Nutrient transport, shear strength and hydraulic characteristics of topsoils amended with mulch, compost and biosolids.

Anthropogenic disturbance of soils can disrupt soil structure, diminish fertility, alter soil chemical properties, and cause erosion. Current remediation practices involve amending degraded urban topsoils lacking in organic matter and nutrition with organic amendments (OA) to enhance vegetative growth. However, the impact of OAs on water quality and structural properties at rates that meet common topsoil organic matter specifications need to be studied and understood. This study tested three commonly available OAs: shredded wood mulch, leaf-based compost, and class A Exceptional Quality stabilized sewage sludge (or biosolids) for nutrient (nitrogen and phosphorus) water quality, soil shear strength, and hydraulic properties, through two greenhouse tub studies. Findings showed that nitrogen losses to leachate were greater in the biosolids amended topsoils compared to leaf-compost, mulch amended topsoils, and control treatments. Steady-state mean total nitrogen (N) concentrations from biosolids treatment exceeded typical highway stormwater concentrations by at least 25 times. Soil total N content combined with the carbon:nitrogen ratio were identified to be the governing properties of N leaching in soils. Study soils, irrespective of the type of amendment, reduced the applied (tap) water phosphorus (P) concentration of ∼0.3 mg-P/L throughout the experiment. Contrary to the effects on N leaching, P was successfully retained by the biosolids amendment, due to the presence of greater active iron contents. A breakthrough mechanism for P was observed in leaf compost amended soil, where the effluent concentrations of P continued to increase with each rainfall application, possibly due to an saturation of soil adsorption sites. The addition of OAs also improved the strength and hydraulic properties of soils. The effective interlocking mechanisms between the soil and OA surfaces could provide soil its required strength and stability, particularly on slopes. OAs also improved soil fertility to promote turf growth. Presence of vegetative root zones can further reinforce the soil and control erosion.

Interactions of particulate- and dissolved-phase heavy metals in a mature stormwater bioretention cell.

Bioretention is an increasingly common stormwater control measure (SCM) for mitigation of stormwater quantity and quality. Studies from lab to field scale have shown successful removal of total metals from stormwater, especially Cu and Zn which are ubiquitous in the urban environment yet detrimental to aquatic ecosystems. While bioretention effectively removes particulate matter and particulate bound (PB) contaminants, removal performance of dissolved metals has been neglected in field studies. After approximately two decades of these systems being implemented, with a typical design-life of 20 years, performance of mature systems is unknown. This study examined the performance of a 16- to 18-year-old bioretention cell by characterizing Cu and Zn partitioning and removal. Flow-weighted composite samples of stormwater and bioretention effluent were collected and analyzed for total and dissolved metals. Size-fractioned road-deposited sediments (RDS) were collected and analyzed for metals and particle size distribution. The comparison of RDS and PB metals showed that PB-Zn was enriched in stormwater, indicating higher mobility of PB-Zn compared to PB-Cu. The mature bioretention system effectively removed particulates and PB-metals with average load reductions of 82% and 83%, respectively. While concentrations for dissolved metals were low (<40 µg/L), no significant difference between influent and effluent was observed. Effluent concentrations of total and dissolved Cu, total organic carbon, and particulates were not significantly different from those measured over 10 years ago at the site, while total Zn effluent concentration slightly increased. MINTEQ speciation modeling showed that Cu was approximately 100% bound with dissolved organic matter (DOM) in all bioretention effluent. While Zn was also mostly bound with DOM in effluent, some events showed free ionic Zn reaching concentrations in the same order of magnitude. Media amendments, maintenance, and monitoring of SCMs should be considered where further removal of dissolved metals is necessary for the protection of aquatic environments.

Distribution and biodegradation potential of polycyclic aromatic hydrocarbons (PAHs) accumulated in media of a stormwater bioretention.

