding removal rate of SSFW with a palm silk substrate were 78.37%, 69.42%, 24.4%; while substrate without a bamboo splint and a palm silk were 66.61%, 58.71%, 22.23%. The above information demonstrates that the removal efficiency of TN with a bamboo splint is better while the removal efficiency of COD and TP with palm silk is better. Bamboo splint and palm silk have their own advantages when they are applied to SSFW as substrate, which provide theoretical basis for the selection and matching of substrate.

RETRACTED: Study on the Effect of Sewage Concentration on Treatment Efficiency of Artificial Wetland of Plateau Lake

Retracted article: Contrast experiments on the effect of concentration of the total phosphorus(TP) on treatment efficiency of artificial wetland are carried out through changing TP concentration of influent water into surface flow(SF) and subsurface flow(SSF) wetlands and growing six plants (purple-leaf canna, water onion, water hyacinth, oenanthe javanica, calamus, reed). Treatment efficiencies of SF and SSF wetlands with the six plants varying with TP concentration change are analyzed, and some indexes of purifying function of SF and SSF wetlands with six plants including theoretically optimum treatment concentrations and removal efficiencies, actual optimum treatment concentrations and removal efficiencies, and TP maximum daily treatment loads, are obtained. The experimental results show that, TP removal efficiencies of SF and SSF wetlands with the six plants decrease with the increases of TP concentrations, different plants have different levels of treatment efficiency reduction, and the reduction of removal efficiency of SSF wetland is smaller than that of SF wetland. Removal efficiency of water hyacinth is little changed with the increase of TP concentration, and treatment efficiencies of SF and SSF wetlands decrease by 1.61% and 1.12% respectively. Removal efficiency of Canna is obvious changed, and treatment efficiencies of SF and SSF wetlands decrease by 2.94% and 2.55% respectively. Purple leaf cama had the least changes in the decrease of removal rate, and the table stream processing rate decreased by 17.07%, and subsurface flow by 15.94%. The decrease in the removal rate of reed was clear and its table stream processing rate decreased by 20.86% and wubsurface flow by 18.2%. The actual optimum removal efficiency of SF wetland with water hyacinth is the largest of these SF wetlands’ and reaches 85.2%, and its corresponding maximum daily treatment load of TP is 81g /( d • m2). But the actual optimal removal efficiency of SF wetland with oenanthe javanica is the smallest of these SF wetlands’ and is 76.88%, and its corresponding maximum daily treatment load of TP is 51.84g/( d • m2). The actual optimum removal efficiency of SSF wetland with water hyacinth is also the largest of these SSF wetlands’ and is just 88.31%, and its corresponding maximum daily treatment load of TP is 102.6g/( d • m2). The actual optimum removal efficiency of SSF wetland with cama is the smallest of these SSF wetlands’ and is only 78.32%, and its corresponding maximum daily treatment load is 55.2g/( d • m2).

Removal of Various Contaminations from Drainage by Using Free Water Surface and Submerged Surface Flow Wetland Systems

This study demonstrates an efficient approach to remove various liquid-phase pollutants from wastewater by using constructed wetland (CW) systems. The plant used in this study was phragmites communis. Two types of CW systems, free water surface flow and submerged surface flow wetlands, are used to compare with their efficiencies for removing chemical oxygen demand (COD), Zn2+, true color, and NH3-N from the drainage. Experimental results confirmed that the CW treatment displays excellent capability for removing the pollutants, i.e., high removal efficiencies. This finding indicates that the growth of phragmites communis enables well-developed root network in CW system, thus leading to a higher adsorption capacity. The growth period of the root network in our case takes about 40 days, forming the bio-membrane. On the basis of the present work, the presence of bio-membrane on the plant root not only enhances but also stabilizes the efficiencies for removing various contaminations from the wastewater.

