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.     

Evaluation of the macroalga,muskgrass, for the phytoremediation of selenium-contaminated agricultural drainage water by microcosms

Previous field studies suggested that the macroalga, muskgrass (Chara canescens Desv. & Lois), plays an important role in the removal of selenium (Se) from agricultural drainage water. This study evaluated the efficiency of Se removal from drainage water by muskgrass-vegetated wetland microcosms, and determined the extent to which muskgrass removed Se through phytoextraction and biovolatilization. Six flow-through wetland microcosms were continuously supplied with drainage water containing an average Se concentration of 22 microg L(-1) over a 24-d experimental period. The Se mass input and outflow and the rate of Se volatilization were monitored daily for each microcosm. Three microcosms containing muskgrass reduced the daily mass Se input in the inflow drainage water by 72.1%; this compared with a reduction of 50.6% of the mass Se input for three unvegetated control microcosms. Selenium accumulated in muskgrass tissues accounted for 1.9% of the total mass Se input in the microcosm, followed by 0.5% via biological volatilization. The low rates of Se volatilization from selenate-supplied muskgrass, which were 10-fold less than from selenite, were probably due to a major rate limitation in the reduction of selenate to organic forms of Se in muskgrass. This conclusion was derived from X-ray absorption spectroscopy speciation analysis, which showed that muskgrass treated with selenite contained 91% of the total Se in organic forms (selenoethers and diselenides), compared with 47% in muskgrass treated with selenate.     

Removal of selenate in simulated agricultural drainage water by a rice straw bioreactor channel system

Removal of selenium (Se) from agricultural drainage water is important in protecting wetland wildlife. Three flow-through bioreactor channel systems (BCSs), each with three channels filled with rice (Oryza sativa L.) straw, were set in the laboratory to determine removal of selenate [Se(VI)] (1020 microg L(-1)) from drainage water with a salinity of 10.4 dS m(-1), a pH of 8.1, and a nitrate (NO3-) range of 0 to 100 mg L(-1). Results showed that the rice straw effectively reduced Se(VI) during 122 to 165 d of the experiments. Calculation of Se mass in the three BCSs showed that 89.5 to 91.9% of the input Se(VI) was reduced to red elemental Se [Se(0)], where 96.6 to 98.2% was trapped in the BCSs. Losses of each gram of rice straw were almost equal to the removal of 1.66 mg of Se from the drainage water as a form of red Se(0), indicating that rice straw is a very effective organic source for removing Se(VI) from drainage water.     

Nitrate Removal in a Restored Spring‐Fed Wetland,Pennsylvania, USA1

ABSTRACT:  In 2001, the 1.04‐ha Hornbaker wetland in south‐central Pennsylvania was restored by blocking an artificial drainage ditch to increase water storage and hydraulic retention time (HRT). A primary goal was to diminish downstream delivery of nitrate that enters the wetland from a limestone spring, its main source of inflow. Wetland inflow and outflow were monitored weekly for two years to assess nitrate flux, water temperature, pH, and specific conductivity. In Year 2, spring discharge was measured weekly to allow calculation of nitrate loads and hydraulic retention time. Surface runoff was confirmed to be a small fraction of wetland inflows via rainfall‐runoff modeling with TR‐55. The full dataset (n = 102) was screened to remove 13 weeks in which spring discharge constituted < 85% of total inflows because of high precipitation and surface runoff. Over two years (n = 89), mean nitrate‐nitrogen concentrations were 7.89 mg/l in inflow and 3.68 mg/l in outflow, with a mean nitrate removal of 4.19 mg/l. During Year 2 (n = 47), for which nitrate load data were available, the wetland removed an average of 2.32 kg N/day, 65% of the load. Nitrate removal was significantly correlated with HRT, water temperature, and the concentration of nitrate in inflow and was significantly greater during the growing season (5.36 mg/l, 64%) than during the non‐growing season (3.23 mg/l, 43%). This study indicates that hydrologic restoration of formerly drained wetlands can provide substantial water quality benefits and that the hydrologic characteristics of spring‐fed wetlands, in particular, support effective nitrogen removal.     

