Tracer and model-based quantification of groundwater inflow along Moselle River

Michael Engel1, Simon Mischel1, Sabrina Quanz1, Sven Frei2, Ben Gilfedder2, Dirk Radny1, Axel Schmidt1
1 Federal Institute for Hydrology
2 University of Bayreuth

P 13.3 in Artificial and natural groundwater recharge (co-organized by IAH)

In times of a changing water cycle and its components precipitation, runoff and groundwater due to a changing climate, the quantification of these components is of uttermost importance for river systems and their runoff dynamics.

In this context, the present study focuses on the surface water-groundwater interaction using the example of the Moselle River, the second most important tributary of the Rhine. The river is classified as a federal waterway and has 12 barrages on German territory to ensure navigability all year round. Geologically, the upper reaches of the Moselle are characterised by shell limestone and sandstone, and in the middle and lower reaches by slate and quartzite of the Rhenish Slate Mountains.

The research approach is based on the assumption that the inflow of groundwater into the Moselle is detectable by increased radon concentrations in the river and that the isotopic composition of the river water approximates that of the groundwater. For this purpose, water samples were taken at intermediate flow conditions (gauge Cochem: 200 m³/s) in October 2020 along the Moselle on a stretch of 242 kilometres at high spatial resolution (every 2 km) to measure stable water isotopes (d2H and d18O) and electrical conductivity. Integrated over the same spatial resolution, in-situ radon measurements (222-Rn) were carried out. Tributaries and selected groundwater monitoring wells were sampled for the same analysis. Stable water isotopes of precipitation were recorded by the station Trier of the German Meteorological Service on a monthly basis. In agreement with to this measurement concept, another sampling took place for selected reaches in August/September 2021 at lower discharges (Cochem gauge: approx. 90 m³/s). The groundwater inflow was calculated using both a mixing model of stable water isotopes and electrical conductivity (Sklash and Farvolden, 1979) and the numerical model FINIFLUX for radon data (Frei and Gilfedder, 2015).

The results show that the measured radon values in October 2020 (n = 126, median = 147 Bq/m³) had a wider range and were slightly lower than those in August/September 2021 (n = 55, median = 174 Bq/m³). The modelling results for autumn 2020 indicate a diffuse groundwater inflow (approx. 0.17 to 0.3 m³/s) for the shell limestone in the upper reaches of the Moselle. In contrast, a locally increased groundwater inflow is identified for the middle reaches in the transition area to the Rhenish Slate Mountains and the Detzem barrage (approx. 1.4 to 2.4 m³/s). An increased groundwater inflow in these areas is also expected for August/September 2021. The evaluation to date indicates that increased groundwater inflow is dependent on geology, fault areas and an increased hydraulic gradient of a barrage. Furthermore, the characterisation and spatial allocation of groundwater as an “end-member” seems to have a high influence on the uncertainties in the modelling.

 



Sklash, M.G. & Farvolden, R.N. (1979): The role of groundwater in storm runoff. J. Hydrol., 43: 45–65.

Frei, S. & Gilfedder, B.S. (2015): FINIFLUX: An implicit finite element model for quantification of groundwater fluxes and hyporheic exchange in streams and rivers using radon, Water Resour. Res., 51: 6776–6786.