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Bioretention Technology: Overview of Current Practice and Future Needs
Allen P. Davis1; William F. Hunt2; Robert G. Traver3; and Michael Clar4
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Abstract: Bioretention, or variations such as bioinltration and rain gardens, has become one of the most frequently used storm-water management tools in urbanized watersheds. Incorporating both ltration and inltration, initial research into bioretention has shown that these facilities substantially reduce runoff volumes and peak ows. Low impact development, which has a goal of modifying postdevelopment hydrology to more closely mimic predevelopment, is a driver for the use of bioretention in many parts of the country. Research over the past decade has shown that bioretention efuent loads are low for suspended solids, nutrients, hydrocarbons, and heavy metals. Pollutant removal mechanisms include ltration, 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 ll media depth, ll media composition and conguration, underdrain conguration, 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.
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CE Database subject headings: Sustainable development; Filtration; Biological treatment; Hydrology; Water quality; Stormwater management; Best Management Practice.
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Introduction
Since its initial development and trial applications over a decade ago, the bioretention system, also referred to as raingardens, bioinltration, and other names, has rapidly become one of the most versatile and widely used storm water best management practices BMPs throughout the United States and many parts of the world. It has recently become identied as a preferred site practice for green building design and leadership in energy and environmental design LEED certication. General features of a bioretention system include 0.7 1 m of a sand/soil/organic media for treating inltrating storm-water runoff, a surface mulch layer, various forms of vegetation, orientation to allow 15 30 cm of runoff pooling and associated appurtenances for inlet, outlet, and overow. Fig. 1 shows a diagram of a typical bioretention system. Several bioretention installations in the mid-Atlantic region are presented in Fig. 2.
Professor, Dept. of Civil and Environmental Engineering, Univ. of Maryland, College Park, MD 20742-3021. E-mail: [e-mail] 2 Assistant Professor and Extension Specialist, Dept. of Biological and Agricultural Engineering, North Carolina State Univ., Raleigh, NC 27695-7625 corresponding author . E-mail: [e-mail] 3 Professor, Dept. of Civil and Environmental Engineering, Villanova Univ., Villanova, PA 19085. E-mail: [e-mail] 4 Principal Engineer, Ecosite, Inc., 6470 Dobbin Rd., Suite F, Columbia, MD 21045. E-mail: [e-mail] Note. Discussion open until August 1, 2009. Separate discussions must be submitted for individual papers. The manuscript for this paper was submitted for review and possible publication on June 18, 2008; approved on October 28, 2008. This paper is part of the Journal of Environmental Engineering, Vol. 135, No. 3, March 1, 2009. ASCE, ISSN 0733-9372/2009/3-1XXXX/$25.00.
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Despite the rapid acceptance of this BMP throughout the United States, detailed performance information and related design guidance are not currently available for many regions. Several state and local governments have adopted bioretention guidelines published by another state agency, often without modifying the guidelines for local conditions, using out-ofdate information, or without a good understanding of the range or limitations of these BMPs. In addition, variations of the original design concept have been developed to promote inltration Heasom et al. 2006 or specic nutrient reduction Kim et al. 2003; Hunt et al. 2006 . After several years of research and installation experience, an improved understanding of bioretention performance is now available. This paper summarizes the state of current knowledge of bioretention in addressing both hydrologic and water quality environmental issues. Both eld demonstration and laboratory mechanistic results are available. Continued research will allow greater renement of bioretention design criteria and encourage wider use of this technology to address many urban runoff challenges.
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Design Issues
A wide range of design issues are associated with the use of the bioretention BMP. Some of these issues stem from the fact that the bioretention system is a new breed of BMP that integrates knowledge from a number of disciplines including engineering, hydrology and hydraulics, surface and groundwater ow, soil science, horticulture, and landscape architecture. Consequently, development of guidelines for this technology requires a team approach. A second set of issues arises from the fact that biore-
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Fig. 1. Diagram of bioretention facility
tention technology is often applied as part of the low impact development LID approach to storm-water management. This approach is still relatively new and gives rise to numerous questions related to design objectives and procedures, and performance metrics. Some of the design issues or elements associated with the application of bioretention technology include the following: 1. Design objectives; Base ow, groundwater recharge; Pollution prevention and removal; Channel protection erosion control ; and Peak ow reduction; 2. Drainage area and BMP location guidelines; 3. Pretreatment requirements; 4. Treatment processes detention, retention, inltration, evapotranspiration, ltration, adsorption, precipitation, biodegradation, phytoremediation ; 5. Sizing, ponding depth; 6. Soil/lter media composition; 7. Media depth; 8. Vegetation; 9. Retention vis--vis inltration; 10. Underdrains; 11. Overow design; 12. Time of concentration issues; 13. Computational methods and models; 14. Pollutant accumulations and fates; and 15. Maintenance/service life/inspection.
