MUNICIPAL WATER EXCHANGES BETWEEN SURFACE WATER AND GROUNDWATER RESOURCES IN AN URBAN CATCHMENT

Urban land use impacts water quantity and quality through a variety of mechanisms, including contributions of municipal waters to natural hydrologic reservoirs. Municipal waters, like untreated wastewater and treated drinking water, can enter the environment through intentional releases, aging infrastructure, or irrigation activities. While untreated wastewater discharging to the environment can threaten human health due to the addition of pathogens or heavy metals, inputs of treated drinking water indicate wasteful water management practices. Municipal waters can contribute directly to surface water or groundwater systems, but, depending on local conditions, these anthropogenic waters and their associated chemistries may further migrate via natural exchanges between surface water and groundwater supplies. To quantify municipal water inputs to an urban watershed (Deer Creek near St. Louis, Missouri) and investigate their movement among natural hydrologic reservoirs, we monitored the stream and three proximal shallow (<5 m) groundwater wells from October 2022 to December 2023. The portion of Deer Creek that drains to the study site has historical water quality problems, which motivated a major sewer renovation that was completed in 2021. While the stream no longer has direct inputs of raw wastewater from combined sewer overflows, residences in the area use treated drinking water for lawn irrigation and buried pipes may leak either municipal water type into the groundwater. Chemical (e.g., F, B, optical brighteners) and biological (e.g., E. coli) tracers were assessed weekly to determine municipal water sourcing to the basin, and stream and well water levels were continuously monitored to ascertain the direction of water exchange between the surface and subsurface systems. Our results for F (a dental hygiene amendment) showed that the stream had higher drinking water inputs compared to the groundwater throughout the year. However, F levels in the stream and groundwater converged in the winter, implying possible mixing of the natural water types or changing municipal water inputs. Mixing models demonstrated that wastewater signatures (e.g., B from detergents) may be confounded by other sources in the basin. Our findings indicate potential transfer of municipal waters among natural hydrologic reservoirs, revealing complex contaminant transport processes.