Characterization of aquifer systems from high-resolution subsurface stratigraphy: the case of the Eastern Valdarno Basin (Tuscany, Italy)

Realistic models of ground water circulation and transport in modern alluvial\nand coastal plains demand an accurate reconstruction of subsurface\nsedimentary architecture. In this study, we propose the characterization of a\nmultilayered aquifer from the subsiding eastern Valdarno Basin, a strategic\narea far both water research and pollution issues, between the city of\nPontedera and S. Croce.\nAn integrated sedimentological, stratigraphic and micropaleontological study of\nsix continuously cored boreholes allows the detailed reconstruction of Pliocene\nto Quaternary stratigraphy in the uppermost 100 m, and shows how a\nmultidisciplinary approach may represent a successful tool to define\nthree-dimensional facies relationships with in sedimentary bodies, and thus\naquifer geometries. Moreover, AMS 14C dates associated to Sr isotope data\nprompt to place the identified stratigraphic units in a worthy\nchronostratigraphic framework, with a coherent depositional evolution in terms\nof space and time.\nThe study area, oriented SW-NE along the Arno River course, is rimmed to the\nsouth and to the north by the Pisa and the Cerbaie Hills, respectively, both\nformed by Plio-Quaternary marine and continental deposits. A fault, located\na long the southern margin of the Cerbaie Hills, causes the NNW-dipping, below\nthe Arno-plain, of the Plio-Quaternary deposits cropping out o n the Pisa Hills.\nStratigraphic architecture in the study area is varied. Except far the Holocene\nsuccession, consisting mainly of fine-grained alluvial-plain deposits lying onto\ntransgressive swamp deposits and showing a homogenous spatial distribution,\nthe pre-Holocene deposits exhibit noticeable distinct features west and east of\nPontedera, respectively, due to the activity of a normal fault with an Apenninic\norientation, causing the lowering of the west side of the study area. A thick\nPleistocene alluvial succession, made up by a cyclic alternation of\ncoarse-grained fluvial channel and fine-grained floodplain deposits, is recorded\nbeneath Pontedera, whereas eastwards the stratigraphic sequence is\ncharacterized by Early-Middle Pliocene deposits, related to coastal-shallow\nmarine sands with very subordinate alluvial facies.\nGiven this structural setting, the reconstruction of stratigraphic architecture\nleads to the identification, in the uppermost 100 m, of a multilayered confined\naquifer consisting of five aquifer systems (A-E) ranging in age between the\nHolocene and the Early-Middle Pliocene.\nAquifer system A developed during the Holocene and exhibits a markedly\nlenticular geometry recording the recent dynamics of the Arno River. The\nunderlying aquifer system (B), Lower-Middle Pleistocene-Holocene in age,\nconsists of sands an d gravels of fluvial origin and is recorded only in the\nwestern sector of the examined area. The lower three aquifer systems (C-D-E),\ncharacterizing the eastern sector of the study area, mainly consist of sandy\nshallow-marine deposits, Early to Middle Pliocene in age, showing a cyclic\narrangement.\nThe Quaternary aquifer systems (A-B) exhibit a strongly lenticular shape,\nreflecting their fluvial origin, while the Early-Middle Pliocene aquifer systems\n(C-D-E) display a higher lateral continuity, as expected from sedimentary\nbodies deposited in coastal-littoral setting.\nThe geological framework and the structural setting suggest the identification\nof water recharge areas in the Middle Pliocene sandy deposits cropping out on\nthe Pisa Hills, taking into account their NNW dipping toward the Arno Plain\n.