Radon emission from soil gases in the active fault zones in the Capital of China and its environmental effects

The release of radon in active fault zones is a sustained radioactive pollution source of the atmospheric environment. The species, concentration and flux of radon emitted in soil gas in active fault zones in the Capital of China were investigated by in-situ field measurements. Two main species of radon discharging from soil gas in active fault zones were identified, including radon diffused and dispersed from permeable soil, and upwelling from faults. Higher concentrations and flux of radon from faults were observed in the Bohai Bay Basin due to the accumulated uranium in the sandstone reservoirs and higher permeability of the strata and bed rocks. Increased radon released by strong earthquakes persists, with the max flux of 334.56 mBq m−2 s−1 observed in FN (Fengnan district) located at the epicenter of the 28 July, 1976 Tangshan MS 7.8 earthquake. The level of radon released in 8 of 22 locations within the Basin and Range Province (to the west of Taihangshan piedmont fault Zone) reached level 2, and 13 of 14 locations within the Bohai Bay Basin reached levels 2–4, according to the Chinese Code (GB 50325–2001, 2006). Corresponding protective and safety measures should be in place to protect the health of nearby residents, due to their exposure to radon emitted from the faults. Also, the concentration of radon in active fault zones should be investigated to assess the possible risk, before land-use is planned.


Results
The concentrations and fluxes of radon in the soil gas from the active fault zones in the seismic areas near Beijing, China are listed in Tables 1 and 2. Descriptive statistics (i.e. min and max values, mean, median, standard deviation (SD), interquartile range (IQR), lower interquartile (LQ) and upper interquartile (UQ)), number of repeated measurements and concentration values of radon are listed in Table 1. The minimum and maximum concentrations of radon in the soil gas ranged from 0.20 kBq m −3 to 5.43 kBq m −3 and 9.07 kBq m −3 to 97.55 kBq m −3 , respectively. The mean concentrations of radon were 4.01 to 31.41 kBqm −3 , comparable to that of the median concentrations with a range from 3.83 kBqm −3 to 30.78 kBq m −3 . The LQ and UQ for the concentration of radon varied from 4.19 kBq m −3 to 34.01 kBq m −3 and 3.83 kBq m −3 to 28.15 kBq m −3 , respectively, and the IQR ranged from 0.36 to 9.32 kBq m −3 . The SD ranged from 1.45 to 22.13 kBq m −3 , and the ρ-values ranged from 0.00 to 0.73.
The flux of radon (i.e. min and max values, mean, median, the quantity of data and times for repeated measurement) are listed in Table 2