Polycyclic aromatic hydrocarbons (PAHs) are a group of organic compounds that can be captured and accumulate in the bioretention cell media, which may lead to secondary pollution and ecological risks. This research aimed to understand the spatial distribution of 16 priority PAHs in bioretention media, identify their sources, evaluate their ecological impact, and assess the potential for their aerobic biodegradation. The highest total PAH concentration (25.5 ± 1.7 µg/g) was observed 1.83 m from the inlet and 10-15 cm deep. The individual PAHs with the highest concentrations were benzo [g,h,i]perylene in February (1.8 ± 0.8 µg/g) and pyrene in June (1.8 ± 0.8 µg/g). Data indicated that primary sources of PAHs were fossil fuel combustion and petroleum. The ecological impact and toxicity of the media were assessed by probable effect concentrations (PECs) and benzo [a]pyrene total toxicity equivalent (BaP-TEQ). The results showed that the concentrations of pyrene and chrysene exceeded the PECs, and the average BaP-TEQ was 1.64 µg/g, primarily caused by benzo [a]pyrene. The functional gene (C12O) of PAH-ring cleaving dioxygenases (PAH-RCD) was present in the surface media, which indicated that aerobic biodegradation of PAHs was possible. Overall, this study revealed the PAHs accumulated most at medium distance and depth, where biodegradation may be limited. Thus, the accumulation of PAHs below the surface of the bioretention cell may need to be considered during long-term operation and maintenance.

Considerations for evaluating innovative stormwater treatment media for removal of dissolved contaminants of concern with focus on biochar.

Stormwater from complex land uses is an important contributor of contaminants of concern (COCs) such as polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), Copper, and Zinc to receiving water bodies. A large portion of these COCs bind to particulate matter in stormwater, which can be removed through filtration by traditional media. However, the remaining dissolved COCs can be significant and require special attention such as engineered treatment measures and media. Biochar is a porous sorbent produced from a variety of organic materials. In the last decade biochar has been gaining attention as a stormwater treatment medium due to low cost compared to activated carbon. However, biochar is not a uniform product and selection of an appropriate biochar for the removal of specific contaminants can be a complex process. Biochars are synthesized from various feedstocks and using different manufacturing approaches, including pyrolysis temperature, impact the biochar properties thus affecting ability to remove stormwater contaminants. The local availability of specific biochar products is another important consideration. An evaluation of proposed stormwater control measure (SCM) media needs to consider the dynamic conditions associated with stormwater and its management, but the passive requirements of the SCM. The media should be able to mitigate flood risks, remove targeted COCs under high flow SCM conditions, and address practical considerations like cost, sourcing, and construction and maintenance. This paper outlines a process for selecting promising candidates for SCM media and evaluating their performance through laboratory tests and field deployment with special attention to unique stormwater considerations.

Removal of stormwater dissolved organic nitrogen through biotransformation using activated carbon.

Conventional bioretention systems are not effectively designed to remove stormwater dissolved organic nitrogen (DON). Biotransformation study on five organic nitrogenous compounds with different values for adsorption on coal activated carbon (AC) and bioavailability revealed that adsorption is a greater controlling factor for ammonification than bioavailability. This study also showed three apparent benefits: enhancement of the ammonification rate, ammonification of the bio-recalcitrant organic nitrogenous compounds, for example, pyrrole, and bio-regeneration of the adsorbent (coal AC). Low temperature (4°C) did not impact ammonification of leucine at a velocity of 34 cm/h, but negatively affected it at 61 cm/h. It was also observed that bed media height > 30 cm would not appreciably increase ammonification. Under intermittent wetting/draining conditions, the DON removal efficiency was more than 90%, indicating that DON was successfully removed through concurrent adsorption/ammonification, although generated ammonium in the effluent must be properly addressed. PRACTITIONER POINTS: Coal activated carbon appears a better material for DON ammonification compared with charcoal and quartz sand. A temperature as low as 4°C may not adversely impact DON ammonification at a velocity of 34 cm/h or less. A bed media depth of 30 cm is considered as adequate to promote DON ammonification. A larger depth may not be expected to improve ammonification. Ammonification of the bio-recalcitrant organic nitrogenous compounds, for example, pyrrole, and bio-regeneration of the adsorbent, for example, coal activated carbon, may be achieved.

