Assessment of radon and potentially toxic metals in agricultural soils of Punjab, India

Accepted Manuscript Assessment of radon and potentially toxic metals in agricultural soils of Punjab, India

Inderpreet Kaur, Akash Gupta, Bhupinder Pal Singh, Sumit Sharma, Ajay Kumar PII: DOI: Reference:

S0026-265X(18)30992-5 https://doi.org/10.1016/j.microc.2019.01.028 MICROC 3599

To appear in:

Microchemical Journal

Received date: Revised date: Accepted date:

10 August 2018 10 December 2018 10 January 2019

Please cite this article as: Inderpreet Kaur, Akash Gupta, Bhupinder Pal Singh, Sumit Sharma, Ajay Kumar , Assessment of radon and potentially toxic metals in agricultural soils of Punjab, India. Microc (2019), https://doi.org/10.1016/j.microc.2019.01.028

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ACCEPTED MANUSCRIPT Assessment of radon and potentially toxic metals in agricultural soils of Punjab, India Inderpreet Kaura*, Akash Guptaa, Bhupinder Pal Singhb, Sumit Sharmac, Ajay Kumarc a

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Department of Chemistry, Centre for Advanced Studies, Guru Nanak Dev University, Amritsar - 143001, Punjab, India b Centre for I.T. Solutions, Guru Nanak Dev University, Amritsar - 143001, Punjab, India. c Department of Physics, DAV College, Amritsar, 143001, Punjab, India *Corresponding author: [email protected]

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Abstract: Radon, a radioactive gas is a primary source of radioactive pollution and directly influence the humans exposed to it. Radon (Rn222) / thoron (Rn220) exhalation and soil gas radon

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concentrations were estimated in agricultural soil samples from different locations of Amritsar district of Punjab, India, by using active techniques i.e. Smart RnDuo monitor and RAD7. Rn222

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mass and Rn220 surface exhalation rate of 22 locations varied from 21.41 to 43.81 mBqkg-1h-1 and 64.73 to 717.03 mBqm-2s-1 with average values of 32.58±1.11 mBqkg-1h-1 and 195.10±53.02

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mBqm-2s-1, respectively. Mean soil radon concentration of 12 agricultural soils samples varied from 0.1±0.04 to 9.7 ±0.40 kBqm-3. Potentially toxic metals such as Zn, Cu, Pb, Cr, and Cd were

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also estimated in agricultural soil samples and soils were found to be drastically suffering from Zn and Cu pollution followed by some Pb contamination. On the basis of average values (in

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mgkg-1), potentially toxic metal contents in soil samples can be arranged in decreasing order as: Zn (78.93) > Cu (42.32) > Cr (36.92) >Pb (7.49) >> Cd. The results indicated the probability of

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occurrence of potentially toxic metal accumulation in edible parts of the crops grown in the study area. Although no direct significant correlation has been observed between soil gas radon and

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mass/surface exhalation rate in soil samples under investigation but a positive correlation was observed among some toxic metals such as Zn:Cu, Zn:Cr, Cu:Cr, Cu: Pb and Pb:Cr. pH and conductivity were negatively correlated whereas, SOM and mass exhalation showed positive correlation. The investigation demonstrated the dominance of potentially toxic metal contamination by Cu, Zn and Pb over Rn222 emanation rate in agricultural soil towards health risks posed to residents of Amritsar. Keywords: Mass exhalation, Surface exhalation rate, Soil gas radon, Potentially toxic metals, RAD7

ACCEPTED MANUSCRIPT 1. Introduction Investigation of natural radioactivity is an important issue on account of its radiological effects on the environment and various forms of life. Most predominant part of natural radioactivity of soil is derived from decay of the primordial radionuclides of

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U and

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Th

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decay series and 40K. The primordial long-lived radionuclides such as 238U, 226Ra, 232Th, and 40K

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are commonly found in rocks, soil present on earth, water and building materials used for construction purposes, significantly contribute to predominant pathway of radiation exposure on

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the populace which is associated with the risk of leukemia, cancers of kidney and prostate [1]. The worldwide annual effective dose received by the population from all natural and artificial

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sources is 2.8 mSv and about 85 % of the dose (2.4 mSv) comes from only natural background

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radiations [2]. The dosage varies from place to place depending upon the concentration of natural radioactive nuclides in the dirt (soil). Natural radioactivity also depends on geological and

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topographical conditions and appears at various levels in soils of different geographical regions.

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The dispersion of natural radioactive nuclides and their radiological impacts are essential to study for the estimation of occurrence of health effects on individuals residing in that area.

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Radon and thoron radiate fundamentally from earth surface into air through the pores of soil.

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More porous soil allows greater diffusion of radon gas and results in higher chances of radon emanation. The concentration of radon gas in soil and radon/thoron exhalation rates from the soil surface depends on many physical parameters related to the internal structure of soil, soil porosity, grain size of soil, type of mineralization, soil permeability, radium contents and emanation coefficient [3]. The indoor radon concentration in some building materials is higher as compared to others depending on their microstructure, higher radon exhalation rates, and uranium/ radium enrichment. In India, buildings are constructed with bricks made of nearly 80%

ACCEPTED MANUSCRIPT soil, which may contain high content of natural radioactive nuclides [4]. This enhanced level provides awareness about indoor 222Rn/ 220Rn dosage exposure due to the natural radioactivity in buildings. So, it is necessary to quantify the exposure level of radiations emitted from soil in order to assess possible health hazards posed to the human population exposed to these

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radiations. The radon transmission from ground into environment relies on radium (Ra226)

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content in mineral grains of the soil. Two main factors responsible for transportation of Rn222 in

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surrounding environment are mass dissemination (it is a concentration gradient between the birth point of radon and the atmosphere) and advection (movements caused via air pressure

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differentials) [1]. According to Etiope and Martinelli (2002), radon transport is controlled by

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geophysical and geochemical parameters while exhalation is controlled by hydro-meteorological conditions [5]. Emanation coefficient also relies on water content in the soil and its radon

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transport parameters. As such, radon gas does not cause any health risk till it decays to short-

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lived daughter products which are solid elements that get lodged in inner layers of the lungs during inhalation [6]. Upon decaying, they emit small radiations which could damage the

