Geophysical and Stratigraphical Research Into Deep Groundwater and Intruding Seawater in the Mediterranean Area (The Salento Peninsula, Italy) S

Geophysical and stratigraphical research into deep groundwater and intruding seawater in the mediterranean area (the Salento Peninsula, Italy) S. Margiotta, S. Negri To cite this version: S. Margiotta, S. Negri. Geophysical and stratigraphical research into deep groundwater and intruding seawater in the mediterranean area (the Salento Peninsula, Italy). Natural Hazards and Earth System Sciences, Copernicus Publ. / European Geosciences Union, 2005, 5 (1), pp.127-136. hal-00299131 HAL Id: hal-00299131 https://hal.archives-ouvertes.fr/hal-00299131 Submitted on 25 Jan 2005 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Natural Hazards and Earth System Sciences (2005) 5: 127–136 SRef-ID: 1684-9981/nhess/2005-5-127 Natural Hazards European Geosciences Union and Earth © 2005 Author(s). This work is licensed System Sciences under a Creative Commons License. Geophysical and stratigraphical research into deep groundwater and intruding seawater in the mediterranean area (the Salento Peninsula, Italy) S. Margiotta and S. Negri Dipartimento di Scienze dei Materiali, Osservatorio di Chimica, Fisica e Geologia Ambientali, Universita` degli Studi di Lecce, Italy Received: 30 July 2004 – Revised: 22 November 2004 – Accepted: 23 December 2004 – Published: 25 January 2005 Part of Special Issue “Natural and anthropogenic hazards in karst areas” Abstract. Geological, geophysical and hydrological surveys of the aquifer due to saline water intrusion. The research also were performed in an area between Lecce to the North and confirms the validity of geophysical methods for the study of Otranto to the South on the Lower Adriatic coast of Italy in hydro-geological issues. order to verify the “state of health” of the ground water in the deep aquifer, which is characterised by Mesozoic carbon- atic formations and is permeable due to fracturing and karstic phenomena. This area was chosen because it is highly ur- 1 Introduction banised, and contains many wells, some of which were inten- sively studied from a geological and hydro-geological point The Salento peninsula (Puglia, Italy) (Fig. 1) possesses very of view in 1987. The first phase of the research consisted few superficial water resources because of its karstic na- of a stratigraphical and geological study, based on geological ture; it does have remarkable underground resources how- surveys of the surface and the subsoil by direct observation ever, which have made settlement and the development of of borehole cores and the collection and analysis of existing productive activities possible across wide areas. stratigraphical data on wells in the area. The stratigraphi- The nature of such underground water resources, the par- cal and hydrogeological data are kept in the archives of lo- ticular phenomena that regulate the outflow and feeding pro- cal agencies. The collected data were organised in a single cesses and the effects exercised by the sea mean that the cor- database managed by an appropriate software (ARCVIEW). rect use and management of underground water resources is Numerous geological cross sections of the territory were essential. Deterioration of underground waters results from studied in order to characterise the subsoil. Subsequently, both saline contamination and human-generated pollution. The last few years have seen a gradual and constant fall in the multi-parameter surveys (O2, pH, temperature, TDS, con- ductivity) were carried out on the water-column of certain volume of underground waters available in the main water- wells selected from those studied in 1987. This was to verify structures of the region (the Gargano, Tavoliere, Murge and possible changes in the state of the groundwater since 1987. Salento areas), consequent of the overexploitation of ground- The surveys were carried out in two different periods (May water resources. This in turn has led to growing salt contam- and September) in order to identify potential differences be- ination resulting from the continental invasion of marine wa- tween rainy and dry periods. Moreover, electrical tomog- ters (Maggiore and Pagliarulo, 2003). Research has shown raphy was used to characterise the various subsurface geo- that the rising salinity levels of the ground water is a more logical formations and possible saline water intrusion where serious problem than the contamination of the soil by vari- direct information collected from the wells was lacking. ous pollutants (Fidelibus and Tulipano 2002; Maggiore and From the research