Characterizing and modeling of urban stormwater nutrient export

Abstract

Urbanization influenced increasing impervious areas, altered hydrologic and ecologicaldynamics are contributing to the increased rate of nutrients accumulation and transportation to the freshwater system through stormwater runoff. Degradation of water quality due to harmful algal bloom from surplus nutrients has led to regulations aimed at reducing nutrient loading to urban rivers. In compliance with U.S. Environmental Protection Agency's (EPA) National Pollutant Discharge Elimination System (NPDES) Stormwater Permit Regulation, the cities with impaired water bodies are required to assess the current situation of pollutants loading in the stormwater and to develop and implement necessary prevention and management programs essential to reducing stormwater pollutant discharge. High-resolution stormwater sampling was conducted to understand the variability in pollutant export from different urban landscapes in the greater Boston area. For this, baseflow (dry weather) grab samples were collected every week, and stormflow (wet weather) samples at an interval of 15-45 minutes were collected when there is a storm event that generated significant runoff in the watersheds. Samples were analyzed for nutrients (soluble reactive phosphorus, total phosphorus, nitrate, nitrite, ammonia, total nitrogen), physical parameters (pH, temperature, dissolved oxygen, specific conductivity, hardness), and total suspended solids. Based on the sampled data obtained, the baseflow and stormflow nutrient species contributions were significantly different across the sites and within the events. In stormflow, phosphorus species contribution was dominated by particulate phosphorus. While determining baseflow (dry weather) and stormflow (wet weather) contribution to the annual storm sewer pollutant loading, it was found that this loading contribution is sensitive to the dry and wet weather definition. In addition, this dissertation develops an optimized diversion and treatment strategy by conducting stormwater sampling to understand the variability in nutrient export from different urban landscapes. One potential treatment approach for dense urban areas with separated sewers is to divert stormwater to some type of treatment system. This study characterized within event (i.e., hourly) total phosphorus and total solids particle size distributions and associated fluxes from urban catchments in the City of Cambridge, Massachusetts, to determine potential total phosphorus and total solids removal based on four different diversion and treatment strategies. The stormwater diversion strategies integrate new insights on temporal variations in particle size distributions and mass loading characteristics. The focus on particle size provides a connection to flow velocities required for transport (i.e., shear stress), which can be modeled and potentially used to trigger flow diversions. The evaluation of four flow diversion strategies suggested that in terms of diverted stormwater, the volume of stormwater treated tends to control more than what water is treated. To further the application of the proposed diversion strategies, results were presented using the NRCS CN model to estimate runoff and a coefficient-based model to estimate phosphorus export. Our findings suggest that relatively simple models have the potential to support efforts in evaluating phosphorus removal strategies. --Author's abstract