Kouli_etal_2008_Groundwater modelling_BOOK.pdf - Pantelis ...

Groundwater Management in the Northern Adriatic Coast (Ravenna, Italy)… 215
the aquifer recharge and study the influence of topography and lithology on the brackishfreshwater
interface depth.
The analytical Dupuit-Ghyben-Herzberg relationship (Dupuit, 1863; Bear et al., 1999;
Fetter, 2001) was used to model the position of the brackish-freshwater interface for the area
of Punte Alberete e San Vitale. We have considered a section from the coast to inland areas
and the estimated recharge along the section was calculated according to measured hydraulic
conductivity, the different land uses, runoff and recharge. The permeability was determined
based on the stratigraphic information given by well logs and the resistivity surveys. The
results of this analytical modeling show that the location of the brackish-freshwater interface
is compatible with the boundary surface obtained by the resistivity measurements, with an
average depth of 5-7 m below mean sea level. The maximum interface (10-13m) depth is
registered in proximity of the fossil dunes and the minimum (2-4 m) closed to the brackish
Piallassa Baiona lagoon.
Various runs of the model using different permeability and recharge values show that the
depth of the interface is controlled much more by the permeability of the aquifer than by the
amount of annual recharge.
Variable density groundwater flow modeling studies are carried out to study how the 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 salinization process.
Two-dimensional models that aimed to quantify the relative contribution to saltwater
intrusion by various boundary conditions such as channels, rivers, lagoons or topography,
showed that it is very important to simulate the hydraulic and hydrologic history of the area.
So many natural and man-made changes have altered the hydrology relatively recently (see
Table 1), that most probably the flow regime is still responding to those changes and the salt
distribution is also still changing as a consequence. The simulations carried out with
MOCDENS3D (Oude Essink, 1998; 1999; 2001; Giambastiani et. al, 2007) to quantify
hydraulic head, salinity distribution, seepage and salt load fluxes to the surface water system
in the coastal aquifer of Ravenna in a cross section 8 km long, show that over the last century
artificial subsidence and heavy drainage started the salinization process in the study area. The
local climatic conditions and the lack of a continuous coastal dune system favor salt wedge
intrusion.
Considering the model outputs, the salt load will affect the entire coastal hydrogeological
system and groundwater flow during the coming decades when sea levels will rise; the mixing
zone between fresh and saline groundwater will be shifted 800 m farther inland. Since the soil
becomes more saline, farmland degradation and problems for the pine forests would also
occur.
A 3 dimensional model was built for an area of 400 by 500 m with SEAWAT (Guo
and Langevin, 2002) to study the effect of dune destruction on saltwater intrusion
(Mollema et al., 2008). The conclusions are that for the area in consideration near Marina
Romea the overall topography with a continuous second dune belt parallel to the first
interrupted dune belt and the recharge are sufficient to sustain freshwater underneath the
dunes. The discontinuity of the dunes closest to shore or a drainage channel at the back of
the dunes (Fig. 2a) cannot explain the high salinity of the groundwater only 200 m more
inland and so there must be other factors contributing to the salinization: the
evapotranspiration of the trees or seepage from the river Lamone to the north or
mechanical drainage.