Name: Grace O'Hara
Date: 04/18/2025
Time (EST/EDT): 09:00 am
Location: Appalachian Laboratory, IVN room 109
Remote Access: email mees@umd.edu

Committee Chair: Dr. David Nelson
Committee Members: Dr. Keith Eshleman and Dr. Joel Bostic

Title: Evaluating the functionality of green stormwater infrastructure using stable isotopes of nitrate in stormwater

Abstract: Excess nutrient loading into water bodies can lead to a suite of negative environmental impacts downstream. Urbanization and suburbanization often lead to excess nitrate (NO3-) in stream water via the rapid routing of stormwater through impervious infrastructure. Green stormwater infrastructure (GSI) promotes temporary storage of water through enhanced soil infiltration to mitigate nutrient pollution to receiving surface waters and downstream ecosystems, but its effectiveness at removing nitrogen pollution is unclear. To evaluate the effectiveness of GSI at promoting physical and biogeochemical processes that retain and/or remove NO3- during varied hydrological conditions we used stable nitrogen (δ15N) and oxygen (δ18O, δ17O, and Δ17O) isotopes of NO3- to distinguish stormwater NO3- sources (i.e., atmospheric and terrestrial NO3-) and infer nitrogen cycling processes in samples (n ≅ 391) collected from 2020-2024 in two adjacent suburban Baltimore watersheds and two adjacent highway swales. There is variability in the modulation and export of NO3- between the bio and grass swale at the site scale. Despite differences in GSI design between the site scale (i.e., bioswale) and watershed scale (i.e., bioretention cells), greater storm magnitude (i.e., event rainfall) limits NO3- processing at both scales. Loads of atmospheric NO3- (NO3-Atm) were lower in the watershed with GSI implementation relative to the watershed with traditional stormwater management, likely because impervious surfaces in the latter cause NO3-Atm to bypass processing to a greater extent. Overall, my results imply GSI is promoting physical and biogeochemical processes that retain NO3-Atm. There is evidence that NO3-Atm is incorporated into the terrestrial nitrogen cycle through the uptake of plants and soil microbes within GSI. Considering increasing frequency and magnitude of storm events as a symptom of climate change, future stormwater management techniques that facilitate processing of NO3-Atm independent of precipitation amount and complete soil saturation to promote denitrification, will likely be the most effective at managing NO3- pollution.