Temporal and Spatial Variation of Radon Concentrations in Environmental Water from Okinawa Island, Southwestern Part of Japan.

Shunya Nakasone,Masahiro Tanaka,Yumi Yasuoka,Natsumi Masuda,Masahide Furukawa,Masahiro Hosoda,Naofumi Akata,Shuhei Shiroma,Yoshitaka Shiroma,Sohei Ooka,Kaori Nakamura,Akinobu Ishimine,Akemi Kato

doi:10.3390/ijerph18030998

Abstract

In this study, to get a better understanding in characterizing groundwater and ensure its effective management, the radon concentrations in water samples were measured through Ryukyu limestone in southern Okinawa Island, Japan. Water samples were collected from a limestone cave (Gyokusendo cave, dropping water) and two springs (Ukinju and Komesu, spring water), and the radon concentrations were measured by liquid scintillation counters. The radon concentrations in the samples from the Gyokusendo cave, and Ukinju and Komesu springs were 10 ± 1.3 Bq L−1, 3.2 ± 1.0 Bq L−1, and 3.1 ± 1.1 Bq L−1, respectively. The radon concentrations showed a gradually increasing trend from summer to autumn and decreased during winter. The variation of radon concentrations in the dripping water sample from the Gyokusendo cave showed a lagged response to precipitation changes by approximately 2–3 months. The estimated radon concentrations in the dripping water sample were calculated with the measured radon concentrations from the dripping water obtained during the study period. Based on our results, groundwater in the Gyokusendo cave system was estimated to percolate through the Ryukyu limestone in 7–10 days, and the residence time of groundwater in the soil above Gyokusendo cave was estimated to be approximately 50–80 days. This work makes a valuable contribution to the understanding of groundwater processes in limestone aquifers, which is essential for ensuring groundwater sustainability.

Highlights

Radon is a radioactive noble gas with an atomic number of 86
Radon concentrations for dripping water and spring water samples were collected in the southern part of Okinawa Island from October 2016 to October 2020
Radon concentrations for the water samples showed a gradually increasing trend from summer to autumn and decreased in the winter. This was noticeable in the radon concentrations measured in the dripping water sample from the Gyokusendo cave

Summary

Introduction

Radon is a radioactive noble gas with an atomic number of 86. there are many isotopes of radon, only three occur in the natural environment: 222 Rn, 220 Rn (Thoron), and 219 Rn (Actinon), with half-lives of 3.8 d, 55.6 s, and 3.9 s, respectively. 222 Rn is the most frequently used radon isotope due to its relatively long half-life compared to thoron and actinon. There are many isotopes of radon, only three occur in the natural environment: 222 Rn, 220 Rn (Thoron), and 219 Rn (Actinon), with half-lives of 3.8 d, 55.6 s, and 3.9 s, respectively. 222 Rn is the most frequently used radon isotope due to its relatively long half-life compared to thoron and actinon. Hereinafter, “radon” refers to 222 Rn. Radon is a natural radionuclide belonging to the uranium (238 U) decay series, with radium (226 Ra) as a parent nuclide. Most radon in the environment is produced by the alpha decay of radium in the soil and rocks. If it is dissipated from soil grains into pore water or voids, it can migrate relatively far from where it is produced by advection or diffusion [2]. Radon, which is widely distributed in the natural environment, is commonly used to study atmospheric transport processes and groundwater flow systems such as groundwater recharge, flow, and discharge [3,4]

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