Effects of stormwater management and stream restoration on watershed nitrogen retention

Restoring urban infrastructure and managing the nitrogen cycle represent emerging challenges for urban water quality. We investigated whether stormwater control measures (SCMs), a form of green infrastructure, integrated into restored and degraded urban stream networks can influence watershed nitrogen loads. We hypothesized that hydrologically connected floodplains and SCMs are “hot spots” for nitrogen removal through denitrification because they have ample organic carbon, low dissolved oxygen levels, and extended hydrologic residence times. We tested this hypothesis by comparing nitrogen retention metrics in two urban stream networks (one restored and one urban degraded) that each contain SCMs, and a forested reference watershed at the Baltimore Long-Term Ecological Research site. We used an urban watershed continuum approach which included sampling over both space and time with a combination of: (1) longitudinal reach-scale mass balances of nitrogen and carbon conducted over 2 years during baseflow and storms (n = 24 sampling dates × 15 stream reaches = 360) and (2) ¹⁵N push–pull tracer experiments to measure in situ denitrification in SCMs and floodplain features (n = 72). The SCMs consisted of inline wetlands installed below a storm drain outfall at one urban site (restored Spring Branch) and a wetland/wet pond configured in an oxbow design to receive water during high flow events at another highly urbanized site (Gwynns Run). The SCMs significantly decreased total dissolved nitrogen (TDN) concentrations at both sites and significantly increased dissolved organic carbon concentrations at one site. At Spring Branch, TDN retention estimated by mass balance (g/day) was ~150 times higher within the stream network than the SCMs. There were no significant differences between mean in situ denitrification rates between SCMs and hydrologically connected floodplains. Longitudinal N budgets along the stream network showed that hydrologically connected floodplains were important sites for watershed nitrogen retention due to groundwater–surface water interactions. Overall, our results indicate that hydrologic variability can influence nitrogen source/sink dynamics along engineered stream networks. Our analysis also suggests that some major predictors for watershed N retention were: (1) streamwater and groundwater flux through stream restoration or stormwater management controls, (2) hydrologic residence times, and (3) surface area of hydrologically connected features.     

Landscape controls on seston stoichiometry in urban stormwater management ponds

1. Stormwater management ponds (SWMPs) are taking the place of natural ponds and wetlands in urban areas. SWMPs have the potential to serve as hotspots for nutrient cycling, yet little is known about how urban catchments affect nutrient chemistry and stoichiometry within these ponds. 2. We sampled 50 SWMPs in Southern Ontario, Canada, to characterise their seston stoichiometry and make comparisons with published lake and pond data and models of seston stoichiometry. We tested (i) whether C : N : P ratios were similar to natural ponds and small lakes, (ii) whether seston stoichiometry was scale dependent and (iii) whether variability in seston chemistry could be explained by landscape and pond characteristics, such as catchment imperviousness and hydrological condition (based on recently received rainfall). 3. Seston C : N and C : P ratios were significantly lower in SWMPs than published ratios for small lakes, likely because of high nitrogen and phosphorus concentrations in SWMPs. Our results also showed no dependency of stoichiometric ratios on pond size. Analyses of ratios versus landscape and pond characteristics revealed significant relationships only when ponds were grouped based on the hydrological condition of the catchments. 4. It is likely that SWMPs function very differently during wet and dry periods. When SWMPs are disconnected from the landscape after a lengthy dry period, internal processes become increasingly important for seston stoichiometry.     

Are cladoceran diversity and community structure linked to spatial heterogeneity in urban landscapes and pond environments?

Biodiversity patterns in cladoceran communities were investigated in urban waterbodies in relation with residential land use, pond management, and waterbody environments. We evaluated species richness in the pelagic and littoral zones of eighteen waterbodies of a large Canadian city. Gamma diversity (26 species) observed at a small scale in the urban survey was important comparatively to large-scale surveys of lakes. Beta diversity ranged from 1 to 8 species among waterbodies. We tested if littoral species greatly contributed to regional diversity in urban waterbodies. Littoral species (Chydoridae, Ilyocryptidae, Macrothricidae, Polyphemidae) accounted for 58% of the total species pool. We distinguished five cladoceran assemblages associated to different waterbodies (temporary ponds, permanent lakes, and wetlands). Cladoceran communities were more diverse and variable in permanent lakes than in temporary ponds. Changes in cladoceran species assemblages among waterbodies were driven by variations in waterbody size and phosphorus enrichment, macrophyte and algal biomass, urban density, pond management practices, and the presence of potential predators as fish and macroinvertebrates. Our study indicates that both artificial ponds and lakes and natural wetlands are valuable habitats for the conservation of cladoceran biodiversity and rare endemic species in urban regions. Further research on pond management strategies promoting urban aquatic biodiversity should be undertaken.     

