Introduction

South Korea has been developed tremendously for not only industrial sector but also agricultural area during the last five decades. Many cultivated lands were converted into industrial complexes where farming is still ongoing near those industrial sites. Therefore, operating industrial business around farming area may influence detrimental effects on the farmland and environment (Kim et al., 2003; Jung et al., 2005; Lee et al., 2010). Soil is a limited essential medium for agricultural production. Soil contamination is one of the most important issues for food safety (Kong, 2014). Monitoring heavy metal concentrations in agricultural soils can help policy makers to improve agricultural environment and manage pollution sources better (Kloke, 1979; Sutherland, 1999; Wu et al., 2010; Ahmadi et al., 2017).

Pollution sources are industrial effluents, sewage discharge, dust, and industrial wastewater (Kim et al., 1996; Lambert et al., 2000; Jung et al., 2005; Lee et al., 2010). The common pollution issues in agricultural soils near industrial complexes is from either wastewater or solid waste input during the industrial process (Chen et al., 2005). They may negatively influence on the environment that result in critical issues on soil, air, water and groundwater pollution (Lambert et al., 2000). Heavy metals are the most environmental pollutants in agricultural fields. Existing heavy metals in the agricultural soil can be negatively influence on crops and easily transferred into food chains and it resides in plants that enter the human body when consumed (Johansson et al., 1988; Jarup, 2003; Duruibe et al., 2007; Jolly et al., 2013). The human health is highly related with agricultural products and agricultural field quality (Jarup, 2003; Akoto et al., 2008). Also, consuming food containing certain levels of heavy metals have detrimental effects on human body such as the heart, liver, kidney and serious health disorders such as gastrointestinal cancer for a long period for time (Jarup, 2003; Duruibe et al., 2007; Jolly et al., 2013). For example, arsenic (As) is one of the most hazardous elements for human health that can result in serious health disorders such as epigenetics, skin neoplasm and cancer along with their respective system (Duruibe et al., 2007; Jolly et al., 2013; Mohammed Abdul et al., 2015; Prakash and Verma, 2021; Jeong et al., 2022).

In South Korea, Ministry of Environment (MOE) has set up the levels of Soil Contamination Warning Standard (SCWS) for cultivated lands (Region 1) (25, 4, 150, 100, 200, and 300 mg kg^-1 for As, Cd, Cu, Ni, Pb, and Zn, respectively. The objective of this study was to monitor heavy metal concentrations in agricultural soils near industrial areas in Jeon-Nam province, Korea.

Materials and Methods

Site description and soil sample

Soil samples (0 - 15 cm depth for top soil and 15 - 30 cm depth for sub soil) were collected at 10 agricultural fields each near 15 industrial complexes each in 15 counties (Yeongam, Muan, Yeonggwang, Hampyeong, Gangjin, Wando, Haenam, Jindo, Jangseong, Damyang, Boseong, Goheung, Hwasun, Gokseong, and Gurye) in Jeon-Nam, Republic of Korea in 2020 (Table 1). Ten agricultural fields near each industrial complex were randomly chosen, with four directions (east, west, south, and north) within 0 - 500 meter radius for five agricultural fields and 500 - 1,000-meter radius for another five agricultural fields. Soil auger was used for collecting five soil cores per depth for each agricultural field (Eijelkamp, Gempler’s, Giesbeek, Netherlands), thoroughly mixed, and stored in a plastic bag. Total three hundred soil samples were collected (10 agricultural field for two depths at 15 industrial complexes) (Table 1). These collected soil samples were air-dried and sieved with a 2-mm sieve before heavy metal analysis (MOE, 2016).

