Saltwater intrusion increasingly jeopardizes groundwater in low-lying coastal plains worldwide, where the combined effects of sea-level rise, land subsidence, and hydraulic regulation further exacerbate aquifer vulnerability and threaten the long-term sustainability of freshwater supplies. To move beyond sparse and fragmented piezometric observations, we propose “integrated coastal supersites”: wells equipped with multiparametric sensors and multilevel piezometers that couple high-resolution vertical conductivity–temperature–depth (CTD) profiling with continuous hydro-meteorological time series to monitor the hydrodynamic behavior of coastal aquifers and saltwater intrusion. This study describes the installation of two supersites and presents early insights from the first monitoring period, which, despite a short observation window limited to the summer season (July–September 2025), demonstrate the effectiveness of this approach. Two contrasting supersites were deployed in the coastal plain between the Brenta and Adige Rivers (Italy): Gorzone, characterized by a thick, laterally persistent aquitard, and Buoro, where the aquitard is thinner and discontinuous. Profiles and fixed sensors at both sites reveal a consistent fresh-to-saline transition in the phreatic aquifers and a secondary freshwater lens capping the confined systems. At Gorzone, the confining layer hydraulically isolates the deeper aquifer, preserving low salinity beneath a saline, tidally constrained phreatic zone. Groundwater heads oscillate by about 0.2 m, and rainfall events do not dilute salinity; instead, pressure transients—amplified by drainage regulation and inland-propagating tides—induce short-lived EC increases via upconing. Buoro shows smaller water-level variations, not always linked to rainfall, and, in contrast, exhibits partial vertical connectivity and faster dynamics: phreatic heads respond chiefly to internal drainage and local recharge, with rises rapidly damped by pumping, while salinity remains steady without episodic peaks. The confined aquifer shows buffered, delayed responses to surface forcings. Although the monitoring window is currently limited to 2025 through the summer season, these results offer compelling evidence that coastal supersites are reliable, scalable, and management-critical relevance platforms for groundwater calibration, forecasting, and long-term assessment.

The river Neretva coastal aquifer system is the largest agricultural area along Croatia's Adriatic Sea coast. With the construction of an embankment, pumping stations, and melioration channels, a once-marshy area has been transformed into an area with favourable agricultural conditions.  Due to its proximity to the sea, numerous anthropogenic impacts in the region, and climate change, groundwater and surface water in the study area are presently significantly influenced by the salinization process. Multiple times per year, groundwater and surface water samples are collected to fully comprehend saltwater processes and the origin of saltwater in the study area. Within each sample, the concentration of major ions (K+, Na+, Ca+, Mg2+, Cl-, HCO3- and SO42-) is determined under laboratory conditions.The analysis of major ions indicates that seawater intrusion is the primary source of salinization in both unconfined and confined aquifers. In contrast to unconfined aquifer, confined aquifer is minimally affected by precipitation and surface water regimes. In addition, samples of groundwater from unconfined aquifer are categorized into three groups based on their degree of contamination with seawater. The samples from the Diga area that are closest to the Adriatic Sea are most significantly influenced by the seawater. The groundwater quality in the Jasenska area varied significantly between dry and rain periods, whereas groundwater samples from the area of Vidrice revealed the lowest level of seawater contamination.   Neretva river surface water samples reveal the presence of a salt wedge, while river Mala Neretva samples indicate that river Mala Neretva is the primary source of freshwater in the study area during dry season.

