Occurrence of minor toxic elements in volcanic-sedimentary aquifers: a case study in central Italy

Abstract

The occurrence of minor toxic elements (arse- nic, vanadium and fluoride among others) in the volcanic aquifers of central Italy, with concentrations often above the national standards for drinking waters, has been recognized since the 1970s. However, these groundwaters are still often used for human consumption. With the aim of providing insight into the water-rock interaction processes, as well as into the evolution of arsenic and other elements, water-sampling campaigns where under- taken within a 100 km 2 area at the eastern margin of the Viterbo region, where volcanites overlie an alluvial aquifer complex. The samples were analysed and geo- chemical modelling was applied to characterize the thermodynamic state of the waters. The results indicate the existence of direct relations among arsenic, vanadium and fluoride concentrations and of their inverse relation- ship with calcium concentration. An evolution scheme of groundwater composition from the upstream volcanic aquifer to the downstream sedimentary aquifer is also discussed. Two hypotheses are proposed which justify the observed decrease of the cited minor element concen- trations, i.e. dilution processes and/or precipitation of fluorapatite, which can include in its structure small amounts of these elements. The precipitation hypothesis is supported by modelling results. Resume La presence de certains elements toxiques mine- urs (arsenic, vanadium et fluor entre autres) dans les aquiferes volcaniques d'Italie centrale, a des concentra- tions souvent superieures aux limites de potabilite nation- ales, est reconnue depuis les annees 70. Ces eaux restent cependant tres utilisees pour la consommation humaine. Afi nd 'apprendre les interactions eau/roche et l'evolution de l'arsenic et des autres elements, des campagnes de prelevement ont ete mises en œuvre dans un secteur couvrant 100 km 2 , sur la bordure orientale de la region de