Vulnerability Evaluation of a Mutilayer Aquifer for a Sustainable Use of the Resources (Western Sicily, Italy)
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BALWOIS 2004 Ohrid, FY Republic of Macedonia, 25-29 May 2004 Vulnerability Evaluation Of A Mutilayer Aquifer For A Sustainable Use Of The Resources (Western Sicily, Italy) V. Liguori, V.Rametta Università di Palermo Palermo, Italy Abstract This paper is intended to describe the water environmental status in the Plain of Castelvetrano and Campobello di Mazara (Western Sicily, Italy) where there are two fields wells (Staglio e Bresciana) for abstraction of drinking water. According to twenty years collected data about quality of drawing it is necessary to determine the potential vulnerability of the aquifer in order to prevent pollution phenomena of the rich body of water. The groundwater chemical composition of the unconfinated aquifer shows high level of nitrate, sulfide and potassium caused by the frequent use of chemical additives in agriculture. In the investigated aquifer the water discharge, mainly represented by pumping to irrigation and civil use, is more or less 15% higher then mean annual recharge. This study consequently is the starting point for the definition of hazardousness and pollution risk, as well as for a correct water management policy. Key-words: hydrogeology; multiaquifer; groundwater mining; vulnerability; sustainability. Introduction The present work is intended to describe the groundwater lowering in the Piana of Castelvetrano and Campobello di Mazara (Western Sicily) and to identify the basic quality of the groundwaters (Civita M, et al.,1993), as a contribution to a correct estimation of the potentially available resources for different uses (Legislative Decree 152/99). The groundwaters resources in the considered zone, are intended for civil and irrigation use; consequently, in the last twenty years, it is occurred a lowering of the piezometric level, also accentuated by the succession of hydrogeologic years with few rainfalls. So, in this area, the intense exploitation of aquifers and the wide use of fertilizers may cause a degradation of their qualitative and quantitative features (Affuso A.M.G.et al.,1999). Besides this, if we add the bad management of the water resources and the more and more growing diffusion of productive activities which represent potential hazardous factors (Civita M et al., 1995), it is clear how important is to consider the hydrogeologic hazard, especially, in a zone where dryness is particularly frequent. Geological setting The area chosen for this study includes the Piana di Castelvetrano and Campobello di Mazara (Western Sicily). The Piana, extended between the River Delia in the west and the River Belice in the east, sloping down softly to south with a slight incline (10o-15o) has a course approximately southwest-northeast and an average altitude of 100 a.m.s.l. The morphology of the zone, quite regular, represents the result both of the action of the quaternary sea and of that one of the rivers which have an incidence more or less deep on it, and of the other exogenous agents which have acted selectively on the different lithological types. The geological features are represented by a series of quaternary marine terraces (D’Angelo et al., 1992,2001) covering a neogenic sequence (Bommarito S.,1984). These terrains are characterized by clayey deposits of Terravecchia Formation, on which the Messinian evaporitics terrains lean on uncorfomably, passing, in their turn, on the upper part, to the pelagic and detritus-organogenic plioquaternary deposits. Water bodies protection and Ecohydrology 1 BALWOIS 2004 Ohrid, FY Republic of Macedonia, 25-29 May 2004 Particularly, the stratigraphy of the studied area is characterized, from the lower part upward: Terravecchia Formation (Upper Tortonian – Lower Messinian): clayey-marley and clayey- sandy terrigeneous deposits, containing sandy-conglomeratic levels; Gessoso Solfifera Formation (Upper Messinian): evaporitic limestones, selenitic chalks, gessareniti and gessopeliti, slightly stratified and sometimes fractured and karsted; Trubi (Lower Pliocene): white marls with foraminifera, marly limestones and marly clays; Valle del Belice Marnoso Arenacea Formation (Upper Pliocene): terrigenous and torbiditics deposits characterized in the upper part by calcarenites and calciruditi, passing downward to sandy clays, marly clays and clayey-sandy mudstones, sandstones and calcarenites; Calcareniti di Marsala Formation (Lower Pleistocene): constituted, in the lower part, of alternation of yellowish calcarenites fine and coarse, clayey sands with levels of sandy clays superiorly followed by calcarenites well cemented (Ruggieri et al., 1975); Marine terraces in different orders (Crotonian and Tyrrhenian). Recent and actual deposits (Olocene). The following stratigrafic column gives an indication about the lithologies: Coastal sandies (Olocene) Marine terraces (Upper Pleistocene) Calcareniti di Marsala Formation (Lower Pleistocene) Calcarenites and calciruditi Sandy-marly clays Marnoso Arenacea And clayey-sandy silt Valle del Belice Formation Sandstones, marls and calcarenites (Upper Pliocene) Marls, marly limestones and marly clays Trubi (Lower Pliocene) Selenitic chalks Marls Gessoso-Solfifera Formation (Upper Messinian) Chalks and marls Cozzo Terravecchia Formation Marly-sandy clays (Upper Tortonian.