The hydrogeology of the Classical Karst (Kras) aquifer of southwestern Slovenia

The Classical Karst (Kras) region of southwestern Slovenia and north of Trieste, Italy is the type locality of karst terranes. This study quantified the sources of water that contribute to the discharge of the Kras aquifer under various hydrologic conditions. Water chemistry and naturally occurring stable isotopes of oxygen, hydrogen, and carbon in the groundwaters of the aquifer and in nearby surface waters were measured over a period of two years, and were employed as tracers of water movement and to identify water sources. Water samples were collected on a monthly basis from November 1998 to October 2000, and daily during storm events in September and October of 1999 and 2000. Principal Components Analysis (PCA) and End-Member Mixing Analysis (EMMA) were conducted to quantify mixing proportions among three end-member sources: (1) allogenic surface water recharge from the sinking Soča River, (2) autogenic recharge, and (3) an anthropogenic component. The anthropogenic component was characterized by anomalously high Cl− and SO₄2− concentrations, and a Cl −/Br− ratio of 460. Hydrograph recession analysis of the Timavo spring flow was used to identify four hydrologic flow regimes of the aquifer resurgence. The PCA/EMMA was performed on the mean chemical compositions of the four flow regimes, as well as on the data obtained from the storm sampling.; The three end-member sources mix and give rise to 4 chemically and isotopically distinct groups of groundwaters. The results show that the Soča River is the largest component of the flow from all the springs and the water supply well at Klariči. The river provides approximately 56% of the total gauged outflow of the system under average flow conditions. During high flow, there is an increased contribution from autogenic recharge. The anthropogenic component most affects the water supply well at Klariči, providing as much as 32% of the well discharge under conditions of elevated hydraulic head during storm events. Time series analysis of the δ18O data indicated that the transit time of the sinking Soča River to the resurgence zone is on the order of 1–2 months.