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Seasonal and spatial characteristics of seawater and sediment at Youngil bay, Southeast Coast of Korea
Marine Pollution Bulletin 57 (2008) 325–334
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Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul
Seasonal and spatial characteristics of seawater and sediment at Youngil bay, Southeast Coast of Korea Mikyung Lee a,c, Wookeun Bae a, Jinwook Chung b, Hoi-Soo Jung c, Hojae Shim d,* a
Department of Civil and Environmental Engineering, Hanyang University, Republic of Korea R&D Center, Samsung Engineering Co., Ltd., Republic of Korea c Marine Geoenvironment and Resources Research Division, Korea Ocean Research and Development Institute, Republic of Korea d Department of Civil and Environmental Engineering, University of Macau, Macau SAR, China b
a r t i c l e Keywords: Estuary I0geo Labile fraction Metals Sediment
i n f o
a b s t r a c t The seasonal geochemical characteristics of the seawater and sediments and the major factors causing heavy metal contamination were investigated at the Youngil bay and the Hyungsan river estuary in the Southeast Coast of Korea, where a world-scale steel-industry complex (Pohang iron and steel industrial complex, POSCO) is located. The seasonal and spatial distribution characteristics of temperature, dissolved oxygen (DO), pH, and nutrients of the seawater were studied at 45 fixed stations, especially focusing on the river mouth area. Sediments at 27 stations were examined during winter and summer to determine the major controlling factors for the distribution of metals, using correlation matrix and R-mode factor analyses, and to evaluate the pollution status, using the modified geoaccumulation ðI0geo Þ index. Temperatures for the effluent from the POSCO located at the Hyungsan river mouth were 2–3 °C higher compared to other sampling areas, due to the thermal discharge from the POSCO. The DO concentration of the surface water at the Pohang old port was as low as 2–4 mg/L. In spring, the DO value at the Hyungsan river mouth was higher than 12 mg/L, by the mass multiplication of phytoplanktons at the river mouth where seawater temperature and nutrients concentrations were relatively high, resulting in the pH value of higher than 8.3. The nitrogen to phosphorus (N/P) ratios at the river mouth were 20–150 times higher compared to other areas, implying that the nitrogen loading into this semi-enclosed bay is significantly higher than phosphorus and the major nitrogen sources are not only the domestic sewage from the city but the industrial wastewater from the POSCO and other steel factories nearby. The phosphorus concentrations at the Pohang old port were shown 3–10 times higher than those at other stations, due to the inflow of pollutants generated from the nearby ships anchoring and the release of phosphate from the bottom sediment. Results from the sediment analysis showed that the major controlling factors for the distribution pattern of each metal are grain size and organic carbon (Corg) content. Based on the factor analysis, Al, Fe, Cr, Li, and Pb were shown strongly correlated with the mean grain size (Mz), whereas Cd, Cu, Zn, and Sn with the Corg content. Results from the fractionation of the sedimentary metals into lattice and labile fractions to characterize the mobility of sediment metals showed that the mineral lattice fraction was high in the order of Al = K > Cr > Li > Sr > Fe, while the labile fraction, which might be released to the overlying water, was in the order of Pb > Zn > Cd > Cu > Ca > Sn. Evaluation of the sediment pollution status by applying ðI0geo Þ of 13 metals showed Cd, Cu, and Zn as high as 1–3 range at the old port. Even though the overall marine pollution mainly by the world-class steel industrial complex in this semi-enclosed bay area studied does not currently pose a serious threat, due to the seawater circulation and the large influx of river discharge, the countermeasures to implement the sediments concentrated with heavy metals, especially at the old port with no seawater circulation, are still warranted for this coastal water environment. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
* Corresponding author. Address: Faculty of Science and Technology, University of Macau, Taipa, Macau SAR, China. Tel.: +86 853 8397 4456; fax: +86 853 2883 8314. E-mail address: [email protected] (H. Shim). 0025-326X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpolbul.2008.04.038
The importance of the estuarine environment management has long been addressed, since there are wide ranges of salinity and land-derived pollutants emitted to the sea as well as the place being where the biodiversity of marine inhabitants exists. Biogeochemically complex phenomena such as the addition and removal
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of dissolved matters, the interaction with suspended matters, including adsorption and desorption, the ingestion and excretion by creatures, and the decomposition of organisms occur in the estuarine environment (Martin et al., 1971; Sholkovitz et al., 1977; Sholkovitz and Coplang, 1981). Such complex aspects can become even more complicated due to the distinctive seasonal variations. In case of sediments, the natural contamination by metals seems a world-wide problem (Niragu and Pacyna, 1988) and this phenomenon is especially significant in the estuarine and coastal sediments, usually as a sink receiving the river-derived metals from weathering rocks and anthropogenic sources (Martin and Windom, 1991; Zwolsman et al., 1996). Trace metals deposited in sediments may be directly available to the benthic fauna (Langston, 1990) or released to the water column through the sediment re-suspension, the adsorption–desorption reactions, the oxidation–reduction reactions, and the degradation of organism (Santschi et al., 1990). These processes may further enhance the dissolved metal concentrations in the environment and threaten the ecosystem. In order to prevent the marine pollution by trace metals, it is necessary to establish the database and to understand the mechanisms influencing the distribution of trace metals in the marine environment (Fang and Hong, 1999). Despite the well known importance of river mouth and coastal management, analyzing the characteristics of the estuarine environment can be a quite task and most previous researches have not dealt with the variations covering distinctive four seasons. Therefore, the main objective of this research was to investigate the seasonal and spatial characteristics of seawater quality from the river estuary to the whole bay area, throughout the year. Mean grain sizes, organic car-
bons, and metal elements in sediments were also analyzed to better understand the impacts of pollutants sources in the area. 1.1. Study area The sea areas around the Korean peninsula are affected by the varying conditions, characteristic of distinctive four seasons. The Youngil bay is located at the far southeast and the Hyungsan river runs through the world’s largest, Pohang iron and steel industrial complex, POSCO, as shown in Fig. 1. This semi-enclosed bay has the gross sea area of 120 km2. The Hyungsan river flows into the Youngil bay through Kyungju City and Pohang City with the population of 300,000 and 500,000, respectively. The Hyungsan river valley (watershed) covers the area of 1,167 km2, its extended river path and average width are 62.2 km and 18.8 km, respectively, and the lower reaches are about 100 m wide. The average rainfall in this area is 17.4 m3/s, the average minimum flow is 4.3 m3/s, and the annual average outflow reaches about 6 108 m3. The Hyungsan river serves as the principal drinking water source for both cities as well as for the industrial and agricultural usages. On the other hand, due to its riverbed with a steep slope, the water flows rapidly into the Youngil bay. Furthermore, as the water travels short, the purification capacity of the river fall short of the average. Especially, for a small port called Pohang old port, located north of the river mouth and with lots of fish markets, due to the poor circulation with outer sea water, the pollution status is currently getting even worse (Fig. 1). According to the pollution status of the Hyungsan river water system (EDWGIS, 2002), 303,735 m3 of sewage and wastewater are generated daily, with domestic, industrial,
Fig. 1. Sampling stations in the Youngil bay, Korea (numbers are for the fixed stations and circles are for sediments sampled).