Effect of Hydraulic Loading on Removal Efficiency of the Folds Stable Surface Flow Wetland

Three folds stable surface flow wetlands (FSSFWs) were operated at different hydraulic loadings of 0.80, 0.53 and 0.27m3/ (m2•d) to treat the decentralized domestic sewage. And we investigated the effect of the hydraulic loading rate (HLR) on the removal efficiency of FSSFWs by measuring five indicators of chemical oxygen demand (COD), total nitrogen (TN), ammonium (NH3-N), total phosphorus (TP) and suspended substance (SS). The research results indicated that the hydraulic loading had great impact on the removal efficiency of FSSFWs, and was looked upon as one crucial factor for FSSFWs design. The removal efficiency of FSSFWs presented a downward trend in various degrees along with the increase of hydraulic loading. Also the hydraulic loading had greater influences on the removal rate of NH3-N and TN than COD, TP and SS. Considering the operation costs, the treatment capacity and the purification effect in this experiment, we could ensure the efficient and stable operation of FSSFWs with HLR at 0.53 m3/ (m2•d) and HRT at 0.4 m3•d-1, which were the most appropriate options.

Nutrient removal and plant biomass of 5 wetland plant species in Singapore

Five native plant species (Typha angustifolia, Scirpus mucronatus, Lepironia articulata, Eleocharis dulcis and Phragmites karka) were investigated for their nutrient removal efficiencies in shallow pond systems in Sg Buloh Wetland Reserve with water depth 0.12-0.30 m of a total 160 m2 in area size. The project aimed to investigate nutrient removal rates, removal efficiencies, nutrient storage in plant biomass, and plant growth. Pond water quality before and after planting in fully vegetated ponds improved significantly at 24.4% TP and 64.4% TN reduction in Pond 6. Scirpus mucronatus, Typha angustifolia, Lepironia articulata and Eleocharis dulcis showed high growth rates. Phragmites karka showed low growth rate due to pest attack by aphids. Nutrient concentration in stem/leaf samples of the 5 species were within the ranges of 1% to 3% of dry weight for nitrogen and 0.1% to 0.3% of dry weight for phosphorus. Average daily mass removal rates ranged from 3.5-9.6 kg P ha−1 day−1 and 13.5-33.5 kg N ha−1 day−1. This field trial showed that the 4 species Lepironia articulata, Eleocharis dulcis, Typha angustifolia and Scirpus mucronatus are suitable species in surface flow wetland or shallow pond systems for nutrient removal in tropical environments.

Nitrous oxide emissions from surface flow and subsurface flow constructed wetland microcosms: Effect of feeding strategies

The effects of continuous and intermittent feeding strategies on nitrogen removal and N 2O emission from surface flow and subsurface flow constructed wetlands were evaluated in this study. Microcosm wetlands planted with Phragmites australis were constructed and operated with different feeding strategies for the 4-month experiment. Results showed the intermittent feeding strategy could enhance the removal of ammonium effectively in the subsurface flow constructed wetlands, although it had no significant effect for the surface flow wetlands. And the intermittent feeding mode could promote the emission of N 2O. The amount of N 2O–N emission from the subsurface flow constructed wetlands with intermittent feeding mode was about 5 times higher than that with continuous feeding strategy and the emission rate ranged from 0.09 ± 0.03 to 7.33 ± 1.49 mg/m 2/h. Compared with the surface flow constructed wetlands, the N 2O emission in the subsurface flow constructed wetlands was affected significantly by the intermittent feeding mode.

Evaluation of PPCPs removal in a combined anaerobic digester-constructed wetland pilot plant treating urban wastewater

The removal efficiency of 16 pharmaceuticals and personal care products (PPCPs) from urban wastewater (dissolved and particulate phases) was evaluated for the first time in a hybrid pilot plant consisting of an upflow anaerobic sludge blanket reactor followed by two sequentially connected horizontal flow constructed wetlands: a surface flow wetland (SF CW) and a subsurface flow wetland (SSF CW). Whereas the PPCP removal associated with the dissolved phase exhibited a seasonal pattern, the fraction associated with the suspended solids showed less seasonality. In the dissolved phase, the overall removal efficiency in summer ranged from 70% to 85% for salicylic acid (SAL), methyl dihydrojasmonate, caffeine (CAF), ketoprofen and triclosan, whereas in winter it declined for most of the PPCPs to between 30% and 50%, except for CAF and SAL (>80%) and carbamazepine and butylated hydroxyl toluene (11–18%). In the suspended solids, the removal exceeded 80% for most of the target PPCPs. The efficiency of the different treatment steps was also compound-dependent, but the SF CW generally exhibited the highest removal efficiency for most of the contaminants analyzed. The characterization of the organic matter retained in the wetland gravel beds revealed the occurrence of hydrophobic contaminants such as phthalate esters and fragrances at moderate concentrations (i.e., up to 3.5 μg kg −1), which declined strongly over the course of the different treatment steps. In the SF CW, the net mass accumulation rates of tonalide and galaxolide were 4 and 23 g y −1 respectively, whereas in the SSF CW they were 0.3 and 1.8 g y −1 respectively.