Microcosm wetlands for wastewater treatment with different hydraulic loading rates and macrophytes

Constructed wetlands (CW) usually require large land areas for treating wastewater. This study evaluated the feasibility of applying CW with less land requirement by operating a group of microcosm wetlands at a hydraulic retention time (HRT) of less than 4 d in southern Taiwan. An artificial wastewater, simulating municipal wastewater containing 200 mg L(-1) of chemical oxygen demand (COD), 20 mg L(-1) of NH4+-N (AN), and 20 mg L(-1) of PO4(3-)-P (OP), was the inflow source. Three emergent plants [reed, Phragmites australis (Cav.) Trin. ex Steud.; water primrose, Ludwigia octovalvis (Jacq.) P.H. Raven; and dayflower, Commelina communis L.] and two floating plants [water spinach, Ipomoea aquatica Forssk.; and water lettuce, Pistia stratiotes L.] plants were tested. The planted systems showed more nutrient removal than unplanted systems; however, the type of macrophytes in CW did not make a major difference in treatment. At the HRTs of 2 to 4 d, the planted system maintained greater than 72,80, and 46% removal for COD, AN, and OP, respectively. For AN and OP removal, the highest efficiencies occurred at the HRT of 3 d, whereas maximum removal rates for AN and OP occurred at the HRT of 2 d. Both removal rates and efficiencies were reduced drastically at the HRT of 1 d. Removals of COD, OP, and AN followed first-order reactions within the HRTs of 1 to 4 d. The efficient removals of these constituents obtained with HRT between 2 and 4 d indicated the possibility of using a CW system for wastewater treatment with less land requirement.     

Characterization of selenate removal from drainage water using rice straw

Removal of selenium (Se) from agricultural drainage water is very important for protecting wildlife in wetland systems. We conducted a series of experiments on selenite [Se(IV)] adsorption and selenate [Se(VI)] reduction to determine Se removal from drainage water amended with 1000 microg/L of Se(VI) or Se(IV) and 5 g of rice (Oryza sativa L.) straw. Under sterile conditions, the added Se(IV) was not adsorbed to the rice straw within 2 d of the experiment and the added Se(VI) was not reduced within 14 d. In contrast, added Se(VI) in a nonsterile rice-straw solution was reduced rapidly, from 930 microg/L at Day 3 to 20 microg/L at Day 5, with an increase in unprecipitated elemental Se [Se(0)] and total Se(0). In the last several days of the experiments, unprecipitated Se(0) was the major Se form in the rice-straw solution, with a small amount of organic Se(-II). This study showed that Se removal from drainage water in the presence of rice straw involves a two-step process. The first is the microbial reduction of Se(VI) to Se(IV) and then to colloidal Se(0). The second is flocculation and precipitation of colloidal Se(0) to the bottom of the experimental flasks and the surface of rice straw.     

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.     

Pesticide applications of copper on perennial crops in California, 1993 to 1998

Inorganic copper is used as a broad-spectrum fungicide and bacteriocide on a variety of agricultural crops. After application, the copper residue typically accumulates in the upper 15 cm of soil. Data from the California Pesticide Use Reports were used to estimate the augmentation of copper in the soil that resulted from pesticide applications for the six years from 1993 to 1998 on 12 crops that are grown without rotation. The estimated mean mg Cu kg(-1) soil added to the upper 15 cm during the six years was the following: walnut (Juglans regia L.), 28; peach [Prunus persica (L.) Batsch var. persica], 22; nectarine [Prunuspersica (L.) Batsch var. nucipersica (Suckow) C.K. Schneid], 19; cherry (Pseudolmedia oxyphyllaria Donn. Sm.), 18; rice (Orvza sativa L.), 16; apricot (Prunus armeniaca L.), 11; orange [Citrus sinensis (L.) Osbeck) and plum (Prunus domestica L. subsp. domestica ), 9; lemon [Citrus limon (L.) Burm.f.] and almond [Prunus dulcis (Mill.) D.A. Webb], 6; pear (Pyrus communis L.), 4; and grape (Vitis vinifera L.), 3. In addition, for the first five of these crops, we estimated the area that was treated with each level of kg Cu ha(-1). For example, for walnut orchards, we estimated that 12 500 ha, or 17% of the planted area, was treated with a quantity of Cu that would increase the total concentration of Cu in the upper 15 cm of soil by at least 50 mg Cu kg(-1) soil. A comparison of the amount of Cu per unit planted area that was applied in the first and second half of the study indicated that the intensity of copper use is either relatively constant or increasing, depending on the crop. The findings are discussed in relation to the potential effect of continued long-term use of Cu pesticides on soil sustainability.     

