Saltwater intrusion in the unconfined coastal aquifer of Ravenna (Italy): A numerical model

Summary

The Ravenna pine forests represent an historical landmark in the Po River Plain. They have great environmental, historical and tourist value. The San Vitale pine forest is located 10 km north of the town. It is surrounded by an urban area, the city industrial infrastructure and the waterworks of the agricultural drainage system. Most land in this area is below mean sea level. As a result, no natural freshwater hydraulic gradient contrasts the density gradient of saltwater. In the last century, many events (land subsidence; land reclamation and drainage; urban and industrial development and gas and deep groundwater extractions; coastal dune destruction) led to the intrusion of large volumes of brackish and saline groundwater. Today the freshwater in this coastal aquifer consists of low salinity water lenses floating on the saltwater wedge. This study is aimed at understanding how past and present human activities have affected the saltwater intrusion process in the phreatic aquifer and how the predicted future sea level rise will affect the salinisation process. We used a numerical model to quantify these effects on the density-dependent groundwater flow, hydraulic head and salinity distribution, seepage and salt load fluxes to the surface water system. The simulations show that over the last century artificial subsidence and heavy drainage started the salinisation process in the study area and a relative sea level rise will accelerate the increase in salt load in the coming decades, affecting the entire aquifer. Climatic conditions in the area result in limited precipitations throughout the year and preclude efficient aquifer recharge, especially in spring and summer when saltwater seepage is extensive. The lack of a continuous coastal dune system favors salt wedge intrusion.

Introduction

The San Vitale pine forest near Ravenna represents an historical landmark in the Po River Plain. It extends north–south for 10 km, beginning north of Ravenna, on an area of about 16.6 km² (Fig. 1). It is surrounded by the urban area of Ravenna, the city industrial infrastructure and the waterworks of the agricultural drainage system. The surface hydrographic system includes the course of the Lamone River and a complex system of drainage canals that are managed by three drainage-pumping machines. In addition there are numerous small drainage canals within the pine forest, with floodgates regulating their flow. Various surface water bodies are present in the study area: the Valle Mandriole and Punte Alberete wetlands and the Pialassa Baiona, a semi-natural lagoon adjacent to the pine forest and directly connected with the sea (Fig. 1).

Several natural and anthropogenic features threaten this area: saltwater intrusion in the phreatic aquifer and seawater encroachment inland along the rivers; natural and anthropogenic land subsidence; direct contamination from water bodies open to the sea; destruction of coastal dunes and reduction of their barrier effect; land reclamation drainage systems; insufficient aquifer recharge and sea level rise.

The natural subsidence rate, due to the compaction of alluvial deposits, is 1 mm/year (Selli and Ciabatti, 1977, Pieri and Groppi, 1981); in the last century, however, the main topographical variation of the area has been due to anthropogenic subsidence. Since 1950 and during the industrial development of Ravenna (1970–1980), gas winning and deep groundwater exploitation have led to a fast subsidence rate (for the values, see Sections “The period 1700–1920 AD: natural development” and “The period 1920–1996 AD: severe land subsidence”) (Preti, 2000). Land subsidence has dropped most of the territory below mean sea level, modifying the river and normal groundwater flow regimes. A drainage system is necessary to lower the phreatic level and keep the agricultural land dry. A second objective of the drainage system is to keep tree roots in the coastal areas above the watertable. The result of the drainage system management, however, is an unstable phreatic level in the aquifer that is not able to contrast saltwater intrusion. In addition, it is not helping much to keep the pine forests healthy, as they are sensitive to salinity. Since the phreatic groundwater level in the area is kept very low and the aquifer is in communication with the sea, saline and brackish groundwater intrudes landwards.

The focus of this paper is on saltwater intrusion in one specific coastal groundwater system where a non-uniform density distribution occurs. The present and future distributions of fresh, brackish, and saline groundwater will be discussed. Firstly, development of the area is discussed, followed by a description of the computer code MOCDENS3D that is used to simulate variable-density groundwater flow in 2D. The results of different scenarios are discussed and some conclusions for possible remediation measures are drawn.