Elsevier

Journal of Hydrology

Volume 501, 25 September 2013, Pages 35-44
Journal of Hydrology

Characterization of the lowland coastal aquifer of Comacchio (Ferrara, Italy): Hydrology, hydrochemistry and evolution of the system

https://doi.org/10.1016/j.jhydrol.2013.07.037Get rights and content

Highlights

  • We propose a full hydrogeochemical characterization of a lowland coastal aquifer.

  • We highlight groundwater dynamics and salinization processes in the aquifer.

  • Hypersalinity derives from fossil seawater trapped in the inter-basins.

  • Lateral intrusion or upconing of present seawater is found only near the coastline.

  • Results could be useful to promote sustainable water management in coastal areas.

Summary

This study delineates the actual hydrogeochemistry and the geological evolution of an unconfined coastal aquifer located in a lowland setting in order to understand the drivers of the groundwater salinization. Physical aquifer parameterization highlights a vertical hydraulic gradient due to the presence of a heavy drainage system, which controls the hydrodynamics of this coastal area, forcing groundwater to flow from the bottom toward the top of the aquifer. As a consequence, relict seawater in stable density stratification, preserved within low permeability sediments in the deepest portion of the aquifer, has been drawn upward. The hydrogeochemical investigations allow identifying the role of seepage and water–sediment interactions in the aquifer salinization process and in the modification of groundwater chemistry. Mixing between freshwater and saltwater occurs; however, it is neither the only nor the dominant process driving groundwater hydrochemistry. In the aquifer several concurring and competing water–sediment interactions – as NaCl solution, ion-exchange, calcite and dolomite dissolution/precipitation, oxidation of organic matter, and sulfate bacterial reduction – are triggered by or overlap freshwater–saltwater mixing The hyper-salinity found in the deepest portion of the aquifer cannot be associated with present seawater intrusion, but suggests the presence of salt water of marine origin, which was trapped in the inter-basin during the Holocene transgression. The results of this study contribute to a better understanding of groundwater dynamics and salinization processes in this lowland coastal aquifer.

Introduction

The recognition of groundwater salinization origin in coastal aquifers is not straightforward; in many cases it is reasonable just to presume the involvement of present seawater. Though, besides present seawater, groundwater salinization can result from other natural salt sources or from human activities (Cartwright et al., 2004, Custodio, 1997, Fakir et al., 2002, Richter and Kreitler, 1993, Sanchez-Martos et al., 2002, Stuyfzand and Stuurman, 1994, Tellam, 1995, Tulipano et al., 2005). The most relevant salt sources/processes recognized worldwide are: evaporation, evaporite leaching, mobilization of salts stored in the unsaturated zone, infiltration of non-marine polluted surface waters, slow-moving saline/salt waters of marine origin (Barbecot et al., 2000, Darling et al., 1997, Fidelibus and Tulipano, 1996, Ng and Jones, 1995), highly mineralized waters from geothermal fields (Fidelibus et al., 2011a), sea spray, hyper-filtration, agricultural practices (return flow, use of fertilizers and irrigation with treated wastewater), cycling wetting and drying. Mixing processes with present seawater and/or saline ground waters are normally enhanced by human actions as over-exploitation (causing lateral seawater intrusion and/or upconing), presence of drainage systems for land reclamation, and construction of navigation channels along rivers or artificial canals (Barlow and Reichard, 2010, Custodio, 2010, Fidelibus et al., 2011b, Kass et al., 2005). In addition, has been recently demonstrated the importance of characterizing vertical variability of aquifer properties for quantifying the magnitude and direction of upward seepage of saline groundwater (de Louw et al., 2010, de Louw et al., 2011).

Whatever the salt source, its recognition is very often masked by the activation of water–rock interactions directly related to increase in water ionic strength, whose effects overlap those ones caused by the salt source. The salt sources have different geochemical and isotopic imprints. Thus, an approach by multiple natural tracers is required for their recognition: the chemical composition of present seawater is used in this case as the end-member of non-reactive freshwater–saltwater mixing for deducing overlapping/triggered water–rock interactions and solute contribution (de Montety et al., 2008, Fidelibus and Tulipano, 1996, Ghiglieri et al., 2009, Giménez and Morell, 1997, Howard and Lloyd, 1983, Yamanaka and Kumagai, 2006).

The Geological Survey of the Emilia-Romagna Region (Northern Italy) has developed a regional monitoring network to monitor saltwater intrusion in the coastal aquifer facing Adriatic Sea (Bonzi et al., 2010). Through the hydrogeologic and hydrochemical characterization of the coastal aquifer, this study, aims at identifying the origin and the mechanism of groundwater salinization, to the ultimate goal of the prevention or mitigation of groundwater resources deterioration, and the promotion of a sustainable water resource management.

Section snippets

Study area

The study area is located in the coastal floodplain of the Po River, in Northern Italy (Fig. 1). This area is characterized by intensive agriculture inland and intensive tourism along the coast during summer periods (June–September). Most of the territory is a recently reclaimed land with flat topography below sea level, and with an altitude ranging from 5 to −11 m above sea level (a.s.l.); the only topographic heights consist of dune systems, paleodunes and river banks.

The surface hydrographic

Materials and methods

This study combines field data collected during two different campaigns (June and November 2011). This section describes the methodology used to characterize the selected transect, in particular the methodology developed via straddle low pressure packers to characterize simultaneously vertical hydraulic gradients, permeability, and groundwater quality with the aim of identifying the aquifer dynamics and the spatial distribution and composition of saline groundwater.

Hydrostratigraphycal characterization

Grain size distribution analysis carried out along the transect reveals the presence of an heterogonous aquifer with very poorly sorted silty clay and peat lenses overlaying a highly permeable well sorted sandy layer typical of high energy depositional environments, such as coastal environment, intercalated with poorly sorted sandy silt lenses. Porosity values range between 19% and 76% with an average value of 30%; the highest values of porosity are associated with the presence of peat layers,

Conclusions

This study contributes to the knowledge of the hydrology, hydrochemistry and evolution of the coastal aquifer of Comacchio (Ferrara, Italy) by shedding lights on the main process of groundwater salinization in this area.

The hydrogeological and hydro-geochemical investigations carried out along the selected transect allow identifying the role of seepage and water–sediment interactions in the aquifer salinization process and in the modification of groundwater chemistry.

The results suggest that

Acknowledgements

We gratefully thank Paolo Severi and Lorenzo Calabrese from the Geological, Seismic and Soil Survey of Emilia-Romagna Region for their technical and scientific support. Enzo Salemi and Davide Rizzati from the University of Ferrara are thanked for their help in the field and laboratory activities.

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