Environ Eng Res > Volume 28(1); 2023 > Article
Thuamthansanga, Sahoo, and Tiwari: Estimation of 238U and 232Th in soil and water of prominent fault region of Mizoram

### Abstract

The study investigates radon and thoron concentrations in soil and water at one of the most prominent faults in Mizoram state, India. The obtained isotope pair data were consequently used for estimating the uranium and thorium content of the region. An indigenously developed and calibrated ZnS(Ag) alpha based scintillation counter (Model: SMARTRnDuo, BARC, India) was deployed for assessing radon and thoron data. Thoron concentration was found to be higher than radon concentration in both soil and water. The isotope pair and their parent nuclei concentration in water were found to be higher than in soil. The uranium and thorium content in soil were estimated to be 17.5 and 22.6 Bqkg−1 respectively, but in water, they were estimated to be 41.6 and 124.8 Bqkg−1 respectively. Comparisons with global averages were also presented in detail and no radiological risk has been observed for the region. A continuous radon data was generated at Mizoram University, Aizawl, Mizoram (India) for cross-analysis with data at Mat fault. It was observed that radon data of the two locations behave similarly during geophysical phenomena, indicating that the region was seismically active. No geophysical properties of thoron were observed.

### 2.1. Geology of the Study Area

Mizoram belongs to the Surma basin which is part of the greater Bengal Basin. The basin is an area of folded sediment, wider to the north and narrower to the south with many NE-SW and NW-SE trending lineaments/faults. The NE-SW Syllhet fault running from near Dhaka (Bangladesh) demarcates the northwestern boundary of the Surma basin while the Gumti fault cut across the basin (Fig. 1(b)). Among the NW-SE trending faults, Mat fault and Tuipui fault lies within Mizoram state at the southern part of the basin (Fig. 1(b)). Mat fault is one of the most prominent faults in Mizoram which obliquely cut across the Indo-Burmese Arc (NS-trend) and is traceable across the entire state from satellite and Google maps [5, 54]. According to the seismic hazard zonation map of India, northeast India lies at zone V, the highest seismic activity and is one of the six most seismically active regions of the world along with Taiwan, Japan, Mexico, California and Turkey [55]. The northeast region based on the distribution of its epicentres, fault plane solutions and geotectonic features was divided into five seismotectonic zones (Fig. 1(b)) [55]. They are as follows, (i) Eastern Himalayan collision zone (zone A) (ii) Indo-Myanmar subduction zone (zone B), (iii) Syntaxis zone of Himalayan arc and Burmese arc (Mishmi Hills, zone C) (iv) Plate boundary zone of the Shillong plateau and Assam valley (zone D) and (v) Bengal basin and plate boundary zone of Tripura-Mizoram fold belt (zone E). In the present study, Mat fault; being the most prominent fault within the state was selected for generating in-situ data (Fig. 1(b) and Fig. 1(c)).

