Model based quantification of salinization dynamics under changing hydrological conditions in the Volturno River (Italy) coastal aquifer

• 1D HEC-RAS numerical model was built up and run for a decade (2010–2020). • Simulated river salinities were used in a 3D SEAWAT model covering the same period. • Increasing groundwater salinization pattern due to intrusion from riverbed leakage. • Paleo seawater trapped in aquitards is the major source of groundwater salinization. This work presents a semi-coupled modelling approach to study salinization dynamics in the Volturno River coastal aquifer (Italy), distinguishing among different salinization mechanisms. The area is of particular interest, given its location in the Mediterranean region, a climate change hot-spot. A 1D HEC-RAS numerical model was built up and run for a decade (2010–2020) to quantify the areal extent and timing of salinization events due to seawater encroachment within the Volturno River mouth. The results were used as input in a 3D SEAWAT model that incorporated salinity variations on a monthly basis for the same period. The SEAWAT model was downscaled from a large calibrated MODFLOW model of the whole Campania region. Both national and worldwide databases were used to constrain the models. The model was then compared with 9 high resolution vertical profiles of porewater salinity obtained using a continuous coring sediment sampler, providing good model performance indicators (R 2 = 0.867, NSE = 0.808, and RMSE = 3.926 g/L). Results highlight an increasing groundwater salinization pattern due to intrusion from the Volturno riverbed. The classical mechanism of seawater wedge intrusion from the coastline was minimal, while large inland portions of the model domain were characterized by high salinity (up to 75 g/l) due to remnant paleo seawater trapped into peaty and silty-clay aquitards. This physically-based modelling approach could be replicated in any coastal porous aquifer (if hydrological and hydrogeological datasets are available) to identify and quantify the salinization mechanisms and to help water managers to implement tailored solutions in the most affected areas.