Multi-tracer experiments to characterise contaminant mitigation capacities for different types of artificial wetlands

Salt tracers (sodium bromide/sodium chloride) and two different fluorescent tracers, uranine (UR) and sulforhodamine-B (SRB), were injected as a pulse into six different surface flow wetlands (SFWs). Salt tracers documented wetland hydraulics. The fluorescent tracers were used as a reference to mimic photolytic decay (UR) and sorption (SRB) of contaminants as illustrated by a comparison to a real herbicide (Isoproturon), which was used as a model for mobile pesticides. Tracer breakthrough curves were used to document residence time distributions, hydraulic efficiencies, peak attenuation and retention capacities of completely different wetland systems. A 530 m2 forest buffer zone showed considerable peak attenuation but limited retention capabilities despite its large area. Approximately 80% of SRB was permanently retained in a re-structured 325 m2 flood detention pond. These two non-steady SFWs indicated long-term tracer washout. The remaining four SFWs displayed constant outflow rates and steady-state flow conditions. Due to photolytic decay in a 330 m2 row of three wetlands, UR was almost entirely degraded, but the SRB breakthrough suggested relatively low sorption. A 65 m2 shallow flow-through wetland yielded negligible photolytic decay but showed considerable sorption losses. Finally two types of vegetated ditches were analysed. In one case, vegetation was removed from a 413 m long ditch immediately prior to tracer injection. A 30% loss by sorption to sediment and plant remnants occurred at the very beginning of the tracer breakthrough. Inside a second ditch, 80 m long and densely vegetated by Phragmites australis, sorption was even higher and yielded eightfold higher specific SRB retention rates. Although the present findings are only valid for low flow conditions, they indicate that a shallow water depth seems to be a key variable which may increase sorption of tracers and therefore contaminants. Large wetlands with deep water bodies may attenuate concentrations efficiently, but unit load reduction was found to be more significant in shallow systems even at much higher flow velocities.

Modeling E.coli fate and transport in treatment wetlands using the water quality analysis and simulation program

Treatment wetlands can be a viable wastewater treatment option, especially in rural and remote regions where centralized wastewater treatment is not feasible. Bacteria fate and transport modeling within wetlands requires further development if they are to become a more reliable and predictable form of wastewater treatment. The goal of this paper was to calibrate and test an unsteady state numerical model for the simulation of E. coli fate and transport within full-scale surface flow (SF) wetlands treating domestic wastewater. The Water Quality Analysis and Simulation Program (WASP) was used to develop the model. Accurately predicting E. coli effluent concentrations using WASP was difficult due to the dynamic nature of the wetland environment including hydraulics, seasonal variability, and wetland maturity. WASP was successful in predicting average E. coli effluent concentrations but did not accurately forecast maximum and minimum values. The model produced better fits with observed E. coli effluent concentrations during the summer months, when observed effluent concentrations were less variable. Hydraulic tracer studies and model results suggest that preferential flow pathways may be affecting E. coli removal due to reduced retention times. Flow channelling or short circuiting may have been caused by high flow conditions and/or dense cattail growth. A more detailed understanding of treatment wetland hydraulics is required before we can accurately predict treatment performance.

Building highway landscapes: innovative directions for urban wastewater treatment in the face of new challenges in China

Urban development in China has caused many problems of urban water management. Today we have to face new challenges about decreasing water resources, wastewater treatment, limited spaces and ecological preservation. Solving these problems should be in sustainable ways with innovative approaches. This paper proposed a highway landscape strategy that contains four major green directions for urban water management. The strategy combined several disciplines of ecology, civil engineering, landscape design and agricultural irrigation. This conceptual idea is demonstrated through a design of a complex constructed wetland system for Huai’an city wastewater plant in Jiangsu province, which sits on the East-route Water Transfer Project of China. The designed project in this paper consists of retention facilities, ecotype corridors, surface flow wetlands and stabilization ponds. Especially, it is designed beside a highway around the city. Normally, the whole system could treat effluents from wastewater plants and irrigation fields. In rainy seasons, it could store and treat extra urban sewer discharges and road surface runoff. During farming season, it is also designed that the outflow of the system would be reused to irrigate. Moreover, this proposal illustrated that the landscape design of the system on a highway side could achieve multiple benefits for the highway surrounding environment. It would provide as much education, recreation, and habitat creation as possible to local community.

