Investigating the hydrological behaviour of a shallow karst aquifer flooding intermittently: the Pivka Karst Aquifer (Slovenia)

Abstract

The Pivka Karst Aquifer is a shallow karst aquifer located under the Upper Pivka Valley, about 40 km SW from Ljubljana (Slovenia). This aquifer is connected to the larger Javorniki Karst Aquifer that borders the Upper Pivka Valley on the NE. While the geometry of the conduit system in the Pivka Karst Aquifer is practically unknown, the geometry of the Javorniki Karst Aquifer is better characterized. Under low water conditions, water from the Pivka Karst Aquifer drains through the Javorniki Karst Aquifer towards the Unica and Malenščica Springs in the N, which are the terminal outlets of the region. Under high-water situations, the regional groundwater level rises up to 45 m, and the regional flow direction is modified. The Pivka Karst Aquifer receives water from the Javorniki Karst Aquifer which provides in the meantime autogenic water to the Unica and Malenščica Springs. The rise of water level in the Pivka Karst Aquifer result in the appearance of 17 intermittent lakes in the Upper Pivka Valley. This work aims establishing a conceptual hydrological model of the Pivka Karst Aquifer to better understand its interaction with the Javorniki Karst Aquifer. To do so, a network of automatic stations recording water level, specific electrical conductivity and water temperature at a 30 min interval has been progressively established in the Upper Pivka Valley since 2020. The four years dataset were analysed with data collected in the water active caves of the Javorniki Karst Aquifer and at the Unica and Malenščica Springs. The interpretation of water level records suggest that the Javorniki Karst Aquifer is a large recharge contributor of the Pivka Karst Aquifer, which act as an overflow of the whole system. However, the southern and western parts of the Pivka Karst Aquifer are also recharged locally. Such finding is supported by the analysis of specific electric conductivity data, which suggests the existence of several preferential flow paths in the Pivka Karst Aquifer that activate during flooding.