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Heavy metal contamination in the vicinity of the Daduk Au–Ag–Pb–Zn mine in Korea
Applied Geochemistry 16 (2001) 1377–1386 www.elsevier.com/locate/apgeochem
Heavy metal contamination in the vicinity of the Daduk Au–Ag–Pb–Zn mine in Korea Churl Gyu Lee a, Hyo-Taek Chon a,*, Myung Chae Jung b a School of Civil, Urban and Geosystem Engineering, College of Engineering, Seoul National University, Seoul, 151-742, Korea Department of Earth Resources and Environmental Geotechnics Engineering, Semyung University, Jecheon, Choongbuk, 390-711, Korea
b
Abstract The heavy metal contamination and seasonal variation of the metals in soils, plants and waters in the vicinity of an abandoned metalliferous mine in Korea were studied. Elevated levels of Cd, Cu, Pb and Zn were found in tailings with averages of 8.57, 481, 4,450 and 753 mg/kg, respectively. These metals are continuously dispersed downstream and downslope from the tailings by clastic movement through wind and water. Thus, significant levels of the elements in waters and sediments were found up to 3.3 km downstream from the mining site, especially for Cd and Zn. Enriched concentrations of heavy metals were also found in various plants grown in the vicinity of the mining area, and the metal concentrations in plants increased with those in soils. In a study of seasonal variation on the heavy metals in paddy fields, relatively high concentrations of heavy metals were found in rice leaves and stalks grown under oxidizing conditions rather than a reducing environment (P<0.05). # 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction Mining is one of the most important sources of heavy metals in the environment. Mining and milling operations together with grinding, concentrating ores and disposal of tailings, along with mine and mill waste water, provide obvious sources of contamination (Adriano, 1986). Therefore, large areas of agricultural land can be contaminated, including paddy fields. Korea, for example, has a long history of metalliferous mining and the most extensive activities occurred during the early twentieth century. As a result, over 1000 metalliferous mines were distributed along mineralized zones; most of the mines were abandoned due to a lack of ore minerals. However, these abandoned mines can become an important point source of toxic elements including As, Cd, Cu, Pb and Zn in the surface environment. Numerous studies have been undertaken into trace element contamination derived from mining activities, in
* Corresponding author. Fax: +82-2-871-7892/8938. E-mail address: [email protected] (H.-T. Chon).
soils, plants, waters and sediments in various countries (Thornton, 1980; Fuge, et al., 1989; Merrington and Alloway, 1994; Pestana et al., 1997). Limited studies of heavy metal concentration derived from mining activities, however, have been carried out in Korea (Jung and Thornton, 1996; Chon et al., 1997). Thus, the objectives of this study are (1) to investigate the extent and degree of trace element contamination of soils, plants, waters and sediments influenced by mining activity of the Daduk mine; (2) to examine the variations of trace elements in paddy fields throughout the rice growing season and those in waters and sediments between the dry and wet seasons; (3) to find out the differences in element concentrations in soils by a chemical decomposition method.
2. Materials and methods The study area, the Daduk Au–Ag–Pb–Zn mine, is located in the middle part of Korea, and was one of the largest Au–Ag–Pb–Zn mines in Korea. During the period of operation, mainly in the 1950s, the mine produced over 20 kg of Au, 50 kg of Ag and a thousand tons of
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Pb and Zn. The geology of the mine is mainly hornblende granite and Chunyang granite. The mineralization, classified as a hydrothermal replacement type with Au–Pb–Zn minerals in quartz veins, is of low grade Au and Ag and contains pyrite, chalcopyrite, pyrrhotite, galena and sphalerite. The mine ceased production in 1984 and large amounts of mine wastes including tailings have been left without proper environmental treatment. Thus, these materials have been dispersed downslope by both surface erosion and wind action and by effluent draining the mine wastes into lower lying land, mainly used for the growth of paddy rice and garden crops. Sampling of soils, plants, waters and sediments was carried out on 2nd August (wet season, average rainfall of 250–300 mm/month) and on 26 September (dry season, average rainfall of 100–150 mm/month), 1998 to investigate the seasonal variation of trace elements. The sampling locations and topography of the study area are shown in Fig. 1. Surface soils (0–15 cm in depth) were sampled by hand auger (2.5 cm in diameter) and trowel in and around the mine, from the tailings, uncultivated and high lands, paddy fields and a nearby control area. Each soil sample comprised a composite of 9 subsamples taken within a 55 m square. Random samples of plants were taken from agricultural land including household gardens and paddy fields. They included red pepper (Capsicum annuum), soybean (Glycine max.) leaves, rice (Oryza sativa L.) stalks and grain. Stream waters and sediments were also collected along a small stream. After air-drying at 25 C for 5 days, soil and sediment samples were disaggregated, sieved to <2 mm and then ground to a fine powder (<180 mm). The finely milled soil and sediment samples were digested in 3:1 concentrated HCl and HNO3 (aqua regia). The solutions were analyzed by inductively coupled plasma–atomic emission spectrometry (ICP–AES) for 23 elements including Cd, Cu, Pb and Zn (Ure, 1995). As a Korean standard method for chemical analysis of soils, the samples were also digested into 0.1 N HCl solution (10 g of soil with 50 ml of the solution) and analyzed by ICP–AES. Samples of plants were thoroughly washed in de-ionized water, dried in a clean room at ambient temperature for 5 days and milled to a fine powder. The samples were decomposed with concentrated fuming HNO3 and HClO4 and leached with 5 M HCl. The solution was analyzed by ICP–AES (Thompson and Walsh, 1988). Water samples were filtered using a hand-pump on site through a 0.45 mm membrane filter paper. After acidification with concentrated HNO3, the samples were stored in a cool box. The physical and chemical properties of the waters were measured in the field including pH, total dissolved solid (TDS), electrical conductivity (EC) and salinity. Filtered waters were also collected and stored in a cool box without acidification for anion
analysis. The chemical analysis of water samples was carried out using ICP–AES for cations and Ion Chromatography (IC) for anions. A rigorous quality control program was implemented, which included reagent blanks, duplicate samples, inhouse reference materials and certified international reference materials (Ramsey et al., 1987). The precision and bias of the chemical analysis was less than 10%.
