S0329 ANNALS OF GEOPHYSICS, 60, 3, 2017, S0329; doi: 10.4401/ag-7057 A study of radon and thoron concentration in the soil along the active fault of NW Himalayas in India Gulshan Kumar1,4, Punam Kumari1, Arvind Kumar2, Sangeeta Prasher3, Mukesh Kumar1 1 Department of Physics, Lovely Professional University, Phagwara, Punjab, India 2 National Center for Research on Earthquake Engineering, Taipei, Taiwan 3 Department of Physics, Kanya Maha Vidyalaya, Jalandhar, Punjab, India 4 Govt. College, Sarkaghat, Mandi, Himachal Pradesh, India Article history Received May 18, 2016; accepted March 01, 2017. Subject classification: Radon concentration, Lithological locations, Active fault line, Main Boundary Thrust. ABSTRACT The Study has been conducted to analyse the radon and thoron flux in the soil of Mandi district, Himachal Pradesh. The detectors have been rooted at seventy one lithological locations in the north-eastern part of the district. The average values of radon concentration has been observed as 4541 Bq/m3 with maximum of 19970 Bq/m3 at lo- cation no. 3 (N 32°00.46': E 76°51.74') and minimum of 867 Bq/m3 at location no. 57 (N 31°45.65': E 76°51.56') and the thoron varia- tion ranges from 37 to 6970Bq/m3 with an average value of 1778 Bq/ m3. The radon liberation at different positions has been correlated to the presence of the active fault to reveal the contributory aspects for abnormal release of radon in the soils. The spatial distribution of radon and thoron gas along the lines passing through the fault zones have unveiled the variances connected to the local tectonic structures. Radon exhalation rates, radium contents and porosity of soil samples have been calculated and a correlation factor of 0.64 has been detected for the observed concentrations of thoron and the porosity of the soil. 1. Introduction Himalayan mountains are one of the most sei- smically active fault zones in the world. The monito- ring of soil gases like hydrogen, helium, radon- tho- ron, methane, carbon dioxide profiles in these zones may provide useful information before the seismic activities. Anomalous changes in the subsurface soil gas concentrations may be used as tectonic activities precursor according to the dilitancy-diffusion model for earthquake occurrence [Scholz et al. 1973]. Mo- reover the soil gas monitoring has been developed as the effective tool in understanding the gas transpor- tation mechanisms in the seismically active zones and many other fields of geosciences. However, the precursor predictions may not be accurate, but may be helpful to study the under soil activities [Han et al. 2014] and is considered as precursor for various deportation processes, such as to discover fault in- terfaces and uranium- thorium ores [Quattrocchi et al. 2000]. Improved permeability of soils along active faults customarily favours the gas escape, hydraulic conductivity (for ground water and thermal fluids), however the thermal conductivity of the soil decrea- ses with increase in the porosity and permeability of soil because the pore filling fluids have lower value of coefficient of thermal conductivity [Poelchau et al. 1997]. The movement of radon through rocks under the earth largely depends on lithology, compaction, porosity and fractural/tectonic features [Choubey et al. 1997, Gunderson et al. 1998]. The presence of va- rious fault systems and thrusts in any region provides secondary porosity for upward migration of thermal fluids. The thermal gradient may be diluted if fresh water is mixed in up flow of thermal