Radiation hazard evaluation and spatial dose rate mapping in mineralized region of Siwaliks: A case study from Una, Kangra and Hamirpur distt, Himachal Pradesh, India

The following work is a part of wider uranium prospecting programme of the Atomic Mineral Directorate for Exploration and Research (AMD) for mapping radioactive anomalies in mineralized regions of Siwalik, Himachal Pradesh. This study reports the analysis of average activity concentration, potential radiological hazards; radium equivalent activity (Raeq), external hazard index (Hex), internal hazard index (Hin) and excess cancer life time risk (ECLR) calculated due to naturally occurring radioactive materials (NORM); Radium ( 226 Ra), Thorium ( 232 Th) and Potassium ( 40 K) in uranium mineralized zone of Siwaliks for health risk assessment. The geometric mean value of calculated indoor and outdoor annual effective dose in the Una, Hamirpur and Kangra region are 0.88 and 0.22; 1.77 and 0.44; 1.86 and 0.46 mSvy -1 respectively. The average air absorbed dose rate were measured to be 118 nGyh -1 in Una, 163 nGyh -1 in Hamirpur and 135 nGyh -1 in Kangra district respectively. Correlation study indicated good correlation of the measured gamma dose rate and the estimated gamma dose rate with a correlation coefficient of (R 2 =0.62). Our data provides important information on the radiation risk and spatial variability of the natural terrestrial gamma radiation in the Siwalik region.


Introduction
Living organisms on the earth have been receiving the radiation exposure continuously from cosmic rays, terrestrial radionuclides ( 235 U, 238 U, 232 Th and 40 K), cosmogenic nuclides ( 7 Be, 14 C, 18 F, 22 Na etc.) and anthropogenic radionuclides ( 137 Cs) (UNSCEAR 1993). The total annual effective dose from the natural radiation sources is 2.3 mSvy -1 in India as compared to the global contribution of 2.4 mSvy -1 (UNSCEAR 1982;UNSCEAR 2000;UNSCEAR 2008;UNSCEAR 2000b). In India, natural sources that account for exposure to environmental radiation include cosmic radiation (15.44%), terrestrial radiation (internal and external)(30.18%), 222 Rn and 220 Rn inhalation (53.72%) and cosmogenic nuclide (inhalation) (0.65%). The worldwide average annual effective dose equivalent due to terrestrial gamma radiation is 0.48mSv. Large evidences are present that support the study that radiation exposure above a certain threshold level could induce heredity disorders, leukemia and cancers of different organs such as lungs, kidney (Editing et. al. 1982;Flodin et. al.1990;Murrihead et al. 1990). Significantly high radiation doses have been received by human population residing in regions of high atomic mineral occurrences. In the previous years, many studies for estimating the natural radiation hazard indices (Ra eq , H ex , H in and ECLR); indoor and outdoor gamma dose rate (γ) have been Uranium exploration is the prime mandate of AMD and there are six prime investigation areas. Airborne (AGRS) and ground based radiometric survey is carried out in the preliminary stages of uranium investigation for delineating and identifying radiation anomalies in uranium prospecting and for baseline environmental survey (Grasty and Lamairre 2014). Since these are mineralized regions with high concentration of primordial activities hence there is high probability of large radiation doses being imparted to the local population. This requires monitoring of gamma dose rate and radionuclide concentration for health risk assessment in the mineralized and in the background region. A detailed investigation on radiation monitoring has already been conducted on various high atomic mineral occurrences sites of Gogi, Dharmapuri and Jaduguda. Srinivass et al. has revealed average dose of 186 nGyh -1 in Chitrial, 130 nGyh -1 in Lambapur-Pedaguutu and 63 nGyh -1 in koppunura in uranium mineralized region of Gogi. The average annual effective dose rate in the Dharmapuri shear zone was estimated by Bhattacharya et al. and was observed to be above the recommended limit of 1 msvy -1 . Siwaliks region is known for hosting sandstone type of uranium deposits. Exploratory mining carried out at Andalada in Siwaliks have delineated six discontinuous ore lenses over 330 x 100 m with thickness of 0.98 to 2.2m.
A total of 3058 tonnes of ore with 0.02-0.045 % eU 3  interpret geophysical activity and radionuclide migration. Previous literature on radiological parameters study has only demonstrated the radionuclide concentration and hazard parameter study in this region; however none of the above studies has demonstrated the spatial variability of radionuclides. Our report is divided into two parts. We have tried to determine the background radiation in the uranium mineralized region of Una, Hamirpur and kangra and evaluate radiation hazard by evaluating different radiological indices and visually characterize the migratory behavior of radionuclides using spatial modeling. The obtained data may be the baseline for future research, environmental monitoring and epidemiological studies in the studied area.