### 2.2. Procedure for Measurement

An indigenously developed and calibrated ZnS(Ag) alpha based scintillation counter named SMARTRnDuo (Model: SMARTRnDuo, BARC, Mumbai, India) was deployed for measuring radon and thoron data [16, 17]. The monitor is operable in three modes; radon, thoron and alpha modes. Only radon and thoron modes were applied in the present study. In thoron mode, the monitor retrieves counts of radon, thoron and additive concentration of the isotope pair. But in radon mode, the monitor gives counts and concentration of radon only as thoron is prevented from entering the scintillation cell. The obtained isotope pair data counts were converted into their respective concentrations using Eq. (1) [46].
##### (1)
$CRn=C3EVe-λt$
where C is the net count rate (count per second, s−1) of 222Rn or 220Rn, E is the efficiency of counting, V is volume of the sampler (m3), λ is the decay constant of 222Rn or 220Rn, t is the time delay post-sampling (s) and 3 represent the three alphas in the respective decay chain of 222Rn and 220Rn.
The site was selected in such a way that the data might be suitable for approximating the isotope pair concentrations of the region and as well for revealing their geophysical properties. For that, 9 location spots were selected across and along Mat fault from where radon and thoron data were generated (Fig. 1(c)). To obtain radon flux, the instrument was operated in radon mode using an accumulator chamber (2.1 × 10−4 m3) with 15 min cycle for 3 h in each of the spots (Fig. 2(a)). The accumulated radon concentration of each spot was least fitted and then averaged out to get the radon concentration built-up rate C(t). Then it was subsequently substituted in Eq. (2) to get radon flux of the region [46].
##### (2)
$C(t)=C0+kAVft$
where C0 is the initial concentration (Bqm−3), k is the factor by which the initial flux drops while the gas inside the accumulator passes through state of uniform mixing prior to deployment to the state of diffusive mixing post to deployment, A is the surface area of the opening of the accumulator (m2), V is the effective volume of the sampling device (m3), f is the flux of radon (Bqm−2s−1) and t is the measurement time (s).
To obtain thoron flux, first, we need the thoron equilibrium concentration. Using the thoron equilibrium concentration, thoron flux of the soil-air interface will be estimated. For that, the accumulator chamber was placed in one of the sampling spots and connected to the monitor. Now the monitor was operated in thoron mode with 15 min cycle for 1 h at each spot (Fig. 2(b)). Average of the last three readings from all the spots were taken as the thoron equilibrium concentration of the region. The first reading of each spot has been neglected to avoid corruption in the data due to external sources. The equilibrium concentration was then substituted in Eq. (3) to estimate thoron flux of the region [46].
##### (3)
$f=CeqVλA$
where Ceq is the 220Rn equilibrium concentration (Bqm−3), V is the effective volume of the sampling device (m3), λ is the thoron decay constant and A is the surface opening area of the accumulator (m2).
To estimate the 238U concentration, first, the radon production rate (Jm) was obtained from the soil sample. For that soil samples from the 9 selected spots (Fig. 1(c)) were collected with a frequency of once a month, between May, 2018 and October, 2018. The soil samples were put in a metal cylinder (5.0 × 10−4 m3) called the mass exhalation chamber. The detector probe of the monitor was then mounted on it using the provided slide tight mechanism which prevents it from leakage (Fig. 2(c)). Now, the build-up radon concentration was monitored with 60 min cycle for 24 h and was least fitted to obtain the radon build-up rate C(t). The build-up rate was then subsequently substituted into Eq. (4) to obtain the radon mass exhalation rate of the region. The mass exhalation rate was further substituted into Eq. (5) for retrieving 226Ra content of the soil samples [46].
##### (4)
$C(t)=(JmMV)t+C∘$
where Jm is the radon mass exhalation rate (Bqkg−1s−1), M is mass of the soil sample (kg), V is the volume of the mass exhalation chamber (m3) and C0 is the radon concentration at t = 0.
##### (5)
$Jm=REλ$
where R is 226Rn content of the soil in Bqkg−1, E is the emanation coefficient of 222Rn (0.1–0.3 in soil) [46], λ is the radioactive decay constant of 222Rn.
On the other hand, the 232Th content of the soil was estimated using equation (6) after substituting the thoron flux from Eq. (3) [46].
##### (6)
$f=λLRρE$
where f is the 220Rn flux at the soil-air interface, λ is the 220Rn decay constant, L is the diffusion length of 220Rn in soil (0.013 m) [46], R is the 224Ra content in the soil, ρ is the density of the soil matrix and E is the emanation coefficient of 220Rn (0.14) [46].
For assessing radon and thoron data from water, all water sources near the vicinity of the above 9 spots were located. A total of 5 such water spots were selected for a sampling spot. The water samples were collected in a glass bottle (2.2 × 10−4 m3) attached with a bubbler. For this, the glass bottle containing the water sample was connected to the monitor in place of the accumulator as shown in Fig. 2(a). The pump was then turned ON for 3 min such that it may cause bubbling in the sample water through the bubbler. These actions will push most of the radon gas dissolves in the water into the scintillation cell through the connecting tube. After that, the monitor was run in radon mode for 1 h with 15 min cycle. The first reading was discarded to avoid corruption in the data. Averages of the last three readings were taken as radon concentrations of the sample water. The exact same procedure was followed for retrieving thoron concentration in water except that the monitor was operated in thoron mode.
The obtained radon and thoron concentrations were then substituted in Eq. (7) for retrieving 238U and 232Th content of the water, respectively [46].
##### (7)
$E=VCMR$
where E is the radon or thoron emanation coefficient, V is the effective volume of the sampling device (m3), C is the radon or thoron concentrations (Bqm−3), M is the total mass of the sample (kg) and R is the 226Ra or 224Ra content of the sample water (Bqkg−1). Since, the 226Ra and 224Ra are in equilibrium concentrations with their parent nuclei, they may be used for depicting 238U and 232Th concentrations of the region, respectively.
To study radon and thoron anomalies due to geophysical phenomena, data of the isotope pair were continuously generated with 15 min cycle between January, 2017 and March, 2017. An accumulator chamber of 5.0 × 10−4 m3 and a soil probe of length 5 cm were connected to the SMARTRnDuo in a closed-loop manner to sample the sub-soil sample gas (Fig. 2(a)). For sampling with a soil probe, the accumulator chamber shown in Fig. 2(a) was replaced with a soil probe of length 5 cm. The sample gas was sampled for 47 days and 33 days using the soil probe and an accumulator chamber, respectively during the said period. The surface radon and thoron gases were drawn into the scintillation cell by the inbuilt pump at the rate of 5–7 L/min for 5 min for each 15 min cycle. During the 5 min sampling, counting of the alpha particles was simultaneously carried out by the monitor. The sample gas was drawn in through a progeny filter preventing progeny of both the gases and trace gases (Ch4, Co2 etc.) from entering into the scintillation cell. Hence the 5 min simultaneous sampling and counting attributes to the sum of radon and thoron concentration of the sample gas. The following 5 min was delayed such that the short-lived (55.6 s) thoron may decay out. After that, counting of alpha particles was resumed for another 5 min, which attributes to the radon concentrations and some marginal long-lived alpha particles. Subsequently, the thoron count was obtained by subtracting the last 5 min counts from the first 5 min counts. In this manner, the radon and thoron data were continuously monitored for 24 h [16, 17].