fluid [Sharma 1977, Shanker 1988, Cinti et al. 2009]. Gas anomalies at active faults can be either ‘direct leak anomalies’ where the gas measured corresponds to the deep gas phase or ‘secondary anomalies linked to different mineralogy having only superficial roots like the anomalous distribution of radium [Toutain and Boubron 1999]. The measurement of various soil gases for earthquake monitoring and prediction of active faults zones has been reported by various re- searchers. [Etiope and Lombardi 1995, Igarashi et al. 1995, Ciotoli et al. 1998, Guerra and Lombardi 2001, Al-Tamimi and Abumurad 2001, Chyi et al. 2005, Fu et al. 2005, Singh et al. 2005, Kumar et al. 2009, 2012, 2013a, 2013b, Pereira et al. 2010, Singh et al. 2010, Sac et al. 2011, Yang et al. 2011, Li et al. 2013, Walia et al. 2013, Koike et al. 2014, Han et al. 2014, Jaishi et al. KUMAR ET AL. 2 2014, Jashank, 2014, Georgy et al. 2015, Piersanti et al. 2015]. Measurement of natural radon in soil is very important to determine because it helps in monitoring changes in natural background activity with time as a result of any radioactivity release [Darko et al. 2015]. The soil gas and water radon has been measured using alpha guards in some areas of Punjab and Himachal Pradesh for health risk assessments [Bajwa et al. and Walia et al. 2003]. Chandrasekharam et al. [2005] and Walia et al. [2005] have conducted studies of the Himachal Pra- desh geothermal sub-province mainly on the famous thermal springs of Manikaran and Kasol along the Par- vati with the aim to characterize the geothermal re- sources with respect to their suitability for electric power production. Other study by various authors [Choubey et al. 1997, 2007, Virk and Walia 2000, Walia et al. 2003] focused on radon monitoring in waters and soils for health hazard assessment and earthquake pre- diction research. Some research papers have reported chemical [Gupta 1996] and isotopic data [Giggenbach et al. 1983] of the thermal waters. The present studies dealt with radon-thoron me- asurements in soils, measurement of radon exhalation rates and radium contents of Mandi district, Himachal Pradesh, NW Himalaya, India using the passive detec- tors LR -115 type 2 films and measurement of porosi- ty of soil samples from sampling sites. The technique Figure 1. Geological map of the study area. SOIL GAS STUDY IN NW INDIAN HIMALAYAS 3 used is cost effective, easily applicable and less distur- bed by different environmental conditions. The stati- stical variation in measurement of radon concentra- tion is larger in summer than in winter [Szabo et al. 2013] keeping in view this fact, study was performed in January and February, 2015 1. Geological Mapping of study Area The study area is Mandi (31°13'26"- 32°04'22" north latitude and 76°36'08" - 70°23'26" east longitu- de) that includes the various thrust and fault systems especially MBT (Main boundary thrust), Chail thrust, Palampur thrust, Galma thrust, Riwalsar thrust and various fault systems (Figure 1). These faults and thrust are formed because of collision of Indian and Eurasian converging Plates [Gansser 1964]. This distri- ct lies partly on rocks belonging to the central Hima- layan zone some part of district lies on tertiary shale and sand stone. Rocks of area represent the Paleopro- terozoic period and are strongly foliated with well-de- veloped augen-gneiss, Sericite- chlorite, carbonaceous slates with lime stone residues, Phyllite quartzite, mylonitic gneiss and Porphyroblastic biotite gneiss with non-foliated granitoids. These