Antecedent Dry Days Research Articles

Quantitative source apportionment of dissolved organic matters in wet weather overflows of storm drainage systems based on degradation potential index and end member mixing model

Dissolved organic matter (DOM) in wet weather overflows (WWFs) of storm drainage systems mainly originates from anthropogenic sources, such as paved runoff, illegally discharged domestic sewage and the retained sediment. This study provides a promising method to quantitatively apportion the WWF DOM of storm drainage systems using degradation potential index (DPI) and end member mixing (EMM) model. DPI is derived from excitation-emission matrix parallel factor analysis (EEM-PARAFAC), which can endow the end members and itself of WWF DOM with numerical features, and thus help quantify the source contributions of WWF DOM in EMM model. Findings show that (1) DPI was a reliable tool in the quantitative source apportionment of WWF DOM, owing to its features of small variance within source and large variances between sources; (2) DPI combined with EMM model could help identify the factors that induce significant impacts on the source contributions of WWF DOM, such as the storm pumping discharge and antecedent dry days in our case study; (3) the identified factors could guide the development of effective strategies for WWF DOM control, e.g. sediment management in our case.

Supervised Machine Learning for Estimation of Total Suspended Solids in Urban Watersheds

Machine Learning (ML) algorithms provide an alternative for the prediction of pollutant concentration. We compared eight ML algorithms (Linear Regression (LR), uniform weighting k-Nearest Neighbor (UW-kNN), variable weighting k-Nearest Neighbor (VW-kNN), Support Vector Regression (SVR), Artificial Neural Network (ANN), Regression Tree (RT), Random Forest (RF), and Adaptive Boosting (AdB)) to evaluate the feasibility of ML approaches for estimation of Total Suspended Solids (TSS) using the national stormwater quality database. Six factors were used as features to train the algorithms with TSS concentration as the target parameter: Drainage area, land use, percent of imperviousness, rainfall depth, runoff volume, and antecedent dry days. Comparisons among the ML methods demonstrated a higher degree of variability in model performance, with the coefficient of determination (R2) and Nash–Sutcliffe (NSE) values ranging from 0.15 to 0.77. The Root Mean Square (RMSE) values ranged from 110 mg/L to 220 mg/L. The best fit was obtained using the AdB and RF models, with R2 values of 0.77 and 0.74 in the training step and 0.67 and 0.64 in the prediction step. The NSE values were 0.76 and 0.72 in the training step and 0.67 and 0.62 in the prediction step. The predictions from AdB were sensitive to all six factors. However, the sensitivity level was variable.

Influence of Antecedent Dry Days on Nitrogen Removal in Bioretention Systems

Bioretention systems have become an optimal technology during the construction of the sponge city, but its nitrogen removal performance can be affected by antecedent dry days (ADD). This study was designed to investigate the effects of different lengths of ADD (1,2,3, 5, 7, 12, and 22 d) on nitrogen removal performance using a series of laboratory-scale bioretention systems to form seven constant alternate drying-rewetting regimes. The influence mechanism was further investigated by analyzing the spatial distribution of nitrogen reductase activity and microbial community structure under different drying-rewetting regimes. The results showed that the ammonium removal efficiency was not significantly affected by ADD, while exhibiting high variation depending on the hydraulic permeability of the filler and plant growth conditions. The nitrate and total nitrogen removal efficiency decreased as the length of ADD increased form 7 d to 22 d. In addition, the spatial distribution of nitrate reductase (NaR), nitrite reductase (NiR), and hydroxylamine reductase (HyR) were affected by ADD to some extent. It was found that the soil moisture of submerged layer (SL) regulated the nitrogen processes. The nitrate dissimilatory reduction to ammonium (DNRA) can occur in the SL through secondary catalytic reduction by nitrogen reductases, thus affecting the removal of ammonium. The soil microbial community structure and its spatial distribution could be altered by ADD significantly, and the removal of multiple nitrogen species was partly affected. Thereinto, under shorter ADD values of 1, 2, 3, and 5 d, the dominant phylum was Firmicutes, a group of denitrifying microbes, and its dominant genus, Clostridium_sensu_stricto_1, also had the function of DNRA. The results of the study confirmed that ADD has a certain effect on the nitrogen removal capacity and nitrogen reductase activity, while resulting in spatial changes in the microbial community structure in the bioretention system under constant drying-rewetting conditions.

