IV. DISTRIBUTION AND SOURCE OF POLYCYCLIC AROMATIC
4.3. Results & Discussion
74
75 carbon). These sources were likely to reach the aquatic environment through soil washout or land abrasion during high precipitation.
4.3.2. Content and Distribution of PAH
The contents and distribution of the 16 PAHs (expressed as PAHs) in the sediments from the three different environments are summarized in Table 2 (p. 132 see also Fig. 2). Owing to a low mud fraction ( 2%), river stations of S104, S105 and S35 were analysed only for the sand fraction. The PAHs in the sand fraction of the Siak river, estuarine and coastal sediments range from 164 to 5474 (median=556) ng g-1 d.w., 208 – 3913 (425) ng g-1 d.w., and 594 – 2495 (1142) ng g-1 d.w., respectively. While in the mud sediment, corresponding contents varied from 319 to 1143 (521) ng g-1 d.w., 126 to 584 (468) ng g-1 d.w., 443 to 1314 (633) ng g-1 d.w. The highest PAHs content (>5000 ng g-1 d.w.) in the sand fraction was found at S42 in the river, followed by S138 in the estuary (~4000 ng g-1 d.w.), S253 and S269 in the coast (>2000 ng g-1 d.w.). In the mud fraction, the highest PAHs contents (1000-1300 ng g-1 d.w.) were observed at S42 in the river and S269 and S226 in the coast. The mud fraction of the estuarine samples with a mean content of 410 ng g-1 d.w. had intermediate PAHs concentrations, which were not appreciably different between sampling locations. The PAHs were generally about two times higher in the sand fraction. Sometimes, the PAHs concentrated up to four times higher in the sand fraction e.g. at S42 (river), S250 (coast) and S232 (coast) , and even to seven times higher at S138 (estuary) suggesting the existence of high affinity organic matter for PAHs associated with the sand fraction. Enriched PAH contents in the coarse/sand fraction were also reported from other riverine, estuarine and coastal systems (e.g. Oen et al., 2006; Wang et al., 2001;
Ahrens & Depree, 2004; Rockne et al., 2002; Yang et al., 2008). These studies figured out the role of carbonaceous geosorbents such as coal-/wood-derived particles and black carbon in sorbing more PAHs in the sand fraction.
76 Fig. 2. Distribution of PAHs in sand and mud fractions of surface sediments from the Siak River, estuary and the coastal areas. Stations are arranged downstream from the Siak tributaries over the estuary to the coastal region.
A bulk PAH content (PAHBulk) was calculated for each station from the content for each fraction and the proportion on the total sediment dry mass. PAHBulk of all aquatic systems ranged from 145 to 1234 ng g-1 d.w.(Table 2). The coastal sediments turn out to contain the highest PAHBulk with a median of 577 ng g-1 d.w., followed by the river and the estuary of 484 and 443 ng g-1 d.w., respectively. This calculation placed S42, S138, S253 and S269 as stations having the highest PAHBulk (>1000 ng g-1 d.w.). In addition, these locations were also identified as having the highest PAH concentration within the sand fraction alone. The spatial distribution of PAHBulk as well as PAHs for the sand and the mud fractions did not show a clear pattern between sampling locations towards the river mouth of the estuary area suggesting various local inputs (Fig 2). However, the highest PAHBulk contamination was found in the vicinity of urban and industrial centres (S42, S226, S269, and S253). In contrast, the lowest PAHBulk contents were detected at the river upstream (S104) and in the estuary (S125).
Because most studies in the literature considered only the bulk assessment, the PAHBulk could bring comparable information on their pollution level. Values of PAH >1000 ng/g d.w.
mostly represent chronically polluted industrialized areas and harbours (Baumard et al., 1998;
Tolosa et al., 2004). Therefore, the river sediment around the capital city of Pekanbaru (S42), the coastal sediments in the vicinity of Dumai (S269, S226) and in the mouth of Selat Panjang (S253) can be considered highly polluted, while the other locations show moderate pollution.
0 1000 2000 3000 4000 5000 6000
S 24 S 101 S 20 S 104 S 105 S 35 S 116 S 42 S 145 S 143 S 142 S 138 S 134 S 252 S 125 S 250 S 251 S 269 S 226 S 227 S 228 S 267 S 266 S 253 S 230 S 231 S 232
River Estuary Coast
PAHs (ng/g d.w.)