types of Geologi- cal formations near MCT are cause of some geother- mal regions in Himachal Pradesh. The high intensity of Thermal energy (>100mW/m2) with temperature gradient of more than 200°C/km have been obser- ved at some places in north west Himalaya [Shanker 1988]. The areas where aquifers are situated near to earth surface geothermal sources like, Manikaran in Kullu and Tatapani in Mandi district formed. Whereas micaceous purple clay and silt with intrusive granite are found near MBT. The region under study has a good average rainfall (about 1331.5mm as compare to Himachal Pradesh’s average of 1251mm) including more than 2000mm in the Jogindernagar belt hence a good quantity of the fresh water is seeped to the ground, when this feature is added to high porosity at certain places, unconfined aquifers situation is for- med, which causes the elevated ground water levels around MCT and MBT in Mandi district [Walia et al. 2005, Chandrasekharam et al. 2008]. These are the ol- dest rocks exposed in Himachal Pradesh comprises dominantly of purple coloured arenaceous sediments with argillites and characterized by interstratified ba- sic lava flows of the Mandi-Darla Volcanic [Geology and Mineral resources of Himachal Pradesh, 2012]. 2. Material and Methods 2.1 Measurements of the radon and thoron concen- tration in soil Polyvinyl chloride pipe of length 0.25m and dia- meter of 0.06m with an air tight aluminium caps at the ends has been used as discriminator for the ra- don-thoron. The detectors LR -115 type -2 films were cut in to the pieces of size of 0.015m × 0.015m and placed at the bottom and top of the discriminator to record alpha particle tracks of thoron and radon and radon, respectively. Figure 2 shows the position of the detectors along with different faults and thrust systems in the study area, whereas figure 3 shows the sketch of radon thoron discriminator used at 71 se- lected sites in the study area. After exposure to standard durations of 15 days the detectors were subjected to chemical processing in a 10 M analytical grade sodium hydroxide solution at (60 ±1)ºC, for 90 min, in a constant temperature water bath to enlarge the latent tracks produced by alpha particles. The washed and dried detectors were observed under an optical microscope (Zeiss at 400× magnification) to count the alpha particle tracks. The counted tracks have been converted in to units of radon concentration of Bq/m3 using calibration factor [Eappen and Mayya 2004]. 2.2 Measurement of the radon exhalation rates and po- rosity from soil samples The soil samples collected from 71 different sites of study area (Figure 2) have been dried and grinded Figure 2. The position of the detectors installed along different faults and thrust system in the study area. KUMAR ET AL. 4 L oc at io n N o. A re a ex ha la ti on ra te (B qm -2 h -1 ) M as s ex ha la ti on r at es (B q kg -1 h- 1 ) R ad iu m c on te n ts (B q kg -1 ) R ad on co n ce n tr at io n (B q/ m 3 ) as m ea su re d in s oi l T ho ro n co n ce n tr at io n (B q/ m 3 ) as m ea su re d in s oi l P or os it y A m ou n t of r ad on a va il ab le fo r tr an sp or t to t he s ur fa ce (r ad on pr od uc ti on r at e pe r un it v ol um e) (B qm -3 h -1 ) 1 1. 14 0 0. 05 0 5. 34 32 73 95 7 0. 36 0. 01 46 2 0. 85 4 0. 03 8 4. 11 43 53 13 87 0. 38 0. 01 15 3 1. 67 0 0. 07 4 7. 98 19 97 0 59 50 0. 39 0. 02 17 4 1. 40 3 0. 06 2 6. 65 16 18 3 41 23 0. 39 0. 01 82 5 1. 32 8 0. 05 9 6. 26 12 74 0 14 90 0. 39 0. 01 76 6 1. 76 9 0. 07 8 8. 20 44 59 24 91 0. 40 0. 