Chemical characterization of urban stormwater: Traditional and emerging contaminants.

Increases in urbanization have led to increased stormwater runoff and mobilization of pollutants from urban watersheds. Discharge of these pollutants often leads to contamination of receiving water bodies. Chemical characterization of urban stormwater is necessary to gain deeper insights into the ecological impacts of urban runoff and to evaluate parameters that influence possible treatment technologies. This study assessed stormwater event mean concentrations and particle size fractions from field studies reported in national/international stormwater quality databases (The National Stormwater Quality and The Best Management Practices databases) and peer-reviewed literature. This characterization of urban stormwater includes statistical evaluation of probability distribution, consideration of dissolved and particulate-bound pollutants and focuses on partitioning and speciation behavior. Solids, nutrients, metals, organic pollutants, and bacterial pathogen indicators were evaluated. A significant fraction of stormwater phosphorus, metals and organic pollutants are particle-bound. Results from the speciation of metals demonstrated that metals are predominantly present as either inner-sphere or electrostatic complexes with dissolved organic matter. This study provides a comprehensive overview of the myriad pollutants found in urban stormwater and provides a starting point for addressing ubiquitous and emerging contaminants. Finally, research needs for further detailed stormwater characterization were identified.

Biological nitrogen removal from stormwater in bioretention cells: a critical review.

Excess nitrogen in stormwater degrades surface water quality via eutrophication and related processes. Bioretention has been recognized as a highly effective low-impact development (LID) technology for the management of high runoff volumes and reduction of nitrogen (N) pollutants through various mechanisms. This paper provides a comprehensive and critical review of recent developments on the biological N removal processes occurring in bioretention systems. The key plant- and microbe-mediated N transformation processes include assimilation (N uptake by plants and microbes), nitrification, denitrification, and anammox (anaerobic ammonia oxidation), but denitrification is the major pathway of permanent N removal. Overall, both laboratory- and field-scale bioretention systems have demonstrated promising N removal performance (TN: >70%). The phyla Bacteroidetes and Proteobacteria are the most abundant microbial communities found to be enriched in biofilter media. Furthermore, the denitrifying communities contain several functional genes (e.g., nirK/nirS, and nosZ), and their concentrations increase near the surface of media depth. The N removal effectiveness of bioretention systems is largely impacted by the hydraulics and environmental factors. When a bioretention system operates at: low hydraulic/N loading rate, containing a saturation zone, vegetated with native plants, having deeper and multilayer biofilter media with warm climate temperature and wet storm events periods, the N removal efficiency can be high. This review highlights shortcomings and current knowledge gaps in the area of total nitrogen removal using bioretention systems, as well as identifies future research directions on this topic.

Unconventional microbial mechanisms for the key factors influencing inorganic nitrogen removal in stormwater bioretention columns.

Bioretention systems are environmentally friendly measures to control the amount of water and pollutants in urban stormwater runoff, and their treatment performance for inorganic N strongly depends on various microbial processes. However, microbial responses to variations of N mass reduction in bioretention systems are complex and poorly understood, which is not conducive to management designs. In the present study, a series of bioretention columns were established to monitor their fate performance for inorganic N (NH4+and NO3-) by using different configurations and by dosing with simulated stormwater events. The results showed that NH4+ was efficiently oxidized to NO3-, mainly by ammonia- and nitrite-oxidizing bacteria in the oxic media, regardless of the configurations of the bioretention systems or stormwater conditions. In contrast, NO3- removal pathways varied greatly in different columns. The presence of vegetation efficiently improved NO3-mass reduction through root assimilation and enhancement of microbial NO3- reduction in the rhizosphere. The construction of an organic-rich saturation zone can make the redox potential too low for heterotrophic denitrification to occur, so as to ensure high NO3- mass reduction mainly via stimulating chemolithotrophic NO3- reduction coupled with oxidation of reductive sulfur compounds derived from the bio-reduction of sulfate. In contrast, in the organic-poor saturation zone, multiple oligotrophic NO3- reduction pathways may be responsible for the high NO3- mass reduction. These findings highlight the necessity of considering the variation of N bio-transformation pathways for inorganic N removal in the configuration of bioretention systems.