conducted it was possible to effect a Pagliarulo, 2002, 2003; Tulipano and Fidelibus 1999, 2002). stratigraphical characterisation of the subsoil and to verify Wasteful uses of water are fundamental causes of desertifi- overexploitation of the water resources resulting from the in- cation and environmental degradation and thus, Puglia, in- creasing number of wells, and the consequent deterioration cluding the Salento, turns out to be one of the regions most threatened by this problems (Glenn et al., 1998; Zito, 2003). Correspondence to: S. Negri Furthermore, the cited literature shows how the karstic nature ([email protected]) of the coastal aquifers of the Salento and the Murge consti- 128 S. Margiotta and S. Negri: Geophysical and stratigraphical research Fig.1 Salento Acquarica Fig. 1. LocationFig.1 of studied area and geological map. tutes a important risk factor, since the underground karstic located south of Lecce, and to verify “the state of health” channels may represent routes of rapid and deep saline intru- of the aquifer holding the deep waters of Salento. This sion. However, in these studies it emerged that the shortage area was characterised in a previous study (Margiotta and of information, due to the limited number of points of obser- Tadolini, 1987), where the authors carried out geological vation, did not allow for a reliable evaluation of the deterio- and hydro-geological research, including multi-parameter ration of underground water resources. surveys of the water-columns of many wells pertaining to the land-improvement consortium known as “Consorzio The objective of this work is the physical and stratigraphi- Bonifica Ugento li Foggi” (CBU). cal characterisation of the subsoil of an Adriatic coastal area S. Margiotta and S. Negri: Geophysical and stratigraphical research 129 Our study sought to verify possible changes to the state of horizontal area, depth and thickness of the lithostratigraphi- the groundwater with respect to the earlier surveys. Strati- cal formations. graphical data on existing wells were collected and analysed The following litostratigraphic formations, from the most in the light of new geological knowledge of the territory. The ancient to the most recent, were recognised (Fig. 1): stratigraphic and geological data are kept in the archives of the following local agencies: “Acquedotto Pugliese, Ente Ir- – Altamura Limestone (Upper Cretaceous) rigazione, Consorzio di Bonifica Ugento li Foggi” and “Con- – Galatone Formation (Upper Oligocene) sorzio di Bonifica dell’Arneo”. The collected data were organised into a single database – Lecce Formation (Upper Oligocene – Lower Miocene) managed by an appropriate software (ARCVIEW). In addi- tion, analysis of the rocks was also carried out and numerous – Pietra Leccese (Lower Miocene – Upper Miocene) geological cross sections of the territory were prepared. – Andrano Calcarenite (Upper Miocene) The hydro-geological surveys were carried out in two different periods (May and September) in order to identify po- – Leuca Formation (Lower Pliocene) tential differences between rainy and dry periods. The wells – Uggiano la Chiesa Formation (Lower – Upper Pliocene) considered in this study have never been used either for drinking water or irrigation. – Gravina Calcarenite (Pleistocene) In order to test the reliability of electrical tomography and induced polarisation testing as methods for the characterisa- Outcrops of Altamura Limestone are extensive in the area tion of subsurface geological formations and the detection north of Lecce and in the area between Caprarica di Lecce of possible saline water intrusion, geophysical surveys were to the north, Zollino to the south-west and Martano to the carried out in an area for which stratigraphical data were al- south-east. These are the highest reliefs in the territory un- ready available. der study. This formation is made up of alternating layers of variable thickness of compact limestones and dolomitic limestones of white and grey colour. The Cretaceous limestones 2 Geological and hydrogeological setting are characterised by folding along a NNW-SSE axis, locally associated with faults. These phenomena have created a sys- The investigated area represents a complex hydro-geological tem of faults forming small horsts (known as “Serre”) and environment. The Salento is characterised by two aquifers: Grabens (Figs. 2 and 3). Karst was able to develop along the first (i.e. the closest to the surface) is made up of Mio- these lines of structural weakness. The Mesozoic rocks are Plio-Pleistocene sediments

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