Constructed Wetland as an Alternative Solution to Maintain Urban Landscape Lake Water Quality: Trial of Xing-Qing Lake in Xi’an City

Abstract This paper describes a pilot study aimed to use constructed wetland (CW) technology for urban lake water treatment to maintain water quality. A SFS (Surface Flow Wetland) and/or a FWS (Subsurface Flow Wetland) CW was continuously operated in autumn and winter. The results were then compared and the factors to affect the results were examined. The results showed that, regarding the overall level of the system stability and the pollutants removal efficiency, SFS constructed wetland is better than the FWS wetland. The average removal efficiency of SFS wetland is 84.2% for COD, 53.8% for NH3-N, 47.9% for TN, 73.3% for TP and 86.6% for SS. It is noted that the NH3-N, TN removal rate is not high in single-stage wetlands. Therefore, some novel actions and other ecological processing technologies such as multi-stage wetland system, effluent recirculation etc should be introduced.

Long-Term Performance of a Constructed Wetland for Metal Removal

Constructed wetlands have the ability to economically remove pollutants from water and retain them in sediment. This paper describes the long-term performance of a constructed wetland for metal removal, including the efficiency of metal removal, and the retention of metals in the wetland sediment. It is based on four years of data collected from the A-01 wetland treatment system, a surface flow wetland planted with Schoenoplectus californicus (giant bulrush). The system is designed to remove Cu and other metals from the A-01 National Pollution Discharge Elimination System (NPDES) outfall effluent at the Savannah River Site near Aiken, SC. Copper, Zn, and Pb concentrations in water were usually reduced 60 to 80% during passage through the A-01 treatment system. Most of the metal removed by the wetland cells was accumulated in the two top layers of the substrate; i.e., the floc and organic layers. This gradient was strongly correlated with percent organic matter, pH, and the concentration of all metals. These results showed that most metals in the A-01 wetland sediments behaved similarly: their concentrations decreased as sediment depth increased.

Wetland treatment of leachate from a closed landfill

A wetland system has operated seasonally at Saginaw Township, MI, USA, for ten years. The system consists of extraction, aeration, settling, intermittent vertical sand filtration, a surface flow wetland treatment with recycle, and discharge to the Tittibawassee River. The 0.85 ha cattail wetland treats the full leachate flow, with a total system detention time of 180 days. The high recycle rate creates a lesser wetland detention time of 60 days. Ammonia is the principal contaminant of concern, because it occurs at high concentrations, typically 300–500 mg/L. Ammonia mass reduction averaged 99.5% for the last nine years, with a 95% mass removal in the startup year. Metals were not present in all samples, with modest reductions in those always present (zinc 16%, arsenic 29%, barium 78%, chromium 67%). Volatile organic compounds were removed to below detection, excepting BTEX, which occurred in only 2% of the outflow samples. Base neutral organics, PCBs and pesticides were also removed to below detection, excepting phthalates with an outlet detection frequency of 29%. No pesticides or PCBs were detected in the system outflow. The ammonia removal rate coefficients for the wetland (12 m/yr) was at the 55th percentile of the distribution for other surface flow wetlands. The vertical filter was likely oxygen limited, and functioned with an apparent oxygen utilization of 30 gO/(m 2 d).

