GEOCHEMICAL TRACERS SHOW THAT MAJOR SEWER RENOVATIONS DECREASE WASTEWATER INPUTS TO AN URBAN STREAM

Municipal waters, including drinking water and wastewater, can enter waterbodies through a variety of pathways like aging infrastructure, irrigation, and intentional sewer overflow releases. While inputs of treated drinking water to the environment waste water resources, untreated wastewater additions to waterbodies threaten human and ecosystem health because of the presence of pathogens and harmful chemicals. These municipal water inputs can be identified in the environment using geochemical tracers unique to those sources. Fluoride (F) can be used to detect treated drinking water due to water fluoridation for dental health, while optical brighteners, whitening and brightening agents in laundry detergents, can be used to identify untreated wastewater. The St. Louis, Missouri, region features the fourth largest sewer system in the United States, which is undergoing a 28-year remediation effort to resolve water quality issues stemming from the area’s century-old combined sewer system. To test the effectiveness of completed infrastructure updates, we employed geochemical tracers in an urban stream (Deer Creek) that has a history of wastewater contamination. Weekly stream samples were collected at low flow (<1 m³/s) downstream of sewer infrastructure updates from 2016 to 2021, encompassing the timeframe before, during, and after renovations. Samples were analyzed for municipal water tracers (e.g., F, optical brighteners) as well as calcium (Ca), magnesium (Mg), and strontium (Sr), which could indicate construction-related perturbations of the limestone host rock. Preliminary results showed a 15% decrease in average optical brightener levels after the renovations began, suggesting that the remediation project decreased wastewater inputs to the stream. In contrast, average F concentrations increased 18% after project completion, implying enhanced drinking water inputs to the system. We also observed increases in the average Ca (42%), Mg (45%), and Sr (71%) concentrations during construction, which we attribute to excavation activities to drill a new sanitary tunnel through the nearby limestone. Our findings suggest wastewater infrastructure renovation projects successfully decrease wastewater contributions but may temporarily increase drinking water and construction-related inputs.