Sea water intrusion (SWI) has been widely recognized as a global problem, significantly influencing coastal aquifers, mostly through reduced water quality and agricultural production indicators. In this paper, we present the outcomes of the implementation of two independent real-time monitoring systems, planned and installed to get insights on groundwater dynamics within the adjacent coastal aquifer systems, one located in the Neretva Valley, southeastern Croatia, the other located south of the Venice lagoon, northeastern Italy. Both systems are presented with technical details and the capacity to observe, store, and transmit (Neretva site) observed values in real-time. Analysis of time series reveals the significant influence of the sea level oscillations onto the observed groundwater electrical conductivity (EC) and piezometric head values, while precipitation rate is detected as a driving mechanism for groundwater parameters in shallow geological units. The installed monitoring systems are shown to be of great importance to provide qualitative and quantitative information on the processes influencing groundwater and surface water dynamics within two coastal systems.

Hydrogeological data availability is often limited to local areas where usual in situ tests or methods are applied (slug/bail or pumping tests, Electrical Resistivity Tomography (ERT)). Because most problems (e.g., saltwater intrusion mitigation) require problem analysis on larger scales (catchment or sub catchment), hydrogeological identification of global character is preferable. This work leads to the determination of aquifer hydrogeological parameters on the basis of observed sea level, groundwater piezometric head found inland, and barometric pressure. When applied to observed signals, the approach led efficiently to final hydrogeological characterization. After identification of dominant tidal constituents from observed signals, barometric efficiency was successfully determined. Following available information on geological settings, an appropriate conceptual model was applied and updated to count for polychromatic signals. Final determination of hydrogeological parameters relied on root mean square error (RMSE) minimization and led to determination of (i) presence of three stratigraphic units: unconfined sandy aquifer on the top, a confining layer made of clay, and a confined gravel layer; (ii) existence of the clay layer under the sea with a total length of 1400 m; (iii) a clay layer has been identified as confining one by both spectral analysis and determined leakance value; and (iv) estimated confined aquifer specific storage ranging from 2.87 × 10⁻⁶ to 4.98 × 10⁻⁶ (m⁻¹), whereas hydraulic conductivity ranged from 7.0 × 10⁻⁴ to 7.5 × 10⁻³ (m s⁻¹). Both range intervals corresponded to previous in situ findings conducted within the area of interest.

In this paper, we present an approach based on the simultaneous use of multilevel monitoring systems for the ground and surface water and wavelet-based analysis of the time series observed to detect the main mechanisms influencing the water level, electrical conductivity (EC) and temperature (T) in ground and surface water of the river Neretva coastal aquifer system. Although the area of interest is under significant impact of seawater intrusion, the presented approach enabled the insight to transient response of the system to external loadings like precipitation and irrigation system primarily. The capacity of the monitoring system has been demonstrated on two different subareas, respectively, Diga and Jasenska. While the Diga area is shown to be under significant influence of the seawater, the Jasenska area shows sensitivity to precipitation and pumping station (PS) Modric operative regime. Groundwater parameters as observed during dry periods at Jasenska show the presence of stratification. Wavelet-based analysis enabled the determination of the interdependence of the variables of interest as well as their temporal dependence. Determined regimes of the surface water, as found within the Jasenska channel, are additionally supported by the in situ profiling results, thus strengthening the findings of the study. Our findings reflect the capacity of the approach to capture for the stratification of the groundwater parameters induced by seawater intrusion and external loadings.

Our work presents a reliable procedure to obtain real-time assessment of the sea water elevation at the Kaštela Bay site to ensure the a priori warning in the case of expected coastal flooding along the site area. In its origin, the presented procedure relies on relevant data sets which are site-specific and locally observed. Observed data sets are used within the procedure to assess sea water surface elevation when induced by barometric pressure changes and wind-generated waves. Tidal-induced changes are introduced into the assessment procedure by a pre-learned algorithm which relies on long-term sea level oscillations from the relevant tidal gauge. Wind-generated wave heights are determined in the near shore area, following the features of the depth and reflection of the shoreline subsections. By coupling three mechanisms, this paper offers a unique real-time procedure to determine the sea water elevation and assess the possibility for coastline structure to be flooded by the sea. Given information is visualized in a form of mobile application that implements the algorithm and allows end users to set the notifications based on the given ruleset.