Design Objectives
The design objectives for any BMP will have a strong inuence on the type and extent of information required for the design of the BMP. Many storm water practitioners perceive small microscale landscape-based practices such as the bioretention BMP to be primarily or, if not exclusively, a water quality management practice. However, it is now clear that bioretention can be used to achieve a wide range of storm-water management objectives including: 1 maintenance of groundwater recharge and base ow; 2 surface and groundwater pollutant removal; 3 channel protection; and 4 peak ow reduction. Base Flow and Groundwater Recharge Maintaining predevelopment groundwater recharge functions is one of the design objectives of the LID approach to storm-water management. Unfortunately, today only a small number of states and local jurisdictions have requirements and design criteria for
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Fig. 2. Photos of bioretention in: a North Carolina; b Maryland; and c Pennsylvania
maintaining groundwater recharge. Maintaining groundwater recharge is not an issue that is exclusive to bioretention design, but rather it is an emerging issue in storm-water management. It is important to note that the inltration bioretention BMP can be used to recharge or restore both the base ow and groundwater components of the hydrologic cycle. The design criteria for the use of bioretention BMPs to maintain base ow and groundwater recharge are not currently well dened. The design elements for which design criteria need to be standardized include: 1 the volume of annual runoff required to maintain predevelopment recharge; 2 how the bioretention system should be designed to optimize recharge; and 3 what limitations, if any, pollutant inputs and the native subsoils place on the ability of the bioretention system to meet recharge requirements. The components of the pollutant input to the BMP deter-
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Table 1. Summary of TSS Removal from Select Field and Laboratory Studies Inuent conc. mg/L Efuent conc. mg/L 18 13 20 1.36E + 07 mga 107 6 Load reduction % 59 54 n/ab 97% 99% n/ab
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Study Davis 2007 Davis 2007 Hunt et al. 2008 UNHSC 2006 USEPA 2006 Hsieh and Davis 2005a
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Site name Haddam, Conn. Greensboro, N.C. Chapel Hill, N.C. Charlotte, N.C. Louisburg, N.C. Durham, N.H.a Pilot boxes Laboratory columns a NO3 N. b n / a = not available.
College Park, Md. 34 College Park, Md. 34 Charlotte, N.C. 49.5 Durham, N.H. Villanova, Pa. 1.21E + 09 mga Laboratory columns 150 a Total mass October 3 December 5 . b n / a = not available.
mines whether inltration is an acceptable practice. Current EPA policy encourages for bioretention inltration in residential areas USEPA Memorandum June 11, 2008 . For industrial and commercial sites, pretreatment or noninltration bioretention BMPs may be required to protect the groundwater. In areas where groundwater recharge is critical, novel designs can be incorporated that would have the subsurface base of the bioretention system wider than the surface. Watershed studies in Pennsylvania have related recharge to preconstruction recharge and water consumption, and show that capturing as little as 1.25 2.5 cm 0.5 1.0 in. of runoff from each storm exceeds this goal Delaware County Planning Commission 2005 . Inltration rates from a bioinltration device were monitored for 4 years in Villanova, Pa. Emerson and Traver 2008 . The rate of inltration did not degrade, supporting the idea that bioretention cells can maintain inltration rates for at least several years. Emerson and Traver did nd that seasonality existed in inltration rate, with reduced inltration occurring in colder months. Inltration and evapotranspiration ET processes are important in the functioning of bioretention systems. To date, however, only limited information has been published related to how these processes function within a bioretention system, or how they can be optimized for hydrologic benet and pollutant removal. A eld study by Sharkey 2006 showed ET accounting for the fate of 2530% of all inow water on an annual basis in Louisburg, N.C. Inltration and evapotranspiration together can account for the fate of 5090% of inow, depending on in situ soil type, media depth and type, and drainage conguration Heasom et al. 2006; Hunt et al. 2006 .