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sensitive tissues in lungs and cause harmful effects on the respiratory function and may lead to

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occurrence of lung cancer. Some fractions of radon progeny may also penetrate into blood from lungs and irradiate the whole human body. In recent times, much attention has been given to

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radon considering it as a radiological health hazard, as human beings are continuously exposed to radon and its decay products in the environment they live in. In addition to radioactive pollution, the contamination of soil by potentially toxic metals from different sources also occurs due to natural as well as anthropogenic activities. Some natural issues like emanations from volcanoes, forest fires and chemical compounds of parent rocks are responsible for potentially toxic metal contamination of soil [7]. On the other hand,

ACCEPTED MANUSCRIPT anthropogenic sources mainly include direct or indirect emissions of trace elements from human activities such as mining, smelting, burning of fossil fuels, waste incineration and disposal, sewage irrigation, leaded gasoline and motor traffic, production and extensive use of chemical fertilizers and pesticide, etc. Due to the disturbance and acceleration of nature’s slowly occurring

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geochemical cycle of metals by man, most soils of rural and urban environments may

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accumulate one or more of the potentially toxic metals above the defined background values,

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high enough to cause risks to human health, plants, and animal’s ecosystems. The emitted contaminations from vehicles scatter into air and stale in earth’s eco-system [8]. Toxic metals in

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soil not only debase the quality of soil but also retain in soil for longer periods. Through soil,

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toxic metals leach into groundwater and may enter into food crops grown in the respective agricultural fields. Different heavy metals such as As, Cd, Cu, Pb, and Hg are well known for

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their toxicity when present beyond permissible limits and cause various health issues in humans

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like hypertension, fever, kidney disorders and DNA damage, etc. The study area has its own importance due to excessive application of agrochemicals which

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may enhance the level of natural radioactivity. So, it is necessary to investigate the level of soil

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gas radon, exhalation rate and potentially toxic metals in soil samples from different locations across the region as it may provide fundamental information about hidden uranium deposits.

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Several sole studies like soil gas radon and potentially toxic metal analysis have been carried out in India and worldwide but no such combined study was carried out earlier in Amritsar district, Punjab, India. The main objective of present work was to carry out an assessment of radon pollution in terms of Rn222& Rn220 exhalation rates, soil gas radon concentration and potentially toxic metal contamination in agricultural soil samples collected from different locations of Amritsar district,

ACCEPTED MANUSCRIPT Punjab, India. Further, data were analyzed to identify correlation (if any) between soil radon, mass exhalation and potentially toxic metals in the soil by performing correlation analysis to understand the relationships between these variables. Further, principal component analysis (PCA) was also applied on data for data reduction and identification of components causing

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maximum variation in data. The present investigation is useful for estimating the level of soil

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pollution due to excessive applications of agrochemicals and other anthropogenic actions. The

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information obtained might be useful to prepare baseline data for general awareness, which

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further allows radiological mapping and potentially toxic metal assessment in Northern India.

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2. Geology of the area

Soil samples were collected from various villages of Amritsar district as shown in Fig. 1.

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Amritsar district is situated in northern part of Punjab state and lies between 31°28' to 32°03'

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North latitude & 74°29' to 75°24' East longitude. It frames a part of the tract known as the Bari Doab or the territory lying between the rivers, Ravi and Beas. Its western side adjoins Pakistan,

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partly separated by the river Ravi. The northeastern side is bounded by the Gurdaspur District,

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and towards its south-east across river Beas, lays Kapurthala and Tarn Taran districts. The physiography of Amritsar district is the product of alleviation by Beas and Ravi rivers.

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There are no hills within the limits of this district and the formation is strictly alluvial though apparently of a uniform level. Amritsar district has two major landforms viz. alluvial plain and flood plain. Alluvial plain constitutes a major part of the district, formed by the alluvial deposits brought by Ravi and other rivers of the Indus system. The flood plains of Ravi and Beas rivers constitute other land forms of the district; occupy western half of the district and accounts for about 7% of the total area. It contains abandoned courses of rivers, patches of marshy land and

ACCEPTED MANUSCRIPT thickly growing grasses. The origin of this district may be attributed to a tectonic uplift which affected the whole of the Indus-Yamuna divide during the Pleistocene age. 3. Materials and methods

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Soil samples were collected from 22 different locations at a depth of 15 to 20 cm from the

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ground level to minimize the external effects of human activities in Amritsar district of Punjab

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state, India. The collected soil samples were dried in an oven at a temperature of 110 °C for 5 hours to remove the moisture content. The dried soil samples were crushed, sieved through mesh

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size equal to 2 mm and then used for exhalation studies.

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3.1. Physicochemical characterization

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20mg soil was broken down in 100ml distilled water (1:5: w/v) and shaked on a mechanical

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shaker for 12 hours. The soil suspension was then filtered through whatman no. 1 filter paper and filtrate so obtained was considered as soil extract. pH and electrical conductivity were measured

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by microprocessor based analyzer kit NPC 362D, Naina Solaries Limited, New Delhi whereas soil organic matter and carbonate content were estimated using standard methods described by

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Trivedi et al. (1987) [9].

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3.2. Dosimeter–radiometer

In the selected locations of Amritsar district, ambient outdoor gamma dose rate was measured by using dosimeter–radiometer MKS-03D gamma detector-meter at about 1 m above the ground surface. It is GM tube based survey-meter with digital display. It can detect the gamma radiation ranged from 0.05 to 3.0 meV and the ambient gamma radiations dose equivalent rate ranged from 0.1 µSvh-1 to 0.1 Svh-1 [10]. Measurements were taken in outdoor environment during collection of soil samples at all locations. The measurements were made

ACCEPTED MANUSCRIPT according to the standard protocol i.e., in the air at a distance of one meter above the ground. Due to the random nature of radioactive decays, the radiation exposure rate varies rapidly with time and the data presented in this article was the mean estimation of measurement carried out for the period of two minute at each location.