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and livestock wastewater at 78%, 20%, and 2%, respectively, whereas among the total biochemical oxygen demand (BOD) pollutant loads of 92,027 kg/day, 23% and 57% in the form of livestock and industrial wastewater, respectively, flow out of the river downstream where the POSCO is situated. In addition, since the river valley has paddy and dry field area and forest land area, 16% and 71% of the gross area, respectively, red tide and eutrophication would result when the river is polluted by non-point sources flowing into the Youngil bay. Deteriorating water quality due to the industrial and domestic wastewaters in this bay area thus makes it imperative for the continuous monitoring of pollutants. 2. Data collection and methods Throughout 45 fixed stations at the Hyungsan river mouth in the Youngil bay area (Fig. 1), field observations and seawater samplings were performed during four different seasons from November 2001 to July 2002. Water temperature, pH, and dissolved oxygen (DO) were measured on site using a multi-probe sensor (YSI-556, USA). For the seawater, nitrite, nitrate, and phosphate were measured following the method of Parsons et al. (1984). In case of nitrite (NO2), sulfanilamide was added to the sample via diazotization, resulting in an azo compound which then reacts with the NED (naphthyl-ethylene-diamine) solution, forming a diazo compound. The absorbance of azo compounds was measured from 543 nm using a UV–Vis spectrophotometer (Shimadzu UV1601, Japan). In case of nitrate (NO3), samples in NH4Cl buffer solutions were passed through the Cu–Cd reductor column, going through the reduction to nitrite. Then, after coupled with sulfanilamide and NED solution, a dye was formed and its absorbance was measured from 543 nm, similar to nitrite. In case of phosphate (PO43), a mixed solution of ammonium molybdate, sulfuric acid, and potassium antimonyl tartrate was added to the sample to transform the existing phosphate into a complex of ammonium
phosphomolybdate in acidic condition. This complex compound was then reduced to ascorbic acid, forming a blue dye for which the absorbance was measured from 885 nm. On the other hand, the total of 27 sediment samples was collected at the Youngil bay, including the Hyungsan river mouth and the Pohang old port (Fig. 1). After removing organic substances and carbonates using diluted hydrogen peroxide and hydrochloric acid (HCl) and then using 4U sieve, the grain size was divided into two groups, coarse and fine. These coarse and fine grains were then analyzed by a dry sieving method after dehydration and by a pipettes method after adding diffusion gas, respectively (Folk, 1968). The grain size was calculated according to Folk and Ward (1975), with percentage in every 1U interval. The total carbon contents of sediments were measured using a CHN-analyzer (CE Instrument, USA; Flash1120) with a tin capsule filled with powder samples, and the inorganic carbon content was obtained by eliminating inorganic carbon after samples mixed with sulfuric acid. The amount of organic carbon was then calculated by subtracting the amount of inorganic carbon from the total carbon. For accuracy, Sulfanilamide Standard Material (SRM) was used. For the metal concentrations of sediment, 0.2 g of dried sample in a teflon bomb was added in 5 mL mixture of nitric acid and perchloric acid (3:1) to oxidize organic matters thoroughly. Dehydrated materials were then dissolved thoroughly by boiling with 5 mL mixture of boric acid and perchloric acid (4:1). This mixed material was dried and then liquefied on a heating plate for 10 min (Tessier et al., 1979). Quantification of metal elements against pretreated samples was done using an inductively coupled plasma mass spectrometer (ICP-MS; VG PlasmaQud ICP-MS, Korea Basic Science Institute). While metal elements with higher recovery rates were shown Cd (115%), Cu (108%), and Pb (114%), metal elements with lower recovery rates were Cr (82%) and Sr (84%), and other minor metal elements were in the vicinity of 88% to 99%, thus showing good representations for most parts. In this study, the labile fraction was defined as the amount extracted by 1 N HCl according to Fang and Hong (1999),
Table 1 Seasonal water properties at the Youngil bay Season
Temp. (oC)
DO (mg/L)
pH
NO2 + NO3 (lM)
PO43 (lM)
Spring Avg.
16.1–21.8 17.9
2.4–12.5 8.2
7.6–8.3 8.0
0.3–16.7 3.7
0.1–1.4 0.2
Summer Avg.
22.8–28.3 24.7
1.6–10.3 7.4
7.6–8.4 8.1
0.2–68.1 17.6
0.1–3.4 0.8
Fall Avg.
15.7–20.8 17.3
2.2–7.1 5.6
8.0–8.4 8.3
1.4–59.4 11.3
0.03–2.1 0.6
Winter Avg.
6.7–10.4 8.5
4.9–8.5 7.9
ND*
5.3–61.4 16.2
0.5–2.6 0.8
*
Not determined.
Table 2 Mean grain size, Corg, and metal elements concentration in this study area (unit in %, lg/g) Area
Season
Mz (phi)
Corg
Al
Fe
K
Ca
Sr
Zn
Cu
Cd
Sn
Cr
Pb
Li
Old port (St. 142, 113, 85)
Summer Winter Avg.
4.4 5.0 4.7
2.0 1.8 1.9
6.2 5.8 6.0
2.6 2.5 2.5
1.8 1.7 1.8
1.3 1.3 1.3
170.0 170.7 170.3
377.0 364.7 370.8
95.6 133.7 114.6
3.4 4.0 3.7
5.6 7.1 6.4
39.0 39.2 39.1
45.3 53.2 49.2
42.8 42.7 42.8
Hyungsan river mouth (St. 273, 193, 175)
Summer Winter Avg.
3.0 2.3 2.6
0.9 0.2 0.6
6.5 5.9 6.2
2.9 1.8 2.3
2.3 2.4 2.4
1.3 1.1 1.2
200.0 196.0 198.0
141.3 146.1 143.7
18.2 19.1 18.7
0.5 0.6 0.6
4.4 2.5 3.5
25.2 15.0 20.1
33.4 22.0 27.7
23.2 16.6 19.9
Other sites
Summer Winter Avg.
4.2 3.8 4.0
0.7 0.6 0.6
5.6 6.2 5.9
2.3 2.5 2.4
2.0 2.2 2.2
1.6 1.9 1.7
211.7 232.4 222.0
96.8 86.6 91.7
17.5 10.9 14.2
0.3 0.4 0.3
3.2 2.8 3.0
26.9 26.4 26.6
34.2 29.6 31.9
26.3 24.6 25.4
Total
Summer Winter Avg.
3.9 3.7 3.8
1.2 0.9 1.0
6.1 6.0 6.0
2.6 2.3 2.4
2.0 2.1 2.1
1.4 1.4 1.4
193.9 199.7 196.8
205.0 199.1 202.1
43.8 54.6 49.2
1.4 1.6 1.5
4.4 4.2 4.3
30.4 26.9 28.6
37.6 34.9 36.3
30.8 28.0 29.4
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and was analyzed as follows. First, 0.4 g of dried samples was mixed with 20 mL of 1 N HCl and warmed up with shaking at 50 °C for 6 h. After centrifugation, supernatants were analyzed by the ICP-MS. 3. Results and discussion 3.1. Seasonal and spatial distribution characteristics of water temperature, DO, pH, and nutrients Water temperatures at 45 fixed stations showed a wide variance, ranging from 6.7 °C to 28.3 °C. Seasonal average temperatures were 8.5 °C in winter and 24.7 °C in summer, thus displaying a relatively large difference between them (Table 1). The higher water temperatures at the Hyungsan river mouth throughout the year, especially, compared to other areas at the Youngil bay (data not shown), were considered due to the thermal discharge from the POSCO, which did not disperse further into the open sea. The seasonal DO concentrations were ranged from 1.6 to 12.5 mg/L and the average DO ranged from 5.6 to 8.2 mg/L (Table 1). Except for spring, DO was distinctively lower at the Hyungsan river mouth and the Pohang old port, compared to the open sea (data not shown). The lower DO concentrations at the river mouth were considered due to the increased water temperatures by the heated water mentioned above. It was also considered that the organic matters flowing into the polluted old port were dissolved but
could not be mixed with the open seawater because of its halfclosed geographical feature, which subsequently caused the DO to be exhausted. The seasonal pH range, except for winter (not measured), were from 7.6 to 8.4 with the average of 8.0–8.3 (Table 1). For the river mouth, pH’s were slightly lower except for spring, whereas pH’s at the old port, ranging from 7.6 to 8.3, were always lower throughout spring, summer, and fall, compared to the sea area (data not shown). The lower pH’s at the heavily polluted old port were due to the active decomposition of organic matters, providing more hydrogen ions to the water. In addition, organic carbon (Corg) contents in sediments at the old port were also much higher than those in nearby sea area (Table 2). The seasonal concentrations of dissolved inorganic nitrogen (NO2–N + NO3–N) ranged from 0.2 to 68.1 lM and the seasonal averages were also shown with a large variation from 3.7 lM in spring to 17.6 lM in summer, whereas the seasonal average value for dissolved inorganic phosphate was from 0.2 to 0.8 lM (Table 1). Both nitrogen and phosphate were shown with almost the same spatial distribution pattern, with much higher concentrations especially at the Hyungsan river mouth and the Pohang old port, throughout the year (data not shown). Such high concentrations of nitrogen and phosphate in these areas were considered caused by domestic sewage, industrial wastewater, and non-point sources from the river. High phosphate concentrations, in particular, were probably caused by the pollution from anchoring ships and the phosphorus release from the anaerobic bottom sediments. In addi-
Fig. 2. Seasonal distributions of DO and pH and their relationship at the Hyungsan river mouth and the Pohang old port.