Geology
The study area (Siwalik) is in northern part of India and covers an area of 2500 sq km. The basin, comprising the Siwalik sediments, extends over >2500 km from Potwar plateau, Pakistan in NW to Arunachal Pradesh. The study area extended from 31º37'30" N to 32º9'0" N latitudes and 75°51'0"E to 76°33'0" E longitude. The Siwalik sediments considered as favourable host for epigenetic sandstone type of uranium mineralization due to their sedimentological character, provenance and depositional environment. The rocks of Siwalik group are divided into upper, middle and lower siwalik groups. The area of present investigation, ie. Rajpura-Nari-Jawar-Parah area falls within the Kangra sub-basin within the middle and upper siwalik the northern limit of which is defined by the Main Boundary Thrust (MBT) whereas the southern margin is bounded by Soan and Barsar thrust.
The geological map and location map of Siwaliks is shown in the Fig 1(a) and Fig 1(b). The Siwalik sediments are deposited along the number of foreland basins of Himalaya. The Siwaik Group (Middle Miocene to Pleistocene) represents ca.6000m thick molasse deposit. These sediments have been traditionally been divided into Lower, Middle and Upper Siwalik. Lower Siwalik is a distinct assemblage of fine sandstone-mudstone, whereas sandstone-conglomerate package represents Middle Siwalik and Upper Siwalik comprises of gravel/boulder bed with minor sandstone. Contacts between the formations are gradual and conglomerate dominance increase towards top. Kangra subbasin possess the thickest deposits of Siwalik rocks and the most promising according to the uranium mineralisation point of view (Kaul et al. 1979;Kaul et al.1993;Bala et al. 2014), Occurrences of number of radioactivity in this basin attracted this study of radiation dose in the area. Investigating area of Rajpura-Nari-Jawar-Parah area falls within the Soan &Barsar thrust and here radioactivity is hosted by Upper Part of Middle Siwalik pebbly sandstone-conglomerate. And the Loarkhar-Sibbal area falls in Barsar &Soan thrust; here radioactivity is hosted by Middle Siwalik pebbly sandstone-conglomerate.
Uranium mostly occur as adsorbed phase with the mud clast, coaly matter, clay minerals. Table 1. describes the sampling areas of Himachal Pradesh.

Materials and Methodology
Based on the activity of the radionuclides (> 3 Bg) and geological background, a total of 218 grab samples were collected from the 33 different locations of Una, Hamirpur and Kangra district, Himachal Pradesh, India during the period of 2016-2017. These sample locations were numbered as S 1 to S 33 . The sampling area, sampling sites, geographical location, dose rate measured and geological parameters are listed in Table 2. In order to collect the natural soil, the soil samples weighing about 250 gm have been collected at a depth of 75 cm from the surface. Distance between each location was 1 km. The corresponding latitudes and longitudes were noted using a GPS make (Global positioning system). The sample were homogenized, pulverized and then sieved through -150 mesh. The samples were then dried in oven for about 24 hours. Quantitative determination of 226 Ra, 232 Th and 40 K was performed by Gamma ray spectrometric system using NaI(Tl) gamma detector of size 5" × 4" coupled with photomultiplier tube and a DSP based 2K MCA system. Spatial distribution modeling analysis using ARCMAP GIS (Geographical Information Software) has been adopted for modeling of terrestrial radionuclide activity (in BqKg -1 ) due to the distribution of gamma dose rate, Ra eq (in ppm), Thorium (in ppm) and K (in %). Statistical analysis was performed with SPSS 20 (Statistical Program for Social Science). The geoelemental values are presented by arithmetic mean (AM), geometric mean (GM), and range (R). The dispersion in the parameters are expressed by standard deviation (SD), Interquartile range (IQR) and median absolute deviation (MAD). The outliers (cases with standardized residual greater than ±3 standard deviations) found in the data were removed for calculation of radiation hazard indices. The asymmetry and tailness of the distribution is indicated by the skewness (Sk) and kurtosis (Ҝ) of the data. Further the data have been tested for normality (Kolmogorov-Smirnov). The frequency distribution indicating the normality of the radioelemental data is visualized by Q-Q (Quantile -Quantile) plots. Pearson correlation analysis, cluster analysis was performed to find correlation between radiological parameters. All tests were performed at 95% confidence interval and value of p<0.05 were considered statistically significant.

