The metal concentrations in surface sediment of the South China Sea off the Terengganu coastline were determined. Cu, Cr and Zn concentrations were comparable to values for offshore regions of the South China Sea and of concentrations of coastal sediments from the more developed west coast of Malaysia. Direct comparisons with earth crust values for Cd and Pb ratios to Al indicate anthropogenic pollution by these metals. As the sediments of the area studied are composed of significant proportions of quartz and carbonate, from shells and coral, direct ratioing to Al would result in anomalous values. Thus enrichment factor (EF) values using Al as a reference element were determined and showed that sampling sites off the major rivers of Terengganu were anthropogenically influenced by Pb and Cd. Sources of pollution are probably sewage, agricultural wastes and atmospheric deposition of Pb from the use of leaded petrol.
The coast of Terengganu State consists of a shallow South China Sea environment and a number of islands that are surrounded by coral reefs. The sediments are composed of sandy muds with a high content of coral debris. Along the coast are found numerous lagoon systems inhabited by mangrove forest, the largest of which is found at the Setiu lagoon. Urban areas that may contribute to anthropogenic input of metals are at Kuala Besut town to the north of the study area (Figure 1) and Kuala Terengganu town to the south of the study area. Some small-scale industries are found only at Kuala Terengganu. Most of the other parts of the coastline are underdeveloped, consisting of coastal villages. The rivers that enter the South China Sea, notably from the northern region of the study area, may be a source of sewage, agricultural chemicals from oil palm and rubber plantations and of waste water from shrimp aquaculture ponds.
Studies on metal concentrations in coastal sediments of the Terengganu coast are limited to investigations of nearshore sediments and of the estuaries of Terengganu River and the Setiu River and other smaller rivers. Although it has been shown that concentrations of anthropogenic metals are relatively low and within earth crust values, there is a need to quantify baseline values in the coastal sediments as the coastal environment of Terengganu State is under intense development. Aquaculture actvities, in earth ponds and in floating cages in coastal lagoons and estuaries of rivers are increasing and mangrove forests being cut down in order to accommodate these activities. Tourism is a major source of income for the Terengganu State and the Redang and Kapas group of islands are undergoing rapid development of hotels and chalets. Methods for the interpretation of metal data to differentiate between anthropogenic and natural effects have yet to be fully understood for the South China Sea coastal sediments. High quartz and carbonate (from corals) contents and their patchy distribution, and grain size variability due to the high energy environment typical for the open coastline of Terengganu, makes interpretation of metal data somewhat complicated.
Geochemical normalization has been used with some success by utilizing conservative elements such as Al (Calvert et al., 1993; Din, 1992; Wood, 2004), Fe (Macias-Zamora et al., 1999) and Li (Aloupi and Angelidis, 2001; Loring, 1991). However the choice of the normalizer depends on the local sedimentary characteristics and a particular normalizing element may not be suitable for all sediment environments. In this study, heavy metal concentrations in sediments from the Terengganu coast were examined and normalized against Al and data further interpreted using enrichment factors, EF (Szefer et al., 1998; Selvaraj et al., 2004).
The objective of the study was to map out the concentrations of metals in the Terengganu coast as a baseline reference for future studies and to determine if Al is suitable for use in interpreting if pollution of the sediment has occurred. As anomalies in their distribution were indicated in previous studies in invertebrates living along the coastline (Shazili et al., 1995) the geochemistry of metals, especially of the pollutant metals Cd and Pb were studied using Al as a reference element.
The methodology for sample collection, preparation and metal analyses used in the present study was similar to that used during the previous work in the South China Sea (Shazili et al., 1999a, b) and that of Wood et al. (1997).
Sediment sample collection
Sediment samples were collected using a Smith McIntyre grab. The top 3 cm of sediment were carefully collected with a clean plastic spatula and kept in acid-cleaned polyethylene bottles. The samples were then kept at −20°C for six months before they were prepared and analyzed as described below.
