## 4 Results
### Tracer Test with Uranine
#### 4.4.2 Second Tracer Test with Uranine (Injection at Ui-1-1 October 2020)  
During the tracer test on 17 April 2020, no significant increase in conductivity was detected at the Urbas stream monitoring site. It was assumed that this was due to high salt dilution. Therefore, a new tracer test was performed on 1 October 2020, at 13:00, using uranine as the tracer, injected below the Urbas spring. Automatic Cyclops fluorimeters were installed at two locations, the PP-8 borehole and the Urbas stream. For comparison, samples were also collected at both sites for laboratory fluorescence analysis. At borehole PP-8, located approximately 200 m downstream from the injection point, significant differences were observed between continuous fluorimeter measurements and laboratory analyses. Tracer concentrations began to rise on 5 October 2020, at 14:30, 97 h and 30 min after injection (Figure 7). The highest tracer concentration (206 ug/L) was recorded on 13 October 2020, at 12:00, 287 h after injection. After reaching the peak concentration, the tracer concentration gradually decreased. Measurements were concluded before the concentration dropped to pre-injection levels due to time constraints. The fluorescence samples, except for the  
Figure 6: Fluorescence concentrations at sampling points after uranine injection on 14 May 2020.  
first, which matched the fluorimeter results, showed a different breakthrough pattern. A significant increase in concentration was already evident in the second sample the day after injection (71.9 \(\upmu\)g/L). By the third sample, collected on 6 October 2020, the concentration had reached 103.1 \(\upmu\)g/L, and by 17 October 2020, it had increased slightly to 103.6 \(\upmu\)g/L. It is worth noting that sampling was conducted using a manual sampler, which remained in the borehole throughout the test to avoid contamination. This sampler allowed for groundwater samples to be taken from a depth of 2 m, the upper part of the water column, while the fluorimeter was installed deeper, in the lower part of the borehole. We suspect that the manual sampling captured the shallow flow zone, while the fluorimeter measured the deeper flow zone, hence the significantly slower breakthrough speeds recorded by the fluorimeter. The most intense inflows into the borehole likely occurred in the lower part, preventing the shallow tracer from mixing within the borehole.  
Figure 7: Fluorescence concentrations at sampling points after uranine injection on 1 October 2020.  
In the Urbas stream, the breakthrough pattern differed significantly from the PP-4 borehole. As expected, the analyzed samples and fluorimeter measurements matched. Tracer concentrations began to rise sharply after about two hours, with the highest value of 440.4 \(\upmu\)g/L reached after 5 h and 20 min. This rapid breakthrough was likely influenced by high water levels and intense surface runoff due to previous rainfall. The concentrations then declined rapidly until 5 October 2020, after which they increased slightly again, matching the measurements from PP-8. The bulk of the tracer thus flowed out before 5 October 2020.  
During the test period, additional samples were taken from the Ui-4 and Ui-6 springs. At Ui-6, the highest analyzed concentration was similar to that in the Urbas stream. Since the sample was taken when the concentration was in decline, the peak concentration at this site was likely much higher. Like the Urbas stream, concentrations dropped rapidly after peaking. At the Ui-4 spring, the highest concentration was recorded the day after injection, but it was four orders of magnitude lower than that at Ui-6 (0.05 \(\upmu\)g/L). Such concentrations are usually attributed to natural water fluctuations, but since the increased concentration coincided with results at other monitoring sites, we can interpret the tracer's presence at Ui-4.