- Lower Messinian) Hydrogeology The present formations have been gathered in six hydrogeological complexes, according to their permeability and hydraulic conductivity (Tab.1). The structural features influence the hydrodynamic relations among the bodies of water, according to their permeability and the mutual spatial relations. The hydrostructure of the Piana di Castelvetrano and Campobello di Mazara is constituted of a multi- layered system composed of: a superficial unconfined aquifer (upper calcarenitic complex); a semiconfined aquifer (lower calcarenitic complex). Water bodies protection and Ecohydrology 2 BALWOIS 2004 Ohrid, FY Republic of Macedonia, 25-29 May 2004 Table 1 –Hydrogeological complex in the Plain HYDROGEOLOGICAL HYDRAULIC FORMAtTION PERMEABILITY COMPLEX CONDUCTIVITY Arenaceous- conglomeratic and sandy terrains of: marines UPPER terraces Calcareniti di CALCARENITIC Porosity and secondary for Marsala, Marnoso Medium COMPLEX fracturing Arenacea della Valle del Belice Formation Clayey terrains of Marnoso Arenacea della CLAYEY COMPLEX Porosity Valle del Belice Vera low Formation Arenaceous-calcarenitic LOWER terrains of Marnoso Porosity and secondary for CALCARENITIC Arenacea Valle del fracturing Medium COMPLEX Belice Formation MARLY Porosity and secondary for Trubi COMPLEX fracturing Very low CHALKY Gessoso Solfifera Low and Fracturing and karst COMPLEX Formation medium CLAYEY-MARLY Porosity Terravecchia Formation COMPLEX Very low The two aquifers are separated from silty and sandy clayey aquitad (clayey complex); in the south of Campobello di Mazara, the multi-layered aquifer becomes a single unconfined aquifer. (Hydrogeological section D-E-F, Fig. 1) Figure 1 Hydrogeological sections of Castelvetrano and Campobello di Mazara Plain (Bonanno et al., 2000) Water bodies protection and Ecohydrology 3 BALWOIS 2004 Ohrid, FY Republic of Macedonia, 25-29 May 2004 The measures of the piezometric levels, taken in November 1999 in 66 drilled wells, show the course of groundwater drainage axis of in the northeast area is oriented NE-SW. In this zone, the preferential drainage axis converges towards località Contrada Staglio, where there is a field wells, and towards River Modione, that represents the preferential direction of flow of the superficial aquifer. In the southern area, the drainage develops from north to south, with two directions of preferential flow towards the groundwaters drawings zones in località Bresciana (Fig 2). Figure 2 Hydrogelogical map of Castelvetrano and Campobello di Mazara Plain (Bonanno et al., 2000) The abstractions of drinking water have been calculated in about 8,6.106m3-year, on the basis of the data provided by the bodies interested in drawings management (Bonanno et al., 2000). The drawings for irrigation are calculated on data provided by Consorzio di bonifica Agrigento 3 on the basis of the adacquamento seasonal volume of (vsa) of the present cultivation in the Piana: citrus, (vsa = it 3000m3-ha year), olive (vsa = 1110 m3-ha year) and vegetables (vsa = 4200 m3-have year). As regards the average annual recharge for effective infiltration (Aureli A., 1995) this has been evaluated in 17,2.106m3 (applying the method Thornthwaite for the calculation of the evapotranspiration), using rainfalls and temperatures data with a 19 years long and a 50% rate of potential infiltration, according to the outcropping formations in the Piana and to its flat morphology. The groundwater lowering recharge is linked to the effective infiltration, subordinately to lateral input whereas the chalky complex, the only one with a certain permeability, is in contact with the groundwater lowering. This has made the definition of the aquifer hydrogeologic balance problematic. However, it results overexploited. It is arrived to such a conclusion considering that the drawings of groundwaters, for civil and agricultural use, estimated in total in 19,9.106m3-year, result to be superior to the average recharge for infiltration, equal to 17,2.106m3-year, corresponding to an average annual deficit of budget equal to 2,7.106m3. It has to be excluded that the possible lateral input could fill such a deficit. A comparison between piezometry in the autumn 1999 and in 1981 (Fig.3), show that in 18 years, the piezometric level of the semiconfined