A multidisciplinary approach to define the hydrogeological model of aquifer systems in the "Fiume Versilia" catchment and the adjacent coastal plain (Northwest Tuscany, Italy)

This PhD-research regards a hydrogeological study carried out on important aquifer systems located in the Versilia area, in which high water demand is present and consequently a detailed knowledge of the same systems is necessary to optimize the groundwater resources management. In this scenario a multidisciplinary approach, geological, hydrogeological and hydro-chemical-isotopic has allowed to achieve significant results as regards the fractured/karst aquifer systems in the "Fiume Versilia" catchment and the aquifer systems of the adjacent coastal plain. Fractured/karst aquifer of the mountain area. The two main aims of the research regarding these aquifers were: i) to describe and to model the water-rock interaction processes to define a geochemical conceptual model; ii) to individuate the feeding areas of the most important springs by means of the analyses of the main geological structures and hydrogeological/geochemical-isotopic considerations. The geochemical conceptual model allowed to draw the following considerations: - The air particulate matter collected reflects the outcropping lithologies in different parts of the study area, but there seems to be no obvious phenomena of interaction between the particles and rainwater, whose chemical composition derives mainly from sea spray; - Stable water isotopes confirm that groundwaters have a meteoric origin and that the wide range of values found depends essentially on the different average altitude of feeding zone; - Based on the major dissolved elements of the sampled springs, 3 main chemical facies were identified: Ca-HCO3, Ca-SO4 and Na-Cl. The first two highlight the interaction of water with limestone and carbonate-evaporite rocks, respectively, for a time sufficient to acquire these chemical compositions and to achieve saturation/supersaturation in calcite and dolomite. The Na-Cl groundwaters show a composition not very different from that of rainwater indicating a circulation in rocks containing minerals not very reactive and/or short interaction time with carbonate rocks. These two main types of water-rock interaction are confirmed by the isotopic ratio 13C/12C. In fact, for the first two types (Ca-HCO3 and Ca-SO4 water), 13C values require a significant contribution of carbon derived from dissolution of calcite, while for Na-Cl water, 13C/12C values are consistent with the addition of biogenic CO2 in rainwater; - As regards the reaction path modeling, the two theoretical trends, which describe the dissolution of a local average rock into a local average rainwater, can explain the chemical characteristics of most springs. Finally, with respect to trace metals the theoretical approaches do not show substantial changes from the initial concentrations in the aqueous solution, suggesting that there is a low input of these chemical components in the aqueous phase during the progressive dissolution of carbonate rock. In conclusion, comparing the geological and hydrogeological features with the results of the geochemical processing, it is reasonable to assume that: i) the Na-Cl springs are representative of the shallow circuits, with small feeding zones and very low residence times in the aquifer; ii) the Ca-HCO3 and Ca-SO4 springs are representative of relatively deep circuits developed in extensive aquifers with high permeability, like Marmi, dolomitic marbles and Grezzoni. The firsts type of springs were used to obtain the relationship between the 18O/16O ratio and the altitude of rainwater infiltration. In fact taking into account that they drain a small basin and considering the regulator effect of the aquifers, the isotopic compositions of these springs are very similar to the annual average isotopic values of the local meteoric waters. This relationship was than used to evaluate the average altitude of the feeding area of the second type of springs. Feeding areas were individuated through a multidisciplinary approach using geological, hydrogeological and geochemical-isotopic tools. In particular 17 hydrogeological sections were elaborated in order to define the geometry of the main hydrostructures and to individuate possible groundwater divides. Moreover, in the hydrogeological domain under examination were considered: the injection and recovering points of the tracer tests performed before this research; the zone included between the altitude + 50 m and – 50 m with respect to the average infiltration altitude evaluated by means of isotopic ratios; the possible groundwater divides individuated by means of hydrostructural considerations. All these elements, together with the indications obtained from geochemical processing on the types of rocks mainly involved in water-rock interaction, allowed us to delimit the hydrogeological basins probably drained by the most important springs under study. In addition for each hydrogeological system, a simplified water balance using meteorological data and the effective infiltration coefficients reported in the literature was performed, verifying that the delimited feeding areas are entirely consistent with the flow rate data of the springs. Aquifer system of the coastal plain As regards the coastal plain aquifer system, the main object of the study was to define a reliable conceptual model based on a multidisciplinary approach that highlight the main feeding components and the main processes occurring in the system, based on geological, hydrogeological, and geochemical considerations. A secondary purpose was the implementation of a hydrogeological numerical model and a preliminary calibration under steady-state condition. The first step was the realization of 9 hydrostratigraphic sections to delineate the geometry and thickness of the various hydrogeological complexes constituting the aquifer system. Once delineated the hydrostructural model, all stratigraphic information was re-processed with the “Groundwater Modeling Software” for 3D reconstruction and calculation of the volume and grainsize for each horizon. The hydrostratigraphic elaboration highlights the multilayer type of the aquifer system due to the alternation of sandy and gravelly permeable layers with low and very-low permeability layers consisting of silty and clayey deposits. However the limited thickness and the lack of continuity of the impervious layers allows locally a direct connection between different permeable horizons giving to the system a monolayer character. As evidenced by both water level data and hydrogeochemical data processing, the hydraulic connection between the aquifer horizons is likely present throughout aquifer system, also probably due to the presence of many multiscreening wells. Taking into account the literature data and the results of pumping test performed during this study, the hydraulic conductivity of the permeable horizons is variable from 10-3 m/s to 10-4 m/s. Water level contours, elaborated both in wet and dry season, highlight that an important feeding component toward the coastal plain