Occurrence, distribution and health risk assessment of potentially toxic elements in soil: A case study from semi-arid region of southwest Punjab, India.

: Numerous groundwater studies exist on the heavy metal (HM) pollution in Punjab, however, research 10 is quite limited in the soil system of the region. In the current work, detailed study has been carried out on the 11 distribution of toxic metals in the agricultural and barren soil (AS & BS) samples in the semi-arid region of Punjab. 12 Pollution level of HMs was determined by using various pollution indices like geo-accumulation index (I geo ), 13 enrichment factor (EF), pollution index and pollution load index (PLI), and potential ecological risk assessment 14 (PERI). A total of 11 toxic elements were measured and 2 fold values were observed in the AS samples. Range 15 of HMs in the AS samples were 3.9 – 28.1, 29.2 - 90, 6.7 – 32.8, 4.2 – 65.6, 13019.5 – 43900, 95.7 – 553, 13.1 – 16 42.1, 16.5 – 25.2, 82 – 267, 25 – 78.6, 25.4 – 131.8 mg/kg for As, Cr, Cu, Co, Fe, Mn, Ni, Pb, Sr, V and Zn 17 respectively. It is seen that there is high variability in the spatial distribution of metals throughout the region 18 indicating the role of anthropogenic activities. Pollution levels reveal the studied region to be moderately 19 contaminated in terms of anthropogenic pollution. Multivariate statistical analysis results also revealed that the 20 majority of HM pollution in the region is due to anthropogenic activities with few elements to be of geogenic 21 origin. Potential health risks assessment is carried out and total hazard index (HI) values were in the acceptable 22 range but the total cancer risks were comparatively higher for children than adults. This study shows the risk of 23 heavy metal contamination in the agricultural regions and the results obtained have global implications.


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Soil is considered to be a fundamental basis for human survival, habitation, and social development (Wei-Xin et 28 al., 2008; Wong et al., 2002). From the last few decades with the rapid expansion of industrialization and 29 anthropogenic activities, heavy metals (HMs) presence in the surface sediments have emerged as a matter of 30 global concern and is growing progressively. This is due to the high toxicity and non-biodegradable nature of 31 heavy metals and once they enter the soil system it takes a long time to eliminate (Alloway, 2012). In general, 32 metals whose density is greater than 5gm/cm 3 are categorized as heavy metals in the environment (Darwesh & anthropogenic activities are accountable for the existence of the heavy metal in nature. Anthropogenic events such 45 as application of pesticides, fertilizers and waste disposal, play a key role in the accumulation of these metals in   better management policies and to protect the human beings exposed to higher levels of contaminants.

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Study area

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The study region lies in the Punjab state of north-western India (Fig. 1). It is among one of the most productive 71 agricultural regions and has been contributing about 25-50% of rice and 38-75% of wheat to the food reserves of 72 India during the last four decades (Singh & Park, 2018). Geographically, the Punjab state is divided into 3 zones 73 i.e. Malwa, Majha and Doaba. Current study region covering an area of 1351.7 km 2 belongs to the Barnala district 74 of Malwa region (30º to 30º52'N; 75º15' 75ºE) and has a population of 190685 as per census of 2011. Agriculture 75 is the main occupation of people with wheat, paddy and cotton as main crops grown in the area. Two well-known 76 industries of textiles and combines is also present in the urban part of the district. Climate of the region is generally 77 semi-arid with an average rainfall of 420mm (CGWB, 2017). Geologically area forms part of Indo-Gangetic 78 alluvial deposits of quaternary age (Singh et al., 2015) and is mainly composed of sands of various grades, clays 79 and silts. Kankar in the form of sheet deposits or in nodular form is present at the depth of 0.75 cm to 2 m below 80 the surface. Soil of the district is loamy sand along with sandy loam kaller spotted at few places (CGWB, 2017).

