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ISSN : 1225-6692(Print)
ISSN : 2287-4518(Online)
Journal of the Korean earth science society Vol.42 No.4 pp.412-424
DOI : https://doi.org/10.5467/JKESS.2021.42.4.412

# Distribution of Heavy Metals in Sediment Cores Collected from the Nakdong River, South Korea

Ntahokaja Magalie1,2, Jiyeong Lee1,2, Jihye Kang2, Jeonghoon Kim1,2, Ho-Jin Park2, Sang Yeol Bae1,2, Seok Jeong1,2, Young-Seog Kim1,2, Jong-Sik Ryu1,2*
1Division of Earth Environmental System Sciences, Pukyong National University, Busan 48513, Korea
2Department of Earth and Environmental Sciences, Pukyong National University, Busan 48513, Korea

Both equally contributed to this study.

*Corresponding author: jongsikryu@pknu.ac.kr Tel: +82-51-629-6624, Fax: +82-51-629-6623
July 24, 2021 July 30, 2021 July 30, 2021

## Abstract

Understanding the distribution of heavy metals in sediment is necessary because labile heavy metals can partition into the water column and bioaccumulate in aquatic organisms. Here we investigated six heavy metals (Co, Cu, Mn, Ni, Pb, and Zn) in sediment cores using a five-step sequential leaching method to examine the occurrence of heavy metals in the sediment. The results showed that all elements, except Mn, are depleted in the exchangeable and carbonate fractions. However, heavy metal concentrations are much higher in the Fe-Mn oxide and organic matter fractions, especially for Cu, indicating enrichment in the organic matter fraction. Furthermore, contamination parameters (contamination factor and geoaccumulation index) indicate that Mn contamination is high, primarily derived from anthropogenic sources, presenting a potential risk to ecosystems in the Nakdong River.

## Introduction

Heavy metals sorbed onto the sediment are one of the most serious environmental issues due to their high toxicity, high persistence, and rapid accumulation in the living organisms (Guevara-Riba et al., 2004). In recent, the contents of heavy metals in the sediment have increased with the urbanization and population growth, exceeding the natural background contents (Guevara-Riba et al., 2004;Lee et al., 2020). Heavy metals released from the highly contaminated sediments eventually enter the food chain, resulting in adverse effects on the human health, biodiversity and environmental quality. It is difficult to determine the source of heavy metals in the sediments because they are originated from both natural and anthropogenic sources (Idris et al., 2007). In a natural environment, heavy metals commonly exist in the mineral as stable oxide forms but under anthropogenic influences, they may occur as exchangeable, carbonate, organic and oxide forms (Passos et al., 2010). Therefore, it is necessary to evaluate the occurrence and content of heavy metals in each fraction in order to fully understand their actual and potential environmental effects (Tessier et al., 1979;Bordas and Bourg, 1998;Cook and Parker, 2006).

Although the sequential extraction procedures are quite variable using various reagents under defined experimental conditions, the five-step method initiated by Tessier et al.(1979) has been commonly used to distinguish fractional heavy metals in the sediments; (1) exchangeable fraction, (2) carbonate fraction, (3) Fe-Mn oxides, (4) organic matter, and (5) residue. Then, with the results from sequential leaching the impact assessment of the heavy metals in the sediments can be quantified using contamination factor (CF), geoaccumulation index (Igeo), and enrichment factor (EF) (Medici et al., 2011). For example, Lee et al., (2020) calculated EF and Igeo values of six heavy metals in the Nakdong estuarine sediments and indicated only Cd was contaminated with EF values of 1-3.

The Nakdong River as the longest river in South Korea drains agricultural, residential, and industrial areas and therefore is highly vulnerable to heavy metal contaminations. With this reason, although many studies have examined heavy metals in the Nakdong River sediments (Kim et al., 2015a, 2015b;Kim et al., 2020;Lee et al., 2020), they measured total contents of heavy metals in the bulk surface sediments. Here, we collected the sediment cores and conducted the sequential leaching in order to examine the vertical variations and pollution intensity assessment of heavy metals in the Nakdong River.

