Groundwaters of Florence (Italy): Trace element distribution and vulnerability of the aquifers

Abstract

Geochemical and hydrogeological research has been carried out on 109 wells in the alluvial plain of Florence, in order to evaluate conductivity and main chemistry of ground waters, the pattern of some possible pollutant chemical species (Fe, Mn, Cr, Cu, Pb, Zn, NO₂, NO₃), and the vulnerability of the aquifers. The plain is made up of Plio-Quaternary alluvial and lacustrine sediments for a maximum thickness of 600 m. Silts and clays, sometimes with lenses of sandy gravels, are dominant, while considerable deposits of sands, pebbles, and gravels occur along the course of the Arno river and its tributary streams, and represent the most important aquifer of the plain. The groundwaters analyzed belong to this aquifer or to the smaller ones, hosted in the gravel lenses. Most waters show conductivity values around 1000–1200 μS, and almost all of them have an alkaline-earth-bicarbonate chemical character; these features are consistent with the mainly calcareous lithology of the aquifers. In the western areas a higher salt content of the groundwaters is evident, probably related to the presence of industrial activities which use water desalinators. Heavy metal and NO₂, NO₃ analyses point out that no important pollution phenomena affect the groundwaters; all the mean values of the chemical considered species are below the maximum admissible concentration (MAC) fixed by the European Community for drinkable waters. Nevertheless, some anomalies of NO₂, NO₃, Fe, Mn, and Zn are present in the plain. Apart from Mn, which seems to be released by certain calcareous gravels, the other anomalies have a local influence, since they disappear even in the nearest wells. The most plausible causes can be recognized in losses of the sewage system (NO₂=3–4 mg/t); use of nitrate compounds in agriculture (NO₃=60–70 mg/l); oxidation of well pipes (Fe ≈ 20 mg/l; Zn ≈ 6 mg/l). As regards Cr, Cu, and Pb, all the observations are below the MAC; therefore, the median values of < 3, 3.9, and 1.1 μg/l, respectively, could be considered reference concentrations for groundwaters circulating in calcareous lithotypes, under undisturbed natural conditions. Finally, a map of vulnerability related to the most superficial and important aquifer has been elaborated on the basis of thickness and permeability of the covers. The map shows that the areas near the Arno river are highly vulnerable, for the minimum thickness (or lacking) of sediments covering the aquifer. On the other hand, in the case of pollution, several factors not considered in the map could significantly increase the self-purification capacity of the aquifer, such as the dilution of groundwaters caused by the feeding of the rivers, the bacteria oxidation of nitrogenous species, and the sorption capacity of clay minerals and organic matter with respect to trace metals.