02 16 7 1. 50 7 0. 06 6 6. 97 10 67 87 3 0. 32 0. 01 90 8 1. 01 4 0. 04 5 4. 77 17 33 15 27 0. 31 0. 01 17 9 1. 07 9 0. 04 8 5. 28 84 47 34 47 0. 38 0. 01 52 10 1. 72 3 0. 07 6 8. 19 33 37 74 7 0. 36 0. 02 28 11 1. 00 9 0. 04 4 4. 71 93 70 20 48 0. 38 0. 01 22 12 2. 07 6 0. 09 2 9. 33 23 70 16 49 0. 38 0. 02 40 13 2. 11 0 0. 09 3 9. 96 39 40 69 7 0. 36 0. 02 69 14 1. 03 5 0. 04 6 4. 81 58 13 24 63 0. 42 0. 01 27 15 1. 64 3 0. 07 3 7. 41 39 83 36 0 0. 37 0. 01 85 16 0. 86 3 0. 03 8 3. 97 23 53 18 77 0. 41 0. 01 06 17 1. 78 2 0. 07 9 8. 25 89 3 36 7 0. 31 0. 02 25 18 0. 91 4 0. 04 0 4. 37 34 17 29 0 0. 35 0. 01 19 19 1. 97 6 0. 08 7 9. 53 13 40 29 6 0. 33 0. 02 68 20 1. 25 4 0. 05 5 5. 60 34 98 45 3 0. 31 0. 01 41 21 1. 28 5 0. 05 7 6. 37 31 80 37 3 0. 24 0. 01 86 22 1. 04 3 0. 04 6 5. 01 10 40 37 0. 34 0. 01 39 23 0. 97 6 0. 04 3 4. 52 48 30 38 27 0. 43 0. 01 23 24 1. 17 1 0. 05 2 5. 67 10 01 7 77 0. 31 0. 01 63 25 1. 69 8 0. 07 5 8. 35 56 55 20 60 0. 27 0. 02 32 26 1. 78 4 0. 07 9 8. 66 47 40 36 50 0. 31 0. 02 50 27 1. 31 6 0. 05 8 6. 29 56 48 43 66 0. 5 0. 01 79 28 0. 67 5 0. 03 0 3. 18 27 70 16 47 0. 38 0. 00 85 29 1. 65 0 0. 07 3 7. 71 42 77 13 23 0. 41 0. 01 99 30 1. 65 8 0. 07 3 8. 02 36 00 32 47 0. 32 0. 02 29 31 2. 63 5 0. 11 6 12 .4 1 43 23 69 3 0. 39 0. 03 32 32 0. 88 1 0. 03 9 4. 21 41 40 12 90 0. 35 0. 01 19 33 1. 49 4 0. 06 6 7. 04 20 30 60 0 0. 39 0. 01 89 34 2. 52 4 0. 11 1 11 .7 3 35 70 33 37 0. 41 0. 02 97 35 1. 55 6 0. 06 9 7. 12 49 23 33 8 0. 32 0. 01 86 SOIL GAS STUDY IN NW INDIAN HIMALAYAS 5 36 1. 57 8 0. 07 0 7. 20 16 47 60 0 0. 34 0. 01 87 37 1. 43 3 0. 06 3 6. 63 20 04 52 5 0. 32 0. 01 67 38 1. 35 5 0. 06 0 6. 61 80 17 48 67 0. 45 0. 01 96 39 1. 86 4 0. 08 2 8. 99 46 00 39 23 0. 42 0. 02 54 40 1. 42 0 0. 06 3 6. 88 39 70 29 07 0. 4 0. 01 92 41 1. 62 7 0. 07 2 7. 37 10 05 2 35 3 0. 36 0. 01 86 42 1. 52 1 0. 06 7 6. 94 14 12 3 69 70 0. 5 0. 01 80 43 2. 00 5 0. 08 9 9. 06 19 83 10 50 0. 39 0. 02 37 44 2. 07 5 0. 09 2 9. 57 10 33 6 17 97 0. 4 0. 02 57 45 0. 93 4 0. 04 1 4. 39 10 27 98 0 0. 38 0. 01 17 46 2. 06 1 0. 09 1 9. 44 73 00 20 33 0. 41 0. 02 47 47 1. 53 8 0. 06 8 6. 99 50 03 18 67 0. 4 0. 01 81 48 0. 89 6 0. 04 0 4. 26 24 06 11 80 0. 36 0. 01 18 49 2. 20 4 0. 09 7 10 .4 4 15 16 98 3 0. 37 0. 02 85 50 1. 98 4 0. 08 8 9. 72 14 73 13 53 0. 32 0. 02 76 51 1. 42 2 0. 06 3 6. 74 18 80 18 63 0. 37 0. 01 84 52 1. 25 5 0. 05 5 5. 81 78 07 58 20 0. 48 0. 01 52 53 1. 56 3 0. 06 9 7. 19 29 20 14 40 0. 39 0. 01 92 54 1. 70 5 0. 07 5 7. 88 43 40 38 47 0. 43 0. 02 04 55 1. 44 3 0. 06 4 6. 79 33 53 32 90 0. 4 0. 01 81 56 1. 15 2 0. 05 1 5. 35 30 00 54 0. 29 0. 01 42 57 1. 11 3 0. 04 9 5. 07 86 7 68 4 0. 36 0. 01 31 58 1. 19 7 0. 05 3 5. 59 99 3 16 7 0. 31 0. 01 51 59 0. 77 8 0. 03 4 3. 66 34 30 16 04 0. 39 0. 00 98 60 1. 44 3 0. 06 4 6. 88 26 74 15 72 0. 38 0. 01 95 61 0. 64 4 0. 02 8 3. 11 36 20 93 7 0. 36 0. 00 88 62 1. 36 9 0. 06 0 6. 72 16 80 17 07 0. 39 0. 01 92 63 1. 20 2 0. 05 3 5. 73 15 74 90 7 0. 36 0. 01 62 64 1. 16 0 0. 05 1 5. 46 43 22 12 34 0. 39 0. 01 46 65 1. 42 2 0. 06 3 6. 67 34 25 65 4 0. 4 0. 01 83 66 1. 57 2 0. 06 9 7. 49 22 32 10 87 0. 37 0. 02 10 67 1. 23 9 0. 05 5 5. 71 24 65 15 43 0. 43 0. 01 47 68 3. 31 7 0. 15 15 .4 1 56 78 24 66 0. 41 0. 04 08 69 0. 91 8 0. 04 1 4. 41 53 76 14 89 0. 37 0. 01 23 70 1. 30 4 0. 05 8 6. 27 34 56 98 7 0. 34 0. 01 75 71 1. 11 8 0. 04 9 5. 17 45 66 11 23 0. 38 0. 01 35 T ab le 1 . T he r ad on a nd th or on c on ce nt ra ti on s al on g w it h ex ha la ti on r at es (A re a an d M as s ex ha la ti on r at es ), ra di um c on te nt s, p or os it y an d ra do n pr od uc ti on r at e pe r un it v ol um e at s am pl in g po si ti on s KUMAR ET AL. 6 to very fine powder. 