Hydrocarbon compounds viz. aliphatic hydrocarbons, polycyclic aromatic hydrocarbons (PAHs), sterols, long chain fatty acids, alkanones and alkanals in surface sediment of Setiu Wetland were analysed and characterized using GCMS. The concentration of total identified resolved n-alkanes (TIRNA) in sediment ranged from 2.99–11.6 μ g g− 1 dry weight. The distribution of the aliphatic fraction showed the presence of n-alkanes ranging from C12 to C36 with high predominance for long chain homologues (C25-C31) and a carbon maximum at C29 and C31 with CPI > 3; these observations provide evidence for the presence of biogenic terrigenous input corresponding to epicuticular plant waxes into the lagoon sediments. Positive and strong correlation between n-alkanes associated with terrigenous input (ALK TER) and TIRNA suggest terrestrial input is the main sources of TIRNA in this study area. The absence of unresolved complex mixture (UCM) in the chromatogram and the absence of hopanes, steranes and PAHs compounds are indicative of uncontaminated sediment by petrogenic and pyrogenic hydrocarbons. The concentration of total identified sterols (TIS) ranged from 1.41 μ g g− 1 dry weight to 3.11 μ g g− 1 dry weight with cholesterol, β -Sitosterol and stigmasterol were generally the most dominant and abundant components detected at almost all stations. A positive and strong correlation was observed between B-Sitosterol and TIS and the distribution of long chain n-alkan-2-ones in the range of C21 to C27 with odd to even predominance and n-alkanals (C20 to C28), n-alkanols (C22–C30) and n-alkanoic acids (C22–C30) with even to odd predominance provide further evidence for biogenic sources of hydrocarbons with terrestrial plant input as the predominant source. It was noted that a minor contribution associated with marine phytoplankton (algae) as well as bacteria were also observed in some of the sediment samples superimposing with the terrigenous plant wax input. In general results from this study clearly showed the lagoon sediment of Setiu Wetland is still in uncontaminated condition where terrestrial plants input are the dominant contributor of organic compounds in the sediments with a minor input from marine organisms.
Hydrocarbons are commonly found in the environment as complex mixtures deriving from multiple sources. Being major constituents of petroleum (Neff, 1979), hydrocarbons may enter the marine environment through riverine discharges, shipping activities, sewage disposal, offshore oil production and transport as well as through accidental oil spills. Other important sources could be from man-induced biomass burning or natural forest fires. Apart from anthropogenic sources, hydrocarbons can also be derived from natural sources, such as terrestrial plant waxes, marine phytoplankton and bacteria and digenetic transformation of biogenic precursors.
Geochemical markers are organic compounds characterized by their source specificity and molecular stability, that maintain the “fingerprint” of their origins (Simoneit, 1984). For instance, petroleum-related hopanes are resistant to environmental alteration and, therefore, can be used to detect oil pollution in recent sediments even after fairly extensive degradation has occurred, and/or even in the presence of an overwhelming abundance of biogenic compounds (Boulobassi et al., 2001).
Coastal seas serve as a final receiver of natural and anthropogenic organic matter derived from land and carried by river and atmosphere (Lipiatou et al., 1997). Consequently, coastal sediments act as temporary or long term sinks for these natural and anthropogenic organic matters from land based sources. These compounds are readily adsorbed onto particulate matter, and bottom sediments act as an ultimate reservoir of hydrophobic contaminants (Szwarzenbach et al., 1993; Budzinski et al., 1997; Pereira et al., 1999). Thus sediments are good sources of integrated samples, exhibiting levels of hydrocarbons several orders of magnitude higher than those found in the water column. Therefore, the confident discrimination between biogenic and anthropogenic origin of hydrocarbons, as well as the further recognition of inputs from biogenic, diagenetic, petrogenic and pyrogenic sources requires the use of geochemical or molecular markers.
Setiu Wetland, located in the east coast of Peninsular Malaysia, is a unique area covering many ecosystems such as estuary, mangrove, wetland and lagoon, offering a vast array of biological diversity and many utilizable natural resources. However at present, the potential disturbance to the pristine environment is of concern due to the rapidly increasing number of fish and prawn culture activities at Setiu Wetland. At present very little scientific study has been carried out in the area, particularly on the organic geochemistry aspect, hence this study has been initiated to provide valuable baseline information regarding the distribution of hydrocarbons in lagoon sediment of Setiu Wetland. An assessment will be made to ascertain the status of the sediment using suitable organic molecular markers.
