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Literature Review

Because bioretention is a fairly new urban stormwater BMP, relatively few studies have been performed on it until recently. This section summarizes the current design standards from the Prince George’s County Bioretention Design Manual (Winogradoff and Coffman, 2002), and results from several other published studies.Davis (2001, 2003, 2005) conducted laboratory studies and a series of field studies of bioretention using synthetic stormwater runoff. More detailed studies are currently underway on a paired system of bioretention cells at the University of Maryland, but those results are not yet available (Davis, 2005).Yu (1999, 2001) has reported short-term studies on a bioretention area in Charlottesville, Virginia, and on a combined bioretention area and wet pond in Warrenton, Virginia. In North Carolina, Hunt and Jarrett (2004) monitored water quality from bioretention areas in Greensboro and Chapel Hill. In Pennsylvania,Emerson and Traver (2004) reported hydrology data from the Bio-Infiltration Traffic Island at Villanova University. Plans are underway to initiate water quality sampling at the same location (Emerson and Traver, 2004). Nordberg and Thorolfsson (2004) have reported on plans for a detailed study of the application of bioretention in cold climates.Rushton (2001) conducted a study of parking areas that involved the incorporation of grassed swales and vegetated areas that functioned similarly to bioretention. On a larger watershed scale, a study is continuing in Prince George’s County, Maryland, to evaluate the hydrologic and water quality benefits of applying Low Impact Development strategies, including bioretention, to an entire subdivision (Cheng et al, 2004). Two years of data are currently available from the paired watersheds in that study, in which an older residential development with conventional curb and gutter and lacking BMPs was compared to a newer watershed that was developed using LID techniques.Christianson, et al. (2004) have reported the development of a model of infiltration and pollutant removal mechanisms in bioretention areas using computer spreadsheets. Lastly, Pitt, et al. (2002) have reported on the effects of soil compaction on infiltration rates in urban areas, particularly with regard to the potential impacts on the effectiveness of an infiltration-based BMP such as bioretention.

Nitrogen Cycle

Several nitrogen species are commonly present in surface and groundwaters. A basic understanding of these nitrogen species and the transformations between them, shown in Figure 2.1, is necessary for any discussion of bioretention area nutrient removal.“Total nitrogen” is the sum of the reduced nitrogen species and the oxidized nitrogen species present in a water sample. The reduced nitrogen species include organic nitrogen and ammonia/ammonium. Of the oxidized nitrogen species, nitrite concentrations are usually insignificant compared to nitrate concentrations. Nitrogen gas is not part of the total nitrogen measurement. Thus, nitrogen “removal” from water generally involves the transformation of all other nitrogen forms into nitrogen gas, followed by the escape of that gas to the atmosphere (Metcalf and Eddy, 2003). Not shown in Figure 2.1, but significant to bioretention processes, is the fact that both organic nitrogen and ammonium can adsorb to organic matter and sediment particles, while nitrate and nitrite are primarily dissolved species. So organic nitrogen and ammonium may also be removed from a water stream through their incorporation with solids (Davis et al., 2001). A reduction in the concentration of a single nitrogen species does not necessarily equal a reduction in the total nitrogen concentration, but may only indicate a transformation to some other aqueous species. Consider the case of a stormwater containing mainly organic nitrogen. In order for this nitrogen to be removed from the water, three different transformations must occur: ammonification to convert it to ammonia, followed by nitrification to convert it to nitrite and then to nitrate, and finally denitrification to convert it back through nitrite to nitrogen gas. Some assimilation of these nitrogen forms into plant organic matter may also occur, and of course the harvesting of those plants would also remove nitrogen from the system. Nitrogen fixation occurs when there is a nitrogen deficit, a condition that is uncommon in stormwater. Nitrification requires an aerobic environment, while denitrification only occurs under anaerobic conditions, so the stormwater must pass through both micro-environments in order for the organic nitrogen to completely transform to nitrogen gas (Metcalf and Eddy, 2003).

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Bioretention Design Principles

The earliest set of design practices identified in this review of the literature is the Bioretention Manual was published by Prince George’s County, Maryland in 1993. It has since been modified and expanded in the most current version (2002), and may be accessed through the county website at www.goprincegeorgescounty.com. A brief summary of the relevant information from that document is included here in Section 2.2. Inclusion of an underlying sandbed was recommended in the 1993 guidelines, atmospheric N2 ammonia (NH3) / ammonium (NH4+) organic nitrogennitrite, NO2-nitrate, NO3- nitrogen fixation ammonification nitrification denitrification reduced nitrogen species, or Total Kjeldahl Nitrogen (TKN) oxidized nitrogen species assimilation  but this recommendation has been removed from the 2002 document. The design guidelines allow for a great deal of variation as bioretention features are altered to fit the characteristics of a specific site. It should be noted, however, that the variation in designs has resulted in variation in the pollutant removal efficiencies observed at different bioretention areas. This variation is especially evident in the cases when a single research team has studied multiple bioretention areas, as in Davis (2003) or Hunt and Jarrett (2004).

List of Figures 
List of Tables
Acknowledgements.
1 Introduction 
2 Literature Review
2.1 Nitrogen Cycle
2.2 Bioretention Design Principles
2.2.1 Size
2.2.2 Design of Components
2.2.3 Facility Maintenance
2.3 University of Maryland Studies
2.3.1 Metal Adsorption
2.3.2 Small and Large Box Experiments
2.3.3 Field Studies
2.3.4 Denitrification in Bioretention Areas
2.3.5 Current Work
2.4 University of Virginia Studies
2.5 North Carolina State University Studies
2.5.1 Greensboro Battleground Crossing Shopping Center
2.5.2 Chapel Hill University Mall
2.5.3 Comparison of Seasonal Flow Reductions
2.6 Villanova Bio-Infiltration Traffic Island
2.7 Filterra®, Concrete Bioretention in a Box
2.8 Trondheim, Norway
2.9 Florida Aquarium Study 
2.10 Somerset Heights Subdivision
2.11 Computer Model at Oklahoma State University
2.12 Effects of Soil Compaction on Infiltration Rates 
2.13 Summary of Pollutant Removal Results
3 Methods and Materials 
3.1 Site Description 
3.1.1 Soils
3.1.2 Plants
3.2 Hydrologic Measurements
3.3 Chemical Measurements
3.4 Statistics
3.4.1 Detection Limit
3.4.2 Confidence Limits on the Mean for a Normal Distribution
3.4.3 Confidence Limits on the Mean for a Lognormal Distribution
4 Results and Discussion 
4.1 Hydrology
4.1.1 Climate
4.1.2 Typical Hydrology
4.1.3 Measurement Issues
4.1.4 Water Balance.
4.2 Soil Chemistry 
4.2.1 Iron
4.3 Pollutant Concentrations and Loads
4.3.1 Load Calculation Methodology
4.3.2 Sensitivity Analysis
4.3.3 An Example: Pollutant Removal Calculations for Total Phosphorus
4.3.4 Solids and Turbidity
4.3.5 Nutrients
4.3.6 Metals
4.3.7 Organic Matter: COD and TOC
4.3.8 Other Measurements
4.3.9 Summary Results for All Pollutants
5 Summary and Conclusions 
6 References
Appendix A: Hydrographs 
Appendix B: Original Chemistry Data 
Appendix C: Water Chemistry Analysis 
Appendix D: Sensitivity Analysis 
Appendix E: Evapotranspiration 
Appendix F: Guide To Abbreviations
Vita 

 

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