Table 2. Summary of TN Removal from Select Field and Laboratory Studies Inuent conc. mg/L 1.2 1.35 1.68 1.70 1.66.0 2.1 Efuent conc. mg/L 0.81.0 4.38 1.14 1.25 1.12.8 0.13 Load reduction % 32.0 40 40 n/ab 65% 97% 3099% n/ab
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Water Quality Improvement A number of outstanding issues and questions are related to water quality and treatment. These include: 1 what size rainfall event should be captured by the BMP?; 2 how should the treatment efciencies be computed?; and 3 can bioretention design be targeted or optimized for the removal of specic pollutants i.e., sediment, metals, nutrients, pathogens ? One of the most common goals of storm-water management includes the reduction and removal of pollutants from the runoff. Many state programs specify that a certain volume of runoff 1.8, 2.5, 5.1 cm, equivalent to 1 / 2, 1, 2 in. be captured and treated. If these volume requirements are met, it is assumed that the associated water quality criteria have been met. Urban runoff measurements have supported the concept of a rst ush and initial volume control for several common pollutants Sansalone and Christina 2004; Ermilio and Traver 2006; Flint and Davis 2007 . Other state programs use a combination of volume control and reported BMP removal rates, for which the data base is very scarce at best, and develop estimates of pollutant removal for individual or groups of pollutants MDE 2000; PaDEP 2006; NC DENR 2007 . Still other practitioners advocate the use of continuous rainfall records to model the volume of ow through a specic structure and subsequently assume pollutant removals based on the total percentage of ow through the BMP. The data base on the pollutant removal performance of bioretention systems is currently small, but is rapidly growing. Performance results from both laboratory and eld studies are summarized in Tables 14 for several pollutants. Overall perfor-
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Table 3. Summary of TP Removal from Select Field and Laboratory Studies Inuent conc. mg/L Efuent conc. mg/L 0.15 0.17 0.0580.060 0.56 0.13 0.18 4.23E + 06 mga 0.060.15 0.462.9 0.05 1.6 Load reduction % 79 77 111 240 65 n/ab 69 28.1% 52 to 99% n/ab 63 to 85%
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Study Davis 2007 Davis 2007 Dietz and Clausen 2006 Hunt et al. 2006 Hunt et al. 2006 Hunt et al. 2008 Sharkey 2006 USEPA 2006 Davis et al. 2006 Hsieh and Davis 2005a Hsieh et al. 2007
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College Park, Md. 0.61 College Park, Md. 0.61 Haddam, Conn. 0.0120.019 Greensboro, N.C. 0.11 Chapel Hill, N.C. Charlotte, N.C. 0.19 Louisburg, N.C. 0.29 Villanova, Pa. 5.89E + 06 mga Pilot boxes 0.280.88 Laboratory columns 3 Laboratory columns 3 a Total mass October 3December 5 b n / a = not available.
mance results are promising and suggest that bioretention systems have the potential to one be of the most effective BMPs in pollutant removal. Nonetheless, reporting of water quality efciency for bioretention and other storm-water BMPs can be problematic because of the variability in conditions experienced during runoff events. While percent or fractional concentration removals may be satisfying and easy to report, they can be misleading in some cases. Specically, when the inuent water quality is relatively good and pollutant concentrations are low, the efuent discharge may not differ greatly from the input. The discharge water quality is good, but the fractional removal may be low. BMP performance should not be interpreted as unsatisfactory in this case. Additionally, pollutant loads are calculated as the product of concentration and ow. Therefore, both ow attenuation and pollutant treatment and removal are important to load reduction. Accordingly, runoff events that have very high or very low ows will deliver different mass loads and should not be treated equally. The link between runoff volume capture and quality performance is strong, and small storm capture is extremely effective. The Villanova bioinltration BMP is designed to capture 2.5 cm 1 in. of runoff, and has been shown to remove over 80% of the annual watershed rainfall input to the surface waters over multiple years Ermelio and Traver 2006; USEPA 2006 . The efciency of pollutant load reduction is similar to that of total runoff captured. Higher removals can be manifest for events in which a rst ush occurs. Suspended solids. Total suspended solids TSS removal via
Table 4. Summary of Zn Removal from Select Field and Laboratory Studies Inuent conc. g/L Efuent conc. g/L 48 44 2.45E + 07 g 17 25
College Park, Md. 107 College Park, Md. 107 Villanova, Pa. 9.43E + 07 gb Charlotte, N.C. 72 Durham, N.H. Pilot boxes 2601290 a One outlier point sequestered. b Total mass October 3December 5
