Innovating Green Stormwater Infrastructure for Nutrient Management: Long-Term Field and Modeling Studies of Conventional and Modified Denitrifying Bioretention Systems

Urban stormwater and nutrient management are increasingly important topics to address globally, as coastal urbanization increases, disturbing the natural landscape, hydrology, and water quality. Untreated urban stormwater runoff carries pollutants that enter our waterways, such as rivers and marine environments, which serve as drinking water sources, recreational sites, and locations for economic livelihood. One pollutant and nutrient of concern for water quality is reactive nitrogen (N). Since pre-industrial time, reactive nitrogen has doubled from human activity. When found in excess in waterways, nitrogen causes an overabundant growth of algae, which can result in eutrophic and hypoxic conditions, impacting ecosystems, human health, and the economy. For this reason, managing the nitrogen cycle, designing a future without pollution, and creating healthy resilient cities have become grand challenges in the 21st century, as listed by the U.S. National Academy of Engineering and by the National Academies of Sciences, Engineering, and Medicine. Green stormwater infrastructure is a suite of low impact development technologies and best management practices that can be applied strategically throughout a watershed to capture stormwater and reduce pollutants from urban runoff to natural waterways. One such technology increasingly being implemented is bioretention, a structural low impact development technology. Bioretention systems consist of a shallow depression with a planting bed and a series of permeable layers where the water that passes is filtered and treated. However, conventional bioretention systems are not designed specifically to remove or recover dissolved nitrogen species found in stormwater. They have poor and inconsistent nitrogen removal, especially for nitrate (NO3-), which can be exported from conventional systems. Nitrogen removal in a bioretention system can be improved by modifying the conventional system to promote biological nitrogen removal processes. Denitrification, the reduction of NO3- to inert gaseous nitrogen (N2), can be enhanced with the inclusion of an internal water storage zone (IWSZ) at the bottom of a bioretention system that contains an electron donor (e.g., an organic carbon source from wood chips). In modified bioretention systems reactive nitrogen is removed from the water and returned to the atmosphere. Prior studies have shown that the use of an IWSZ with a carbon source resulted in total nitrogen removal efficiencies greater than 88% under laboratory conditions. However, there have not been previous long-term field studies conducted on modified denitrifying bioretention systems for treating stormwater runoff assessing their continuous performance and stability. Two bioretention systems, a conventional and a modified design, were installed side-by-side using locally available materials. The field-scale site is located in East Tampa in the property of a community partner, the Corporation to Develop Communities of Tampa, Inc. The bioretention systems were monitored for four years, with experiments conducted in years three and four. The results from simulated storm events in the field demonstrate that the biological and chemical processes that occur within the modified bioretention system significantly improve nitrogen removal. The modified system successfully removed over 75% of total N (TN) while the conventional system removed approximately 40%. Hydraulic loading rate had the most significant impact on nitrogen removal efficiency in the modified system, greater than the addition of plants or increased antecedent dry conditions. Greater NO3- removal efficiencies were observed when the hydraulic loading rate was the lowest pertaining to a longer hydraulic retention time of the stormwater in the IWSZ. For the conventional system, the addition of plants contributed to greater removal of NO3- than the lowest hydraulic loading rate. With the experimental data from the field, tracer studies, denitrification kinetic data from other studies employing the same media used in the IWSZ of this study, and rainfall data, a model for sub-tropical regions, such as Florida, was created for modified bioretention systems. The model uses local rainfall data and an event mean N concentration typical for the specific land use where the bioretention system is being placed to determine an annual N load for a specified impervious area. Rainfall data were analyzed to determine the frequency distribution of rain event depths, duration, and antecedent dry periods. Adequate detention time within the IWSZ is crucial for the design and performance of modified denitrifying bioretention systems, as concluded from the field experiments. The model results can be used to guide the sizing of modified bioretention systems by indicating the N removal efficiency of a system and identifying if annual N removal goals can be met for conditions of a site. Considering the local climate conditions in the design, practitioners can more accurately aim to meet not only hydrologic goals, but also water quality goals; which can also allow policy makers to provide incentives or credits when these are met. Installing effectively designed bioretention systems has the potential to improve the water quality of watersheds in the long term. This type of research also provided and continues to provide an ongoing opportunity to engage with residents, urban planners, engineers, and government officials to adopt bioretention systems not only as a stormwater and nitrogen control measure but also as a way to beautify and add more green spaces to communities. The researcher and author of this dissertation partook in service learning activities within the community were the two bioretention cells were installed. The objective was to integrate green infrastructure research conducted in the local community and in collaboration with a community partner with service learning and community-based participatory research activities that would actively involve members of the community and provide beneficial experiences for both the student and the community, such as co-learning and capacity-building. These experiences were recorded and reflected on based on the six qualities of service-learning described by Clevenger-Bright et al., (2012), which are integrative, reflective, contextualized, strength-based, reciprocal, and lifelong. The main goal is to be able to obtain a broader appreciation of conducting green infrastructure research within communities and enhancing a sense of civic engagement