land Article Influence of Carbonate-Flysch Contact and Groundwater Dynamics on the Occurrence of Geohazards in Istria, Croatia Sanja Dugonji´ c Jovanˇ cevi´ c* , Josip Rubini´ c , Igor Ruži´ c and Maja Radiši´ c   Citation: Dugonji´ c Jovanˇ cevi´ c, S.; Rubini´ c, J.; Ruži´ c, I.; Radiši´ c, M. Influence of Carbonate-Flysch Contact and Groundwater Dynamics on the Occurrence of Geohazards in Istria, Croatia. Land 2021, 10, 441. https:// doi.org/10.3390/land10050441 Academic Editors: Domenico Calcaterra, Diego Di Martire, Luigi Guerriero and Roberto Tomás Received: 15 March 2021 Accepted: 18 April 2021 Published: 21 April 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Faculty of Civil Engineering, University of Rijeka, Rijeka 51000, Croatia; jrubinic@gradri.uniri.hr (J.R.); iruzic@uniri.hr (I.R.); maja.radisic@gradri.uniri.hr (M.R.) * Correspondence: sanja.dugonjic@uniri.hr Abstract: This research focuses on the analysis of soil-water interaction at the carbonate-flysch contact on the Istrian peninsula in Croatia. As a result of the interaction of surface and groundwater and the position of flysch and carbonate rocks in the geotechnical profile, two problems occur in the study area: numerous instabilities and the occasionally high turbidity of drinking water. As an example, the St. Ivan spring was considered. The paper presents a complex mechanism of groundwater circulation in geological structures at carbonate-flysch contacts, differences in runoff through karst aquifers and flysch rocks during heavy rainfall under current and predicted (climate change) conditions, and the mentioned geohazards as a result of extreme precipitation. The analyses carried out showed the decisive influence of the existing geological structure on the dynamics of infiltration and precipitation runoff, as well as the risks of pronounced spring water turbidity and instability events. The main drivers of these geohazards are continuous long-term precipitation for landslides and intense daily precipitation for turbidity. Possible consequences of climate change are the increase in precipitation intensity, amount and higher variation, which subsequently brings risks such as the increase in maximum runoff, i.e., the expected more frequent occurrence of high turbidity and the more frequent occurrence of higher cumulative precipitation triggering instabilities in the area. Keywords: flysch; karst; groundwater dynamics; turbidity; landslides; climate change; Istria 1. Introduction According to its geological and geomorphological structure, the peninsula of Istria is divided into three different units: White, Red and Gray Istria. White Istria, named after the white color of the limestone, includes the area of the ´ Ci´ carija massif in the northeast and the Uˇ cka massif in the east, and is characterized by the alternation of carbonate and siliciclastic rocks from the Cretaceous and Palaeogene periods. The study area is located around St. Ivan (Figure 1), at the contact of White and central Gray Istria (named after the gray color of the bare soil in the flysch marls). It is an area of complex geological structure, developed surface and groundwater morphology and complex groundwater flow systems. The aim of this paper is to show how these different geological structures, especially at their contact points, cause very different manifestations of geohazards with precipitation of different durations as their triggers. Both current climatic conditions and potential climate change impacts are considered in the analysis. Frequent landslide phenomena have occurred in flysch rocks, mostly triggered by longer continuous precipitation under current climatic conditions. There is a need to understand how climate change affects these geohydrological hazards, so the occurrence of landslides under climate change conditions in the study area [1] and worldwide [2–5] is increasingly discussed. Gariano and Guzzetti have shown the increasing number of articles on landslide-climate studies published in scientific journals and their geographical distribution [6]. The occurrence of increased turbidity in karst water sources fed by the flysch parts of the basin, associated with changes in the precipitation regime due to the effects of climate change, has been analyzed in several Land 2021, 10, 441. https://doi.org/10.3390/land10050441 https://www.mdpi.com/journal/land