100 gm of each powdered sam- ples were placed at bottom of the cylinder of radius 3.5 cm and length 7.5 cm [Singh et al. 1997] (Figure 4). LR-115 type-II SSNTD (0.015m × 0.015m) were placed at the top of the cylindrical enclosures and the container was sealed tightly for 90 days to establish the equilibrium. The detectors have been etched in 10M NaOH at (60±5) ºC, for 90 min, in a constant tem- perature water bath to enlarge the latent tracks and counted using optical microscope (400 X). The tracks are converted in to radon activity using the calibration factor of 0.02tracks/cm2/day= 1Bq/m3 [Eappen and Mayya 2004]. The area and mass radon exhalation rate has been calculated using the formula [Amrani and Sr. No. Profile Porosity (average) Average Area Exhalation rates Average Radium Contents Average value (Bq/m3 ) Standard deviation value(Bq/m3 ) Radon Thoron Radon Thoron 1 1 to 6 0.39 1.36 6.42 10163 2733 7111 1947 2 7 to 11 0.35 1.27 5.98 4791 1728 3862 1094 3 12 to 16 0.39 1.55 7.10 3692 1409 1431 865 4 17 to 22 0.35 1.46 6.81 2221 303 1267 143 5 23 to 27 0.36 1.39 6.70 6178 2796 2190 1746 6 28 to 34 0.38 1.65 7.76 3530 1734 856 1126 7 35 to 37 0.33 1.52 6.98 2858 488 1797 135 8 38 to 42 0.43 1.56 7.36 8152 3804 4165 2443 9 43 to 47 0.40 1.72 7.89 5130 1545 3826 492 10 48 to 51 0.36 1.63 7.79 1819 1345 432 377 11 52 to 55 0.43 1.49 6.92 4605 3599 2216 1803 12 56 to 60 0.35 1.14 5.31 2193 816 1184 744 13 61 to 63 0.37 1.07 5.19 2291 1184 1152 453 14 64 to 67 0.40 1.35 6.33 3111 1130 958 370 15 68 to 71 0.38 1.66 7.82 4769 1516 993 668 Table 2. Values of various soil parameters in different profiles (as shown in figure 2). Average values along MCT Average values along MBT Sr. No. Serial no. of detectors as in figure Radon con- centration (Bq/m3) Thoron concen- tration (Bq/m3) Serial no. of detectors as in figure Radon concentration (Bq/m3) Thoron concentration (Bq/m3) 1 Monitoring station no. 5, 9, 14, 21, 25, 62 very near to MCT 6253 1923 Monitoring station no. 69, 65, 59, 49, 34, 29, 36, 35, 37, 54, 46 Very near to MBT 3801 1521 2 Above serial no. 1 (towards District Kullu) station no. 6, 10, 16, 22, 26, 61 6158 1297 Above serial no. 1 (towards District Kullu and MCT) 68, 64, 60, 48, 28, 53, 45 3113 1502 3 Next nearest stations above serial no.2 from Chail thrust 2401 1661 4 Nearest Station from chail thrust (towards MBT) i.e. Below MCT (Station No. 4, 8, 13, 20, 24, 63) 3258 1623 The stations below MBT towards Palampur thrust) 70, 66, 56, 50, 30, 55, 47 3159 1698 5 Next nearest stations below chail thrust ( towards MBT) from serial no. 4 5217 2186 The stations below MBT towards Palampur thrust. Station No. 71, 67, 57, 51 2445 1303 Table 3. Average values of radon-thoron concentration of a relative distance from MBT and MCT. SOIL GAS STUDY IN NW INDIAN HIMALAYAS 7 Cherouati 1999]. Whereas the porosity η of the soil is calculated using following formula η=1-ρbulk/ρparticle [Morgan et al. 2005]. 