M. Lee et al. / Marine Pollution Bulletin 57 (2008) 325–334
tion, pollutants such as domestic wastewater and non-point sources continuously flow into the river. The current at the Youngil bay is from the northwestern, via the inside of the bay along the coastline, into the open sea merging with the Hyungsan river and the nitrogen dispersion was shown expanded along with the seawater current system (data not shown). 3.2. Changes in DO and pH Fig. 2 shows the seasonal distributions of DO and pH and their relationships around the Youngil bay, especially at the Hyungsan river mouth and the Pohang old port, except for winter. In spring, the water temperature at the river mouth was above 20 °C and the DO concentration was above 12 mg/L, about 3 mg/L higher compared to the sea area (data not shown). In addition, the green tide was also observed during spring, resulted from the multiplication of phytoplanktons at the river mouth. As the eutrophication causes phytoplanktons to multiply, the active photosynthesis by the multiplied phytoplanktons subsequently caused the DO to increase. On the other hand, pH at the river mouth during spring was above 8.3, about 0.3 higher compared to the sea area. In general, CO2, HCO3, and CO32 exist at 1%, 92%, and 7%, respectively, in seawater. However, phytoplankton multiplication causes the exhaustion of CO2 and HCO3 couples with H+ to form H2CO3, which results in favoring the reverse reaction. Subsequently, H+ in seawater is exhausted, followed by the pH increase (David, 1995). Therefore, the increases of DO concentration and pH at the Hyungsan river mouth during spring were considered due to the mass multiplication of phytoplanktons.
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3.3. N/P ratio in seawater The ratio of nitrogen (NO2 + NO3) to phosphate (PO43) was P20 at the Hyungsan river mouth throughout the year (Fig. 3). Since the seasonal average phosphate concentration was relatively constant at 0.2–0.8 lM (Table 1), this high N/P ratio might have been resulted from the mass inflow of nitrogen into the river mouth. The Hyungsan river runs through two big cities, Pohang and Kyungju (Fig. 1), with a total population of 800,000 and the world’s largest iron and steel industrial complex nearby. Such excess nitrogen may also flow out via domestic wastewater into the Hyungsan river. Especially in spring, the ratio was as high as 150 (Fig. 3), most probably due to the less flow of phosphorus compared to other seasons or the alkaline pH coupled with phosphate settling with such compounds as calcium or iron (Stanley et al., 2000). 3.4. Grain size composition, organic carbon (Corg) contents, and distribution characteristics of metals in sediment
3.4.1. Grain size Sediments were sampled during summer and winter, to assess the primary causes for the elevated trace metal concentrations. Mean grain size (Mz) values were 3U at the Hyungsan river mouth, 4U at the Youngil bay, and 5U at the Pohang old port (Table 2). According to Folk’s triangle (1968), grain sizes were divided into four types, sand, muddy-sand, sandy-mud, and mud. The sand and muddy-sand zones were distributed primarily on the northern edge of the bay, whereas the sandy-mud and mud zones were
Fig. 3. Seasonal ratios of nitrogen to phosphate at the Hyungsan river mouth and the Pohang old port.
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distributed dominantly throughout the center of the bay, along with the Hyungsan river mouth and the Pohang old port (Fig. 4). This difference was mainly due to the influence by the current system in the bay area and the amount of the river discharge. The current at the Youngil bay is from the northwestern, via the inside of the bay along the coastline, into the open sea, merging with the flow from the river. From the northwestern to the inside of the bay, sandy type sediments were predominant because of the strong speed of current and the lower depth of water. On the other
hand, for the center of the bay, sediments were mainly composed of fine grains due to the weak speed of current and the higher depth of water. 3.4.2. Corg contents The highest organic carbon (Corg) content was observed at the old port (2.0%) and the correlation coefficients (r2) between Mz and Corg were shown 0.74 and 0.88 during summer and winter, respectively (Table 2 and Fig. 5).
Fig. 4. Grain size distribution of sediments at the Youngil bay.
Fig. 5. Relationship between mean grain size and TOC content of sediment at the Youngil bay.
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3.4.3. Distributional characteristics of metal elements As shown in Table 2, the average concentrations of 12 metal elements in sediment were shown Al (6.0%), Fe (2.4%), K (2.1%), Ca (1.4%), Sr (196.8 ppm), Zn (202.1 ppm), Cu (49.2 ppm), Cd (1.5 ppm), Sn (4.3 ppm), Cr (28.6 ppm), Pb (36.3 ppm), and Li (29.4 ppm), respectively. To determine the controlling factors for their distributions in sediment, the correlation matrix and the Rmode factor analyses were carried out for 15 items, including metal elements, mean grain size (Mz), and Corg content. As a result, four primary factors were determined characterizing 97% of the total variance and there were no significant distributional differences between summer and winter (Table 3). In summer, Factor 1 takes up to 37.6% of the total variance and shows high loading values for Mz, Corg, Al, Fe, Cr, Li, and Pb, which were more concentrated at the central bay area and the Pohang old port. The distribution of Al, Fe, Cr, Li, and Pb was quite high as the grain size of sediment became finer (Fig. 6a). Factor 2 comprising 37.5% of the total vari-
ance mostly consists of Cd, Cu, Sn, and Zn (Table 3). The levels of these toxic metals were significantly higher at the Pohang old port compared to other areas despite of no industrial complex, due to the high organic carbon concentration and being the anaerobic environment (Fig. 6b). These organophilic metals could be accumulated to and released from the bottom sediments at the old port, where the metals might be present in forms of such metal sulfides as CdS, CuS, and ZnS. The pH and oxidation–reduction potential (Eh) for those metal sulfide complexes were 6–6.5 and 0.2–0 mV, respectively, which were thus feasible to occur in the anaerobic environment of the old port. For Factor 3, the major elements were Ca and Sr (Table 3 and Fig. 6c), which generally occur in the carbonates and are highly concentrated in shells within the sea sediment (Emelyanov and Shimkus, 1986; Cho et al., 1999). Factor 4 shows that K was the highest loading element (Table 3 and Fig. 6c), and Cho et al. (1999) reported that K can be highly concentrated in sediment which contains illite and feldspar.
Table 3 Rotated component matrix for the sediment during (a) summer and (b) winter Component 1
Component 2
Component 3
Component 4
Communality
Panel a Mz Corg Al Fe Cr Li Pb Cd Cu Sn Zn Sr Ca K Eigen value (%)
0.92 0.73 0.91 0.91 0.80 0.72 0.69 0.18 0.36 0.46 0.45 0.02 0.07 0.06 37.6
0.17 0.64 0.25 0.31 0.54 0.65 0.64 0.94 0.91 0.83 0.88 0.16 0.02 0.55 37.5
0.26 0.08 0.16 0.14 0.11 0.17 0.09 0.10 0.15 0.07 0.08 0.98 0.99 0.56 17.6
0.17 0.13 0.28 0.09 0.19 0.14 0.16 0.16 0.11 0.24 0.06 0.12 0.02 0.59 4.9
0.98 0.97 0.99 0.96 0.99 0.98 0.90 0.94 0.99 0.95 0.99 0.99 0.99 0.97
Panal b Mz Corg Al Fe Cr Li Pb Cd Cu Sn Zn Sr Ca K Eigen value loading (%)
0.85 0.65 0.95 0.95 0.79 0.74 0.67 0.08 0.11 0.35 0.32 0.13 0.29 0.27 35.6
0.16 0.52 0.13 0.14 0.34 0.44 0.52 0.91 0.96 0.86 0.88 0.15 0.10 0.35 30.9
0.33 0.26 0.05 0.12 0.23 0.24 0.17 0.11 0.06 0.08 0.16 0.97 0.93 0.13 15.8
0.32 0.43 0.18 0.16 0.43 0.40 0.35 0.26 0.10 0.14 0.12 0.09 0.13 0.86 12.1
0.96 0.94 0.95 0.97 0.98 0.96 0.87 0.91 0.94 0.88 0.92 0.99 0.98 0.96
Fig. 6. Metal elements distributions of sediment at the Youngil bay.
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Overall, two major controlling factors for the metal distribution of sediment in this bay area are mean grain size and organic carbon content (Table 3). Since the mean grain size is usually controlled by the seawater current system as mentioned above, the reduction of organic carbon content may play a key role in controlling the pollution status especially at the Pohang old port area.
3.5. Metal content in labile fraction Sedimentary trace metals present in the labile fraction can be re-dissolved into the water column under the favorable environmental conditions, including Eh or pH changes (Santschi et al., 1990). The concern of sedimentary trace metals polluting the
Fig. 7. Average percentage of acid (1 N HCl) extractable metal (labile) fractions from the sediment at the Youngil bay.
Table 4 Modified geoaccumulation index ðI0geo Þ for total metals in study area
Old port Hyungsan river Youngil bay Avg.
