Saltwater has invaded the coastal aquifer along the southern Adriatic coast of the Po Plain in Italy. The topography, morphology and land use of the region is complex: rivers, canals, wetlands, lagoons, urban, industrial and agricultural areas and tourist establishments all coexist in a small area. Water table and iso-salinity maps show that in four study areas (Ancona-Bellocchio, Marina Romea, San Vitale Forest, Cervia) out of five, the water tables are below sea level and saltwater has replaced freshwater in the aquifer. The fifth area (Classe Forest) has a relatively pristine freshwater aquifer thanks to an average water-table height of 2m above sea level, a lower hydraulic conductivity (< 7.7m/day) and a continuous dune system along the coast. Only in this area is the topography high enough to maintain freshwater heads that can counteract saltwater intrusion according to the Ghyben-Herzberg principle. Furthermore, the climate, with an average yearly precipitation of 606mm and an average temperature of 14.4°C, allows for little recharge of the aquifer. Ongoing subsidence, encroachment of sea water along rivers and canals, as well as drainage from agricultural land also enhance the salinization process

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Rivers and aquifers are, in many cases, a connected resource and as such the interactions between them need to be understood and quantified for the resource to be managed appropriately. The objective of this paper is to advance the understanding of riveraquifer interactions processes in semi-arid environments stressed by groundwater abstraction. This is performed using data from a specific catchment where records of precipitation, evapotranspiration, river flow, groundwater levels and groundwater abstraction are analysed using basic statistics, hydrograph analysis and a simple mathematical model to determine the processes causing the spatial and temporal changes in river-aquifer interactions. This combined approach provides a novel but simple methodology to analyse river-aquifer interactions, which can be applied to catchments worldwide.The analysis revealed that the groundwater levels have declined (~3 m) since the onset of groundwater abstraction. The decline is predominantly due to the abstraction rather than climatic changes (r = 0.84 for the relationship between groundwater abstraction and groundwater levels; r = 0.92 for the relationship between decline in groundwater levels and magnitude of seasonal drawdown). It is then demonstrated that, since the onset of abstraction, the river has changed from being gaining to losing during low-flow periods, defined as periods with flow less than 0.5, 1.0 or 1.5 GL/day (1 GL/day = 1 Â 10 6 m 3 /day). If defined as < 1.0 GL/day, low-flow periods constitute approximately 65% of the river flows; the periods where the river is losing at low-flow conditions are thus significant. Importantly, there was a significant delay (> 10 years) between the onset of groundwater abstraction and the changeover from gaining to losing conditions. Finally, a relationship between the groundwater gradient towards the river and the river flow at low-flow is demonstrated. The results have important implications for water management as well as water ecology and quality.

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