Migration and fate of polycyclic aromatic hydrocarbons in bioretention systems with different media:experiments and simulations

Polycyclic aromatic hydrocarbons(PAHs)present significant risks to human health owing to their carcinogenic,teratogenic,and mutagenic properties.The contamination of surface water with PAHs via runoff has become a prominent source of water pollution.While the capacity of bioretention systems to remove PAHs from runoff is recognized,the dynamics of PAH migration and degradation in these systems are not well-understood.This study aims to explain the migration and fate of PAHs in bioretention systems through a series of experiments and model simulations.This study constructed bioretention systems with three different media types and found that these systems achieved PAH load reductions exceeding 92%.Notably,naphthalene(NAP),fluoranthene(FLT),and pyrene(PYR)tended to accumulate in the media’s upper layer,at depths of 10 to 40 cm.To further analyze the migration and fate of PAHs during multi-site rainfall events and across prolonged operation,we applied the HYDRUS-1D model under three distinct scenarios.The findings of this study indicated that NAP degraded in 40 d,whereas FLT and PYR showed incomplete degradation after 120 d.During continuous rainfall events,there was no clear pattern of PAH accumulation;however,FLT and PYR persisted in the bioretention systems.The combination of experimental and simulation findings highlights the inevitable accumulation of PAHs during extended use of bioretention systems.This research provides a theoretical basis for improving operational efficiency,advancing PAH degradation in bioretention systems,and reducing their toxicity.

Long-term nitrogen and phosphorus removal,shifts of functional bacteria and fate of resistance genes in bioretention systems under sulfamethoxazole stress

To understand the long-term performance of bioretention systems under sulfamethoxazole (SMX) stress, an unplanted bioretention system (BRS) and two modified BRSs with coconut-shell activated carbon (CAC) and CAC/zero-valent-iron (Fe^(0)) granules (CAC-BRS and Fe/CAC-BRS) were established. Both CAC-BRS and Fe/CAC-BRS significantly outperformed BRS in removing total nitrogen (TN)(CAC-BRS:82.48%;Fe/CAC-BRS:78.08%;BRS:47.51%), total phosphorous (TP)(CAC-BRS:79.36%;Fe/CAC-BRS:98.26%;BRS:41.99%),and SMX (CAC-BRS:99.74%, Fe/CAC-BRS:99.80%;BRS:23.05%) under the long-term SMX exposure (0.8 mg/L, 205 days). High-throughput sequencing revealed that the microbial community structures of the three BRSs shifted greatly in upper zones after SMX exposure.Key functional genera, dominantly Nitrospira, Rhodoplanes, Desulfomicrobium, Geobacter,were identified by combining the functional prediction by the FAPROTAX database with the dominant genera. The higher abundance of nitrogen functional genes (nirK, nirS and nos Z) in CAC-BRS and Fe/CAC-BRS might explain the more efficient TN removal in these two systems. Furthermore, the relative abundance of antibiotic-resistant genes (ARGs)sul I and sulII increased in all BRSs along with SMX exposure, suggesting the selection of bacteria containing sul genes. Substrates tended to become reservoirs of sul genes. Also,co-occurrence network analysis revealed distinct potential host genera of ARGs between upper and lower zones. Notably, Fe/CAC-BRS succeeded to reduce the effluent sul genes by1-2 orders of magnitude, followed by CAC-BRS after 205-day exposure. This study demon-strated that substrate modification was crucial to maintain highly efficient nutrients and SMX removals, and ultimately extend the service life of BRSs in treating SMX wastewater.