3. Results and discussions The radon and thoron concentration along with exhalation rates, radium contents, porosity and radon production rate per unit volume recorded at 71 loca- tions in the study area are shown in the table 1. The average concentration of radon and thoron gases in the study area has been found to be 4541 Bq/m3 and 1778 Bq/m3within a range of 867-19970 Bq/m3 and 37-6970 Bq/m3, respectively. In order to identify pos- sible threshold values of anomalous radon and thoron concentration, various statistical methods have been used by different authors in the past [Guerra et al. 2001, Walia et al. 2005, Fu et al. 2005]. In the present context, statistical threshold values of gas anomalies are fixed at average (µ) plus one standard deviation (σ). Figures 5 and 6 shows the variation of radon and thoron concen- tration (Bq/m3) at different sampling locations in com- parison to average (µ) and average+standard deviation value (µ+σ). The anomalous value of radon has been observed at 8 locations (3, 4, 5, 11, 24, 41, 42 & 44). The locations 3, 4, 5, 11 & 24 are close to MCT and locations 41 & 42 are close to MBT-II. The location 44 is close to local fault in the study area. The anomalous value of thoron has been observed in 12 locations (3, 4, 9, 23, 26, 27, 34, 38, 39, 42, 52 & 54). The locations 3, 4, 9, 23 & 26 are close to MCT and location 27 is close to MBT-I. Whereas locations 39 & 42 are close to MBT-II and locations 34, 38, 52 & 54 are close to the local fault in the study area. Anomalies in measurement of radon and thoron concentration are more along and across MCT than MBT and any fault system. More anomalies have been found in measurement of thoron concen- trations. It may be due to shallower gas source in the study area [Yang et al. 2005, Kumar et al. 2013b]. The area and mass exhalation rates of radon have been calculated for each site and have been reported in Table 1. The average value of area and mass exhalation rates have been found to be 1.46 Bq/m2 h and 0.064 Bq/kg h with a variation of 0.644Bq/m2 h and 0.028 Bq/kg h, respectively at location number 61 to 3.317 Bq/m2 h and 0.15 Bq/kg h respectively at location number 68. The radium content of the soil has been ranged between 3.11- 15.41 Bq/kg with an average va- lue for the area as 6.84 Bq/kg. The porosity of the soil has been found to vary with a minimum (0.24) at location 21 to a maximum of 0.5 at two locations 27 and 42 with an average of 0.37 at five locations labelled with numbers 15, 49, 51, 66, 69. The correlation factor of 0.59 has been observed between radon and thoron concentration of the study area and a good correlation (0.64) between thoron and porosity has also been detected. Similar kind of cor- relation has been reported by Al Jarallah et al. [2005] in a study related to construction materials (especially granite) used in Saudi Arabia. In this study very less correlation (0.03) between porosity and amount of radon available for transport to the surface has been recorded. However, at the sam- pling sites 51, 62 and 65 the values of radium contents, porosity and amount of radon available for transport to the surface recorded are 6.74 Bq kg-1, 0.37, 0.0184 Figure 3. Sketch of radon-thoron discriminator (plastic cane with side cap including LR-115 films at the top of the cane and at the bottom of the cane) used in the present study. Figure 4. Schematic diagram of a container utilized for radon exhalation rate measurements in the present study. KUMAR ET AL. 8 Bqm-3 h-1, 6.72 Bq kg-1, 0.39, 0.0192 Bqm-3 h-1 and6.67 Bq kg-1, 0.4, 0.0183 Bqm-3 h-1 respectively. For sampling sites 17 and 18 these values are 8.25 Bq kg-1, 0.31, 0.022 Bqm-3 h-15 and 8.02 Bq kg-1, 0.32, 0.0229 Bqm-3 h-1 and at sampling sites 57 and 71 values are 5.17 Bq kg-1,0.38, 0.0135 Bqm-3 h-1 and 5.07 Bq kg-1,0.36, 0.0131 Bqm-3 h-1. These observations shows that if the radium con- tents of some soil samples are comparable then with the porosity of the soil radon transport factor to sur- face will