3. Results and discussion 3.1. The trace element concentrations in soil The range and mean concentrations of Cd, Cu, Pb and Zn in surface soils extracted by aqua regia and 0.1 N HCl are shown in Table 1. As shown in the table, elevated levels of those metals extracted by aqua regia were found in samples of tailings, with average values of 8.57 Cd, 481 mg/kg Cu, 4,450 mg/kg Pb and 753 mg/kg Zn. These levels in the tailings are significantly higher than those in uncontaminated soils reported by Bowen (1979). In addition, some tailings samples contain high levels of those heavy metals extracted by a 0.1 N HCl solution, which is the standard method for the Korean Soil Environmental Conservation Act established in 1996. The relationship of heavy metal concentrations in soil samples using both the methods are illustrated in Fig. 2. Although there are variations from metal to metal, the concentrations of Cd, Cu, Pb and Zn in soils extracted by both methods are statistically highly correlated (P<0.001). Jung and Chon (1997) also found these relationships in the paddy soils contaminated by an abandoned metal mine in Korea. According to the Korean Soil Environmental Conservation Act, soils containing over 12 mg/kg of Cd, 200 mg/kg of Cu and 400 mg/kg of Pb extracted by 0.1 N HCl solution need to be continuously monitored and not used for agricultural purposes such as crop planting. As shown in Table 1, some of the tailings exceed these guidelines. Elevated levels of Cd, Cu, Pb and Zn are also found in soils sampled in paddy fields and the forest area due to dispersion of the metals from the tailings by clastic movement through wind and water. Therefore, these contaminated soils can influence the metal uptake by plants grown on the soils. In comparison with the contaminated area, relatively low contents of heavy metals which are similar to those in background levels reported by Bowen (1979) are found in soils sampled at a nearby control area with similar geology. Results from a t-test indicate there is a statistical difference between the average concentrations in the contaminated area and the control site (P<0.05). The heavy metal concentrations in soils are shown in Fig. 3. Wide dispersion patterns are found in metal concentrations in soils. This may be due to transport clastic from tailings, especially during the wet season.
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It is well-known that most heavy metal contamination in the surface environment is associated with a cocktail of contaminants rather than one metal. Thus, the concept
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of a pollution index has been introduced in many studies to identify multi-element contamination (Jung, 1995; Chon et al., 1998). According to Chon et al.
Fig. 1. Sampling location map of the Daduk Au–Ag–Pb–Zn mine.
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Table 1 The range and mean concentrations of Cd, Cu, Pb and Zn in soils extracted by aqua-regia and 0.1N HCl(mg/kg) Extracted by 0.1N HClc
Extracted by aqua-regia Cd M S.D.b (range)
Cu M S.D. (range)
Pb M S.D. (range)
Zn (M S.D. range
Cd M S.D. (range)
Cu M S.D. (range)
Pb M S.D. (range)
Zn M S.D. (range)
Contaminated area Paddy soil 1.78 1.09 n=34a (0.40-4.76) Forest soil 1.30 0.49 n=8 (0.80-2.20) Tailings 8.57 11.7 n=12 (1.56-36.0)
42.2 30.6 (9.6-143.6) 42.7 27.0 (13.6-86.0) 481 515 (168-1,708)
133 105 (21-484) 155 227 (33-708) 4,450 3,372 (2,304-12,560)
328 251 (60-1,064) 198 77 (113-332) 753 459 (248-1,464)
0.78 0.66 (0.10-2.49) 0.38 0.39 (0.13-1.21) 5.67 13.3 (0.05-38.4)
13.3 13.0 (1.2-56.5) 9.3 14.1 (1.9-41.0) 130 278 (8.5-815)
33.2 29.4 (3.8-124) 36.3 66.5 (3.3-187) 84.4 149 (2.4-443)
73.6 69.9 (5.5-280) 33.4 30.1 (12-93) 100 158 (5.9-480)
Control area Paddy soil n=24 Forest soil n=4
28.3 20.2 (3.6-97.2) 11.4 6.4 (6.9-20.4)
81 84 (23-367) 33 7 (27-43)
183 188 (43-936) 75 22 (55-106)
0.35 0.48 (0.07-2.35) 0.09 0.01 (0.07-0.09)
5.5 7.6 (0.5-39.0) 0.80 0.67 (0.4-1.8)
20.8 29.5 (2.1-115) 3.2 0.7 (2.3-3.9)
27.3 34.7 (3.9-165) 7.2 2.2 (5.2-9.8)
a b c
1.18 0.79 (0.48-4.24) 0.77 0.14 (0.64-0.96)
Number of samples. Arithmetic mean standard deviation. Extraction method regulated by the Korean Soil Environmental Conservation Act in 1996.
Fig. 2. Metal concentrations in solis extracted by aqua-regia and 0.1 NHCl (n=68).
