Anchialine cave-submarine spring system hydrological dynamics traced by temperature – example of Litno, Krka Estuary, Croatia

Hydrological dynamics of a submarine spring and its coastal karst aquifer in Krka Estuary (Croatia) were studied in order to evaluate the potential of temperature as a tracer of hydrological processes. To study the system we monitor the anchialine cave (Litno Cave) that connects the aquifer with the sea, and installed a series of thermometers and depth loggers along vertical profiles in order to continuously monitor the system. The experiment was conducted during three weeks at the end of the dry season and captured the response of the system to the first significant precipitation events. Due to stratification of waters in Krka Estuary, the studied profiles captured the vertical evolution of the thermocline in the estuary and the cave. Temperature was highly correlated to conductivity enabling this parameter to be used as a proxy to identify the source of water in the system. Our results showed that the location of the thermocline in the system depends on the recharge of aquifer via infiltration of precipitation, the evolution of the thermocline in the estuary and the tidal oscillations. As well, we identified the source of water discharging in the submarine spring. At the end of the dry season, the outflow was mostly brackish water resulting from (1) a shallower and inland displaced saline wedge and (2) the aquifer being flooded with estuarine waters due to absence of a significant hydraulic head. However, the aquifer was partially recharged with freshwater after the first significant precipitation event, preventing estuarine waters to flood the aquifer due to the gravitational barrier of the freshwater hydraulic head and the displacement of the saline wedge towards offshore and to deeper positions. Thus, after the initial significant precipitation events the spring discharged freshwater from the aquifer.