Saltwater intrusion in coastal aquifers is a global problem recently worsened by anthropogenic activities (e.g., aquifer overexploitation, hydraulic reclamation and drainage of low-lying areas) and climate change effects (e.g., severe droughts, sea level rise) that contribute to reduce groundwater natural recharge, water quality, and agricultural production. Many low-lying coastal plains facing the Adriatic Sea are strongly affected by saltwater intrusion with serious consequences on agricultural activities and tourism that may become dramatic in a relatively short time due to climate change. In this framework, this work aims to identify monitoring strategies to characterize the process of saltwater intrusion under the effects of climate change and recommend appropriate countermeasures in two Adriatic low-lying coastal plain: south of the Venice Lagoon (north-eastern Italy), and at the Neretva River mouth (south-eastern Croatia).Geomorphologic, stratigraphic, hydrogeologic, and agricultural data were collected to characterize the aquifer system at both sites and assess the effects of seawater intrusion on agricultural productivity. Saltwater intrusion was monitored and analysed through monitoring systems that provide qualitative and quantitative information on the processes influencing groundwater and surface water dynamics within the two coastal systems. Moreover, laboratory physical models were developed to serve as benchmarks for the numerical models used to simulate the field results. Numerical modelling reliably implements boundary and initial conditions defined in-situ on both sites, simulates existing states, specifies different scenarios, and predicts salinization dynamic changes caused by climate changes.The results of the research activities include the development of specific tools for the management of agriculture-related activities and freshwater resources in coastal areas including vulnerability assessment, mitigation plans, and countermeasures against salt contamination. These results were obtained by integrating the findings gained on both sites, considering differences and peculiarities of the specific areas that are representative of many low-lying plains located on both sides of the Adriatic coast.This study has been funded by the contribution from the EU cofinancing and the Interreg Italy–Croatia Cross Border Collaboration (CBC) Programme 2014–2020 (Priority Axes: Safety and Resilience) through the European Regional Development Fund as a part of the projects MoST (AID: 10047742) and SeCure (AID: 10419304).

Our study stems from in situ observations of temperature, EC and salinity performed during 2021-2023 along the river Neretva bed within the Republic of Croatia territory. Apart from the local water column profiling performed several times per year, mostly during dry period, continuous observations of EC, and temperature have been performed at fixed locations and for variable depths. As a main driving forces controlling the termohaline stratification of the Neretva water column caused by seawater intrusion, river Neretva discharge and Adriatic Sea level have been observed continuously. The data sets offer insight to changes in seawater-freshwater interface (SFI) and its shape ranging from typical salt wedge to complete stratification diminishing conditions. Hereby, within the data sets we identify three main scenarios of mechanisms controlling the seawater intrusions and thus the SFI: i) dominant influence of the mean sea level during the dry period with natural discharge kept below 250 m3/s, ii) dominant influence of intermittent discharge events caused by upstream hydropower plant operation and iii) rain period with on average annual duration of app. 25 % when river Neretva natural discharge controls the salinity vanishing from the river bed downstream. In this way, specific changes in the salinity corresponding to different time scales have been determined. Natural hydrologically induced changes in the salinity are identified to correspond to largest time scales of several days and even weeks, unless intermittent discharge caused changes occur very fast decrease in the salinity, typically less than three hours, with recovery time corresponding to app. three to six hours.Although the data sets offer the definition of different time scale changes in the river salinity as mentioned above, an improvement in continuous stratification observation has been suggested and implemented as a result of conducted study.

Područje doline rijeke Neretve suočava se s izraženom intruzijom morske vode, osobito tijekom sušnog perioda godine. Ova pojava predstavlja značajnu prijetnju za poljoprivrednu proizvodnju, opskrbu pitkom vodom i očuvanje ekološke ravnoteže područja. Sustavno praćenje hidroloških parametara i parametara stanja voda na području vodonosnog sustava delte rijeke Neretve te na profilima duž rijeke, ukazuje na intezivan prodor morske vode kroz samo korito rijeke. Uspostava numeričkog modela postojećeg stanja naglašava sezonske razlike režima slanosti i intezitet intruzije morske vode kroz korito rijeke Neretve. Ovaj rad prikazuje rezultate podataka prikupljenih monitoring aktivnostima i korištenih u analizirama numeričkim modelom te predlaže tehnička i operativna poboljšanja za unaprjeđenje upravljanja vodnim resursima u području delte.