Estimation of 226 Ra, 40 K and 232 Th concentration by gamma ray spectrometry
The activity concentration of 226 Ra, 232 Th and 40 K were determined by NaI(Tl) detector. The detector has an active volume of 1286.38 cm 3 , a resolution of 9.5% and an efficiency of 14.5 % at 662 keV. The detector was surrounded by lead shield with dimensions of 2"x 4"x8" to reduce background. The detector was calibrated w.r.t the 667 KeV peak of 137 Cs and 1. The stripping ratio α, β, γ related to the concentration as Th n = Th g -aU n (2) are given by , , , , The minimum detection limit of 226 Ra, 232 Th and 40 K are 2.65 Bq kg -1 , 3.53 Bq kg -1 and 15.8 Bq kg -1 respectively.

Estimation of Radiological Hazard indices
The radiological Hazard indices Radium equivalent(Ra eq ), Air absorbed dose rate, D(nGyh -1 ) , annual effective dose equivalent,

Geostatistical Interpolation
Spatial distribution modeling was performed using IDW (Inverse Distance weighted) algorithm on Arc map GIS (Geographical Where p is an arbitrarily positive number called the power parameter and are the distances from the dispersion points to the interpolation points, given by Where , are the coordinates of the interpolation points and and are the coordinates of each dispersion points. There are 190 sampling points that are measured at different locations of Una, Hamirpur and Kangra districts of Himachal Pradesh. The survey points were transferred to a basemap to create geodatabase.

Soil Radioactivity ( 226 Ra , 232 Th and 40 K) :
The descriptive statistics of geo-elemental radioactivity concentration of 226 Ra, 232 Th and 40 K in Una, Hamirpur and Kangra region are given in Table 3. The concentration of 226 Ra, 232 Th and 40  Barsar thrust in the east with yellowish brown oxidized sandstone and whitish grey homogeneous sandstone rock type. No significant variation was observed in the activity concentration of 232 Th in studied region ( Table 3). The overall mean activity concentration of 40 K was also observed to be homogeneous in the studied region. The 40  shows the radionuclide concentration in the Siwalik region.
The Q -Q (Quantile -Quantile) probability plots have been used to confirm the statistical distribution of data around their mean.  and represent the general trend observed for naturally occurring radionuclides as given in Table 3. (Shacklette and Boerngen 1984).
The ratio of mean value of 226 Ra to 232 Th was 6.5 for Una region, 12.5 for Kangra region and 11.8 for Hamirpur region. This activity ratio is due to uranium mineralization in this area. The samples were collected from depth between 75 cm and 1 m and hence the mineralization is present at shallow regions. Further, high disequilibrium factor ( 238 U/ 226 Ra) suggests that the process of uranium mineralization is still under dynamic state and uranium is constantly being moved from clay oxidizing zone and getting precipitated with enrichment in the reduced zone.

Radium Equivalent(Ra eq ):
Radium Equivalent activity Ra eq is a common radiological hazard index to assess the radiation hazards due to non uniform activity distribution of 226 Ra, 232 Th and 40 K in the soil samples. Ra eq is estimated using the following relation ( The statistical parameters of Ra eq measurement are given in Table 3. The arithmetic mean of Ra eq in Una, Hamirpur and Kangra region was observed to be 581.3, 964.0 and 941.6 Bq kg -1 . These values are higher than the maximum permissible value of 370 Bq kg -1 as recommended by Organization for Economic Cooperation and Development (OECD 1979). This high Ra eq is due to high mineralization in the localized region.