increase. Also, if there is secondary porosity in any region due to presence of fault systems then permeability/ emanation factor of the soil will incre- ase, which will further increase the radon transport to surface [Cinti et al. 2009, Ciotoli et al. 2016]. The average value of radon, thoron along with average exhalation rates, average radium contents and average porosity in different profiles (as shown in fig 2) has been reported in table 2. Profiling 1, 2, 3, 4, 5 and 13 has been made along and across the main cen- tral thrust (MCT) and profiling 6, 7, 8, 9, 10, 11, 12, 14 and 15 has been made along and across MBT and local faults in the study area. The radon has been found to vary in the range of 1680-12740 Bqm-3 with an average of 6253 Bqm-3 along the MCT. The concentration of radon and thoron at MBT has varied in the range of 1516-7300 Bqm-3 with an average value of 3980 Bqm- 3 and 338-3847 Bqm-3 with an average value of 1621 Bqm-3, respectively. The values of radon concentration are decreasing by factor of 1.01 to 2.56 on both side of MCT and MBT, while thoron concentrations are de- creasing by factor of 1.01 to 1.5 on moving distance of 2 to 3 km from MBT and MCT, with exceptions at some stations towards Palampur thrust, this may be due to the fact that there exist Numbers of local faults in between MBT and MCT. The radon and thoron concentrations have been detected to increase with a movement from Chail thrust (MCT) to MBT in the Jogindernagar region that may be attributed to the geological stress and strain in this region. The concentrations of radon and thoron along the MBT have been found lesser than that of the MCT. This may be due to the reason that MCT is under more geological stress and strain as compared to the MBT. The average values of soil radon measured along the MCT (Main central thrust) in the profiles 1, 2, 3, 4, 5 and 13 have been recorded as 10163 Bq/m3, 4791 Bq/ m3, 3692 Bq/m3, 2221 Bq/m3, 6178 Bq/m3, 2291 Bq/ m3 whereas the average thoron concentration in the same profiles has been observed as 2733 Bq/m3, 1728 Bq/m3, 1409 Bq/m3, 303Bq/m3, 2796Bq/m3, 1184Bq/ m3. The profile 1 has exceptionally high average values because the presence of sling zones along this profile. The radon concentrations have been noticed to decre- ase from Jogindernagar to Mandi along MBT-1 due to the decrease in porosity of the soil. The same trend has been observed when the observer has moved along the profile 8, 11, 10 that may be attributed to the cross presence of other faults systems. The decreasing trend of radon and thoron con- centrations has been observed along the route fol- lowed through the profile 15, 3 and 4 that may be due to the higher radon exhalation rates, radium contents and more or less due to porosity of soil in the area of profile 15 than in soil of profiles 3 and 4. The slight elevation in the concentration along the sampling si- tes of profile 6 and 7 has been spotted that may have been caused by the presence of secondary fault sy- stems (Sundernagar fault)in the vicinity of Sunder- nagar [Mahajan et al., 2010] and may be due to high porosity in the soil of profile 6 (0.38) and profile 7 (0.33) and relatively high values of radon exhalation rates1.65 Bq/m2 h (in profile 6) and 1.52 Bq/m2 h (in profile 7) and high values of radium contents 7.76Bq/ kg (in profile 6) and 6.98Bq/kg (in profile 7). The results also shows that the MBT and MCT of Himalayan region is more active than Turkish faults as reported by Sac et al. [2011] as the values of soil radon concentration measured