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 nitrate (NO3-) pollution 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 NO3- 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 I used stable nitrogen (δ15N) and oxygen (δ18O, δ17O, and Δ17O) isotope ratios 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. Results indicate that 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) limited NO3-Atm processing at both scales. The factors influencing the modulation and export of NO3- at the grass swale were less clear. 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 promotes physical and biogeochemical processes that retain NO3-Atm. Reduced NO3-Atm loads in the total load exported at the GSI watershed indicate 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, will likely be the most effective at managing NO3- pollution.