Biofilms can be simply defined as communities of microbes attached to a surface. There are various types of biofilm, which can be either beneficial or harmful to an ecosystem. Good biofilm offers valuable services to society or in the function of natural ecosystems such as those that contribute to controlled bioremediation of ground water and soils in Low Impact Development approaches called bioretention cell. This thesis researched ways to model biofilm activity at the field-scale and used experimental data (BOD5 and NO3-) to verify these models. Two mathematical models are presented in this work. The first model provides and tests the solution of substrate and biomass concentration while the second model modified the expression for the substrate flux into the biofilm. They are analyzed using a sensitivity analysis and their performance is compared using field-scale data. The solution for concentration is computed with some selected values of dimensionless biofilm thickness (0.0375 and 3.75) and dimensionless substrate concentration outside of the biofilm (0.005 to 0.5), which shows these two variables significantly affect model results. The simulations illustrate that biofilm activity mostly occurs in the summer while the substrate flux is normally stable at similar levels in the same season. / Graduate

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The performance of an urban stormwater bioretention management practice in the Kingsbrooke Subdivision of Prince William County, Virginia was examined over a one-year period. Bioretention is a relatively new urban stormwater best management practice (BMP) intended to mimic the pollutant-removal characteristics of an upland forest habitat. Typical bioretention areas utilize shallow ponding and highly-infiltrative sandy soils to treat the stormwater runoff from small commercial or residential drainage sites. The Kingsbrooke bioretention area was found to be atypical in several ways, including its relatively large, 14 acre, drainage area and the high clay content of its topsoil. Hydrologic and chemical data were collected by Virginia Tech staff for a total of 8 months in 2003 and 2004. Analysis of pollutant loading data was complicated by the presence of three unmeasured water flows: overland inflow bypassing the inflow gage, and groundwater flows both entering and exiting the bioretention soils. The BMP did reduce peak runoff rates for some storms, but did not significantly reduce total storm volumes because of the combined effects of the large drainage area to BMP area ratio and the poor infiltration capacity of the soil. Pollutant load calculations determined that the site removed about 28% of total suspended solids, 32% of total phosphorus, and about 15% of total nitrogen. Removals of approximately 16% and 7% were observed for lead and zinc, respectively. Although the Kingsbrooke bioretention area did improve water quality, the pollutant removal efficiencies were lower than those reported in the literature from more conventional bioretention areas. / Master of Science

Virginia's new stormwater regulations involve the use of the Runoff Reduction Method (RRM), which requires the product of the peak flow rate and runoff volume (Q*RV) from the one-year storm event in the post-development condition to be reduced to eighty percent of the pre-development Q*RV to protect against channel erosion. This study models different bioretention cell sizes in a developed watershed in Blacksburg, Virginia to determine the "performance" at both the sub-watershed and watershed levels. In addition, models of "optimal" bioretention cells sized to meet the RRM for each sub-watershed are evaluated. A direct relationship is determined between the size of the cell required to meet the RRM and the sub-watershed's Natural Resources Conservation Service (NRCS) curve number. However, the required size for some of the cells is much larger than those typically seen. With the RRM applied for all of the sub-watersheds, the resulting hydrograph at the watershed outlet has a lower peak than the pre-development condition. / Master of Science

