Information on groundwater and aquifer conditions is essential for the analysis of groundwater systems. Fluid sampling and pumping tests in boreholes are used as the standard methods for collecting this information. However, the contamination of groundwater by invaded drilling mud is a serious problem when taking samples from boreholes. While drilling a research borehole in the saltwater-freshwater transition zone on the German North Sea coast, uranine tracer was added to the drilling mud to identify possible contamination. Push-pull-type pumping tests were carried out in the open borehole at depths of 53 and 87 m using a new test design. The uranine concentration of the pumped water decreased exponentially with increasing recovery volume and dropped to 1% of initial concentration after the recovery of 10 invasion volumes. The total fluid loss in the test interval was calculated from the test results and supports the assumption that mud loss can be mainly attributed to the deepest (freshly drilled) part of the borehole. Breakthrough curves from two-dimensional numerical calculations using FEFLOW were fitted to the test data by varying the dispersivity α and the effective groundwater velocity va. The best results were achieved when α = 0.02 m and va = 0.28 m/d (values which correspond well with the scale of the experiment and other determinations of groundwater velocity). Thus the mud tracer test procedure not only provides information on the fate of the drilling mud but also on aquifer properties.