A Nearly Decade‐Long Assessment of Urban Stream Physicochemical Responses to a Regional‐Scale Wastewater Infrastructure Renovation

ABSTRACT As populations shift towards urban areas and precipitation patterns alter with climate change, the demands on ageing sewer infrastructure increase. These strains have led to more frequent occurrences of sewer overflows and leaks that adversely impact water resources. To address water quality challenges, ecosystem degradation, and human health risks associated with outdated wastewater infrastructure, cities worldwide are modernising their wastewater systems (e.g., removing sewer overflows and installing sanitary tunnel systems). However, critical information regarding the immediate and long‐term impacts of these regional‐scale infrastructure projects on urban waterbodies remains limited. Our study therefore evaluated changes in water quality and wastewater tracers in response to a regional‐scale sewer renovation project in St. Louis, Missouri, United States. The project was designed to reduce untreated wastewater inputs to stream systems through the removal of sewer overflow sites and the construction of a sanitary tunnel. To accomplish our study objectives, we collected weekly physicochemical data from an urban stream within the renovation project area over an 8.7‐year period (i.e., from 2 September 2016 to 1 May 2025), encompassing 2.5 years before, 3.4 years during, and 2.8 years after the infrastructure updates. Our results demonstrated that the sewer infrastructure renovation effort successfully reduced the amount of untreated wastewater in the stream as evidenced by significant decreases in the average values for wastewater tracers like optical brighteners (15%) and potassium (K) (13%), along with a significant 26% increase in average dissolved oxygen (O 2 ) saturation from pre‐construction to post‐construction conditions. While the sanitary tunnel project effectively reduced untreated wastewater contributions to the urban stream, it also had immediate impacts on certain stream physicochemical variables during the construction phase of the renovation, including increases in turbidity and major element (e.g., calcium Ca, magnesium Mg, strontium Sr, and silicon Si) concentrations. Our findings highlight the importance of designing major wastewater infrastructure projects that protect surface waterbodies from the immediate impacts of construction while still addressing pervasive and long‐term water quality concerns.