## Material and Methods

### Study area

The Nakdong River is approximately 525 km in length with a basin area of approximately 23,389 km2. It faces the southeastern coastal area of the Korean peninsula passing through the metropolitan city (Busan), which is a major water supply for domestic water (23.8%), agricultural activity (60.3%), and industry (15.9%) (Jung, 2009). The mean annual precipitation is 1,150 mm, and most precipitation (60-70%) occurs during the summer monsoon and typhoon seasons. To manage the water supply and seawater intrusion, the estuary dam was constructed at the mouth of the river in 1987 (Jeong et al., 2007;Lee and Kim, 2018). Many industrial complexes are located around the Nakdong River estuary. Furthermore, a wastewater treatment plant constructed in 1990 has discharged an effluent into the Nakdong River estuary.

### Sampling and sequential leaching

Four sediment cores (YFs 1-3 and YDF1) were collected from the Nakdong River in October 2020. The YFs 1-3 cores were vertically drilled but YDF1 core was drilled at 80° (Fig. 1). Total core depths are 100 m for YFs and 250 m for YDF1. All cores were sectioned at 1 m interval, put in tubes and kept in boxes. After prescreening using the Vanta handheld XRF (Olympus, Japan), the samples were taken and dried in the oven at 60°C.

According to the sequential extraction scheme (Tessier et al., 1979), the exchangeable fraction defines the heavy metals in weak binding sites particularly onto the surfaces, the carbonate fraction does them in carbonate minerals, the Fe-Mn oxides fraction does them in easily reducible Mn oxides and amorphous Fe oxyhydroxides, the organic matter fraction does them in oxidizable species including iron sulfides, and the residue does them in the silicate minerals. In order to distinguish the distribution of different heavy metal fractions in the sediment samples, we conducted a five-step leaching and digestion (Tessier et al., 1979); (1) magnesium chloride (MgCl2) leach (exchangeable fraction), (2) sodium acetate (NaOAc) leach (carbonate fraction), (3) hydroxyl amine hydrochloride (NH2OH-HCl) leach (Fe-Mn oxide leach), (4) mixture of HNO3 and H2O2 leach (organic matter leach), and (5) complete digestion of the silicate residue.

### Sample preparation and elemental analysis

About 1.0 g of the sediment samples was reacted with 8mL of 1 M MgCl2 for 6 h using a Thermo Scientific Vari-MixTM Test Tube Rocker. The mixture was centrifuged, and the supernatant was passed through a 0.2 μm polypropylene syringe filter, dried, and re-dissolved in 5% HNO3. The residue was then reacted with 8 mL of 1M NaOAc for 6 h (pH=5) prior to repeating the same procedures above, followed by reaction with 20 mL of 0.04 M NH₂OH-HCl in 25% (v/v) HOAc at 96°C for 6 h, and 8 mL of a mixture of 0.02M HNO3 and 30% H2O2 for 6 h (pH=2). Finally, the residue was dried and a 0.1 g sub-sample was powdered and completely digested in a 5:3 mixtures of HF and HNO3.

Heavy metal concentrations were measured using an iCAP RQ ICP-MS at the Korea Institute of Ocean Science & Technology (KIOST). Repeated analyses of USGS rock standard powders (BCR-2, BHVO-2, and BIR-1) yielded external reproducibilities better than ±5%.

### Evaluation of pollution indices

We used two different indices in order to assess the degree of heavy metal contamination in the sediment samples; (1) contamination factor (CF), and (2) geoaccumulation index (Igeo).

The CF is defined as the ratio of heavy metal concentration in samples to that in upper continental crust (UCC) (Hakanson, 1980) and calculated as follows:

$C F = C s a m p l e C U C C$
(1)

where CF < 1 indicates low contamination, 1 ≤ CF < 3 moderate contamination, 3 ≤ CF < 6 considerable contamination, and CF ≥ 6 very high contamination.

The Igeo is used to determine whether heavy metals come from either natural or anthropogenic sources and calculated as follows:

$I g e o = log 2 ( C s a m p l e C U C C × 1.5 )$
(2)

, where the factor of 1.5 is the background matrix correction factor due to lithospheric effects. The Igeo can be classified into seven classes depending the value (Müller, 1986); Class 0 (practically unpolluted) is defined as Igeo≤ 0, Class 1 (unpolluted to moderately polluted) as 0 < Igeo< 1, Class 2 (moderately polluted) as 1 < Igeo< 2, Class 3 (moderately to heavily polluted) as 2 < Igeo< 3, Class 4 (heavily polluted) as 3 < Igeo < 4, Class 5 (heavily to extremely polluted) as 4 < Igeo< 5, and Class 6 (extremely polluted) as Igeo≥ 5.

