Assessing tritium and persistent organic micropollutants as tracers for investigating surface and groundwater interaction in a large river system (Moselle River)

Understanding the couplings of surface-groundwater interaction as well as their environmental impact is crucial for sustainable water management. However, water fluxes vary depending on external factors like water levels or heavy rain events and may alter the quantity and quality of surface and groundwater. As direct measurements of the ongoing mixing processes are challenging, various tracers are utilized to estimate water fluxes and transit times. Tritium as an environmental radioactive tracer introduced into the environment via nuclear bomb tests in the late 1950s has widely been used for water flux and transit time analyses. However, the tritium concentrations in surface waters in most regions declined to background concentrations due to the nuclear decay of tritium. Therefore, scientists are searching for alternatives like stable water isotopes or other chemical tracers to investigate surface-groundwater fluxes. Persistent organic micropollutants emitted into surface waters might present suitable alternative tracers.

The Moselle River has its source in the southern Vosges mountains and flows through France, along Luxembourg and through western Germany. The river contains high tritium concentrations (up to 50 Bq/l) induced by the French nuclear power plant Cattenom. Hence, tritium concentrations of the Moselle River surface water surpass the naturally abundant tritium concentrations ( ~1 Bq/l) found in groundwater reservoirs adjacent to the river. The German part of the Moselle River was monitored in 2020 to 2022 with monthly to quarterly intervals. Two spatially distributed sampling campaigns along the German Moselle River as well as continuous monthly investigations of a barrage site at Lehmen roughly 20 km upstream of Koblenz were conducted. The analysis of the water samples involves on-site parameters, cations, anions, metals, dissolved organic carbon, stable water isotopes, radon-222, tritium, and organic trace substances like pharmaceuticals. The study found significant surface-groundwater interaction at Lehmen. Thus, we evaluated correlations between tritium and detected organic micropollutants at this site. So far, seven organic micropollutants including the corrosion inhibitor benzotriazole and its derivative 5-methyl-1H-benzotriazole as well as the pharmaceuticals carbamazepine, lamotrigine, tramadol, candesartan and olmesartan were selected for this investigation. These pollutants enter the environment via wastewater release.

In this study, we explored the capability of tritium and persistent organic micropollutants tracers to reflect surface-groundwater interaction. So far, we compared the suitability of different organic micropollutants to reflect the observed water fluxes and transit time estimations with estimated results from tritium. Furthermore, we discuss the possible utility of benzotriazole or other organic compounds for future investigations of surface-groundwater-interaction.