<p>Water in the Kiel Canal flows from the Baltic Sea westward to the Lower Elbe, crossing three hydrogeological areas: the eastern hilly region, the Geest, and the marsh. In contrast to the other two areas, direct hydraulic contact can be observed in the Geest area between the Kiel Canal and surrounding groundwater, as determined by the analysis of hydrogeological storage conditions. The high salinity levels in the east (greater than 10&#xa0;PSU; practical salinity units), which exceed the local groundwater concentrations, decrease with increasing flow distance to the west and are already nearly balanced in the marsh, often falling below 5&#xa0;PSU. Data from over 1000 official, and third-party operated groundwater monitoring wells have been reviewed, and a&#xa0;multi-stage selection algorithm was developed to identify a&#xa0;likely saline influence from the Kiel Canal. At 40 monitoring wells close to the canal (less than 3 km away), influent conditions may exist — temporarily in the hilly region and the Geest, and even locally permanent in the marsh — and saline transport from surface water into the groundwater is possible, as the hydraulic potentials of the groundwater are temporarily or permanently lower than those of the Kiel Canal. In the hilly region, only two monitoring wells show higher salinities due to construction measures at the eastern section of the Kiel Canal. However, this finding clearly demonstrates the potential risk that the Kiel Canal poses to the groundwater. In the marsh, some state monitoring wells show elevated salt concentrations in groundwater. These concentrations are not caused by the Kiel Canal but have a&#xa0;geogenic origin, and exceed the salt concentrations in the Kiel Canal.</p>