Identifying urban wastewater in streams and connected groundwater through a multi-tracer approach (boron and boron isotopes, stable isotopes of O and H, gadolinium anomaly, pharmaceuticals) and non-target screening

Domestic wastewater (WW) may constitute a significant part of the baseflow of rivers and contribute to groundwater recharge into connected alluvial aquifers. Even treated, WW impacts both surface and groundwater quality by a large range of nutrients, inorganic and organic contaminants, including contaminants of emerging concern (CECs). There are overlaps with the compound range from other sources of contamination, notably agriculture, so that their distinction is challenging, especially for the nitrogen input. We combine in our study of the Rhine valley alluvial aquifer a range of tracers that are specific to WW, notably boron and its isotopes, gadolinium and a selection of human pharmaceuticals. This allows us to detect and quantify urban WW contribution from treatment plants to small streams and associated groundwater in a context of multiple, complex human pressures on water quality. We distinguish local and regional recharge components and identify stream-aquifer connection through stable water isotopes. Non-target screening of a large range of organic compounds by LC-TOF-MS allows to establish contaminant fingerprints under high- and low water conditions and, together with targeted tracers, provides insight into the annual variations of water and contaminant dynamics.