Importance of sediment deposition and denitrification for nutrient retention in floodplain wetlands

Questions: Various floodplain communities may differ in their relative abilities to influence water quality through nutrient retention and denitrification. Our main questions were: (1) what is the importance of sediment deposition and denitrification for plant productivity and nutrient retention in floodplains; (2) will rehabilitation of natural floodplain communities (semi‐natural grassland, reedbed, woodland, pond) from agricultural grassland affect nutrient retention? Location: Floodplains of two Rhine distributaries (rivers Ussel and Waal), The Netherlands. Methods: Net sedimentation was measured using mats, denitrification in soil cores by acetylene inhibition and bio‐mass production by clipping above‐ground vegetation in winter and summer. Results: Sediment deposition was a major source of N and P in all floodplain communities. Highest deposition rates were found where water velocity was reduced by vegetation structure (reedbeds) or by a drop in surface elevation (pond). Sediment deposition was not higher in woodlands than in grassland types. Denitrification rates were low in winter but significantly higher in summer. Highest denitrification rates were found in an agricultural grassland (winter and summer) and in the ponds (summer). Plant productivity and nutrient uptake were high in reedbeds, intermediate in agricultural grasslands, ponds and semi‐natural grasslands and very low in woodlands (only understorey). All wetlands were N‐limited, which could be explained by low N:P ratios in sediment. Conclusions: Considering Rhine water quality: only substantial P‐retention is expected because, relative to the annual nutrient loads in the river, the floodplains are important sinks for P, but much less for N. Rehabilitation of agricultural grasslands into ponds or reedbeds will probably be more beneficial for downstream water quality (lower P‐concentrations) than into woodlands or semi‐natural grasslands.     

Lagoonal stormwater detention ponds as promoters of harmful algal blooms and eutrophication along the South Carolina coast

In the rapidly urbanizing coastal zone of South Carolina, intensive landscape maintenance and turf management are significant sources of nonpoint source pollutant loadings. The best management practice of choice for stormwater in this region is wet detention ponds, the majority of which are brackish lagoons. Typically, stormwater is piped directly into the ponds, but ponds have limited capacity for processing pollutants. These eutrophic brackish ponds are “hot spots” for harmful algal blooms (HABs)—over 200 blooms from 23 different species were documented over the last 4 years, many associated with measured toxins, fish kills or shellfish health problems. Because these ponds exchange with tidal creeks, they are potential sources for HAB dispersion into adjacent estuaries. Also, flux measurements indicated that groundwater was both a source of nutrients to the ponds and a mechanism for transporting nutrients from the ponds. These findings suggest that manmade ponds as presently designed along the SC coast may contribute to estuarine eutrophication and HAB prevalence. A summary of HAB occurrences in SC lagoonal ponds from 2001 to 2005 is presented, and a project is described that simulates the effectiveness of constructed wetlands as a supplementary best management practice designed to process stormwater and groundwater and remove nutrients prior to entering wet detention ponds.     

Phosphorus cycling in urban aquatic ecosystems: connecting biological processes and water chemistry to sediment P fractions in urban stormwater management ponds