Bioretention systems for stormwater management: Recent advances and future prospects.

Bioretention is a popular stormwater management strategy that is often utilized in urban environments to combat water quality and hydrological impacts of stormwater. This goal is achieved by selective designing of a system, which consists of suitable vegetation at the top planted on an engineered media with drainage system and possible underdrain at the bottom. Bibliometric analysis on bioretention studies indicates that most of the original research contributions are derived from a few countries and selected research groups. Hence, most of the bioretention systems installed in diverse geographical locations are based on guidelines from climatically different countries, which often lead to operational failures. The current review critically analyzes recent research findings from the bioretention literature, provides the authors' perspectives on the current state of knowledge, highlights the key knowledge gaps in bioretention research, and points out future research directions to make further advances in the field. Specifically, the role and desired features of bioretention components, the importance of fundamental investigations in laboratory, field-based studies and modeling efforts, the real-time process control of bioretention cells, bioretention system design considerations, and life cycle assessment of full-scale bioretention systems are discussed. The importance of local conditions in guiding bioretention designs in difference climates is emphasized. At the end of the review, current technical challenges are identified and recommendations to overcome them are provided. This comprehensive review not only offers fundamental insights into bioretention technology, but also provides novel ideas to combat issues related to urban runoff and achieve sustainable stormwater management.

Characterizing laboratory-scale clinoptilolite bio-columns for removal and nitrification of ammoniacal nitrogen in simulated stormwater.

Due to the diverse speciation and biochemical characteristics of nitrogen in urban runoff, excess nitrogen continues to be a major source of eutrophication in receiving waters. The performance of a nitrifying-sorptive Clinoptilolite (ZT) was examined for use in a media-based stormwater control measure (SCM) for ammonium removal. Results suggested that columns operated under continuous feed showed more nitrification as the media approached ammonium exhaustion. Influent concentrations of 2.5 and 5 mg NH4 + -N/L tested under continuous flow regimes both showed steady-state operation after media exhaustion, with the average effluent [NO3 - N] of 1.2 and 1.7 mg/L, respectively. The performance of the media under intermittent flow regime showed lower effluent ammonium, nitrification between simulated saturated periods, and could treat an additional 70 bed volumes of simulated runoff when compared to a column receiving identical continuous feed. However, nitrification was not sufficient to prevent desorption of ammonium during drops in influent NH4 + -N concentrations. Use of Clinoptilolite for ammonium sorption/nitrification is a systematic approach for capture and transformation of incoming/mineralized ammonium to nitrate prior to reaching an anoxic/denitrifying zone within SCMs. PRACTITIONER POINTS: Clinoptilolite can accumulate stormwater ammonium, allowing it to be nitrified. Ammonium nitrification will regenerate exchange sites on the clinoptilolite. Intermittent flow conditions allowed more nitrification between stormwater events.

Activated carbon column adsorption of compounds that mimic urban stormwater dissolved organic nitrogen.