Bioretention cells remove microplastics from urban stormwater

Microplastic pathways in the environment must be better understood to help select appropriate mitigation strategies. In this 2-year long field study, microplastics were characterized and quantified in urban stormwater runoff and through a bioretention cell, a type of low impact development infrastructure. Concentrations of microparticles ranged from below the detection limit to 704 microparticles/L and the dominant morphology found were fibers. High rainfall intensity and longer antecedent dry days resulted in larger microparticle concentrations. In addition, atmospheric deposition was a source of microplastics to urban runoff. Overall, these results demonstrate that urban stormwater runoff is a concentrated source of microplastics whose concentrations depend on specific climate variables. The bioretention cell showed an 84% decrease in median microparticle concentration in the 106–5,000 µm range, and thus is effective in filtering out microplastics and preventing their spread to downstream environments. Altogether, these results highlight the large contribution of urban stormwater runoff to microplastic contamination in larger aquatic systems and demonstrate the potential for current infiltration-based low impact development practices to limit the spread of microplastic contamination downstream.

Effect of roof materials and weather patterns on the quality of harvested rainwater in Shanghai, China

As the supplementary of domestic water and even as potable water, roof harvested rainwater seems increasingly probable worldwide due to the shortage of fresh water resource. In this study, we compared the quality of harvested rainwater under different meteorological conditions and roofing materials (i.e., galvanized metal, ceramic tile, concrete and asphalt). Results showed that the quality of roof harvested rainwater in the rainwater tank was superior to the first-flush tank, but reusing directly without diversion was not recommended. Compared with other three roof types, ceramic tile was ideally qualified for roof rainwater harvesting, which achieved the highest quality of recycled rainwater (e.g., turbidity (5.03 ± 1.73 NTU), TOC < 22.86 mg·L−1, TN < 4.23 mg·L−1, NH4+-N < 1.85 mg·L−1, NO3−-N < 1.56 mg·L−1, Al < 0.01 mg·L−1, Fe < 0.02 mg·L−1, Zn (No detection), Pb (No detection)). However, asphalt was the last choice as roof material to harvest rainwater due to the worst recycled water quality. Additionally, weather patterns and features could have significant influence on the water quality parameters, and rainfall was the principal meteorological factor. Moreover, antecedent dry days had a great interaction with the quality of rainwater in first-flush tank for asphalt roof. This study lays the foundation for roof rainwater harvesting by selecting suitable roof materials under various weather patterns. Based on the results of the economic analysis, investing roof rainwater harvested system with large catchment area may have low risk and can quickly recover costs in the life cycle.

Predicting Event-Based Sediment and Heavy Metal Loads in Untreated Urban Runoff from Impermeable Surfaces

Understanding the amount of pollutants contributed by impermeable urban surfaces during rain events is necessary for developing effective stormwater management. A process-based pollutant load model, named Modelled Estimates of Discharges for Urban Stormwater Assessments (MEDUSA), was further developed (MEDUSA2.0; Christchurch, New Zealand) to include simulations of dissolved metal loadings and improve total suspended solids (TSS) loading estimations. The model uses antecedent dry days, rainfall pH, average event intensity and duration to predict sediment and heavy metal loads generated by individual surfaces. The MEDUSA2.0 improvements provided a moderate to strong degree of fit to observed sediment, copper, and zinc loads for each modelled road and roof surface type. The individual surface-scale modelling performed by MEDUSA2.0 allows for identification of specific source areas of high pollution for targeted surface management within urban catchments.

Novel design of volume of detention tanks assisted by a multi-source pollution overflow model towards pollution control in urban drainage basins.

