Distinguishing between different sources of recharge in a complex binary karst aquifer: a case study of the Unica springs (SW Slovenia)

The characteristics of groundwater flow and solute transport in karst aquifers differ considerably from those in intergranular and fissured aquifers. To understand how they function, appropriately adapted hydrogeological research techniques and analyses are required. In this study, a binary karst aquifer in the recharge area of the Malenščica and Unica springs, which covers an area of about 820 km² in southwestern Slovenia, was selected as the study area. A monitoring network was set up to obtain data on precipitation and discharge at the two springs, two sinking streams and two water-active caves in their catchment over a period of two hydrological years. First, a classical approach of correlation and spectral analysis of these time series data was applied to determine and compare the flow characteristics and storage capacity of selected springs and their recharge areas. The allogenic and autogenic recharges were considered separately as input functions and the results of the analysis were compared. Although these widely used methods provided a good characterization of the studied karst system, the interpretation can be ambiguous due to the interference of the two input components. To avoid this problem, an innovative method of partial cross-correlation analysis was used, which has previously only been applied to separate the influence of different precipitation stations in karst. Here, its application was extended to the evaluation of the influence of allogenic recharge. By controlling the input parameters precipitation and discharge of one of the sinking streams, it was possible to determine the contribution of the other sinking stream to the observed spring. The differences in the recharge characteristics of the Unica and Malenščica springs were revealed, and the ability of this innovative approach to provide additional insights into the functioning of binary karst aquifers was confirmed.

 

Key words: karst aquifer, autogenic and allogenic recharge, time series analysis, partial cross-correlation, Slovenia.