Stratigraphic architecture and aquifer systems in the eastern Valdarno Basin, Tuscany

Developing a realistic model of high-resolution stratigraphy from the subsurface of modern alluvial and coastal plains is an important first step toward a successful three-dimensional representation of aquifers and aquifer systems. An integrated (stratigraphic, sedimentological and micropalaeontological) study of six cores, 100-115 m long, from the eastern Valdarno Basin, Tuscany, enables the detailed reconstruction of Pliocene to Quaternary subsurface architecture between Pontedera and S. Croce sull'Arno. Pollen data from lagoonal sediments and strontium isotope dating of shallow-marine deposits provide the basis for the construction of a reliable chronologic framework for the study succession. Stratigraphic correlations, based upon detailed facies anal ysis, show a varied facies architecture in the study area. Beneath the Holocene succession, which is about 40 m thick and consists of alluvial deposits resting onto transgressive swamp clays, pre-Holocene deposits display remarkably different characteristics from Pontedera toward the basin margin, likely as a result of tectonic activity due to a normal fault running parallel to the Apenninic chain. In the Pontedera area, a thick succession of Pleistocene alluvial deposits, showing a cyclic alternation of fluvial-channel (gravel/sand) and floodplain (clay) facies associations is the dominant stratigraphic feature. By contrast, lower-middle Pliocene deposits, characterized by alternating coastal and shallow-marine deposits, with very subordinate alluvial facies, are recorded at shallow depths, west of S. Croce. A significantly improved stratigraphic architecture in the uppermost 100 m enables detailed reconstruction of aquifers geometry in the eastern Valdarno Basin. With respect to previous work, documenting the presence of one aquifer only, a multilayered confined aquifer, made up of five aquifer systems ranging in age from Middle Pliocene to Holocene, is identified in this study. Lenticular and sheet-like geometries of these aquifer systems are reconstructed, as a function of the alluvial versus littoral origin of their constituent deposits. According to the geological framework, the Pliocene to Quaternary sands cropping out on the Pisa Hills are likely to represent the meteoric-water recharge area for the aquifers of the eastern Valdarno Basin.