Hydrologic experiments and modeling of two laboratory bioretention systems under different boundary conditions

Hydrologic performance of bioretention systems is significantly influenced by the media composition and underdrain configuration. This research measured hydrologic performance of column-scale bioretention systems during a synthetic design storm of 25.9 mm, assuming a system area：catchment area ratio of 5%. The laboratory experiments involved two different engineered media and two different drainage configurations. Results show that the two engineered mediawith different sand aggregates were able to retain about 36% of the inflow volume with tree drainage conlaguratlon. However, the medium with marine sand is better at delaying the occurrence of drainage than the one with pumice sand, denoting the better detention ability of the former. For both engineered media, an underdrain configuration with internal water storage （IWS） zone lowered drainage volume and peak drainage rate as well as delayed the occurrence of drainage and peak drainage rate, as compared to a free drainage configuration. The USEPA SWMM v5.1.11 model was applied for the tree drainage configuration case, and there is a reasonable fit between observed and modeled drainag.e-rates when media-specific characteristics are available. For the IWS drainage configuration case, air entrapment was observed to occur in the engineered medium with manne sand. F^lhng ot an IWS zone is most likely to be influenced by many factors, such as the structure of the bioretention system, medium physical and hydraulic properties, and inflow characteristics. More research is needed on the analysis and modeling of hydrologic process in bioretention with IWS drainage configuration.

Nitrogen removal by three types of bioretention columns under wetting and drying regimes

The behaviors of inorganic nitrogen species in three types of bioretention columns under an intermittently wetting regime were investigated. The mean NH+4—N, NO-3—N and total N(TN) removal efficiencies for the conventional bioretention column(Col. T1) are 71%, 1% and 41%, for layered bioretention column with less permeable soil layer(Col. T2) the efficiencies are 83%, 84% and 82%, and for the bioretention column with submerged zone(Col. T3) the values are 63%, 31% and 53%, respectively. The best nitrogen removal is obtained using Col. T2 with relatively low infiltration rate. Adsorption during runoff dosing and nitrification during the drying period are the primary NH+4—N removal pathways. Less permeable soil and the elevated outlet promote the formation of anoxic conditions. 30%–70% of NO-3—N applied to columns in a single repetition is denitrified during the draining period, suggesting that the draining period is an important timeframe for the removal of NO-3—N. Infiltration rate controls the contact time with media during the draining periods, greatly influencing the NO-3—N removal effects. Bioretention systems with infiltration rate ranging from 3 to 7 cm/h have a great potential to remove NO-3—N.

Global analysis of sensitivity of bioretention cell design elements to hydrologic performance

Analysis of sensitivity of bioretention cell design elements to their hydrologic performances is meaningful in offering theoretical guidelines for proper design. Hydrologic performance of bioretention cells was facilitated with consideration of four metrics： the overflow ratio, groundwater recharge ratio, ponding time, and runoff coefficients. The storm water management model （SWMM） and the bioretention infiltration model RECARGA were applied to generating runoff and outflow time series for calculation of hydrologic performance metrics. Using a parking lot to build a bioretention cell, as an example, the Morris method was used to conduct global sensitivity analysis for two groups of bioretention samples, one without underdrain and the other with underdrain. Results show that the surface area is the most sensitive element to most of the hydrologic metrics, while the gravel depth is the least sensitive element whether bioretention cells are installed with underdrain or not. The saturated infiltration rate of planting soil and the saturated infiltration rate of native soil are the other two most sensitive elements for bioretention cells without underdrain, while the saturated infiltration rate of native soil and underdrain size are the two most sensitive design elements for bioretention cells with underdrain.

Strategies for Enhancing the Efficiency of Bioretention Swales and Basins for Urban Stormwater Management in Temperate Region

Among various schemes to reuse and recycle the limited urban fresh water resources, sustainable urban stormwater management, such as water sensitive urban design and rainwater harvesting, has been recognized as one of the most efficient and economically viable approaches. Storm runoff shall be treated as close as possible to its source before it is reused or discharged into public drainage network or receiving waters to enhance the water environment quality. Bioretention swale/basin, which has been commonly applied to treat runoff from roads, car parks, cyclist and pedestrian paths, rooftops, etc., is recognized to be the most efficient and aesthetic pleasing way to harvest rainwater in urban settings, and other longish shape runoff catchment area. This paper studied over 10 bioretention swales/basins applications in temperate region worldwide covering China, Germany, Norway, Austria, USA, and Australia. Key steps in bioretention swale/basin design and implementation in temperate region were investigated, such as strategic bioretention scheme selection, flow conveyance and hydraulic capacity design, filtering media profile, vegetation scheme selection and maintenance scheme, and suggestion and key design parameters. The critical path and parameters of bioretention swale/basin design which enhanced the effectiveness and efficiency of its application for rainwater harvesting in temperate regions were derived.