Bioavailability of organic phosphorus in a submerged aquatic vegetation-dominated treatment wetland

Enzymatic hydrolysis and mineralization of organic phosphorus (P) were determined in surface water samples collected from inflow and outflow of a submerged aquatic vegetation (SAV)-dominated treatment wetland of the Florida Everglades. Water samples were fractionated into three size fractions (> 0.4 micron, < 0.4 to > 0.05 micron, and < 0.05 micron) with a sequential flow filtration technique. The fractionated water samples were incubated to hydrolyze with alkaline phosphatase (APase) and phosphodiesterase (PDEase), and to mineralize at different redox and pH. Unlike APase, which hydrolyzed < or = 10% of organic P, PDEase hydrolyzed > or = 71% of organic P in unfiltered water from both inflow and outflow waters, suggesting the domination of bioavailable diester P in the water. Phosphodiesterase completely hydrolyzed organic P in the < 0.4- to > 0.05-micron and < 0.05-micron fractions, as compared with < or = 35% in the > 0.4-micron fraction. However, the P mineralization in inflow and outflow waters at different redox and pH showed that P associated with particulate > 0.4 micron had been mineralized the most. Phosphorus-31 nuclear magnetic resonance (NMR) spectroscopy showed that surficial sediments from the inflow region contained a high proportion of polynucleotides, nucleoside monophosphates, and previously unreported glycerophosphoethanolamine and phosphoenolpyruvates. However, at the outflow, the relative proportion of polynucleotides and nucleoside monophosphates was reduced substantially. This suggests that the SAV wetland may sequester P via accretion of organic matter.     

Nutrient and sediment removal by a restored wetland receiving agricultural runoff

Few studies have measured removal of pollutants by restored wetlands that receive highly variable inflows. We used automated flow-proportional sampling to monitor the removal of nutrients and suspended solids by a 1.3-ha restored wetland receiving unregulated inflows from a 14-ha agricultural watershed in Maryland, USA. Water entered the wetland mainly in brief pulses of runoff, which sometimes exceeded the 2500-m3 water holding capacity of the wetland. Half of the total water inflow occurred in only 24 days scattered throughout the two-year study. Measured annual water gains were within 5% of balancing water losses. Annual removal of nutrients differed greatly between the two years of the study. The most removal occurred in the first year, which included a three-month period of decreasing water level in the wetland. In that year, the wetland removed 59% of the total P, 38% of the total N, and 41% of the total organic C it received. However, in the second year, which lacked a drying period, there was no significant (p > 0.05) net removal of total N or P, although 30% of the total organic C input was removed. For the entire two-year period, the wetland removed 25% of the ammonium, 52% of the nitrate, and 34% of the organic C it received, but there was no significant net removal of total suspended solids (TSS) or other forms of N and P. Although the variability of inflow may have decreased the capacity of the wetland to remove materials, the wetland still reduced nonpoint-source pollution.     

Escherichia coli load reduction from runoff by vegetative filter strips: a laboratory-scale study

Vegetative filter strips (VFS) are commonly used best management practices for removing contaminants from runoff. Additional research is warranted to determine their efficiency and the most appropriate metrics for predicting fecal bacteria reductions. The objective of this research was to determine VFS effectiveness in removing from runoff relative to inflow rate, infiltration capacity, and flow concentration. This research also investigated the presence of in runoff from clean water runon after diluted manure runon events. A laboratory-scale VFS soil box (200 cm long, 100 cm wide, 7.5% slope) was packed with a sandy loam soil. Ten constant-flow VFS experiments were conducted with and without vegetation (8-10 cm ryegrass [ L.]) at low (20-40 cm s), medium (40-60 cm s), and high (85-120 cm s) flow rates and for a full (100 cm) or concentrated (40 cm) VFS flow width to simulate a channelizing flow condition. Two runon events were investigated for each experimental condition: (i) diluted liquid swine manure runon and (ii) clean water runon 48 h afterward. was used as an indicator of fecal contamination and was quantified by the most probable number (MPN) technique. No concentration reductions were observed based on peak outflow concentrations, and only small concentration reductions were observed based on outflow event mean concentrations. The mass reductions ranged from 22 to 71% and were strongly correlated to infiltration or runoff reduction ( = 0.88), which was dependent on the degree of flow concentration. Little to no effect of sedimentation on transport was observed, hypothesized to be due to minimum attachment to sediment particles because the bacteria originated from manure sources. Therefore, the design of VFS for bacteria removal should be based on the infiltration capacity in the VFS and should prevent concentrated flow, which limits total infiltration. The event mean concentrations in clean water runon experiments were between 10 and 100 MPN per 100 mL; therefore, under these conditions, VFS served as a source of residual from previous runon events.     