### 3.1. 238U, 232Th, 222Rn and 220Rn Content of the Region

The radon and thoron concentrations of the soil were found to be 5,649.4 and 11,858.2 Bqm−3, respectively while their concentrations in water were observed to be 7,557.0 and 12,091.2 Bqm−3, respectively (Table 1). At the soil-air interface, radon and thoron fluxes were found to be 0.016 and 1.25 Bqm−2s−1, respectively with radon mass exhalation rate of 7.36 × 10−6 Bqkg−1h−1 (Table 1). The 238U and 232Th content of the soil were estimated to be 17.5 and 22.6 Bqkg−1, respectively (Table 1). On the other hand, the 238U and 232Th content in water were estimated to be 41.6 and 124.8 Bqkg−1, respectively (Table 1). When compared to that of the worldwide average given by UNSCEAR and IAEA [4346] the obtained isotope pair concentrations in soil and water, fall within the range (103–105 Bqm−3in soil) given by IAEA [46]. Also, the radon and thoron concentrations in water were respectively higher than their concentrations in soil (Table 1). But the thoron concentration in both the media was higher than its isotope pair (Table 1). The isotope pair fluxes were also in close agreement with the worldwide average (15–20 mBqm−2s−1 for radon and 1–1.9 Bqm−2s−1 for thoron) given by UNSCEAR [44]. The 238U and 232Th content of the soil was lower than that of the worldwide average (35 and 30 Bqkg−1 for 238U and 232Th, respectively) given by UNSCEAR [43]. But their concentrations in water were higher than the reported average [43]. The higher 232Th content reflects the observed higher concentrations and flux of its daughter nuclei to its isotope pair in both the media. Again when compared to that of the critical value set by IAEA [45] (1,000 Bqkg−1) no radiological hazards from 238U and 232Th were observed in the region. The obtained result was in close agreement with the previous studies [4042] of the region and some recent reports from southern India [13, 35] where no radiological risk due to radon and thoron were observed. It also confirmed that radon and thoron data of fault and non-fault areas were in close agreement in the present study region.