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3.3. Exhalation rate

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Rn222 mass exhalation rate in soil was estimated by using advanced smart RnDuo Monitor (SRM), manufactured and calibrated at Bhabha Atomic Research Center (BARC), Mumbai,

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India [11]. Samples were collected and treated following standard procedure described by

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Sharma et al., (2018) [12]. Exhalation chamber was associated with SRM through diffusion mode as shown in Fig. 2a. Alpha particles released from decay of radon were detected in the

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detector volume (153 cc) by scintillation with ZnS(Ag) through progeny filter and Rn220

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discriminator eliminates Rn222 progeny and Rn220. Rn220 discriminator based on diffusion-time delay and does not allow the short lived Rn220 to pass through. The total scintillation counts

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obtained in measurements cycle (60 minutes) were converted to Rn222 activity concentration

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(Bqm-3) by utilizing an algorithm implemented in the micro-controller. The monitor has a sensitivity factor of 1.2 counts h-1Bqm-3 with a detection range from 8 Bqm-3 to 10 mBqm-3. The

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device did not get affected by humidity and trace gases. The measurement of soil samples has been taken for an average of 24 hours for proper Rn222 growth and saturation. The growth of Rn222 mass exhalation in chamber was allowed till the saturation of radon concentration obtained for each sample and radon mass exhalation rate (Jm) was estimated by using the least square fitting method [13] for radon concentration Ct at time ‘t’ in equation (1) as shown in fig. 2b. Ct =

𝐽𝑚 𝑀 𝜆𝑒 𝑉

( 1- 𝑒 −𝜆𝑒𝑡 ) + 𝐶0 𝑒 −𝜆𝑒𝑡

(1)

ACCEPTED MANUSCRIPT where, Jm represents the radon mass exhalation rate in (Bqkg-1h-1), M is the mass of soil sample (Kg), V is the residual volume of the chamber (m3) and λe is the radon decay constant. C0 is the Rn222 concentration (Bqm-3) in the chamber volume at ‘t’ = 0.

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3.4. Thoron Surface Exhalation rate

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Rn220 surface exhalation rate was also determined by using a smart RnDuo monitor. Soil

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samples were directly placed in exhalation chamber as discussed above which was connected with pump inlet of the smart RnDuo monitor in a closed loop circuit [12]. The measurement

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cycle was of 15 minutes, of which for initial five minutes pump was ON for the measurement of

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thoron and the background, next five minutes ensured the entire decay of thoron and in last five minute gave the measurement of the background of that cycle. The background count was

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subtracted from the initial 5 min count to get the thoron concentration. The monitoring of thoron

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concentration was carried out for 2 to 3 hours. Rn220 surface exhalation rate (JT) was obtained from the mean equilibrium concentration of thoron (CT) build up in the chamber by using the

𝐶𝑇 𝑉𝜆 𝐴

(2)

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JT =

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following expression [12]:

where CT is the equilibrium concentration of thoron (Bqm-3) and V is the residual air volume of

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the set up (m3), A is the surface area of the sample (m2) and λ is 220Rn decay constant . 3.5. Soil gas Radon

The concentration of soil gas radon was determined by RAD7, semiconductor based detector (Durridge Company, USA). In this method, a hollow steel probe (diameter =3 cm and length = 10 cm) with a disposable sharp edge that can make a steel probe favorable for drilling into the soil, was used. It additionally keeps the dirt (soil) away from hindering the probe. The probe was

ACCEPTED MANUSCRIPT immersed into the ground at a depth of 50 cm where the hard shake basement was located with a gentle stroke of hammer. The probe was associated with RAD7 through a desiccant in the closed loop circuit as shown in Fig. 2c. The detector operates in external relative humidity ranging from 0 to 95% and internal humidity of 0-10% with lower detection threshold of 4 Bqm-3 and an upper

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linear detection limit of 400 kBqm-3 [14]. As the relative humidity reached at 10%, the inlet of

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RAD7 was connected with the stainless probe using plastic tubing through desiccant and made

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the outlet open to the environment. The Grab mode and Sniff protocol was setup for this process. The measurement cycle was about 30 minutes. The soil was sucked through the tube into the

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measuring instrument for 5 min pumping phase. The instrument waits for another 5 min and then

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started the counts for four 5 min cycles. At the end of 30 min period, the RAD7 provide a

5-min cycle measurements [14, 3].

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summary of measurement, including an average radon concentration in the soil-gas from the four

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3.6. Potentially toxic metal analysis

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To estimate potentially toxic metals such as chromium (Cr), copper (Cu), cadmium (Cd),

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zinc (Zn) and lead (Pb), 1 gm of each soil sample was digested in glass digestion tube of 250 ml with 15 ml of aqua regia (HNO3:HCl::1:3) at 80 °C till a transparent solution was obtained [15].

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After digestion, the contents were cooled, filtered and diluted with deionized water up to 20 ml which was used for toxic metal analysis using Microwave induced plasma atomic emission spectrophotometer (MPAES). MPAES was opertated at wavelengths of 425.43 nm, 324.75 nm, 228.80 nm, 213.86 nm, and 405.78 nm, for Cr, Cu, Cd, Zn and Pb analysis, respectively [16]. The working standards were prepared by dilution of certified reference materials (CRMs) i.e 10, 000 mg/L in 5% HNO3 procured from Agilent Technologies, USA, to construct the calibration

ACCEPTED MANUSCRIPT curve for each metal under investigation. The limit of quantification for each targeted metal was set at 0.1 mg/kg. 3.7. Correlation analysis and Principal component analysis (PCA)

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Correlation analysis was performed on the data obtained in the present study including different

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physico-chemical properties, contents of potentially toxic metals, mass exhalation and soil radon

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concentrations to draw associations among these variables, using SPSS Version 16.0 (SPSS Inc., Chicago, USA) and Microsoft Office Excel Version 12.0 (Microsoft Corporation, Washington,

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USA). Correlations were considered significant only if p ≤ 0.05. Further, the data was also

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subjected to Principal component analysis (PCA) to reduce the number of variables which can explain maximum variation in the data. Principal components obtained with Eigen value > 1

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Results and discussion

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were considered for further analysis and used for Varimax rotation and Kaiser Normalization.

