The influence of hydrogeological properties, seawater intrusion and refreshening on the quality of groundwater used for irrigation in an agricultural coastal plain in North Sardinia, Italy

Intensive agriculture can influence groundwater composition, aquifer biogeochemistry and soil properties; this is especially true in coastal areas where groundwater is the main source of irrigation water. Depending on the irrigation efficiency, the aquifers may suffer from pollutant contamination and salinization caused by the encroaching sea. Information about the chemical composition of the groundwater is not sufficient to permit an evaluation of either the suitability of the water for irrigation or its negative impacts on the quality of the aquifer. In this study, a method for evaluating the suitability of groundwater for irrigation, from the perspective of water quality, is proposed suggesting that groundwater use should depend on the degree of saltwater contamination. A classification of the quality of irrigation water, based on quantifying elements potentially harmful for agricultural productivity, was revised to consider the dynamic movement of the seawater interface (intrusion and freshening phases) and hydrogeological properties. The effectiveness of this method was tested in a cropland-dominated coastal alluvial aquifer (Turritana Plain, Sardinia, Italy) that is irrigated by unconfined groundwater stored in sedimentary deposits. Results show that freshening processes cover almost 53 % of the alluvial aquifer. While the groundwater may be suitable for irrigation from a quality perspective, seawater intrusion affects 47 % of the aquifer, mainly the coastal margins. This method, based on index technique, is a useful tool to help policy makers understand the important role of water resources in an area that is vulnerable to desertification and where semiarid climatic conditions are developing.