*3.1. Physical Parameters of Soil and 226Ra and 228Ra Activity Concentrations at the Study Site*

The percentages of sand, silt and clay for each soil sample collected at the study site were evaluated as 63 ± 4%, 16 ± 2%, and 21 ± 3%, respectively. As a result, the textural class of all soil samples was decided as sandy clay loam (SCL) based on a soil texture triangle. In general, the characteristics of SCL are reported to be high water retention and low air permeability [32]. Dry bulk density, soil particle density and porosity were evaluated as 1340 ± 19 kg m−3, 2657 ± 27 kg m−3, and 0.50 ± 0.07, respectively. These obtained values were not significantly different from the typical values of 1300−1350 kg <sup>m</sup>−<sup>3</sup> for dry bulk density, 2600−2700 kg m−<sup>3</sup> for soil particle density, and 0.3−0.6 for porosity [33]. Activity concentrations of 226Ra and 228Ra were evaluated to be 24.1 ± 0.4 Bq kg−1and 34.0 ± 0.9 Bq kg−1, respectively. According to the UNSCEAR [1], the Japanese mean activity concentrations of 226Ra and 228Ra (assuming radioactive equilibrium with 232Th) are reported as 33 Bq kg−<sup>1</sup> and 28 Bq kg−1, respectively. Thus, radium activity concentrations in soil at the study site were found to be slightly lower than those of the national mean.

#### *3.2. Radon and Thoron Concentration in the Ventilated-Type Accumulation Chamber*

An example of the vertical distribution of thoron concentration inside the ventilated type accumulation chamber is shown in Figure 2. Thoron concentration decreased exponentially with the height above the ground surface. On the other hand, radon concentrations inside the accumulation chamber did not depend on the height above the ground. These observations were similar to the previously reported findings [16,17,19]. The results obtained at the 10 cm and above height from the ground were not considered in the calculation of thoron exhalation rate because the thoron concentrations at these heights were below the lower limit of detection. Additionally, the air exchange rate of the accumulation chamber was evaluated as 0.30 h−<sup>1</sup> which was similar to the literature [9].

**Figure 2.** Example of the vertical distribution of thoron concentration inside the ventilated type accumulation chamber. The thoron concentration at ground level (*z* = 0 cm) was determined to be <sup>416</sup> <sup>±</sup> 42 Bq m<sup>−</sup>3.

#### *3.3. Comparison of Radon and Thoron Exhalation Rates Obtained by the Present System to Those Obtained by the Other Methods*

Comparison of radon and thoron exhalation rates obtained by the ventilated-type accumulation chamber, accumulation chamber with scintillation cell and in situ monitor are shown in Table 1. Radon exhalation rates obtained by the passive method were in relatively good agreement with the results measured by the accumulation chamber with scintillation cell and in situ monitor (MSZ) taking into account the measurement uncertainty. Statistical analyses were conducted using the "EZR software" (Easy R) [34]. The difference was considered significant for *p* < 0.05. A one-way ANOVA test was performed for the comparison of radon exhalation rates obtained by each technique. Consequently, the radon exhalation rate measured by the present system and those obtained by the other methods are not significantly different (*p* = 0.128). Furthermore, the thoron exhalation rate obtained by the ventilated-type accumulation chamber in the first measurements was also in agreement with the result obtained by the in situ monitor (*p* = 0.156). However, in the second measurements, thoron exhalation rate obtained by the ventilated-type accumulation chamber was approximately half that of the value obtained by the in situ monitor. Theoretically, the diffusion length of thoron is reported as a few centimeters which is much shorter than that of radon due to the short half-life of thoron. Therefore, thoron exhalation rate is considered to be more strongly affected by the soil surface condition compared with the radon exhalation rate. Thus, it is necessary to make intercomparison experiments repeatedly to ensure the quality of the data obtained by the present ventilated-type accumulation chamber. However, the present method can offer an easy and low-cost system for the measurements of both radon and thoron exhalation rates, as no electric power supply is needed and operation and maintenance are easy.


**Table 1.** Comparison of radon and thoron exhalation rates obtained by the ventilated-type accumulation chamber, accumulation chamber with scintillation cell, and in situ monitor (MSZ) method.