Heavy metal analysis preparation

Dried soil samples were finely ground with Zirconia balls (DAIHAN, Korea) by mill machine at 10,000 rpm. Three grams of soils were prepared and mixed with 21 mL of hydrochloric acid (HCI) and 7 mL of nitric acid (HNO₃), then heated for 2 hours at 120°C on the heating plate (Block Heating Sample Preparation System, US/MHB-250, Ctrl-M Science, USA). After the wet digestion, total volume of digested samples was filled up to 100 ml, then filtered through No. 2 filter paper (Advantec No.2, 110 mm). Digested soil samples were analyzed for heavy metals, such as As, Cd, Cr, Cu, Ni, Pb, and Zn (MOE, 2016) using an ICP-OES (Optima 7300DV, Perkin Elmer, USA). The ICP grade multi-element standard solutions for Cd, Cr, Cu, Ni, Pb, and Zn (Inorganic Ventures, USA) and single-element standard for As (Kriat Co., Ltd., Korea) was used. Individual heavy metal standard solutions were prepared. In detail, single-element standard As concentrations were 0.1, 0.5, 1, 5, 10, 20, 50 and 100 mg kg^-1 and multi-element standard (Cd, Cr, Cu, Ni, Pb and Zn) were 0.1, 0.5, 1, 5, 10, 20, 50 and 100 mg kg^-1, respectively. For quality control, a batch of 20 samples were used. Each batch consists of a blank, control (lab soil standard), BAM (BAM-U112a) (Federal Institute for Materials Research and Testing, Germany), a soil sample, duplicate the first soil sample, and rest of soil samples.

Soil contamination index and soil pollution index analysis

For evaluating soil quality, pollution index (PI) and pollution load index (PLI) were used for soil contamination. Hakanson (1980) reported the PI using the modified formula below.

where, Cn is the concentration of the examined element in the soil, Bn is the geochemical background value. The background values were used from a study and the PI values are classified into 4 classification index (Table 2). For this study, the background values were from 2070 paddy rice fields (Yun et al., 2018; Lee et al., 2019). The background values serve to assess contamination by comparing current and pre-industrial concentration of heavy metal concentrations (Suwanmanon and Kim, 2021). The averaged background concentrations of As, Cd, Cu, Ni, Pb, and Zn were 3.68, 0.22, 14.9, 14.1, 16.7, and 56.8 mg kg^-1, respectively.

The contamination level of heavy metals was assessed by PLI. The pollution load index was evaluated for the extent of the heavy metal pollution. Where n is the number of heavy metals studied that is 7 elements. The PLI was calculated using the modified formula based on Angulo (1996) and Jorfi et al. (2017) below. The PLI values are classified into 4 classification index (Table 3).

Statistical analysis

All experiments data were statistically analyzed using Microsoft Office program (Excel version 2010, Microsoft, USA) for general statistics, such as average, minimum (min.), maximum (max.), standard deviation (stdev.), coefficients of variation (C.V. (%)), kurtosis, and skewness based on for top soil (0 - 15 cm depth) and sub soil (15 - 30 cm depth). The measurements were expressed in term of means and standard deviation.

Results and Discussion

Assessment of heavy metals in soil

The summarized statistics of heavy metal concentration in 300 agricultural soils near the industrial complexes in Jeon-Nam province are shown in Table 4. Heavy metals, such as As, Cd, Cr, Cu, Ni, Pb, and Zn, respectively were observed in collected agricultural soils near industrial complexes. Total averaged heavy metals concentrations for top and sub soils were 4.5, 4.8 mg kg^-1 for As, 0.6, 0.6 mg kg^-1 for Cd, 27.7, 27.7 mg kg^-1 for Cr, 13.9, 13.6 mg kg^-1 for Cu, 12.8, 13.1 mg kg^-1 for Ni, 13.3, 13.7 mg kg^-1 for Pb, and 96.9, 95.2 mg kg^-1 for Zn, respectively (Table 4). The warning levels of SECL are 25, 4, 150, 100, 200, and 300 mg kg^-1 for As, Cd, Cu, Ni, Pb, and Zn, respectively (MOE, 2016). Therefore, heavy metal concentrations in all 300 soil samples were not higher than the levels of Soil Contamination Warning Standard (SCWS) for cultivated lands under the regulations of Soil Environment Conservation Law (SECL), Ministry of Environment (MOE) in Korea.

