Multi-layered aquifers from the susburface of tha Arno coastal plain (Tuscany, Italy)

Management and conservation of water resources represent a priority in coastal pia in areas where social and economica! resource, because of their strong anthropization, are concentrated. T o define realistic models of groundwater circulation and transport the knowledge of subsurface depositional architecture fixed in a worthy chrono-stratigraphic framework represents a key point that cannot be renounced. The study area (Arno coastal plain, Tuscany, Italy) constitutes the southern portio n of the Viareggio Basin, which formed a long the Tyrrhenian margin since Late Tortonian, in response to the opening of the Tyrrhenian Sea and the counter-clockwise migration of the chain foredeep-foreland system. The Viareggio Basin is bounded by the Versilia coastal plain and Pisani Mountains to the north, and by Pisa and Leghorn hills to the south. Integrated stratigraphic, sedimentological, and micropalaeontological studies of severa l continuously-cored boreholes drilled in the Arno coastal plain prompt to individuate two transgressive-regressive sequences (T -R) attributed to the last two interglacial-glacial cycles. The lower portion of these T-R sequences consists of coastal to shallow-marine sandy to muddy sediments deposited during rising and highstand sea-level conditions (OIS l and Se). Phases of sea-level fai l, linked to glacial periods (OIS 4-2 and 6, respectively), are instead characterized by fine to coarse-grained alluvial deposits (upper part of T-R sequences). During OIS 3-2 transition an incised-valley, 35-40 m deep and 5-7 km wide, formed and successively, since about 12 kyr BP, was filled by transgressive muddy estuari ne to coastal pia in deposits. Reconstruction of stratigraphic architecture allows identification, in the uppermost 100 m of the Pisa coastal pia in subsurface, of a sand-gravel multi-layered confined aquifer of Late Pleistocene to Holocene age. In its shallowest portion, this aquifer becomes unconfined. Aquifers within coastal-marine sediments (OIS l and Se) display a landward-wedging geometry and a higher lateral extent relative to aquifers of fluvial origin (OIS 4-2 and 6), which are instead characterized by a marked lenticular geometry. The wedge-shaped geometry of sand coastal bodies reflects the landward migration of a barrier-lagoon system and the following seaward progradation of a delta-strandplain complex. Transgressive and highstand sands are separated by shallow-marine muddy sediments that thin aut landwards, implying that these two aquifers become interconnected to a certa in extent. Concerning aquifers of fluvial origin, the high silt/clay ratio within the fine-grained overbank units separating sand and gravel channel bodies suggests that floodplain sediments do not probably form true aquicludes, and consequently that aquifers could be locally interconnected. In addition, these aquifers of fluvial origin are cut by an incised valley system filled with about 35 m of muddy deposits, which works as permeability barrier, thus constituting a further complication in the 3D aquifers geometry.