The commonly employed design of detention tanks cannot effectively control overflow pollution because of non-stormwater entry and sewer sediments in the urban drainage system. Herein, a multi-source overflow model considering both overflow water quality and quantity has been developed for simulating real overflow events. Subcatchment and drainage information is extracted through geographic information system (ArcGIS) and a multi-source overflow model is developed in Stormwater Management Model (SWMM) by coupling runoff mode, non-stormwater mode, and sediment mode. This model is successfully calibrated and validated with the reasonable root-mean-square error (RMSE) of 8.2 and 5.8% for water quality and quantity, respectively. The simulated results suggest that the misconnected non-stormwater entry can affect overflow contaminant concentrations over the period of overflow due to its continuous pollution, while sewer sediments mainly exert effects on the peak pollution period of overflow. Based on model prediction, an approach called overflow peak pollution interception (OPPI) is proposed for model application and design optimization. The OPPI designed detention tank is suitable for high non-stormwater entries and long antecedent dry days (large amount of sediment). A case study is conducted in a high-density urban area of Shanghai, and compared with two commonly employed design methods in Germany and China, which have the similar design principle of volume, relying on amount of precipitation multiplying area of region, the combination of overflow model and OPPI approach enables to offer more accurate and effective design of detention tanks for pollution control in urban areas. Graphical abstract .

A Comparison Study of Runoff Characteristics of Non-Point Source Pollution from Three Watersheds in South Korea

Three watersheds in Korea (Dochoncheon, Gongjicheon, Seolseongcheon) with different land cover characteristics were selected for non-point source pollution monitoring. Event mean concentration (EMC) was calculated, and runoff characteristics were compared through first-flushing and statistical analyses. The mean of the water quality parameters was the highest in Seolseongcheon during dry days among the three watersheds. EMCs of biochemical oxygen demand (BOD) and total nitrogen (TN) were higher in Dochoncheon and Gongjicheon during rainy days, respectively. The upper Seolseongchun watershed showed overall greater values of chemical oxygen demand (COD), suspended solids (SS), total organic carbon (TOC), and total phosphorus (TP). First-flush analyses indicated that SS had the strongest and TN had the weakest effects on the first flush. BOD was the highest in Dochoncheon (urban watershed) and increased with increased number of antecedent dry days. Rainfall intensity appeared to affect SS runoff strongly in Gongjicheon and Seolseongcheon. COD showed strong correlation with SS and TOC in all watersheds, and organic matter (COD and TOC) demonstrated high factor loads during dry and rainy days. Thus, organic matter–related factors were classified as the major factors in pollutant loads. TP and TN were separately classified during dry days in Gongjicheon and Seolseongcheon, whereas these were the secondary factors during rainfall when the influence of non-point pollution was substantial. Cluster analyses showed that the monitoring sites in Dochoncheon and Gongjicheon watersheds were closer than Seolseongcheon. As a result of the comparison of non-point source pollution runoff in the three watersheds, it was difficult to explain the non-point source pollution runoff by specific characteristics such as land cover. For science-based management of non-point pollution, it is necessary to obtain additional survey data considering the climatic, geographical and major industries.

A Novel Approach for Delineation of Homogeneous Rainfall Regions for Water Sensitive Urban Design—A Case Study in Southeast Queensland

The delineation of homogeneous regions is primarily based on long-term overall rainfall characteristics and therefore does not necessarily consider the homogeneity of event-based rainfall characteristics. However, event-based rainfall characteristics including antecedent dry days, rainfall intensity, total rainfall and total duration of rainfall events are critical for Water Sensitive Urban Design (WSUD). Accordingly, this study presents a novel approach to objectively identify homogeneous rainfall regions based on event-based rainfall characteristics. This approach uses cluster analysis and Hosking–Wallis heterogeneous tests collectively to test the homogeneity of event-based rainfall characteristics. A case study conducted for southeast Queensland (SEQ), Australia is also presented in this article. This study compares the results of the novel modified approach against results of the conventional approach for the delineation of homogeneous regions. It was evident from the results that the entire SEQ could be treated as a homogeneous rainfall region based on the conventional approach. In contrast, based on the modified approach, the coast and the inland of SEQ were identified as separate homogeneous regions. Further, antecedent dry days and rainfall intensity were recognized as the deciding rainfall characteristics in the delineation of homogeneous rainfall regions.

