Bioretention and Stormwater Research Publications

 Olszewski, J.M. and Davis, A.P. “Comparing the Hydrologic Performance of a Bioretention Cell with Pre-Development Values,” J. Irrig. Drain. Engg. Accepted for publication, July, 2012.






.       Jones, P.S. and Davis, A.P. “Spatial Accumulation and Strength of Affiliation of Heavy Metals in Bioretention Media,” J. Environ. Eng, ASCE, accepted for publication, July 2012.






    Sample, D.J., Grizzard, T.J., Sansalone, J., Davis, A.P., Roseen, R., Walker, J. “Assessing Performance of Manufactured Treatment Devices for the Removal of Phosphorus from Urban Stormwater,” J. Environ. Mgmt 113 279-291 (2012).

Abstract --Nutrients such as nitrogen and phosphorus in urban runoff can be controlled through a variety of nonstructural and structural controls commonly known as best management practices (BMPs). Manufactured treatment devices (MTDs) are structural BMPs that may be used in portions of a site, often when space is limited. MTDs use a variety of technologies to achieve potentially greater treatment efficiency while reducing spatial requirements. However, verifying the performance of MTDs is difficult because of the variability of runoff water quality, the variability in treatment technologies, and the lack of standardized protocols for field testing. Performance testing of MTDs has focused almost exclusively upon removal of sediment; however MTDs are now being applied to the task of removing other constituents of concern, including nutrients such as phosphorus. This paper reviews current methods of assessing treatment performance of MTDs and introduces the Virginia Technology Assessment Protocol (VTAP), a program developed to evaluate the removal of phosphorus by MTDs. The competing goals of various stakeholders were considered when developing the VTAP. A conceptual framework of the tradeoffs considered is presented; these tradeoffs require compromise among the competing interests in order that innovation proceeds and benefits accrue. The key strengths of VTAP are also presented and compared with other existing programs.





    “Geosynthetic Filters for Water Quality Improvement of Urban Stormwater Runoff,”

         Franks, C.A., Davis, A.P., and Aydilek, A.  J. Environ. Eng, ASCE, 138 (10) 1018-1028 (2012).

Abstract --Suspended solids in highway storm water runoff create a wide range of water quality problems; their removal lessens the deleterious impact of storm water runoff on aquatic ecosystems. In this study, three geotextiles were tested in a laboratory setup with influent
suspensions having a hydraulic loading rate, total suspended solids (TSS) concentration, and particle size distribution similar to those reported for urban highway storm water runoff. Results indicated that after a short ripening period, a geotextile with an apparent opening size of 150 μm can effectively remove suspended solids to below a target concentration of 30 mg=L. The results also showed that the change in hydraulic conductivity of the geotextile filter system can be related to suspended solids captured via a power function, which may be used to predict the flow rate through the filter during its life span. New retention criteria specifically for storm water filtration are introduced for geotextiles.





“Performance of Grass Swales for Improving Water Quality from Highway Runoff,”  

     Stagge, J.H., Davis, A.P., Jamil., E., and Kim, H. Water Research, 46(20), 6731-6742 (2012).

Abstract --The performance of grass swales for treating highway runoff was evaluated using an experimental design that allowed for influent and effluent flow and pollutant concentration measurements to be taken at specific intervals through each storm event. Two common swale design alternatives, pre-treatment grass filter strips and vegetated check dams, were compared during 45 storm events over 4.5 years. All swale alternatives significantly removed total suspended solids and all metals evaluated: lead, copper, zinc, and cadmium. The probability of instantaneous concentrations exceeding 30 mg/L TSS was decreased from 41e56% in the untreated runoff to 1e19% via swale treatment. Nutrient treatment was variable, with generally positive removal except for seasonal events with large pulses of release from the swales. Nitrite was the only consistently removed nutrient constituent. Chloride concentrations were higher in swale discharges in nearly every measurement, suggesting accumulation during the winter and release throughout the year. Sedimentation and filtration within the grass layer are the primary mechanisms of pollutant treatment; correspondingly, particles and particulate-bound pollutants show the greatest removal via swales. Inclusion of filter strips or check dams had minimal effects on water quality.






“Hydrologic Performance of Grass Swales for Managing Highway Runoff,”  

     Davis, A.P., Stagge, J.H., Jamil., E., and Kim, H. Water Research, 46(20), 6775-6786 (2012).

Abstract --The hydraulic performance of grass swales as a highway stormwater control measure was evaluated in a field-scale study adjacent to a Maryland highway. Two common swale design alternatives, pretreatment grass filter strips and vegetated check dams, were compared during 52 storm events over 4.5 years. Swale performance is described via three regimes, dependent on the relative size of the rainfall event. Overall, half of the events were small enough that the entire flow was stored, infiltrated, and evapotranspirated by the swales, resulting in no net swale discharge. Swales significantly reduced total volume and flow magnitudes generally during events with rainfall less than 3 cm. While the majority of improvement can be attributed to the swales, inclusion of check dams increases swale effectiveness. Pretreatment grass filter strips produced mixed effects. The swales demonstrated essentially no volumetric reduction during large storm events, functioning instead as conveyance, and smoothing fluctuations in flow.






“Meeting Hydrologic and Water Quality Goals through Targeted Bioretention Design,”      

      Hunt, W.F., Davis, A.P., and Traver, R.G., J. Environ. Eng, ASCE., 138(6), 698-707 (2012).

Abstract --Bioretention is one of the most commonly used stormwater control measures (SCMs) in North America and Australasia. However,

current design is not targeted to regulatory need, often reflecting an outdated understanding of how and why bioretention works. The purpose

of this manuscript is to synthesize research to recommend a suite of design standards focused on the purpose of bioretention SCM.

Both hydrologic (peak flow mitigation, infiltration, annual hydrology, and stream stability) and water quality [total suspended solids

(TSS) and particulates, pathogen-indicator species, metals, hydrocarbons, phosphorus, nitrogen, and temperature] regulatory and stream

ecology needs are addressed. Bioretention cells designed to meet a prioritized subset of those measures would be substantially different

than cells that are designed for a different subset of needs. Designers have the ability to adjust bowl volume, media composition, media

depth, underdrainage configuration, and vegetation type. This study examines how each of those design parameters can be adjusted such that

a one size fits all approach is no longer the norm






“Effects of Temperature on Bacterial Transport and Destruction in Bioretention Media,”  

     Zhang, L, Seagren, E.A., Davis, A.P., and Karns, J.S. Water Environ. Res, 84(6), 485-496 (2012).