## Results and Discussion

### Exchangeable fraction

The heavy metals in the exchangeable fraction of sediments are mainly concerned because they are labile, highly toxic and the most bioavailable (Raj and Jayaprakash, 2008;Wang et al., 2010). A sequential leaching showed that concentrations of five metals (Cu, Co, Ni, Pb, and Zn) in the exchangeable fraction are quite variable in each core but their concentrations are relatively low, ranging from 0.002 mg/kg for Pb in the YF1 to 17.7 mg/kg for Cu in the YF3 (Table 1). In contrast, Mn concentrations are much higher than the others, ranging from 0.58 mg/kg in the YDF1 to 92.8 mg/kg in the YF1 (Table 1).

In the YF1 core, although concentrations of most elements except for Mn and Zn are highly variable, their concentrations are much less than 1 mg/kg in all depths (Fig. 2). Zinc gradually increases up to 9.70 mg/kg at the depth of 30 m and then decreases. Although Mn also display a similar pattern with Zn, Mn concentrations are one-order higher than Zn having the maximum value at the depth of 25 m (Fig. 2). The YDF1 core also display a similar pattern with the YF1 core. On the contrary, in the YF2 core Mn concentration decreases up to less than 1 mg/kg with the depth, while in the YF3 core it gradually increases with the depth. The results indicate that only Mn having high concentration of up to 92.8 mg/kg in the exchangeable fraction could be a threat to the ecosystem of the Nakdong River.

### Carbonate fraction

Compared to the exchangeable fraction, the carbonate fraction displays relatively lower concentrations in five metals (Cu, Co, Ni, Pb and Zn), ranging from 0.006 mg/kg for Pb in the YDF1 to 48.7 mg/kg for Cu in the YF3. On the contrary, Mn has much higher concentrations, ranging from 0.63 mg/kg in the YDF1 to 196mg/kg in the YF1 (Table 1), which is consistent with previous study that Mn can be adsorbed onto the surfaces of carbonate minerals followed by incorporation into the crystal lattice to form solid solution (Billon et al., 2002).

In the YF1 core, concentrations of five elements are highly variable with the depth but are more or less 1 mg/kg even though Zn concentration is 5.21 mg/kg at the depth of 35 m (Table 1). However, Mn displays the highest concentration in the upper and bottom parts with lower concentration in the middle parts (Fig. 3). Likewise, all elements except Mn show a similar pattern in other cores (YFs 2-3) having around 1 mg/kg. However, Mn displays the saw-tooth pattern in the YF2 and YDF1 cores but are relatively constant up to at the depth of 20 m in the YF3 core. This result indicates that Mn may precipitate as MnCO3 in the carbonate fraction.

### Fe-Mn oxide fraction

The Fe-Mn oxides fraction are one of the most important geochemical phases impacting the mobility and behaviour of trace metals due to their large surface areas (Turner, 2000;Swallow et al., 1980). All elemental concentrations in this fraction are much higher than those in two previous fractions, ranging from 0.037 mg/kg for Ni in the YF2 to 659 mg/kg for Mn in the YF1, indicating strong adsorption capacity of Fe-Mn oxides (Table 1).

In the YF1 core, Mn concentrations decrease up to at the depth of 20 m and are spiked at the depths of 25 and 55m (Fig. 4). On the other hand, concentrations gradually decrease with the depth in the YF2 core and are relatively constant in the YF3 core. The YDF1 core displays the saw-tooth pattern having the highest Mn concentration of 463 mg/kg at the depth of 25 m. Altogether, this suggests that Fe-Mn oxides may play a major role in controlling the fate and transport of heavy metals in the sediment cores of the Nakdong River.