is the groundwater flow from the upper zone of the Versilia River fan. On the base of the piezometric surface morphology and the transmissivity value of 1,4 E-02 m2/sec, deriving from a pumping test performed on May 2010, the flow rate of this component was evaluated to be closed to 0,38 m3/s and 0,18 m3/s, for high level and low level conditions respectively. Analysing groundwater flow paths, minor inputs toward the coastal plain are individuated on the fans of the secondary rivers and where the limestone complexes of the mountain zone are directly in contact with the permeable horizons of the coastal plain aquifer system. Two principal piezometric minima, with hydraulic head below sea level, are located in the middle part of the plain and near the shoreline, respectively. The first one is caused by drinking water well pumping, whereas the second one is linkable to artificial drainage of the humid area located in the south-western part of the study zone. Geochemical data confirm that Ca-HCO3 waters have meteoric origin and shallow circulation. It is likely that their chemical composition is the result of dissolution of calcite, in a limited or relatively limited time, due to the high kinetic of dissolution of this carbonate mineral. The recharge from the mountain provided by low salinity Ca-HCO3 waters is particularly evident from the foothills to the central strip. Samples collected in the coastal strip have Ca-HCO3 composition only at shallow depths (maximum 10 m). Among Ca-HCO3 waters some samples have a high sulphate content due to partial interaction with carbonate-evaporite rocks and/or other reasons. The Na-Cl-HCO3, mix-Cl-HCO3, mix-HCO3 and Na-HCO3 water are located in the coastal strip and especially near of Forte dei Marmi. It should be emphasized that the presence of these waters must be understood as a positive signal, that is an improvement of water quality previously deteriorated by sea water intrusion. Finally, Na-Cl waters (usually found in relatively deep wells, >30m) are located near the coastline and are likely related to sea water intrusion. The wells that have these characteristics are generally 30m deep or more. The 18O/16O isotopic ratio allowed to identify the different components involved in the aquifer system of the coastal plain. In particular: i) A component characterized by the low isotopic ratios is recognizable starting from the upper part of the Versilia River fan up to the middle strip of the coastal plain. The relatively large extension of territory in which this component found highlights the importance of this input, in agreement with the indication deriving from piezometric surfaces. The low isotopic ratio is not compatible with a local origin of water but requires an input from the mountain area. Considering the hydrostructural features, a groundwater flow from rock reliefs to the porous aquifer of the plain is unlikely in this zone; consequently the input of water from the mountains is through the Versilia River, as confirmed by water levels; ii) a further component, even if less important than the last one, is represented by contributions from permeable carbonate rocks, where these are close to the coastal plane, and from other minor alluvial fans, as T.Baccatoio in the southern part and T.Montignoso in the northern part; iii) a further and significant component is the local rainwater characterized by a specific isotopic ratio. This input is well recognizable in the part of the plain close to the coastline, although its influence on the other input is evident also in the inner part. Finally, the study of isotopes has contributed to investigate in more detail the sea water intrusion phenomenon occurring both directly and through inflow along stream courses, as well as the relationship between groundwaters and main rivers. Direct marine ingression is evident in wells with depths higher than 25-30 m and located in a strip of about 1 km from the coastal line. The ingression inside the water courses, instead, is particularly evident in the Versilia river where the sea water reaches the intermediate strip (travelling for more than 2 km along the course of the river, with about 15% of mixing). The ingression inside the water course occurs also in the T.Baccatoio/F.Motrone with a maximum distance of about 1 km from the coastline. Finally, for the Fiumetto ditch the ingression is stopped by the Vinciane Port at 500 m from the coastline. In agreement with stream water level/groundwater head level relationship, geochemical and isotopic features of water show that rivers/streams/ditches are in hydraulic connection with the aquifer almost throughout the coastal plain. Since the Versilia River has an important role in aquifer feeding, a detailed geochemical survey was carried out to better investigate the stream water-groundwater relationship. Obtained results indicate that the river feeds the aquifer from the upper part of the plain up to the middle one (ex-Lago di Porta zone) and in the final stretch of the river course. In the ex-Lago di Porta zone the river is likely not connected with the aquifer. In conclusion, based on hydrostructural, hydrogeological, and geochemical-isotopic considerations the main input to the aquifer system are: - an important component from the Versilia River alluvial fan, which is essentially recharged by the river itself; - a component, less important than the previous one, developed in minor alluvial fans and some carbonate rocks in the foothill strip; - an important local rainwater infiltration; - stream water infiltration; - a sea water component, even if of minor importance, developed both directly from the shoreline to the aquifer and indirectly through of the stream courses. Based on the above described hydrogeological conceptual model, a numerical flow model was implemented and calibrated under steady state condition. The results of the calibration generally confirm the conceptual model, although in some zones differences between observed and model values are significant and suggest that model refinement is necessary. This is essentially due to the insufficient and not homogeneous distribution of some type of data, as hydraulic parameters and stratigraphic information. Following these possible improvements, a transient flow model and transport model could be implemented also thanks to the amount and quality of data produced during this research. Lastly, the results of this PhD-research not only are scientifically interesting but can also have practical implications for the protection and the correct management of the groundwater resource. For example, the multidisciplinary approach used for delimiting the hydrogeological basin of some important springs in the mountain area responds to the need to define the protection zones of water resources, as recommended by the European Community normative and the national normative. On the other hand, the conceptual/numerical model obtained for the coastal plain aquifer system represents an important tool for a correct planning and regulation of groundwater exploitation, to preclude a qualitative and quantitative deterioration of this indispensable resource