are more as compared to the values measured near the fault in the western Turkey. However the values of soil radon concentration obser- ved in the present study have been found to be lower as compared to the values measured in complex tectonic and seismic Tangshan area of northern China (where earthquake Ms 7.8 was occurred in 1976) as reported by Li et al. [2013]. The average values of radon and thoron concentration in Dharamshala near MBT and MCT in Himachal Pradesh were 5992 Bq/m3 and Thoron values of 901 Bq/m3 [Kumar et al. 2013b]. The trend in obser- ved value of radon in Dharamshala region of Himachal Pradesh are similar to the present study, however the thoron values in Mandi region are found to be almost double than in Dharamshala region. The values of Radon concentration are decrea- sing by factor of 1.01 to 2.56 on both side of MCT and MBT, while Thoron concentrations are decreasing by factor of 1.01 to 1.5 on moving distance of 2 to 3 km from MBT and MCT, with exceptions at some stations towards Palampur thrust (Table 3), this may be due to the fact that there exist numbers of local faults in between MBT and MCT. The present study may also be helpful to study SOIL GAS STUDY IN NW INDIAN HIMALAYAS 9 other natural features of study area like geothermal potential and ground water reservoirs, since high ther- mal energy flow with good thermal gradient have been observed by various researchers in NW Hima- laya. This heat flow may be due the melting and re- ductions of intrusive granites near to MBT and MCT in Mandi area and presence of uranium, thorium and potassium contents in the soil texture [Rao et al. 1976, Das et al. 1979, Walia et al. 2005]. These reductions in basic strata of area may create faults which are the cau- se of secondary porosity and hence increase in the ga- ses like Radon and thoron along with thermal energy fluids with a path of flow of water to the surface. The Beas valley, Uhl valley and region from Jogindernagar to Mandi have potentially good source of ground wa- ter. Thus elevated levels of radon and thoron gases in seismically active and faulty area may be used detect secondary porosity, so the monitoring of these radio nuclides will further help to study the other gases tran- sport through porous medium of the soil. 4. Conclusions The radon-thoron measurement in soil, measure- ment of radon exhalation rates and radium contents of Figure 5. The variation of radon concentration (Bq/m3) at different measuring stations in comparison to average (µ) and average+ standard deviation value (µ+σ). Figure 6. The variation of thoron concentration (Bq/m3) at different measuring stations in comparison to average (µ) and average+ stan- dard deviation value (µ+σ). KUMAR ET AL. 10 Mandi district, Himachal Pradesh, NW Himalaya, India has been measured using the passive detectors LR-115 type-2 films. The anomalous value of radon-thoron has been reported along and around MCT, MBT and local faults in the study area. Radon concentration along MCT have been found higher than that along MBT, this may be due to the reason that MCT is under more geological stress and strain as compared to MBT. More anomalies have been recorded in the measurement of thoron con- centration. The thoron concentrations values are very low at some places; this may be due to presence of deeper source in earth at these places. Good correlation between porosity and thoron has been recorded in this study, whi- ch shows presence of local fault in the area. 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