Abstract --Microbial activities are significantly influenced by temperature. This study investigated the effects of temperature on the capture and destruction of bacteria from urban stormwater runoff in bioretention media using 2-year field evaluations coupled with controlled laboratory column studies. Field data from two bioretention cells show that the concentration of indicator bacteria (fecal coliforms and Escherichia coli) was reduced during most storm events, and that the probability of meeting specific water quality criteria in the discharge was increased. Indicator bacteria concentration in the input flow typically increased with higher daily temperature. Although bacterial removal efficiency was independent of temperature in the field and laboratory, column tests showed that bacterial decay coefficients in conventional bioretention media (CBM) increase exponentially with elevated temperature. Increases in levels of protozoa and heterotrophic bacteria associated with increasing temperature appear to contribute to faster die-off of trapped E. coli in CBM via predation and competition.





“Hydrologic Performance of Bioretention Stormwater Control Measures,” .     

        Davis, A.P., Traver, R.G., Hunt, W.F., Brown, R.A., Lee, R. and Olszewski, J.M.J. Hydrologic Eng, ASCE., 17(5), 604-614 (2012).

Abstract -- The transportation and urban infrastructure relies heavily on impervious surfaces. Unmitigated rainfall runoff from impervious

surfaces can lead to a myriad of environmental problems in downgradient areas. To address this issue, novel stormwater control measures

(SCMs) are being emphasized and implemented widely to mitigate some of the impacts of impervious surface. Bioretention is a soil/mediabased

SCM that is often used for this purpose, but current design practices are highly empirical. This study compiles work from three research

sites in three states to provide some fundamental underpinnings to bioretention design. Although all sites demonstrate different levels of

performance, water volumetric performance trends are common to all. These trends are based on the available storage in the bioretention cell,

termed herein as the Bioretention Abstraction Volume (BAV). The BAV is directly related to available media porosity and storage in the

surface bowl. A finite capacity to completely store all runoff from smaller events is defined by the BAV. Normalization for this storage

provides prediction for volumetric performance. Recommendations for bioretention design are provided.





 “Water Treatment Residual as a Bioretention Amendment for Phosphorus.  I.  Evaluation Studies,”

         O’Neill, S.W. and Davis, A.P.  J. Environ. Eng, ASCE. 138(3), 318-327 (2012).

Abstract -- Urban stormwater runoff has been implicated as a major source of excess nutrients to surface waters, contributing to the development

of eutrophic conditions. Bioretention, a promising technology for urban stormwater pollution treatment, was investigated to determine

whether an aluminum-based water treatment residual (WTR) amended bioretention soil media (BSM) could adsorb sufficient P at low concentrations

(120 μg P/L), extrapolated for a 20-year facility lifetime. Batch and minicolumn studies were employed to determine both an

optimal BSM mixture and media performance. Media tests demonstrated P adsorption proportional to WTR addition. Additional amendments

such as quartz sand and hardwood bark mulch produced further significant variations in adsorptive behavior. WTR-amended media

showed excellent P removal, meeting the developed benchmark for adsorptive behavior. Predominantly, media met minimal P adsorption

requirements when it contained at least 4 to 5% WTR by mass (air dried). The use of an oxalate-extractable aluminum-, iron-, and

phosphorus-based metric to predict media P adsorption capacity, the oxalate ratio, is proposed. A media oxalate ratio of 2040 is expected

to meet the P adsorption requirement.





  “Water Treatment Residual as a Bioretention Amendment for Phosphorus.  II.  Long-term Column Studies,”

         O’Neill, S.W. and Davis, A.P. J. Environ. Eng, ASCE. 138(3), 328-336 (2012).

  Abstract -- Bioretention is an EPA-designated best management practice developed to mitigate negative ecological effects from urban stormwater.

However, while these facilities perform well for the removal of a multitude of pollutants, in many cases they are ineffective in treating

excessive storm-water nutrients such as phosphorus (P) that may induce surface water eutrophication. This work builds on the results of a

previous paper, which describe initial studies on the use of aluminum-based water treatment residual (WTR) as a bioretention soil media

(BSM) amendment. A 5% WTR-, 3% triple-shredded hardwood bark mulch-amended loamy sand BSM was investigated in a large-scale

(0.9 m) column to determine the media P adsorption performance under varying hydrologic and pollutant concentration conditions. Results

indicate that the WTR-amended media adsorbed 88.5% of the applied P mass, relative to a non-WTR-amended control media for which

effluent P mass increased by 71.2%. The amended media consistently produced total phosphorus effluent event mean concentrations

< 25 μg/L and exhibited a maximum effluent concentration of only 70 μg/L. Media oxalate ratio (OR) correlated well with the observed

media P adsorption performance, providing additional support for the effectiveness of this metric in predicting media P adsorption capacity.







   “  "Influent Pollutant Concentrations as Predictors of Effluent Pollutant Concentrations for Mid-Atlantic Bioretention,”

.           McNett, J.K., Hunt, W.F. and Davis, A.P.   J.Environ. Eng, ASCE. 137(9), 790-799 (2011).

Abstract -- The water quality performance of Best Management Practices (BMPs) has been frequently assessed by the removal efficiency metric.  Recent findings show that the removal efficiency metric is flawed because it does not account for background water quality, eco-region differentiation, background or "irreducible" concentrations. Additionally, the removal efficiency metric inherently assumes a definite association exists between influent and effluent pollutant concentrations. Such a relationship between influent and effluent concentrations has been minimally studied for bioretention, the most common stormwater control measure associated with Low Impact Development (LID). This study analyzes influent and effluent TN and TP concentrations from 11 bioretention cells in the Mid- Atlantic United States. Pooled data showed only a slight association between influent and effluent TN.  Essentially no relationship exists between influent and effluent TP concentration. Both findings indicate that the percent removal metric is a faulty means of evaluating bioretention performance.  Twelve general linear models (GLMs) were created where influent TN and TP were the predictors of respective effluent TN and TP concentrations. Only one GLM was considered to be "good", defined as 67% to 90% of the variation in effluent concentrations being explained by respective influent concentrations (R2 = 0.72). In addition, there were 2 "fair" models, 5 "poor" models and 4 "very poor" models. No "very good" models were found for TN or TP. Furthermore, as influent nutrient concentration in runoff increases, the removal efficiency increases for TN and TP. "Dirtier" influent TP concentrations were effectively reduced; conversely, "cleaner" TP influent concentrations increased, both tending toward a (possibly media controlled) baseline effluent concentration (0.10 to 0.18 mg/l).  TN data also may have been tending toward a common concentration; however, the value was not as discernible.