### Organic matter fraction

The organic matter fractions of sediments are important factor controlling the mobility and bioavailability of heavy metals (Wang et al., 2010). All elemental concentrations in this fraction range from 0.01 mg/kg for Co in the YF2 to 149 mg/kg for Mn in the YF1 (Fig. 5; Table 1). In this study, concentrations of all elements in the organic matter fraction are much higher than other labile fractions (the exchangeable and carbonate fractions) but those of four elements (Co, Ni, Pb and Zn) are relatively similar to those in the Fe-Mn oxide fraction (Fig. 5). The observation that only Cu is enriched in this fraction indicates that organic matter is the primary factor impacting the behaviour of Cu in the sediment of Nakdong River. In all cores, variations of all elements versus the depth are quite similar as shown in other fractions.

### The residue

The heavy metals in the residues of sediments are generally much less toxic for organisms in aquatic environment because they cannot be easily released from the crystal lattice of the minerals (Savvides et al., 1995). All elemental concentrations in the residue are mostly the highest compared to the other fractions, ranging from 0.29 mg/kg for Ni in the YF2 to 527 mg/kg for Mn in the YF3 (Fig. 6; Table 1). As shown in the other fractions, Mn concentrations are significantly higher than the other metals, ranging from 119 mg/kg in the YF2 to 527 mg/kg in the YF3. In all cores, variations of concentrations are quite constant at all depth, while each core displays different concentrations of all elements (Fig. 6).

### Contamination parameters

High amounts of heavy metals in labile fractions may be harmful to ecosystems in the Nakdong River basin. In order to assess pollution intensity, the parameters, such as CF and Igeo, can be used. The CFs of all elements are quite variable, ranging from 0 for Co, Cu and Ni to 20 for Mn (Table 2). Although the CFs are quite variable in all depths and cores, only two elements (Pb and Mn) have the average CF of more than 1, indicating moderate contamination for them. Surprisingly, the CF of Mn ranges from 2 to 20 with the average of 7, implying that very high contamination of Mn may results in heavy accumulation of Mn in the aquatic organisms.

The calculated Igeo allows us to specify whether the sources of heavy metals are natural or anthropogenic. Average Igeo values of all elements except Mn are negative indicating natural sources for them, while average Igeo value of 2 for Mn indicates moderately to heavily polluted from anthropogenic sources. However, most samples are classified into Class 3 (moderately to heavily polluted). Therefore, additional studies should be conducted to explore detailed causes affecting the Mn concentration levels of sediments observed in the Nakdong River.

## Conclusions

In order to understand the status of heavy metal contaminations in the sediment cores collected at the Nakdong River, four sediment cores were investigated using a five-step leaching. The results showed that the distributions of heavy metals are quite site- and fraction-dependent. Except Mn, all elements are depleted in both exchangeable and carbonate fractions. However, in both Fe-Mn oxide and organic matter fractions concentrations of all elements become much higher, especially Cu is much enriched in the organic matter fraction. Consistent with the result that only Mn displays much high concentration in all fractions and cores, the contamination parameters (CF and Igeo) indicate that Mn is highly contaminated from anthropogenic sources. This study suggests that Mn can be a potential risk to ecosystems in the Nakdong River and therefore more detailed studies should be conducted to explore causes affecting the Mn concentration levels of sediments observed in the Nakdong River.

## Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korea government (MSIT) (No. NRF-2019R1A2C 2085973) as well as the University of Rwanda Capacity Building Project (CD20191211). We thank H. Jeong and K. Ra for their help in ICP-MS analysis.

## Figure

Map showing location of sampling sites in the study area.

Concentrations of heavy metal in the exchangeable fraction versus depth. (a) YF1, (b) YF2, (c) YF3, and (d) YDF1.

Concentrations of heavy metal in the carbonate fraction versus depth. (a) YF1, (b) YF2, (c) YF3, and (d) YDF1.

Concentrations of heavy metal in the Fe-Mn oxide fraction versus depth. (a) YF1, (b) YF2, (c) YF3, and (d) YDF1.

Concentrations of heavy metal in the organic matter fraction versus depth. (a) YF1, (b) YF2, (c) YF3, and (d) YDF1.

Concentrations of heavy metal in the residue versus depth. (a) YF1, (b) YF2, (c) YF3, and (d) YDF1.

## Table

Heavy metal concentrations in five fractions of sediment cores collected at the Nakdong River.

Calculated values of the contamination factor (CF) and geoaccumulation index (Igeo) for each element

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