Impacts of rainfall and catchment characteristics on bioretention cell performance

Although many studies have evaluated the impacts of bioretention cell (BRC) design elements on hydrologic performance,few have investigated the roles played by site characteristics and rainfall patterns.The objectives of this study were to assess the impacts of rainfall and catchments with different characteristics on the hydrologic performance of BRCs and identify important factors in sizing bioretention when hydrologic performance was oriented for the design using a modeling approach.A 10-year record of rainfall data was used to identify the frequency and magnitude of rainfall events.The results showed that although the small and medium rainfall events were dominant they contributed less to the total rainfall depth than the large rainfall events.The ratio of runoff coefficient to imperviousness can be used as an indicator to explain why BRCs perform differently with the same design strategy under the same rainfall events.Rainfall patterns had significant impacts on the hydrologic performance of BRCs by influencing the overflow and underdrain flow.BRCs performed better for rainfall events with a longer duration and lower rainfall intensity because they generated smoother runoff processes into the BRCs.On the basis of these results,the runoff coefficient is suggested for BRC surface design.

DESIGN AND PERFORMANCE OF BIORETENTION BEDS FOR REMOVAL OF STORMWATER CONTAMINANTS

Bioretention basins hold a volume of water that is filtered through sandy soil and allowed to infiltrate into the subsoil or drained to an outlet(Figure 1).Originally,bioretention basins were intended as a site-scale tool to improve the quality of urban stormwater runoff,but they have a demonstrated impact on reduction of runoff volume and time of concentration.Therefore,the design of basins are complicated by a range of possible goals.Recharging groundwater,improving runoff water quality to maintain or restore aquatic ecosystem health,reducing peak storm flows,extending time of concentration,and reducing runoff volume to prevent channel erosion and sedimentation are all possible goal options.Only a few states have guidelines or regulations for bioretention basins.Furthermore,some existing state design requirements do not reflect the range of goals or the research demonstrating the design and performance of bioretention basins.For example,the Idaho Department of Environmental Quality recommendations were published in 2005 but based information from 1993(IDEQ,2005).The first section below considers the original purpose-pollutant removal.

Plant Traits for Phytoremediation in the Tropics

Water is a limited and valuable resource.Singapore has four national sources of water supply,one of which is natural precipitation.Pollutants collected in stormwater runoff are deposited into drainage systems and reservoirs.Major nutrient pollutants found in local stormwater runoff include nitrate and phosphate,which may cause eutrophication.Bioretention systems are efficient in removing these pollutants in the presence of plants.This paper discusses plant traits that can enhance the phytoremediation of nutrient pollutants in stormwater runoff for application in bioretention systems.The plant species studied showed variations in chlorophyll florescence,leaf greenness,biomass production,and nitrate and phosphate removal.In general,dry biomass was moderately correlated to nitrate and phosphate removal(r=0.339–0.501).Root,leaf,and total dry biomass of the native tree species showed a moderate to strong correlation with nitrate removal(r=0.811,0.657,and 0.727,respectively).Leaf dry biomass of fastgrowing plants also showed a moderate to strong relationship with the removal of both pollutants(r=0.707 and 0.609,respectively).Root dry biomass of slow-growing plants showed a strong relationship with phosphate removal(r=0.707),but the correlation was weaker for nitrate removal(r=0.557).These results are valuable for choosing plants for application in bioretention systems.

