Geochemical and isotopic analysis allows for investigating of the origin of the water samples analysed, discriminating the different types of water, and understanding their interactions. This approach is widely used in literature especially to date water resources with wide applications both in Italy (Mayer et al., 2014; Romano et al., 2018; Telloli et al., 2022), in Europe (Brkić et al., 2020; Malov 2021; Balocchi et al., 2022), and all over the word (Copia et al., 2020; Varol et al., 2020).
As better known, the knowledge of the isotopic composition (δD, δ18O and 3H) in surface and groundwater in relation to the isotopic composition of the precipitations allows to determine the average residence time of the water in the aquifers (Campbell et al., 2021; Krajcar Bronić and Barešić, 2021), the possible interconnections between different aquifers (Mahlangu et al., 2020; Andries et al., 2021), the origin of waters (Catalano et al, 2014; Andries et al., 2021) and the dynamics of processes in surface waters (Ramaroson et al., 2018; Schmidt et al., 2020).
In addition, although the 3H concentration in the atmosphere in recent years has reached values in line with the environmental threshold, thanks to the electrolytic enrichment methodologies it has been possible to determine the low concentrations of 3H using the liquid scintillation counting technique (Belachew et al., 2018; Lin et al., 2020).
For this reason, in this research work, an isotopic approach was adopted which is useful for implementing geochemical knowledge and optimally defining and characterizing the interactions between aquifers and the surface waters (e.g., wastewater or canal water).
Stable and radioactive isotopes allow for discrimination of the presence of phreatic and artesian water, which are the most important water resources in the Fiume Veneto area, and which need to be protected caused to climate change and to the water need.
In the sampling site investigated there are a lot of domestic water wells that have not been reported to the competent authorities. The domestic wells data present in the public geodatabase are a close underestimation of the real situation representing the best knowledge framework (Zini et al., 2013). The number of wells reported at authorities is still lower than the actual one, as the transposition of Article 10 of the Legislative Decree 275 (1993) by the population remained largely disregarded (Zini et al., 2011). This is important to know in order to better understand critical aquifer depletion values.
The water-budget scheme of the Friuli-Venezia Giulia region proposed by Zini et al. (2011) shows a slightly positive water budget (+ 2.6 m3 / s) which could be lower, or even negative, if we consider the withdrawals from wells not reported. However, the water-budget is not homogeneous in the Friuli-Venezia Giulia region. In detail, in the Friuli plain on the right hydrographic of the Tagliamento river (Fiume Veneto area), Zini et al. (2011) have estimated that the discharges of the springs belt are a totally of 45.9 m3/s of the 58.4 m3/s deriving from the High Plain. From the difference between the inputs of the High Plain and the springs belt discharges, was estimated the input of the Low Plain as 12.5 m3/s. Considering a withdrawal from aquifers equal to about 31.7 m3/s, the water budget in this area of the plain (and also in the Fiume Veneto area) is greatly affected by the overexploitation of the aquifers.
In addition, considering the data surveyed in the last decades by Granati et al. (2000), a mean discharge per artesian well of 0.8 l/s has been estimated.
Accordingly to these researches, the total withdrawals in the Friuli-Venezia Giulia Region is about 62.4 m3/s. The total withdrawals amount from the confined aquifer systems is 44.4 m3/s, of which 3.3 m3/s in the High Plain and 41.1 m3/s in the Low Plain areas (38.6 m3/s in Friuli-Venezia Giulia Region and 2.5 m3/s in Veneto Region, which is closed to Friuli-Venezia Giulia Region). So, almost 50% of the withdrawals are in the aquifer systems (Zini et al., 2013).
Figure 12 shows the map of the water wells reported to the authorities. The Fiume Veneto area has a lot of domestic wells (1861 – coloured in green in the map of Fig. 12) which represent the 88.3% of the total wells in the same area. But on most of these wells, the depth construction and the quantity of withdrawals are unknown.
Figure 13, on the opposite, shows the map of the wells which have the authority to water withdrawals. In detail, Fig. 13a shows which and how many of these wells draw from groundwater (coloured in blue in the map of Fig. 13a) or superficial courses (coloured in red in the map of Fig. 13a). The withdrawals on the superficial course are very low respect to those on the groundwater.
Regarding the superficial water wells (7 in total), these kinds of wells are used for hydroelectrical activity (95.6%) and less for agricultural processes (4.4%) (Fig. 13b).
On the other side, agricultural processes (38,5%), drinking water for potable uses (39.5%), industrial activity (7.3%) and public fountains (0.3%) exploit wells from groundwater aquifers (Fig. 13b).
Most of the authorized water wells are related to agricultural processes and potable uses. The domestic water wells represented in the maps of Fig. 13 are very low and underestimated compared to reality because they are not authorized, but only reported by the authorities. This confirms the lack of knowledge of how much water the domestic wells can drain and / or waste and above all what type of water they collect based on the aquifer in which they draw (phreatic or artesian).
Based on this data and information, licensed wells have been correlated with depth. Figure 14 shows the average withdrawals in aquifer expressed in m3 / s for the wells reported to the competent authorities and which have allowed them to be taken and which are built at different depth: 0–30 m (coloured in blue – n. of wells 182), 30–90 m (coloured in orange– n. of wells 76) and 90–220 m (coloured in grey – n. of wells 132). The graph in Fig. 14 shows that the maximum withdrawals are related to wells with depths less than 30 m (superficial water). Probably, since the activities declared for the withdrawal are mainly agricultural and industrial, it is cheaper to build a shallow well than a deep well.
However, it must be remembered that this calculation (the results of which are expressed in Fig. 14) was made considering only water wells with sampling licenses and not private ones, therefore most of the domestic wells, which characterize the Low Plain, are missing.
In general, all wells considered for the calculation of the withdrawals are mainly interested the shallower artesian aquifer systems (A, B and C in Fig. 14) and have continuous withdrawal seen that are naturally gushing wells and generally not equipped with discharge reducers. Accordingly, the withdrawals of all the wells considered (also for industrial and agricultural processes) are far higher than the real demands of the population (Zini et al., 2013).
Due to all these gaps in knowledge, it is not possible to accurately quantify the critical values of the over-exploitation of the aquifers, but they can be deduced by reasoning using the data provided by isotopic geochemistry.
Zini et al. (2011) stated that surface aquifers are critical due to high over-exploitation (mainly related to agricultural activities). However, the effective quantity of withdrawal, from both surface and aquifers, is underestimated because of the non-reporting of wells to the competent authorities present on the territory, especially water wells for domestic use. Just think that the estimate of the average withdrawals in the locality of Fiume Veneto was made based on approximately 350 wells declared with known withdrawals (mainly for agricultural activity) compared to the 2.107 wells declared to the authorities without knowing the average withdrawals. To these are added all the wells still not declared to the authorities. For this reason, it is not possible to calculate the correct amount of water withdrawn from these wells, which increases the criticality of the aquifers.
To be able to calculate this gap, it is necessary to take into account the waters of the superficial canals, into which the domestic wells that are not reported are discharged.
The data provided by the isotope analyses in this research work, in fact, indicate that the waters of the superficial canals are correlated with groundwaters. Taking into consideration that the domestic wells not reported are continuous flow, therefore, they withdraw water without any interruption, this suggests that not only the superficial aquifers are over-exploited, but also the deep aquifers, which instead should be protected.