### 3.2. Correlation of Radon with Geophysical Phenomena

A total of 7,160 data were recorded for each isotope between January, 2017 and March, 2017. Thoron data was neglected for geophysical studies as the data remain constant throughout the measuring period and have no geophysical meaning (Fig. 4(a) and (b)). Exclusively average of all the diurnal and nocturnal radon data was taken after removing the peak period data. This average value was taken as the base count (CB) of radon for the study period (Fig. 4(c) and (d)). And it was assumed as the count of radon data in the absence of any geophysical or meteorological perturbation. In the real-time data curve, the CB line was represented by a horizontal line passing through zero (Fig. 4(c) and (d)). The rise of any radon counts was a measure from this value (CB).
Now the radon anomaly peaks were identified using the mean plus ‘n’ times standard deviation (SD) method (where n = 1, 2, 3,...). The radon variation was considered as an anomaly peak when it crosses +2.6SD or the anomaly line (AL) (Fig. 4(c) and (d)). This turned out to be 1.1 times and 3 times the CB (xmean) for radon data at 5 cm depth and soil-air interface respectively (Fig. 4(c) and (d). Using the above method 21 radon anomaly peaks were observed during the period. Seismic data of the study period were assessed from USGS archive (United State Geological Survey, https://eartquake.usgs.gov/earthquakes/map/) and were selected using Dobrovolsky and Fleihcher criteria [55] given by Eq. (8) and (9), respectively.
##### (8)
$D=100.43MkmD=100.813M16.6km for M≤3$
##### (9)
$D=100.48M1.66km for M≥3$
Using Eq. (8) and (9) 46 earthquakes were selected and all of them were located within 1,000 km radius from the monitoring station. Details of the selected earthquakes were given in Table 3 and inserted as vertical lines in Fig. 4. Upon analysis, no post-cursory radon peak was observed but only a precursory one. The recorded earthquakes have an occurrence time range of 0:39:02 min – 8 days after radon anomalies. Out of the 46 earthquakes 23 of them occur within 1 day from the anomaly peaks; 8 within 2 days; 3 within 3 days; 5 within 4 days; 2 within 5 days; 1 within 6 days; 2 within 7 and 8 days from the peaks (Table 3). In general, most earthquakes occur close to the anomaly peaks with an average of 2.4±2.3 days after the peaks. In other words, it may be stated as 50% of the earthquakes occur within 2 days after the radon anomaly peaks; 39% of them between 3–5 days after the anomaly peaks and 11% of them after 5 days from the anomaly peaks. Thoron data, on the other hand, shows no anomaly peaks. Hence correlation of thoron with seismic activity was neglected in the study.
The observation supports and agrees well with the experimentally demonstrated analytical model of Sahoo and Gaware [60] at the sub-soil. Their model suggested that due to relatively low radon concentration at the sub-soil, perturbation in its concentration due to external source was more pronounced compared to that of the deep soil where it attains asymptotic value. The study affirmed the significance of monitoring sub-soil radon anomaly as premonitory gas to seismic activity within 1,000 km radius from the monitoring site. Correlations of air and soil radon data with other parameters like tidal loading, atmospheric electromagnetic radiation, meteorological etc for seismic precursor were recently proposed by some others [38, 39, 4753, 59]. The study agrees well in observing radon anomaly especially to those authors [38, 39, 52, 59] who also observed radon anomaly at 2σ above the mean concentrations.

### 4. Conclusions

The study shows that radon and thoron concentrations and fluxes of the region were in agreement with the worldwide averages reported by IAEA and UNSCEAR [4346]. The thoron concentration in soil and water were found to be higher than radon. The 238U and 232Th contents of the soil were observed to be lower than the worldwide average [43]. However, their concentrations in water were observed to be higher than the worldwide average [43] but far below the critical value given by IAEA [43] respectively. Hence, no radiological risk due to the isotope pair and their parent nuclei was observed for the region. The study also highlights the advantages of monitoring sub-soil radon data as a premonitory gas to nearby earthquakes. The seismic activities were found to succeed the radon anomaly peaks within 2.4±2.3 days on average and with a range of 00:39:02 min–8 days. It has been observed that 50% of the earthquakes occurred within 2 days from the anomaly peaks, 39% between 3–5 days from the peaks and 11% after 5 days from the anomaly peaks. The inter-correlation analysis also shows that data generated at the fault region (Mizoram University) could also be used for seismic study, avoiding the hardship in assessing the fault line. Despite the short study period, the observation clearly reveals that the region was seismically active and suitable for monitoring radon data as a forecasting gas. Although this region has been declared as the second-highest seismically active region of the world, Mizoram in particular has no continuous online data in the past. Hence, the author hopes that the present study will serve as a significant baseline data for future reference. We also hope that the obtained result might conveniently be replicated to other similar tectonic zones around the globe.