“Long-term Sustainability of Escherichia coli Removal in Conventional Bioretention Media,”  

     Zhang, L, Seagren, E.A., Davis, A.P., and Karns, J.S. J. Environ. Eng., ASCE, 137(8), 669-677 (2011).

Abstract -- Bioretention has significant potential for reducing bacterial levels in urban stormwater discharge. The long-term performance of bacteria removal was evaluated using column studies over an 18 month period, during which synthetic urban stormwater runoff was loaded to conventional bioretention media (CBM) columns once every two weeks.  CBM initially achieved a mean of 72% removal efficiency for Escherichia coli O157:H7 strain B6914.  The removal efficiency improved over time, achieving 97% or higher efficiency after six months.  The trapped B6914 cells died off rapidly between runoff application events.  Mechanistic studies indicated that decreased porosity and increased hydrodynamic dispersion observed in mature CBM are favorable for improving the physical straining of cells and for bacterial adhesion.  The temporal change in surface charge on CBM may not be a key factor in the improved bacterial removal.  Indigenous protozoa in the CBM grew logistically, and may play an important role in the enhancement of bacterial capture and the rapid decline in the numbers of trapped bacteria via predation.  Overall, the long-term bacterial removal process in CBM can be efficient and sustainable.






“Unit Process Modeling of Stormwater Flow in a Bioretention Cell,”  

     He, Z. and Davis, A.P., J. Irrig. Drain. Engg, ASCE, 137(3), 701-714 (2011).

Abstract -- A two-dimensional variable saturated flow model was developed to simulate subsurface flow in bioretention facilities employing the Richards’ Equation.  Variable hydrologic performances of bioretention are evaluated using the underdrain outflow hydrographs, outflow volumes for ten storms with various duration and depth, and flow duration curves for 25 different storms. The effects of some important design parameters and elements are tested, including media type, surrounding soils, initial water content, ratio of drainage area to bioretention surface area, and ratio of cell length to width. Model results indicate that the outflow volume via underdrain is less than the inflow; the flow peak is significantly reduced and delayed.  Underdrain outflow volume from loamy sand media (with larger Ks) is larger than that from sandy clay loam media.  The saturated hydraulic conductivity, storage capacity and exfiltration into surrounding soils contribute to the hydrologic performance of a bioretention cell. Initial media storage capacity is affected by the hydraulic properties of media soils, initial water content, and bioretention surface area. The exfiltration volume is determined by the surrounding soil type and exfiltration area, dominated by flow through the bottom of the media.





“The Capture and Destruction of Escherichia coli from Simulated Urban Runoff Using Conventional Bioretention Media and Iron Oxide-Coated Sand,”  

     Zhang, L, Seagren, E.A., Davis, A.P., and Karns, J.S. Water Environ. Res., 82(8) 701-714 (2010).

Abstract -- The performance, sustainability, and mechanisms of bacterial removal from stormwater runoff by bioretention systems are poorly understood. The potential for removal of microorganisms in bioretention systems was evaluated using column studies and simulated urban stormwater runoff. Conventional bioretention media (CBM) removed 82% of Escherichia coli O157:H7 strain B6914 cells; iron-oxide coated sand (IOCS) significantly enhanced capture, with 99% efficiency.  This improvement possibly was because of the greater positive surface charge and roughness of the IOCS. Trapped strain B6914 cells decayed more rapidly in CBM, however, with more than 99.98% die-off within one week compared with the IOCS in which approximately 48% of trapped cells survived. Predation and competition from native microorganisms in CBM were verified to play a dominant role in rapid destruction of trapped strain B6914. In particular, protozoan grazing appeared to play an important role, with the die-off of trapped B6914 increasing with increasing concentrations of protozoa.





Thermal Reduction by an Underground Stormwater Detention System,”  

     Natarajan, P. and Davis, A.P. J. Environ. Eng., ASCE, 137(5) 520-526 (2010).

Abstract -- Increases in stream temperatures by heated storm-water runoff from impervious surfaces are a serious environmental problem.

An underground detention with slow-release facility is a versatile storm-water best management practice BMP for buffering high flows.

Temperature reductions in underground storm-water storage BMPs, however, have not been quantified. A field study on an underground

detention BMP located in Maryland was undertaken to characterize its effect on storm-water runoff temperatures. In colder months, when

the runoff temperature ranged from 5 to 15°C, small or no temperature change was observed. Runoff produced during summer storm

events, however, with event mean temperatures over 20°C, exhibited mean temperature reductions of 1.6°C through the BMP. While

statistically significant, the reductions were not sufficient to cool the summer runoff discharges below the Maryland Class III temperature

standard 20°C 100% of the time. The results indicate that underground facilities can moderate high runoff temperatures, but that more

efficient designs are needed for heat transfer.





Simulation of the Performance of a Stormwater BMP,”  

     Wild, T.B. and Davis, A.P. J. Environ. Eng., ASCE, 136(12) 1257-1267 (2009).