Metals in surface specific urban runoff in Beijing

Metals are among the most toxic pollutants in urban stormwater. To investigate the concentration of dissolved and particulate fractions, the temporal variation during rain events, the effect of wash-off surface, and to assess the pollution status of metals in urban runoff, a total of 155 samples were collected mainly from trafficked areas, roofs and parking lots in Beijing from March to November 2015. Most of the metals were found mainly in the particulate fraction (68–96%) from trafficked surfaces, while for roof runoff Cd, Fe, Mn and Zn were found more equally in dissolved and particulate fractions. Metal concentrations were higher during start of a rain event than later (p < 0.05), and also were higher the longer the period of antecedent dry days. The mean concentration of all metals in trafficked areas exceeded both the Chinese standard Level III (swimming and fishery waters) and the European standards (surface water). Mean concentrations of Cd, Mn, Zn, Al, Fe, Pb and Ni from trafficked areas were 2–10 times higher due to higher traffic intensity and substantial atmospheric deposition, while Sb was 20 times higher than in any other reported data for urban runoff. Cluster analysis (CA) and principal component analysis (PCA) together with Pearson's correlation co-efficient suggested that Cd, Cr, Cu, Mn, Ni, Pb, and Zn mainly originates from vehicular activities, while Mn and Zn in roof runoff is due to atmospheric deposition. The geo-accumulation and pollution indices show that runoff from trafficked areas are moderately to heavily polluted by most metals, except Cu and Zn. Thus, Beijing urban runoff presents an environmental risk towards lakes, bathing water and drinking water. The results can be used as basis for development of stormwater and pollution control strategies.

Trend analysis of rainfall characteristics and its impact on stormwater runoff quality from urban and agricultural catchment

Climate change has significantly affected the rainfall characteristics which can influence the pollutant build-up and wash-off patterns from the catchment. Therefore, this study explored the influence of varying rainfall characteristics on urban and agricultural runoff pollutant export using statistical approaches. For this purpose, Mann-Kendall and Pettitt\'s test were applied to detect the trend and breakpoint in rainfall characteristics time series. In addition, double mass curve and correlation analysis were used to drive the relationship between rainfall-runoff and pollutant exports from both catchments. The results indicate a significant decreased in total rainfall and average rainfall intensity, while a significant increased trend for antecedents dry days and total storm duration over the study periods. The breakpoint was determined to be 2013 which shows remarkable trend shifts for total rainfall, average rainfall intensity and antecedents dry days except total duration. Double mass curve exhibited a straight line with significant rainfall-runoff relationship indicates a climate change effect on both sites. Overall, higher pollutant exports were observed at both sites during the baseline period as compared to change periods. In agricultural site, most of the pollutants exhibited significant (p< 0.05) association with total rainfall, average rainfall intensity and total storm duration. In contrast, pollutants from urban site significantly correlated with antecedent dry days and average rainfall intensity. Thus, total rainfall, average rainfall intensity and total duration were the significant factors for the agricultural catchment while, antecedents dry days and average rainfall intensity were key factors in build-up and wash-off from the urban catchment.

Investigation of the Influence of Hydrogel Amendment on the Retention Capacities of Green Roofs