### Acknowledgment

This work was supported financially by DAE-BRNS, BARC, Mumbai, India [Sanction Order No.:36(4)/14/66/2014-BRNS/36024 Dt.26.02.2016. This article was presented at 2nd Annual Convention of North East (India) Academy of Science & Technology (NEAST) held on 16–18 November 2020 in Mizoram, India.

### Notes

Conflict of Interest

The authors declare that they have no conflict of interest.

Authors Contributions

T.T.T.S (PhD student) carried out the fieldwork, performed data analysis, wrote and revised the manuscript. B.K.S (Scientist) provided research ideas and designs the experiment. R.C.T (Professor) guide the research work and acquired the research grant.

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##### Fig. 1
(a) Schematic map of India (b) Northeast India and its geotectonic setup (c) formation of 9 locations grid at Mat fault (edited after Jaishi et al. [56]).
##### Fig. 2
Set up of SMARTRnDuo for sampling radon data (a) at the continuous monitoring station (b) at Mat fault and (c) at the Lab for mass exhalation.
##### Fig. 3
Analysis plot between radon isotopes (radon and thoron) and Meteorological data recorded at Mizoram University, Aizawl (India), showing the extent of meteorological influence on the isotope pair data by displaying the correlation strength r (Pearson’s correlation coefficient value) between the isotope pair and each meteorological parameters. Plot of radon data versus (a) temperature (b) pressure (c) rainfall (d) humidity and (e) wind speed. Plot of thoron data versus (f) temperature (g) pressure (h) rainfall (i) humidity and (j) wind speed.
##### Fig. 4
Sampling time versus 15 min cycle (a) thoron data at 5 cm depth, (b) thoron data at soil-air interface, (c) radon data at 5 cm depth and (d) radon data at soil-air interface, along with earthquakes data during the measurement period within 1,000 Km radius from the monitoring station (represented by vertical lines).
##### Fig. 5
Plot of sampling dates versus (a) continuous radon data and (b) continuous thoron data between May, 2018 and October, 2017 at Mizoram University; (c) Average radon count in soil at Mat fault during anomaly period and non-anomaly period and (d) Average radon count in water at Mat fault during anomaly period and non-anomaly period.
##### Table 1
Detail Estimated Value of 238U and 232Th Concentrations and Fluxes of Their Daughter Nuclei in Soil and Water at Mat Fault
Soil Water ratio

222Rn 220Rn ratio 222Rn 220Rn ratio 222RnW/222RnS 220RnW/220RnS
Bqm−3 5,649.4 11,858.2 2.1 7,557.0 12,091.2 1.6 1.3 1.02
Bqm−2s−1 0.016 1.25 79.3

Soil Water ratio

238U 232Th ratio 238U 232Th ratio 238UW/238US 232ThW/232ThS
Bqkg−1 17.5 22.6 1.3 41.6 124.8 3 2.4 5.5
##### Table 2
Details of Correlation between Meteorological Parameters and Radon and Thoron Data Recorded between January, 2017 and March, 2017
Temperature (°C) Pressure (Pa) Rainfall (mm) Humidity (%) Wind speed (Kmh−1) 222Rn 220Rn
Temperature (°C) r 0.12 −0.22 −0.26 0.08 −0.18 0.11
Sig. 0.30 0.05 0.02 0.46 0.11 0.33

Pressure (Pa) r −0.23 −0.34 −0.44 0.05 0.49
Sig. 0.04 0.002 5.9 × 10−5 0.66 4 × 10−6

Rainfall (mm) r 0.37 0.27 0.16 −0.35
Sig. 0.0007 0.016 0.15 0.0014

Humidity (%) r 0.30 −0.04 −0.33
Sig. 0.008 0.73 0.003

Wind speed (Kmh−1) r −0.03 −0.15
Sig. 0.8 0.17

222Rn r −0.03
Sig. 0.8
##### Table 3
Details of the Selected Earthquakes within 1,000 km Radius Using Debrovolsky and Fleischer Criteria [55] Represented by Vertical lines in Fig. 4
Dates of Radon Peak Dates of Earthquakes Lat, Long Depth (km) Magnitude Distance (km)
10-01-2017 07:47:00 11-01-2017 18:51:14 28.3, 94.1 10 3.3 527
12-01-2017 15:02:07 26.5, 95.4 75 4.7 413

