4D Monitoring of sea water intrusion by Electrical Resistivity. Tomography: case study in the coastal alluvial plain of the Volturno River, Italy.

A common problem of coastal aquifers is saltwater intrusion, induced by the flow of seawater into freshwater aquifers due to the groundwater development near the coast. Several factors affect the ingression of sea water. Among these, the most important are the coastal subsidence, the lowering of the sea level, coastal erosion and excessive pumping of groundwater. In fact, where the groundwater is pumped from coastal aquifers, the induced gradients may cause the migration of salt water from the sea to the well, making the freshwater unusable; being the fresh water less dense than salt water, it floats on top. According to the Integrated Coastal Zone Management (ICZM) of the European Commission, coastal areas are of great environmental, economic, social and cultural relevance. Therefore, the implementation of suitable monitoring and protection actions is fundamental for their preservation and for assuring the future use of this resource. Such actions have to be based on an ecosystem perspective for preserving coastal environment integrity and functioning and for planning sustainable resource management of both the marine and terrestrial components. Planning and management of natural resources through a dynamic process has to set, as its objective, the promotion of economic and social welfare of coastal zones. Unfortunately coastal plains are often contaminated by sea water intrusion, and the vulnerability to salinization is probably the most common and diffused problem in an aquifer. The boundary between salt water and fresh water is not distinct; the dispersion and transition zone, or salt-water interface are brackish with salt water and fresh water mixing. Under normal conditions fresh water flows from inland aquifers and recharge areas to coastal discharge areas to the sea. In general, groundwater flows from areas with higher groundwater levels (hydraulic head) to areas with lower groundwater levels. This natural movement of fresh water towards the sea prevents salt water from entering freshwater coastal aquifers (Barlow, 2003). Sea water intrusion in the water table can cause significant worsening in vegetation status. A likely related soil salinization would cause detrimental environmental and socioeconomic impacts. Hence, monitoring the sea water intrusion represents a priority for the safeguard of coastal areas. Identify changes in the freshwater-saltwater interface position can be a useful element for the rationalization of water resources and to guide the choice of use of these areas. Among the geophysical method applied for the location and movement of saltwater intrusion, best results were obtained by electrical methods (Al-Sayed and Al-Quady, 2007; Chitea et al., 2011). Many hydrological processes can be expected to provide significant contrasts in resistivity, consequently, Electrical Resistivity Tomography (ERT) has been adopted as a tool for new research within the hydrology field. Previous workers have demonstrated the ability of ERT to visualize hydrological structure within laboratory cores (Binley et al., 1996a, 1996b), monitor fluid or contaminant migration at the field scale (Daily et al., 1992, 1995; Schima et al., 1993), and to ascertain the efficiency of new contaminant remediation processes (Daily and Ramirez, 1995; LaBrecque et al., 1996). This paper outlines the results of a 3D ERT experiment obtained in the coastal alluvial plain of the Volturno river to assess changes in the freshwater-brine interface. The main aim was to investigate spatial and temporal variations of groundwater salinity. Acquisitions have been carried out in the months of May and October 2013 and in May 2014. This acquisition has allowed to obtain a monitoring 4D salt wedge in a "volume" specific subsurface.