Phosphorus (P) dynamics in urban landscapes may differ from that in natural landscapes due to different P sources and unique environmental conditions. However, many aspects of P cycles in urban areas, especially within engineered aquatic ecosystems, remain largely unknown. Through this work, we aim to contribute to better understanding of P cycling in urban aquatic ecosystems by investigating P fractions in sediment and their relationship with ambient chemistry in surface water from six urban stormwater management ponds located in Ontario, Canada. We found that organic P contributed up to 75% of total P in pond sediment, but this percentage decreased significantly between our two sampling events in June and September 2012. This decrease coincided with increased rates of extracellular enzyme (especially phosphatase) activities, which is indicative of fast mineralization processes in these ecosystems. Moreover, the decreased sediment organic P was matched by increased water column P concentration. This inverse relationship suggests that the large organic P pool in pond sediment, and its fast decomposition, contributed to internal release of P from sediment and increased water column P concentrations. The dominance of organic P in sediment and the putative role of relevant biological processes (i.e., decomposition and productivity) in urban ponds found in this study strongly contrast with classic water management expectations of physicochemically controlled P dynamics and long-term P storage in sediment of aquatic ecosystems. This difference suggests that urban stormwater ponds may perform poorly in terms of P retention and thereby contribute to poor water quality in terms of P pollution to downstream urban watersheds. Thus, stormwater pond design and future management strategies should consider these biogeochemical features of urban ponds, including internal P release, to help prevent eutrophication of downstream ecosystems.     

A catchment-oriented and cost-effective policy for water protection

Governmental programmes and international agreements to counteract eutrophication have largely not attained agreed objectives (e.g. reduction by half of the anthropogenic nitrogen load on Swedish coastal waters). Important components of such programmes are improved removal of nitrogen in municipal treatment plants and changed agricultural practices. In addition, increased N-removal during runoff, i.e. restoration of ponds and wetlands, is an important strategy. One explanation of the fact that the objectives have yet not been achieved might be that the most effective step to counteract diffuse pollution has not been fully implemented. It is therefore important to stress the potential of effective measures and find ways to fully implement them at the watershed level. It is important to avoid excessive applications of fertilizers because this leads to an exponential increase in leaching. Field experiments indicate that the use of winter crops or an undersown catch crop outside the main cropping season has reduced nitrate losses by up to 75% in single years, and by nearly 50% over successive years. In southern Sweden, the area of wetlands has been reduced considerably (more than 90%) by melioration activities. In a recent project, budget studies with restored ponds verified the importance of ponds and wetlands in nitrogen retention. Per unit area, increased nitrogen loading implied increased nitrogen retention, but often a decrease in the percent retained. Ponds with depths of 0.4–2.0 m and hydrological loads of 0.14–5.2 m³ m⁻² day⁻¹ were created. One hundred and fifty to seven thousand kg N ha⁻¹ year⁻¹ was removed in ponds loaded by streams dominated by agricultural run off. A pond receiving pre-treated municipal wastewater removed 8000 kg N ha⁻¹ year⁻¹. The upper limit for N-removal is set by the hydrological conditions. Sedimentation of organic material must be favoured in order to obtain adequate conditions for denitrification. To achieve the governmental objective in nitrogen load reduction changed cultivation practices within the agricultural sector must be combined with restoration of ponds/wetlands.     

Impact of drying and re-wetting on N,P and K dynamics in a wetland soil

As increased nutrient availability due to drainage is considered a major cause of eutrophication in wetlands rewetting of drained wetlands is recommended as a restoration measure. The effect of soil drying and rewetting on the contribution of various nutrient release or transformation processes to changed nutrient availability for plants is however weakly understood. We measured effects of soil drying and re-wetting on N mineralization, and denitrification, as well as on release of dissolved organic nitrogen (DON), phosphorus, and potassium in incubated soil cores from a wet meadow in southern Sweden. Additionally, the impact of re-wetting with sulphate-enriched water was studied. Soil drying stimulated N mineralization (3 times higher) and reduced denitrification (5 times lower) compared to continuously wet soil. In the wet cores, denitrification increased to 20 mg N m^–2 d^–1, which was much higher than denitrification measured in the field. In the field, increased inorganic-N availability for plants due to drainage seemed primarily to be caused by increased N mineralization, and less by decreased denitrification. Soil drying also stimulated the release of DON and K, but P release was not affected. Re-wetting of dried soil cores strongly stimulated denitrification (up to 160 mg N m^–2 d^–1), but N mineralization was not significantly decreased, neither were DON or K release. In contrast, the extractable P pool increased upon soil wetting. Re-wetting with sulphate-enriched water had no effect on any of the nutrient release or transformation rates. We conclude that caution is required in re-wetting of drained wetlands, because it may unintendently cause internal eutrophication through an increased P availability for plants.     

