“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., accepted for publication, July 2008.

“Heavy Metal Capture and Accumulation in Bioretention Media,” 

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

“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