Abstract -- Vegetated storage-infiltration best management practices BMPs have become an increasingly popular means of attenuating

and treating runoff from developed land. However, the hydrologic and pollutant removal performances of these facilities can be highly

variable. A mathematical model of an idealized BMP was developed in order to quantify the impact of variable hydrologic and pollutant

concentration input on BMP performance by simulating the treatment performance of the model system during 1,250 non-steady-state

storm events generated based on historic Maryland rainfall data. The model BMP was effective in attenuating volume (42% total volume

reduction) and peak flow (median peak output to peak input flow ratio was 0.058). The simulated mean effluent pollutant event mean

concentration was much less than the influent (0.284 compared with 1.51 mg/L) and the overall mass load reduction was 92%. However,

the performance parameters demonstrated significant variability. Consequently, the results suggest a need to incorporate into BMP

performance guidelines the impact of the variable influent hydrologic and pollutant concentration characteristics. Emphasis should be

placed on discharge water quality and statistical distributions rather than on single-percent removal values.





 “Water Quality Improvement through Reductions of Pollutant Loads using Bioretention,”

     Li, H. and Davis, A.P.  J. Environ. Eng., ASCE, 135(8) 567-576 (2009)

Abstract -- As an increasingly adopted stormwater best management practice to remedy hydrology and water quality impairment from urban development, bioretention facilities need rigorous investigation to quantify performance benefits and to allow design improvements.  This study examines water quality improvements (total arsenic, total cadmium, chloride, total chromium, total and dissolved copper, E. Coli., fecal coliform, lead, mercury, nitrogen species, Oil & Grease, phosphorus, total organic carbon (TOC), total suspended solids, and total zinc) via monitoring for a 15-month period at two bioretention cells in Maryland.  Both bioretention cells effectively removed suspended solids, lead, and zinc from runoff through concentration reduction.  Runoff volume reduction promotes pollutant mass removal and links BMP water quality benefits with hydrologic performance.  From a load perspective (kg/ha-yr), all but TOC at one cell showed pollutant reduction.  Bioretention effluents exhibited good water quality for all significant pollutants except for nitrate, copper and phosphorus in one cell, the latter two of which may be attributed to media organic matter dissolution.  Copper dissolved/particulate analyses showed that significant changes in copper speciation behavior results from transport through the bioretention media.





Mitigation of Impervious Surface Hydrology using Bioretention in Maryland and North Carolina,” 

     Li, H., Sharkey, L., Hunt, W.F., and Davis, A.P. J. Hydrologic Engg, ASCE., 14(4) 407-415 (2009)

Abstract -- As an increasingly adopted stormwater best management practice to remedy hydrologic impairment from urban imperviousness, bioretention facilities need rigorous field performance research and monitoring to confirm performance and improve design and maintenance recommendations.  This study investigated hydrologic performance at six bioretention cells in Maryland (College Park (CP), a 181 m2 cell, 50-80 cm media depth, monitored for 22 events, and Silver Spring (SS), a 102 m2 cell, 90 cm media depth, monitored for 60 events) and North Carolina (Greensboro (G1 and G2), each approximately 317 m2, 120 cm media depth, both monitored for 46 events, and Louisburg, (L1-surface area of 162 m2, L2-surface area of 99 m2), each had 50-60 cm fill depths, monitored for 31 and 33 events, respectively) over 10- to 15-month periods.  Outflow from each cell was recorded and inflow was either recorded or calculated from rainfall data.  In Louisburg, L2 was lined with an impermeable membrane to eliminate exfiltration while L1 was unlined to allow both exfiltration and evapotranspiration.  Results indicate that bioretention facilities can achieve substantial hydrologic benefits through delaying and reducing peak flows and decreasing runoff volume.  A large cell media volume: drainage area ratio and adjustments to the drainage configuration appear to improve the performance. Media layer depth may be the primary design parameter controlling hydrologic performance.  Performance diminishes as rainfall depths increase and rainfall durations become longer. Annual water budget analysis suggests that approximately 20% to 50% of runoff entering the bioretention cells was lost to exfiltration and evapotranspiration.





 “Bioretention Technology: An Overview of Current Practice and Future Needs,”

     Davis, A.P., Hunt, W.F., Traver, R.G., and Clar, M. J. Environ. Eng, ASCE., 135(3) 109-117 (2009).

Abstract -- Bioretention, or variations such as bioinfiltration and rain gardens, has become one of the most frequently used stormwater management tools in urbanized watersheds.  Incorporating both filtration and infiltration, initial research into bioretention has shown that these facilities substantially reduce runoff volumes and peak flows.  Low Impact Development (LID), which has a goal of modifying post-development hydrology to more closely mimic pre-development, is a driver for the use of bioretention in many parts of the country.  Research over the past decade has shown that bioretention effluent loads are low for suspended solids, nutrients, hydrocarbons and heavy metals. Pollutant removal mechanisms include filtration, adsorption, and possibly biological treatment.  Limited research suggests that bioretention can effectively manage other pollutants, such as pathogenic bacteria and thermal pollution, as well. Reductions in pollutant load result from the combination of pollutant removal and runoff volume attenuation, linking water quality and hydrologic performance.  Nonetheless, many design questions persist for this practice, such as maximum pooling bowl depth, minimum fill media depth, fill media composition and configuration, underdrain configuration, pretreatment options, and vegetation selection. Moreover, the exact nature and impact of bioretention maintenance is still evolving, which will dictate long term performance and life cycle costs. Bioretention usage will grow as design guidance matures as a result of continued research and application.





 “Removal and Fate of Polycyclic Aromatic Hydrocarbon Pollutants in an Urban Stormwater Bioretention Facility"

     DiBlasi, C.J., Li, H., Davis, A.P., Ghosh, U. Environ. Sci. Technol., 43(2) 494-502 (2009).

Abstract -- This research investigated the removal and fate of sixteen USEPA priority pollutant polycyclic aromatic hydrocarbons (PAHs) from urban stormwater runoff through a bioretention cell.  Bioretention is an infiltration/filtration practice containing a mixed layer of about 90 cm of soil, sand, and organic matter, planted with appropriate vegetation. Field water quality monitoring, as well as bioretention media core analyses were performed.  The results indicate that bioretention is a promising management practice to control runoff PAH pollutants.  The PAH event mean concentration (EMC) reduction ranged from 31 to 99%, with a mean discharge EMC of 0.22 mg/L.  The mass load decreased from a mean value of 0.0180 kg/ha-yr to 0.0025 kg/ha-yr, suggesting an average PAH mass load reduction of 87% to the discharging watershed.  The most dominant PAH species monitored were fluoranthene and pyrene.  Influent PAHs indicated strong affiliation with runoff total suspended solids (TSS).  As such, PAH removal positively correlated with TSS removal.  Low rainfall depth was associated with high influent PAH concentration and resulted in favorable PAH removal.  Source investigation suggested that the PAHs measured in the monitored cell were from pyrogenic sources, likely resulting from vehicle combustion processes.   Sealers used in parking lots and driveway coatings were also a possible source of PAHs.  Media core analyses indicated that the intercepted PAH compounds transported only a few centimeters vertically in the soil near the runoff entrance location, suggesting that a shallow cell design may be adequate for systems focusing on PAH removal.




