Monitoring dissolved radon-222 in groundwater in a volcanic island (Tenerife, Canary Islands)

Radon-222, ²²²Rn, is a naturally radioactive noble gas with a half-life of 3.82 days. It is a product of the ²³⁸U disintegration chain with ²²⁶Ra as parent isotope. Radon gas has traditionally been linked to magmatic and seismic activity.

In order to improve volcanic surveillance networks, ²²²Rn monitoring in volcanic areas is carried out by measuring its concentration in air and soils. Also, since radon-222 is soluble in water, the gas may also be dissolved into groundwater flows. The quantity of dissolved ²²²Rn depends on different factors such as the characteristics of the aquifer, water-rock interaction, water residence time and material content of radium.

In Tenerife there are a vast number of excavated galleries, subhorizontal water mining tunnels, that drain the main aquifer in the island. Thirteen of them have been selected in order to monitor the dissolved radon-222 concentration since October 2019 every three months. This set of galleries surrounds the main volcanic complex in the island, Teide-Pico Viejo, located in Las Cañadas caldera in central Tenerife. 

Before sampling, 10 ml of Opti-Fluor O scintillation cocktail for selective extraction of radon-222, two-phase counting method, was transferred into a 20 ml vial. During the sampling, 10 ml of water were injected into the bottom of the vial. The vial was then tightly capped, vigorously shaken and transported to the laboratory for analysis. Water samples were measured using the liquid scintillation system Quantulus 1220 from PerkinElmer.

In this work, we show the main and preliminary results, which includes both spatial and temporal distribution of the dissolved radon concentration measured in the selected sampling points for the studied period. There are galleries showing a high stability, with radon-222 activity concentrations around 20 Bq/L, whereas others show a clear seasonal influence. The maximum dissolved ²²²Rn value detected was 29.2 ± 2.1 Bq/L. The detection limit is 0.5 Bq/l using a 30 minutes counting time. Besides, this dataset has been compared to data previously reported by other authors, in order to find changes in dissolved radon-222 emission across time. Finally, in the aim of finding interesting relationships, dissolved ²²²Rn values have been studied together with in situ groundwater parameters measured in the field (temperature, pH and electric conductivity) and, in some of the sampling points, also with dissolved CO₂ concentration and isotopic ratio ³He/⁴He corrected for air contamination, (R/R_a)_c, which are parameters directly related to volcanic activity.