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For the present work 17 sampling sites were selected from the agricultural land and barren areas on the basis of 85 the systematic sampling strategy. Each sample was collected on the basis of grid pattern covering the entire 86 district. Barren areas are those that has been largely unaffected by any of the anthropogenic activity. Detailed 87 information of sampling sites as per their land-use is given in Fig. 1 & Table 1 along with their geographical   88 coordinates. Composite soil sample of around 1 kg was collected from each site using a stainless steel auger.

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Collected samples were tightly packed in double zip plastic bags (pre cleaned with double distilled water) and 90 transported to the laboratory. Sample preparation is carried out by following the EPA method 3050B (EPA, 1996).

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Samples were oven dried in the laboratory until all the moisture content was driven out. After complete drying 92 each sample was passed though 2mm polyethylene sieve to remove any of the plant roots or other coarser material 93 present. Subsample of the dried soil was homogenized by using mechanical agate mortar and fine particles (<200 94 µm) were obtained for heavy metal analysis.

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Strong acid digestion procedure (HNO3: HCL: HF) was employed for the complete decomposition and extraction 96 of HMs on a hot plate. After the complete digestion, samples were filtered and final volume of 50 ml was obtained 97 with ultra-pure distilled water. The HM analysis was carried out by using triple quadropole inductively coupled 98 plasma mass spectrophotometer (ICP-MS). High grade reagents, pre-cleaned glassware and ultra-pure de-ionised 99 water was used for the analysis. For quality assurance and quality control (QA/QC) purpose certified reference

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Geochemical baseline plays vital role in identifying the natural and anthropogenic sources of these metals in soil.

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Due to the lack of background data in this study region, metal concentration of barren land is utilized as the 121 baseline concentration in this study. These are the sites unaffected by any of the urban, industrial or agricultural 122 activities and thus can be utilized for measuring pollution levels.

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Geo-accumulation index is widely used to quantify the extent of environmental pollution. It reflects the amount 125 of heavy metal absorption in soil and was determined by using the following equation (Muller, 1979).
Where n is the number of heavy metals studied and Cf represents the contamination factor of single metal.

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Pollution levels are classified into four classes based on PLI values (Table 2).

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It defines the threat of multi-metals to organisms in contaminated areas (Hakanson, 1980). This expression not 156 only defines the HM pollution status in the sediments but also provides cumulative effect of environment and 157 ecology with toxicity. PERI is expressed as: Where Cf represents the contamination factor of metal, Tr is the biological toxicity response factor and is taken as 161 5 for Cu, Pb and Ni, 1 for Zn, 2 for Cr, V 10 for As (Hakanson, 1980;Zhang et al., 2014;Zhu et al., 2013). Er i 162 indicates the potential ecological risk of each metal and RI is the combined effect of studied metals. RI is classified 163 into four types on the basis of ecological risk: Low ecological risk (<150), Moderate risk (150-300), Considerable 164 risk (300-600) and very high risk (>600) (Hakanson, 1980).

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Health Risk Assessment

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Health risk assessment is calculated to determine the health risk posed to human beings through the exposure of 167 different metals (NRC, 1983). In the current study Hazard quotient (HQ) and cancer risk (CR) were calculated to Where Sf is the key factor called carcinogenic slope factor (mg/kg/day) for single metal of concern. Sf Ingestion (As

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Hotspot areas of contamination for the studied heavy metals were demarcated by using spatial distribution maps.

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Zn and Sr however significant enrichment (EF>5) is seen in all the samples of Cr with EF value ranging from 6-266 12 (Fig. 4b). Generally the input source of particular metal is considered to be crustal if its EF value is between     As, accounting for 47.3% in adults and 47% in children of the total HI value, followed by Cr>Pb>Ni>Cu>Zn via 302 all the three pathways (Fig. 5). The computed HI values also follows the same order and are below 1 for all the 303 metals indicating negligible non-carcinogenic risk of heavy metals in the study region (USEPA, 2001). However 304 younger age group is at greater health risk with HI values an order of magnitude higher than that of adults. Thus 305 heavy metals in soil poses negligible non-carcinogenic health effect to the exposed population.  On the other hand, slope factor is only available for As, Pb and Ni thus, carcinogenic health risk assessment is 313 carried out for these three heavy metals only and results are shown in Table 8