Abstract Progressive economic development as well as urbanisation influence the characteristics of the stormwater runoff. Progressive sealing of drainage basin surface prompts the decrease of rainwater infiltration, thus increasing the runoff intensity. This results in an increase of flood risk. Thus, in urban areas the sustainable urban drainage systems (SUDS) are used in addition to the traditional sewer systems. The examples of SUDS strategy are, inter alia, the roofs covered with vegetation (the green roofs). The paper presents the results of research of retention capacities of 4 diverse green roof models with following growing media: (1) the typical green roof substrate without any additions, (2) the substrate with addition of about 1 % by weight of hydrogel (the cross-linked potassium polyacrylate), (3) the substrate with addition of about 0.25 % by weight of hydrogel, (4) the substrate with addition of expanded clay and perlite. The models did not have the vegetation layers in order to explore only the retention capacities of drainage layers and substrates. The aim of the first part of research was to investigate the retention capacities of green roof models after 1, 2, 6, 8 and 10 antecedent dry days. In the case of 1 and 2 antecedent dry days the best medium retention capacity had green roof model 2 (with substrate with addition of 1 % by weight of hydrogel), and the weakest medium retention capacity had green roof model 1 (without any additions). In the cases of precipitations which occurred after 6 as well as 8 and 10 antecedent dry days the best retention capacity had green roof model 3 (with addition of about 0.25 % by weight of hydrogel). The weakest retention capacity had in these cases green roof model 4 (with addition of expanded clay and perlite). The aim of the second part of research described in the paper was to investigate the retention capacities of green roof models during precipitations that occurred after long antecedent dry periods of time (34, 59 and 106 antecedent dry days). The substrates and drainage layers were air-dry directly before precipitations. The best retention capacity had in this case green roof model 3 (with the substrate with addition of about 0.25 % by weight of hydrogel). The second largest retention capacity had model 2 (with the substrate with addition of about 1 % by weight of hydrogel). The definitely weakest retention capacity had model 4 containing the substrate with addition of expanded clay and perlite. The results may indicate that the efficacy of hydrogel decreased over time probably due to its decay under the influence of solar radiation.

Dynamics of Clay Particles in Non-vegetated Stormwater Biofilters

Stormwater biofilters manage quantity and quality of urban stormwater runoff. Particulate solids from natural and anthropogenic sources accumulate on paved surfaces and eventually reach receiving waters. Retention of suspended solids in stormwater management systems ensures the quality of stormwater runoff to water resources. Stormwater biofilters are similar in most of design parameters to sand filters employed in water treatment systems. The understanding and design of stormwater biofilters are often based on generic models of sand filters. Unlike water treatment sand filters, which are continuously fed, stormwater biofilters operate intermittently with spontaneously alternating wetting and drying cycles. This results in dynamic pollutant removal pattern that employs different mechanisms during and across rainfall events. As such, pilot scale biofilter columns fabricated with a layer of organic material were operated. Removal of suspended solids was very dynamic, where impact of age of filter, antecedent dry days, and inflow quality varied during and across events. Flush of retained solids and filter material occurred during the stabilisation period during each event while very high removal percentages (more than 90%) were observed after stabilisation, during an event. Clogging was not observed due to re-entrainment, re-distribution, and flush of retained solids during intermittent wetting and drying cycles.

Variability and uncertainty of particle build-up on urban road surfaces

Particle build-up is a key stormwater pollutant process that is typically replicated using a power function with increasing antecedent dry days. Though the use of a power function is recommended by a range of researchers, its applicability is demonstrated primarily for residential roads. Particle build-up process is also subjected to significant variability due to catchment heterogeneity and variability associated with source characteristics such as traffic and land use. Variability in the build-up process and use of stereotypical coefficients can lead to significant model uncertainty. This study evaluates particle build-up characteristics on urban road surfaces using an extensive field investigation program, giving specific priority to industrial and commercial roads. Based on the outcomes, particle build-up process characteristics and respective uncertainties were evaluated and compared for road surfaces in residential, industrial and commercial areas. The study primarily found that both, industrial and commercial land uses generally manifested greater particle build-up loads compared to residential land uses. The study provides estimates for build-up coefficients for a range of land uses, including industrial and commercial with their potential uncertainties in build-up predictions. This provides new knowledge to improve stormwater quality modelling. Aside from land use, the proximity of sites to major road networks was also identified as a critical factor influencing the variability and uncertainty in particle build-up. Variability of the fraction of particles in the <75 μm size range with antecedent dry days exerts the most distinct influence on particle build-up variability across all land uses. The outcomes of this research study are expected to enhance stormwater quality monitoring, modelling and remediation, and thereby promoting greater protection of human and aquatic ecosystem health.

