Seawater Intrusion in the Plain of Oristano (sardinia, Italy)

The site under study covers an area of 273 km in the Oristano plain on the west coast of Sardinia (Italy). Since 1950 the plain, formed during the Quaternary by alluvial deposits, has been under intensive cultivation and is an important dairy farming centre. In the coastal region, the large number of wells (25,000 tapped and not) some very close to each other, has resulted in over-exploitation of the aquifer system creating a circular circuit of water between the shallow phreatic aquifer and the deeper semi-confined aquifer. As a consequence, a significant deterioration of groundwater quality due to seawater encroachment has been observed in three successive monitoring campaigns (1989, 1995, 2000). This study aims to investigate the causes of saltwater intrusion by developing a model supported by a geographic information system. A three-dimensional finite element model (CODESA 3D) is used to simulate coupled flow and solute transport processes in variably saturated porous media. The data processing steps undertaken to set up the computational model are briefly described and available data are critically reviewed. One of the issues to be addressed is the importance of accurately describing the properties of the semi-confining clayey layer between the two aquifers, as its presence may create a barrier to saltwater intrusion but may also hinder attempts to replenish the deeper aquifer or, vice versa, its local discontinuities may have a strong impact in enhancing salt content.