Local Peculiarities in the Dynamic Behaviour of the Salinity Regime in the Neretva Valley, Croatia

The adverse effects of seawater intrusion (SWI) in coastal aquifers are considered a major threat worldwide, primarily through reductions in water quality and agricultural crop production. These negative impacts are expected to increase as climate change progresses. The Neretva Valley is located in the south-eastern part of Croatia on the coast of the Adriatic Sea. The potential of the valley for agricultural activity was recognised long ago and melioration and construction of infrastructure in the Neretva Valley began in the 1960s. Nowadays, about 30 000 people live near the valley and their main income comes from agriculture. The Neretva Valley in its present state consists of a complex network of melioration channels, embankment, pumping stations which together with the sea and two rivers forms different microsites within the study area, each microsite with different salinity regime. To understand the dynamic behaviour of the salinity regime in the area, two microsites (Jasenska, Vidrice) were used for further analysis. Both are located in the central part of the study area with different boundary conditions. Borehole profiles were conducted at both sites using a multiparameter probe, mainly during the dry period of the year when salinity is expected to be higher in the study area. In addition to the borehole profiles, continuous measurements of groundwater level (GWL) and electrical conductivity (EC) in the unconfined aquifer are observed at each microsite and analysed in temporal and frequency domain to point out the transient nature of the SWI. By analysing all the results at both microsites, the conclusions can be summarised as follows: (i) Jasenska shows significant vertical stratification during the dry period and significant change of EC between rain and dry seasons, indicating that the SWI process in Jasenska is present with seasonal periodicity, (ii) The temperature profiles in Jasenska show that the temperature at the bottom of the aquifer in Jasenska is relatively constant during the dry season and has a pronounced vertical stratification, further highlighting the inert nature of the SWI process, (iii) Vidrice has relatively constant values of EC and temperature with depth change, indicating that the Vidrice microsite is dominated by advection, further emphasising the complexity of the Neretva Valley.