Date of Award

Fall 2025

Project Type

Thesis

Program or Major

Civil and Environmental Engineering

Degree Name

Master of Science

First Advisor

Thomas Ballestero

Second Advisor

James Houle

Third Advisor

Daniel Macadam

Abstract

Bioretention systems are widely used in stormwater management to mitigate nutrient pollution, particularly nitrogen and phosphorus. Over time, these systems undergo changes in nutrient retention efficiency due to organic matter accumulation and biogeochemical transformations. This study examines the distribution and long-term retention of total nitrogen (TN), total Kjeldahl nitrogen (TKN), nitrate (NO3-), total phosphorus (TP), and orthophosphate (PO43-) within a 16-year-old bioretention system. A combination of k-means clustering and non-parametric statistical tests (Kruskal-Wallis and Wilcoxon rank-sum tests) were employed to analyze nutrient concentrations across different soil depths and media types. Results indicate that TN and TKN are concentrated in the upper soil layers, correlating strongly with organic matter content, while NO3- is more prevalent in deeper, more permeable layers. This distribution may reflect nitrate mobility through the profile and potentially limited denitrification in lower zones, although further investigation would be required to confirm this. Phosphorus accumulation, particularly TP and PO43-, indicates signs of media saturation, as concentrations increase with depth and correlate poorly with organic matter. This suggests adsorption sites in the media matrix have become exhausted over the system's 16-year lifespan, reducing long-term P retention capacity. The findings highlight the importance of periodic media replacement, optimized soil composition, and improved phosphorus-binding amendments to sustain nutrient removal efficiency in aging bioretention systems. Future research should explore strategies such as biochar amendments, engineered media blends with high P-sorption capacity (e.g., iron oxides, gypsum), and targeted microbial enhancements, such as inoculation with denitrifying bacteria or organic carbon amendments, to improve long-term nutrient retention and minimize nutrient export risk.

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