“Heavy Metal Capture and Accumulation in Bioretention Media,” 

     Li, H. and Davis, A.P. Environ. Sci. Technol., 42(14) 5247-5253 (2008).

Abstract -- Heavy metal capture and accumulation in bioretention media were investigated through the use of a one dimensional filtration equation for particulate metals, advection/dispersion/adsorption transport equations for dissolved metals, and sequential extractions.  Predicted spatial profiles and partitioning patterns of captured metals were compared to data derived from a bioretention cell in the District of Columbia, USA.  Zinc, lead, and copper profiles showed a high surface accumulation, significantly decreasing with the media depth.  Surface street particle-enriched areas had the highest heavy metal levels, demonstrating the close relationship between the capture of metals and runoff particles.  Sequential extractions suggested that most captured metals were of anthropogenic origin.  “Soluble-exchangeable” bound metals from the sequential extraction correlate well with predicted aqueous dissolved metals; the more strongly associated metal fractions correlated with modeled runoff and media particulate metals.  A simple risk evaluation indicates that lead is the limiting metal in bioretention accumulation.  Based on information collected in this study, a shallow bioretention cell design is suggested for systems with focus on metal capture.




Urban Particle Capture in Bioretention Media I: Laboratory and Field Studies,”

     Li, H. and Davis, A.P.  J. Environ. Eng., ASCE, 134(6) 409-418 (2008).

  Abstract -- Bioretention is a novel stormwater best management practice that uses a mixture of soil/sand/mulch as adsorptive filtration media that can capture both urban particulates and dissolved pollutants while promoting infiltration.  This study conducted a series of laboratory column experiments and field observations which showed that (1) bioretention media stratification occurs with runoff percolation due to particulate deposition, (2) bioretention filter media are clogging limited, instead of breakthrough limited, and (3) both depth filtration and cake filtration significantly contribute to urban particle capture.  Because of the fine size of bioretention media, incoming suspended solids cannot significantly penetrate below 5~10 cm of the media in the column tests and approximately 20 cm in the monitored field facility.  Bioretention filters under intermittent flow conditions exhibited higher solids loading capacity (in kg/m2) before clogging than under continuous flow conditions.  The clay components in incoming TSS assume critical responsibility for bioretention media clogging.   The media resistance due to solids deposition was estimated through Darcy’s law.  The hydraulic conductivity of two media types decreased from 54±23 and 72±46 cm/hr to less than 10 cm/hr due to particle capture.  Experimental results suggest that a 20-cm media depth is sufficient for bioretention design and maintenance procedures (media replacement) for runoff particles capture.






Urban Particle Capture in Bioretention Media II: Theory and Model Development,”

    Li, H. and Davis, A.P.  J. Environ. Eng., ASCE, 134(6) 419-432 (2008).

Abstract -- As a novel storm water best management practice, using a soil/sand/mulch mixture to capture urban pollutants while promoting infiltration, the unique media composition renders bioretention significantly different from conventional sand filters.  In this work, a three-layer model is presented to describe particulate capture in bioretention media employing parameter calibration and pre-sensitivity analysis.  Since the fine size of bioretention media strictly limits the particulate penetration distance, the media column is modeled as a pristine zone (bottom), a working zone (middle), and a cake zone (top).  Mechanisms of both depth filtration and cake filtration are examined through mass balances, which show that both are significant.  The developed resistance of each layer due to solid deposition was also estimated.


    Experimental data for different media/TSS type combinations of selected experimental trials were used in parameter calibration.  The calibrated model successfully predicted the effluent TSS and media hydraulic conductivity of subsequent trials with appropriate boundary and initial conditions as input.  A weighted combination of calibrated parameters from different TSS types also agreed well with media behavior for treating a complex TSS mixture.  The results of media replacement (top-removal and refill) simulation also reasonably fit experimental data.  Using proper assumptions, a long term scenario analysis for permeability reduction was performed for a field bioretention facility.  Based on modeling results, this study recommends a shallow bioretention media depth design, an annual or biannual field inspection schedule, and periodic media replacement maintenance.




“Field Performance of Bioretention:  Hydrology Impacts,” 

  Davis, A.P. J. Hydrology, ASCE., 13(2), 90-95 (2008)

Abstract -- Flows into and out of two bioretention facilities constructed on the University of Maryland campus were monitored for nearly two years, covering 49 runoff events.  The two parallel cells capture and treat stormwater runoff from a 0.24 ha section of an asphalt surface parking lot.  The primary objective of this work was to quantify the reduction of hydrologic volume and flow peaks and delay in peak timing via bioretention.  Overall, results indicate that bioretention can be effective for minimizing hydrologic impacts of development on surrounding water resources.  Eighteen percent of the monitored events were small enough so that the bioretention media captured the entire inflow volume and no outflow was observed.   Underdrain flow continued for many hours at very low flow rates.  Mean peak reductions of 49 and 58% were noted for the two cells.  Flow peaks were significantly delayed as well, usually by a factor of two or more.  Using simple parameters to compare volume, peak flow, and peak delay to values expected for undeveloped lands, it was found that probabilities for bioretention Cell A to meet or exceed volume, peak flow, and peak delay hydrologic performance criteria were 55%, 30%, and 38%, respectively.  The probabilities were 62%, 42%, and 31%, respectively, for Cell B





“Field Performance of Bioretention:  Water Quality,”

  Davis, A.P.,  Environ. Engg. Sci., 24(8), 1048-1063 (2007)