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(1998), the pollution index of soil is calculated by the average ratio of metal concentrations in soil to the guideline values. Tolerable levels in soil (Kloke, 1979) are used as guideline values, and the pollution index of soil samples are shown in Fig. 4. In this study, tolerable levels are the threshold values of elements in soil, concentrations above which can produce crops that are unsafe for human or animal consumption. As shown in the figure, the tailings site and a nearby ore dressing plant site are areas with a pollution index >1 and are considered as contaminated. Thus it can be seen that the large amounts of heavy metals present in the tailings and associated soils provide a source for continuing dispersion downstream and downslope, and have led to a moderate degree of contamination of paddy soils distributed below the tailings. 3.2. The trace element concentrations in various plants The range and mean concentrations of Cd, Cu, Pb and Zn in various plants grown on paddy soils and household gardens are shown in Table 2. It is well known that metal concentrations in plants vary with plant species (Adriano, 1986; Alloway, 1995). In general, leafy plants tend to accumulate higher metal concentrations than root, grain or fruit crops (Alloway, 1995; Jung and Thornton, 1996). The results of this study confirmed that metal concentrations in leafy crops such as rice stalks and soybean leaves are much higher than those in rice grain and red pepper. Jung and Thornton (1996) also reported that soybean leaf is an accumulator of heavy metals. They found 0.39, 10.0, 3.95and 200 mg/ kg, respectively, of Cd, Cu, Pb and Zn in soybean leaves
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grown on contaminated soils. In this study, higher concentrations of the metals are also found in soybean leaves. High levels of the metals are also found in red peppers and rice stalks, which is similar to the study of Jung (1995). According to Jung and Thornton (1997), the concentrations of Cd, Cu, Pb and Zn in rice grain grown on soils contaminated by Pb–Zn mining activity in Korea averaged 0.21, 3.0, 0.22 and 22.5 mg/kg, respectively. Although there is some variation of Cd concentration, rice grain sampled in the study area contains similar concentrations of the metals reported by Jung and Thornton (1997). In comparison with plants grown on contaminated soils, relatively lower concentrations of Cd, Cu, Pb and Zn are found in plants grown on soils sampled at a control area. The accumulation ratio (defined as the metal concentration in plant divided by those in soil) provides a useful indication of the relative metal availability from soils to plants (Alloway et al., 1988; Jung, 1995). The range and mean accumulation ratios for Cd, Cu, Pb and Zn in plant samples are shown in Fig. 5. It can be seen that the values for Cd, Cu and Zn are much higher than those for Pb, due to the different solubility of those metal species in soils governing their availability to plants. The ratios are also different between plant species. 3.3. The trace element concentrations in sediments and waters The range and mean concentrations of Cd, Cu, Pb and Zn in stream sediments are shown in Table 3. Elevated levels of the metals are found in the sediments, especially for Pb and Zn and to some degree for Cd and
Fig. 3. Concentrations of Cd, Cu, Pb and Zn in surface soil samples from the Daduk mine area.
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Table 2 The range and mean concentrations of Cd, Cu, Pb and Zn in crop plants (mg/kg, dry weight) Cd M S.D.b (range)
Cu MS.D. (range)
Pb MS.D. (range)
Zn MS.D. (range)
Contaminated area Red peppers n=8a Soybean leaves n=4 Rice stalks and leaves n=20 Rice grains n=8
0.610.43 (0.22-1.50) 1.981.95 (0.49-4.67) 0.801.27 (0.11-5.85) 0.150.20 (0.03-0.65)
8.74.3 (4.1-17.4) 40.758.5 (8.7-128.4) 11.19.6 (1.9-39.9) 3.11.0 (1.6-4.6)
1.92.4 (0.3-7.2) 5.00.81 (4.0-5.8) 4.44.2 (1.1-16.6) 0.320.15 (0.2-0.7)
35.126.3 (11.4-85.8) 271.3252.4 (46.3-494.8) 86.785.4 (15.4-324.8) 16.13.4 (11.8-21.8)
Control area Red peppers n=7 Soybean leaves n=5 Rice stalks and leaves n=10 Rice grains n=6
0.390.20 (0.15-0.64) 0.500.11 (0.36-0.62) 0.640.92 (0.12-2.95) 0.090.08 (0.01-0.24)
10.34.8 (5.5-19.5) 15.314.0 (7.3-39.9) 5.21.4 (2.5-7.8) 2.10.9 (1.0-3.0)
1.10.49 (0.5-2.1) 4.40.52 (3.8-5.1) 2.91.4 (1.7-6.0) 0.290.13 (0.2-0.5)
23.512.4 (13.6-47.9) 77.471.7 (32.5-202.2) 42.323.2 (16.5-79.2) 13.93.6 (8.4-18.4)
a b
Number of samples. Arithmetic mean standard deviation.
Cu. These high levels of the metals are probably influenced by clastic movement from the tailings in the mine waste dumps. However, relatively low contents of the metals are found in sediments sampled at a control site. The range and mean values of physical and chemical properties of waters and concentrations of Cd, Cu, Pb and Zn in stream waters are shown in Table 4. A wide range of water pH from 3.2 to 6.3 is found in the water samples from the contaminated area. The total dissolved solids (TDS) range from 62 to 1373 mg/l and the electrical conductivity (EC) ranges from 120 to 1930 mS/cm. Lower values for pH (<4) and higher levels of TDS(>1000mg/l) and EC (>1000 mS/cm) are found in waters sampled immediately downstream of effluents passing through tailings. Furthermore, high levels of heavy metals are found in the water. Thus, it can be understood that the main source of heavy metals in waters is the tailings containing elevated levels of the metals (see Table 1). In addition, over 1000 mg/l of SO4 is also found in the effluents, mainly due to leaching and oxidizing of sulfide minerals including pyrite, arsenopyrite, galena and sphalerite in tailings. Like dispersion patterns of soils, heavy metal concentrations in waters exponentially decreased with increasing distance from the tailings and they reach background levels at 3.3 km downstream. 3.4. Seasonal variation on metals in paddy fields As mentioned above, this study assesses the seasonal variation on heavy metal concentrations in paddy fields. Because rice is generally grown under both reducing
Fig. 4. Pollution index of heavy metals in surface soil samples from the Daduk mine area.