The potential of rhizosphere microbes isolated from a constructed wetland to biomethylate selenium

The potential of rhizosphere microbes isolated from common reed [Phragmites australis (Cav.) Trin. ex Steud] plants grown in a subsurface-flow constructed wetland to biomethylate selenate or selenite was studied in liquid cultures under controlled conditions. Total mean percentages of volatilized Se from half-strength Hoagland culture solutions (low C content) supplemented with selenate or selenite and inoculated with cultured rhizosphere microbes after 15 d of incubation were 7.9 and 49.1%, respectively. There was a relative best fit (r = 0.87) between total number of rhizosphere and cultured microbes and the percentage of volatilized Se in Hoagland solution after 15 d of incubation. However, when the same microbes were cultured in tryptic soybean broth (TSB) medium (high C content), the percentages of volatilized Se from selenate and selenite were 1.3 and 1.9%, respectively. The volatilization percentages of Se from selenate or selenite in culture solutions inoculated with rhizosphere suspension instead of cultured rhizosphere microbes were very low (1.2-3.0%) in both cultivation media. In all experiments, selenite was volatilized significantly (p < 0.05) in higher amounts by cultured rhizosphere microbes after 15 d of incubation compared with selenate. Dissolved biomethylated dimethylselenide (DMSe) in water samples taken from the subsurface-flow bed was determined by purging with helium. The DMSe in water samples was indirectly detected up to 2.4 microg Se L(-1), which indicates that part of the produced DMSe was dissolved in the matrix before being released into the atmosphere. Our results show that rhizosphere microbes isolated from common reed plants have a high potential of Se biomethylation and volatilization from selenate and selenite.     

Soil sulfur amendments suppress selenium uptake by alfalfa and Western wheatgrass

Selenium (Se) is a potential soil contaminant in many parts of the world where it can pose a health risk to livestock and wildlife. Phosphate ore mining in Southeast Idaho has resulted in numerous waste rock dumps revegetated with forages to stabilize the dumps and support grazing. Alfalfa (Medicago sativa L.), smooth brome (Bromus inermis Leyss.), and western wheat grass [Pascopyrum smithii (Rydb.) A. L?ve] are the dominant forage species on these lands. To demonstrate the feasibility of using sulfur (S) as a soil amendment to restrict plant Se uptake, 3 kg pots containing 50:50 w/w soil and waste shale were uniformly mixed with 0, 0.5, 1.0, or 2.0 Mg ha(-1) S as either elemental S or gypsum. Pots were seeded with alfalfa or western wheat grass. Dry mass and tissue Se were monitored over several clippings. Soils were sampled at the conclusion of the study and analyzed for water-soluble, oxalate-extractable, and total Se. Sulfur amendments as either elemental S or gypsum at 1.0 Mg ha(-1) or greater equally suppressed Se uptake over 60% in both forage species. Alfalfa accumulated more Se than western wheat grass. Plant removal via successive clippings resulted in lower tissue Se accumulation over time than the use of S soil amendments alone. Alfalfa-planted soils contained lower water-soluble and oxalate-extractable Se than did the non-planted controls while western wheat grass-planted soils contained lower water-soluble Se. Applying S to these shale-based soils may be an economically viable option for treating Se-impacted, revegetated lands.     

Metal distribution and stability in constructed wetland sediment

The A-01 wetland treatment system (WTS) is a surface flow wetland planted with giant bulrush [Schoenoplectus californicus (C.A. Mey.) Palla] that is designed to remove Cu and other metals from the A-01 National Pollution Discharge Elimination System (NPDES) effluent at the Savannah River Site near Aiken, SC. Copper, Zn, and Pb concentrations in water were usually reduced 60 to 80% by passage through the treatment system. The Cu concentrations in the wetland sediments increased from about 4 to 205 and 796 mg kg(-1), respectively, in the organic and floc sediment layers in cell 4A over a 5-yr period. Metal concentrations were higher in the two top layers of sediment (i.e., the floc and organic layers) than in the deeper inorganic layers. Sequential extraction was used to evaluate remobilization and retention of Cu, Pb, Zn, Mn, and Fe in the wetland sediment. Metal remobilization was determined by the potentially mobile fraction (PMF) and metal retention by the recalcitrant factor (RF). The PMF values were high in the floc layer but comparatively low in the organic and inorganic layers. High RF values for Cu, Zn, and Pb in the organic and inorganic layers indicated that these metals were strongly bound in the sediment. The RF values for Mn were lower than for the other elements especially in the floc layer, indicating low retention or binding capacity. Retention of contaminants was also evaluated by distribution coefficient (Kd) values. Distribution coefficient (Kd) values were lower for Cu and Zn than for Pb, indicating a smaller exchangeable fraction for Pb.     