Materials and methods
In this study 5 sampling sites (ST 1–ST 5) were selected along Setiu Lagoon covering an area from Setiu River to Benting Lintang (Figure 1). Water depth is generally between 0.5 to 2m depending on tidal conditions and locations within the lagoon. Surficial samples were collected using a grab sampler (Ekman Grab Sampler) between 21st February and 24th April 2004. The coordinates of sampling station are shown in Table 1. The sediment was recovered, wrapped with aluminium foil, frozen and stored at −30°C until analysis.
The sediments were freeze-dried and sieved using a 500 μ m sieve. Analysis was carried out on the fraction ≤ 500 μ m.
Extraction and fractionation
Total organic carbons (TOC) of the sediment were analysed using Wakley and Black!s titration method (Moris and Singh, 1971). Freeze-dried sediment was extracted by using Soxhlet extraction technique with dichloromethane (DCM) as solvent. Before extraction, four internal standards (n-C18:1,n-C32 for aliphatic fraction, 9,10-dihydroanthracene for PAHs fraction and 5α -Androstanol for sterols fraction) were spiked into the sediment for recovery assessment. Sulphur content in the sediment was removed using mercury treatment (Beg et al., 2003). The extracts were concentrated to about 1 ml using a rotary evaporator and then fractionated into subfractions on a silica-alumina column deactivated with 4% deionized water. 25 ml of n-hexane was then used to elute the n-alkanes (first fraction), followed by 25 ml of 50% DCM in hexane to elute the second fraction (PAHs), and the third fraction was eluted by 40 ml of 10% methanol in DCM. The third fraction was further treated with bis-(trimethylsilyl)-trifluoroacetamide (BSTFA) (100 μ l, 70°C, 60 min) to convert the alcohols, fatty acids and sterols to their corresponding trimethylsilyl esthers, prior to the instrumental analysis.
Aliphatic hydrocarbons (F1) were analysed using a Thermo Finnigan capillary gas chromatography equipped with a flame ionization detector (GC-FID) and Altech 13638 capillary column (30 m × 0.25 mm i.d). Helium was used as the carrier gas and the oven temperature was programmed from 50°C (1 min) to 140°C at 5°C min− 1 and finally to 290°C at 4°C min− 1 with a final hold time of 13 min. Second and third fractions were injected in the GCMS Shimadzu QP2010 fitted with a BPX5 SGE 30 m fused silica column (30 m × 0.25 mm i.d). The column temperature for second fraction was programmed as follows: hold at 50°C for 1 minute; 5°C min− 1 to 140°C, 4°C min− 1 to 300°C and held for 15 minutes. For the third fraction, the following temperature program was employed: hold at 60°C for 0.5 min; 6°C min− 1 to 290°C, hold for 16 min. Helium was used as the carrier gas on a constant pressure at 14.2 kPa. The F1 was also injected into the GCMS for identification of hopanes and confirmation of n-alkanes.
Result and discussion
Total organic carbon
Surface sediments in the study area contained relatively low organic carbon with values ranging from 1.18% to 1.99%. Station 2 has the highest percentage of organic matter while Station 5 exhibited the lowest percentage of organic matter. Samples from St 2, where fine sediment accumulates, exhibit relatively high OC values (1.99%). This is most likely due to the increase in biological activities at ST 2, since this station is in close vicinity of prawn culture cages. Enhanced organic carbon content at ST 1 (1.97%) may be due to a direct riverine influence.
Fraction 1 and 2
The nonpolar fractions in this study were analyzed for n-alkanes, isoprenoids (pristane and phytane), hopanes, steranes and unresolved complex mixture of polycyclic aliphatic compounds (UCM). Table 2 gives the total concentration of the total identified resolved n-alkanes (TIRNA), individual isoprenoids concentrations and selected biomarker fingerprinting parameters. The TIRNA obtained in surface sediments of Setiu Wetland lagoon ranged from 2.99 μ g g− 1 dry weight to 11.6 μ g g− 1 dry weight, respectively. The n-alkanes distribution obtained ranged from C12 – C36. The compositional profiles showed a high predominance of long chain homologues in the range of C25 to C31 with an elevated odd to even carbon number preference as reflected in their CPI values with Cmax generally at C31(Figure 2A). The sum of the most abundant n-alkanes related to biogenic terrestrial sources (C27, C29 and C31) referred in Table 2 as ALK (ter) account for 30%–65% of total n-alkanes in the sediment samples, indicating a prominent terrigenous input derived from higher plant waxes (Colombo et al., 1996). For isoprenoids, pristane was detected at all stations with concentrations ranging from 0.03 μ g g− 1 to 0.14 μ g g− 1 dry weight while phytane was only detected at three stations with concentration of 0.01 μ g g− 1 to 0.06 μ g g− 1 dry weight. No hopanes, steranes and UCM were detected in this study. The high Pris/Phy ratio (2.10 to 2.66) and the absence of hopanes, steranes and UCM strongly indicated a predominance of biogenic hydrocarbon inputs to the Setiu Lagoon sediment rather than petrogenic hydrocarbons input. In addition, no PAHs compounds were detected in the second fraction (aromatic fraction) of all samples again confirming the absence of petrogenic contamination in studied area.
