A geochemical atlas of the ground- and running waters of Arezzo (Tuscany, Italy)

Abstract

Since 2000 a joint project between the Department of Earth Science of Florence and the Unit of Environment and Health of the Municipality of Arezzo has allowed to geochemically characterise the superficial and ground waters of the municipal territory of Arezzo in order to establish the water quality and to investigate the main natural and anthropic processes responsible of their composition. The available geochemical data-base consists of more than 500 samples sites (90% of which are private wells and 7 and 3% are springs and running waters, respectively) on which physical parameters (temperature and electrical conductivity) and major, minor and trace dissolved species (pH, Ca, Mg, Na, K, NH4, HCO3, SO4, NO3, NO2, Cl, Br, F, heavy metals) have been performed by using the same sampling procedure and analytical methodology in order to have a consistent set of data. Fifteen selected sites have been analysed twice per year to evidence possible seasonal effects. No significant differences have been recorded. The Arezzo Basin, formed since Upper Pliocene, is a structural depression limited to the North and to the East by the Pratomagno and Chianti belts, respectively, and to the South and to the East by two tectonic lineaments (Val d’Arbia-Val Marecchia transversal and Chitignano normal faults). Along these tectonic discontinuities CO2-rich manifestations either seep out or exploited by private companies. Hydrogeologically, three main aquifers are recognised: i) a relatively deep aquifer hosted in Tertiary sandstone formations; ii) an intermediate aquifer hosted in Quaternary fluvio-lacustrine sediments and iii) a shallow aquifer in recent alluvional sediments. The content in Total Dissolved Solids (TDS) allows to classify the Arezzo waters in: oligomineral (69%), medium-mineral (30%) and mineral (1%) and they can be regarded as Ca(Mg)-HCO3 (87 %), Na(K)-HCO3 (7%), Ca(Mg)-SO4 (5%) and Na(K)-Cl (1%). It is noteworthy to point of that the Na(K)-HCO3 waters are aligned along the above mentioned tectonic systems. The quality of Arezzo waters has been referred to the Italian legislation that is addressed to the definition of the Maximum Admissible Concentration (MAC, DPR 236/88, Dlgs 31/01) and the Reference Value (RV, DPR 236/88) in terms of waters for the human consumption. Waters from the northern area of Arezzo overcome MAC for chlorides, sulphates and sodium; if we consider nitrogen species (NH4, NO2, NO3) the values overcome CMA for those waters collected into the city, its peripheral areas and in the south-western suburbs.

Thematic maps has been produced, on the basis of the principles of linear Geostatistics, in order to analyse the spatial behaviour of the analysed variables. The aim was to find correlations with lithology, use of the soils, drainage density, pressure of antrophic activities and so on, and to identify sensible areas to monitor in their time evolution. The investigation has been developed starting from a detailed variographic analysis by means of the geochemical behaviour of each variable has been analysed in the different directions of the space while the estimation procedure to obtain the maps has been based on the application of sequential Gaussian simulation procedures.