Removal of selenate from water by zerovalent iron

Zerovalent iron (ZVI) has been widely used in the removal of environmental contaminants from water. In this study, ZVI was used to remove selenate [Se(VI)] at a level of 1000 microg L(-1) in the presence of varying concentrations of Cl-, SO(2-)4, NO(-)3, HCO(-)3, and PO(3-)4. Results showed that Se(VI) was rapidly removed during the corrosion of ZVI to iron oxyhydroxides (Fe(OH)). During the 16 h of the experiments, 100 and 56% of the added Se(VI) was removed in 10 mM Cl- and SO(2-)4 solutions under a closed contained system, respectively. Under an open condition, 100 and 93% of the added Se(VI) were removed in the Cl- and SO(2-)4 solutions, respectively. Analysis of Se species in ZVI-Fe(OH) revealed that selenite [Se(IV)] and nonextractable Se increased during the first 2 to 4 h of reaction, with a decrease of Se(VI) in the Cl- experiment and no detection of Se(VI) in the SO(2-)4 experiment. Two mechanisms can be attributed to the rapid removal of Se(VI) from the solutions. One is the reduction of Se(VI) to Se(IV), followed by rapid adsorption of Se(IV) to Fe(OH). The other is the adsorption of Se(VI) directly to Fe(OH), followed by its reduction to Se(IV). The results also show that there was little effect on Se(VI) removal in the presence of Cl- (5, 50, and 100 mM), NO(-)3 (1, 5, and 10 mM), SO(2-)4 (5 mM), HCO(-)3 (1 and 5 mM), or PO(3-)4 (1 mM) and only a slight effect in the presence of SO(2-)4 (50 and 100 mM), HCO(-)3 (10 mM), and PO(3-)4 (5 mM) during a 2-d experiment, whereas 10 mM PO(3-)4 significantly inhibited Se(VI) removal. This work suggests that ZVI may be an effective agent to remove Se from Se-contaminated agricultural drainage water.     

Dissolved phosphorus retention and release from a coastal plain in-stream wetland

Dissolved phosphorus (DP) can be released from wetlands as a result of flooding or shifts in water column concentrations. Our objectives were to determine the long-term (1460 d) DP retention and release characteristics of an in-stream wetland, and to evaluate how these characteristics respond to flooding, draining, and changes in DP concentrations. The studied in-stream wetland drains an agriculturally intensive subwatershed in the North Carolina Coastal Plain region. The wetland's DP retention and release characteristics were evaluated by measuring inflow and outflow DP concentrations, DP mass balance, and DP movement across the sediment-water column interface. Phosphorus sorption isotherms were measured to determine the sediment's equilibria P concentration (EPCo), and passive samplers were used to measure sediment pore water DP concentrations. Initially, the in-stream wetland was undersized (0.31 ha) and released 1.5 kg of DP. Increasing the in-stream wetland area to 0.67 ha by flooding resulted in more DP retention (28 kg) and low outflow DP concentrations. Draining the in-stream wetland from 0.67 to 0.33 ha caused the release of stored DP (12.1 kg). Shifts both in sediment pore water DP concentrations and sediment EPCo values corroborate the release of stored DP. Reflooding the wetland from 0.33 to 0.85 ha caused additional release of stored DP into the outflowing stream (10.9 kg). We conclude that for a time period, this in-stream wetland did provide DP retention. During other time periods, DP was released due to changes in wetland area, rainfall, and DP concentrations.     