Al
Fe
Ca
K
Cr
Zn
Cu
Cd
Sn
Pb
Sr
Li
0.6 0.2 0.3 0.2
0.4 0.3 0.1 0.0
0.7 0.0 0.0 0.2
1.1 0.1 0.5 0.5
0.2 0.4 0.4 0.3
1.5 0.9 0.2 0.9
2.4 0.6 0.2 1.1
2.9 1.0 0.5 1.5
0.4 0.3 0.1 0.2
0.2 0.2 0.5 0.3
0.9 0.2 0.2 0.3
0.0 0.3 0.4 0.2
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marine environment is that the amount of these metals can be released into the water column during the early diagenesis, thus enhancing the potential deterioration of water quality in the environment. Many one-step extraction procedures have been developed to assess this phenomenon and the sedimentary trace metals determined by these techniques are generally divided into two fractions, namely labile and lattice (non-labile) or bioavailable and non-bioavailable (Luoma, 1990). To estimate influences of surrounding environments in this study, the amount of metal elements was analyzed by the single acid extraction with 1 N HCl. This method was appropriate especially to judge the contamination derived from anthropogenic activities, due to the fact that it is possible to obtain metal elements corresponding to those in labile fractions (Chester and Voutsinou, 1981). Results from the fractionation of the sedimentary metals into lattice and labile fractions to characterize the mobility of sedimentary metals showed that the mineral lattice fraction was high in the order of Al = K (94.5%) > Cr (78.3%) > Li (73.7%) > Sr (64.3%) > Fe (64.1%), while the labile fraction, which might be released to the overlying water, was in the order of Pb (83.8%) > Zn (82.1%) > Cd (78.5%) > Cu (72.0%) > Ca (66.0%) > Sn (58.7%), as shown in Fig. 7. The simple approach to the normalization of geochemical data is to compare the total metal concentration of surface sediments with concentrations characteristic of natural background levels or anthropogenically uncontaminated sediments. Among such various methods as enrichment factor (EF), geoaccumulation index ðIgeo Þ, concentration enrichment ratio (CER), and metal pollution index, ðIgeo Þ was employed in this study since the regional geometrical characteristics can be reflected, and the contamination degree of the sea area studied was quantified accordingly. ðIgeo Þ, suggested by Müller (1979), is used to estimate the contamination degree of sediments and calculated through the following equation. Igeo ¼ log2 C n =1:5Bn where Cn is the content of sample analyzed and Bn is the reference value. In general, the metal content increases as the particle size becomes finer. Therefore, it is appropriate to calculate the contamination index by considering the regional Mz even though the metal contents in sediment are same. In this study, the contamination index was calculated by the following modified equation after considering the regional Mz in Müller’s equation. Modified I0geo ¼ log2 C n =1:5 Bn Mz where Mz indicates Mzc/MzB, mean grain size ratio of investigation station to reference area. Therefore, the average values for stations 25, 31, and 44 (Fig. 1) were considered least influenced by Mz and inflowing contamination sources. The ðIgeo Þ value of metals is classified into five types: class 1 (0–1, unpolluted to moderately polluted), class 2 (1–2, moderately polluted), class 3 (2–3, moderately to strongly polluted), class 4 (3–4, strongly polluted), and class 5 (4–5, strongly to very strongly polluted). In case of ðIgeo Þ value less than 0, it is classified as uncontaminated area. Results showed the ðI0geo Þ value for the Pohang old port decreased after the Mz adjusted, due to the fine granularity. Especially, such metals as Cr, Li, and Pb were found not contaminated with after adjusting the grain size (Table 4). In comparison, due to the coarse granularity, the ðI0geo Þ value for the Hyungsan river mouth increased after the Mz adjusted. For the open sea areas at the Youngil bay, it was found uncontaminated with the ðI0geo Þ value less than 0. However, the ðI0geo Þ values of Zn, Cu, and Cd at the old port were shown 1–3, implying more serious heavy metal contamination than sea areas.
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4. Conclusion (1) Compared to other seasons and other areas at the Youngil bay, dissolved oxygen and pH levels at the Hyungsan river mouth during spring were distinctively higher, around 12.0 mg/L and 8.3, respectively. In addition, when the water temperature was relatively high in spring, phytoplanktons multiplied massively resulting in the green tide. (2) Compared to other areas at the bay, the Hyungsan river mouth was shown with very high N/P ratios (up to 150 in spring) throughout the year, probably due to the nitrogen pollution by the domestic wastewater generated from two neighboring big cities and the industrial wastewater generated from the world’s largest Pohang iron and steel industrial complex (POSCO) and other related industries nearby. Therefore, to prevent the nitrogen loadings from flowing out into the open sea, the optimal sewage treatment, including the advanced nutrient treatment, is needed and the industrial complex in this area needs to treat its own wastewater more properly at sources. (3) Despite the dredging was conducted about two years ago, the seawater around the Pohang old port does not appear to circulate well. Thus, compared to other areas at the bay, pH and dissolved oxygen levels of the water were lower, sometimes even with odors such as hydrogen sulfide generated, whereas the phosphate concentration was 3–10 times higher resulting from the massive phosphorus inflow from the neighboring fish market, many anchoring ships, and the anaerobic state of sediments. Such heavy metals as Cd, Cu, and Zn were also highly concentrated around the old port despite no industrial complex nearby, as well as with finer grain size and higher organic carbon content, compared to other bay areas. In addition, compared to the geoaccumulation index, ðI0geo Þ, indicating the relative metal pollution, of less than 0 for other studied areas, the ðI0geo Þ values of Zn, Cu, and Cd for the old port was 1–3, implying severe pollutions. This further suggests Cd2+, Cu2+, and Zn2+ in seawater be accumulated in forms of such metal sulfide complexes as CdS, CuS, and ZnS in sediments, resulting from the high organic matter content in sediment and the anaerobic environment verified by the pH and Eh values at the old port. Based on the overall results, the marine pollution status in this semi-enclosed bay area may not show the direct negative effect of the world-class steel industrial complex, probably due to the circulating seawater current system. However, results also suggested the nitrogen control especially around the complex still be required. The major controlling factors for the distribution of sedimentary metals at the Youngil bay area were shown mean grain size and organic carbon content. Reducing organic carbon content will further relieve toxic heavy metal contents in the sediment, which may subsequently be the solution to control the pollution status around the Youngil bay. Acknowledgements This study was supported by a Grant from the Korea Ocean Research Institute through the POSCO and the Pohang City Council. Help of Ms. Hye-Jin Rho is also appreciated for the sample collection and preparation. References Chester, R., Voutsinou, F.G., 1981. The initial assessment of trace metal pollution in coastal sediment. Marine Pollution Bulletin 12, 84–91.