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bioretention appears to be very efcient. In some cases, newer facilities will leak TSS, but this appears to be from an initial washout of nes from the media Hsieh and Davis 2005b . Longer-term data from mature facilities indicate that sedimentation and ltration through the media are very efcient. A comprehensive set of eld tests at the University of New Hampshire Stormwater Center UNHSC documents 97% TSS removal through bioretention UNHSC 2006 . These results are duplicated at the Villanova site USEPA 2006 . A Maryland eld study of two cells has documented 54 and 59% mass removals of TSS Davis 2007 . Preliminary work on ultimate fates of captured solids and clogging issues has been recently presented Li and Davis 2008b,c . Nitrogen and phosphorus. For nutrients, the results have been variable, likely because of the complexity of the chemistry of these species. In some instances very good removal has been documented, but in others, the treatment efciency has been low. Nutrient pollution removal is complicated by possible leaching by the soil and vegetation within the BMP. Laboratory bioretention box studies have shown 7085% phosphorus removal Davis et al. 2006 . Column and grab eld studies have shown phosphorus removal to be the most variable of six pollutants examined Hsieh and Davis 2005a . Field results in Maryland have shown 7779% phosphorus mass removal Davis 2007 . It appears that the phosphorus content of the soil used in the original bioretention media is critical to phosphorus removal performance. The media used in bioretention studies in North Carolina have demonstrated ranges of performance for phosphorus, from removal up
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to 65% to the addition of 240% at various eld sites Hunt et al. 2006 . The variation was dependent upon initial levels of soil phosphorus. Nitrogen behavior is complex because of the biogeochemical complexity of the nitrogen species. Ammonia a cation capture is somewhat variable, with removal likely related to the cation exchange capacity CEC of the media Davis et al. 2001 . Organic nitrogen is captured well, apparently by the organic material in the media. Total Kjeldahl nitrogen removal in box studies was good, at 5565% Davis et al. 2006 . Nitrate, as an anion, is not held by soil media and is generally quite mobile in soil/water systems It appears, however, that biological nitrication and denitrication processes can take place in bioretention media, depending on design and operating conditions. Captured ammonia and organic nitrogen can be nitried to nitrate during interevent periods, resulting in excess nitrate washout during subsequent events. Then again, if the bioretention media is allowed to remain saturated for a signicant period, either naturally or through an engineering modication of the design, some denitrication of the nitrate is possible Kim et al. 2003; Dietz and Clausen 2006; Hunt et al. 2006, Hsieh et al. 2007 . This concept of controlling redox conditions, organic content, and retention time could prove very benecial in areas where nutrient pollution is a major concern, and it deserves more investigation. Field monitoring from the University of New Hampshire has demonstrated 44% nitrate removal through bioretention UNHSC 2006 . Multiyear eld studies in North Carolina show TN mass removal ranging from 33 to 66% Hunt et al. 2006; Sharkey 2006; Hunt et al. 2008 . These results are again duplicated at the Villanova eld site with a nitrate removal of 46% USEPA 2006 . Heavy metals. Both dissolved and particulate-bound metals appear to be very efciently removed by bioretention. Laboratory and eld data are available for copper, lead, and zinc, and some cadmium Davis et al. 2003; Davis 2007 . Total metal concentrations exiting bioretention facilities are generally in the low g / L ppb levels. Dietz and Clausen 2005 found heavy metal concentrations from their rain garden to be very low, but the input concentrations were also low. Zinc removal at the UNH site was reported at 99% UNHSC 2006 , and 74% at the Villanova site USEPA 2006 . Through both ltration of particulate metals and adsorption of dissolved forms, most of the metal removal appears to occur in the upper surface layers of the media Li and Davis 2008a . Oil and grease. Used motor oil was completely removed from storm-water runoff 96% using bioretention columns with several mixes of media Hsieh and Davis 2005a,b . Laboratory studies have indicated that motor oil and two petroleum hydrocarbons, specically, toluene and naphthalene, can be readily sorbed from incoming simulated storm-water ows by a layer of composted leaf mulch Hong et al. 2006 . More importantly, native bacteria in the mulch can biodegrade the captured hydrocarbons in a few days, indicating that the mulch layer may be a sustainable hydrocarbon pollutant management resource in bioretention. Hydrocarbon removal is corroborated by the New Hampshire eld data, which indicate 99% removal of total petroleum hydrocarbons-diesel UNHSC 2006 . Chlorides. Bioretention facilities may be exposed to very high episodic chloride loads resulting from road deicing operations. Care is recommended when measuring total dissolved solids TDS in the early spring or after snow fall in northern climates as the presence of chlorides may dominate, masking other TDS pro-
cesses and interactions. Increasing evidence indicates chloride leaching from bioretention facilities year round. Pathogenic bacteria. Particularly in coastal areas, pathogenic bacteria are a major water quality concern. Conceptually, bioretention should remove most species of bacteria due to its design to collect and lter water, and then dry out, exposing bacteria to dry conditions and sunlight. Initial studies in Charlotte, N.C., show signicant reduction of indicator species. Fecal coliform and E.coli removal rates were approximately 70% Hunt et al. 2008 . A recent laboratory study found very high fecal coliform removal rates, with a mean of 91.6% less fecal coliform concentrations leaving than were applied to the column Rusciano and Obropta 2007 . More studies and long-term performance data are needed. To our knowledge many gaps still remain related to how best to optimize bioretention designs to enhance pollutant removal. Additional research, monitoring, and testing are clearly needed to supplement the existing data base and to develop predictive methods for various design strategies.