Concentrations of total n-alkanes observed in the study area were slightly higher than those found in surface sediments from Changjiang (Yantze River) (0.16 μ g g− 1 to 1.88 μ g g− 1 dry weight) (Boulobassi et al., 2001) and Cretan Sea (0.08 μ g g− 1–0.89 μ g g− 1 dry weight) (Gogou et al., 2000). However, the present results were slightly lower than n-alkanes observed from sediments of the coastal environment of Egypt (7.1 μ g g− 1–142.8 μ g g− 1 dry weight) which has been attributed to petroleum contamination (Aboul-Kassim and Simoneit, 1996) and closer in range with those found in Florida Key sediment (4.29 μ g g− 1–8.44 μ g g− 1 dry weight) (Snedakar et al., 1995) and Yellow Sea (0.7 μ g g− 1 −15.8 μ g g− 1 dry weight) (Wu et al., 2001).
The concentration of total identified sterol (TIS) in the surface sediment varied from 1.41 μ g g− 1 to 3.11 μ g g− 1 dry weight. The highest concentration was observed at station 1 whilst station 3 exhibited the lowest concentration. Table 3 shows the concentration of TIS and the percentage of individual identified sterol compounds. Results showed that cholesterol was the most abundant sterol compound followed by β -Sitosterol and stigmasterol. The high concentration of cholesterol (26.8% to 51.8% of TIS), indicated major contribution of marine algal sterol at the studied area (Volkman, 1986) followed by β -Sitosterol and stigmasterol which are generally found in epicuticular waxes of vascular plants (Jeng et al., 2003). This distribution suggested that terrestrial input is the major source of TIS in the studied area.
Other compounds such as n-alkanals, n-alkan-2-ones, n-alkanols and n-alkanoic acids were also identified in this study using the MS library but not quantified due to lack of internal standards. The distribution of alkan-2-ones observed in this study ranged from C21–C27 with a strong odd over even predominance (Figure 2B), was believed to be derived from higher plants. Long chain alkanals (C18–C28), alkanols (C16–C28) and n-fatty acids (C16–C28) distributions with a strong even to odd predominance (Figure 2C–E) also suggested an input of epicuticular higher plant waxes.
The results of this study clearly indicate that the sources of hydrocarbons in lagoon sediment of Setiu Wetland are mainly derived from biogenic inputs viz. from epicuticular plant waxes associated with terrestrial plants and a minor contribution associated with marine phytoplankton (algaes) as well as bacteria. Total identified resolved n-alkanes (TIRNA) in sediment ranged from 2.99–11.6 μ g g− 1 dry weight. Distribution of n-alkanes showed an odd to even carbon number predominance (CPI > 1) and Cmax at C29 and C31. Positive and strong correlation between n-alkanes associated with terrigenous input (ALK TER) and TIRNA suggest terrestrial input is the main sources of TIRNA in this study area. The absence of unresolved complex mixtures (UCM), hopanes, steranes and PAHs compounds are indicative of uncontaminated sediment. The concentration of total identified sterols (TIS) ranged from 1.41 μ g g− 1 dry weight to 3.11 μ g g− 1 dry weight, dominated by cholesterol, β -Sitosterol and stigmasterol which indicated contribution from marine algal sterol and epicuticular waxes of vascular plant. It is speculated that these natural hydrocarbons are primarily delivered by riverine discharges and also from terrestrial plants that exists in the lagoon such as mangroves. Thus, it can be concluded that the lagoon sediments of Setiu Wetland are still in a relatively uncontaminated condition.