conditions (early August in this study) and oxidizing conditions (late September), these conditions may influence metal uptake by rice plants (Bingham et al., 1976; Jung, 1995). Many investigators have demonstrated that the availability of metals decreased under submerged conditions due to the precipitation of hydride, carbonate, sulfide and iron compounds (Kitagishi and Yamane, 1981; Dutta et al., 1989). Jung and Thornton (1997) also reported that metal concentrations in rice stalks and leaves grown under oxidizing conditions are higher than those under reducing conditions. The seasonal variations of heavy metal concentrations in rice stalks and leaves are presented in Fig. 6. The figure shows that rice stalks and leaves sampled in late September (oxidizing conditions) contain higher metal concentrations than those sampled in early August (reducing conditions). A statistical test for differences in average concentrations
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between each season confirms this (P<0.05). However, there is little variation in metal concentrations in paddy soils throughout the period of rice growing (P> 0.05). Variations on heavy metals are found in stream sediments (Table 3) and waters (Table 4). t-Test results show that, relatively high concentrations of Cd, Cu, Pb and Zn are found in water and sediment samples in the
dry season (September) (P<0.05). Lower metal concentrations in waters in the wet season (August) may be due to a dilution effect by heavy rain in summer. The dispersion patterns and seasonal variations of Cd, Cu, Pb and Zn in stream waters and sediments are shown in Figs. 7 and 8, respectively. The figures show that the concentrations of the metals decrease with increasing distance from the tailings.
Fig. 5. The range and mean values of accumulation ratios for Cd, Cu, Pb and Zn in plants. Accumulation ratio (AR) defined as metal concentation in plant divided by metal concentration extracted by aqua-regia in soil. (RP: red pepper; SL: soybean leaves; RS: rice stalks and leaves; RG: rice grain). Table 3 The range and mean concentration of Cd, Cu, Pb and Zn in stream sediments and its seasonal variation (mg/kg) Cd M S.D.b(range)
Cu M S.D. (range)
Pb M S.D. (range)
Zn M S.D. (range)
Contaminated area n=24a Samples on 2 Aug n=12 Samples on 26 Sep. n=12
4.93 15.1 (0.17-75.6) 2.09 0.93 (1.08-4.40) 7.77 21.4 (0.17-75.6)
136 166 (12.8-700) 121 98.9 (13.6-332) 152 218 (12.8-700)
685 887 (32.3,024) 616 873 (34-3,024) 754 934 (32,2,748)
417 330 (42-1,356) 356 221 (42-948) 479 412 (136-1,356)
Control area n=16 Samples on 2 Aug. n=8 Samples on 26 Sep. n=8
0.75 0.55 (0.28-2.52) 0.99 0.71 (0.28 2.52) 0.51 0.12 (0.32-0.68)
11.2 6.3 (4.0-26.4) 12.2 5.4 (6.0-22.8) 10.3 7.2 (4.0-26.4)
44 31 (12.121) 53 37 (23-121) 34 21 (12-67)
145 128 (36-532) 195 161 (45-532) 94 57 (36-216)
a b
Number of samples. Arithmetic mean standard deviation.
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Table 4 The range and mean concentrations of Cd, Cu, Pb and Zn and hydrogeological properties of waters and their seasonal variation Contaminated area Early August n=10 b
pH Conductivity (mS/cm) TDS (mg/l) Cd (mg/l) Cu (mg/l) Pb (mg/l) Zn (mg/l) a b c d
Control area a
5.4 0.8 4.1-6.3c 379 192 120-824 190 97 62-413 29 48 1-159 243 365 3-1,197 26 27 1-90 6.6 7.0 0.02-24.0
Late September n=10
Early August n=11
Late September n=11
4.4 1.1 3.3-6.2 719 491 213-1,930 512 349 151-1,373 45 26 18-112 536 329 187-1,219 42 24 15-86 13.2 9.5 4.7-38.4
6.9 0.6 5.5-8.0 109 66 51-222 54 33 25-110 1 1 nd-3d 2 2 nd-5d 2 2 1-7 0.19 0.29 0.01-0.88
6.9 0.33 6.5-7.4 157 99 71-378 111 71 51-269 1 2 nd-6d 11 5 7-23 3 1 2-4 0.25 0.52 0.01-1.73
Number of samples. Arithmetic mean standard deviation. Range. nd, Not determined.
Fig. 6. Seasonal variation of metal concentrations in agricutural crops (red peppers: 1–4; soybean leaves: 5–8; rice stalks: 9–17).
Fig. 7. Variations of PH and heavy metal concentrations in water with a distance and their seasonal variation in the Daduk area.
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in stream water and sediments decreased with increasing distance from the tailings. As a result of seasonal variations, rice stalks and leaves grown under oxidizing conditions contained higher metal concentrations than those grown under reducing conditions (P<0.05). In addition, relatively high concentrations of metals were found in water and sediment samples collected during the dry season (P<0.05).
Acknowledgements We would like to acknowledge the Center for Mineral Resources Research (CMR) of Korea and the Research Division of Seoul National University/Hanyang University for Social Infrastructure and Construction Technology (BK-21) for their financial support during this study. Many thanks are extended to reviewers who improved our original manuscript.
References
Fig. 8. Seasonal variation of metal concentrations in stream sediments in the Daduk mine area.
