Heterogeneous temperature sensitivity of effective radium concentration from various rock and soil samples

Abstract

Temporal variations of radon concentration, or spatial variations around geothermal systems, are partly driven by the effect of temperature on the radon source term, the effective radium concentration (EC_Ra). EC_Ra from 12 crushed rock and 12 soil samples from Nepal was measured in the laboratory using the radon accumulation method and Lucas scintillation flasks at three temperatures: 7, 22 and 37 °C. For each sample and at each temperature, 5 or 6 measurements were carried out, representing a total of 360 measurements, with an EC_Ra average varying from 1.1 to 75 Bq kg⁻¹. While the effect is small, EC_Ra was observed to increase with temperature in a significant and sufficiently reproducible manner. The increase was approximately linear with a slope (temperature sensitivity, TS) expressed in % °C⁻¹. We observed a large heterogeneity of TS with average values (range min-max) of 0.79 ± 0.05 (0.16–2.0) % °C⁻¹ and 0.61 ± 0.05 (0.10–2.0) % °C⁻¹, for rock and soil samples, respectively. While this range overlaps with the results of previous studies, our values of TS tend to be smaller. The observed heterogeneity implies that the TS, rather poorly understood, needs to be assessed by dedicated experiments in every case where it is of consequence for the interpretation.