<p>Hydraulic engineering structures like locks affect the natural hydraulic conditions and have a relevant impact on surface water – groundwater interactions due to enlarging the hydraulic gradient. For this, these sites are excellent areas to study associated flow paths, mass transport and their spatial and temporal variability in higher detail. However, no large-scale study at an inland waterway is available in Germany until now.</p><p>Our work aims to close this gap by applying a multiparameter approach for analyzing surface water-groundwater-interactions by using pH, electrical conductivity, major ions in combination with various other tracers like stable water isotopes, 222-Rn, and tritium. In this context, we also investigate the usability of organic trace compounds and their associated transformation products as potential new tracers.</p><p>The main study approach is based on the hypothesis that i) gaining stream sections show relatively high 222-Rn concentrations originating from discharging groundwater and ii) losing stream sections which are characterized by low 222-Rn concentrations as well as lower tritium and organic trace compounds inventories compared to unaffected areas.</p><p>During different flow-scenarios of the river Moselle, we test these hypotheses by means of a high-resolution longitudinal sampling at 2 km intervals of the main stream (along 242 km) and its major tributaries in combination with groundwater sampling at numerous wells.</p><p>Here, we present the first results of the longitudinal sampling campaign of the river Moselle in October 2020, which took place during intermediate flow conditions (Q=200 m³/s). We used on-site and in-situ 222-Rn measurements and electrical conductivity as a tracer to immediately identify zones along the Moselle with increased groundwater inflow.</p><p>With the use of these tracers, we will deepen the conceptual process understanding of surface water – groundwater interactions occurring at larger streams and during different flow conditions, which may lead to a general river characterization of losing and gaining stream reaches. Moreover, understanding the sources of water compounds and the processes involved during transportation and transformation is crucial for maintaining a good quality of the water body, which is key for proper water management. The findings obtained in the region of the Moselle river might be further transferred to other waterways and support decision making.</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>