Quantifying nonpoint source emissions and their water quality responses in a complex catchment: A case study of a typical urban-rural mixed catchment

As two key threats to receiving water bodies, the generation mechanisms and processes of urban and agricultural nonpoint sources (NPSs) show clear differences, which lead to distinct characteristics of water quality responses with mixed land-uses catchments compared to single land-use ones. However, few studies have provided such insights in these characteristic or quantified different water environment responses to NPS pollution. In this study, an integrated modelling approach was developed for those complex catchments by combining three commonly used models: SWMM (Storm Water Management Model), SWAT (Soil and Water Assessment Tool) and MIKE 11. A case study was performed in a typical urban–rural catchment of Chao Lake, China. The simulated results indicated that urban NPS pollution responded sensitively to rainfall events and was greatly affected by the antecedent dry days. Compare to urban NPS, agricultural NPS pollution was characterized with the time-lag to rainfall depended on soil moisture and the post-rain-season emissions carried by lateral flows, and were also affected by the local farm-practice schedule. With comprehensive impacts from urban-rural land-uses, the time-interleaved urban and agricultural NPS pollution emissions and more abundant pollution accumulation both led to a decrease in the responsive time and an increase in the frequency of peak pollution concentration values even during the dry season. These obtained characteristics can provide guidance for drafting watershed management plans in similar mixed land use catchments.

Role of residence time on the transformation of Zn, Cu, Pb and Cd attached to road dust in different land uses

The adsorption behaviour of metals deposited on road surfaces undergo changes during dry weather periods, with the bioavailability varying compared to the original species, prior to incorporation in stormwater runoff. This study investigated the role of antecedent dry days on the transformation characteristics of Zn, Cu, Pb and Cd attached to different geochemical forms of road dust, in order to predict potential stormwater quality impacts. The study outcomes showed that the exchangeable fraction generally decreased with the increase in antecedent dry days, but the reducible, oxidisable and residual fractions increased. This implies that there is a time-dependent transformation of weakly bound metals in road dust to more persistent chemical forms. The transformation rate of metals was found to be in the order of Pb > Cu > Zn > Cd. Significant changes in metal distribution among the geochemical fractions were observed up to seven dry days, suggesting that during the initial antecedent dry days, the transformations can be relatively more significant. Among the analysed metals, Cd present on road surfaces has a higher potential for being bioavailable during the antecedent dry days.

Intrinsic and extrinsic factors which influence metal adsorption to road dust

The adsorption behaviour of metals deposited on road surfaces is a complex process and influenced by a range of factors common to the urban environment. However, all factors do not have the same level of importance. It is therefore important to identify the most crucial factors for accurate stormwater quality predictions and to implement effective stormwater pollution mitigation measures. Accordingly, this study investigated the extrinsic and intrinsic factors in terms of their degree of influence on the adsorption of individual metal cations to particulates. The variability associated with the adsorption of Zn, Cu, Pb, Cd, Cr and Ni to road dust was found to be influenced by changes to the antecedent dry days and land use characteristics. The initial dry days after a storm event exerts a significant influence on adsorption compared to the later dry days in all land uses. In terms of the intrinsic physico-chemical properties of road dust, the parameters that influence the adsorption process differ in terms of the type of metal cation and particle size fractions of solids. Based on the influential parameters identified, the bioavailability characteristics of Zn, Cd, Pb and Ni in <150μm size fraction of road dust and potential stormwater quality impacts can be highlighted.

