<p>The residence time of groundwater is an essential parameter for water resource management. In the presented study, environmental tracers (Rn-222, S-35, H-3, <em>δ</em><sup>18</sup>O) and hydrogeochemical groundwater components are used for assessing groundwater mean residence times in a near surface aquifer.</p><p>At a barrage station at the River Moselle, four groundwater monitoring wells and two surface water spots were sampled at a 4-week interval over an 18-month period. At each sampling event, isotopes as well as other hydrogeochemical parameters (e.g. water level, water temperature, oxygen, pH value and electrical conductivity) were measured and evaluated.</p><p>All tracers showed different concentrations and signatures in ground- and surface water samples. As expected, Radon showed high concentrations in groundwater (up to 25 Bq/L) and low concentrations (about 0.2 Bq/L) in surface water. The tritium content of groundwater (13 Bq/L; ~110 TU) was similar to the long-term average concentration measured in surface water (~14 Bq/L); these comparatively high concentrations are way above the natural background concentration of about 1 Bq/L and result from the release of tritium from the French nuclear power plant Cattenom (situated about 250 km upstream of the sampling site). S-35, produced in the atmosphere and entering the hydrological cycle via precipitation, could be determined only once (January 2021) due to technical obstacles. The S-35 concentration measured in surface water (0.035 Bq/L) was about 4 times higher than the concentration in groundwater (0.0093 Bq/L). Finally, the median δ<sup>18</sup>O signature in surface water (-8.13 ‰) was similar to the signature found in groundwater (-7.78 ‰).</p><p>The selected isotopes and water parameters indicate that (i) the aquifer is predominantly recharged by surface water and (ii) the groundwater mean residence times varies between 5 and 6 months based on S-35 and <em>δ</em><sup>18</sup>O.</p><p>Hence, it can be concluded that the selected isotopes are suitable as tracers for estimating groundwater mean residence times. However, further studies are needed, especially to minimize the time gap between the established tracers Radon (useful for up to 40 days) and tritium (useful from about one year). The novel tracer S-35 seems promising, but long-term data series of S-35 in surface and precipitation water are still missing to establish the necessary input functions.</p>

<p>Groundwater represents a major component for runoff generation of large rivers systems. Its quantification is of uttermost importance during low flow periods and in the context of changing runoff dynamics due to climate change.</p><p>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.</p><p>The research approach is based on the assumption that local groundwater inflow into the Moselle is detectable by increased 222-Rn concentrations in the river and that the δ<sup>18</sup>O composition of the river water approximates that of the groundwater. Therefore, we applied a numerical model for solving the 222-Rn and Tritium mass balance and a mixing model of δ<sup>18</sup>O and electrical conductivity.</p><p>For this purpose, water samples were taken at intermediate flow conditions (gauge Cochem: about 220 m³/s) in October 2020 along the Moselle on a stretch of 242 kilometers at high spatial resolution (every 2 km) to measure stable water isotopes and electrical conductivity. Integrated over the same spatial resolution, in-situ 222-Rn measurements were carried out. Tributaries and selected groundwater monitoring wells were sampled for the same analysis. Precipitation was collected at the station Trier of the German Meteorological Service on a monthly basis. In agreement with this measurement concept, another sampling campaign took place for selected reaches in August/September 2021 at lower discharges (Cochem gauge: about 94 m³/s).</p><p>In autumn 2020, diffuse groundwater inflow (approx. 0.17 to 0.3 m³/s) was detected for the shell limestone of the upper Moselle reaches and locally increased groundwater inflow 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). These estimates translate into groundwater contribution of the total Moselle discharge of 0.3 and 1.2 % respectively, which is much lower than those calculated by the mixing model (about 10 and 5 %, respectively). For August/September 2021, higher groundwater inflows in these areas are expected for both methods.</p><p>The evaluation to date indicates that 222-Rn is the most sensitive tracer to locations with increased groundwater inflow compared to tritium and stable water isotopes. While tritium results seem to strongly depend on the current flow conditions and the propagating river wave, stable isotope results are affected by the appropriate characterization of end-member hydrochemistry.</p>