Column Analyses of Chloride Breakthrough and Lead Retention in Amended Sandy Soils

Stormwater quality design manuals lack scientifically creditable bases for many novel LID （Low-Impact Development） designs presently proposed to meet stormwater runoff management requirements. Potential stormwater pollutant adsorption and infiltration enhancement capabilities of forest-derived by-products （e.g. woodchips, bio-char） provide an opportunity to combine these readily availability materials into stormwater quality designs. On-site stormwater runoff treatment through determination of the soil-water transport and lead （Pb） retention capacity of two sandy soils from Oregon is considered. Using synthetic stormwater （-120 mg CI/L and -5 mg Pb/L） displacement tests in pairs ofABS （Acrylonitrile butadiene styrene） soil columns （75 mm dia by 0.46 m tall） with upward flow （to minimize air entrapment） saturated hydraulic conductivity, chloride dispersion and Pb retention by plain and amended soils is evaluated. Generally, soil amendment incorporation （woodchips, compost or biochar） as compared to an amendment layer resulted in improved hydraulic conductivities as compared to that of the soil alone. Chloride breakthrough curves verified that resident soil-water displacement occurred with 0.9 to 1.6 pore volumes and residence times for most columns were 15-30 minutes. No synthetic stormwater Pb ＂breakthrough＂ within the displaced or replaced soil-water was found, rather most Pb was adsorbed within the first 150 mm of soil.

A critical literature review of bioretention research for stormwater management in cold climate and future research recommendations

Bioretention is a popular best management practice of low impact development that el/ecUvely restores urban hydrologic characteristics to those ofpredevelopment and improves water quality prior to conveyance to surface waters. This is achieved by utilizing an engineered system containing a surface layer of mulch, a thick soil media often amended with a variety of materials to improve water oualitv, a variety of vegetation, and underdrains, depending on the surrounding soil characteristics.Bioretention systems have been studied quite extensively for warm climate applications, but ctata strongly supporting their long-tema efficacy and application in cold climates is sparse. Although it is apparent that biorelention is an effective stormwater management system, its design in cold climate needs further research. Existing cold climate research has shown that coarser media is required to prevent concrete frost from forming. For spring, summer and fall seasons, if sufficient permeability exists to drain the system prior to freezing, peak flow and volume reduction can be maintained. Additionally. contaminants that are removed via filtration are also not impacted by cold climates. In contrary, dissolved contaminants, nutrients, and organics are significantly more variable in their ability to be removed or degraded via bioretention in colder temperatures. Winter road maintenance salts have been shown to negatively impact the removal of some contaminants and positively impact others, while their effects on properly selected vegetation or bacteria health are also not well understood. Research in these water quality aspects has been inconsistent and therefore requires further study.

Hydrologic and water quality performance of a laboratory scale bioretention unit

A bioretention unit （BRU） or cell is a green infrastructure practice that is widely used as a low impact development （LID） technique for urban stormwater management. Bioretention is considered a good fit for use in China＇s sponge cit.y construction,projects. However, studies on biooretention design, whichincorporates site-specific environmental and social-economic conditions in China are still very much needed. In this study, an experimental BRU, consisted of two cells planted with Turf grass and Buxus shfica,was tested with eighteen synthesized storm events. Three levels （high, median, low） of flows and concentrations of pollutants （TN, TP and COD） were fed to the BRU and the performance of which was examined. The results showed that the BRU not only delayed and lowered the peak flowsbut also removed TN, TP and COD in various ways and to different extents. Under the high, medium and low inflow rate conditions, the outflow peaks were delayed for at least 13 minutes and lowered at least 52%. The two cells stored a maximum of 231 mm and 265 mm for turf grass and Buxus sinica, respectively. For both cells the total depth available for storage was 1,220 mm, including a maximum 110 mm deep ponding area. The largest infiltrate rate was 206 mrn/h for both cells with different plants. For the eighteen events, TP and COD were removed at least 60% and 42% by mean concentration, and 65% and 49% by total load, respectively. In the reservoir layer, the efficiency ratio of removal of TN, TP and COD were 52%, 8% and 38%, respectively, within 5 days after runoff events stopped. Furthermore, the engineering implication of the hydrological and water quality performances in sponge city construction projects is discussed.