Urban Wet Environment as Mosquito Habitat in the Upper Midwest

Sampling of Culex larval habitat plays an important role in West Nile virus surveillance and control programs. Although many cities have established mosquito sampling programs and abatement districts, there is relatively little information describing the extent and ecology of urban surface waters and stormwater systems in different geographic areas and how these parameters affect mosquito communities and control strategies. An aerial survey of the city of Madison, Wisconsin revealed 521 above-ground wet sites. These included both constructed stormwater systems (ditches, retention ponds, detention ponds) and natural wetlands (marshes, flood areas, creeks, and rivers). Repeat sampling of 351 of these sites was conducted during 2004 and 2005. The majority of sites, 58% in 2004 and 72% in 2005, yielded no mosquito larvae, suggesting that physical and biological features of these wet sites limit the development of mosquito larvae. For both years, analysis of the positive samples revealed that less than 25% of sites produced Culex spp. while a small number of ditches and detention ponds were consistent “superproducers” of Culex larvae from year to year. This information will facilitate comparisons across geographical areas and provides insight into local variation in the public health risk due to mosquito transmission of human disease agents. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The role of pollutant accumulation in determining the use of stormwater ponds by amphibians

Urbanization often results in the creation of habitats such as stormwater management ponds. Although stormwater ponds are designed to retain runoff and associated pollutants, they are frequently colonized by wildlife including pond-breeding amphibians. Understanding of the ecological function of these created habitats is limited. This study investigated the role of pollutants in shaping use of stormwater ponds by amphibians. A survey of 68 stormwater ponds in Baltimore County, Maryland, and statistical modeling found wood frogs (Rana [= Lithobates] sylvatica) were more likely to breed in ponds with longer hydroperiods and Cl^? concentrations below approximately 250 mg/L. American toad (Bufo [= Anaxyrus] americanus) use of ponds was primarily influenced by hydroperiod; toads were more likely to use longer hydroperiod ponds. To confirm use was a result of toxicity and differential sensitivity among species, wood frog and American toad embryos and larvae were exposed to sediment from six stormwater ponds spanning the range of pollutant conditions documented in the field. Survival of wood frogs through metamorphosis was related to metal and salt levels of pond sediments, but survival of American toads was not. In agreement with the field study, no wood frog larvae survived to metamorphosis when Cl^? levels were above 260 mg/L. The results suggest that pollutants that accumulate in stormwater ponds, specifically road deicing salts, are acting as local filters capable of creating unique assemblages of anuran larvae in urban areas.     

Stormwater ponds and biofilters for large urban sites: Modeled arrangements that achieve the phosphorus reduction target for Boston's Charles River, USA

Urban rivers daily receive tons of phosphorus and other pollutants from stormwater generated by impervious surfaces. Constructed detention ponds and biofiltration cells (biofilters) are often effective for localized stormwater treatment, yet less is known about their effectiveness for large built areas. Our goals were to assess stormwater phosphorus-removal relative to total percent cover, number, size, and configuration of detention ponds and biofilters. Two approximately 200-ac. (80 ha) industrial and institutional sites near Boston's Charles River containing diverse smaller drainages, land uses, and runoff sources were analyzed. Using the model WinSLAMM, P-reduction percents were calculated and compared for detention ponds (1-40 per site; covering 5-15% of their drainage areas) and biofilters (two sizes, with and without underdrains; ∼900-4300 per site; 5-10% cover). The government's proposed TMDL target of 65% P-reduction was only achieved with designs that treated 100% of urban land with a pond or biofilter. The 65% target was met by a single pond covering 5% of the site and by several multi-pond or biofilter arrangements with coverage ranging from 5% to 10%. A stringent water quality goal of 75% P-reduction was also attained with certain consolidated and dispersed pond and biofilter designs. Configuration of treatment landscapes appeared to be more important than total treatment area. Results were generally similar for the large institutional and industrial sites. Stormwater P-reduction goals can be creatively met with diverse, realistic land allocations for ponds and biofilters, which also provide enhanced aesthetics, recreation opportunities, and other benefits beyond water quality.     