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4.1. Physico-chemical parameters

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The outcomes of physico-chemical analysis of soil samples are summarized in Table 1. Soil pH is a vital property which measures its acidity or basicity. pH significantly influences the

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solute concentration and its absorption in the soil and further assures accessibility of fundamental nutrients to plants required for growth and development. pH of all soil samples was estimated in soil extract and all soil samples were found to be alkaline as pH values ranged from 8.2 to 9.0. These outcomes are in conformity with earlier studies reported on the soils of Kano Urban agricultural land [17] and agricultural soils of Vishakapatnam [18]. However, in some studies,

ACCEPTED MANUSCRIPT this pH range was viewed as higher when compared with the ideal pH range of soil for rice cultivation i.e. 5.5-6.5 [19]. Soil electrical conductivity (EC) is a measure of the amount of salts in soil (salinity of soil) and acts as an essential indicator of soil health. It influences crop yields, crop suitability, plant

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nutrient accessibility and activity of soil micro-organisms. It also plays a significant role in the

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key soil processes including the emission of green-house gases such as nitrogen oxides, methane

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and carbon dioxide. EC of all soil samples was found to be ranged from 0.20 to 1.00 mScm-1 (Table 1) which was found to be <4.5 mScm-1 indicating non-saline nature of soils in the study

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area [20].

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Soil organic matter (SOM) is the natural organic matter component of soil, comprising of plants and animal residues at different phases of decomposition; cells and tissues of soil

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organisms and substances synthesized by soil organisms. SOM in the soil samples under

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investigation ranged from 1.91% to 7.85%. The outcomes were in conformity with the earlier studies performed on agricultural soils in Amritsar, Punjab [7]. For ideal agricultural soil, about

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4% to 6% SOM is preferable and 12 soil samples contained SOM in this acceptable range which

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indicated that these soils possess both the physical and chemical properties favorable for growth and development of crops. Only 6 samples out of 22 contained SOM more than 6% which may

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pose enhanced beneficial effects on agricultural soils. On the other hand, 7 samples were found to possess SOM less than 4% and may not serve plentiful chemical, physical and biological benefits. Low organic matter could be attributed to the low retention capacity of sandy loam texture of the soils in the studied region. Carbonates in soils are important constituents of soil mineral matrix and the presence of CaCO3 in calcareous soils has been described as an organic matter stabilization agent mainly due

ACCEPTED MANUSCRIPT to chemical stabilization mechanisms. The carbonate content in the soil gives a direct indication about soil quality and plays a critical role in metal uptake which may lead to metal accumulation in edible parts of the crops grown in that soil. In soil samples under investigation, the carbonates contents were found to vary from 1.80 to 8.08% with mean value of 5.06%, which indicated that

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soils in the study area are slightly calcareous in nature and may adversely affect the availability

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of macronutrients to crops cultivated in these soils. Carbonate content in soils of the study area

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ranged from 1.80 to 8.08% which was comparable to the range observed for the agricultural soils of Punjab around the rivers Beas and Sutlej i.e., 3.81 to 8.31% [21]. Soil carbonate content

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depends on a number of factors such as temporal and geographical placement of soils and human

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activities in the area. Observed carbonate contents were quite high, therefore expected to allow significant accumulation of trace metals in the crops cultivated in agricultural fields of the study

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area. This allows one to conclude that the crops of the study area may be unsafe for consumption

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for the population residing in these villages.

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4.2 Outdoor gamma

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The overall value of outdoor gamma dose rate observed at various sites of Amritsar district is listed in Table 1. The outdoor gamma dose rate was found to be ranged from 0.11 to 0.22 µSvh-1.

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The maximum outdoor gamma dose rate i.e., 0.22 µSvh-1 was observed at Chogawan (A3) and Baniye (A16). The outdoor gamma dose rate observed in Amritsar was compared with the already reported data of Jammu and Udhampur districts adjoining Jammu & Kashmir state [10]. The range of outdoor gamma dose rate of Amritsar district was found to be comparatively lower than the adjoining state. Therefore, outdoor air of the studied region is safe for the population as far as the radioactivity is concerned.

ACCEPTED MANUSCRIPT 4.3. Rn222mass and Rn220 surface exhalation rate The Rn222 mass and Rn220 surface exhalation rates in the soils of the study region have been calculated and listed in Table 1. The Rn222 mass and Rn220 surface exhalation rate varied from 21.41 to 43.81 mBqkg-1h-1 and from 64.73 to 717.03 mBqm-2s-1 with average of 32.58±1.11 195.10±53.02 mBqm-2s-1, respectively. Box Whisker plot to present the

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mBqkg-1h-1 and

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variation of Rn222 mass and Rn220 surface exhalation is shown in Fig. 3a. As the soil particles are

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not uniform, the variation acquired in the outcome might be due to their grain size. As the grain size of particles diminishes, the specific surface area increases which bring about decrease in the

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pore space between grains making wider space available for radon and thoron exhalation from

observed to be

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the surface [22]. In the present investigation, the values of radon mass exhalation rate were higher than those reported in Mohali, Punjab (0.32-2.6 mBqkg-1h-1) [23],

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Southern Punjab (1.15-8.69 mBqkg-1h-1) [24], Kangra district (15.16-35.11 mBqkg-1h-1) [25] and

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Nurpur, Himachal Pradesh (15.16-35.11 mBqkg-1h-1) [26] and Pakistan occupied Kashmir (5.1610.38 mBqkg-1h-1) [27]. On the other hand, the observed values were found to be quite lower

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than those observed in Hamirpur district (9.64-54.12 mBqkg-1h-1) [28], Reasi district (8.38 –

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62.91 mBqkg-1h-1) and Udhampur district (11.57 – 65.62 mBqkg-1h-1) of Jammu &Kashmir (J&K) [3] and Northern Haryana (28.2 – 91.2 mBqkg-1h-1) [29]. The elevated level of mass

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exhalation in Himachal may be due to the leaching of radioactive elements from the uranium enriched zones of the Himachal Himalayas [28]. Similarly, higher radon mass exhalation was reported in Reasi district, may be due to the presence of sedimentary rocks and different geological structures [3]. On the other hand, the elevated levels of radon mass in Northern Haryana may be attributed to the presence of these districts in the footage of Shivalik hills and in the vicinity of the Yamuna river sand bed containing sand mixed soil, both these factors are

ACCEPTED MANUSCRIPT accountable for high radionuclide contents [29]. In this study, the observed values were lower than that of the world average value i.e. 57 mBq Kg-1 h-1, prescribed by United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR 2000) [2].

