The alluvial aquifer of the Lower Magra Basin (La Spezia, Italy): conceptual hydrogeochemical-hydrogeological model, behavior of solutes, and groundwater dynamics

摘要

The alluvial aquifer of the Lower Magra Basin (La Spezia Province, Italy) is mainly recharged by two main rivers of the region, Magra and Vara, with a subordinate contribution from rain waters infiltrating in the eastern horst adjacent to the graben of the Lower Magra River. This conceptual model is consistent with the constraints provided by both piezometric maps and chemical and isotopic data of local groundwaters and fluvial waters. Chemical data also suggest that, in the considered system, Cl and SO_4 have mobile (conservative) behaviour, and Na, K, and Sr have close-to-mobile behaviour; in other words, the contents of these chemical components are essentially fixed by mixing relationships between Magra and Vara river waters. In contrast, Mg, Ca, Ba, HCO_3, and SiO_2 are largely contributed by water-rock interaction. Unexpectedly, Li has non-mobile behavior owing to either incorporation into secondary minerals or sorption onto solid phases or both. The discovery of two natural sources, one of Cl and the other of SO_4, in the basin of the Aullella Creek, a tributary of the Upper Magra River, made possible a key understanding of groundwater dynamics. Every year, during the summer, a mass of Cl and SO_4-rich waters originating from the Aullella Creek enters the alluvial aquifer of the Lower Magra Basin. Average groundwater velocity and average permeability, representative of the entire alluvial aquifer of interest, were derived based on the movement of this high-salinity water mass.