Using concentration-discharge hysteresis to infer recharge processes in karst springs: A case study of the Malenščica and Unica springs, SW Slovenia.

Karst aquifers are recharged both by diffuse infiltration through soil and rock (autogenic recharge) and by concentrated inflow through swallow holes and sinking streams (allogenic recharge). Differences in the origin of water are reflected in rapid changes in its physicochemical and microbiological characteristics. Various approaches are used to monitor these changes and analyse the data obtained. In this paper, concentration-discharge hysteresis analysis is used to define and compare the recharge characteristics of two karst springs. The study was conducted in a binary karst aquifer in the catchment of the Malenščica and Unica springs in south-western Slovenia, an area known for complex hydrodynamics with both autogenic and allogenic recharge. The area offers extensive prior hydrogeological knowledge and is equipped with a dense monitoring network that has been operating since 2016. High-frequency monitoring of temperature, specific conductance, and discharge in springs, water-active caves, and sinking streams was supplemented by event-based sampling for chemical and microbiological analyses. The data obtained were first analysed using descriptive statistics to compare the characteristics of individual monitoring sites and to infer connections between them. In the next step, individual storm events under various hydrological conditions were analysed in more detail. Hysteresis plots between discharge and specific conductance of the springs were compared with those of the sinking streams and underground channels in their catchments to characterise the mixing of groundwater originating from different recharge sources. Additionally, for a selected storm event, hysteresis plots between discharge and water quality parameters (specific conductance, temperature, turbidity, [Ca2+/Mg2+] ratio, Cl-, and bacteria) were analysed. Significant differences in recharge characteristics, water transfer through the system, and the chemical and microbiological composition between the two springs were detected. These findings provided information about the differences in the spatial and temporal distribution of autogenic and allogenic recharge of the two springs, which is very important for planning the protection of these karst water sources.