Nutrients mobilized by stormwater can exacerbate eutrophication in receiving waters. While bioretention systems are increasingly employed to improve stormwater quality, they do not normally incorporate design attributes for removal of dissolved organic nitrogen (DON). Thus, the current study concentrated on continuous column adsorption of stormwater DON using a media mixture of coal activated carbon and quartz sand. Adsorption of eight model organic nitrogenous compounds was studied and only pyrrole showed an appreciable adsorption performance; other organic nitrogen compounds were weakly adsorbed. The breakthrough depth for pyrrole was 88 m (equivalent to 4.4 m simulated rainfall depth), at a superficial velocity of 61 cm/hr and influent DON concentration of 1 mg N/L. Subsequent experiments revealed that adsorption of pyrrole was minimally affected by superficial velocity, such that its DON removal efficiency was greater than 91% for all tested superficial velocities (7-489 cm/hr). Accordingly, adsorption processes may be employed for removing stormwater DON fractions behaving similarly to pyrrole; data suggest DON removal initially at greater than 95%, gradually falling to 30% through 25 years of service. PRACTITIONER POINTS: Adsorption of eight different organic nitrogenous compounds onto coal-based activated carbon was investigated. Amino acids and an amino sugar were weakly adsorbed onto the activated carbon. Pyrrole, a moderately hydrophobic heterocyclic organic nitrogen compound was effectively adsorbed. A 30-cm depth was considered as adequate for removal of pyrrole and compounds that would similarly adsorb. Evidence of biological ammonification was present in all studies except for pyrrole.

Impact of vegetation selection on nitrogen and phosphorus processing in bioretention containers.

A year-long bioretention container study in Maryland, USA, measured the relationship between three plant species (Eutrochium dubium, Iris versicolor, and Juncus effusus) and N ( NO 3 - , NO 2 - , NH 4 + , total nitrogen [TN], total dissolved nitrogen [TDN], dissolved organic nitrogen, particulate organic nitrogen [PON]) and total phosphorus (TP) removal from synthetic stormwater. Statistically significant removal was only found for NO 3 - and TP. Plant-independent NO 3 - removal occurred 9 months after planting, and then changed to removal only by the least-densely planted Juncus treatment. Removal in higher-density Juncus plantings was suspected to be limited by preferential pathways created by high root density. Juncus' low-density NO 3 - removal success correlates with its high growth rate, root mass and length, and large biomass, matching previous literature. TP removal was plant-independent. Shoot harvesting of one plant of each species after 1 year would remove 0.61 g N. Of the plant species in this study, Juncus effusus is most highly recommended for bioretention for its nutrient removal dynamics and year-round green aesthetics. PRACTITIONER POINTS: Only the one-Juncus density treatment had significant NO 3 - removal. All Juncus treatments as well as non-Juncus treatments prevented the PON, TN, or TDN export seen in the No-plants control. TP removal was plant-independent. Juncus had the greatest biomass increase and biomass N. Shoots contain the majority of biomass N for each plant species. Juncus and Iris had high survivorship. Joe Pye had low survivorship. These, and all other study results, need field-scale verification.

Evaluation of an enhanced treatment media and permeable pavement base to remove stormwater nitrogen, phosphorus, and metals under simulated rainfall.

An enhanced stormwater treatment media, developed previously by the authors, was shown to effectively retain dissolved phosphorus (DP) and total Cu and Zn under simulated rainfall. The media comprises expanded shale aggregate, aluminum-based water treatment residual (WTR), and a psyllium-based binder. A 5-cm layer of media was installed as a permeable pavement base layer in a laboratory mesocosm and subjected to rainfall simulations using synthetic stormwater. At rainfall intensity of 0.66 cm/h, effluent DP event mean concentration (EMC) fell from an average of 0.22 mg/L P to below the EPA water quality criterion of 0.037 mg/L in 8 of 9 storms. DP retention increased at lower rainfall intensity and lower pH. Effluent EMC was lowered to less than 30 µg/L for total Cu and less than 92 µg/L for total Zn, on average, relative to average influent EMCs above 61 and 255 µg/L for total Cu and Zn, respectively. Effluent total Al EMCs were below the 25 µg/L detection limit for all storm simulations, indicating Al leaching from the WTR-containing media not to be an issue. Inclusion of an internal water storage (IWS) zone resulted in a 33% total nitrogen (TN) load reduction when adequate carbon was present to advance denitrification. This study provides an evaluation and demonstrates expected treatment performance of a novel stormwater treatment media under conditions representative of urban stormwater.

Polychlorinated biphenyls in stormwater sediments: Relationships with land use and particle characteristics.