One of the areas of prior interest for the Republic of Croatia in terms of water conservation and water needs, especially for irrigation of agricultural crops, is the river Neretva delta. This area represents the largest agricultural area on the Croatian coast and is the source of existence for the local population. Managing the area has become a complex task given the intensive agricultural activities, the large number of people living near the area of interest, significant seawater intrusion (SWI) and ongoing climate changes manifested by indicators such as sea level rise and increasingly unpredictable precipitation. Therefore, the need to understand the processes responsible for creating favorable/unfavorable conditions in both, surface and groundwater, is self-evident. A review of the available literature reveals that SWI in the coastal aquifer system of the Neretva Valley, with a focus on external loadings and their effect on ground and surface water has not been sufficiently investigated. The influence of the operative regime of the melioration system, the hydrological regime of the river Neretva and the tidal characteristics of the Adriatic Sea to ground and surface water fluctuations along the study area was established as the primary scope of this thesis. A variety of methodological approaches was used throughout the thesis to analyze and investigate the impact of SWI on the Neretva coastal aquifer system. Despite the strong effect of the SWI, the area of interest is defined as a complex system that can be controlled naturally or by management. In order to investigate the interaction between factors controlling the surface and ground water along the Neretva Valley in conjunction with SWI, a monitoring system was designed and implemented to collect pertinent data sets and time series. An investigation was conducted utilizing time series in the temporal and frequency domains. The application of cross correlation methods, the Discrete Fourier Transform and the Continuous Wavelet Transform provided insights into the interdependence of analyzed ground and surface water parameters. Based on past in-situ work and time series of sea water elevation and piezometric head, the study area was hydrogeologically characterized. Previously collected in-situ data were used for the conceptualization of the coastal aquifer system. Tidal methods were upgraded to incorporate tidal signals with more than one harmonic constituent to improve the method’s capacity. Findings are focused on determining specific storage, hydraulic conductivity and leakance between shallow unconfined and deeper confined aquifer units. Novel in-situ and laboratory results were utilized to bolster these findings. The confinement of the deeper aquifer was confirmed in two ways: i) by using frequency analysis of constituents typical for confined conditions which confirm confinement of deeper aquifer and ii) by using ERT results which do not evidence the absence of confining clay layer along the study area. The determined values of hydraulic conductivity were consistent with the gravely material, which was confirmed by interpreting the characteristic grain values determined in the laboratory from borehole cores taken from the study area. Based on the hydrogeological characterization and determination of the connectivity between the geological units of the aquifer, the predominant SWI corridors characterizing rain and dry seasons were identified. To comprehend the transient nature of surface and groundwater parameters such as temperature, piezometric head, and electrical conductivity, a number of external loadings were considered. The confined aquifer appears to maintain its electrical conductivity regime independently from the unconfined aquifer above and vice versa. The electrical conductivity in the confined aquifer shows no significant correlation with tidally-induced oscillations, while piezometric head as observed within the confined aquifer is shown to be driven by tidal oscillations. In addition to the confined aquifer, it was demonstrated that the Adriatic Sea is the dominant factor controlling the groundwater regime in the Diga area, as found in the unconfined aquifer. The melioration system’s operative regime creates favorable agricultural conditions in the study area. As a result, groundwater has been delineated below sea levels, thereby creating active SWI conditions that increase the inflow of seawater from the Adriatic Sea. The majority of precipitation occurs during the rain season, which creates an imbalance in the natural aquifer recharge. In both rain and dry seasons, the river Neretva's hydrological regime plays a dominant role but with opposite effects. During the dry season, seawater intrudes upstream along the Neretva riverbed, generating a source of seawater that impacts the Jasenska region. Specific attention has been given to the sub melioration systems of Vidrice and Opuzen ušće. The groundwater of the coastal area of Opuzen ušće, called Diga, was identified as dominantly tide controlled, while the Vidrice area is predominantly influenced by the presence of upstream springs of brackish water which enter the melioration system. It was discovered that only the Vidrice area has higher salinity during the rain season than during the dry season. The operative regime of the melioration system controls the dynamics of water quality as found along the Vidrice area, while the groundwater in the Jasenska area is salinized primarily by the Neretva River.

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).