Life cycle assessment of low impact development technologies combined with conventional centralized water systems for the City of Atlanta, Georgia

Low-impact development （LID） technologies, such as bioretention areas, rooftop rainwater harvesting, a_nd xeris_caping can co_ntrol stormwater runoff, supply non-potable water, and landscape open space.TillS study examines a hybrid system （HS） that combines LID technologies with a centralized water system to lessen the burden on a conventional system （CS）. CS is defined as the stormwater collection and water supply infrastructure, and the conventional landscaping choices in the City of Atlanta. The study scope is limited to five single-family residential zones （SFZs）, classified R-1 through R-5, and four multi-family residential zones （MFZs）, classified RG-2 through RG-5. Population density increases from 0.4 （R-1） to 62.2 （RG-5） persons per 1,000 m2. We performed a life cycle assessment （LCA） comparison of CS and HS using TRACI 2.1 to simulate impacts on the ecosystem, human health, and natural resources. We quantified the impact of freshwater consumption using the freshwater ecosystem impact （FEI） indicator. Test results indicate that HS has a higher LCA single score than CS in zones with a low population density; however, the difference becomes negligible as population density increases. Incorporating LID in SFZs and MFZs can reduce potable water use by an average of 50%. and 25%,respectively.; however, water savings are negligible in zones with high population density （i.e., RG-5） due to the diminished surface area per capitaavailable for LID technoogies. The results demonstrate that LID technologies effectively reduce outdoor water demand and therefore would be a good choice to decrease the water consumption impact in the City of Atlanta.

RESIDENTIAL STORMWATER:METHODS FOR DECREASING RUNOFF AND INCREASING STORMWATER INFILTRATION

INTRODUCTION Humans and plants depend on an adequate supply of clean water for numerous reasons,from food production to sustaining terrestrial and aquatic life.The average Virginia resident uses about 47 gallons(178 L)of fresh water daily(VDEQ 2008).While a majority of Virginians are provided water from a centralized,public utility,there are nearly two million Virginia residents who depend on well water as their main source(VDH 2008).Replenishing groundwater withdrawals depends on recharge(water moving from the surface to groundwater)from infiltration of precipitation through permeable surfaces in the environment;an important part of the hydrologic,or water,cycle(VDEQ 2010).Forests and grasslands provide much of the available recharge area due to their high capacities to infiltrate precipitation.However,the urbanization process is rapidly converting forested areas and grasslands to commercial,residential,or industrial developments.This conversion creates a significant increase in impervious surfaces such as concrete,asphalt,building roofs,and even compacted vegetated sites(U.S.EPA 2003).Impervious surfaces decrease infiltration and groundwater recharge.They also generate increases in stormwater runoff;defined as any precipitation from a rain or snow event that flows off of an impervious surface.As water runs off urban impervious surfaces,it picks up sediment,oils,debris,nutrients,chemicals,and bacteria.The runoff is then collected in a conveyance system,transported,and discharged to surface waters such as creeks and rivers;most of the time without any type of water quality treatment(U.S.EPA 2003;Paul and Meyer 2001).In addition to carrying pollutants,the runoff is also typically warmer than the receiving surface waters.The increased volume and velocity of the stormwater runoff erodes soil and stream channels and can lead to stream“blow out.”Water quality is degraded and aquatic habitats are adversely altered(Meyer,et al.2005,Booth and Jackson 1997).Due to the interconnected nature of watersheds,the degraded water travels downstream causing subsequent problems.The effect of increased development is an increase in stormwater runoff and associated pollutants into surface waters and a decrease in infiltration for groundwater recharge and stream base flows.Traditional practices for mitigating stormwater runoff impacts have targeted the management of peak runoff by using storage facilities such as detention and retention ponds.Mounting evidence that these methods are inadequate prompted the National Research Council in 2008 to advocate a shift to Low Impact Development(LID)practices to better meet stormwater quality and quantity management goals.LID is based on a set of techniques used in Prince Georges County,Maryland(Prince Georges County 1999).LID seeks to restore the natural hydrology of a site by minimizing the creation of impervious surfaces and increasing infiltration of runoff volume.The ineffectiveness of conventional management approaches and the implementation of the Chesapeake Bay and other critical watershed Total Maximum Daily Loads(TMDLs)caused Virginia to revise its entire process for regulating stormwater.LID and Environmental Site Design(ESD)practices are now used to design sites to meet hydrologic goals and to treat runoff to meet a net site nutrient export standard(Battiata et al.2010).As of the date of this paper,15 of these best management practices,or BMPs,have been approved for use by Virginia(Virginia Stormwater BMP Clearinghouse 2011).Similar approaches are being considered and adopted in other Chesapeake Bay jurisdictions,as well as nationally.The responsibility of stormwater management can be fragmented between state,local,and municipal government(Roy,et al.2008),often differing from watershed to watershed.Because LID is decentralized,it changes the management focus from a large,regional scale to a site scale.Changes at the residential lot level can generate much greater infiltration over the watershed.Each homeowner can significantly reduce the stormwater load leaving their property,thereby improving surface water quality and helping to recharge groundwater reserves.From a green building perspective,LID techniques can provide a substantial credit under the LEEDS-ND(Leadership in Energy and Environmental Design-Neighborhood Development)program.The objective of this paper is to provide a relative context for runoff at the site scale,and an overview of the available BMPs that may be applicable.