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Unit Processes Employed by Bioretention
Perhaps the main reason bioretention cells are more effective relative to other BMPs in improving water quality is their employment of several pollutant removal processes, including sedimentation, ltration, chemical sorption, biological activity nitrication and denitrication , and heat transfer. While current design standards do not specically focus on employing the unit processes independently, nearly all design standards do incorporate these unit processes. Future design considerations may look to optimize certain embedded pollutant removal processes. Channel Protection Erosion Control, Flood Control, and Hydrologic Modication
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Current issues associated with the use of bioretention systems for channel protection include the following questions: 1 can bioretention BMPs be used to effectively provide channel protection; 2 how much control is required and what are the computational methods to be used; and 3 how close can bioretention implementation get to reproducing predevelopment hydrology? Many storm water practitioners perceive bioretention to be primarily a water quality BMP and do not value the ability of bioretention systems to be effective with respect to peak discharge control and providing management for channel protection. While the database related to this level of control is also very small, the computational methods and recent examples clearly indicate that bioretention systems used within an LID design framework can be effective in controlling peak discharge rates and providing channel protection Clar 2003; PGCo 2001 . Field hydrologic data from the UNH Center support peak ow control. An average peak reduction of 85% is reported from an underdrained bioretention site treating a large parking area UNHSC 2006 . Median eld peak reductions of 40 and 48% were noted from two underdrained University of Maryland sites treating mostly roadway and parking lot runoff Davis 2008 . A lag of 615 min between the center of mass of the bioretention output ow volume compared to the input has been reported UNHSC 2006 . Field data from Villanova demonstrate that inltration for larger storms occurs continuously throughout the event. This site designed for 2.5 cm 1 in. of runoff rarely experiences over-
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ow until rainfall levels exceed 5.7 cm 2.25 in. Ermelio and Traver 2006 . Additionally, peak ows from storms up to 3.8 cm 1.5 in. have been reduced by over 99% by cells in North Carolina Hunt et al. 2008 . The ability of bioretention to mitigate peak ow is highly dependent upon the soil inltration rate and capture volume. Field data, physical testing, and mathematical modeling of BMPs similar to bioretention, namely an ecology ditch and a partial exltration reactor, have quantied ow peak reductions of up to 90% and peak delays up to 125 min Barber et al. 2003; Sansalone and Teng 2004; 2005 . The incorporation of shallow bioretention areas into residential lot developments has been shown using HEC-HMS modeling to assist in producing hydrologic conditions close to those of predevelopment Williams and Wise 2006 . Greater benets were noted for smaller storm events and results were found to be very sensitive to antecedent moisture conditions. Hydrologic improvements are smallest for large events, high antecedent water contents, and colder months. The highest performance measured at the Villanova sites is the late summer and fall Heasom et al. 2006 . A recent study by Johnston and Staeheli 2006 documented the potential of LID designs to provide ood control benets. Unlike detention, when viewed on a watershed scale, these BMPs if located across the watershed reduce the cumulative volume of downstream ow. The use of bioretention facilities can provide considerable benets with respect to managing the time of concentration and the corresponding peak discharge values. A time of concentration value in the range of 5 10 min would be calculated for a parking lot 0.2 0.4 ha 0.5 1.0 acre in size draining directly to a storm drain inlet. In the contrast, placement of a bioretention facility in front of the inlet will increase the time of concentration for the runoff to reach the inlet by several hours, depending on the ow rates through the treatment media Heasom et al. 2006; Hunt et al. 2008; Davis et al. 2008 . In addition to reducing ow peaks and increasing times of concentration, bioretention cells modify the water balance, decreasing direct discharge by increasing evapotranspiration and exltration. Outow volumes were shown to be less than 50% those of inuent runoff on an annual basis Hunt et al. 2006 . Li et al. 2008 showed that inuent to efuent volume ratios over a 24 h period ranged from 0.60 to less than 0.10 for six underdrained cells in Maryland and North Carolina. A ratio of 0.10 indicates that 90% of water entering the facility did not leave by way of underdrains or overow during the 24-h period during and immediately following a storm event. Larger soil media:drainage area ratios appear to reduce the runoff volume discharge Li et al. 2008 .