#### *3.4. Seasonal Variations of the Radon and Thoron Exhalation Rates*

Seasonal variations of radon and thoron exhalation rates are shown in Figure 3. The median values (range) of radon exhalation rate in spring, summer, autumn and winter were estimated to be 3.5 ± 0.5 (2.4−5.8), 5.4 ± 1.1 (4.0−6.4), 5.6 ± 0.3 (4.0−7.1), and 3.5 ± 0.3 (2.1−4.8) mBq m−<sup>2</sup> <sup>s</sup>−1, respectively. The median values (range) of thoron exhalation rates were evaluated as 614 ± 126 (159−848), 555 ± 110 (295−1110), 563 ± 395 (61−1524), and 593 ± 138 (318−797) mBq m−<sup>2</sup> <sup>s</sup>−<sup>1</sup> for spring, summer, autumn and winter, respectively. Annual means of the radon and thoron exhalation rates were evaluated to be 4.5 ± 0.3 mBq m−<sup>2</sup> <sup>s</sup>−<sup>1</sup> and 581 ± 113 mBq m−<sup>2</sup> <sup>s</sup>−1, respectively. According to the results of a large-scale survey in Japan [12,35], average radon and thoron exhalation rates from the ground were 8.6 mBq m−<sup>2</sup> s−<sup>1</sup> (*N* = 111) and 790 mBq m−<sup>2</sup> s−<sup>1</sup> (*N* = 405), respectively. Therefore, radon and thoron exhalation rates at the present measurement site were 52% and 74% of the Japanese averages. Furthermore, average radon exhalation rates in summer and autumn were higher than the annual mean. The ratio of the radon exhalation rate in summer to the radon exhalation rate in winter (or spring) is 1.5 and the ratio of the radon exhalation rate in autumn to the radon exhalation rate in winter (or spring) is 1.6. A oneway ANOVA was performed to determine whether there are any statistically significant differences between the means of each season. Consequently, it showed no statistically significant difference in the average radon exhalation rate between the seasons in this study (*p* = 0.103). However, the median values of radon exhalation rate tend to be higher from summer to autumn and lower from winter to spring (Figure 3a). Zhuo et al. [36] reported a similar seasonal variation of radon exhalation rate from the ground in China. Zhuo et al. [9] have also reported a negative correlation between radon exhalation rate and precipitation. On the other hand, Hosoda et al. [12] reported that when the variation of moisture saturation was small, the soil temperature appeared to induce a strong effect on the exhalation rate. However, when the variation of moisture saturation was large, the influence of moisture saturation appears to be larger than the soil surface temperature [12]. Furthermore, it has been also reported that an increase in the soil temperature markedly decreased the amount of adsorption of gases which contributed to the increase of emanation and diffusion coefficients [37,38]. Precipitation data from a location near the monitoring site were reported by the Japan Meteorological Agency [39], and cumulative precipitations in spring, summer, autumn, and winter during the measurement period were 424, 586, 441, and 177 mm, respectively. Additionally, their respective mean temperatures were 14.1, 25.2, 17.9, and 4.2 ◦C. Therefore, the radon exhalation rate in winter at the measurement site might be affected by low precipitation and temperature. On the other hand, in summer the high temperature might affect the radon exhalation rate.

**Figure 3.** Seasonal variations of radon (**a**) and thoron (**b**) exhalation rates. The lines from top to bottom of the box-plot from top to bottom are defined as maximum, third quartile (75th percentile), median (50th percentile), first quartile (25th percentile), and minimum values.

The statistical analysis using the one-way ANOVA test showed that there was no significant difference between the averages of thoron exhalation rate for the different seasons (*p* = 0.982) (Figure 3b). According to the report by Prasad et al. [40], radon and thoron exhalation rates in summer and autumn were higher than those in spring and winter. However, the reported diffusion length of radon and thoron were a few meters and a few centimeters, respectively [11,26]. That is, thoron exhalation rate from the ground would be affected by such environmental parameters as moisture saturation and temperature around the surface soil. Therefore, the soil temperature and moisture saturation were measured continuously for three months in summer at 10 cm depth from the surface inside and outside of the accumulation chamber. The average surface soil temperature inside and outside the accumulation chamber were 22.3 ± 3.5 (RSD: 18%) and 22.2 ± 3.9 ◦C (RSD: 16%), respectively. The results suggested that the accumulation chamber setup was not affected by the surface soil temperature as same with the previous

report [10]. On the other hand, the average moisture saturation (convert from volumetric water content using porosity) inside and outside the accumulation chamber were evaluated to be 0.364 ± 0.172 (RSD: 47%) and 0.114 ± 0.028 (RSD: 24%), respectively. As we mentioned above, Zhuo et al. reported that the water potential inside and outside of the stainless-steel box shown nearly the same throughout the year [10]. It is well known that the water potential is related parameter to volumetric water content which is influential parameter of exhalation rate. However, both the average moisture saturation and its variation inside the accumulation chamber were smaller than those outside the chamber. Additionally, the accumulation chamber was embedded 15 cm into the soil. That is, it is considered that this measurement condition was not easy for water due to rainfall to move from outside the chamber to under the chamber by passing through pore spaces in the surface soil. However, the thoron exhalation rates obtained in this study may be considered as baseline level at the measurement site. Thus, we will develop the correction method of thoron exhalation rates with the variation of the environmental factors. Additionally, it might be possible to evaluate the seasonal variations of thoron exhalation rate if a passive type radon and thoron discriminative monitor was set in a small size accumulation chamber.

#### **4. Conclusions**

In this study, radon and thoron exhalation rates from the ground were simultaneously evaluated for four years by applying the naturally ventilated accumulation chamber of a previous report. The results were compared to the data obtained by the accumulation chamber with scintillation cell and in situ radon and thoron exhalation rate monitor. The results of the present method had relatively good agreement with results of the other methods. Relationships between radon exhalation rates with environmental parameters were also observed and their variations with seasons were determined. The baseline level of thoron exhalation rate at the measurement site was evaluated. However, thoron exhalation rates did not show clear seasonal variations, most likely due to limitations of the present methodology. Therefore, the methodology will be modified based on the present results to allow the season variation of thoron exhalation rate from the ground to be obtained.

**Author Contributions:** Conceptualization, M.H. and S.T.; methodology, W.Z., N.A., M.H., and S.T.; validation, C.K., K.I., N.A., and M.H.; formal analysis, Y.T., T.S., and R.Y.; investigation, Y.T., T.S., R.Y., K.I., N.A., M.H., and S.T.; resources, N.A., M.H., and S.T.; data curation, T.S. and M.H.; writing original draft preparation, O.B.M.; writing—review and editing, Y.T., T.S., R.Y., W.Z., C.K., K.I., N.A., M.H., and S.T.; visualization, T.S. and M.H.; supervision, C.K., N.A., M.H., and S.T.; project administration, N.A., M.H., and S.T.; funding acquisition, M.H. and S.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work has been partially supported by the Research Foundation for the Electrotechnology of Chubu, the NIFS Collaboration Research Program (NIFS17KLEA034), the Radiation Effects Association, the JSPS KAKENHI (Grant Number 16H02667, 16K15358, and 18K10023), and the Hirosaki University Institutional Research Grant.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

