Geochemically Heterogeneous Shallow Groundwater in an Urban Stream's Floodplain Contributes Negligible Quantities of Municipal Water Types to the Stream

In urban environments, municipal water sources (e.g., drinking water and wastewater) can contribute considerably to regional groundwater resources. However, little is known about municipal water fluxes between streams and shallow groundwater at the floodplain scale. Understanding surface water–groundwater interactions in urban floodplains is essential for protecting both human and ecosystem health, sustainably managing water resources, and informing efforts to upgrade municipal water infrastructure as cities continue to expand. To address this research gap, we investigated whether the shallow groundwater in an urban floodplain transports drinking water and untreated wastewater to a stream. Our study focused on the urban Deer Creek watershed, near St. Louis, Missouri, United States, where we collected samples from the stream and three shallow (< 4 m deep) groundwater wells situated in its floodplain < 0.5 km apart. Stream and shallow groundwater samples were obtained weekly over 2.6 years and analysed for municipal water tracers (e.g., optical brighteners, faecal coliform bacteria, and F⁻) as well as other water quality parameters (e.g., temperature, specific conductivity, major anions, and total element concentrations) to understand water sourcing and exchange in this urban catchment. Although municipal water tracer signatures were elevated in Deer Creek, the shallow groundwater in its floodplain exhibited low concentrations for these tracers. Our findings suggest that the shallow groundwater is not a meaningful source of drinking water or untreated wastewater to the stream. However, if municipal water types migrate deeper into the groundwater profile than the depths we sampled, deeper groundwater could still deliver drinking water and untreated wastewater to Deer Creek, potentially explaining the elevated municipal water signatures observed in the stream. Nevertheless, we found that the shallow groundwater was geochemically heterogeneous despite the proximity of the observation wells. Variations in shallow groundwater characteristics among the wells are likely driven by differences in water depth, floodplain geomorphology, water residence time, mineral precipitation and dissolution processes in the substrate, and localised urban land use. Such differences over the small spatial extent we explored imply that municipal water types could still locally impact shallow groundwaters in urban catchments.