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Cho, Y.G., Lee, C.B., Choi, M.S., 1999. Geochemistry of surface sediments off the southern and western coasts of Korea. Marine Geology 159, 111–129. David, H.M., 1995. Marine Life and the Sea. Wadsworth Publishing Company, pp. 51–79. Emelyanov, E.M., Shimkus, K.M., 1986. Geochemistry and Sedimentology of the Mediterranean Sea. Springer, Berlin, p. 553. EDW (Environmental Data Web) GIS, 2002.. Fang, T.H., Hong, E., 1999. Mechanisms influencing the spatial distribution of trace metals in surficial sediments off the south-western Taiwan. Marine Pollution Bulletin 38, 1026–1037. Folk, R.L., 1968. Petrology of Sediment Trace Elementary Rock. Hemphill’s, Drawer M., University Station, p. 170. Folk, R.L., Ward, W.C., 1975. Brazos river bar: a study in the significance of grain size parameters. Journal of Sedimentary Petrology 27, 3–26. Langston, W.J., 1990. Toxic effects of metals and the incidence of metal pollution in marine ecosystems. In: Furness, R.W., Rainbow, P.S. (Eds.), Heavy Metals in the Marine Environment. CRC Press Inc., pp. 101–122. Luoma, S.N., 1990. Processes affecting metal concentrations in estuarine and coastal marine sediments. In: Furness, R.W., Rainbow, P.S. (Eds.), Heavy Metals in the Marine Environment. CRC Press Inc., pp. 51–66. Martin, J.M., Jednacak, I., Pravdic, V., 1971. The physico-chemical aspects of trace elements behavior in estuarine environment. Thalassica Jugoslavica 7, 619–637. Martin, J.M., Windom, H.L., 1991. Present and future roles of ocean margins in regulating marine biogeochemical cycles of trace elements. In: Montora, R.F.C.,
Martin, J.M., Wollast, R. (Eds.), Ocean Marginal Processes in Global Change. Wiley, UK, pp. 45–68. Müller, G., 1979. Schwernetalle in den sedimenten des rheins-veranderungen seit. Umschau 79, 778–783. Niragu, J.O., Pacyna, J.M., 1988. Quantitative assessment of worldwide contamination of air, water and soils by trace metals. Nature 333, 134–139. Parsons, T.T., Maita, Y., Lalli, C.M., 1984. A Manual of Chemical and Biological Methods for Seawater Analysis. Pergamon Press, New York, p. 173. Santschi, P., Hohener, P., Benoit, G., Brink, M.B., 1990. Chemical processes at the sediment-water interface. Marine Chemistry 30, 269–315. Sholkovitz, E.R., Boyle, E., Price, N.B., Edmond, K.M., 1977. Removal of dissolved material in the Amazon estuary. American Geophysical Union 58, 423. Sholkovitz, E.R., Coplang, D., 1981. The coagulation, solubility and adsorption properties of Fe, Mn, Cu, Ni, Co and humic acids in a river water. Geochimica et Cosmochimica Acta 45, 181–189. Stanley, I.D., Timothy, F.H.A., Stephen, R.C., Anthony, R.I., Robert, L.J., James, F.K., Nancy, E.L., Monica, G.T., 2000. Ecology. Oxford University Press Inc., pp. 146– 150. Tessier, A., Campbell, P.G.C., Bisson, M., 1979. Sequential extraction procedure for the speciation of particulate trace metals. Analytical Chemistry 51, 844– 851. Zwolsman, J.J., van Eck, G.T.M., Burger, G., 1996. Spatial and temporal distribution of trace metals in sediments from the Sheldt Estuary, South-west Netherlands. Estuarine, Coastal and Shelf Science 43, 55–79.
Contents lists available at ScienceDirect
Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul
Seasonal and spatial characteristics of seawater and sediment at Youngil bay, Southeast Coast of Korea Mikyung Lee a,c, Wookeun Bae a, Jinwook Chung b, Hoi-Soo Jung c, Hojae Shim d,* a
Department of Civil and Environmental Engineering, Hanyang University, Republic of Korea R&D Center, Samsung Engineering Co., Ltd., Republic of Korea c Marine Geoenvironment and Resources Research Division, Korea Ocean Research and Development Institute, Republic of Korea d Department of Civil and Environmental Engineering, University of Macau, Macau SAR, China b
a r t i c l e Keywords: Estuary I0geo Labile fraction Metals Sediment
i n f o
a b s t r a c t The seasonal geochemical characteristics of the seawater and sediments and the major factors causing heavy metal contamination were investigated at the Youngil bay and the Hyungsan river estuary in the Southeast Coast of Korea, where a world-scale steel-industry complex (Pohang iron and steel industrial complex, POSCO) is located. The seasonal and spatial distribution characteristics of temperature, dissolved oxygen (DO), pH, and nutrients of the seawater were studied at 45 fixed stations, especially focusing on the river mouth area. Sediments at 27 stations were examined during winter and summer to determine the major controlling factors for the distribution of metals, using correlation matrix and R-mode factor analyses, and to evaluate the pollution status, using the modified geoaccumulation ðI0geo Þ index. Temperatures for the effluent from the POSCO located at the Hyungsan river mouth were 2–3 °C higher compared to other sampling areas, due to the thermal discharge from the POSCO. The DO concentration of the surface water at the Pohang old port was as low as 2–4 mg/L. In spring, the DO value at the Hyungsan river mouth was higher than 12 mg/L, by the mass multiplication of phytoplanktons at the river mouth where seawater temperature and nutrients concentrations were relatively high, resulting in the pH value of higher than 8.3. The nitrogen to phosphorus (N/P) ratios at the river mouth were 20–150 times higher compared to other areas, implying that the nitrogen loading into this semi-enclosed bay is significantly higher than phosphorus and the major nitrogen sources are not only the domestic sewage from the city but the industrial wastewater from the POSCO and other steel factories nearby. The phosphorus concentrations at the Pohang old port were shown 3–10 times higher than those at other stations, due to the inflow of pollutants generated from the nearby ships anchoring and the release of phosphate from the bottom sediment. Results from the sediment analysis showed that the major controlling factors for the distribution pattern of each metal are grain size and organic carbon (Corg) content. Based on the factor analysis, Al, Fe, Cr, Li, and Pb were shown strongly correlated with the mean grain size (Mz), whereas Cd, Cu, Zn, and Sn with the Corg content. Results from the fractionation of the sedimentary metals into lattice and labile fractions to characterize the mobility of sediment metals showed that the mineral lattice fraction was high in the order of Al = K > Cr > Li > Sr > Fe, while the labile fraction, which might be released to the overlying water, was in the order of Pb > Zn > Cd > Cu > Ca > Sn. Evaluation of the sediment pollution status by applying ðI0geo Þ of 13 metals showed Cd, Cu, and Zn as high as 1–3 range at the old port. Even though the overall marine pollution mainly by the world-class steel industrial complex in this semi-enclosed bay area studied does not currently pose a serious threat, due to the seawater circulation and the large influx of river discharge, the countermeasures to implement the sediments concentrated with heavy metals, especially at the old port with no seawater circulation, are still warranted for this coastal water environment. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
* Corresponding author. Address: Faculty of Science and Technology, University of Macau, Taipa, Macau SAR, China. Tel.: +86 853 8397 4456; fax: +86 853 2883 8314. E-mail address: [email protected] (H. Shim). 0025-326X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpolbul.2008.04.038
The importance of the estuarine environment management has long been addressed, since there are wide ranges of salinity and land-derived pollutants emitted to the sea as well as the place being where the biodiversity of marine inhabitants exists. Biogeochemically complex phenomena such as the addition and removal
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of dissolved matters, the interaction with suspended matters, including adsorption and desorption, the ingestion and excretion by creatures, and the decomposition of organisms occur in the estuarine environment (Martin et al., 1971; Sholkovitz et al., 1977; Sholkovitz and Coplang, 1981). Such complex aspects can become even more complicated due to the distinctive seasonal variations. In case of sediments, the natural contamination by metals seems a world-wide problem (Niragu and Pacyna, 1988) and this phenomenon is especially significant in the estuarine and coastal sediments, usually as a sink receiving the river-derived metals from weathering rocks and anthropogenic sources (Martin and Windom, 1991; Zwolsman et al., 1996). Trace metals deposited in sediments may be directly available to the benthic fauna (Langston, 1990) or released to the water column through the sediment re-suspension, the adsorption–desorption reactions, the oxidation–reduction reactions, and the degradation of organism (Santschi et al., 1990). These processes may further enhance the dissolved metal concentrations in the environment and threaten the ecosystem. In order to prevent the marine pollution by trace metals, it is necessary to establish the database and to understand the mechanisms influencing the distribution of trace metals in the marine environment (Fang and Hong, 1999). Despite the well known importance of river mouth and coastal management, analyzing the characteristics of the estuarine environment can be a quite task and most previous researches have not dealt with the variations covering distinctive four seasons. Therefore, the main objective of this research was to investigate the seasonal and spatial characteristics of seawater quality from the river estuary to the whole bay area, throughout the year. Mean grain sizes, organic car-
bons, and metal elements in sediments were also analyzed to better understand the impacts of pollutants sources in the area. 1.1. Study area The sea areas around the Korean peninsula are affected by the varying conditions, characteristic of distinctive four seasons. The Youngil bay is located at the far southeast and the Hyungsan river runs through the world’s largest, Pohang iron and steel industrial complex, POSCO, as shown in Fig. 1. This semi-enclosed bay has the gross sea area of 120 km2. The Hyungsan river flows into the Youngil bay through Kyungju City and Pohang City with the population of 300,000 and 500,000, respectively. The Hyungsan river valley (watershed) covers the area of 1,167 km2, its extended river path and average width are 62.2 km and 18.8 km, respectively, and the lower reaches are about 100 m wide. The average rainfall in this area is 17.4 m3/s, the average minimum flow is 4.3 m3/s, and the annual average outflow reaches about 6 108 m3. The Hyungsan river serves as the principal drinking water source for both cities as well as for the industrial and agricultural usages. On the other hand, due to its riverbed with a steep slope, the water flows rapidly into the Youngil bay. Furthermore, as the water travels short, the purification capacity of the river fall short of the average. Especially, for a small port called Pohang old port, located north of the river mouth and with lots of fish markets, due to the poor circulation with outer sea water, the pollution status is currently getting even worse (Fig. 1). According to the pollution status of the Hyungsan river water system (EDWGIS, 2002), 303,735 m3 of sewage and wastewater are generated daily, with domestic, industrial,
Fig. 1. Sampling stations in the Youngil bay, Korea (numbers are for the fixed stations and circles are for sediments sampled).
