Hydrogeological Studies and Mapping on West Milan Area

Transcription

1 Hydrogeological Studies and Mapping on West Milan Area R.Bergamo, W.Danieli, D.de Bartolo, A.P.Gatti ARPA Lombardia, Dipartimento di Milano 1, Area di Tutela Ambientale Via Spagliardi 19, Parabiago (MI) ABSTRACT By means of GIS ArcView software, a map of the territory with wells, classified depending on their depth, was developed; in this way six best drawings for the hydrostratigraphic sections were chosen. The sections obtained enabled us to identify the geometry and the relations among the various groundwater layers in the whole territory. It was then possible to classify the water wells of the territory depending on the aquifer catchment; other maps, showing isolines of the depth of the layers bottom, were also implemented. INTRODUCTION AND PURPOSE This project was conceived and developed in order to define a detailed hydrogeologic structure of the territory of ARPA Lombardia Milano 1, with peculiar interest in the geometry and the relations among the groundwater layers. In fact, even if the whole territory extends in Po plain and is all made up of fluvioglacial and alluvial deposits, there are significant differences in hydrogeological structure of the various zones. So, it is a fundamental requirement, for those who deal with environmental subsoil problems, to have a flexible and preliminary tool, which can give swift and synthetic information on the aquifer geometry. Starting from this point, the aquifer characterization could be further investigated ( hydrodynamical properties, vulnerability, etc.) CHARACTERISTICS OF THE AREA Geology and hydrogeology of the territory This work has been developed starting from the general knowledge of geology and hydrogeology of Lombardy plain and from the wells database of the SIF ( Sistema Informativo Falda Groundwater Information System), a team coordinated by the Provincia of Milan and working with ARPA Milano 1 and various public agencies. The area under study extends in the west Milan Area. The subsoil, at water wells catchment depths, is made up of plio-pleistocenic sediments, which were classified by various authors in lithological or hydrostratigraphic or hydrogeological units (PROVINCIA DI MILANO and POLITECNICO DI MILANO, 1995); summing up, it is possible to see, from bottom to top, a clayey layer of marine origin, a sandy-clay layer of transition and continental origin (Villafranchiano), a conglomerate and sandstone layer (Ceppo), a sandy-gravel layer of fluvioglacial Riss-Mindel origin and a gravellysand layer of fluvioglacial Wurm origin (AVANZINI et al. 1995). Fluvioglacial Wurm deposits outcrop in almost the whole territory and constitute the main level of the plain; recent drifts along the Ticino and Olona river and the Mindel-Riss fluvioglacial terrace, which is located to the north of Garbagnate, almost ten meters high above the plain, are an exception. The Ceppo layer is just observed in the east part of the territory under study. It is possible to notice, passing downstream and increasing the depth, a progressive prevailing of the silty-clay terrain.

2 The plio-pleistocenic sediments are also influenced by deep tectonic structures which concern the premesozoic basement. A series of faults and overthrusts determine a progressive raising of the basement going eastward up to a maximum value in Monza area and whereas the overlying layers, even the pleistocenic ones, grow thinner in connection with the Monza horst (AVANZINI et al. 1995). For the subdivision of the groundwater layers, the classification of the aquifers in hydrostratigraphic units has been adopted (FRANCANI and POZZI, 1981); according to this classification, the first aquifer finds place in the gravelly-sand layer, the second in the sandy-gravel layer and in the Ceppo and the third in the sandy-clay layer. The first and the second aquifer, in such a way defined, constitute the traditional aquifer. In SIF database, about 4000 wells and 1000 piezometers of the territory under study are codified, with data relative to the principal characteristics (stratigraphy, x y coordinates, property, condition of the wells, usage, etc ). Almost 1250 of these points ( 1000 wells and 250 piezometers circa) have x y coordinates and stratigraphy and constitute the basis for this work. GIS APPLICATIONS By means of ArcView GIS software it was possible to develop a map of the whole territory, in which wells and piezometers are located and classified by means of different colors depending on the stratigraphy depth. The deepest well is about 275 m, whereas the most frequent depth class takes place between 100 and 150 m. This map ( see fig. 1 for a detail), enabled us to define the best layout for six stratigraphic sections which cross the whole territory according to the criterion to form a regularly spaced grid with three sections parallel to the principal flow lines (NNW-SSE) and three perpendicular, and to involve the most and deepest wells with known stratigraphy Another criterion which has led to choose the layout, was the intersection with most wells of the three piezometric monitoring SIF networks (first, second and third aquifer layer), because these wells were already classified depending on the water catchment. By utilizing the finding features within a specified distance Arcview function, the wells located within 300 m from the section drawing were selected; among them the most suitable, because of location and depth, were selected to add new data to the six stratigraphic sections. A long and exacting work was done to draw stratigraphic sections and to correlate the lithological levels. In that way the principal aquifers and aquicludes of the territory were located, also by means of a critical comparison with the existing bibliography and the pre-existing classification of the monitoring SIF net wells. The six sections were then schematically developed, by representing only the limits between the aquifer layers, the filter position of each well, and the piezometric level deduced by the piezometric map of the phreatic zone at September 2000 (SIF). Fig. 2 shows a detail of one schematic section. RESULTS AND CONCLUSIONS The study of the sections showed very interesting situations: In the northern part of the territory there are no interbedded silty-clay layers for a nearly 100 m depth; we have just one phreatic aquifer that only downstream divides in first and second aquifer, separated by an impervious layer. This fact is very well shown in fig.2. The bottom of the traditional aquifer (first and second layers) becomes less deep going from north to south.

3 Near the limit of the Mindel-Riss fluvioglacial terrace it is possible to point out a rapid change in aquifers depth. Going eastward it is possible to observe the gradual thinning of the traditional aquifer, because of the raising of its basis. The schematic reconstruction of the hydrogeologic structure, defined along the section drawings, consents to individuate the aquifer caught by all the wells of the territory, depending on the filter positions, even if not included in the sections: during this procedure we realized the need to add 4 hydrogeologic sections: two in the NNW-SSE and two in WSW-ENE direction. Fig. 3 shows, for a part of the territory, the result of the work: the wells which were in the beginning only classified depending on depth, can now be differentiated depending on aquifer catchment. A series of map with isobaths of the limits between aquifers was also developed by means of SURFER interpolation software and then imported by ArcView. The final result is a work tool which consents a rapid framing of the hydrogeologic condition, very useful for the development of the functions of ARPA, for instance: the individuation of the vulnerability degree of the wells in an area in which a spill occurred, the evaluation of the best depth of new catchments, the choice of how to close unused wells, the individuation of the recharge area of deep catchments. REFERENCES AVANZINI, M., BERETTA, G.P., FRANCANI, V., and NESPOLI, M., 1995, Indagine preliminare sull uso sostenibile delle falde profonde nella Provincia di Milano. FRANCANI, V., and POZZI, R., 1981, Condizioni di alimentazione delle riserve idriche del territorio milanese. La rivista della strada, L303, Milano PROVINCIA DI MILANO and POLITECNICO DI MILANO, 1995, Le risorse idriche sotterranee nella Provincia di Milano vol.1 : Lineamenti idrogeologici

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