Fluorine in the rocks and sediments of volcanic areas in central Italy: total content, enrichment and leaching processes and a hypothesis on the vulnerability of the related aquifers

Abstract

Rock, sediment and water samples from areas characterised by hydrothermal alterations in the Sabatini and Vico Volcanic Districts, near Rome and the large city of Viterbo, respectively, were collected and analysed to determine the total fluorine (F) content and to understand the F geochemical background level in the volcanic districts of central Italy. Leaching and alteration processes controlling the high concentration of F in water were also investigated. Fluorine concentrations were directly determined (potentiometrically) by an F selective electrode in water samples, while the procedure for rock samples included preliminary F dissolution through alkaline fusion. F concentrations higher than 800 mg kg(-1) were commonly found in the analysed rocks and sediments; the concentration depended on the lithology and on the distance from the alteration areas. A specific successive sampling campaign was conducted in three areas where the F content in sediments was particularly high; in the same areas, measurements of CO(2) flux were also performed to investigate the possible deep origin of F. To verify the relationships among the high F contents in rocks and sediments, the leaching processes involved and the presence of F in the aquifer, we also collected water samples in the western sector of the Sabatini Volcanic District, where hydrothermal manifestations and mineral springs are common. The data were processed using a GIS system in which the F distribution was combined with morphological and geological observations. The main results of our study are that (1) F concentrations are higher in volcanic and recently formed travertine (especially in hydrothermally altered sediments) than in sedimentary rocks and decrease with distance from hydrothermal alteration areas, (2) F is more easily leached from hydrothermally altered rocks and from travertine and (3) sediments enriched with F may indicate the presence of deep regional fractures that represent direct pathways of hydrothermal fluids from the crust to the surface.

Groundwater chemical status assessment considering geochemical background: an example from Northern Latium (Central Italy)

“…The hydrogeological circulation is complex due to the overlapping series of pyroclastic, lavas, and sedimentary units with different thickness and geometry. In addition, the regional fractures may play a role in producing hot spots of fluoride or arsenic concentration in groundwater in this area, as suggested by De Rita et al (2012) and Cinti et al (2015). All the above-mentioned geological factors contribute to the explanation of the high spatial variability of the concentration of the elements such as arsenic and fluoride and should be taken into account.…”

Anomalous concentrations of arsenic, fluoride and radon in volcanic-sedimentary aquifers from central Italy: Quality indexes for management of the water resource

“…High F À concentrations in groundwater are generally related to natural sources, whereas only occasionally is of anthropogenic origin (Ozsvath, 2009;WHO, 2011). The occurrence of F À in volcanic-hydrothermal fluids is related to i) release of magmatic fluorine as HF through volcanic degassing, ii) upwelling of geothermal fluids and iii) water-rock interaction processes involving F-rich minerals (e.g., D'Alessandro, 2006;Sawyer and Opperheimer, 2006;Ozsvath, 2009;De Rita et al, 2011). High fluorine contents are generally found in volcanic rocks and hydrothermal deposits commonly associated with fluorite (CaF 2 ), fluorapatite (Ca 5 (PO 4 ) 3 F) and F-rich micas and/or amphiboles (where F À largely replaces OH À within the mineral structures).…”

“…The concentrations of fluorine tend to decrease with distance from hydrothermal alteration areas. Sedimentary deposits are usually characterized by low contents (De Rita et al, 2011). Radiometric characterization of volcanic and sedimentary rocks from SVD and VCVD (Trevisi et al, 2005) showed that the mean content of Rn-generating radionuclides 238 U and 226 Ra of tuffs (160 and 147 Bq/kg, respectively) and lavas (134 and 124 Bq/kg, respectively) from the Roman Magmatic Province is significantly higher than that of sedimentary formations (up to 49 and 24 Bq/kg, respectively) (Locardi and Mittempergher, 1971;Voltaggio et al, 2001).…”

“…above the concentration limit for drinkable waters (>1.5 mg/L), may be related to i) mixing with thermal F-rich waters and ii) leaching of F-rich deposits and/or hydrothermally altered rocks. On the other hand, F À mobility is limited in the absence of active hydrothermal and alteration processes, as suggested by results of leaching tests on SVD and VCVD rocks (De Rita et al, 2011), likely due to the low solubility of common F-bearing minerals of volcanic rocks (e.g. fluorite, fluorapatite).…”

“…As far as the study area is concerned, a few geochemical maps on the As spatial distribution, though limited to the VCVD, were published by Vivona et al (2007), Angelone et al (2009), Armiento et al (2015) and Cinti et al (2015) whereas maps related to the distribution of F À (Vivona et al, 2007;De Rita et al, 2011;Preziosi et al, 2016) and 222 Rn (Cinti et al, 2013) were poorly constrained. To fill this gap, in this paper spatial distribution maps of As, F À and 222 Rn concentrations, based on a large and original dataset consisting of 659 water samples collected in the SVD and VCVD, are shown and discussed to i) investigate the sources and the main factors controlling their geochemical behavior and distribution and ii) evaluate the contribution of these pollutants due to the geogenic source.…”