Trout are an important game species that provide a substantial economic impact in Virginia. Along with other cold-water fish species, trout are extremely susceptible to changes in stream temperatures. Urban development and the increase in impervious surfaces alter the hydrologic cycle in urban watersheds, limiting infiltration and increasing surface runoff. Impervious surfaces absorb and store solar radiation, resulting in higher surfaces temperatures, and then transfer this thermal energy to runoff during a rainfall event, resulting in higher runoff temperatures. Bioretention cells are a common stormwater control practice identified as a possible thermal mitigation practice in urban watersheds harboring cold-water fish species. However, design specifications vary by locality and few studies have explored how design characteristics impact the temperature reduction potential. The goal of this study was to investigate changes in stormwater thermal load due to bioretention cells. In this study two bioretention cells with differing design approaches were monitored to quantify the thermal reduction impact that the bioretention cells have on stormwater from impervious surfaces. Both cells significantly reduced stormwater outflow volume, event mean temperatures and heat loads; however, outflow temperatures repeatedly exceeded the 21°C temperature threshold for cold-water fish species. This finding indicates this practice alone may not be sufficient to reduce runoff temperatures below biological stress thresholds. In addition, previous literature suggested that deeper cells may provide more cooling benefits as deeper soil layers are cooler and have more stable temperatures. In this study, the deeper cell was not as effective in reducing runoff temperatures, likely due to surface overflow and a shorter residence time in the bioretention cell. This finding indicates there is a limit to the effectiveness of cell depth in runoff thermal reduction and that other cell characteristics, such as subsurface drainage system length, may play an important role in runoff temperature reduction. / Master of Science / Cold-water fish species such as trout are a game species of large economic value that are very susceptible to changes in water temperature. Due to warmer runoff temperatures from urban watersheds stream temperatures are increasing, posing a potential impact on the cold-water fish found in these watersheds. Bioretention cells are a common method for treating and reducing pollutants from stormwater in urban areas. Recently, research has focused on the potential of bioretention cells to reduce runoff temperatures in urban watersheds. However, research is limited and does not fully address the bioretention design characteristics that may be beneficial for reducing runoff temperatures. In this study two bioretention with differing design approaches were monitored during summer months to quantify and assess the potential for runoff temperature reduction. Both cells reduced runoff volume, temperature, and overall heat energy leaving the cell. However, outflow temperatures were typically above the stress temperature threshold for many cold-water fish species, indicating that this practice may reduce runoff temperatures to a level that will not stress these fish species. Previous research has suggested that deeper cells may provide more cooling benefits as deeper soil layers are experience cooler and more stable temperatures. In this study, the deeper cell was not as effective in reducing runoff temperatures as the shallow cell with a greater overall volume. This finding suggests that there is a limit to the effectiveness of deeper cells and that other cell characteristics, such as cell volume, play an important role in runoff temperature reduction.

Bioretention cells (BRCs) are low-impact development stormwater management structures that integrate water quantity and quality management. Although BRCs have a predicted design life of about 25 years, most current research focuses on performance of cells less than two years old. This project evaluated the effectiveness of a BRC installed in 2007 to treat a 0.16-ha parking lot in Blacksburg, VA. After installation, this BRC was monitored for five months to determine initial flow reduction and total suspended solids, and nutrient removal. By monitoring for the same parameters, changes in cell performance since installation were quantified. ISCO automated stormwater samplers collected inflow and outflow composite samples from the cell, which were then analyzed for fecal indicator bacteria (total coliforms, E. coli, and enterococci), total suspended solids (TSS), total nitrogen (TN), and total phosphorus (TP). To determine if denitrification is occurring within the BRC, media samples taken throughout the cell were analyzed using qPCR. The bioretention media was also sampled to quantify changes in media nutrient content and particle size over the past seven years. Results indicate the bioretention media has not accumulated nitrogen and phosphorus since installation, and that the BRC remains effective at reducing flow volume and peak flow rates, as well as TSS, TN, TP, total coliforms, E. coli, and enterococci loads. Bacterial analysis of the media show most of the denitrifiers are present in the top layers of the bioretention media, despite an internal water storage layer and the bottom of the cell designed specifically for denitrification. / Master of Science