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4.4. Soil gas radon

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The measurement of soil radon gas concentration in 12 representative sites of the region

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under investigation was carried out and the results so obtained are summarized in Table 2. The recorded Rn222 concentration varied from 106 to 9714 Bqm-3 with an average value of 3212

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Bqm-3, respectively. The variation of soil gas radon at different locations is shown in Fig.3b.

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Some locations (A8, A11, and A13) have shown comparatively higher values as compared to other locations which may be attributed to different soil types, compact packing of

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sandstones/bedrocks after a particular depth, resulting in poor migration of radon gas. The

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primary sources of soil gas Rn222 are uranium, radium and their circulation, penetrability, moisture content and porosity of the soil. The principal sources of uranium and radium in the soil

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are minerals (salts), iron oxide coatings on rocks, soil grains, organic materials, sediments,

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phosphate and carbonate compounds in the soil [30]. These lithological units may possibly be the cause of high soil gas radon concentrations in these areas.

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Table 3 shows a comparison of soil gas radon data of agricultural soils from Amritsar having regional geology with other locations from national and international places which are already available in the literature. The overall results of soil gas radon concentration (0.1 - 9.71kBqm-3) obtained in the present study are comparatively lower than that in Islamabad, Pakistan [31], Upper Siwalik [32], Tusham ring, Haryana [33], Kangra region of Himachal Pradesh [34], Dharamshala [35], Malwa belt of Punjab [36] and Budhakedar of Tehri Garhwal [37]. On the other hand, observed values are higher than those reported in Murree region of Pakistan [31],

ACCEPTED MANUSCRIPT Southern Punjab of Pakistan [24], Hamirpur of Himachal Pradesh [38] and Garhwal Himalayas [39]. The higher value of soil gas radon concentration in Islamabad region of Pakistan may be due to the presence of different geographical formations such as Hangu, Lackhart, Chichali, Lumshiwal and Samana Suk formations [31] while in Upper Siwalik, it may be due to the

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presence of active faults [32]. In Tusham ring of Haryana, it may be due to the presence of

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underlying granite rocks [33] while in Kangra, the presence of transverse Dehar lineament which

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is tectonically active [34] may be responsible. Alluvium geological formations of Dharamshala

4.5. Potentially toxic metal contamination

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be attributed to high porosity of the soil [35].

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have high soil gas concentrations in comparison to other formations of Dharamshala which may

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Potentially toxic metals Zn, Cr, Pb, Cu, and Cd were estimated (in triplicates) in soil samples

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of the study area and results so obtained are summarized in Table 4. Zn, Cu, Pb, Cr contents in soil samples ranged 41.00 to 212.10 mgkg-1, 12.07 to 198.97 mgkg-1, 0.93 to 23.07 mgkg-1 and

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6.03 to 88.90 mgkg-1, respectively while Cd was found at one location (1.07 mgkg-1) only and

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soil samples from other locations found to have Cd content below the detection limit. Fig. 4 displays the contour plots showing the distribution of trace metals in the study area. In the

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present study, on the basis of average values (in mgkg-1) of potentially toxic metals contents observed in soil samples, different metals can be arranged as: Zn (78.93) > Cu (42.32) > Cr (36.92) > Pb (7.49) >> Cd This order shows the presence of higher Zn content in the soil samples in comparison to other toxic metals under investigation. Observed Zn content in soils is also significantly higher in comparison to the typical Zn soil content of 50.00 mgkg-1, given by Aggarwal (2009) [40]. Elevated Zn content in soils can be attributed to calcareous and alkaline nature of soils in the

ACCEPTED MANUSCRIPT study area which allow the chemisorption of Zn by adsorptive calcium carbonates present in the soil [41]. The soils of the study area were also found to be contaminated with copper with an average copper content of 42.32 mgkg-1 which is very higher in comparison to the typical Cu soil content of 20.00 mgkg-1. The main sources of Cu in these soils include an excessive application

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of agrochemicals, especially the use of fungicidal sprays that reach the soil directly or indirectly

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as leaf litter [42]. On the other hand, mean Cr content of 36.92 mgkg-1, was observed in soil

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samples which may be due to the presence of chromium in natural bedrocks but it was found to be below the safe limit of 100 mgkg-1. Pb and Cd are generally known for their toxicity for plants

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and animals [43]. As far as Pb contamination of the soil is concerned, the mean Pb content was

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7.49 mgkg-1 which is less than the safe limit (10.00 mgkg-1) given by Aggarwal (2009) [40] which reduces the probability of occurrence of health problems via lead poisoning to the

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residents of the study area. Further, no Cd content was found in the soil samples except one in

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the considered area (Table 4).

Higher levels of potentially toxic metals were found in the soil samples collected from

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some locations of Amritsar such as A2, A3, A5, A6, A9 and A21 in comparison to other

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agricultural soils which may be due to natural factors such as sedimentation of dust and suspended substances from atmosphere; dry and wet precipitation of other pollutants and

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anthropogenic factors which includes excessive application of agrochemicals; disposal of domestic wastes and vehicular pollution in the rural areas [44]. It can be seen that among all the potentially toxic metal under investigation, 20, 18, 06 out of 22 soil samples were found to have Zn, Cu and Pb contents, respectively above the typical soil concentration limits of 50 mgkg-1, 20 mgkg-1 and 10 mgkg-1, respectively [40]. On the other hand, all 22 soil samples were observed with Cr content below the typical Cr soil concentration

ACCEPTED MANUSCRIPT of 100 mgkg-1. Therefore, it can be concluded that the soil of the study area is significantly suffering from potentially toxic metal pollution due to Zn and Cu which could serve as a source of toxic metal accumulation in edible parts of the crops, i.e food grains: wheat and rice, fruits and vegetables grown in these agricultural fields. On the basis of the above results, it can be

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concluded that Zn and Cu via food crops grown in agricultural fields under investigation may

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pose a health risk to the population residing in these areas.