DESIGNING FOR ENVIRONMENTAL AND INFRASTRUCTURE SUSTAINABILITY: ONTARIO CASE STUDIES FOR RETROFITS AND NEW DEVELOPMENTS

The Low Impact Development (LID) approach has been implemented worldwide for managing stormwater quantity and quality within the context of land development, re-development, and retrofits within an existing development site. Since the inception of the concept in the 1990s, the application of LID has covered different land uses, spatial scales, and environmental objectives, leading to an expanded vision for applying and testing the LID approach. Recently, holistic methodologies and frameworks have linked land planning to key ecological landscapes larger than the previous site scale practice. This new emerging paradigm considers the watershed, subwatershed, and neighbourhood, in addition to the site scale, and consequently, recommends a landscape-based LID and broader Green Infrastructure (GI) solutions (Benedict and McMahon, 2002;Tzoulas et al, 2007;NRDC, 2011). As part of the holistic understanding of land planning and environmental features and functions within the intended spatial scale, LID and GI measures have been designed and constructed as retrofit measures (i.e., measures implemented within existing development) and as measures implemented within new development areas. Under this new paradigm, the land planning context is linked to environmental objectives to provide end points for environmental conservation and restoration within an ecological landscape such as watersheds, subwatersheds, and stream corridors. This paper presents three case studies for the design and construction of LID and GI measures within different land use contexts and for providing multiple environmental objectives.

Initial impacts of rain gardens＇ application on water quality and quantity in combined sewer： field-scale experiment

Green infrastructures such as rain gardens can benetit onsite reduction ot stormwater runott, leading to reduced combined sewer overflows. A pilot project was conducted to evaluate the impact of rain gardens on the water quality and volume reduction of storm runoff from urban streets in a combined sewer area. The study took place in a six-block area on South Grand Boulevard in St. Louis, Missouri. The impact was assessed through a comparison between the pre-construction （2011/2012） and the post-construction （2014） phases. Shortly after the rain gardens were installed, the levels of total suspended solids, chloride, total nitrogen, total phosphorous, zinc, and copper increased. The level of mercury was lower than the detection level in both phases. E. coli was the only parameter that showed statistically significant decrease following the installation of rain gardens. The likely reason for initial increase in monitored water quality parameters is that the post-construction sampling began after the rain gardens were constructed but before planting, resulted from soil erosion and wash-out from the mulch. However, the levels of most of water quality parameters decreased in the following time period during the post-construction phase. The study found 76% volume reduction of stormwater runoff following the installation of rain gardens at one of studied sites. Statistical analysis is essential on collected data because of the encountered high variability of measured flows resulted from low flow conditions in studied sewers.