Longevity of simultaneous operation of aquaculture and mangrove forestry as explained in terms of water and sediment qualities

A silvofishery system (SFS) of 5.2 ha, simultaneously combining aquaculture (shrimp, crab, and fish) and forestry, was studied to understand how the water/sediment qualities had remained viable for 30 years. The long life of this SFS pond contrasts sharply with a short life of many conventional, intensively managed shrimp ponds (5 years on average). Total ammonia nitrogen in the SFS water (0.06 mg l^?1) was much lower than the Thai environmental safety standard for shrimp ponds (1.0 mg l^?1) and approximately 0.05 % of an average conventional, intensively managed shrimp pond. Total organic nitrogen of the pond sediment was 1.47 mg g^?1 which was almost half of conventional, intensively managed shrimp ponds. The flux study revealed that NH₄–N was the dominant form of nitrogen, with lesser amounts of NO₂–N and NO₃–N, and that NH₄–N was being released from the sediment into the water. Nitrogen loss from the pond, which was regarded as the denitrification rate, was estimated to be 71.5 mgN m^?2 d^?1, corresponded to 55 % of the total nitrogen input. As the average denitrification rate in a conventional, intensively managed shrimp pond is 13.4 %, the SFS was shown to be relatively efficient in removing accumulated nitrogen from the pond. Assuming accepted feed conversion rates, 3,340 kg of feed would have been necessary for the amount of fishery production recorded during 5 May 2005 and 22 March 2006. However, only 380 kg of trash fish was added, representing a saving of 2,960 kg of feed. Such a saving could be attributed to detritus from the mangrove trees that have been growing within the pond and algae encouraged to bloom by the shallow water depth. Therefore, it is suggested that the efficient nitrogen removal due to the high denitrification rate as well as the reduced feed input from mangrove detritus substitution, have contributed to maintaining favourable water and sediment qualities, resulting in the longevity of SFS pond.     

Is consolidation drainage an indirect mechanism for increased abundance of cattail in northern prairie wetlands?

In the Prairie Pothole Region of North America, disturbances to wetlands that disrupt water-level fluctuations in response to wet–dry climatic conditions have the potential to alter natural vegetative communities in favor of species that proliferate in stable environments, such as cattail (Typha spp.). We evaluated the effect of water-level dynamics during a recent fluctuation in wet–dry conditions on cattail coverage within semipermanently and permanently ponded wetlands situated in watersheds with different land use and amounts of wetland drainage. We found that ponded water depth increase was significantly greater in wetlands where water levels were not near the spill point of the topographic basin, where banks were steeper, and in larger wetlands where past dry conditions had less influence on change in pond area. Proportion of the wetland covered by cattail was negatively correlated with increased water depth, bank slope and pond area. Our observations provide evidence that cattail coverage in prairie wetlands is regulated by water-level fluctuations and that land use surrounding the wetland might have an indirect effect on cattail coverage by altering water-level response to wet–dry climate conditions. For example, drainage of smaller wetlands into larger wetlands that are characterized by more permanent hydroperiods, leads to stabilized water levels near their spill point and is therefore a potential mechanism for increased cattail abundance in the northern prairie region.     

How hydrological and hydraulic conditions affect performance of ponds

During the 1990s there has been a growing interest in (re)constructed wetlands and ponds for treatment of agricultural runoff, sewage water, and stormwater. Some of these facilities, however, are poorly designed in terms of hydrological and hydraulic performance, which strongly affects their treatment capacity. This paper gives an overview of the present knowledge regarding pond hydraulics in terms of effective volume ratio and dispersion. The importance of these parameters is demonstrated by modelling of nitrogen removal in 13 ponds of different design. The results show that effective volume ratio has a clear influence, which increases when removal rate coefficients increase, while dispersion is of minor importance for removal efficiency. Finally, the concept of hydraulic efficiency is discussed in the light of these results.     

