HYPOXIA AND EUTROPHICATION - A CLOSER LOOK AT THE WATER QUALITY PROBLEMS FACING THE CHICAGO AREA WATERWAY SYSTEM (CAWS) THROUGH A 3-D ENVIRONMENTAL FLUID DYNAMICS MODEL
Advisor: Professor Marcelo H. Garcia
With a population of over 9 million, the metropolitan area of Chicago generates, on average, 1.3 billion gallons of treated wastewater each day. The treated water, a significant nutrient input source, gets conveyed to the Illinois River Basin, then the Mississippi River Basin, and eventually the Gulf of Mexico through the Chicago Area Waterway System (CAWS). Located at the mid-continental divide between the Great Lakes Basin and the Mississippi River Basin, the CAWS is a highly engineered water conveyance system that connects the two basins. Ever since the completion of the system's renowned flow reversal project in the early 20th century, the water in the CAWS generally flows west towards the Illinois River. Nevertheless, during extreme weather events, treated and untreated wastewater, and contaminants from legacy nutrient sources, such as Bubbly Creek, can be released into Lake Michigan to prevent flooding in the city. Nutrients from CSOs and legacy sources can threaten the lake’s ecosystem and add to the nutrient load released into the Illinois River Basin during extreme rainfall events.
Eutrophication is the process by which a water body is enriched with nutrients such as nitrogen (N) and phosphorus (P) and is typically characterized by the proliferation of algal mass on the water surface. It is a natural process that occurs over long-time scales in aging aquatic systems such as a lake. However, anthropogenic activities have significantly expedited eutrophication rates by substantially increasing the nutrient loads into ecosystems. Excessive eutrophication has been observed in many parts of the world and can lead to undesirable phenomena such as harmful algal blooms (HABs) and depleted dissolved oxygen (hypoxia) in the water column. A need to better understand nutrient dynamics during extreme rainfall events and normal weather conditions and assess the potential for HABs in the CAWS provided the motivation for this research.
An Environmental Protection Agency (EPA) supported three-dimensional Environmental Fluid Dynamics Code (EFDC) model was developed to simulate the hydrodynamics and water quality characteristics of the CAWS. The model was first calibrated and validated for two consecutive water years (2020/2021) under different weather conditions and then applied to examine the water quality impacts of flow reversal events on the CAWS during an extreme event in May 2020. The extent and duration of the flow reversals are analyzed, and the pollutant loads to Lake Michigan are calculated. A notable reduction in lake-ward propagation of contaminated water from Bubbly Creek was observed. However, untreated sewage still has a profound local influence over an extended period within the CAWS. The 3D model was then utilized to locate reaches prone to
unnatural algal growth. Various remediation scenarios were considered to evaluate their respective efficacy and identify limiting factors for algal reproduction. Finally, a mid-system flow separation alternative proposed by the Great Lakes and Mississippi River Interbasin Study (GLMRIS) report aimed at preventing the migration of invasive species to the Great Lakes was implemented in the EFDC model. Different scenarios with various augmented flow rates are examined to investigate their impacts on water quality characteristics in the CAWS.
As decision-makers face ever more challenging management scenarios, the development of the comprehensive three-dimensional hydrodynamics and eutrophication model presented in this study can provide them with a very useful tool. The findings and insights from this modeling effort will also assist state and federal agencies in better management of the CAWS