Villanova University has incorporated this concept into their building plans due to the savings in piping and inlets. Recently, however, the denition and drainage area criteria associated with distributed storm-water management and LID technology have become topics of debate. The minimum and maximum drainage area guidelines for LID practices are being challenged. An example of this can be found in design criteria developed by the state of New Jersey, which allows a contributing drainage area of 8.1 ha 20 acres . Yet, bioretention siting is limited by seasonally high water table elevations. In most cases bioretention cells located in small watersheds do not have excessively high water tables. BMPs treating larger watersheds, however, are necessarily lower in the landscape and tend to be closer to the water table. Drainage area guidelines clearly are a function of regional rainfall patterns, amount of impervious surface, and usage of the contributing drainage area. Facilities should be installed to ensure that soil pore spaces of the bioretention BMP will empty within 72 96 h for the design rainfall. The original bioretention designs offered minimum sizing criteria for a number of deign parameters, including the width, length, and depth, based on theoretical considerations Clar and Green 1993 . As implementation has expanded, design criteria include ponding storage volume, ponding plus media storage, and drainage area:bioretention area ratios. With more than 10 years of design experience, eld monitoring and research, existing sizing procedures and criteria should be reevaluated and updated. Sizing criteria may depend on the inltration characteristics of the media employed in the facility, ood mitigation, and pollutant removal needs. Siting of bioretention requires consideration of the soil type and geology of the site. If good draining soils and nonhazardous runoff is anticipated, then inltration-style bioretention is recommended. To maximize recharge and water quality performance, facilities should be located to capture runoff directly from impervious areas. If inltration is considered, the site selection should include soil inltration testing. Use of a bioretention cell as a retrot in a brown eld redevelopment will require that the cell perimeter be impermeably lined. Pretreatment
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Design, Sizing, and Siting Considerations
Drainage Area and BMP Location Guidelines The bioretention BMP concept was originally developed as a water quality control measure for small development sites in the range of 0.4 1.2 ha 1 3 acres . Consequently, the guidance developed by Prince Georges County Maryland recommended that the contributing drainage area be kept to 0.81 ha 2.0 acres or less. Many other jurisdictions have followed this guidance. The use of small drainage areas and microscale BMPs which can be integrated into a sites landscape elements is a fundamental concept of distributed storm-water management technology, and if planned within the minor drainage system will reduce costs.
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The requirements and criteria for the use of pretreatment measures for bioretention systems is another area of technical debate. The requirement for pretreatment measures such as 3 6 m 10 20 ft wide grass lter strips can use up as much surface area as the actual bioretention facility, and may often preclude the use of this BMP. Some states, such as North Carolina, require 1 1.5 m 3 5 ft wide sod lter strips, swales, or small forebays Hunt and Lord 2006 . At the same time there are numerous examples of bioretention systems that have been constructed without pretreatment devices and that function very effectively Heasom et al. 2006 . For these facilities, the surface mulch layer acts as the pretreatment, and may need to be removed and replaced on a periodic basis. The need for, as well as, the type and size of pretreatment requirements for bioretention systems need to be analyzed and updated. Clearly, stable or otherwise clean drainage areas e.g., rooftops with limited sediment and gross solid input require less pretreatment as compared to relatively unstable watersheds, such as those with active ongoing construction. Additionally, distributed water input such as sheet ow into
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the bioretention cell warrants less pretreatment, such as a velocity stilling zone, than when discrete concentrated inow points are designed in the system. Ponding Depth
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As previously discussed, the bioretention BMP was initially developed as a landscape-based water quality control measure for small sites. Since the goal was to integrate this BMP into the landscape using site features, such as landscape islands, the initial concept for bioretention design envisioned a very shallow ponding of 0.15 m 6 in. Clar and Green 1993 . This ponding depth was based on the use of a soil media with inltration rates of 1.3 5.1 cm/ h 0.5 2.0 in./ h and a subsoil inltration rate of 0.5 cm/ h 0.2 in./ h . The goal was to have the entire storm storage volume inltrated to a depth of 60 cm 24 in. in 48 h. Subsequent design manuals have modied the allowable ponding depth MDE 2000; DNREC 2005 . The Maryland 2000 Stormwater Management Design Manual MDE 2000 allows a ponding depth of 30 cm 12 in. and the use of bioretention as either a lter or an inltration device. In order to be designed as an inltration device, the subsoil inltration rate must be at least 1.3 cm/ h 0.5 in./ h . Sites with subsoils less than 1.3 cm/ h are considered to be lter BMPs and are required to use an underdrain system which provides positive drainage to a dened outfall point. It should also be noted that the Maryland design manual requires that bowl storage be provided for 70% of the total design volume to the BMP. The Delaware Green Technologies Design Manual and Model DNREC 2005 provide design guidance for bioretention systems and allow a maximum ponding depth of 45 cm 18 in. . It should be noted that the hydraulic conductivity of ll media is such that water empties the storage bowl at a rate of at least 2.5 cm/ h 1 in./ h . Provided the ll media does not clog, deeper ponding depths, such as up to 0.45 m 18 in. , may be reasonable. Ponding depth criteria should at a minimum consider the following design elements: 1. The characteristics of the inow hydrograph to the BMP, including the ow rate, storm duration, and the total volume; 2. The long-term annual