Discussion
Radon, a by-product of the natural radioactive decay of 238 U, occurs widely in soil and rock 32 . It can escape upwards to the shallow crust by diffusing and dispersing in permeable soils, or by migration upward along preferential pathways, such as fractures and faults 33,34 . Therefore, radon observed along the gas profiles across active faults is primarily from two origins: (1) the radon diffuses and disperses in the soil, generated by the decay of 238 U accumulated in the soil (Fig. 2a), (2) the radon migrates upward through the faults and fractures transferred by carrier gases (CO 2 , N 2 , etc.) (Fig. 2b), which can be produced by the decay of 238 U from the deeper crust and in the CaSiO 3 perovskite phase in the mantle 26,35 . Due to the diffusion coefficient in dry soil (5 × 10 −6 m 2 s −1 ) and half-life (3.82 days) of radon 36 , the detectable distance for radon diffusing and dispersing in soil is within several metres 37 . The radon concentration in superficial soil is usually low, thousands of Bq m −3 , and is subject to dilution by air with extremely low radon concentration, several Bq m −3 32 . Faults are the preferred and fastest pathway for the uprising of gases from deep within earth 13,38 , enabling the escape of gaseous radon generated from the decay of 238 U in rocks deep within the crust to the soil surface 39,40 . This is very efficient in the presence of carrier gases (CO 2 , N 2 , etc.), and results in high radon concentration in active fault zones 41,42 . High radon concentrations in the range of 20 to 80 kBq m −3 are reported in active fault zones worldwide 21,23,35 . Therefore, the upward migrating radon through faults is usually at a higher concentration than radon diffusing and dispersing in the soil surface. A radon origin distinction analysis was carried out using the Kurtosis-Skewness test and Q-Q (quantile-quantile) plots. All the ρ-values of the concentration of radon measured between 2012 and 2016 were < 0.05, with the exception of 6 locations (ZZK, LTTC, BYC, CJA, NYC and WFS) ( Table 1), indicating that the radon concentrations were mostly non-normal distribution. The Q-Q plots of radon concentration from 36 locations show single linear distribution patterns (Fig. 3a, ZZK, LTTC, BYC, CJA, NYC and WFS), 3 linear segments ( Fig. 3c, ZJY, YXZ, DYZ, NKC, XHZ, YHM and FN) and 2 linear segments (Fig. 3b).
Based on the analysis, the radon gases observed in the 6 locations (ZZK, LTTC, BYC, CJA, NYC and WFS) were primarily from the first type (Fig. 2a). The gaseous radon observed in the other 7 locations (ZJY, YXZ, DYZ, NKC, XHZ, YHM and FN) should be a mix of the two species; the gas radon with concentration between T 1 and T 2 could be supplied by the first population, the origin of radon with concentration over T 2 was dominated primarily by the second population. Those below T 1 , with much smaller concentrations (ranging from 0 to 3.19 kBq m −3 ) (Table 1), could be subjected to dilution by air through the conglomerate clay covering the 7 locations (Fig. 2c). The radon observed in the remaining 23 locations should be a mix of the two species too. The origin of radon with concentration over T 2 was dominated by the second type (Fig. 2b), and a concentration under T 2 was supplied by the first population.
Fault-origin radon was observed in 30 locations (as described above), with a mean value of fault-origin radon concentrations (MOF) in the range of 7.65 kBq m −3 to 64.14 kBq m −3 (Table 1). Great spatial variations of MOF were observed across the study area (Fig. 4). Higher values of MOF were observed to the east of the Taihangshan piedmont fault zone in the Bohai Basin, in the range of 23.29 to 64.14 kBq m −3 , which were significantly higher than those to the west of Taihangshan piedmont fault zone in the Basin and Range Province (7.65 to 32.11 kBq m −3 ) ( Table 1).      Table 2). In addition, the MF had a positive correlation with MOF (Y = 0.42x + 5.21, R = 0.66) (Fig. 6), suggesting that radon emitted from the locations where radon flux measurements had been performed could originate from radon migrating upwards through the faults and fractures.
As a product of the natural radioactive decay of 238 U 22 , the radon concentration in soil gas should correlate to the amount of underground 238 U. However, sufficient 238 U source in the Basin and Range Province areas around the basin and the wide gentle slopes in the boundary belt between the Basin and Range Province and basin could be favorable in accelerating the transfer of 238 U from the Basin and Range Province to the basin where is accumulates 43 . In the study area, abundant intermediate-acid intrusive rocks containing uranium-bearing minerals were widely distributed around the basin (Fig. 1), with rugged hypsographic features to the west of the study area. When subjected to weathering, uranium-bearing rock fragments and dissolved uranium (U 6+ ) could be transported from the boundary belt to the basin via syn-sedimentary groundwater, and accumulate in sandstone reservoirs (Fig. 7). Sandstone enriched with uranium has been reported in the Bohai Bay Basin 44 . Therefore, the radon generated from the decay of 238 U accumulates in the sandstone reservoirs of the Bohai Bay Basin, escaping to the surface by upward migrating through faults, resulting in higher MOF and MF in the Bohai Bay Basin (Figs 4 and 5).
In addition, the high permeability of the fault zones could be another important factor contributing to the high concentrations and fluxes of gas emitted from active faults 26,45,46 . The permeability of the strata in the Bohai Bay basin could be higher than those in the Basin and Range Province, inferred by a higher Poisson's ratio of the strata in the Bohai Bay Basin compared to that in the Basin and Range Province zone to the west of the   1 and 2, Fig. 8).
Previous studies report that strong earthquakes can enhance the radon degassing from deep in the earth through faults 26,48,49 . However, the water-rock interaction and transportation by groundwater, can result in clay minerals accumulating and clogging the fractures in the faults 38,50 , inhibiting the release of gas from the faults. Historically, 109 earthquakes with M S ≥ 5.0 have occurred in the study area, and the Tangshan M S 7.8 earthquake (28 July, 1976) was the strongest earthquake in the study area since 1680, which was followed by 4 aftershocks with M S ≥ 6.0 and the epicenter located at the FN site along the TSF fault (https://earthquake.usgs.gov/earthquakes/search/). The highest MOF/MB value (3.90) was observed at the same FN site near the epicenter of the 28 July, 1976 Tangshan M S 7.8 earthquake. Therefore, the radon degassing from the faults could have been enhanced by strong earthquakes over decades; 42 years in the case of the Tangshan M S 7.8 earthquake.
As a natural and radioactive gas, the sudden and catastrophic, or quiet and continuous release of radon into the near-surface environment can result in health risks to the inhabitants living in adjacent areas 3,6,51 .
Higher soil radon concentrations and fluxes have been widely observed in fault zones 8,11,14 , due to (1) fault displacement during the late Quaternary Era, and (2) recent earthquakes in nearby faults 15,51 . In this study, non-negligible radon exhalations from active fault zones in the seismic zones near Beijing were observed   (Table 3, Fig. 9). Both the high radon concentrations and fluxes indicate that attention should be given to the environmental effects of radon emission from active fault zones in the seismic zones near Beijing, China. The Chinese code for indoor environmental pollution control of civil building engineering (GB 50325-2001(GB 50325- , 2006) was used to divide the study area into 4 zones, including one "A", two "B" and one "C" (Fig. 10). The "C" zone was the most highly radon polluted area from the faults in the study area, which included all the locations along the faults in the Bohai Bay Basin.  Tables 1, 2 and 4 show the radon protective measures that should be required to protect the inhabitants from radon risk in buildings located along the faults in block "C" due to the levels of radon emitted from the faults. The two blocks "B" were meso-polluted areas caused by radon exhalation from the faults in the study area, which covered the locations distributed to the northwest and northeast of the Basin and Range Province, west of the Taihangshan piedmont fault zone (Fig. 10). The level of radon gaseous releases in 8 (CFY, WQX, QBK, HJP, YHM, SJC, SHZ and DTHS) out of 12 locations were level 2, although 4 locations (XHZ, DYZ, LTTC and BYC) in the northeast zone "B" were level 1 (Table 4), uniform radon protective measures should be   Table 3. List of concentration and fluxes of soil radon gas from different sources globally. "-": no data.  Table 3. recommended considering the close proximity to the 5 sites (CFY, WQX, QBK, HJP and YHM). Therefore, it is suggested that the underlying surface of the structure should be fixed in order to prevent cracking along the faults located in zone "B". The zone "A" is a pollution-free area, which includes 10 locations (NCY, DHZK, YJY, YLK, ZZK, YJG, ZJY, NKC, YXZ and BKC) in the Basin and Range Province to the west of the Taihangshan piedmont fault zone (Fig. 10), where the levels of radon concentration and fluxes in all these locations were level 1, indicating that no pollution prevention need to be carried out up to now.