4. Conclusions In this study of an area around a disused Au–Ag–Pb– Zn mine, soils, plants, waters and sediments have been contaminated by past mining activity. Elevated levels of heavy metals including Cd, Cu, Pb and Zn were found in tailings with arithmetic mean values of 8.57, 481, 4450 and 753 mg/kg, respectively. These significant concentrations greatly influenced the metal concentrations in paddy soils and forest soils in the vicinity of the tailings by surface erosion, wind action and by effluent draining the tailings into lower lying land mainly used for growth of paddy rice and garden crops. Comparison with extraction methods by both 0.1 N HCl and aqua regia showed heavy metal concentrations in soils to have a statistically significant correlation between the two methods (P<0.001). Metal concentrations in crop plants varied between plant species. Soybean leaves and rice stalks and leaves had elevated levels of heavy metals, especially for Cd and Zn. Red peppers and rice grain also accumulated the metals. Relatively high metal accumulation ratios were found in soybean leaves, with over 1.0 for Cd, Cu and Zn. Rice stalks and leaves also show high accumulation ratios. Heavy metal concentrations
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Heavy metal contamination in the vicinity of the Daduk Au–Ag–Pb–Zn mine in Korea Churl Gyu Lee a, Hyo-Taek Chon a,*, Myung Chae Jung b a School of Civil, Urban and Geosystem Engineering, College of Engineering, Seoul National University, Seoul, 151-742, Korea Department of Earth Resources and Environmental Geotechnics Engineering, Semyung University, Jecheon, Choongbuk, 390-711, Korea
b
Abstract The heavy metal contamination and seasonal variation of the metals in soils, plants and waters in the vicinity of an abandoned metalliferous mine in Korea were studied. Elevated levels of Cd, Cu, Pb and Zn were found in tailings with averages of 8.57, 481, 4,450 and 753 mg/kg, respectively. These metals are continuously dispersed downstream and downslope from the tailings by clastic movement through wind and water. Thus, significant levels of the elements in waters and sediments were found up to 3.3 km downstream from the mining site, especially for Cd and Zn. Enriched concentrations of heavy metals were also found in various plants grown in the vicinity of the mining area, and the metal concentrations in plants increased with those in soils. In a study of seasonal variation on the heavy metals in paddy fields, relatively high concentrations of heavy metals were found in rice leaves and stalks grown under oxidizing conditions rather than a reducing environment (P<0.05). # 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction Mining is one of the most important sources of heavy metals in the environment. Mining and milling operations together with grinding, concentrating ores and disposal of tailings, along with mine and mill waste water, provide obvious sources of contamination (Adriano, 1986). Therefore, large areas of agricultural land can be contaminated, including paddy fields. Korea, for example, has a long history of metalliferous mining and the most extensive activities occurred during the early twentieth century. As a result, over 1000 metalliferous mines were distributed along mineralized zones; most of the mines were abandoned due to a lack of ore minerals. However, these abandoned mines can become an important point source of toxic elements including As, Cd, Cu, Pb and Zn in the surface environment. Numerous studies have been undertaken into trace element contamination derived from mining activities, in
* Corresponding author. Fax: +82-2-871-7892/8938. E-mail address: [email protected] (H.-T. Chon).
soils, plants, waters and sediments in various countries (Thornton, 1980; Fuge, et al., 1989; Merrington and Alloway, 1994; Pestana et al., 1997). Limited studies of heavy metal concentration derived from mining activities, however, have been carried out in Korea (Jung and Thornton, 1996; Chon et al., 1997). Thus, the objectives of this study are (1) to investigate the extent and degree of trace element contamination of soils, plants, waters and sediments influenced by mining activity of the Daduk mine; (2) to examine the variations of trace elements in paddy fields throughout the rice growing season and those in waters and sediments between the dry and wet seasons; (3) to find out the differences in element concentrations in soils by a chemical decomposition method.
2. Materials and methods The study area, the Daduk Au–Ag–Pb–Zn mine, is located in the middle part of Korea, and was one of the largest Au–Ag–Pb–Zn mines in Korea. During the period of operation, mainly in the 1950s, the mine produced over 20 kg of Au, 50 kg of Ag and a thousand tons of
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Pb and Zn. The geology of the mine is mainly hornblende granite and Chunyang granite. The mineralization, classified as a hydrothermal replacement type with Au–Pb–Zn minerals in quartz veins, is of low grade Au and Ag and contains pyrite, chalcopyrite, pyrrhotite, galena and sphalerite. The mine ceased production in 1984 and large amounts of mine wastes including tailings have been left without proper environmental treatment. Thus, these materials have been dispersed downslope by both surface erosion and wind action and by effluent draining the mine wastes into lower lying land, mainly used for the growth of paddy rice and garden crops. Sampling of soils, plants, waters and sediments was carried out on 2nd August (wet season, average rainfall of 250–300 mm/month) and on 26 September (dry season, average rainfall of 100–150 mm/month), 1998 to investigate the seasonal variation of trace elements. The sampling locations and topography of the study area are shown in Fig. 1. Surface soils (0–15 cm in depth) were sampled by hand auger (2.5 cm in diameter) and trowel in and around the mine, from the tailings, uncultivated and high lands, paddy fields and a nearby control area. Each soil sample comprised a composite of 9 subsamples taken within a 55 m square. Random samples of plants were taken from agricultural land including household gardens and paddy fields. They included red pepper (Capsicum annuum), soybean (Glycine max.) leaves, rice (Oryza sativa L.) stalks and grain. Stream waters and sediments were also collected along a small stream. After air-drying at 25 C for 5 days, soil and sediment samples were disaggregated, sieved to <2 mm and then ground to a fine powder (<180 mm). The finely milled soil and sediment samples were digested in 3:1 concentrated HCl and HNO3 (aqua regia). The solutions were analyzed by inductively coupled plasma–atomic emission spectrometry (ICP–AES) for 23 elements including Cd, Cu, Pb and Zn (Ure, 1995). As a Korean standard method for chemical analysis of soils, the samples were also digested into 0.1 N HCl solution (10 g of soil with 50 ml of the solution) and analyzed by ICP–AES. Samples of plants were thoroughly washed in de-ionized water, dried in a clean room at ambient temperature for 5 days and milled to a fine powder. The samples were decomposed with concentrated fuming HNO3 and HClO4 and leached with 5 M HCl. The solution was analyzed by ICP–AES (Thompson and Walsh, 1988). Water samples were filtered using a hand-pump on site through a 0.45 mm membrane filter paper. After acidification with concentrated HNO3, the samples were stored in a cool box. The physical and chemical properties of the waters were measured in the field including pH, total dissolved solid (TDS), electrical conductivity (EC) and salinity. Filtered waters were also collected and stored in a cool box without acidification for anion
analysis. The chemical analysis of water samples was carried out using ICP–AES for cations and Ion Chromatography (IC) for anions. A rigorous quality control program was implemented, which included reagent blanks, duplicate samples, inhouse reference materials and certified international reference materials (Ramsey et al., 1987). The precision and bias of the chemical analysis was less than 10%.
