Radioactivity and volcanic areas: radon concentrations in waters from the unresting Campi Flegrei caldera and other volcanoes in Southern Italy using a RAD7 radon detector

This study, conducted in the frame of the Pianeta Dinamico project funded by Istituto Nazionale di Geofisica e Vulcanologia, aims to improve the knowledge of radon concentration dissolved in several different waters from some of the Italian volcanic areas and its relationships with lithological and structural settings, fluid chemistry and circulation, volcano dynamics.

The study mainly focuses on ²²²Rn variations over time in thermal waters of the Campi Flegrei caldera at NW of Naples, an active volcanic field hosting an important geothermal system and one of the most dangerous calderas in the world. The caldera, part of the SW-NE trending fissure-like feeding system of the Phlegraean Volcanic District including also Ischia island, lastly erupted in 1538 and, since 2000, has been under a permanent unrest with up 1 metres of uplift, thousands of earthquakes and tons of CO₂ emissions.

25 sites were sampled and analysed for dissolved ²²²Rn levels at least twice per year since October 2021; starting from October 2022, 5 waters were then selected for monthly monitoring with the goal of unravelling exogenous (e.g., temperature or rainfall) versus endogenous processes relationships.

In order to compare radon concentrations in the caldera with those in the volcanic context of southern Italy, we sampled and analysed as follows: a) 23 cold waters at Somma-Vesuvius, the stratovolcano placed at the intersection of regional NW–SE and NE–SW fault systems east of Naples, showing mild gaseous emissions after the 1944 eruption; b) 8 cold waters from the north of Campi Flegrei caldera, i.e. 7 from a multi-layer aquifer at the base of the extinct Roccamonfina stratovolcano and 1 water at Minturno (northern edge of Roccamonfina); c) 1 thermal water belonging to Ischia at the NW corner of the Gulf of Naples; d) 3 thermal waters of the Vulcano island from the Sicilian Aeolian Arc, characterised by vigorous seismic and fumarolic degassing activities that has shown an abrupt increase on August 2021.

The waters are springs, lakes, pools, and groundwater; two are from Le Fumose (Campi Flegrei) and Porto Ponente (Vulcano) submerged emissions.

Measurements have been performed by a Radon-in-air detector (RAD7^®, Durridge Co.) equipped with Big Bottle RAD H2O and DRYSTIK accessories and processed using the CAPTURE program.

Campi Flegrei reaches the highest radon concentrations, varying from 0.20 ± 0.03 to ~1887 ± 13 Bq/L. Somma-Vesuvius shows from almost no radon to 24 ± 1 Bq/L, and Ischia is at 54 ± 2 Bq/L.

Roccamonfina area has a discrete variability from 0.2 ± 0.1 to ~ 71 ± 7 Bq/L at SE, being 9.0 ± 0.4 Bq/L at Minturno.

Vulcano island attains the lowest detected concentrations, less than 2.4 ± 1.0 Bq/L.

Dealing with different areas, the presented results lay the groundwork for better understanding radon behaviour and evaluating the implications on environmental and volcanic hazards assessment. At Campi Flegrei where, in 2012, the alert level was raised from base to warning, radon should be useful to investigate processes from which volcanic dynamics originates and to corroborate the monitoring outcomes.