Detailed conceptual models of the aquifer systems are an essential base working for groundwater modelling and for a correct groundwater resources management. At the same time the right approach to obtain reliable conceptual models is certainly interdisciplinary, which guarantees the knowledge of geological, hydrogeological and geochemical features of the aquifer systems. In this contribute an example of multidisciplinary study carried out on the Versilian coastal plain aquifers (NW Tuscany) is presented, underlining as the comparison between geological framework and hydraulic, hydrodynamic, chemical and isotopic data allowed at understanding the feeding mechanism of groundwater flow and at defining the seawater-freshwater mixing process. The study area has an extension of about 55 kmq and it has delimitated by Poveromo and Motrone ditch, northward and southward respectively, and by shoreline and Apuan Alps, in the western and eastern parts. The Versilian plain constitutes a recent succession of alternating continental and marine deposits. Hydrostratigraphic cross-sections reaching a depth of about 80-100 metres were elaborated. Aquitards/aquicludes are locally interbedded to the prevalent aquifer terms, which are mainly made up by sand and gravel. Gravel is shallow in the inner portion of the plain, where the alluvial fan of Versilia River is present. Seaward gravel deepens and is overlain by sands. For this framework the aquifer results throughout a phreatic monolayer, although the local presence of superficial silty-clayey or peaty deposits may determine confined or semi-confined conditions. Moreover, the presence of a discontinuos subsurface layer of conglomerate and silty-clayley lenses may locally separate the groundwater flow in multiple levels. In order to achieve the purpose of the present work, multiple types of hydrogeological and hydrodinamics surveys and chemical and isotopic analyses on water samples were performed. In details, two piezometric surveys and physical-chemical analyses (temperature, electrical conductivity, pH, Eh) were carried out in April 2009 (high level condition) and in September 2009 (low level condition). On the whole, 192 wells and 14 stream water points were examined. Piezometric and iso-conductive maps suggest as the recharge area of the coastal plain is mainly the alluvial fan. Afterwards, more detailed hydrogeochemical analyses were performed. Major elements (Ca, Mg, Na, K, Cl, NO3, SO4, HCO3), some minor elements (Br, F, B, As, Fe, Mn, Pb, Cr, Cu, Ni, Zn) concentration and some isotopic ratios (2H/1H, 18O/16O for water and 13C/12C of total inorganic carbon dissolved) were estimated. Vertical logs were also performed in order to measure the electrical conductivity and temperature into 10 wells next to the coastline. In addition, in the apex of the fan a long term pumping test was executed, determining the aquifer hydraulic parameters (K, T, S). The last, together with the potentiometric surface, allowed us estimating the groundwater flow rate at the section corresponding to the Versilia River entrance into the plain. Finally, comparing all data, it was possible to identify the principal component which supplies the coastal aquifer system starting from the upper part of the plain, where the Versilia River feeds the groundwater hosted in its alluvial fan. Secondary contributes are linkable to the local rainfall infiltration, well recognized in the dune sand, and to the groundwater flow coming from some fractured complexes bordering the plain. Freshwater-seawater mixing processes were also individuated.

Multidisciplinary approach to provide detailed conceptual model of the aquifer systems: an example in the Versilian coastal plain (NW Tuscany)

GIANNECCHINI, ROBERTO;
2011

Abstract

Detailed conceptual models of the aquifer systems are an essential base working for groundwater modelling and for a correct groundwater resources management. At the same time the right approach to obtain reliable conceptual models is certainly interdisciplinary, which guarantees the knowledge of geological, hydrogeological and geochemical features of the aquifer systems. In this contribute an example of multidisciplinary study carried out on the Versilian coastal plain aquifers (NW Tuscany) is presented, underlining as the comparison between geological framework and hydraulic, hydrodynamic, chemical and isotopic data allowed at understanding the feeding mechanism of groundwater flow and at defining the seawater-freshwater mixing process. The study area has an extension of about 55 kmq and it has delimitated by Poveromo and Motrone ditch, northward and southward respectively, and by shoreline and Apuan Alps, in the western and eastern parts. The Versilian plain constitutes a recent succession of alternating continental and marine deposits. Hydrostratigraphic cross-sections reaching a depth of about 80-100 metres were elaborated. Aquitards/aquicludes are locally interbedded to the prevalent aquifer terms, which are mainly made up by sand and gravel. Gravel is shallow in the inner portion of the plain, where the alluvial fan of Versilia River is present. Seaward gravel deepens and is overlain by sands. For this framework the aquifer results throughout a phreatic monolayer, although the local presence of superficial silty-clayey or peaty deposits may determine confined or semi-confined conditions. Moreover, the presence of a discontinuos subsurface layer of conglomerate and silty-clayley lenses may locally separate the groundwater flow in multiple levels. In order to achieve the purpose of the present work, multiple types of hydrogeological and hydrodinamics surveys and chemical and isotopic analyses on water samples were performed. In details, two piezometric surveys and physical-chemical analyses (temperature, electrical conductivity, pH, Eh) were carried out in April 2009 (high level condition) and in September 2009 (low level condition). On the whole, 192 wells and 14 stream water points were examined. Piezometric and iso-conductive maps suggest as the recharge area of the coastal plain is mainly the alluvial fan. Afterwards, more detailed hydrogeochemical analyses were performed. Major elements (Ca, Mg, Na, K, Cl, NO3, SO4, HCO3), some minor elements (Br, F, B, As, Fe, Mn, Pb, Cr, Cu, Ni, Zn) concentration and some isotopic ratios (2H/1H, 18O/16O for water and 13C/12C of total inorganic carbon dissolved) were estimated. Vertical logs were also performed in order to measure the electrical conductivity and temperature into 10 wells next to the coastline. In addition, in the apex of the fan a long term pumping test was executed, determining the aquifer hydraulic parameters (K, T, S). The last, together with the potentiometric surface, allowed us estimating the groundwater flow rate at the section corresponding to the Versilia River entrance into the plain. Finally, comparing all data, it was possible to identify the principal component which supplies the coastal aquifer system starting from the upper part of the plain, where the Versilia River feeds the groundwater hosted in its alluvial fan. Secondary contributes are linkable to the local rainfall infiltration, well recognized in the dune sand, and to the groundwater flow coming from some fractured complexes bordering the plain. Freshwater-seawater mixing processes were also individuated.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11568/149858
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