## 2 Geological and Hydrogeological Setting  
The study area (Figure 1) was located in the Versilia plain, a 5 km wide strip of flat coastline between the Apuan Alps, a mountain range reaching almost 2000 m a.s.l., to the east-north-east and the Ligurian Sea to the west-south-west. The Versilia plain finds its natural continuation in the Apuan Riviera to the north-west, and in the Pisa plain to the south-east. The area is well-known worldwide for its tourist attraction in relation to its famous beaches and cultural heritage [37], frequented since Roman and Etruscan times, and for the famous Apuan marble extraction and processing in the territory of Carrara (Figure 1). The area is 85 km west of Florence, about 30 km north of Pisa and 25 km north-west of Lucca. The Versilia plain corresponds to the easternmost side of a large extensional basin called Viareggio Basin, which represents the post-orogenic filling of a graben structure. The sedimentation processes started in the Upper Miocene, with deposition of incoherent detrital sediments of marine origin, marine-transitional and continental. The paleogeographic evolution of the area determined the formation of a composite depositional apparatus, consisting of alluvial and palustrine sediments of different grain sizes (from pebble and gravel to silt and clay, and sometimes peat). From a hydrogeological point of view, this structure determines a complex aquifer system, with unconfined and confined aquifers arranged on several horizons, often interconnected. The aquifer structure object of this study is located at the top of this post-orogenic deposits succession.  
The study area falls within the distal part of the fan of the Baccatoio stream that drains the southern portion of the Apuan Alps exposed to the sea (Figure 1). The tectonic and geological origin of the Apuan Alps is linked to the Apennine orogen, with a mainly compressive first phase and a prevalently extensional second phase. The tectonic structure includes, from top to bottom, Tuscan Nappe, Massa Unit and Apuane Unit (metamorphic units of the Tuscan Domain), and the underlying Paleozoic metamorphic basement [38]. The carbonate rocks of the Apuan Unit (Marbles and Grezzoni Fms.) and Tuscan Nappe (mainly Calcare Cavernoso and polygenic preccias Fm., and cataclasites) host the two most important aquifer systems of the mountainous area. The medium-high permeability of these rocks and the annual rainfall of more than 2000 mm/year determine a significant groundwater flow that feeds several springs, with an average flow rate up to some tens L/s [39]. Some of these springs also feed the Baccatoio stream.  
The Baccatoio stream catchment is rich with mineralization [40], especially with two main types of ore deposits: the Pb-Zn-(Ag) ore bodies, composed of galena + sphalerite + chalcopyrite \(\pm\) Pb sulfosalts, and pyrite \(\pm\) baryte \(\pm\) iron-oxide ore bodies. Pyrite was found to be enriched in several potentially toxic elements (PTEs), particularly TI [41] and As, with the latter reaching concentrations of about 1 g/kg. At present, there are several abandoned mine sites in the upper part of the catchment feeding acid drainages highly contaminated by PTEs, including TI and As [42]. In particular, thallium contaminated the potable water distribution system, posing serious public health threats [43; 44]).