Conclusions
Radon in soil gas from two main sources in the active fault area in the capital of China was identified: (1) radon diffusing and dispersing from the permeable soil, and (2) radon upwelling from faults. Atmospheric dilution occurred in locations ZJY, YXZ, DYZ, NKC, XHZ, YHM and FN, as a result of air circulating through the conglomerate clay. Spatial variations in radon concentration and fluxes across the study area were observed, including higher radon concentration and fluxes in the Bohai Bay Basin compared to those in the Basin and Range Province to the west of the Taihangshan piedmont fault zone. This phenomenon could be a result of the uranium accumulation in the sandstone reservoirs and the higher permeability of strata in the Bohai Bay Basin area.
The radon degassing from faults was enhanced by strong earthquakes, with the concentration of radon from the faults being from 1.  Measuring apparatus and procedure. The mechanism of measurement using RAD 7 and RTM 2200 radon detectors was based on an energy spectrum analysis. 222  With each transformation, the nucleus emits radiation (alpha and beta particles, or gamma rays) with a characteristic energy. 218 Po has a half-life of 3.05 min, and decays by the emission of an alpha particle of 6.00 MeV. Due to its short half-life, a radioactive equilibrium can be achieved in 15 min, which reduces the background and improves the sensitivity of the apparatus. Thus, RAD 7 and RTM 2200 are designed to detect radon concentrations based on energy spectrum analysis using a solid-state detector as it decays to 218 Po. The energy is transformed to an electrical signal, amplified and converted to digital information using electronic circuits. The radon concentration can be calculated from the accumulative decay information. Radon concentration was performed by inserting a stainless-steel sampling tube with a diameter of 3 cm into the ground to a depth of 80 cm (Fig. 11). The sampler was connected to the radon detector using a rubber tube. The radon concentration was measured in the field using a SARAD RTM 2200 radon detector (Fig. 11a). Radon values were obtained 15 min after measuring (time necessary to reach Po and Radon nuclei equilibrium, approximately 5 times the half-life of 218 Po). An inlet filter and molecular sieve were used to protect the detector from dust and soil moisture (>10%). The detection limit and measurement error of the SARAD RTM 2200 were 500 Bq m −3 and ±5%, respectively. The soil gas radon flux was measured using a static closed chamber method. The instrument contained an inverted circular accumulation hemispherical chamber, with a volume of 1.68 × 10 −2 m 3 and radius of 0.2 m, and a RAD7 radon monitor (with detection limit of 14.8 Bq m −3 and accuracy of ± 4%) 52 (Fig. 11b). The gas circulated from the chamber to the monitor and then back into the chamber via a small-diameter plastic tube (3 mm inner diameter). In order to ensure the immediate and homogeneous mixture of the gas in the chamber, an 8-channel deconcentrator was installed onto the inner wall of the chamber to re-inject the circulating gas. The variation of radon concentration inside the chamber during flux measurements was recorded every 5 min. Statistical analysis. Kurtosis-Skewness test and Q-Q plots are usually used together to determine the species in the soil gas 35,46,53 . Statistical analyses for the collected data were subjected to the Kurtosis-Skewness test and Q-Q (normal quantile-quantile) plots. The data set with a normal distribution is usually from a single origin. Kurtosis-Skewness test was used to test the normal distribution for a data set; the ρ-values of a data set > 0.05 indicated that the data were of normal distributed. A Q-Q plots is effective in distinguishing different or overlapping species 18 . It revealed approximating linear segments (identifying gaps or inflection points) of a probability curve, a single linear distribution in the Q-Q plots indicating a normal distribution and single type for a data set, the points between different straight line segments indicated an abnormal distribution and different species for a data set, and the threshold values were determined using abscissa levels 34 .

Data Availability
All data included in the manuscript are available upon request by contacting with the corresponding author.