The averaged heavy metal concentrations for top and sub agricultural soils at each the industrial complex in Jeon-Nam province in Korea was listed in Table 5. Heavy metals were observed in monitored soils and they are As, Cd, Cu, Ni, Pb, and Zn, respectively. The highest As concentration was shown at Haggyo followed by Jukcheong and Hwawon agricultural Industrial Complex. The As concentrations for top and sub soils were 7.7 and 8.6 mg kg^-1 at Haggyo, 6.9 and 7.1 mg kg^-1 at Jukcheong, and 6.2 and 6.4 mg kg^-1 at Hwawon, respectively. The Cd concentrations for top and sub soil were 0.8 and 0.9 mg kg^-1 at Maryang, 0.8 and 0.8 mg kg^-1 at Jukcheong, respectively. The greatest Cr, Cu, and Ni concentrations for top and sub soil were 67.8 and 61.4 mg kg^-1 at Seokgok, 42.7 and 46.0 mg kg^-1 at Beolgyo, and 17.2 and 17.8 mg kg^-1 at Jukcheong. The highest Pb and Zn concentrations for top and sub soil were 27.1 and 31.8 mg kg^-1 at Maryang, 21.8 and 21.7 mg kg^-1 at Jukcheong, and 18.1 and 17.7 mg kg^-1 at Pungyang. The highest Zn concentrations for top and sub soil were 143.9 and 142.6 mg kg^-1 at Jukcheong, followed by 135.9 and 143 mg kg^-1 at Maryang, and 109.8 and 106.1 mg kg^-1 at Mujeong. Although heavy metals were observed at studied agricultural soils, the heavy metal concentrations did not exceed the SCWS level for cultivated lands, which is regulated by SECL, MOE in Korea.

Contamination index of heavy metals in soil

Two pollution indices, PI and PLI (Eqs. 1, 2) were used to evaluate and classify the contamination level for studied soils. Pollution index (PI) and pollution load index (PLI) for top and sub agricultural soils near the industrial complexes in Jeon-Nam province in Korea was listed in Table 6. The calculated PI values using seven heavy metals ranged from 0.1 to 0.7 for top and sub soils. The contamination level of heavy metal using pollution index was classified into Group 1: PI < 1 (uncontaminated) (Table 2) which is not polluted for any heavy metals (Wu et al., 2014; Mohammadpour et al., 2016; Jorfi et al., 2017). Kim et al. (2016) reported that PI ranged from 0.3 to 0.5 for top and sub soil. For this study, the highest PI values of Cr were 0.7 and 0.6 for top and sub soils at Seokgok and the lowest PI values of Cr were 0.1 and 0.1 at Mujeong. The PI values ranged from 0.1 to 0.4 for As, from 0.1 to 0.3 for Cd, from 0.1 to 0.7 for Cr, from 0.1 to 0.2 for Cu, from 0.1 to 0.6 for Ni, from 0.1 to 0.3 for Pb, and from 0.2 to 0.5 for Zn (Table 6). These results suggested that collected agricultural soils in this study near industrial complexes in Jeon-Nam province were not contaminated by heavy metals based on PI index.

The PLI for all heavy metals ranged from 0.14 to 0.30 for sub and top soils (Table 6). The pollution load index (PLI) was classified into Group 2: 0 < PLI ≤ 1 (unpolluted) (Table 3) (Jorfi et al., 2017). Suwanmanon and Kim (2020) reported that PLI ranged from 0.1 to 0.2 for top and sub soil. For this study, the highest PLI value for top and sub soils was 0.3 and 0.3 at Jukcheong that were less than 1. Overall, monitored agricultural soils for this study near industrial complexes in Jeon-Nam province were unpolluted by heavy metals.

Conclusions

Three hundred agricultural soil samples near industrial complexes in Jeon-Nam province in 2020 were collected and analyzed for seven heavy metals, including arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn) to evaluate soil pollution, PI, and PLI. Overall, heavy metal concentrations for this study were below the SCWS under the regulations of SECL for cultivated lands, MOE in Korea. Two pollution indices, PI and PLI were tested. The PI value of all seven heavy metals ranged from 0.1 to 0.7 that was classified into Group 1: PI < 1 (uncontaminated) and PLI ranged from 0.14 to 0.30 that was classified into Group 2: 0 < PLI ≤ 1 (unpolluted). PI and PLI indicated that monitored soils near industrial complexes in Jeon-Nam province in 2020 are unpolluted. Further research is needed to continue to investigate heavy metal concentrations at other agricultural soils around industrial complexes under the regulations of SECL, MOE in Korea.