5. Correlation Analysis and Principal Component Analysis (PCA)

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Correlation analysis was carried out between soil radon, mass exhalation and potentially

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toxic metals to find their dependence on each other and the results so obtained are summarized in Table 5. No correlation was observed between soil radon and mass exhalation. Similarly, no

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correlation of soil radon and mass exhalation with potentially toxic metal contents in soil was

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observed. On the other hand, a positive correlation appears to exist among some trace metals. Correlations between Zn:Cu, Zn:Cr, Cu:Cr, Cu:Pb and Pb:Cr were found to be statistically

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significant as their Pearson’s coefficient were greater than 0.423 (critical value). Cr has a

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geogenic origin but Cu, Zn, and Pb are anthropogenic in nature [45]. Important sources of trace metals such as, Zn, Cu and Pb to agricultural soils could be an excessive application of

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agrochemicals such as phosphate fertilizers, pesticides, fungicidal sprays etc., related to specific agronomic practices and vehicular pollution around the agricultural fields. These results indicated that not only natural processes but anthropogenic activities are also responsible for potentially toxic metal contamination in the study area. A positive correlation was observed between SOM and mass exhalation which indicated that the soil containing high organic content may possess higher affinity for radon [46]. On the other hand, a negative correlation was

ACCEPTED MANUSCRIPT observed between pH and conductivity. Lower the pH (i.e., higher acidity) and higher the conductivity in soil may be due to higher content of H+ ions in the soil. Principal component analysis (PCA) was performed on data obtained from physicochemical analysis, radon and potentially toxic metal estimations in agricultural soils of Amritsar district

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and results so obtained are shown in Figure 5. PCA analysis revealed three components i.e., PCI,

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PCII and PCIII with Eigen values 2.748, 2.227 and 1.658, respectively (Table 6). These

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components explained 66.328% variance of the data (PCI = 26.499, PCII = 20.111 and PCIII = 19.718). PCI comprised of heavy metals such as, Cr, Cu, Zn and Pb. This was supported by the

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positive correlations between these metals (Table 7). PCII constituted of soil radon, SOM and

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mass exhalation. Correlation analysis also revealed positive association between SOM and mass exhalation. Whereas, PCIII comprised of pH, conductivity and carbonates. Negative correlation

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between pH and conductivity has already been discussed above. A positive association between

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pH and carbonates in soil in PCIII could be due to the reason that these anions contribute to the

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alkalinity of soil leading to increase in pH of the soil.

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6. Conclusions

Soil gas radon measurements were carried out in Amritsar district, plain area having

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approximately same topological conditions but with varying physical and chemical composition. The overall soil gas radon concentration ranged from 0.1 to 9.17 kBqm-3. This large variation in radon concentration might be due to compact packing of sandstones and subsoil structure. The relatively higher values of soil gas radon concentration in some locations could be due to the natural occurrence of high radium content in soils or excessive application of agrochemicals. On the other hand, moderate soil gas radon levels indicated the compact soil in underlying bedrocks.

ACCEPTED MANUSCRIPT The results suggested that radon and thoron exhalation rates are emphatically affected by underground soil structures and their physical or chemical properties. Soils in the study area were found to be heavily polluted with the potentially toxic metals such as Zn and Cu followed by some Pb contamination which may be due to anthropogenic

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activities such as injudicious agricultural practices and vehicular pollution in the vicinity of

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agricultural fields. Therefore, Zn and Cu contents should be monitored time to time for the sake

crops grown in the agricultural fields under investigation.

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of public health as they could pose health risks to the population residing in these areas via food

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However, as the potentially toxic metal content and soil radon are not ultimate indicators

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of their availability, mobility and natural radioactivity in soils, further research is required to appraise the pool of potentially available species (e.g., extractable fraction) to determine the

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probability of their transportation from the soil to other ecosystem components (e.g., crops,

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underground water).

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Acknowledgments

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The authors are also thankful to Guru Nanak Dev University, Amritsar for providing the research

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facilities.

Conflict of interest

The authors declare that they do not have any conflict of interest.

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town (central Greece), Environmental Monitoring and Assessment 185 (2013) 9603-9618. [46] R. Kritsananuwat, H. Arae, M. Fukushi, S.K. Sahoo, S. Chanyotha, Natural radioactivity

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survey on soils originated from southern part of Thailand as potential sites for nuclear power plants from radiological viewpoint and risk assessment, Radioanalytical and Nuclear Chemistry, 305 (2015) 487-499.

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Fig. 1. Map showing the studied locations of Amritsar district.

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Fig. 2. Schematic diagrams of (a) setup for radon mass exhalation, (b) Growth of 222Rn in exhalation chanmber and (c) set up for RAD7-soil gas measurements

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Fig.3. Variation of (a) Rn222 mass (mBqkg-1h-1) and Rn220 surface exhalation (mBqm-2s-1) rate in soil samples and (b) Soil gas radon in Amritsar district.

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Fig.4. Contour plots representing distribution of potentially toxic metals in soils of Amritsar district.

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Figure 5. Principal component plot in rotated space for principal component analysis for various physicochemical parameters, potentially toxic metals and radon concentration in soil samples.

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Table 1. Outdoor gamma dose, Exhalation and Physio-chemical parameters of agricultural soils from Amritsar district, India Outdoor gamma Locations

Mass exhalation

Code GPS

(µSvh-1)

(mBqkg-1h-1)

Surface exhalation

Conductivity (mScm-1)

Soil organic matter (%)

Carbonates (%)

0.35

6.8

2.79

0.33

6.0

5.40

8.4

0.37

6.0

5.59

pH

T P

(mBqm-2s-1)

I R

Kohali

A1

N 31°42'10.5" E 74°41'17.7"

0.17±59%

32.85±1.03

109.18±29.01

GNDU backside

A2

N 31°39'45.2" E 74°44'33.5"

0.20±34%

30.06±0.93

231.32±67.09

Chogawan

A3

N 31°42'19.2" E 74°40'20.1"

0.22±53%

40.93±1.19

206.95±70.71

Boparai

A4

N 31°39'11.7" E 74°44'11.8"

0.16±55%

31.11±0.97

91.24±44.62

8.6

0.25

7.85

5.16

Khasa

A5

N 31°37'21.5" E 74°43'56.0"

0.11±47%

32.26±0.97

130.59±48.75

8.9

0.70

4.0

6.42

Naringarh

A6

N 31°37'45.7" E 74°46'15.8"