runoff volumetric percent capture; 3. The surface storage ponding volume available in the BMP; 4. The inltration rate of the soils/lter media; 5. The voids storage space in the soils/lter media; 6. The inltration rate of the subsoil; and 7. Anticipated maintenance schedule. As deeper pond depths are allowed, the need for bioretention maintenance increases. Bioretention cells in practice have been observed to clog usually slowly . Clogging coupled with a high ponding depth may lead to hazards, including areas that promote mosquito breeding. It should be noted that the Villanova bioinltration site has operated successfully with no measurable degradation of performance for 7 years, with an inltration rate of between 0.64 and 1.3 cm/ h 0.25 0.5 in./ h , and is close to 51 cm 20 in. deep Ermilio and Traver 2006 . Soil/Filter Media Composition and Depth The initial bioretention design specications envisioned the use of natural soils with high permeability Clar and Green 1993 . Three soil textural classications were specied which included: loamy sand f = 5.1 cm/ h 2.0 in./ h , sandy loam f = 2.5 cm/ h 1.0 in./ h , and loam f = 1.3 cm/ h 0.5 in./ h . These specications are still being used by many jurisdictions. Some problems,
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however, have been reported with the use of the loam textural classication because it can have a clay content exceeding 30% by volume, which can lead to failure of the system. As a response to this problem, several alternatives have been recommended, usually specifying soil/media mixes with high inltration rates. For example, Prince Georges County Maryland has developed a recommended mix consisting of 50% sand, 30% topsoil, and 20% well-aged organic material such as pine nes, or composted leaf mulch. This mixture seems to support adequate inltration rates, but increases the cost of the media. The state of Delaware recommends a mix consisting of 1 / 3 sand, 1 / 3 peat moss, and 1 / 3 double-shredded mulch DNREC 2005 . This mix also appears to work well, but further raises the cost of the mix. In North Carolina, the media specied is 8588% sand; 812% nes clay+ silt ; and 35% organic material Hunt and Lord 2006 . The costs of this lter media are moderate and vary locally by proximity to media source. Water quality column studies by Hsieh and Davis 2005a,b have shown that bioretention media characteristics do not appear to play a signicant role in the removal of particulates, particulate-bound material metals and some phosphorus , and oil/grease. The bioretention media, even media with a high inltration rate, readily lters and removes all of these pollutants. Treatment of nitrogen and dissolved phosphorus species, however, appears to be much more sensitive to media characteristics. Field studies by Hunt et al. 2006 and Sharkey 2006 conrm that proper media selection is critical for effective phosphorus removal. A low phosphorus index, relatively high CEC media is required in phosphorus-sensitive watersheds throughout North Carolina Hunt and Lord 2006 . While it is expected that dissolved metal capture is media sensitive, no eld studies are available at this time to verify this and most soils have signicant metal adsorption capacity. Media layering may provide water quality benets, albeit at greater costs. In order to obtain tighter ranges for pollutant removals, soil/media specications will likely have to be more specic. Initially a 1.2 m 4 ft media depth was recommended to provide an adequate amount of soil for tree and shrub roots to expand Clar and Green 1993 . However, as plant specication has become more rened, vegetation is chosen that can survive and ourish in shallower media systems. Media depth selection may also depend upon desired pollutant removal. Davis et al. 2003 have shown high metal removal within the top 20 cm of media. Field studies in Louisburg, N.C. Sharkey 2006 show N and P removal rates exceeding 60% for bioretention cells with media depths of 0.75 m 2.5 ft . New guidance used in North Carolina provides media depth selection per pollutant type and has begun to be adopted by localities in several states Hunt and Lord 2006 . The media volume to drainage area ratio appears to be an important factor controlling runoff mitigation reduction of outow . Studies by Jones and Hunt 2008 and Li et al. 2008 both show that increasing the volume of media relative to the drainage area yield less frequent and reduced volumes of outow. Bioretention cells with what would be considered oversized media volumes by current design standards have eliminated outow from up to 87% of storms that produced inow in clayey soil portions of Maryland and North Carolina. Selection of a suitable soil mix and depth includes consideration of a number of different objectives and parameters which include the following: 1. Ability to support and sustain the selected vegetation; 2. Ability to dewater the ponded water in 24 h;
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Ability to remove the projected hydrologic and pollutant load; 4. Life cycle and durability of the media; and 5. Media cost. An urgent need exists for science-based analyses of the advantages and disadvantages of different media materials, mixes, and layerings with respect to meeting these design objectives. Underdrains and Overow Design
Maintenance/Inspection
Inspection and maintenance requirements for bioretention continue to be developed. Hunt and Lord 2006 discuss principal bioretention inspection and maintenance activities and their frequency. Much bioretention maintenance is aesthetic in nature removing trash, pruning, adding mulch, and mowing are examples . Other maintenance activities are hydrologic performance based, such as removing debris from the overow inlet and occasionally removing the mulch layer and top 2.5 5 cm 1 2 in. of media to maintain required bioretention inltration rates. Removal of deposited sediment at and near inlets is necessary so the inlet ow characteristics are not compromised. If pretreatment is part of the bioretention design, some maintenance tasks can be isolated to certain portions of the bioretention cell such as a stilling zone . Laboratory and eld studies indicate that sediment and heavy metals accumulate only in the top 5 10 cm of bioretention media Li and Davis 2008b,c , so that removal and replacement of surface layers may also revitalize water quality performance. The exact type and kinds of maintenance are very much dependent on the catchment use and stability and the presence of pretreatment.
