The study area belongs to the North Tuscany Regional Basin and has an extension of about 55 Km

Abstract

In this work, the hydrogeological and hydrogeochemical study on a portion of the Versilian coastal plain aquifers, between Fosso Poveromo (northward) and Motrone (southward), was carried out. The study mainly aimed at analyzing the seawater-freshwater mixing processes and understanding the feeding mechanism of groundwater flow.

The study area belongs to the North Tuscany Regional Basin and has an extension of about 55 Km

. The Versilian plain constitutes a recent succession of alternating continental and superficial-marine deposits. The available borehole descriptions and the stratigraphic sections carried out in this work, representing the first 80-100 metres of depth, do not individuate the lower limit of the unconfined aquifer, which is considerable as a mono-layer with internal changes in permeability, due to the presence of both sand and gravel. Gravel is shallow in the inner portion of the plain, where the alluvial fan of Versilia River is present.

Seaward gravel become deeper and is overlied by sands. For this reason the aquifer results a phreatic monolayer, altough the localy presence of superficial silty-clayey or peaty deposits can determine confined or semi-confined conditions. Moreover, the presence of a discontinuos subsurface layer of conglomerate and silty-clayley lenses can locally separate the groundwater flow on multiple levels.

In order to achieve the purpose of the present work, multiple types of hydrogeological and hydrodinamics surveys and geochemical and isotopic analyses on water table samples were performed. In details, two piezometric surveys and physical-chemical analyses (temperature, electrical conductivity, pH, Eh) were carried out in April 2009 (high level conditions) and in September 2009 (low level conditions). Totally, 192 wells and 14 stream water points were examined.

Piezometric and electrical conductivity maps highlighted as the recharge area of the coastal plain is chiefly the alluvial fan, that there are large piezometric depressions due to exploitation and some areas affected by salt-water.

Afterwards, more detailed hydrogeochemical analyses were performed on a smaller area, sited on the alluvial fan axis zone, from its apex to the present coastline. Major elements (Ca, Mg, Na, K, Cl, NO3, SO4, HCO3), some minor elements (Br, F, B, As, Fe, Mn, Pb, Cr, Cu, Ni, Zn) concentration and some isotopic ratios (

H/

H,

O/

O for water and

13

C/

C of total inorganic carbon dissolved) were especially estimated.

Vertical logs were also performed to mesaure eletrical conductivity and temperature

into 10 wells next to the coast-line. In addition, in the apex of the fan a long term pumping

test were executed, determining the aquifer hydrodinamic parameters (K, T, S). The last, together with the potentiometric surface, allowed us estimating the groundwater flow rate at the section corresponding to the Versilia River entrance into the plain.

Finally, on the basis of chemical data, it was possible to classify the water collected and to recognize ionic exchange processes inside the aquifer between water and deposits.

Freshwater-seawater mixing processes were also individuated, combining chemical and isotopic data. The isotope ratios were also used as natural tracers to define the main components of groundwater flow.

The hydrogeological and hydrochemical data obtained by means of this work

improve the present hydrogeological frameworks of the area and can contribute at defining a

more detailed hydrogeological model and to realize a possible numerical model.