Polychlorinated biphenyls (PCBs) are classified as persistent organic pollutants (POPs). Concentrations of 209 PCB congeners as well as profiles of the ten homologues were determined in stormwater sediments collected from various (primarily roadway) sites with different land use. The total PCB concentrations ranged from 8.3 to 57.4 ng/g dry weight (dw), with a mean value of 29.2 ng/g dw. PCB concentrations varied with nearby land use. Higher stormwater sediment PCB concentrations were found in dense urban areas (average: 39.8 ±â€¯10.5 ng/g) and residential areas (average: 35.3 ±â€¯6.2 ng/g) compared to highways passing through greenspace (average: 18.0 ±â€¯0.4 ng/g). The number of chlorines per biphenyl ranged from 3.63 to 5.39 and the toxic equivalency (TEQs) of the PCBs were between 1.5 and 18.0 pg/g at all sites. A non-Aroclor congener, PCB 11, was detected in all samples and was dominant at two sites. PCBs were sorbed to smaller stormwater particulate matter (≤75 µm) at higher concentrations compared to larger particles (>75 µm). PCB sorption tended to increase with the total organic carbon (TOC) of the particulate matter in the sediment samples. However, greater PCB mass (almost 80%) was present in the larger particles. Information on sediment PCB concentrations from different land uses, along with stormwater particulate matter data can allow the estimation of PCB loads and load reductions using stormwater control measures.

Ettringite and monosulfate formation to reduce alkalinity in reactions of alum-based water treatment residual with steel slag.

Aged steel slag has a potential use as a highly durable aggregate in roadway construction; however, its high capacity for creating alkaline leachates (pH > 12.4) poses a severe environmental risk. In batch and column leach tests, 10% alum-based water treatment residual (WTR) addition to aged steel slag resulted in a 67% decrease in acid neutralizing capacity of steel slag and leachate alkalinity, but this alkalinity mitigation effect was accompanied with markedly increases in dissolved Al concentrations in the leachates (<4.6 mM) compared to steel slag-only samples. Measurements of dissolved ions, saturation index evaluations, and results of geochemical modeling analysis indicated that ettringite and monosulfate formations were favored and that it is probably the responsible mechanism for the observed mitigation of alkalinity and Ca release under alkaline conditions.

Understanding urban stormwater denitrification in bioretention internal water storage zones.

Conventional free-draining bioretention systems promote nitrate production and continual leaching to receiving waters. In this study, laboratory tests demonstrated the efficacy of an internal water storage zone (IWSZ) to target nitrate removal via denitrification. Experimental results confirmed that the carbon substrate characteristics (Willow Oak woodchip media) and the hydraulic retention time of nitrified stormwater affected nitrate removal performance. A 2.6-day batch treatment time reduced 3.0 mg-N/L to <0.01 mg/L, corresponding to a first-order denitrification rate constant of 0.0011 min-1 . Under various flow conditions, the associated hydraulic retention time may be used as a predictive measurement of nitrate removal performance. Scanning electron microscopy and 16S rRNA analysis of the woodchips showed that biofilms were present that could be responsible for anaerobic lignocellulose degradation and denitrification. This knowledge, along with evaluation of the biofilm community composition, reinforced the notion of a heterogeneous structure due to nutrient availability and hydrodynamic conditions. PRACTITIONER POINTS: Denitrification can occur using woodchips in a bioretention internal water storage zone. The denitrification rate is slow and may be limited during field-scale applications. A woodchip pretreatment did not provide long-term enhancement to the denitrification rate. Denitrification bacteria were found in the internal water storage zone.

Ammonium Removal from Synthetic Stormwater using Clinoptilolite and Hydroaluminosilicate Columns.