Vegetation fluctuation in mediterranean dune ponds in relation to rainfall variation and water extraction

Abstract. The structure of dune ponds hygrophytic vegetation has a spatial organisation in belts around the pond basin, closely related to water level and flooding regime. Doñana National Park has an important representation of temporal dune ponds, which are subjected to rainfall fluctuations and may be suffering the impact of water extraction from the neighbouring tourist resort. Permanent transects in a vegetation complex of five dune ponds have been monitored during a eight year period (1990-1997). This period was characterised by a number of dry years (annual rainfall around 300 mm), located between two wet cycles (800-900 mm). Transects were analysed in 1990 (wet period), 1994 (dry) and 1997 (wet) by hierarchical agglomera-tive clustering. During the dry period hygrophytic species showed regression, with a high mortality of some species like Ulex minor, while the xerophytic species advanced to lower areas. Seedlings of some xerophytic species colonised the dry surface of the pond basin. The lowering of the water table varied in the different ponds, ranging from 312 to 190 cm depending on topography and the distance to the pumping area. The new period of flooding during 1995-96 and 1996-97 cycles provided the opportunity for hygrophytic spe cies to re-establish themselves in their original places. This study suggest that changes in vegetation are caused by the interaction between weather conditions and human disturbance (water extractions). In our example man-made disturbance is more marked during the dry periods while wet periods tend to obscure the effects of water extractions. From a management perspective, long-term monitoring of water table and vegetation structure is revealed as a key procedure to the management of land-water ecotones on pressured areas and threatened habitats.     

Streetscale bioretention basins in Melbourne and their effect on local biodiversity

Development of water sensitive urban design (WSUD) in Australia is rapidly changing urban landscapes by incorporating engineered stormwater management systems such as bioretention basins. Despite these landscape changes, little is known about their effect on urban biodiversity. The biodiversities of six bioretention basins and six corresponding paired greenspaces (divided into two subgroups) in the Melbourne area were compared using ground-dwelling terrestrial invertebrates as biodiversity indicators. Overall, the number of species, species richness and diversity displayed a decreasing trend from bioretention basins to gardenbed and lawn-type greenspaces. This trend may reflect the possible decreasing habitat resources. Species composition was significantly different in these landscape types. The differences in low-stratum vegetation, pH, leaf litter depth and gravel were the main habitat factors influencing the invertebrate communities of these landscapes. The transition from traditional urban greenspaces to bioretention basins potentially promises to enhance urban biodiversity. Landscape planning at the urban-design scale should consider reducing lawn as environmentally unsustainable urban greenspaces. At the streetscape scale, a combination of bioretention basins and gardenbed-type greenspaces may provide ecologically robust and aesthetically pleasant urban streets. This study was conducted in a summer season only and caution should be taken in generalizing the outcomes over an entire year.     

Distinct Optical Chemistry of Dissolved Organic Matter in Urban Pond Ecosystems

Urbanization has the potential to dramatically alter the biogeochemistry of receiving freshwater ecosystems. We examined the optical chemistry of dissolved organic matter (DOM) in forty-five urban ponds across southern Ontario, Canada to examine whether optical characteristics in these relatively new ecosystems are distinct from other freshwater systems. Dissolved organic carbon (DOC) concentrations ranged from 2 to 16 mg C L^-1 across the ponds with an average value of 5.3 mg C L^-1. Excitation-emission matrix (EEM) spectroscopy and parallel factor analysis (PARAFAC) modelling showed urban pond DOM to be characterized by microbial-like and, less importantly, by terrestrial derived humic-like components. The relatively transparent, non-humic DOM in urban ponds was more similar to that found in open water, lake ecosystems than to rivers or wetlands. After irradiation equivalent to 1.7 days of natural solar radiation, DOC concentrations, on average, decreased by 38% and UV absorbance decreased by 25%. Irradiation decreased the relative abundances of terrestrial humic-like components and increased protein-like aspects of the DOM pool. These findings suggest that high internal production and/or prolonged exposure to sunlight exerts a distinct and significant influence on the chemistry of urban pond DOM, which likely reduces its chemical similarity with upstream sources. These properties of urban pond DOM may alter its biogeochemical role in these relatively novel aquatic ecosystems.     