The sediment samples were dried at 60°C, lightly ground to break up the particles, and then sieved through a 63 μ m nylon mesh. About 0.3 g aliquots of this silt and clay fraction were digested in a mixture of nitric, perchloric, hydrofluoric and hydrochloric acids in closed Teflon pressure vessels in a microwave heating system (Milestone Ethos). The digest was then made up to 50 ml with deionised water.
For quality assurance, a standard reference material (1646a Estuarine Sediment) from the National Institute of Standards and Technology was digested as above and analysed for metals.
Metals were analysed using a Varian Vista simultaneous ICPOES. Analyses of NBS 1646a Estuarine Sediment (National Bureau of Standards) demonstrated good accuracy, precision and recovery for all metals (Table 2). Metal concentrations were mapped out using GIS to enable a better interpretation of their distribution in sediments off the Terengganu coast.
Results and discussion
The concentrations of metals in coastal sediments of Terengganu are presented in Table 1 and as concentration isopleths in Figure 2. The large standard deviation values for all metals indicate large spatial variability of metal concentrations in the sediment as a result of variability of sediment granulometry (1.11–7.21 phi) and observed content of coral debris and shells. Stations TR12 and TR13 with sediment mean size of 1.11 and 1.18 respectively consisted of moderately coarse sand, while stations with phi values of between 2.39 and 3.80 were composed mainly of fine sand and low amounts of silt and clay. Nine of the 24 stations had sediments where silt and clay dominated (phi values > 6.3).
Al ranged in concentration from 0.61 to 8.59%, with the lowest concentrations found at the stations (TR16, 17 and 26) furthest from the mainland, where the sediments are composed of mainly of sand. The concentrations of Cu, Cr and Zn are similar to a previous study of near-shore sediments along the Terengganu coast (Mohamed and Shazili, 1998) and similar or lower than those determined for offshore sediments of the South China Sea (Shazili et al., 1999a, b), which are generally finer in size.
The concentrations of Cr, Cu and Zn found in this study are comparable to previous studies of the coastal sediments of the Malacca Strait in the more intensely developed areas of Penang and Johor on the west coast of Malaysia (Wood et al., 1997; Wood et al., 2004). The Pb concentrations in this study however are lower than expected, while Cd values seem to be anomalously higher. However concentrations of 0.32–0.50 μ g g− 1 dry wt. of Cd were reported by Shazili et al. (2003) in sediments in the Setiu lagoon just inland of station TR11.
The concentrations of several metals (Al, Cr, Cu, Zn) were positively correlated with sediment particle size (Figure 3) indicating that metal concentrations tend to be higher in fine sediments. However, neither Cd nor Pb concentrations were related to sediment particle size.
Thus for the anthropogenic metals, Cd and Pb, their distribution in Terengganu coastal sediments do not seem to be associated with fine sizes. As the Terengganu sediments are mostly composed of coarse sandy and carbonate material with only a small proportion of the sediment of detrital origin and of fines, correlations of metals (at least for Cd and Pb) with sediment particle size does not follow normal trends found for muddy sediments such as those of the Malacca Straits (Din, 1992).
Normalization of trace metal data to conservative elements such as Al, Fe and Sc (Loring, 1991; Wood et al., 2004) has been shown to reduce grain-size effects on metal distribution in sediments and thus provide a better means for identifying anthropogenic from natural levels. Cr, Cu and Zn are fairly strongly correlated with Al (Figure 4) while Cd is negatively correlated and Pb showing a very weak correlation. For Cd, 15 sites recorded Cd/Al ratios greater than average shale (3.4 × 10− 6), with values of up to 8.8 × 10− 6 and 108 × 10− 6. Pb/Al ratios exceeded average shale value (2.5 × 10− 4) for 15 sites with values of up to 13.5 × 10− 4 (Figure 5). However, Pb values in sediment are higher than earth crust value at 4 sites only while Cd values exceed earth crust values for 8 sites. Clearly direct comparisons of concentration values with earth crust concentrations are unsatisfactory as the high quartz and carbonate content of the Terengganu sediments dilutes the Al concentrations. Thus ratios of metals to Al in this particular study, especially Cd will seem to be too high. This also shows that Al is not a suitable normalizer element and other elements such as Li, Sc or Fe may be examined in the future as for their suitability for nearshore South China Sea sediments.
