The influence of river bottom topography on salt water encroachment along the Lamone River (Ravenna, Italy), and implications for the salinization of the adjacent coastal aquifer

This study was carried out to quantify the presence of saltwater along the Lamone River in the province of Ravenna, Italy, to estimate the seasonal dynamics of the salt wedge and to establish its contribution to the salinization of the adjacent aquifer. The Lamone River is a small river with a low average discharge of 8 m3/sec that flows from the Apennines to the Adriatic Sea through the southern Po Plain; the salt water encroachment has been monitored by means of monthly vertical electrical conductivity profiles in 32 stations in the centre of the river, along its terminal 8 kilometres. Measurements showed that there is a salinity stratification that is typical of low discharge river estuaries in micro tidal conditions. The electrical conductivity at the bottom of the river equals that of seawater (45 mS/cm) almost all year long. Only the spring flood events that in some cases reach 80 – 100 m3/sec, are able to push the salt water out. Fluvial fresh water in most months is present as a one meter thick layer floating on the dense salt water. At 6.5 km from the sea, the presence of a rocky sill represents an efficient threshold that blocks the salt wedge. The bathymetry of the river was surveyed both indirectly with CTD probe depth measurements and with a coupled DGPS/Ecosounder campaign. Results showed that the Lamone has an unusual inverse riverbed slope, with depth increasing inward from the mouth ranging from –1.5 m.s.l. to – 4 m.s.l. at 6.2 km from the mouth, and a very irregular bottom surface characterized by the presence of holes up to several meters deep. Both the inverse riverbed slope and the deep holes in the river bottom explain why saltwater easily enters the river and remains trapped for most of the year. The salt pools on the river bottom may represent one of the major sources of salinization of the adjacent aquifer. Continuous monitoring of piezometric head and conductivity in observation wells and in the river shows that the two systems are connected and the river transmits the tidal fluctuations to the shallow unconfined aquifer.