3. Results and discussion 3.1. The trace element concentrations in soil The range and mean concentrations of Cd, Cu, Pb and Zn in surface soils extracted by aqua regia and 0.1 N HCl are shown in Table 1. As shown in the table, elevated levels of those metals extracted by aqua regia were found in samples of tailings, with average values of 8.57 Cd, 481 mg/kg Cu, 4,450 mg/kg Pb and 753 mg/kg Zn. These levels in the tailings are significantly higher than those in uncontaminated soils reported by Bowen (1979). In addition, some tailings samples contain high levels of those heavy metals extracted by a 0.1 N HCl solution, which is the standard method for the Korean Soil Environmental Conservation Act established in 1996. The relationship of heavy metal concentrations in soil samples using both the methods are illustrated in Fig. 2. Although there are variations from metal to metal, the concentrations of Cd, Cu, Pb and Zn in soils extracted by both methods are statistically highly correlated (P<0.001). Jung and Chon (1997) also found these relationships in the paddy soils contaminated by an abandoned metal mine in Korea. According to the Korean Soil Environmental Conservation Act, soils containing over 12 mg/kg of Cd, 200 mg/kg of Cu and 400 mg/kg of Pb extracted by 0.1 N HCl solution need to be continuously monitored and not used for agricultural purposes such as crop planting. As shown in Table 1, some of the tailings exceed these guidelines. Elevated levels of Cd, Cu, Pb and Zn are also found in soils sampled in paddy fields and the forest area due to dispersion of the metals from the tailings by clastic movement through wind and water. Therefore, these contaminated soils can influence the metal uptake by plants grown on the soils. In comparison with the contaminated area, relatively low contents of heavy metals which are similar to those in background levels reported by Bowen (1979) are found in soils sampled at a nearby control area with similar geology. Results from a t-test indicate there is a statistical difference between the average concentrations in the contaminated area and the control site (P<0.05). The heavy metal concentrations in soils are shown in Fig. 3. Wide dispersion patterns are found in metal concentrations in soils. This may be due to transport clastic from tailings, especially during the wet season.
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It is well-known that most heavy metal contamination in the surface environment is associated with a cocktail of contaminants rather than one metal. Thus, the concept
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of a pollution index has been introduced in many studies to identify multi-element contamination (Jung, 1995; Chon et al., 1998). According to Chon et al.
Fig. 1. Sampling location map of the Daduk Au–Ag–Pb–Zn mine.
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Table 1 The range and mean concentrations of Cd, Cu, Pb and Zn in soils extracted by aqua-regia and 0.1N HCl(mg/kg) Extracted by 0.1N HClc
Extracted by aqua-regia Cd M S.D.b (range)
Cu M S.D. (range)
Pb M S.D. (range)
Zn (M S.D. range
Cd M S.D. (range)
Cu M S.D. (range)
Pb M S.D. (range)
Zn M S.D. (range)
Contaminated area Paddy soil 1.78 1.09 n=34a (0.40-4.76) Forest soil 1.30 0.49 n=8 (0.80-2.20) Tailings 8.57 11.7 n=12 (1.56-36.0)
42.2 30.6 (9.6-143.6) 42.7 27.0 (13.6-86.0) 481 515 (168-1,708)
133 105 (21-484) 155 227 (33-708) 4,450 3,372 (2,304-12,560)
328 251 (60-1,064) 198 77 (113-332) 753 459 (248-1,464)
0.78 0.66 (0.10-2.49) 0.38 0.39 (0.13-1.21) 5.67 13.3 (0.05-38.4)
13.3 13.0 (1.2-56.5) 9.3 14.1 (1.9-41.0) 130 278 (8.5-815)
33.2 29.4 (3.8-124) 36.3 66.5 (3.3-187) 84.4 149 (2.4-443)
73.6 69.9 (5.5-280) 33.4 30.1 (12-93) 100 158 (5.9-480)
Control area Paddy soil n=24 Forest soil n=4
28.3 20.2 (3.6-97.2) 11.4 6.4 (6.9-20.4)
81 84 (23-367) 33 7 (27-43)
183 188 (43-936) 75 22 (55-106)
0.35 0.48 (0.07-2.35) 0.09 0.01 (0.07-0.09)
5.5 7.6 (0.5-39.0) 0.80 0.67 (0.4-1.8)
20.8 29.5 (2.1-115) 3.2 0.7 (2.3-3.9)
27.3 34.7 (3.9-165) 7.2 2.2 (5.2-9.8)
a b c
1.18 0.79 (0.48-4.24) 0.77 0.14 (0.64-0.96)
Number of samples. Arithmetic mean standard deviation. Extraction method regulated by the Korean Soil Environmental Conservation Act in 1996.
Fig. 2. Metal concentrations in solis extracted by aqua-regia and 0.1 NHCl (n=68).
