Changes in stormwater N:P ratios through green stormwater infrastructure depend on climatology

Abstract:

Nitrogen (N) and phosphorus (P) cycles are accelerated in urban systems due to enhanced anthropogenic inputs and rapid overland transport to receiving waters via impervious surfaces. To address excess nutrient pollution, stormwater engineers have prioritized green stormwater infrastructure (GSI), designed to exploit plant uptake and soil processes to remove N and P from stormwater. N:P stoichiometry is a critical driver of ecosystem productivity, diversity, and resilience; however, it is yet unknown how GSI impacts N:P export from urban watersheds. Analysis of 2042 precipitation events across 124 varying GSI practices from the stormwater BMP database showed alteration of N:P ratios from stormwater runoff to GSI effluent. The direction and degree of N:P alteration depends on climate and individual storm characteristics. GSI in humid climates decreased stormwater N:P ratios, driven by ortho-P release from soil to stormwater, while in sub-humid climates, GSI increased stormwater N:P ratios. The stormwater runoff contribution to ortho-P and NO­_x effluent was highest during frequent, low intensity storms, whereas ortho-P release from bioretention soils was highest during infrequent, high intensity storms, and NO_x release was highest during intermediate storms. These results demonstrate climate and storm intensity as mediators of GSI nutrient removal efficiency and it is recommended that variability in climate and storm characteristics is considered in the planning and design process.