Groundwater characterization focusing on the seawater intrusion risk in the Metaponto coastal plain (Basilicata, southern Italy)

This paper aims to assess and model the seawater intrusion (SWI) risk in the Metaponto coastal aquifer (Basilicata, southern Italy) by characterising its intrinsic geological and hydrogeological setting and analysing the external factors such as excessive withdrawals and climate change causing modifications in the recharge process. Groundwater resources are crucial for the economic development of intensive agricultural activities and tourism in the Metaponto plain. It represents for the entire Basilicata region a relevant economic area for its marked agricultural vocation and the presence of residential settlements, tourist facilities, and natural areas such as woodlands and wetlands. During the 20th century, the anthropogenic impacts, mainly linked to the construction of modern irrigation systems, land reclamation works, overexploitation of wells, and the development of agricultural and industrial activities, have significantly modified the plain. Indeed, the hydrogeological system was negatively impacted by the changes in land use, threatening groundwater availability and quality along the coastal plain and magnifying the risk of aquifer pollution. These modifications also increased soil salinization and SWI risks. Nowadays, groundwater resources are particularly exposed to quantitative degradation due to the historically unfavourable climatic conditions worsened by climate change and the growing water demand and to qualitative degradation also caused by the SWI. The previous hydrochemical investigations, which examined the distribution of dissolved ions in groundwater, showed a progressive mixing between freshwater and seawater, highlighting the areas with higher SWI proneness. The groundwater vulnerability to SWI of the entire coastal plain was evaluated through the GALDIT method. Furthermore, numerical simulation modeling was performed for the portion of the plain characterized by a greater propensity to SWI based on the results of hydrochemical analyses and the GALDIT method application. Once the study area was selected, the geological, hydrogeological, hydrological, climatic, and hydrochemical data were acquired to define the accurate conceptual model of the study area, which is the first step for the subsequent groundwater flow numerical simulations, and the modeling of the SWI phenomenon. The MODFLOW and SEAWAT codes were used within the Visual MODFLOW Flex 7.0 software (© 2021 by Waterloo Hydrogeologic) to implement the input data, define the boundary conditions and carry out the numerical modeling of groundwater flow with the following variable-density transport. Groundwater flow simulations were conducted in steady-state and transient conditions. Afterward, starting from the electrical conductivity values measured on selected groundwater samples, the total dissolved solids (TDS) concentrations expressed in mg/l were derived. Subsequently, the transient transport model was implemented. Three scenarios were simulated, considering the impact of climate change on the aquifer recharge and different pumping rates effects. The most significant hydrogeological condition on the SWI risk is attributable to the pumping rate scenario of 100 l/s. Under this condition, the SWI risk is not negligible in the future, and it can cause negative effects on the quality of the coastal aquifer groundwater. The results highlighted that the aquifer requires reliable management strategies to prevent the SWI phenomenon progression.