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Vegetation
The use of underdrains is another area of technical debate among bioretention designers. Some practitioners and local jurisdictions perceive bioretention facilities strictly as ltering devices and mandate the use of underdrains in all designs. This approach fails to recognize that bioretention was initially conceived as an inltration BMP Clar and Green 1993 , as well as recent reports on the performance of the inltration bioretention system at Villanova University Heasom et al. 2006; Ermilio and Traver 2006 . Bioretention cells with underdrains have been shown to inltrate water, even in clayey soils Sharkey 2006 . Facilities with internal storage zones IWZ that create a sump in the bottom of the bioretention cell and underdrains may allow for a substantial amount of inltration. Some of the technical details that need to be resolved include the following: When should underdrains be used?; What are the minimum soil inltration criteria that determine the use of underdrains?; Should underdrains be enveloped by geotextiles?; At what depth should IWZs be designed?; and Do underdrains affect the area of the bioretention footprint? Bioretention systems can be designed as either on- or off-line facilities. In both instances care must be exercised in evaluating overow conditions and ow paths associated with large storm events. It is important that stable outlets be provided for these facilities.
The engineering benets of bioretention vegetation have not been well quantied. Theoretically, the plants will promote short- and long-term bioretention performance in a number of ways. From a hydrologic perspective, roots should help to promote media permeability. Surface vegetation can be strategically employed to divert and slow surface ow and to lter sediments. Roots should support microbiological populations that may be benecial to pollutant degradation and water quality. Phytoremediation processes may prove benecial in the breakdown of carbon- and nutrientbased pollutants and in the uptake of nonbiodegradable pollutants such as metals Dietz and Clausen 2006 . A study by Lucas and Greenway 2008 did show that the presence of vegetation in bioretention mesocosms improved nutrient removal when compared to mesocosms void of vegetation. The use of grass as the sole vegetation type in bioretention has recently become an issue among many developers. Grassed bioretention beds typically require shallower media depths, making these bioretention systems considerably less expensive. Concerns with using grass as the sole vegetated cover include long-term permeability, soil compaction, and potential reduction in pollutant removal abilities. However, a eld study in Graham, N.C., shows N and P removal from a pair of grassed bioretention cells to be comparable to that of conventional bioretention systems comprised of trees, shrubs, and mulch Passeport et al. 2008 .
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Summary and Conclusions
In the past 15 years, bioretention has become one of the most popular storm-water BMPs in the United States and is now a key component of the LID storm-water management philosophy. Recent research and monitoring studies highlight the ability of bioretention facilities to reduce ood peaks, runoff volumes, and pollutant loads, while increasing runoff lag times, groundwater inltration, and evapotranspiration. Nonetheless, this technology is still immature and additional research is needed to provide quantitative design and performance information. Emphasis areas for research include the following: Fill media composition; Fill media depth and conguration; Drainage conguration specically underdrain layout and the creation of IWZs ; Basin geometry such as perimeter area to surface area ratio ; Maximum bowl ponding depths; Vegetation selection; Maintenance recommendations and their relationship to pretreatment; and Determining costs and benets of alternative bioretention designs. By addressing these questions with focus on hydrology impacts and pollutant removal, more efcient design guidelines can be developed with respect to water quality, water quantity, and life cycle costs. Research investigating other water quality parameters such as temperature attenuation and specic pollutants e.g., pathogenic bacteria, polycyclic aromatic hydrocarbons is necessary to evaluate, improve, and model bioretention performance. Performance metrics to match LID goals may need updating. Construction and maintenance guidelines will also evolve. Bioretention holds much promise as a storm-water measure for use in sustainable development.
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