Phosphorus Dynamics along River Continuum during Typhoon Storm Events

Information on riverine phosphorus (P) dynamics during typhoon storm events remains scarce in subtropical regions. Thus, this study investigates the spatial and temporal dynamics of riverine phosphorus in a headwater catchment during three typhoon events. Continuous sampling (3 h intervals) of stormwater samples and discharge data were conducted at five locations, which represent the upstream, transitional zone, and downstream areas of the main inflow river. The results revealed that the average event mean concentrations (EMCs) for total dissolved phosphorus (TDP) and particulate phosphorus (PP) in the upstream catchment of Fei-Tsui reservoir were 15.66 μg/L and 11.94 μg/L, respectively. There was at least a 1.3-fold increase in flow-weighted concentrations of TDP and PP from the upper to lower reaches of the main stream. PP and TDP were transported either in clockwise or anticlockwise directions, depending on storm intensity and source. The transport of TDP was primarily regulated by the subsurface flow during the storm event. Soluble reactive phosphorus (SRP) contributes more than 50% of the TDP load in moderate storms, while extreme storms supply a greater dissolved organic phosphorus (DOP) load into the stream. TDP accounted for approximately 50% of TP load during typhoon storms. Mobilization of all P forms was observed from upstream to downstream of the river, except for DOP. A decrease of DOP load on passing downstream may reflect the change in phosphorus form along the river continuum. Peak discharge and antecedent dry days are correlated positively with P fluxes, indicating that river bank erosion and re-suspension of within-channel sediment are the dominant pathways of P during typhoon storm periods.

Geochemical phase and particle size relationships of metals in urban road dust

Detailed knowledge of the processes that metals undergo during dry weather periods whilst deposited on urban surfaces and their environmental significance is essential to predict the potential influence of metals on stormwater quality in order to develop appropriate stormwater pollution mitigation measures. However, very limited research has been undertaken in this area. Accordingly, this study investigated the geochemical phase and particle size relationships of seven metals which are commonly associated with urban road dust, using sequential extraction in order to assess their mobility characteristics. Metals in the sequentially extracted fractions of exchangeable, reducible, oxidisable and residual were found to follow a similar trend for different land uses even though they had variable accumulation loads. The high affinity of Cd and Zn for exchangeable reactions in both, bulk and size-fractionated solid samples confirmed their high mobility, while the significant enrichment of Ni and Cr in the stable residual fraction indicated a low risk of mobility. The study results also confirmed the availability of Cu, Pb and Mn in both, stable and mobile fractions. The fine fraction of solids (<150 μm) and antecedent dry days can be highlighted as important parameters when determining the fate of metals associated with urban road dust. The outcomes from this study are expected to contribute to the development of effective stormwater pollution mitigation strategies by taking into consideration the metal-particulate relationships.

Effects of Three Bioretention Configurations on Dissolved Nitrogen Removal from Urban Stormwater

Multiple chemical forms of nitrogen in urban storm water make its management challenging. Three types of bioretention systems were constructed in 2015 with loamy sand as filter media, including a conventional freely drained bioretention (CB), a modified bioretetion incorporated a submerged zone (MB1), and a modified bioretention incorporating a submerged zone with woodchips addition (MB2). This study investigated the role of vegetation, the use of submerged zone and carbon addition in achieving co-optimized dissolved nitrogen removal in bioretention systems. Twelve bioretention columns were monitored over a 12-month period of dosing with synthetic storm water under varying hydrology and nitrogen loading rates. All the studied bioretention systems could achieve very good ammonia removal (more than 95%) at an average inflow ammonia concentration of (5.45±2.21) mg·L-1. The filter media sorption, nitrification and plants uptake were the main removal pathways for incoming ammonia. The effluent nitrate concentrations of the CB, MB1 and MB2 were (4.04±2.64)mg·L-1 (31.3%), (0.84±1.18) mg·L-1 (85.7%), and (0.26±0.48) mg·L-1 (95.6%), respectively, at the average inflow nitrate concentration of (5.88±2.32) mg·L-1. The use of the native species P. alopecuroides, a submerged zone and woodchips addition could effectively decrease the effluent nitrate concentration, reduce the washout and achieve high nitrate removal. Both plants uptake and denitrification were the two major pathways for removal of inflow nitrate. Inflow magnitude, antecedent dry days and inflow nitrate concentration were the main factors influencing the effluent nitrate concentrations for the three bioretention systems. The results highlighted that the bioretention design of the native species P. alopecuroides incorporated a submerged zone with 10% woodchips addition could consistently and effectively remove storm water nitrate under hydrological regime and nitrogen loading rates.