Study of seawater intrusion in the coastal areas of Circeo National Park and Litorale Romano Natural Reserve, for the implementation of numerical modeling methods

Abstract

Coastal plains represent the most populated areas of the world because they allow an industrial, urban and agricultural development. Groundwater is the main resource in coastal plains and it can satisfy the water demand for these activities. Coastal aquifers represent the place where continental freshwater and seawater meet. Freshwater tends to overly seawater because of its lower density, creating a fragile balance. Nevertheless some processes, both natural and anthropic, can perturb this balance determining groundwater salinization. The main human induced salinization process is the upconing. Natural processes can be the lateral intrusion, evaporite dissolution, fossil water contamination, sea spray aerosol deposition and salt-wedge intrusion in river mouths. Last two processes were deepened in the present study because they are usually not so diffused in regional researches. The aim of this thesis was the study of groundwater salinization induced by rivers or anthropic canals, determined by the wind action or the variation of fluvial discharge rate in time, in areas where protected and anthropic areas coexist: the Tiber Delta and the Pontina Plain. The areas were chosen because they exhibit analogies: both are composed by multi-layered sedimentary aquifers and coastal morphology is strongly affected by reclamation works of the past centuries. The major difference is represented by the presence of different hydrological bodies: in the Tiber Delta area a great river is present and it influences salinization mechanisms in the hydrogeological system, whereas in the Pontina Plain lakes and reclamation canals determine the distribution and movement of seawater. This study was developed with several methods, such as hydrological and hydrogeological surveys, multitemporal chemo-physical surveys of groundwater, hydrochemical surveys, remote sensing techniques, statistical analyses about meteorological data and the use of the most innovative techniques of numerical modeling. In the Tiber Delta area the importance of the westerly winds for the salinization processes was investigated. In fact the winds determine both a salt-wedge emplacement in the Tiber mouth during the falls, and a groundwater salinization in the Litorale Romano Natural Reserve pine forest because of sea spray aerosols deposition. The salt-wedge presence in the Tiber River, noticed up to 8.8 km from the Tiber mouth, is supposed to be the more likely reason of the seawater intrusion processes identified in the alluvial area of Ostia Antica. Furthermore regional hydrochemical analyses permitted to identify freshening processes ascribable to ionexchange, related to the presence of historical solar salt works in the Ostia Antica area. In the Pontina Plain, along the coastal area which hosts the Circeo National Park, salinization resulted linked to the presence of seawater in reclamation canals and lakes. As suggested by the dissertation title, the final aim of the work was the implementation of a numerical model, to detect salinization processes. The use of numerical modeling is a further investigation tool to analyze and quantify what happens in a complex hydrogeological system. The implementation of numerical modeling was performed using the software FEFLOW 6.0, to understand the relationships between groundwater and a reclamation canal salinization. The 2D model, implemented in a transient flow-mass transport state, was located at the Moscarello Canal mouth (Pontina Plain), where a salt-wedge intrusion was noticed. The canal resulted to be subject to a strong discharge modification, which is able to determine a hydraulic gradient and a consequent mass transport in the adjacent aquifer. This transport through the canal bottom resulted able to induce the movement of a saline plume present in the aquifer, determining hence an enhancement of salinity values collected in a piezometer.