MICROINQUINANTI DI ORIGINE NATURALE NEGLI ACQUIFERI VULCANICI: UN APPROCCIO INTEGRATO QUANTITATIVO E QUALITATIVO NEL LAZIO SETTENTRIONALE

Risk assessment in the management of drinking water is one of the aim of the GNDCI (National Group for the Defense from Hydrogeological Hazards), and in particular of MIMA project (Integrated Methodologies of Monitoring Aquifers), leaded by IRSA-CNR (Water Research Institute). In this study the preliminary results of a research programme conducted in cooperation with the University of Rome "La Sapienza" are shown. The main objective is the quality assessment of groundwaters hosted in Northern Latium volcanic complexes (Sabatini, Vico and Vulsini volcanoes), which represent important water resources reservoirs at a regional scale. In northern Latium (central Italy) anomalous concentrations of minor toxic elements, such as As and F, of natural origin in ground waters are known since several years (Brondi 1986, Dall'Aglio et al., 2001); in this area, volcanic alkaline-potassic rocks of quaternary age largely outcrop, hosting an important regional aquifer exploited also for human consumption. Concentrations in these elements are often above the threshold values for drinking waters established by the European Directive 98/83/EC (i.e.10 ?g/l for As and 1.5 mg/l for F). In order to shed light on the hydrochemical processes leading to the enrichment in the above cited elements, a comprehensive study of the groundwater system was conducted in a representative area north of Rome. The aim is the characterization of the hydrodynamic and hydrochemical features of the groundwater system by integrating a quantitative approach (river discharge and piezometric head measures) with a qualitative one (study of the behaviour of chemical elements and their inter-correlation, analysis of hydrochemical facies). The relative variation of minor elements in groundwater has been considered a helpful tool to understand the local hydrogeological setting, to show the relationships between local and regional aquifers and to differentiate their contributions to the basal flow of rivers' discharge. Northern Latium is characterized by a widespread outcrop of volcanic units of Quaternary age that overlay a sedimentary substratum (Baldi et al., 1974) represented by Mesozoic limestones, Cretaceous-Paleogenic flysch sequences and Plio-Pleistocene clay, sand and gravel deposits. The volcanic complex represents an important hydrogeological structure which discharges about 30 m3/s in an area of 3500 km2 (Boni et al., 1986). The basal unconfined aquifer, hosted in the volcanic units, is separated from the geothermal reservoir of the Mesozoic limestones beneath by Pliocene clays and flysch sequences. Some of the volcano-tectonic depressions formed during the last phases of activity host now the main lakes of this area. The study area is located about 100 km north of Rome between Vico lake and the Tiber valley. In this area volcanic products are mostly related to Vico volcano activity; the "Red Tuff with black scoria" is the mainly outcropping unit. The Pleistocene deposits underneath, represented by the sand and gravel deposits of the Paleo-Tiber (Alvarez, 1972), outcrop only at the eastern edge of the area. In July and September 2002-2003 two hydrogeochemical surveys were conducted in drought periods; 60 private wells, 5 springs and 16 river sites were sampled using standard methodologies for sampling and for chemical analysis. Eh, pH, electric conductivity, alkalinity and piezometric levels of the boreholes were analysed in situ. At river sites discharge measures were performed in order to evaluate the contribution of groundwater to the rivers at different elevation: measuring sites were located possibly where the stratigraphic boundaries outcrop. The samples were transported to the IRSA laboratory in refrigerated containers and analysed for major anions (F-, Cl-, PO43-,