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and livestock wastewater at 78%, 20%, and 2%, respectively, whereas among the total biochemical oxygen demand (BOD) pollutant loads of 92,027 kg/day, 23% and 57% in the form of livestock and industrial wastewater, respectively, flow out of the river downstream where the POSCO is situated. In addition, since the river valley has paddy and dry field area and forest land area, 16% and 71% of the gross area, respectively, red tide and eutrophication would result when the river is polluted by non-point sources flowing into the Youngil bay. Deteriorating water quality due to the industrial and domestic wastewaters in this bay area thus makes it imperative for the continuous monitoring of pollutants. 2. Data collection and methods Throughout 45 fixed stations at the Hyungsan river mouth in the Youngil bay area (Fig. 1), field observations and seawater samplings were performed during four different seasons from November 2001 to July 2002. Water temperature, pH, and dissolved oxygen (DO) were measured on site using a multi-probe sensor (YSI-556, USA). For the seawater, nitrite, nitrate, and phosphate were measured following the method of Parsons et al. (1984). In case of nitrite (NO2), sulfanilamide was added to the sample via diazotization, resulting in an azo compound which then reacts with the NED (naphthyl-ethylene-diamine) solution, forming a diazo compound. The absorbance of azo compounds was measured from 543 nm using a UV–Vis spectrophotometer (Shimadzu UV1601, Japan). In case of nitrate (NO3), samples in NH4Cl buffer solutions were passed through the Cu–Cd reductor column, going through the reduction to nitrite. Then, after coupled with sulfanilamide and NED solution, a dye was formed and its absorbance was measured from 543 nm, similar to nitrite. In case of phosphate (PO43), a mixed solution of ammonium molybdate, sulfuric acid, and potassium antimonyl tartrate was added to the sample to transform the existing phosphate into a complex of ammonium
phosphomolybdate in acidic condition. This complex compound was then reduced to ascorbic acid, forming a blue dye for which the absorbance was measured from 885 nm. On the other hand, the total of 27 sediment samples was collected at the Youngil bay, including the Hyungsan river mouth and the Pohang old port (Fig. 1). After removing organic substances and carbonates using diluted hydrogen peroxide and hydrochloric acid (HCl) and then using 4U sieve, the grain size was divided into two groups, coarse and fine. These coarse and fine grains were then analyzed by a dry sieving method after dehydration and by a pipettes method after adding diffusion gas, respectively (Folk, 1968). The grain size was calculated according to Folk and Ward (1975), with percentage in every 1U interval. The total carbon contents of sediments were measured using a CHN-analyzer (CE Instrument, USA; Flash1120) with a tin capsule filled with powder samples, and the inorganic carbon content was obtained by eliminating inorganic carbon after samples mixed with sulfuric acid. The amount of organic carbon was then calculated by subtracting the amount of inorganic carbon from the total carbon. For accuracy, Sulfanilamide Standard Material (SRM) was used. For the metal concentrations of sediment, 0.2 g of dried sample in a teflon bomb was added in 5 mL mixture of nitric acid and perchloric acid (3:1) to oxidize organic matters thoroughly. Dehydrated materials were then dissolved thoroughly by boiling with 5 mL mixture of boric acid and perchloric acid (4:1). This mixed material was dried and then liquefied on a heating plate for 10 min (Tessier et al., 1979). Quantification of metal elements against pretreated samples was done using an inductively coupled plasma mass spectrometer (ICP-MS; VG PlasmaQud ICP-MS, Korea Basic Science Institute). While metal elements with higher recovery rates were shown Cd (115%), Cu (108%), and Pb (114%), metal elements with lower recovery rates were Cr (82%) and Sr (84%), and other minor metal elements were in the vicinity of 88% to 99%, thus showing good representations for most parts. In this study, the labile fraction was defined as the amount extracted by 1 N HCl according to Fang and Hong (1999),
Table 1 Seasonal water properties at the Youngil bay Season
Temp. (oC)
DO (mg/L)
pH
NO2 + NO3 (lM)
PO43 (lM)
Spring Avg.
16.1–21.8 17.9
2.4–12.5 8.2
7.6–8.3 8.0
0.3–16.7 3.7
0.1–1.4 0.2
Summer Avg.
22.8–28.3 24.7
1.6–10.3 7.4
7.6–8.4 8.1
0.2–68.1 17.6
0.1–3.4 0.8
Fall Avg.
15.7–20.8 17.3
2.2–7.1 5.6
8.0–8.4 8.3
1.4–59.4 11.3
0.03–2.1 0.6
Winter Avg.
6.7–10.4 8.5
4.9–8.5 7.9
ND*
5.3–61.4 16.2
0.5–2.6 0.8
*
Not determined.
Table 2 Mean grain size, Corg, and metal elements concentration in this study area (unit in %, lg/g) Area
Season
Mz (phi)
Corg
Al
Fe
K
Ca
Sr
Zn
Cu
Cd
Sn
Cr
Pb
Li
Old port (St. 142, 113, 85)
Summer Winter Avg.
4.4 5.0 4.7
2.0 1.8 1.9
6.2 5.8 6.0
2.6 2.5 2.5
1.8 1.7 1.8
1.3 1.3 1.3
170.0 170.7 170.3
377.0 364.7 370.8
95.6 133.7 114.6
3.4 4.0 3.7
5.6 7.1 6.4
39.0 39.2 39.1
45.3 53.2 49.2
42.8 42.7 42.8
Hyungsan river mouth (St. 273, 193, 175)
Summer Winter Avg.
3.0 2.3 2.6
0.9 0.2 0.6
6.5 5.9 6.2
2.9 1.8 2.3
2.3 2.4 2.4
1.3 1.1 1.2
200.0 196.0 198.0
141.3 146.1 143.7
18.2 19.1 18.7
0.5 0.6 0.6
4.4 2.5 3.5
25.2 15.0 20.1
33.4 22.0 27.7
23.2 16.6 19.9
Other sites
Summer Winter Avg.
4.2 3.8 4.0
0.7 0.6 0.6
5.6 6.2 5.9
2.3 2.5 2.4
2.0 2.2 2.2
1.6 1.9 1.7
211.7 232.4 222.0
96.8 86.6 91.7
17.5 10.9 14.2
0.3 0.4 0.3
3.2 2.8 3.0
26.9 26.4 26.6
34.2 29.6 31.9
26.3 24.6 25.4
Total
Summer Winter Avg.
3.9 3.7 3.8
1.2 0.9 1.0
6.1 6.0 6.0
2.6 2.3 2.4
2.0 2.1 2.1
1.4 1.4 1.4
193.9 199.7 196.8
205.0 199.1 202.1
43.8 54.6 49.2
1.4 1.6 1.5
4.4 4.2 4.3
30.4 26.9 28.6
37.6 34.9 36.3
30.8 28.0 29.4
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and was analyzed as follows. First, 0.4 g of dried samples was mixed with 20 mL of 1 N HCl and warmed up with shaking at 50 °C for 6 h. After centrifugation, supernatants were analyzed by the ICP-MS. 3. Results and discussion 3.1. Seasonal and spatial distribution characteristics of water temperature, DO, pH, and nutrients Water temperatures at 45 fixed stations showed a wide variance, ranging from 6.7 °C to 28.3 °C. Seasonal average temperatures were 8.5 °C in winter and 24.7 °C in summer, thus displaying a relatively large difference between them (Table 1). The higher water temperatures at the Hyungsan river mouth throughout the year, especially, compared to other areas at the Youngil bay (data not shown), were considered due to the thermal discharge from the POSCO, which did not disperse further into the open sea. The seasonal DO concentrations were ranged from 1.6 to 12.5 mg/L and the average DO ranged from 5.6 to 8.2 mg/L (Table 1). Except for spring, DO was distinctively lower at the Hyungsan river mouth and the Pohang old port, compared to the open sea (data not shown). The lower DO concentrations at the river mouth were considered due to the increased water temperatures by the heated water mentioned above. It was also considered that the organic matters flowing into the polluted old port were dissolved but
could not be mixed with the open seawater because of its halfclosed geographical feature, which subsequently caused the DO to be exhausted. The seasonal pH range, except for winter (not measured), were from 7.6 to 8.4 with the average of 8.0–8.3 (Table 1). For the river mouth, pH’s were slightly lower except for spring, whereas pH’s at the old port, ranging from 7.6 to 8.3, were always lower throughout spring, summer, and fall, compared to the sea area (data not shown). The lower pH’s at the heavily polluted old port were due to the active decomposition of organic matters, providing more hydrogen ions to the water. In addition, organic carbon (Corg) contents in sediments at the old port were also much higher than those in nearby sea area (Table 2). The seasonal concentrations of dissolved inorganic nitrogen (NO2–N + NO3–N) ranged from 0.2 to 68.1 lM and the seasonal averages were also shown with a large variation from 3.7 lM in spring to 17.6 lM in summer, whereas the seasonal average value for dissolved inorganic phosphate was from 0.2 to 0.8 lM (Table 1). Both nitrogen and phosphate were shown with almost the same spatial distribution pattern, with much higher concentrations especially at the Hyungsan river mouth and the Pohang old port, throughout the year (data not shown). Such high concentrations of nitrogen and phosphate in these areas were considered caused by domestic sewage, industrial wastewater, and non-point sources from the river. High phosphate concentrations, in particular, were probably caused by the pollution from anchoring ships and the phosphorus release from the anaerobic bottom sediments. In addi-
Fig. 2. Seasonal distributions of DO and pH and their relationship at the Hyungsan river mouth and the Pohang old port.
