## 4 Results
### Tracer Tests  
#### 4.3.1 Tracer Tests with Salt  
First Tracer Test with Salt  
At the Ui-6 spring, the electrical conductivity before injection was 250 \(\upmu\)S/cm. The first tracer breakthrough was detected at the Ui-6 spring on 18 April 2020, at 16:30, some 26 h and 30 min after injection (Figure 4). The highest electrical conductivity was recorded on 21 April 2020, at 7:45, measuring 327 \(\upmu\)S/cm. At the Ui-7 spring, the electrical conductivity before injection was 265 \(\upmu\)S/cm. The first tracer breakthrough was recorded on 18 April 2020, at 20:30, some 30.5 h after injection. The peak tracer concentration was recorded on 21 April 2020, at 21:45, measuring 332 \(\upmu\)S/cm. After reaching the peak concentration, the values at the springs began to decline but did not return to pre-injection levels until 23 May 2020. This suggests that the tracer could have lingered at lower concentrations, slowly seeping through the predominantly low-permeability Permo-Carboniferous layers. Water from both springs flows into the Urbas stream, where another monitoring point was set up. No significant changes in the conductivity of the Urbas stream were observed during the tracer test that could be linked to the arrival of the tracer. Minor fluctuations were most likely due to the outflow of various water sources draining into the Urbas stream under different hydrological conditions. The tracer was diluted in the Urbas stream to such an extent that it could not be detected. Similarly, no changes in tracer concentration were observed at the other monitoring points, with any minor changes observed involving a degree of uncertainty. Second Tracer Test with Salt  
On 1 October 2020, around 12:00, we also injected a tracer into borehole PP-12A. However, due to the close proximity of the Urbas spring, which is used as a private drinking water supply, we used salt. Conductivity measurements were carried out at four monitoring locations: the springs Ui-1, Ui-4, and Ui-6, as well as borehole PP-4. We injected 100 kg of salt into borehole PP-12A diluted with approximately 200 L of water. The closest spring to the borehole, located 40 m away, is Ui-1. As expected, the concentration and speed of tracer arrival were the highest at this monitoring location. Conductivity before the injection was 207 \(\upmu\)S/cm (Figure 5). The first elevated conductivity values were detected 4.5 h after injection, with the highest value occurring after 10 h, reaching 379 \(\upmu\)S/cm (\(\Delta\)EC = 172 \(\upmu\)S/cm). After reaching its peak, the conductivity steadily  
Figure 4: Electroconductivity at sampling points after NaCl injection on 17 April 2020.  
decreased, and after eight days returned to pre-injection levels. Following the Ui-1 spring is borehole PP-4, located some 200 m from the injection point. The increase in conductivity at this monitoring location was the least pronounced, with a rise of just 5 \(\upmu\)S/cm at the highest concentration (338 \(\upmu\)S/cm). The highest recorded value of 338 \(\upmu\)S/cm was also the first elevated value we observed after the injection, which occurred 40 h after injection, and remained at this same rate for more than two days. More noticeable tracer arrivals were observed at the Ui-4 and Ui-6 springs, which are located at similar distances from borehole PP-12A. At the Ui-4 spring, located 240 m away, conductivity before the injection was 237 \(\upmu\)S/cm, with the highest recorded value at 299 \(\upmu\)S/cm (\(\Delta\)EC = 62 \(\upmu\)S/cm). The tracer concentration gradually increased from the beginning of the test to its peak, after 100 h. The concentration then decreased for some time and reached significantly elevated levels at least two more times during the test. At the Ui-6 spring, located 250 m from borehole PP-12A, conductivity before the injection was 207 \(\upmu\)S/cm, and the highest recorded value was 232 \(\upmu\)S/cm (\(\Delta\)EC = 25 \(\upmu\)S/cm).