Increases in impervious surfaces caused by urbanization has led to higher volumes and rates of stormwater runoff that transports urban pollutants directly into natural waterways. Bioretention cells (BRCs) are vegetated soil systems designed to intercept stormwater runoff and reduce loads of water and contaminants discharged to surface waters. Nitrogen removal efficiency is highly variable and improvements are constrained by a poor understanding of the physical, biological, and chemical processes that occur within a BRC. The objectives of this study are to characterize and quantify the microbial communities in a range of existing BRCs, and determine which design factors have the greatest impact on denitrification, a microbial process responsible for removing nitrogen from stormwater. We sampled 23 BRCs throughout MD, VA, and NC, and quantified patterns in populations of denitrifying bacteria, denitrification potential, and microbial community structure within the soil medium. We found the greatest denitrifier populations and denitrification potential in the upper layer of the soil medium, which does not coincide with the internal water storage zone that is engineered to harbor anaerobic conditions favorable to denitrifying bacteria at the bottom of recent BRC designs. Results indicate that BRC vegetative cover, soil media nitrogen, and organic carbon concentrations are among the variables that facilitate nitrifying and denitrifying bacteria populations in BRCs. Bacterial community composition was most different between the top and bottom samples of the BRCs while fungal community composition differed most by BRC vegetative cover. Both fungal and bacterial community compositions were influenced by nitrogen and carbon concentrations. / Master of Science

The expansion of urban and suburban areas is a world-wide phenomena. One product of this development is a dramatic increase in impermeable surfaces and a consequent increase in stormwater runoff. Bioretention cells are one best management practice frequently used to mitigate the environmental impacts of urban stormwater runoff. To ensure that a bioretention cell will continue to perform adequately in the long term, it is imperative that the environmental conditions it will experience and their effect on its performance through time are considered during its design. Although bioretention cells are frequently used for stormwater management, very few quantitative data exist on how they perform through time and in varied physical environments. In regions with seasonal freeze-thaw cycles, it is important to understand the effects of freeze-thaw cycles on the infiltration rate of bioretention cell soil mixtures so that the integrity of the design will not be compromised by seasonal change. This project uses laboratory tests to investigate the effects of freeze-thaw cycles and sediment input on the infiltration capacity of three different bioretention cell soil mixtures. These results will provide an analog for long-term changes in bioretention cell infiltration rates due to freeze-thaw cycles, providing critical data on which soil mixture would be best implemented in geographic regions susceptible to freeze-thaw activity. Furthermore these results will inform design standards for bioretention cells to ensure their long-term performance.

Bioretention cells are a stormwater treatment technology that uses soil and vegetation to remove pollutants from runoff and improve downstream water quality. While bioretention has been shown to be effective at removing certain stormwater pollutants such as sediment and heavy metals, removal of nutrients has been more variable. Design components of bioretention such as vegetation and soil media amendments can influence pollutant removal performance. In my experiment, I isolate the effects of low-phosphorus compost and a Switchgrass (Panicum virgatum) monoculture on bioretention performance. In fall 2016, three bioretention cells were installed at the University of Vermont Miller Research Complex, a mixed-use research and agricultural production facility located in South Burlington, VT. Each bioretention had a unique experimental treatment that allowed for the comparison of the presence of the following design components: (1) compost with planted vegetation, (2) no compost and vegetation, and (3) no compost or vegetation. Results suggest that the presence of a low-P compost layer had a small deleterious effect on nutrient removal performance, as the bioretention cell with an added compost layer exported higher concentrations of phosphorus and nitrogen and exhibited a higher concentration of water extractable phosphorus in the bioretention media. The bioretention cell with vegetation and no compost was the only treatment to significantly reduce total nitrogen and phosphorus concentrations; however, there was no effect on media phosphorus concentration. The presence of low-P compost significantly increased the above-ground biomass growth of Switchgrass, but had no effect on the total number of plants surviving in the first year. Switchgrass proved to be a durable plant, capable of surviving in bioretention media without compost, but was slow to grow and required additional watering through droughty conditions.

This study examines the use of bioretention as a strategy to reduce the thermal impact associated with urban stormwater runoff in developing cold water stream watersheds. Temperature and flow data were collected during ten controlled trials at a bioretention facility located in Blacksburg, Virginia. It was determined that bioretention has the ability to reduce the temperature of thermally charged stormwater runoff received from an asphalt surface. Significant reductions in average and peak temperatures were observed. However, this facility was unable to consistently reduce the temperature below the threshold for trout health. The ability of bioretention to reduce runoff flow rates could also serve to reduce the thermal impact. Based on these results it was concluded that bioretention appears to have the capability to reduce the thermal impact of urban stormwater runoff on cold water stream ecosystems. / Master of Science