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(1998), the pollution index of soil is calculated by the average ratio of metal concentrations in soil to the guideline values. Tolerable levels in soil (Kloke, 1979) are used as guideline values, and the pollution index of soil samples are shown in Fig. 4. In this study, tolerable levels are the threshold values of elements in soil, concentrations above which can produce crops that are unsafe for human or animal consumption. As shown in the figure, the tailings site and a nearby ore dressing plant site are areas with a pollution index >1 and are considered as contaminated. Thus it can be seen that the large amounts of heavy metals present in the tailings and associated soils provide a source for continuing dispersion downstream and downslope, and have led to a moderate degree of contamination of paddy soils distributed below the tailings. 3.2. The trace element concentrations in various plants The range and mean concentrations of Cd, Cu, Pb and Zn in various plants grown on paddy soils and household gardens are shown in Table 2. It is well known that metal concentrations in plants vary with plant species (Adriano, 1986; Alloway, 1995). In general, leafy plants tend to accumulate higher metal concentrations than root, grain or fruit crops (Alloway, 1995; Jung and Thornton, 1996). The results of this study confirmed that metal concentrations in leafy crops such as rice stalks and soybean leaves are much higher than those in rice grain and red pepper. Jung and Thornton (1996) also reported that soybean leaf is an accumulator of heavy metals. They found 0.39, 10.0, 3.95and 200 mg/ kg, respectively, of Cd, Cu, Pb and Zn in soybean leaves
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grown on contaminated soils. In this study, higher concentrations of the metals are also found in soybean leaves. High levels of the metals are also found in red peppers and rice stalks, which is similar to the study of Jung (1995). According to Jung and Thornton (1997), the concentrations of Cd, Cu, Pb and Zn in rice grain grown on soils contaminated by Pb–Zn mining activity in Korea averaged 0.21, 3.0, 0.22 and 22.5 mg/kg, respectively. Although there is some variation of Cd concentration, rice grain sampled in the study area contains similar concentrations of the metals reported by Jung and Thornton (1997). In comparison with plants grown on contaminated soils, relatively lower concentrations of Cd, Cu, Pb and Zn are found in plants grown on soils sampled at a control area. The accumulation ratio (defined as the metal concentration in plant divided by those in soil) provides a useful indication of the relative metal availability from soils to plants (Alloway et al., 1988; Jung, 1995). The range and mean accumulation ratios for Cd, Cu, Pb and Zn in plant samples are shown in Fig. 5. It can be seen that the values for Cd, Cu and Zn are much higher than those for Pb, due to the different solubility of those metal species in soils governing their availability to plants. The ratios are also different between plant species. 3.3. The trace element concentrations in sediments and waters The range and mean concentrations of Cd, Cu, Pb and Zn in stream sediments are shown in Table 3. Elevated levels of the metals are found in the sediments, especially for Pb and Zn and to some degree for Cd and
Fig. 3. Concentrations of Cd, Cu, Pb and Zn in surface soil samples from the Daduk mine area.
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Table 2 The range and mean concentrations of Cd, Cu, Pb and Zn in crop plants (mg/kg, dry weight) Cd M S.D.b (range)
Cu MS.D. (range)
Pb MS.D. (range)
Zn MS.D. (range)
Contaminated area Red peppers n=8a Soybean leaves n=4 Rice stalks and leaves n=20 Rice grains n=8
0.610.43 (0.22-1.50) 1.981.95 (0.49-4.67) 0.801.27 (0.11-5.85) 0.150.20 (0.03-0.65)
8.74.3 (4.1-17.4) 40.758.5 (8.7-128.4) 11.19.6 (1.9-39.9) 3.11.0 (1.6-4.6)
1.92.4 (0.3-7.2) 5.00.81 (4.0-5.8) 4.44.2 (1.1-16.6) 0.320.15 (0.2-0.7)
35.126.3 (11.4-85.8) 271.3252.4 (46.3-494.8) 86.785.4 (15.4-324.8) 16.13.4 (11.8-21.8)
Control area Red peppers n=7 Soybean leaves n=5 Rice stalks and leaves n=10 Rice grains n=6
0.390.20 (0.15-0.64) 0.500.11 (0.36-0.62) 0.640.92 (0.12-2.95) 0.090.08 (0.01-0.24)
10.34.8 (5.5-19.5) 15.314.0 (7.3-39.9) 5.21.4 (2.5-7.8) 2.10.9 (1.0-3.0)
1.10.49 (0.5-2.1) 4.40.52 (3.8-5.1) 2.91.4 (1.7-6.0) 0.290.13 (0.2-0.5)
23.512.4 (13.6-47.9) 77.471.7 (32.5-202.2) 42.323.2 (16.5-79.2) 13.93.6 (8.4-18.4)
a b
Number of samples. Arithmetic mean standard deviation.
Cu. These high levels of the metals are probably influenced by clastic movement from the tailings in the mine waste dumps. However, relatively low contents of the metals are found in sediments sampled at a control site. The range and mean values of physical and chemical properties of waters and concentrations of Cd, Cu, Pb and Zn in stream waters are shown in Table 4. A wide range of water pH from 3.2 to 6.3 is found in the water samples from the contaminated area. The total dissolved solids (TDS) range from 62 to 1373 mg/l and the electrical conductivity (EC) ranges from 120 to 1930 mS/cm. Lower values for pH (<4) and higher levels of TDS(>1000mg/l) and EC (>1000 mS/cm) are found in waters sampled immediately downstream of effluents passing through tailings. Furthermore, high levels of heavy metals are found in the water. Thus, it can be understood that the main source of heavy metals in waters is the tailings containing elevated levels of the metals (see Table 1). In addition, over 1000 mg/l of SO4 is also found in the effluents, mainly due to leaching and oxidizing of sulfide minerals including pyrite, arsenopyrite, galena and sphalerite in tailings. Like dispersion patterns of soils, heavy metal concentrations in waters exponentially decreased with increasing distance from the tailings and they reach background levels at 3.3 km downstream. 3.4. Seasonal variation on metals in paddy fields As mentioned above, this study assesses the seasonal variation on heavy metal concentrations in paddy fields. Because rice is generally grown under both reducing
Fig. 4. Pollution index of heavy metals in surface soil samples from the Daduk mine area.