0.13±46%

32.54±1.16

156.57±52.67

8.9

0.27

5.89

6.04

Dodi pind

A7

N 31°36'31.8" E 74°38'26.8"

0.18±59%

39.24±1.16

116.67±50.58

9.0

0.22

5.80

7.47

Garinda

A8

N 31°36'52" E 74°40'07.8"

0.11±45%

28.29±0.92

137.17±62.08

8.9

0.37

2.00

8.08

GNDU main

A9

N 31°38'1.32" E 74°49'38.1"

0.17±38%

29.68±1.38

141.09±24.96

8.8

0.28

5.77

4.56

Ram tirath

A 10

N 31°40'22.8" E 74°45'00.4"

0.12±44%

39.24±1.47

207.90±69.84

8.8

0.20

7.60

6.70

Bhullar

A 11

N 31°44'24.5" E 74°41'20.5"

0.17±39%

24.38±0.88

154.22±58.88

8.7

0.31

6.00

5.21

Kotla

A 12

N 31°41'3.2"

E 74°44'22.4"

0.13±49%

38.76±1.03

717.03±81.09

8.4

0.70

7.40

5.59

Blaggan

A 13

N 31°42'15.6" E 74°43'42.6"

0.16±49%

36.99±1.16

82.09±39.30

8.5

1.00

5.20

5.88

Attari

A 14

N 31°36'35.0" E 74°37'02.4"

0.14±66%

24.31±0.99

138.76±44.65

8.5

0.78

1.91

1.90

Bhittewad

A 15

N 31°43'41.9" E 74°42'47.5"

0.15±49%

35.44±1.17

193.29±46.47

8.2

0.59

5.00

2.90

Baniye

A 16

N 31°38'54.6" E 74°37'57.5"

0.22±35%

28.86±1.34

64.73±25.46

8.3

0.56

3.91

5.33

T P

E C

C A

D E

U N

A

M

C S

9.0 8.8

ACCEPTED MANUSCRIPT Cheiddan

A 17

N 31°37'09.3" E 74°42'25.5"

0.13±43%

24.01±0.88

258.82±53.41

8.6

0.47

2.20

3.86

RorawalaKhurd

A 18

N 31°38'54.6" E 74°37'52.7"

0.19±45%

21.41±1.08

217.42±54.16

8.4

1.00

4.10

7.54

Lahori Mal

A 19

N 31°36'2.07" E 74°41'33.8"

0.16±47%

33.93±1.17

172.90±50.71

8.5

0.62

5.60

6.72

Kathanian

A 20

N 31°37'59.2" E 74°45'15.1"

0.17±44%

25.65±1.27

113.75±46.44

8.0

0.83

6.81

3.10

Ranike

A 21

N 31°36'24.3" E 74°44'48.5"

0.13±43%

43.08±1.10

416.46±85.90

8.6

0.78

3.74

1.80

Pallan

A 22

N 31°36'52.7" E 74°37'45.1"

0.18±36%

43.81±1.22

167.56±59.71

0.28

7.80

3.27

C S

I R

A

U N

D E

T P

C A

E C

M

T P

8.8

ACCEPTED MANUSCRIPT Table 2. Soil gas radon concentration in selected locations of Amritsar district. Code

Soil gas radon (Bqm-3)

GPS N 31°42'10.5"

E 74°41'17.7"

598±124

A3

N 31°42'19.2"

E 74°40'20.1"

106±45

A5

N 31°37'21.5"

E 74°43'56.0"

240±29

A8

N 31°36'52"

E 74°40'07.8"

A 10

N 31°40'22.8"

E 74°45'00.4"

A 11

N 31°44'24.5"

E 74°41'20.5"

7425±324

A 13

N 31°42'15.6"

E 74°43'42.6"

7950±191

A 14

N 31°36'35.0"

E 74°37'02.4"

961±105

A 16

N 31°38'54.6"

E 74°37'57.5"

4177±299

A 17

N 31°37'09.3"

A18

N 31°38'54.6"

A 21

N 31°36'52.7"

IP

CR

US

AN

9714±402 628±132

E 74°42'25.5"

2752±363

E 74°37'52.7"

2054±345

E 74°37'45.1"

1942±142

M

ED PT CE AC

T

A1

ACCEPTED MANUSCRIPT

Range (kBqm-3)

References

Islamabad, Pakistan

17.34 – 72.52

[31]

Murree, Pakistan

0.61 – 3.89

[31]

Southern Punjab, Pakistan

0.42 – 3.56

Budhakedar, Tehri Garhwal, India

1.10 – 31.80

Hamirpur district, HP, India

0.03 – 2.28

Garhwal Himalaya, India

0.01 – 2.33

Upper Siwaliks, India

11.50 – 78.47

[32]

Malwa belt, Punjab, India

1.90 – 16.40

[36]

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Amritsar, India

IP CR

[24]

US

[37]

ED

AN

CE

Dharamshala, HP, India

PT

Tusham ring, Haryana, India Kangra district, HP, India

T

Regions

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Table 3 Comparison of soil gas radon in study region and neighboring areas.

[38] [39]

42.80 – 71.50

[33]

1.10 – 82.20

[34]

13.60 – 110.80

[35]

0.10 – 9.71

Present study

ACCEPTED MANUSCRIPT

Zn (mgkg-1)

Cd (mgkg-1)

Cu (mgkg-1)

Pb (mgkg-1)

Cr (mgkg-1)

