Coupled application of single-storm events and tracer analyses to characterize the flow and transport features of a karst aquifer

Karst areas are characterized by the solubility of carbonate rocks, resulting in predominant fast drainage through numerous fractures and conduits. There are important karst aquifers that supply water to a large part of the world's population. Recharge conditions, together with the internal characteristics of the karst aquifer system, are the most important factors determining the functioning of karst aquifers. Water infiltrating a binary karst aquifer may originate from precipitation (rain and snow) in the karst area itself (autogenic recharge) or it may accumulate in adjacent regions of relatively insoluble rock and flow into the karst as sinking streams (allogenic recharge). Understanding these processes is therefore essential for planning appropriate use and protection of karst water resources. A typical example of a binary karst aquifer is located on the southern edge of Planinsko polje in southwestern Slovenia and is drained by the Malenščica and Unica springs. The catchment area of the springs is characterised by a complex structure and a network of sinking streams, springs and groundwater flows. In addition, the Malenščica spring is used to supply water to almost 23,000 inhabitants. Previous geomorphological, speleological, geological and hydrogeological studies as well as several tracer tests have defined and confirmed underground flow connections in the selected study area of the Malenščica spring catchment. In addition, natural tracers were occasionally monitored and chemical and microbiological analyses of water samples were regularly performed. Unfortunately, no simultaneous measurements were made in sinking streams and water-active caves in the spring recharge area to additionally investigate these relationships. The multidisciplinary approach to analyse single storm events has not yet been carried out in the study area. The proposed project will therefore focus on the Malenščica spring, a regional water resource that is potentially at risk and needs to be better understood in order to ensure water quality in the future. The main objective is to investigate the impact of single storm events on the response of a karst aquifer through multidisciplinary analyses, to determine the role of recharge processes under different hydrological conditions and to investigate how water quantity and quality change during single storm events at different observation points. In addition, improved conceptual model of the studied karst aquifer will present a basis for appropriate action plans to protect the Malenščica spring as a regionally important source of drinking water. As part of the work, a complete hydrological and meteorological measurement campaign will be carried out, including continuous measurements of physical and chemical parameters, water quality samples, discharge measurements and precipitation measurements. These measurements will be complemented by high-frequency water sampling for major ions and microbiological indicators of two storm events under significantly different hydrological conditions. Under hydrological conditions that allow for rapid transfer of pollution, a combined tracer test will also be conducted where the two injection points will be allogenic and autogenic recharge points. A time series analysis of several individual storm events will be conducted to extract a range of hydrological and hydrochemical parameters that can be used for multivariate statistics. The results will contribute to a better understanding of the characteristics of water flow and solute transport in the catchment of karst aquifers. Lumped-parameter modelling will be used for additional evaluation of the results. The comprehensive study could be used in the future when preparing the risk assessment in the framework of planning the protection of the Malenščica spring as a water resource.