Nutrient transport in a restored riparian wetland

We determined the water quality effect of a restored forested riparian wetland adjacent to a manure application area and a heavily fertilized pasture in the Georgia Coastal Plain. The buffer system was managed based on USDA recommendations and averaged 38 m in width. Water quality and hydrology data were collected from 1991-1999. A nitrate plume in shallow ground water with concentrations exceeding 10 mg NO3-N L(-1) moved into the restored forested riparian wetland. Along most of the plume front, concentrations were less than 4 mg NO3-N L(-1) within 25 m. Two preferential flow paths associated with past hydrologic modifications to the site allowed the nitrate plume to progress further into the restored forested riparian wetland. Surface runoff total N, dissolved reactive phosphorus (DRP), and total P concentrations averaged 8.63 mg N L(-1), 1.37 mg P L(-1), and 1.48 mg P L(-1), respectively, at the field edge and were reduced to 4.18 mg N L(-1), 0.31 mg P L(-1), and 0.36 mg P L(-1), respectively, at the restored forested riparian wetland outlet. Water and nutrient mass balance showed that retention and removal rates for nitrogen species ranged from a high of 78% for nitrate to a low of 52% for ammonium. Retention rates for both DRP and total P were 66%. Most of the N retention and removal was accounted for by denitrification. Mean annual concentrations of total N and total P leaving the restored forested riparian wetland were 1.98 mg N L(-1) and 0.24 mg P L(-1), respectively.     

Phosphorus removal in vegetated filter strips

Vegetated filter strips (VFS) are used recently for removal, at or near the source, of sediment and sediment-bound chemicals from cropland runoff. Vegetation within the flowpath increases water infiltration and decreases water turbulence, thus enhancing pollutant removal by sedimentation within filter media and infiltration through the filter surface. Field experiments have been conducted to examine the efficiency of vegetated filter strips for phosphorus removal from cropland runoff with 20 filters with varying length (2 to 15 m), slope (2.3 and 5%), and vegetated cover, including bare-soil plots as control. Artificial runoff used in this study had an average phosphorus concentration of 2.37 mg L(-1) and a sediment concentration of 2700 mg L(-1). The average phosphorus trapping efficiency of all vegetated filters was 61% and ranged from 31% in a 2-m filter to 89% in a 15-m filter. Filter length has been found to be the predominant factor affecting P trapping in VFS. The rate of inflow, type of vegetation, and density of vegetation coverage had secondary influences on P removal. Short filters (2 and 5 m), which are somewhat effective in sediment removal, are much less effective in P removal. Increasing the filter length beyond 15 m is ineffective in enhancing sediment removal but is expected to further enhance P removal. Sediment deposition, infiltration, and plant adsorption are the primary mechanisms for phosphorus trapping in VFS.     

Fate of colloidal-particulate elemental selenium in aquatic systems

Bacterial reduction of selenate [Se(VI)] to elemental Se [Se(0)] is considered an effective bioremediation technique to remove selenium (Se) from agricultural drainage water. However, the fate of the newly formed Se(0) in aquatic systems is not known when it flows out of the treatment system. A set of laboratory experiments was conducted to determine the fate of the colloidal-particulate Se(0) in a water column and in a water-sediment system. Results showed that the newly formed colloidal-particulate Se(0) followed two removal pathways in aquatic systems: (i) flocculation-sedimentation to the bottom of the water and (ii) oxidation to selenite [Se(IV)] and Se(VI). During 58 d of the experiments, 51% of the added Se(0) was precipitated to the bottom of the water and 47% was oxidized to Se(IV) in the water column. In the water-sediment system, Se(IV) in the water accounted for 21 to 25% of the added Se(0). Adsorption of Se(IV) to the bottom sediment resulted in a relatively low amount of Se(IV) in the water. This study indicates that the newly formed Se(0) may be an available form of Se for uptake by organisms if it flows to aquatic systems from a treatment site. Therefore, an effective bioremediation system for removing Se from drainage water must reduce Se(VI) to Se(0) and remove Se(0) directly from the drainage water.     

Hydrologic and Water Quality Functions of a Disturbed Wetland in an Agricultural Setting1

Abstract: In 2006, we collected flow, sediment, and phosphorus (P) data at stream locations upstream and downstream of a small degraded wetland in south‐central Wisconsin traversed by a stream draining a predominantly agricultural watershed. The amount of sediment that left the wetland in the two largest storms, which accounted for 96% of the exported sediment during the observation period, was twice the amount that entered the wetland, even though only 50% of the wetland had been inundated. This apparently anomalous result is due to erosion of sediment that had accumulated in the low‐gradient channel and to the role of drainage ditches, which trapped sediment during the wetland‐filling phase. In the case of total P, the inflow to the wetland approximately equaled the outflow, although the wetland sequestered 30% of the incoming dissolved reactive P. The discrepancy is almost certainly due to net export of sediment. Many wetlands in the glaciated midwestern United States are ditched and traversed by low‐gradient channels draining predominantly agricultural areas, so the processes observed in this wetland are likely to be common in that region. Knowledge of this behavior presents opportunities to improve water quality in this and similar regions.