conditions (early August in this study) and oxidizing conditions (late September), these conditions may influence metal uptake by rice plants (Bingham et al., 1976; Jung, 1995). Many investigators have demonstrated that the availability of metals decreased under submerged conditions due to the precipitation of hydride, carbonate, sulfide and iron compounds (Kitagishi and Yamane, 1981; Dutta et al., 1989). Jung and Thornton (1997) also reported that metal concentrations in rice stalks and leaves grown under oxidizing conditions are higher than those under reducing conditions. The seasonal variations of heavy metal concentrations in rice stalks and leaves are presented in Fig. 6. The figure shows that rice stalks and leaves sampled in late September (oxidizing conditions) contain higher metal concentrations than those sampled in early August (reducing conditions). A statistical test for differences in average concentrations
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between each season confirms this (P<0.05). However, there is little variation in metal concentrations in paddy soils throughout the period of rice growing (P> 0.05). Variations on heavy metals are found in stream sediments (Table 3) and waters (Table 4). t-Test results show that, relatively high concentrations of Cd, Cu, Pb and Zn are found in water and sediment samples in the
dry season (September) (P<0.05). Lower metal concentrations in waters in the wet season (August) may be due to a dilution effect by heavy rain in summer. The dispersion patterns and seasonal variations of Cd, Cu, Pb and Zn in stream waters and sediments are shown in Figs. 7 and 8, respectively. The figures show that the concentrations of the metals decrease with increasing distance from the tailings.
Fig. 5. The range and mean values of accumulation ratios for Cd, Cu, Pb and Zn in plants. Accumulation ratio (AR) defined as metal concentation in plant divided by metal concentration extracted by aqua-regia in soil. (RP: red pepper; SL: soybean leaves; RS: rice stalks and leaves; RG: rice grain). Table 3 The range and mean concentration of Cd, Cu, Pb and Zn in stream sediments and its seasonal variation (mg/kg) Cd M S.D.b(range)
Cu M S.D. (range)
Pb M S.D. (range)
Zn M S.D. (range)
Contaminated area n=24a Samples on 2 Aug n=12 Samples on 26 Sep. n=12
4.93 15.1 (0.17-75.6) 2.09 0.93 (1.08-4.40) 7.77 21.4 (0.17-75.6)
136 166 (12.8-700) 121 98.9 (13.6-332) 152 218 (12.8-700)
685 887 (32.3,024) 616 873 (34-3,024) 754 934 (32,2,748)
417 330 (42-1,356) 356 221 (42-948) 479 412 (136-1,356)
Control area n=16 Samples on 2 Aug. n=8 Samples on 26 Sep. n=8
0.75 0.55 (0.28-2.52) 0.99 0.71 (0.28 2.52) 0.51 0.12 (0.32-0.68)
11.2 6.3 (4.0-26.4) 12.2 5.4 (6.0-22.8) 10.3 7.2 (4.0-26.4)
44 31 (12.121) 53 37 (23-121) 34 21 (12-67)
145 128 (36-532) 195 161 (45-532) 94 57 (36-216)
a b
Number of samples. Arithmetic mean standard deviation.
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Table 4 The range and mean concentrations of Cd, Cu, Pb and Zn and hydrogeological properties of waters and their seasonal variation Contaminated area Early August n=10 b
pH Conductivity (mS/cm) TDS (mg/l) Cd (mg/l) Cu (mg/l) Pb (mg/l) Zn (mg/l) a b c d
Control area a
5.4 0.8 4.1-6.3c 379 192 120-824 190 97 62-413 29 48 1-159 243 365 3-1,197 26 27 1-90 6.6 7.0 0.02-24.0
Late September n=10
Early August n=11
Late September n=11
4.4 1.1 3.3-6.2 719 491 213-1,930 512 349 151-1,373 45 26 18-112 536 329 187-1,219 42 24 15-86 13.2 9.5 4.7-38.4
6.9 0.6 5.5-8.0 109 66 51-222 54 33 25-110 1 1 nd-3d 2 2 nd-5d 2 2 1-7 0.19 0.29 0.01-0.88
6.9 0.33 6.5-7.4 157 99 71-378 111 71 51-269 1 2 nd-6d 11 5 7-23 3 1 2-4 0.25 0.52 0.01-1.73
Number of samples. Arithmetic mean standard deviation. Range. nd, Not determined.
Fig. 6. Seasonal variation of metal concentrations in agricutural crops (red peppers: 1–4; soybean leaves: 5–8; rice stalks: 9–17).
Fig. 7. Variations of PH and heavy metal concentrations in water with a distance and their seasonal variation in the Daduk area.
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in stream water and sediments decreased with increasing distance from the tailings. As a result of seasonal variations, rice stalks and leaves grown under oxidizing conditions contained higher metal concentrations than those grown under reducing conditions (P<0.05). In addition, relatively high concentrations of metals were found in water and sediment samples collected during the dry season (P<0.05).
Acknowledgements We would like to acknowledge the Center for Mineral Resources Research (CMR) of Korea and the Research Division of Seoul National University/Hanyang University for Social Infrastructure and Construction Technology (BK-21) for their financial support during this study. Many thanks are extended to reviewers who improved our original manuscript.
References
Fig. 8. Seasonal variation of metal concentrations in stream sediments in the Daduk mine area.
4. Conclusions In this study of an area around a disused Au–Ag–Pb– Zn mine, soils, plants, waters and sediments have been contaminated by past mining activity. Elevated levels of heavy metals including Cd, Cu, Pb and Zn were found in tailings with arithmetic mean values of 8.57, 481, 4450 and 753 mg/kg, respectively. These significant concentrations greatly influenced the metal concentrations in paddy soils and forest soils in the vicinity of the tailings by surface erosion, wind action and by effluent draining the tailings into lower lying land mainly used for growth of paddy rice and garden crops. Comparison with extraction methods by both 0.1 N HCl and aqua regia showed heavy metal concentrations in soils to have a statistically significant correlation between the two methods (P<0.001). Metal concentrations in crop plants varied between plant species. Soybean leaves and rice stalks and leaves had elevated levels of heavy metals, especially for Cd and Zn. Red peppers and rice grain also accumulated the metals. Relatively high metal accumulation ratios were found in soybean leaves, with over 1.0 for Cd, Cu and Zn. Rice stalks and leaves also show high accumulation ratios. Heavy metal concentrations
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