M. Lee et al. / Marine Pollution Bulletin 57 (2008) 325–334
tion, pollutants such as domestic wastewater and non-point sources continuously flow into the river. The current at the Youngil bay is from the northwestern, via the inside of the bay along the coastline, into the open sea merging with the Hyungsan river and the nitrogen dispersion was shown expanded along with the seawater current system (data not shown). 3.2. Changes in DO and pH Fig. 2 shows the seasonal distributions of DO and pH and their relationships around the Youngil bay, especially at the Hyungsan river mouth and the Pohang old port, except for winter. In spring, the water temperature at the river mouth was above 20 °C and the DO concentration was above 12 mg/L, about 3 mg/L higher compared to the sea area (data not shown). In addition, the green tide was also observed during spring, resulted from the multiplication of phytoplanktons at the river mouth. As the eutrophication causes phytoplanktons to multiply, the active photosynthesis by the multiplied phytoplanktons subsequently caused the DO to increase. On the other hand, pH at the river mouth during spring was above 8.3, about 0.3 higher compared to the sea area. In general, CO2, HCO3, and CO32 exist at 1%, 92%, and 7%, respectively, in seawater. However, phytoplankton multiplication causes the exhaustion of CO2 and HCO3 couples with H+ to form H2CO3, which results in favoring the reverse reaction. Subsequently, H+ in seawater is exhausted, followed by the pH increase (David, 1995). Therefore, the increases of DO concentration and pH at the Hyungsan river mouth during spring were considered due to the mass multiplication of phytoplanktons.
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3.3. N/P ratio in seawater The ratio of nitrogen (NO2 + NO3) to phosphate (PO43) was P20 at the Hyungsan river mouth throughout the year (Fig. 3). Since the seasonal average phosphate concentration was relatively constant at 0.2–0.8 lM (Table 1), this high N/P ratio might have been resulted from the mass inflow of nitrogen into the river mouth. The Hyungsan river runs through two big cities, Pohang and Kyungju (Fig. 1), with a total population of 800,000 and the world’s largest iron and steel industrial complex nearby. Such excess nitrogen may also flow out via domestic wastewater into the Hyungsan river. Especially in spring, the ratio was as high as 150 (Fig. 3), most probably due to the less flow of phosphorus compared to other seasons or the alkaline pH coupled with phosphate settling with such compounds as calcium or iron (Stanley et al., 2000). 3.4. Grain size composition, organic carbon (Corg) contents, and distribution characteristics of metals in sediment
3.4.1. Grain size Sediments were sampled during summer and winter, to assess the primary causes for the elevated trace metal concentrations. Mean grain size (Mz) values were 3U at the Hyungsan river mouth, 4U at the Youngil bay, and 5U at the Pohang old port (Table 2). According to Folk’s triangle (1968), grain sizes were divided into four types, sand, muddy-sand, sandy-mud, and mud. The sand and muddy-sand zones were distributed primarily on the northern edge of the bay, whereas the sandy-mud and mud zones were
Fig. 3. Seasonal ratios of nitrogen to phosphate at the Hyungsan river mouth and the Pohang old port.
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distributed dominantly throughout the center of the bay, along with the Hyungsan river mouth and the Pohang old port (Fig. 4). This difference was mainly due to the influence by the current system in the bay area and the amount of the river discharge. The current at the Youngil bay is from the northwestern, via the inside of the bay along the coastline, into the open sea, merging with the flow from the river. From the northwestern to the inside of the bay, sandy type sediments were predominant because of the strong speed of current and the lower depth of water. On the other
hand, for the center of the bay, sediments were mainly composed of fine grains due to the weak speed of current and the higher depth of water. 3.4.2. Corg contents The highest organic carbon (Corg) content was observed at the old port (2.0%) and the correlation coefficients (r2) between Mz and Corg were shown 0.74 and 0.88 during summer and winter, respectively (Table 2 and Fig. 5).
Fig. 4. Grain size distribution of sediments at the Youngil bay.
Fig. 5. Relationship between mean grain size and TOC content of sediment at the Youngil bay.
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3.4.3. Distributional characteristics of metal elements As shown in Table 2, the average concentrations of 12 metal elements in sediment were shown Al (6.0%), Fe (2.4%), K (2.1%), Ca (1.4%), Sr (196.8 ppm), Zn (202.1 ppm), Cu (49.2 ppm), Cd (1.5 ppm), Sn (4.3 ppm), Cr (28.6 ppm), Pb (36.3 ppm), and Li (29.4 ppm), respectively. To determine the controlling factors for their distributions in sediment, the correlation matrix and the Rmode factor analyses were carried out for 15 items, including metal elements, mean grain size (Mz), and Corg content. As a result, four primary factors were determined characterizing 97% of the total variance and there were no significant distributional differences between summer and winter (Table 3). In summer, Factor 1 takes up to 37.6% of the total variance and shows high loading values for Mz, Corg, Al, Fe, Cr, Li, and Pb, which were more concentrated at the central bay area and the Pohang old port. The distribution of Al, Fe, Cr, Li, and Pb was quite high as the grain size of sediment became finer (Fig. 6a). Factor 2 comprising 37.5% of the total vari-
ance mostly consists of Cd, Cu, Sn, and Zn (Table 3). The levels of these toxic metals were significantly higher at the Pohang old port compared to other areas despite of no industrial complex, due to the high organic carbon concentration and being the anaerobic environment (Fig. 6b). These organophilic metals could be accumulated to and released from the bottom sediments at the old port, where the metals might be present in forms of such metal sulfides as CdS, CuS, and ZnS. The pH and oxidation–reduction potential (Eh) for those metal sulfide complexes were 6–6.5 and 0.2–0 mV, respectively, which were thus feasible to occur in the anaerobic environment of the old port. For Factor 3, the major elements were Ca and Sr (Table 3 and Fig. 6c), which generally occur in the carbonates and are highly concentrated in shells within the sea sediment (Emelyanov and Shimkus, 1986; Cho et al., 1999). Factor 4 shows that K was the highest loading element (Table 3 and Fig. 6c), and Cho et al. (1999) reported that K can be highly concentrated in sediment which contains illite and feldspar.