Algal Growth Control Within Natural Water Purification Systems: Macrophyte Light Shading Effects

Natural water purification systems including oxidation ponds and surface flow wetlands are commonly used to treat wastewater in tropic regions, however, the water quality of the effluent fluctuated and often failed to comply with regulatory water criteria due to algae bloom. Separation of algae is inevitable to produce high effluent quality and to comply with local effluent standards. Algal growth control measures including emergent and floating macrophytes as well as back cloth physical light shading and algaecide application were studied and were aimed to reuse the effluent for water resource conservation and groundwater recharge. The results demonstrated that emergent and free-floating macrophytes light shading on algal growth control were viable in both batch experiments and field-scale hybrid natural water purification systems such as surface flow wetlands. Water quality parameters including COD, turbidity, SS were positively correlated with the algal concentrations and could be used as indictors of the trophic status of aquatic systems. Emergent macrophytes assisted control algal growth by preventing sunlight from reaching the water column in surface flow wetlands. Results of this study can be used by similar hybrid natural water purification systems to control algal activity and to prevent effluent deterioration.

Greenhouse Gas Emissions from Surface Flow and Subsurface Flow Constructed Wetlands Treating Dairy Wastewater

Agricultural wastewater treatment is important for protecting water quality in rural ecosystems, and constructed wetlands are an effective treatment option. During treatment, however, some C and N are converted to CH(4), N(2)O, respectively, which are potent greenhouse gases (GHGs). The objective of this study was to assess CH(4), N(2)O, and CO(2) emissions from surface flow (SF) and subsurface flow (SSF) constructed wetlands. Six constructed wetlands (three SF and three SSF; 6.6 m(2) each) were loaded with dairy wastewater in Truro, Nova Scotia, Canada. From August 2005 through September 2006, GHG fluxes were measured continuously using transparent steady-state chambers that encompassed the entire wetlands. Flux densities of all gases were significantly (p < 0.01) different between SF and SSF wetlands changed significantly with time. Overall, SF wetlands had significantly (p < 0.01) higher emissions of CH(4) N(2)O than SSF wetlands and therefore had 180% higher total GHG emissions. The ratio of N(2)O to CH(4) emissions (CO(2)-equivalent) was nearly 1:1 in both wetland types. Emissions of CH(4)-C as a percentage of C removal varied seasonally from 0.2 to 27% were 2 to 3x higher in SF than SSF wetlands. The ratio of N(2)O-N emitted to N removed was between 0.1 and 1.6%, and the difference between wetland types was inconsistent. Thus, N(2)O emissions had a similar contribution to N removal in both wetland types, but SSF wetlands emitted less CH(4) while removing more C from the wastewater than SF wetlands.

Attenuation and diel cycling of coal-mine drainage constituents in a passive treatment wetland: A case study from Lambert Run, West Virginia, USA

Abstract This study reports changes in coal-mine drainage constituent concentrations through a passive treatment wetland and over diel cycles. The purpose of the study was to determine what physiochemical mechanisms control attenuation of metals and if they varied by location and through time. The source water was slightly acidic (average pH 5.43); downstream degassing of CO2 contributed to an increase in pH prior to discharge from the site (average pH 7.05). Aluminum, Fe, rare earth elements (REE) and Y were removed to a greater extent than were Mn, Co and Ni. At acidic pH, the REE and Y were generally complexed by SO4. At higher pH, carbonate complexes became more important. The REE and Y concentrations were normalized to the North American Shale Composite standard; the normalized patterns were coherent near the source but anomalies of Ce and Y were present further downstream indicating oxidation and sorption processes. Four sets of diel-based samples were collected, one from a shallow, surface-flow wetland and three from a deeper, newly constructed wetland. Well-defined diel cycles were observed for concentrations of Si, Mn, Fe, Co, Ni, As, REE and Y in the shallow wetland. In all cases, the concentrations increased (up to 863%) during night and decreased during the day. These cycles had an inverse relationship with the temperature cycle (pH had no discernable cycle). The consistency of concentration cycles suggests a common mechanism, most likely associated with the formation of Fe oxyhydroxides. The increased rate of Fe2+ oxidation in warm water can account for the cycles in Fe; scavenging of the other elements by the Fe precipitate can account for the consistency of the cycles even though the elements include cations, anions ( H 2 AsO 4 - or HAsO 4 - 2 ), and neutral species ( H 4 SiO 4 0 ). While REE and Y had clear cycles in the constructed wetland, the other elements did not. This is partially due to the lower elemental concentrations (including Fe) but the cycles may also be damped by the deeper, slower-moving water. This study illustrates the dynamic nature of metal removal in passive treatment systems. Furthermore, it suggests that grab samples collected during daytime hours may underestimate the concentrations and flux of metals in these systems.