Abstract -- Two bioretention facilities with different designs were installed on the University of Ma ry land campus and were monitored from Summer 2003 through Fall 2004 to quantify water quality improvements to parking lot stormwater runoff.  One cell was a standard design and the other had an anoxic sump.  Twelve inflow/outflow water quality data sets were successfully collected and analyzed for Total Suspended Solids (TSS), phosphorus, and zinc.   Nine sets were collected for copper and lead, and three for nitrate.  In 2 of the events, all of the runoff flow was attenuated by the bioretention media and no flow exited the cells, resulting in zero pollutant discharge.  In all cases, the median pollutant output is lower than the input, indicating successful water quality improvement through the bioretention media.  Statistically insignificant differences were noted between the two cells for all pollutants examined.  Median values for effluent event mean concentrations and percent removals based on combined data sets (both cells) were TSS, 17 mg/L and 47% TP, 0.18 mg/L and 76%, copper, 0.004 mg/L and 57%, lead, 0.004 mg/L and 83%, zinc, 0.053 mg/L and 62%, and 0.02 mg-N/L and 83% removal of nitrate (based on limited data).  Mass removals were higher than those based on concentrations due to flow attenuation.  These values are in reasonable agreement with those previously published from bioretention field and laboratory studies. 





Nitrogen Removal from Removal from Urban Stormwater Runoff through Layered Bioretention Columns,” 

  Hsieh, C.-h., Davis, A.P. and Needelman, B.A., Water Environ. Res., 79(12),2404-2411 (2007)  

Abstract -- Bioretention is a low-impact technology used for the treatment of stormwater runoff in developed areas. The fates of mineral nitrogen compounds in two bioretention columns (RP1 and RP2) with different media-layering characteristics were investigated under multiple loadings of simulated urban runoff. The immediate capture of nitrogen was evaluated, with nitrogen transformation reactions that occurred during the drying periods between rainfall events. A greater proportion of ammonium was removed from runoff in RP2 (68 ± 16%), which had a high permeability layer over a lower permeability layer, than in RP1 (12 ± 6%), which had the inverse configuration. Both column systems demonstrated nitrate export (9 ± 32% and 54 ± 22% greater than input for RP1 and RP2, respectively), attributed to washout of nitrate resulting from nitrification processes between runoff loading events. Bioretention media with a less permeable bottom soil layer could form an anoxic/anaerobic zone for promoting nitrification/denitrification processes.






“Pollutant Mass Flushing Characteristics of Highway Stormwater Runoff from an Ultra Urban Area,”

   Flint , K. and Davis, A.P. J. Environ. Eng., ASCE, 133(6), 616-626 (2007)

Abstract --  Water quality of highway stormwater runoff from an ultra-urban area was characterized by determining the event mean concentration (EMC) for several pollutants and by evaluating pollutant flushing.  Thirty-two storm events were monitored between June 2002 and October 2003.  Mean EMCs in mg/L were 0.035, 0.11, 0.22, 1.18, 420, 3.4, 0.14, 1.0, and 0.56 for Cd, Cu, Pb, Zn, TSS (total suspended solids), TKN (total Kjeldahl nitrogen), NO2-N, NO3-N, and TP.  First flush as defined by flushing of 50% of the total pollutant mass load in the first 25% of the event runoff volume occurred in 33% of the storm events for NO2-, 27% for TP, 22% for NO3- and TKN, 21% for Cu, 17% for TSS, 14% for Zn, and 13% for Pb.  Median values for the mass flushed in the first 25% of runoff volume were greater than the mass flushed in any 25% portion beyond the first for all pollutants.  The mass in later 25% volume portions were greater than in the first 25% volume in at least 17% of the events for all pollutants, indicating that a significant amount of the pollutant load can be contained in later portions of the runoff volume.  Nonetheless, management of the first 1.3 cm (1/2 in) of runoff was able to capture 81-86% of the total pollutant mass.






“Heavy Metal Fates in Laboratory Bioretention Systems,” 

   Sun, X. and Davis , A.P., Chemosphere, 66(9), 1601-1609 (2007).

Abstract --  Key to managing heavy metals in bioretention is to understand their fates in bioretention facilities.  In this study, pot prototypes filled with bioretention media were built to simulate the conditions of natural growth of plants.  Synthetic runoff with different heavy metal loadings (copper, cadmium, lead, and zinc) was periodically applied.  Metal accumulations in tissues of grasses - Panicum virgatum, Kentucky-31, and Bromus ciliatus, were investigated after 230 days of growth and multiple runoff treatment events.  After 183 d of periodic runoff application, the concentrations of Zn, Cu, Pb and Cd with low and high loadings had the same trends in the plant tissues, Zn > Cu > Pb > Cd, following the trend of the input metal concentrations.  The fates of input metals were 88 to 97% captured in soil media, 2.0 to 11.6% not captured by bioretention media, and 0.5 to 3.3% accumulated in plants.  Compared to the metals retained by the soil, the percentages of input metals taken up by plants were relatively low due to the low plant biomass produced in this study.  Greater biomass density would be required for the vegetation to have a valuable impact in prolonging the lifetime of a bioretention cell.  






“Bioretention Column Studies of Phosphorus Removal from Urban Stormwater Runoff,” 

   Hsieh, C.-h., Davis, A.P. and Needelman, B.A., Water Environ. Res., 79(2), 177-184 (2007).

Abstract -- This study investigated the effectiveness of bioretention as a stormwater management practice using repetitive bioretention columns for phosphorus removal. Bioretention media, with a higher short-term phosphorus sorption capacity, retained more phosphorus from infiltrating runoff after 3 mg/L phosphorus loading. A surface mulch layer prevented clogging after repetitive total suspended solids input. Evidence suggests that long-term phosphorus reactions will regenerate active short-term phosphorus adsorption sites. A high hydraulic conductivity media overlaying one with low hydraulic conductivity resulted in a higher runoff infiltration rate, from 0.51 to 0.16 cm/min at a fixed 15-cm head, and was more efficient in phosphorus removal (85% mass removal) than a profile with low conductivity media over high (63% mass removal). Media extractions suggest that most of the retained phosphorus in the media layers is available for vegetative uptake and that environmental risk thresholds were not exceeded.