Ammonium can enter stormwater control measures (SCMs) with the influent, but is also the intermediate product between organic nitrogen and nitrate, and it is important to retain and treat ammonium within the SCM. In this study the use of aluminosilicate aggregates (CA) and clinoptilolite zeolite (ZT) was investigated under SCM (column) conditions. ZT was found to have the highest capacity (0.45 mg -N/g ZT vis-à-vis 0.33 mg -N/g CA) at 2.5 mg NH4-N/L. The presence of Ca2+ and K+ was found to reduce the capacity of the media significantly. Increasing the contact time from 10 minutes to 47 minutes enhanced the removal efficiency of the system by 70% for CA and 23% for ZT, respectively. Finally, changes in the influent ammonium concentration resulted in successful removal during concentration increases, but desorption of ammonium for sudden concentration reduction. The use of ZT in media-based SCMs is recommended for ammonium removal.

Adsorption of Compounds that Mimic Urban Stormwater Dissolved Organic Nitrogen.

Stormwater runoff carrying nitrogen can accelerate eutrophication. Bioretention facilities are among low impact development systems which are commonly used to manage urban stormwater quality and quantity. They are, however, not designed to remove dissolved organic nitrogen (DON) and may become a net DON exporter. Adsorption of seven organic nitrogenous compounds onto several adsorbents was examined. Batch adsorption study revealed that coal activated carbon (AC) exhibited the best performance in adsorption of the selected organic nitrogenous compounds. The highest adsorption capacity of coal AC was 0.4 mg N/g for pyrrole at an equilibrium concentration of 0.02 mg N/L, while adsorption was not detectable for urea at the same equilibrium concentration. The fastest compound to reach equilibrium adsorption capacity onto the coal AC was pyrrole (1 hour). The adsorption capacity of the coal AC for pyrrole and N-acetyl-d-glucosamine and 1-hour contact time is recommended for designing bioretention systems targeting organic nitrogenous compounds.

A unified look at phosphorus treatment using bioretention.

Phosphorus (P) is a water pollutant of significant concern as it limits the productivity of most freshwater systems, which can undergo eutrophication under heavy phosphorus inputs. Bioretention has shown great potential for stormwater quantity and quality control. However, phosphorus removal has been inconsistent in bioretention systems, with phosphorus leaching observed in some systems. This paper examines P removal mechanisms and performance in bioretention systems through published data. A temporal concept of P fate for different time-scales is proposed. A model is developed to predict effluent P concentrations (Ce) based on a media equilibrium concentration (Ceq) assumption, which is suitable for both short and long-term simulation. Ceq is well correlated with Ce for P data for different time-scales. Ceq varies less in media containing Al and Fe than in typical bioretention soil media. Although significant change in Ceq may occur during an event and long-term, Ceq variation in short-term is small. During the event and short terms, for high-P containing media, the concentration relationship is Ceq > Ce > C0 (influent P concentrations); for low-P containing media, Ceq < Ce < C0. During long-term, as media equilibrates with the influent runoff, Ceq ≈ Ce ≈ C0. The model explains concentration changes of P with depth. With proper selection of media and amendments, Ceq can be driven towards zero for P in bioretention media.

Performance of a 'Transitioned' Infiltration Basin Part 2: Nitrogen and Phosphorus Removals.

Infiltration basins have been widely used for stormwater runoff management. However, their longevity could be compromised over time, up to the point of operational failure. This research study showed that a 'failed' infiltration basin can 'transition' into a wetpond/wetland-like practice and provide water quality benefits. Performance evaluation over three years showed that the transitioned infiltration basin reduced the discharge event mean concentrations of total phosphorus (TP), dissolved phosphorus (DP), particulate phosphorus (PP), NOx-N (nitrate+nitrite), total Kjeldahl nitrogen (TKN), organic-N (ON), and total nitrogen (TN) during most storm events. Exports of TP, DP, ON, and TKN masses were observed only during the coldest periods. The cumulative mass removals were 61% TP, 53% DP, 63% PP, 79% NOx-N, 51% TKN, 45% ON, and 64% TN. The dry-weather nutrient concentrations combined with the environmental conditions at the transitioned basin indicated that sedimentation, adsorption, denitrification, and volume reduction were the removal mechanisms.