Radon (222Rn) levels in thermal waters of the geothermally active Campi Flegrei volcanic caldera (Southern Italy): A framework study using a RAD7 radon detector

Abstract

The present investigation provides measurements of radon (222Rn) concentration levels in 20 thermal waters at the Campi Flegrei volcanic caldera, an important geothermal system with hydrothermal manifestations in the Neapolitan area (Southern Italy). We used a RAD7® Radon-in-air detector equipped with Big Bottle RAD H2O and DRYSTIK accessories (Durridge Co. Inc.). Water samples with different chemical and/or physical conditions, not used as drink waters, are taken from continental thermal groundwaters, springs, lakes, pools and one sub merged thermal spring. The waters are mostly chlorine to bicarbonate, except of a few sulphate types sampled at the hydrothermal discharge areas of Solfatara and Pisciarelli, central in the caldera. Water temperature and pH values range from 18.1 to 91.3 ◦C and from 2 to 8, respectively. Sampling and measurement of radon in groundwater are complicated by the high volatility of the gas; a method is here proposed. In some of the 20 sites double or triple samples were collected by using different volume polyethylene terephthalate (PET) bottles, diluting sample with blank water, and modifying flow of pumped wells. We suggest that dilution can be considered when water is i) not enough to fill in the PET, resulting in large head space in the sampler, ii) too hot determining damage of the PET or iii) too saline to clog the Big Bottle System. Dissolved radon concentrations vary from 0.1 ± 0.1 to 1146 ± 57 Bq/L with an average value of 152 Bq/L, using the CAPTURE program, the default RAD7 data acquisition program. Similar values in radon concentration are obtained using the method proposed in De Simone et al. (2015) ranging between 0.1 ± 5.8 and 1286 ± 98 Bq/L with an average value of 167 Bq/L. The hottest and most acidic sulphate waters refer to a small boiling pool at Pisciarelli hydrothermal discharge area and have nearly zero 222Rn content. 222Rn concentrations from this study are mostly below the reference level of 1000 Bq/L recommended for human health protection by the European Commission and the most adopted in the scientific community (Catao ˜et al., 2022). No correlation has been observed between temperature, pH, major anions and radon content values, nor between rock composition since it is almost homogeneous trachyte at the study sites. 222Rn levels therefore appear to reflect the local sedimentological, structural or hydrogeological conditions. The levels of 222Rn here presented are an important background for the scientific community that will intend to define the natural fluctuations of dissolved 222Rn in relation with seasons, environment, hydrogeology or volcanic dynamics at the geohazardous Campi Flegrei area.