 “Water Quality Improvement through Bioretention Media: Nitrogen and Phosphorus Removal,” 

   Davis, A.P., Shokouhian, M., Sharma, H., and Minami, C.Water Environ. Res., 78(3), 284-293 (2006).

Abstract -- High nutrient inputs and eutrophication continue to be one of the highest priority water quality problems. Bioretention is a Low Impact Development technology that has been advocated for use in urban and other developed areas. This work provides an in-depth analysis on removal of nutrients from a synthetic storm water runoff by bioretention. Results have indicated good removal of phosphorus (70-85%) and TKN (55-65%). Nitrate reduction was poor (<20%) and in several cases, nitrate production was noted. Variations in flow rate (intensity) and duration had a moderate affect on nutrient removal. Mass balances demonstrate the importance of water attenuation in the facility in reducing mass nutrient loads. Captured nitrogen can be converted to nitrate between storm events and subsequently washed from the system. Analysis on the fate of nutrients in bioretention suggests that accumulation of phosphorus and nitrogen may be controlled by carefully managing growing and harvesting of vegetation.






 “Sustainable Oil and Grease Removal from Synthetic Storm Water Runoff Using Bench-Scale Bioretention Studies,” 

Hong, E., Seagren, E.A., and Davis, A.P., Water Environ. Res., 78(2), 141-155 (2006).

Abstract -- One of the principal components of the contaminant load in urban stormwater runoff is oil and grease (O&G) pollution, resulting from vehicle emissions. A mulch layer was used as a contaminant trap to remove O&G (dissolved and particulate naphthalene, dissolved toluene, and dissolved motor oil hydrocarbons) from a synthetic runoff during a bench-scale infiltration study. Approximately 80~95% removal of all contaminants from synthetic runoff via sorption and filtration. Subsequently, approximately 90% of the sorbed naphthalene, toluene, oil, and particulate-associated naphthalene was biodegraded within about 3, 4, 8, and 2 days after the event, respectively, based on decreases in contaminant concentrations coupled with increases of microbial populations. These results indicate the effectiveness and sustainability of placing a thin layer of mulch on the surface of a bioretention facility for reducing O&G pollution from urban stormwater runoff.





  “Green Engineering for Land Development: Low Impact Development,” 

    Davis, A.P., Environ. Sci Technol. 39(16), 338A-344A (2005)

How do we accommodate the needs of a growing population yet minimize negative impacts on the environment and local ecology? Low-impact development (LID) integrates environmental concerns with land development, focusing on water and pollutant balances. Also known by other names, such as environmentally sensitive design, LID represents a fundamental change in the way residential, commercial, and institutional properties are developed. Allen P. Davis at the University of Maryland explains the benefits and drawbacks of this concept.






“Evaluation and Optimization of Bioretention Media for Treatment of Urban Storm Water Runoff,” 

  Hsieh, C.-h and Davis, A.P.J. Environ. Eng., ASCE, 131(11), 1521-1531 (2005).

Abstract -- Bioretention is a relatively new urban storm water best management practice.  The objective of this study is to provide insight on media characteristics that control bioretention water management behavior.  Eighteen bioretention columns and six existing bioretention facilities were evaluated employing synthetic runoff.  In columns, the runoff infiltration rate through different media mixtures ranged from 0.28 to 8.15 cm/min at a fixed 15 cm head.  For pollutant removals, the results showed excellent removal for oil/grease (> 96%).  Total lead removal (from 66 to > 98%) decreased when the total suspended solids level in the effluent increased (removal from 29 to > 96%).  The removal efficiency of total phosphorus ranged widely (4 to 99%), apparently due to preferential flow patterns, and both nitrate and ammonium were moderate to poorly removed, with removals ranging from 1 to 43% and 2 to 49%, respectively.  Two more on-site experiments were conducted during a rainfall event to compare with laboratory investigation.  For bioretention design, two media design profiles are proposed; > 96% TSS, > 96% O/G, > 98% lead, > 70% TP, > 9% nitrate and > 20% ammonium removals are expected with these designs.






“Multiple-Event Study of Bioretention for Treatment of Urban Storm Water Runoff,” 

  Hsieh, C.-h and Davis, A.P.Water Sci. Technol., 51(3-4), 177-181 (2005).

Abstract -- Bioretention is a novel best management practice for urban storm water, employed to minimize the impact of urban runoff during storm events. Bioretention consists of porous media layers that can remove pollutants from infiltrating runoff via mechanisms that include adsorption, precipitation, and filtration.  However, the effectiveness of bioretention in treating repetitive inputs of runoff has not been investigated. In this study, a bioretention test column was set up and experiments proceeded once every week for a total of 12 tests. Through all 12 repetitions, the infiltration rate remained constant (0.35 cm/min). All 12 tests demonstrated excellent removal efficiency for TSS, oil/grease, and lead (99%). For total phosphorus, the removal efficiency was about 47% for the first test, increasing to 68% by the twelfth test. For ammonium, the system removal efficiency ranged from 2.3% to 23%. Effluent nitrate concentration became higher than the influent concentration during the first 28 days and removal efficiency ranged from 9% to 20% afterward.  Some degree of denitrification was apparently proceeding in the bioretention system. Overall, the top mulch layer filtered most of TSS in the runoff and prevented the bioretention media from clogging during 12 repetitions. Runoff quality was improved by the bioretention column.






"Engineered Bioretention for Removal of Nitrate from Stormwater Runoff," 

  Kim, H., Seagren, E.A., and Davis, A.P. Water Environ. Res., 75(4), 355-367 (2003).

Abstract -- A bioretention unit is a simple, plant- and soil-based, low impact treatment and infiltration facility for treating storm water runoff in developed areas. Nitrate, however, is not attenuated in conventional bioretention facilities. Thus, this study systematically evaluated a reengineered concept of bioretention for nitrate removal via microbial denitrification, which incorporates a continuously submerged anoxic zone with an overdrain. Experimental studies were performed in four phases. In the first two phases, column studies demonstrated that, overall, newspaper is the best solid-phase electron-donor substrate for denitrification out of the set studied (alfalfa, leaf mulch compost, newspaper, sawdust, wheat straw, wood chips, and elemental sulfur) based on superior nitrate removal and effluent water quality. The nitrate loading and hydraulic loading studies in the second phase provided design information. In the third phase, system viability after 30- and 84-day dormant periods was evaluated in column studies, demonstrating that newspaper-supported biological denitrification should be effective under conditions of intermittent loadings. Finally, in the fourth phase, pilot-scale bioretention studies demonstrated the effectiveness of the proposed design, showing nitrate plus nitrite mass removals of up to 80%. These results indicate that engineered bioretention for the removal of nitrogen from storm water runoff has the potential for successful application as an urban storm water treatment practice. 






