Monitoring of the spatial and temporal evolution of marine intrusion in the estuary area of the river Magra and estimation of its effects on the groundwater.

Marine intrusion is one of the main consequences of climate change in coastal areas. The progression of the salt wedge inland compromises the quality and quantity of groundwater, seriously damaging agriculture and gradually desertifying the territory. Marine intrusion is often enhanced by rivers. During the summer period, when the discharge is lower, tidal oscillations can favour the progression of the salt wedge along the riverbed, even for several km inland. This phenomenon is increased in rivers with estuarine morphology. In this research, we present the results of a study conducted in the lower Val di Magra, a coastal territory in central Italy. The river Magra is one of the main estuarine rivers in Italy. In its final part, Magra River flows through a wide alluvial plain. Here it feeds an important coastal aquifer currently exploited for drinking water purposes. The objective of this research is the monitoring of the spatial and temporal evolution of marine intrusion in the estuarine zone of the R. Magra and the estimation of its effects on the coastal aquifer. The monitoring covered a period of 2 years, from 2022 to 2023. The drought conditions during the monitoring years were determined using the monthly Standard Precipitation Index (SPI). The index was calculated using data of the Sarzana weather station from 1932 to the present. The SPI indices were calculated with aggregation at 3, 6, 12 and 24 months. In all scenarios they show a severe-extreme drought condition during 2022 and 2023. River waters were monitored (water electrical conductivity (EC) measurements) and sampled periodically in a series of stations located from the sea until upstream of the estuarine zone. The same type of periodic monitoring also involved a series of wells located along the Magra riverside and instrumented with CTD multi-parameter probes. The monitoring data were collected in maps showing the variations over the year of the surface water EC of the R. Magra. The measurements performed confirm that a natural hydraulic barrier, located between the towns of Romito and Sarzana, currently defines the limit of the R. Magra estuary. The water samples were analysed using an IRMS to obtain Oxygen isotopic values. O18 is used as a natural tracer of the R. Magra waters. The correlation between EC and O18 effectively highlights seawater-freshwater mixing and validates the EC data. The monitoring results show that the river water can be chemically summarised as a 3-element system: R. Magra, R. Vara (a major tributary) and the sea. For each sample, the contribution of the three members can be determined as a function of the pairs of EC-O18 measurements in relation to the values of the pre-mixing end-embers. Using the Montecarlo simulation, the probability of all possible natural values of the end-embers was calculated. A chemical map (EC-O18) of the mixing values between the end-members was produced. The chemical map was compared with data recorded in the wells along the riversides to identify mixing water. This method revealed a fraction of seawater in the aquifer near Romito and a source area downstream of the monitored well.