Table 3 Rotated component matrix for the sediment during (a) summer and (b) winter Component 1
Component 2
Component 3
Component 4
Communality
Panel a Mz Corg Al Fe Cr Li Pb Cd Cu Sn Zn Sr Ca K Eigen value (%)
0.92 0.73 0.91 0.91 0.80 0.72 0.69 0.18 0.36 0.46 0.45 0.02 0.07 0.06 37.6
0.17 0.64 0.25 0.31 0.54 0.65 0.64 0.94 0.91 0.83 0.88 0.16 0.02 0.55 37.5
0.26 0.08 0.16 0.14 0.11 0.17 0.09 0.10 0.15 0.07 0.08 0.98 0.99 0.56 17.6
0.17 0.13 0.28 0.09 0.19 0.14 0.16 0.16 0.11 0.24 0.06 0.12 0.02 0.59 4.9
0.98 0.97 0.99 0.96 0.99 0.98 0.90 0.94 0.99 0.95 0.99 0.99 0.99 0.97
Panal b Mz Corg Al Fe Cr Li Pb Cd Cu Sn Zn Sr Ca K Eigen value loading (%)
0.85 0.65 0.95 0.95 0.79 0.74 0.67 0.08 0.11 0.35 0.32 0.13 0.29 0.27 35.6
0.16 0.52 0.13 0.14 0.34 0.44 0.52 0.91 0.96 0.86 0.88 0.15 0.10 0.35 30.9
0.33 0.26 0.05 0.12 0.23 0.24 0.17 0.11 0.06 0.08 0.16 0.97 0.93 0.13 15.8
0.32 0.43 0.18 0.16 0.43 0.40 0.35 0.26 0.10 0.14 0.12 0.09 0.13 0.86 12.1
0.96 0.94 0.95 0.97 0.98 0.96 0.87 0.91 0.94 0.88 0.92 0.99 0.98 0.96
Fig. 6. Metal elements distributions of sediment at the Youngil bay.
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Overall, two major controlling factors for the metal distribution of sediment in this bay area are mean grain size and organic carbon content (Table 3). Since the mean grain size is usually controlled by the seawater current system as mentioned above, the reduction of organic carbon content may play a key role in controlling the pollution status especially at the Pohang old port area.
3.5. Metal content in labile fraction Sedimentary trace metals present in the labile fraction can be re-dissolved into the water column under the favorable environmental conditions, including Eh or pH changes (Santschi et al., 1990). The concern of sedimentary trace metals polluting the
Fig. 7. Average percentage of acid (1 N HCl) extractable metal (labile) fractions from the sediment at the Youngil bay.
Table 4 Modified geoaccumulation index ðI0geo Þ for total metals in study area
Old port Hyungsan river Youngil bay Avg.
Al
Fe
Ca
K
Cr
Zn
Cu
Cd
Sn
Pb
Sr
Li
0.6 0.2 0.3 0.2
0.4 0.3 0.1 0.0
0.7 0.0 0.0 0.2
1.1 0.1 0.5 0.5
0.2 0.4 0.4 0.3
1.5 0.9 0.2 0.9
2.4 0.6 0.2 1.1
2.9 1.0 0.5 1.5
0.4 0.3 0.1 0.2
0.2 0.2 0.5 0.3
0.9 0.2 0.2 0.3
0.0 0.3 0.4 0.2
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marine environment is that the amount of these metals can be released into the water column during the early diagenesis, thus enhancing the potential deterioration of water quality in the environment. Many one-step extraction procedures have been developed to assess this phenomenon and the sedimentary trace metals determined by these techniques are generally divided into two fractions, namely labile and lattice (non-labile) or bioavailable and non-bioavailable (Luoma, 1990). To estimate influences of surrounding environments in this study, the amount of metal elements was analyzed by the single acid extraction with 1 N HCl. This method was appropriate especially to judge the contamination derived from anthropogenic activities, due to the fact that it is possible to obtain metal elements corresponding to those in labile fractions (Chester and Voutsinou, 1981). Results from the fractionation of the sedimentary metals into lattice and labile fractions to characterize the mobility of sedimentary metals showed that the mineral lattice fraction was high in the order of Al = K (94.5%) > Cr (78.3%) > Li (73.7%) > Sr (64.3%) > Fe (64.1%), while the labile fraction, which might be released to the overlying water, was in the order of Pb (83.8%) > Zn (82.1%) > Cd (78.5%) > Cu (72.0%) > Ca (66.0%) > Sn (58.7%), as shown in Fig. 7. The simple approach to the normalization of geochemical data is to compare the total metal concentration of surface sediments with concentrations characteristic of natural background levels or anthropogenically uncontaminated sediments. Among such various methods as enrichment factor (EF), geoaccumulation index ðIgeo Þ, concentration enrichment ratio (CER), and metal pollution index, ðIgeo Þ was employed in this study since the regional geometrical characteristics can be reflected, and the contamination degree of the sea area studied was quantified accordingly. ðIgeo Þ, suggested by Müller (1979), is used to estimate the contamination degree of sediments and calculated through the following equation. Igeo ¼ log2 C n =1:5Bn where Cn is the content of sample analyzed and Bn is the reference value. In general, the metal content increases as the particle size becomes finer. Therefore, it is appropriate to calculate the contamination index by considering the regional Mz even though the metal contents in sediment are same. In this study, the contamination index was calculated by the following modified equation after considering the regional Mz in Müller’s equation. Modified I0geo ¼ log2 C n =1:5 Bn Mz where Mz indicates Mzc/MzB, mean grain size ratio of investigation station to reference area. Therefore, the average values for stations 25, 31, and 44 (Fig. 1) were considered least influenced by Mz and inflowing contamination sources. The ðIgeo Þ value of metals is classified into five types: class 1 (0–1, unpolluted to moderately polluted), class 2 (1–2, moderately polluted), class 3 (2–3, moderately to strongly polluted), class 4 (3–4, strongly polluted), and class 5 (4–5, strongly to very strongly polluted). In case of ðIgeo Þ value less than 0, it is classified as uncontaminated area. Results showed the ðI0geo Þ value for the Pohang old port decreased after the Mz adjusted, due to the fine granularity. Especially, such metals as Cr, Li, and Pb were found not contaminated with after adjusting the grain size (Table 4). In comparison, due to the coarse granularity, the ðI0geo Þ value for the Hyungsan river mouth increased after the Mz adjusted. For the open sea areas at the Youngil bay, it was found uncontaminated with the ðI0geo Þ value less than 0. However, the ðI0geo Þ values of Zn, Cu, and Cd at the old port were shown 1–3, implying more serious heavy metal contamination than sea areas.
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4. Conclusion (1) Compared to other seasons and other areas at the Youngil bay, dissolved oxygen and pH levels at the Hyungsan river mouth during spring were distinctively higher, around 12.0 mg/L and 8.3, respectively. In addition, when the water temperature was relatively high in spring, phytoplanktons multiplied massively resulting in the green tide. (2) Compared to other areas at the bay, the Hyungsan river mouth was shown with very high N/P ratios (up to 150 in spring) throughout the year, probably due to the nitrogen pollution by the domestic wastewater generated from two neighboring big cities and the industrial wastewater generated from the world’s largest Pohang iron and steel industrial complex (POSCO) and other related industries nearby. Therefore, to prevent the nitrogen loadings from flowing out into the open sea, the optimal sewage treatment, including the advanced nutrient treatment, is needed and the industrial complex in this area needs to treat its own wastewater more properly at sources. (3) Despite the dredging was conducted about two years ago, the seawater around the Pohang old port does not appear to circulate well. Thus, compared to other areas at the bay, pH and dissolved oxygen levels of the water were lower, sometimes even with odors such as hydrogen sulfide generated, whereas the phosphate concentration was 3–10 times higher resulting from the massive phosphorus inflow from the neighboring fish market, many anchoring ships, and the anaerobic state of sediments. Such heavy metals as Cd, Cu, and Zn were also highly concentrated around the old port despite no industrial complex nearby, as well as with finer grain size and higher organic carbon content, compared to other bay areas. In addition, compared to the geoaccumulation index, ðI0geo Þ, indicating the relative metal pollution, of less than 0 for other studied areas, the ðI0geo Þ values of Zn, Cu, and Cd for the old port was 1–3, implying severe pollutions. This further suggests Cd2+, Cu2+, and Zn2+ in seawater be accumulated in forms of such metal sulfide complexes as CdS, CuS, and ZnS in sediments, resulting from the high organic matter content in sediment and the anaerobic environment verified by the pH and Eh values at the old port. Based on the overall results, the marine pollution status in this semi-enclosed bay area may not show the direct negative effect of the world-class steel industrial complex, probably due to the circulating seawater current system. However, results also suggested the nitrogen control especially around the complex still be required. The major controlling factors for the distribution of sedimentary metals at the Youngil bay area were shown mean grain size and organic carbon content. Reducing organic carbon content will further relieve toxic heavy metal contents in the sediment, which may subsequently be the solution to control the pollution status around the Youngil bay. Acknowledgements This study was supported by a Grant from the Korea Ocean Research Institute through the POSCO and the Pohang City Council. Help of Ms. Hye-Jin Rho is also appreciated for the sample collection and preparation. References Chester, R., Voutsinou, F.G., 1981. The initial assessment of trace metal pollution in coastal sediment. Marine Pollution Bulletin 12, 84–91.
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