Szefer et al. (1998) and Green-Ruiz and Paez-Osuna (2001) suggested the use of enrichment factor (EF) in which Al is the reference element against which a metal [M] can be compared according to the formula:
where [M] and [Al] refer to the concentrations of element M and Al in surface sediments of the area studied or in the earth crust. EF values within the range 0.1 to 1 are indicative of natural input of metals while values greater than 1 indicate anthropogenic input or pollution. The concentration of trace metals from earth crust follows average shale values (Taylor, 1964).
Calculated EF values (Table 3) for Cr and Zn indicate these metals are found at naturally occurring values, while for Cu only one station showed a value of being slightly polluted with an EF of 2.67 at Station TR 26, just off the Ibai River which drains most of the sewerage outflows from Kuala Terengganu Township. Calculated EF values for Cd shows that 17 stations have values greater than unity (1.0) and thus polluted, of which eight stations are considered moderately polluted with EF 1.0–2.0. Interestingly, the stations with high EF values are found just off the rivers. EF values for Cd of 3.41 and 7.37 at TR1 and TR2 respectively, were found for the sediments off the Besut River, in the north. No measurements have ever been made of the metal levels in the environment of the Besut River and thus it is not possible to confirm if the Besut River is the major source of Cd. At TR7 and TR11, EF values of 3.29 and 4.01 were found for these stations off the Setiu River, which drains most of the agricultural areas where major aquaculture activities are located. Our own study (unpublished) of the Setiu River estuary indicates sediment total Cd concentrations in the range of 0.32 to 0.50 μ g g−1 dry wt. in the 63 μ m fraction which would correlate well with the high concentrations seen at stations TR7 and TR11 off the Setiu River.
The highest Cd EF value of 27.05 was found for station TR26 just off the Ibai River. Unfortunately no data of previous measurements exist, against which this results can be compared with. Lead concentrations in the Ibai River estuary ranged between 17.3 and 46.6 μ g g− 1 dry wt. while in the Terengganu River estuary values ranged between 14.4 and 61 μ g g− 1 dry wt.
For Pb, the EF values indicate that 9 stations, almost all nearshore, are moderately contaminated with EF values mostly < 3. As for Cd, the contaminated areas are found mostly off the rivers Besut, Setiu and Ibai and in addition off the Terengganu River. The two main areas of Pb deposition are off the Kuala Terengganu town, the capital of Terengganu State and off the Setiu agricultural region. It is possible that sources of Pb contamination include atmospheric input of Pb from the use of leaded petrol by motor vehicles as well as fishing boats that frequent the fishing ports located at Besut river mouth (Kuala Besut) and Kuala Terengganu town as well as from sewage. However, unleaded petrol has been in use for a number of years, although leaded petrol is also still in use.
In this study, the metal concentrations in surface sediments of the South China Sea off the Terengganu coastline were studied and mapped. Concentrations of Cu, Cr and Zn in surface sediments off Terengganu coast are comparable to other areas of the South China Sea and the sources of these metals considered natural. However Cd and Pb showed high metal/Al ratios that indicates pollution and/or anomalies as a result of the typically high quartz and carbonate content of the sediments in this region. Examination of EF values however indicates that Pb concentrations off the Ibai, Terengganu, Setiu and Besut Rivers are higher than average earth crust value and thus, indicative of pollution by anthropogenic sources. The sources of these metals are probably the rivers Besut, Setiu, Ibai and Terengganu that drains agricultural areas and sewage from townships that are situated around these rivers. Sources of Pb pollution are thought to be from atmospheric deposition as a result of the use of leaded petrol as well as from sewage. High EF values for Cd in sediments in the sea off the Ibai River point to anthropogenic input from this river.
This work is supported by the Ministry of Science, Technology and Environment Ministry Malaysia Intensified Research Priority Area (IRPA) with grant No: 08-02-12-0041-EA20100. The authors appreciate the help of Mr. Suffian Idris in preparing the GIS maps of metal distribution.