River infiltration into karst aquifer, Vipava Valley case study, Slovenia

Abstract

<p>When and how the exchange of water between a river and an aquifer occurs is still poorly understood and investigated in many areas of the world, especially in complex karst areas. The same applies to Slovenia. As part of our research, we study the relationship between surface water and groundwater or the impact of infiltration of a non-karst watercourse on the water balance of a karst aquifer. The research is taking place in the south-western part of Slovenia, in the area where the Vipava River and the Classical Karst aquifer come into contact in the lower Vipava Valley. In the study area we have a well-defined Karst aquifer, which in its northern part is bounded by the Vipava River, which represents a hydrogeological boundary. The contact between the river Vipava and the Karst aquifer can thus also be considered as a contact karst where water losses occur.</p><p>The amount of water inflow into the Karst aquifer is determined by comparing the measured flows in successive sections in the Vipava River. For flow measurements, we use the Acoustic Doppler Current Profiler (ADCP) StreamPro with the associated WinRiver II software. Flow measurements are performed on selected cross-sections, where the exact velocity flow profile from the surface to the bottom of the profile is recorded at regular time intervals with the ADCP moving slowly and evenly across the river in both directions.</p><p>Based on the field work, it was found that the banks of the Vipava River are mostly quite overgrown and steep, so the riverbed is difficult to access. The river is about 5 m deep and about 30 m wide. The speed of the river is very slow, a few cm/s. The measurement (movement of the ADCP across the river) must be very slow due to the slow flow of the river; one passage of the river takes about 30 minutes. An important factor in performing measurements is also the wind and the increased amount of sediment in the water. The slow flow of the Vipava River, wind and the sediment are so far the main factors influencing the quality of the measured flows.</p><p>Flow measurements are currently performed on six cross-sections at 800 m of the Vipava River. The results of the measurements showed that on the first three consecutive profiles the flows in different water conditions in the downstream direction increase. The results of measurements on the remaining three consecutive profiles, however, showed that the flows between the profiles are comparable and do not show a trend. Measurements at different water conditions also showed that at lower flows (up to 3 m<sup>3</sup>/s) ADCP measurements are performed along the entire profile to the river bottom, while at higher flows the measurements are performed only up to a certain depth in the profile. The reason for this is the excessive scattering of the signal and therefore the rapid drop in intensity, which is due to the increased amount of sediment in the water. These are preliminary findings as measurements continue.</p>