A1

70.33±1.17

1.07±0.12

69.30±0.25

23.03±0.32

44.17±0.12

A2

54.23±0.67

BDL

41.03±0.20

12.13±0.35

43.13±0.13

A3

67.07±0.52

BDL

144.23±0.39

11.90±0.10

73.17±0.17

A4

75.80±0.99

BDL

30.43±0.38

9.83±0.38

43.17±0.12

A5

43.33±0.88

BDL

12.07±0.35

BDL

7.10±0.15

A6

111.67±0.88

BDL

36.83±0.38

17.13±0.13

72.30±0.35

A7

75.83±0.73

BDL

28.23±0.23

9.23±0.23

41.33±0.33

A8

53.43±0.43

BDL

36.90±0.10

6.23±0.23

32.27±0.27

A9

61.90±0.80

BDL

30.33±0.77

1.90±0.10

19.83±0.17

A 10

41.00±0.29

BDL

18.03±0.09

3.13±0.13

6.03±0.15

A 11

54.40±0.72

BDL

50.17±0.12

9.20±0.12

31.97±0.09

A 12

212.10±1.13

BDL

198.97±0.55

12.03±0.15

84.17±0.27

A 13

124.30±0.21

BDL

25.10±0.38

6.93±0.12

41.17±0.38

A 14

81.57±0.99

BDL

23.33±0.44

5.07±0.18

88.90±0.10

A 15

60.27±0.59

BDL

21.97±0.15

2.07±0.07

13.17±0.27

A 16

142.37±0.58

BDL

21.30±0.21

4.10±0.10

65.97±0.20

A 17

69.03±0.38

BDL

28.17±0.12

0.93±0.07

7.00±0.12

54.37±0.27

BDL

26.27±0.13

7.93±0.07

32.17±0.33

69.93±0.46

BDL

24.17±0.27

8.87±0.13

30.07±0.18

A20

92.73±0.48

BDL

36.07±0.18

12.20±0.20

19.03±0.03

A 21

60.17±0.12

BDL

15.17±0.12

BDL

8.07±0.07

A 22

60.70±0.51

BDL

13.07±0.18

0.96±0.03

8.10±0.10

Min.

41.00±0.29

1.07±0.12

12.07±0.35

0.93±0.07

6.03±0.15

Max.

212.10±1.13

1.07±0.12

198.97±0.55

23.03±0.32

88.90±0.10

A 19

CR

US

AN M

ED PT

CE

AC

A 18

T

Code

IP

Table 4. Potentially toxic metal contents estimated in agricultural soil samples from Amritsar district.

ACCEPTED MANUSCRIPT Mean

78.93±0.063

-

42.32±0.04

7.49±0.02

36.92±0.04

Aggarwal, 2009 [40]

50.00

0.06

20.00

10.00

100.00

AC

CE

PT

ED

M

AN

US

CR

IP

T

*BDL = Below detection limit

ACCEPTED MANUSCRIPT Table 5. Correlation coefficient matrix for radon concentration, potentially toxic metal contents and physicochemical parameters of agricultural soils of Amritsar district.

Mass exhalation

Soil radon

pH

Conductivity SOM

T P

Carbonates

Zn

Cu

I R

Mass exhalation

1

Soil radon

-0.31

1

pH

0.19

0.1

1

Conductivity

-0.21

0.11

-0.62

SOM

0.46

-0.42

0.05

Carbonates

-0.06

0.39

T P

Zn

0.12

Cu Pb

C S

1

D E

U N

A

M

-0.39

1

0.29

-0.16

0.03

1

0.04

-0.36

0.29

0.16

0.01

1

0.23

-0.13

-0.18

-0.02

0.31

0.08

0.59

1

-0.09

-0.09

0.11

-0.17

0.41

0.11

0.28

0.47

E C

C A

Pb

33

1

Cr

ACCEPTED MANUSCRIPT Cr

-0.08

0.02

-0.13

0.05

-0.02

0.08

0.64

Underlined values are significant at p ≤ 0.05

T P

I R

C S

A

U N

D E

M

T P

E C

C A

34

0.58

0.54

1

ACCEPTED MANUSCRIPT Table 6. Principal component analysis for various physicochemical parameters, potentially toxic metal contents and radon concentration in agricultural soil of Amritsar district. Initial Eigenvalues

Extraction Sums of Squared Loadings

Principal component Total % of Variance Cumulative %

Total

PCI

2.748

27.476

27.476

2.748

27.476

27.476

PCII

2.227

22.272

49.748

2.227

22.272

PCIII

1.658

16.581

66.329

1.658

16.581

Rotation Sums of Squared Loadings

% of Variance Cumulative %

M

I R

26.499

26.499

49.748

C S

2.011

20.111

46.611

66.329

1.972

19.718

66.328

*Extraction Method: Principal Component Analysis with Varimax rotation and Kaiser normalisation.

D E

T P

E C

C A

35

% of Variance Cumulative %

2.650

U N

A

T P

Total

ACCEPTED MANUSCRIPT Table 7. Rotated component matrix of principal component analysis for various physicochemical parameters, potentially toxic metal contents and radon concentration in soil samples

T P

Principal components Parameter PCI

PCII

Cr

0.850

-0.134

Cu

0.814

Zn

0.779

Pb

0.708

Soil Radon

Mass exahalation pH Conductivity

A

-0.051 -0.045 -0.346

0.107

0.298

T P

-0.783

0.155

0.282

0.743

0.258

0.026

0.708

0.110

-0.184

0.042

0.843

0.048

-0.341

-0.785

E C

C A

0.224

PCIII

0.018

0.027

SOM

U N

C S

I R

D E

M

36

ACCEPTED MANUSCRIPT 0.201

Carbonates

-0.384

0.574

#

Extraction Method: Principal Component Analysis with Varimax rotation and Kaiser Normalisation.

T P

I R

C S

A

U N

D E

M

T P

E C

C A

37

ACCEPTED MANUSCRIPT

Assessment of radon and potentially toxic metals in agricultural soils of Punjab, India Inderpreet Kaura*, Akash Guptaa, Bhupinder Pal Singhb, Sumit Sharmac, Ajay Kumarcc Department of Chemistry, Centre for Advanced Studies,Guru Nanak Dev University,

Amritsar - 143001, Punjab, India.

*Email: [email protected]

T

a

Centre for I.T. Solutions, Guru Nanak Dev University, Amritsar - 143001, Punjab, India.

c

Department of Physics, DAV College, Amritsar143001, Punjab, India

CR

IP

b

US

Highlights

Natural radioactivity and toxic metal contamination in soils of Punjab were examined.



Abnormal distribution of soil radon at some locations observed.



No dependence of soil radon on exhalation and potentially toxic metals.



A positive correlation was observed among some metals such as Zn:Cu, Zn:Cr, Cu:Cr, Cu:

M

AN



CE

PT

A positive correlation was observed between soil organic matter and mass exhalation.

AC



ED

Pb and Pb:Cr.

38

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5