15-01-2017 04:48:00 17-01-2017 20:52:16 27.6, 88.6 10 3.6 592
16-01-2017 08:03:23 18-01-2017 07:16:10 23.9, 93 27 3.7 39
17-01-2017 06:47:43 18-01-2017 08:33:17 24.5, 94.8 22 4.2 233

19-01-2017 08:47:23 19-01-2017 15:29:42 28.9, 88.2 10 4.1 729
19-01-2017 20:48:36 28.1, 92.6 40.21 4.1 494

20-01-2017 07:31:41 21-01-2017 03:19:18 19.8, 94.7 71.98 4.3 487

22-01-2017 08:19:33 23-01-2017 15:03:05 30.8, 78.2 10 3.5 1629
24-01-2017 17:55:38 25.6, 91.7 15 3.4 229
24-01-2017 23:44:29 25.5, 94.6 50 3.1 277
29-01-2017 14:39:04 24.8, 92.8 10 3.2 119
29-01-2017 03:06:08 25.9, 96.4 41.14 4.5 457
31-01-2017 10:58:42 26.4, 93.5 22 3.2 308
31-01-2017 11:46:04 31.5, 94.1 32.7 4.5 885

04-02-2017 06:18:26 05-02-2017 18:24:59 27.9, 93.8 10 3.8 477
08-02-2017 13:44:28 26.9, 92.9 15 3.6 353
08-02-2017 02:16:16 22.5, 94.7 121 4.5 253

10-02-2017 10:04:12 11-02-2017 23:42:51 23.9, 91.8 10 3.5 90

12-02-2017 08:56:11 12-02-2017 09:35:30 25.6, 90.8 10 4.5 280

16-02-2017 08:11:00 16-02-2017 20:43:10 26.2, 92.8 20 3.6 274
19-02-2017 00:23:42 26.6, 93 30 3.4 320

19-02-2017 04:19:45
23-02-2017 18:35:17
24-02-2017 01:46:07 23.7, 94.5 82 3.4 187
24-02-2017 03:09:16 27.3, 88.1 150 3.5 606
24-02-2017 17:32:49 24.1, 93.4 20 5.2 85
25-02-2017 02:30:44 28.7, 96 36 3.2 644
25-02-2017 05:30:44 28.7, 96 10 3.5 644
25-02-2017 12:32:19 24.1, 92.1 33 4 70
27-02-2017 09:07:47 27.3, 85.9 10 5 786
27-02-2017 09:51:45 27.3, 85.9 10 4.7 786

03-03-2017 00:42:58 04-03-2017 05:08:13 24.3, 94.2 70 3.5 168
04-03-2017 07:41:52 25.2, 94.6 70 5 255
04-03-2017 12:20:44 25.5, 90.9 10 3.3 265
06-03-2017 03:00:06 25.1, 95.1 89.95 4.3 294
07-03-2017 15:29:16 26.8, 90.5 30 4.1 405
07-03-2017 15:58:56 26.9, 89.1 10 4 503
09-03-2017 08:25:15 25, 94.2 36 4.1 210

13-03-2017 00:14:45 13-03-2017 19:49:06 17.3, 95.9 10 5.1 785
14-03-2017 22:09:56 27.2, 96.7 54.22 4.4 573

17-03-2017 05:24:42
18-03-2017 00:56:28
21-03-2017 07:03:31
21-03-2017 21:10:44 24.9, 92.1 37 3.9 142

22-03-2017 04:28:14
24-03-2017 02:35:17
25-03-2017 03:31:45
25-03-2017 07:35:55 25, 95.1 82 5 284
26-03-2017 05:10:34 25.8, 99.9 33.07 4.6 769
26-03-2017 05:25:06 25.9, 99.8 27.61 5 764
27-03-2017 03:12:09 27.3 88.6 10 4.6 569
27-03-2017 06:40:25 25.9, 100 10 4.1 779
28-03-2017 15:48:49 26.5, 93.5 10 3 319
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