Ecological risk assessment of urban stormwater ponds: Literature review and proposal of a new conceptual approach providing ecological quality goals and the associated bioassessment tools

Stormwater ponds are a common feature of the urban landscape in many countries with advanced stormwater management. Built to control the impacts of urbanization in the form of increased runoff flows, volumes and pollution loads, stormwater ponds are exposed to strong anthropogenic pressures. Meanwhile, as open water systems, they represent new aquatic habitats potentially enhancing the biodiversity of urban areas and balancing the transformation of original ecosystems existing prior to urbanization. In the current context of sustainable development, assessing the ecological risks of stormwater ponds serving as aquatic habitats is therefore crucial for ensuring both the preservation and rehabilitation of biodiversity in urban areas. During the last decade, ecological risk assessments applied to stormwater ponds lacked adoption of integrated interdisciplinary approaches. This prevented advances in developing adaptive methodologies for assessing the ecological quality of stormwater ponds and for providing quality objectives for the management of these facilities. Also, the application of established integrated assessment methodologies, such as the Sediment Quality Triad widely used in North America, based on comparisons with reference sites, is challenged by the man-made features of urban stormwater ponds. The search for a more specific and effective methodology led to the proposal of supplementing the Sediment Quality Triad with the Oligochaete methodology, which was developed and standardized in France for determining the biological status of sediments in stagnant water ecosystems. The benefits of this approach are discussed in a conceptual framework providing ecological quality goals for urban stormwater ponds.     

Urban nitrogen biogeochemistry: status and processes in green retention basins

Nitrogen (N) cycling has been poorly characterized in urban ecosystems. Processes involving N are of specific concern due to increasing anthropogenic inputs from fertilizer uses and fossil fuel combustion in cities. Here we report on a study of N biogeochemistry in city green retention basins and city parks in the Phoenix metropolitan area, Arizona, USA. City retention basins receive N inputs from street runoff, and along with city parks, fertilizer input from management, making these urban patches potential hot spots for biogeochemical cycling. We sampled soils from six retention basins and two non-retention city parks and measured soil organic matter (SOM) content, net N mineralization, net nitrification, denitrification potential, and intact core denitrification flux and nitrate retention. Our results showed significantly higher SOM, extractable nitrate, nitrification rates and potential denitrification rates in surface soils (0–7.5 cm; soil that is directly affected by fertilizer N input, irrigation, and storm runoff) than in deeper soils. We also observed a distinct horizontal trend of decreasing SOM and denitrification potentials from inlet to outlet (dry well) in the retention basins. Denitrification rates, measured both as potential rates with substrate amendment (390–1151 ng N₂O-N g^–1 soil h^–1), and as intact core fluxes (3.3–57.6 mg N m ^–2 d^–1), were comparable to the highest rates reported in literature for other ecosystems. Management practices that affect biogeochemical processes in urban retention basins thus could affect the whole-city N cycling.     

Modeling nitrate-nitrogen removal process in first-flush reactor for stormwater treatment

Stormwater runoff is one of the most common non-point sources of water pollution to rivers, lakes, estuaries, and coastal beaches. While most pollutants and nutrients, including nitrate-nitrogen, in stormwater are discharged into receiving waters during the first-flush period, no existing best management practices (BMPs) are specifically designed to capture and treat the first-flush portion of urban stormwater runoff. This paper presents a novel BMP device for highway and urban stormwater treatment with emphasis on numerical modeling of the new BMP, called first-flush reactor (FFR). A new model, called VART-DN model, for simulation of denitrification process in the designed first-flush reactor was developed using the variable residence time (VART) model. The VART-DN model is capable of simulating various processes and mechanisms responsible for denitrification in the FFR. Based on sensitivity analysis results of model parameters, the denitrification process is sensitive to the temperature correction factor (b), maximum nitrate-nitrogen decay rate (K (max)), actual varying residence time (T (v)), the constant decay rate of denitrifiying bacteria (v (dec)), temperature (T), biomass inhibition constant (K (b)), maximum growth rate of denitrifiying bacteria (v (max)), denitrifying bacteria concentration (X), longitudinal dispersion coefficient (K (s)), and half-saturation constant of dissolved carbon for biomass (K (Car-X)); a 10% increase in the model parameter values causes a change in model root mean square error (RMSE) of -28.02, -16.16, -12.35, 11.44, -9.68, 10.61, -16.30, -9.27, 6.58 and 3.89%, respectively. The VART-DN model was tested using the data from laboratory experiments conducted using highway stormwater and secondary wastewater. Model results for the denitrification process of highway stormwater showed a good agreement with observed data and the simulation error was less than 9.0%. The RMSE and the coefficient of determination for simulating denitrification process of wastewater were 0.5167 and 0.6912, respectively, demonstrating the efficacy of the VART-DN model.