Radon levels in soil and in groundwater in the Alhama de Murcia fault area, Spain

Abstract

Radon and CO₂ in groundwater, radon and thoron in soil, soil radon exhalation rates, and 14 electrical resistivity imaging were measured in the Alhama de Murcia Fault area. The study 15 aims to (i) characterize radon levels in groundwater and soil and (ii) assess whether variations 16 in gas emissions are related to the presence of an active fault. Forty water sampling points 17 exhibited ²²²Rn concentrations ranging from 0.4 to 66.8 Bq L⁻¹, while 22 soil points along two 18 profiles crossing the fault showed low annual mean values of (8 ± 1) kBq m⁻³ and (12 ± 2) 19 kBq m⁻³. Soil radon exhalation rates at the two profiles, (36 ± 8) and (53 ± 12) Bq m⁻² h⁻¹, 20 respectively, are consistent with these concentrations. Spatial variability of soil radon along 21 both profiles, [1–22] kBq m⁻³, appears associated with the main fault location. Temporal 22 variations of ²²²Rn recorded at a monitoring station from early 2013 to mid-2015 closely match 23 punctual measurements. While fluctuations are largely controlled by barometric pressure and 24 atmospheric temperature, detailed analyses combining multiple radon anomaly detection 25 methods indicate a possible influence of nearby seismic events.