):
For health risk assessment, the absorbed dose rate are calculated from the activity concentration of 226 Ra, 232 Th and 40 K at a height of about 1 m above the ground using the dose conversion coefficient of 0.461, 0.623 and 0.0414 nGyh -1 Bq -1 kg -1 respectively as given in Eq. (3) (Saito and Jacob, 1995). Secular equilibrium has been assumed in between 238 U and 226 Ra so that the activity concentration of uranium is assumed to be similar to that of radium.
( ) The mean, median, std. deviation of the absorbed dose rate has been calculated from the activity concentration of 226 Ra, 232 Th and 40 K for the different areas of Una, Hamirpur and Kangra has been given in

External and Internal Hazard Indices (H ex and H in ):
The primary objective of external hazard index (H ex ) is to limit the radiation exposure due to the natural radionuclide to a permissible extent of 1 mSvy -1 .

Gamma level Index ( ):
The gamma level index ( ) used to estimate the γradiation hazard level of in the soil samples has been calculated using Eq.(13) given by European Commission (European Commission, 1999).

Excess lifetime cancer risk (ELCR)
The Excess lifetime cancer risk (ELCR) has been calculated for the assessment of extra risk of developing cancer due to exposure of a toxic substances acquired over the lifetime. ELCR has been calculated using Eq.(14).
The calculated range of ELCR in Una region varied from 0.21-7.3 x10 -3 , in Hamirpur it was from 0.39-7.49 x10 -3 and in Kangra, the value varied from 0.60-4.24 x10 -3 . The average value of ELCR in the investigation area was higher than the world average value of 0.29 × 10 -3 .

Spatial distribution of gamma dose rate
The air absorbed gamma dose measurements were carried out during the field season 2016-17. Gamma exposure rates at the sampling sites were measured by using Radiation Survey Meter (RSM). The RSM has 1" x 2" NaI(Tl) detector. The device has an energy range of 30 KeV to 3000KeV and the dose measurement range is from 1.0 μRh -1 to 300 mRh -1 . The device is calibrated using 667 keV energy of source 137 Cs. The air absorbed gamma dose rates D(nGyh -1 ) at the sampling sites in air at 1m above the ground surface for a collection time of 200 seconds for the uniform distribution of radionuclides ( 232 Th, 238 U and 40 K) were computed on the basis of guidelines provided by UNSCEAR (1993UNSCEAR ( , 2000. About 5 readings are taken at a height of 1m and the arithmetic mean of the value is taken as the representative value for the gamma dose rate. These exposure rates were converted to AEDE(mSvy -1 ) using an outdoor occupancy factor of 0.2. Various statistical parameters like minimum, maximum and mean of AEDE values for all the three areas are shown in Una, Hamirpur and Kangra. The measurement using RSM is affected by the mineralization near the soil (around 1m from the topsoil) and radius of 10 m around its location. So the total dose rate is generated by the integration near a concentrated mineralized zone (around 1 m) and a distributed non mineralized zone (10 m). This variability in the correlation at the localized scale is counterbalanced at the global scale and a relatively low dose rate is obtained. Spatial distribution of gamma dose rate were plotted area-wise to show the variation of gamma dose rate in the given region. (Fig 10). A positive correlation (R 2 = 0.63) has been observed between measured dose rate and the absorbed dose rate and the best fit is shown in Fig 11. A correlation between the corresponding measured dose rates and calculated dose rate has been already studied Uranium occurs in the earth's crust either as secondary minerals or in adsorbed form and is soluble in U(VI) oxidation state and also in minerals such as zircon and monazite (Dickson and Scott, 1997). Normally thorium is immobile due to its low solubility, U, Th and K occurs in the earth as a crustal abundance of 6 ppm, 12 ppm and 1.5%. The dose rate contribution to gamma dose is primarily due to 238 U. The similarity of gamma dose rate to 226 Ra concentration can be attributed to high mineralization in this region.

Correlation studies between the individual radionuclides
A weak correlation was observed between the activity concentrations of 226 Ra and 232 Th with a correlation coefficient of r 2 = 0.51. No correlation was observed between the measured radionuclides 232 Th and 40 K and 226 Ra and 232 Th (Fig 12) Authors have observed contrasting correlation among the radionuclides (Kovacs et al. 2013;Baeza et al. 2016, Hassan et al. 2018 however in this case it indicates that these radionuclides 226 Ra , 232 Th and 40 K are of different geochemical origin.

Conclusion
A foot based radiometric survey and soil sampling was carried out in the mineralized region of Una, Hamirpur and Kangra districts of Himachal Pradesh. The radionuclide concentration was found to be higher than the global value.     Grey colour siltstone with clay patches Table 3 Descriptive statistics of radiological concentration and radiological hazard indices in each sampling location (n=218).