## 4 Results  
### Hydrostratigraphic Structure  
The coastal plain can be morphologically divided into three roughly parallel bands (Figure 3). The inner one, bordering the foot of the Apuan Alps, is characterized by coalescing alluvial fans of little slope. These mainly consist of gravels, pebbles and sands, sometimes in a medium-fine matrix in the areas close to the hills, sands and silty sands in the intermediate part and silts and clays in the more distal parts. The intermediate zone includes lowland areas, in some cases even below sea level. It corresponds to ancient marshy and lagoon areas isolated from the sea by the development of a beach ridges system [58]. Most of the swampy areas have been reclaimed since the Roman times. The sediments characterizing this strip are both of a lacustrine/marshy environment, such as peats, silts and/or peaty clays, and, locally, alluvial, such as silts and clayey-sandy silts, usually with an abundant organic fraction. The coastal strip is formed by the current beach, and inwards by the system of beach ridges and coastal dunes, mostly dismantled by human action. From a sedimentary point of view, the current beach is entirely composed of medium to fine sand, as are the beach ridges and coastal dunes, of marine and windy origin, respectively. The stratigraphic database available allowed the representation of the lithoststratigraphic structure down to a depth of about 30-40 m (Figure 4). In particular, two hydrostratigraphic sections are shown in Figure 4: one perpendicular (A-A\({}^{\prime}\)) and one parallel (B-B\({}^{\prime}\)) to the alignment of the coastal plain (see Figure 3). Section A-A\({}^{\prime}\) includes the apical zone of the alluvial fan of the Baccatoio stream and is NE-SW directed, whereas section B-B\({}^{\prime}\) is NW-SE directed. In the sections, the vertical scale is exaggerated compared with the horizontal scale to better appreciate narrow horizons. In agreement with the general stratigraphic structure, three main facies were recognized:  
* deposits associated with fluvial processes, mainly consisting of coarse material (gravel) in a finer matrix (sand, silt, clay), especially in the fan area, whereas the fine matrix seems to decrease towards the SW;
* deposits resulting from marine processes, mainly sandy with some lenses of finer grain size (silty sands);
* mixed lacustrine and marsh deposits, consisting of mainly clayey and silty sediments, including peaty levels; these deposits can also occur as lenses within other deposits. Figure 4: Hydrostratigraphic cross-section A-A\({}^{\prime}\) and B-B\({}^{\prime}\) (section tracks in Figure 3) (the vertical scale is exaggerated compared with the horizontal scale). The black lines represent boreholes; the red lines indicate the wells in which the highest As concentrations were found. Figure 3: Geological sketch map of the study area. The section tracks are also shown (see Discussion) (1: beach ridge; 2: marsh area; 3: alluvial fan (mixed grain size); 4 alluvial fan (mainly silty grain size); 5: alluvial fan (sandy grain size); 6: alluvial fan (gravelly grain size): 7 current beach; 8: Apuan hills (bedrock); 9: boreholes (from Geological Map of Tuscany, 1:50,000 scale)). The alluvial fan area (section A-A\({}^{\prime}\), Figure 4) is dominated by a thick, predominantly gravelly, but locally more heterogeneous, horizon, consisting of fine matrix gravels (silty and clayey sand). The average thickness investigated was about 40-50 m. In the upstream part, these deposits lie directly on the bedrock, which is found at a depth of 45 m. The main gravel horizon deepens starting from the apical area of the conoid towards the SW. Its roof was identified at a depth of about 20 m in the terminal part of section A-A\({}^{\prime}\). The gravel horizon appears to locally include lenses of finer material. However, as can be observed in section B-B\({}^{\prime}\) (Figure 4), this horizon is laterally limited by finer deposits.