"Water Quality Improvement through Bioretention: Lead, Copper, and Zinc,"  

Davis, A.P., Shokouhian, M., Sharma, H., Minami, C., and Winogradoff, D.  Water Environ. Res., 75(1), 73-82 (2003).

Abstract -- Intensive automobile use, weathering of building materials, and atmospheric deposition contribute lead, copper, zinc, and other heavy metals to urban and roadway runoff. Bioretention is a low impact- development best management practice that has the potential to improve storm water quality from developed areas. The practice represents a soil, sand, organic matter, and vegetation-based storage and infiltration facility used in parking lots and on individual lots to treat runoff. Investigations using pilot-plant laboratory bioretention systems and two existing bioretention facilities documented their effectiveness at removing low levels of lead, copper, and zinc from synthetic storm water runoff. Removal rates of these metals (based on concentration and total mass) were excellent, reaching close to 100% for all metals under most conditions, with effluent copper and lead levels mostly less than 5 ug/L and zinc less than 25 ug/L. Somewhat less removal was noted for shallow bioretention depths. Runoff pH, duration, intensity, and pollutant concentrations were varied, and all had minimal effect on removal. The two field investigations generally supported the laboratory studies. Overall, excellent removal of dissolved heavy metals can be expected through bioretention infiltration. Although the accumulation of metals is a concern, buildup problems are not anticipated for more than 15 years because of the low metal concentrations expected in runoff.





 “Loadings of Lead, Copper, Cadmium, and Zinc in Urban Runoff from Specific Sources,” 

   Davis, A.P., Shokouhian, M., and Ni, S., Chemosphere, 44(5), 997-1009, (2001).

Abstract --  Urban stormwater runoff is being recognized as a substantial source of pollutants to receiving waters.  A number of investigators have found significant levels of metals in runoff from urban areas, especially in highway runoff.  As an initiatory study, this work estimates lead, copper, cadmium, and zinc loadings from various sources in a developed area utilizing information available in the literature, in conjunction with controlled experimental and sampling investigations.  Specific sources examined include building siding and roofs; automobile brakes, tires, and oil leakage; and wet and dry atmospheric deposition.  Important sources identified are building siding for all four metals, vehicle brake emissions for copper and tire wear for zinc.  Atmospheric deposition is an important source for cadmium, copper, and lead.  Loadings and source distributions depend on building and automobile density assumptions and the type of materials present in the area examined.  Identified important sources are targeted for future comprehensive mechanistic studies.  Improved information on the metal release and distributions from the specific sources, along with detailed characterization of watershed areas will allow refinements in the predictions.   





"Laboratory Study of Biological Retention for Urban Storm Water Management" 

   Davis, A.P., Shokouhian, M., Sharma, H. and Minami, C., Water Environ. Res., 73(1), 5-14 (2001).

Abstract -- Urban storm water runoff contains a broad range of pollutants which become transported into natural water systems. A practice known as bioretention has been suggested to manage storm water runoff from small developed areas. Bioretention facilities consist of porous soil, a topping layer of common hardwood mulch, and establishment of a variety of different plant species.


A detailed study of the characteristics and performance of bioretention systems for the removal of several heavy metals (Cu, Pb, Zn) and nutrients (P, TKN, NH4+-N, NO3--N) from a synthetic urban storm water runoff was completed using batch and column adsorption studies, along with pilot scale laboratory systems. The roles of the soil, mulch, and plants in the removal of heavy metals and nutrients were evaluated to estimate the treatment capacity of laboratory bioretention systems. Reductions in concentrations of all metals was excellent (>90%) with specific metal removals of 15 to 145 mg/m2 per event. Moderate reductions of TKN, ammonium, and phosphorus levels were found (60-80%). Little nitrate was removed and nitrate production was noted in several cases. The importance of the mulch layer in metal removal was identified. Overall results support the use of bioretention as a storm water best management practice and advocate the need for further research and development.






“Evaluation of Lead Concentration in Runoff from Painted Structures” 

   Davis, A.P. and Burns, M., Water Res., 33(13), 2949-2958 (1999).  

Abstract --  Urban stormwater runoff is considered to be a major input source of heavy metals to surface waterways.  In this study, lead runoff from painted structures in an urban setting was assessed.  In many cases, high lead concentrations were found.  Lead concentrations (100 mL over 1600 cm2) from 169 different structures followed the order (geometric mean, median, Q10-Q90): wood (40, 49, 2.6-380 ug/L) > brick (22, 16, 3.3-240 ug/L) > block (9.7, 8.0, <2-110 ug/L).  Lead concentration depended strongly on paint age and condition.  Lead levels from washes of older paints were much higher than from freshly painted surfaces, which were demonstrated quantitatively as:  paint age [>10 yrs] (77, 88, 6.9-590 ug/L) >> [5-10 yrs] (22, 16, <2-240 ug/L) > [0-5 yrs] (8.4, 8.1, <2-64 ug/L).  Lead from surface washes was found to be 70% or greater in particulate lead form, suggesting the release of lead pigments from weathered paints.  High intensity washes were found to liberate more particulate lead than lower intensities.  Old surface paints can contribute high masses of lead into a watershed, targeting these structures for source preventive actions to curtail future lead input into the environment. 





Stormwater Management for Smart Growth

Allen P. Davis and Richard H. McCuen.  Springer, 375 pages, published August 2005


1 Introduction

2 Water Quality Parameters

3 Statistical Methods for Data Analysis

4 Stormwater Hydrology

5 Introduction to Modeling

6 Stormwater Quality

7 Improvement of Stormwater Quality

8 Storage and Flow Control

9 Vegetative Control Methods

10 Traps, Basins, and Filters

11 Wetlands

12 Low Impact Development