Sand Mud Bulk
77 Direct comparison to those of other studies is somewhat difficult due to variation in numbers of studied-PAHs (not all studies investigated all the 16 US EPA compounds), the analytical performances and the background conditions. However, for a general idea of the PAH level, relevant information from studies in several Asian and European countries are presented in Table 3. The median values of PAH Bulk in surface sediments of Riau aquatic environments were higher by a factor of two or more than those on the opposite side of the Malacca Strait (Zakaria et al., 2002), the industrialized Gulf of Thailand (Boonyatumanond et al., 2006), and the Pearl River Estuary plus its adjacent coastal areas of China (Luo et al., 2006). Higher PAH in sediments from this study than in areas from the neighboring countries are assumed to be due to greater inputs of PAHs from widespread, intense plantation and peat burnings. These sources produced significant amount of PAH compared to other pyrogenic sources such as combustion of fossil fuels which are typical sources of sedimentary PAH in urban and industrial environments (e.g. Kakareka & Kukharchyk, 2003; Miguel et al., 1998).
4.3.3. PAH & OC Relationship
The PAHs and the OC content in the sand fraction are generally correlated (R2 = 0.70, Fig. 3). In this correlation, the S253 (coast), S269 (Dumai port), and S142 (estuary) are treated as outliers due to high PAHs coupled with low OC and vice versa, and thus omitted for this regression. In contrast, there was no correlation between PAHs and the OC content in the mud fraction. These results suggest that organic matter (OM) associated with the sand fraction has a stronger affinity for PAHs. On the other hand, in the fine fraction PAHs was independent on the OC content suggesting that PAHs were reluctant to associate with the OM, or that the OM content was relatively high for given PAHs content.
Furthermore, we figure out that there are actually two groups of the PAH-OC association regardless of the fractions. First, the PAHs with <1% OC was not correlated (Fig. 4a), and second, PAHs with >1% OC was linearly correlated (R2= 0.78, Fig. 4b). The first type seems to affirm a specific PAH-organic particle interaction which has nothing to do with the OC content. It might be that PAH-particles came from the same sources entering the aquatic systems. The second type supports the idea that PAHs in both fractions was controlled by its OM content and properties. But, the association was in favor of the coarse-attached OM. As no PAH-OC correlation observed for the fine fraction (Fig. 3), this type emphasizes that the OM of both fractions have different source and composition which control the PAHs sequestration. The coarse fraction-OM was most probably attributed to those from combustion-derived particles or black carbon, vascular plant debris and peat, while the fine fraction-OM was derived mainly from humification. The reluctance of PAH to associate with the fine fraction-OM is probably due to less aromatic fraction of the OM structure stemming from highly degraded peatland. As associations of PAH with the OM of the fine fraction occurs mostly as a direct function of fine
78 particle surface area, it could be then assumed that the low C/N ratio of the fine fraction would represent the condition of dissolved organic matter (DOM). Kalbitz & Geyer (2002) observed that low C/N ratio of DOM is one characteristic property of a degraded peatland. Prahl and Carpenter (1983) also observed evidences for a lack of correlation between PAHs and humic substances that were enriched in the fine (<64μm) fraction of the sediment samples from the Washington coastal region, although no specific reason was mentioned.
To sum up, this PAH-OC association could suggest three modes of which the PAH integrated into the aquatic systems. First, the PAH entered the aquatic environment favorably binding to the OM with high affinity for PAH (i.e. black particles, vascular plant debris, or peat) as found in S42, S116, S143, S138, S250, S232. Second, the PAH-OM came together from similar sources, as present in the coarse fraction comprised of low OC content and low total sediment weight. It was observed at the S230, S231, S269, and probably S226, and S253. Third, the PAHs were unfavorable associated with the fine fraction due to uncondensed, less aromatic fraction of the humic substances from the degraded peatland.
Fig. 3. Correlation between PAHs and the organic carbon contents in the sand () and mud () fractions of the sediments from the Siak River, the estuary and the coast (N=24, R2 =0.70 for the sand fraction,
= outliers).S142 S269: Dumai Port
S253: the Coast
0 1000 2000 3000 4000 5000 6000
0 5 10 15 20 25
PAHs (ng/g d.w.)
OC (%)
Sand Fraction
R2= 0.7
PAHs = 295.4*OC + 349.1
Estuarine Sediment
79 Fig. 4. Specific type of PAH-OC correlations for both fractions: (a) PAHs to low (<1%) OC content suggesting that the PAH and the OC entered the aquatic systems from the same sources, and (b) PAHs to high (>1%) OC indicating that the PAH sorption to the organic matter was a function of the OC content and composition.
4.3.4. Relative Composition of PAHs
The relative composition of PAHs was examined by ringed-group PAHs classified as 2 ring (NAPH), 3 rings (ACYN, ACEN, FLU, PHEN, ANTH), 4 rings (FLA, PYR, BaA, CHRY), 5 rings (BbFLA, BkFLA, BaP, DANTH), and 6 rings (BPERY, IPYR). The composition for both the sand and the mud fractions was largely identical with 3 to 5-ringed PAHs being most abundant. The 3-ringed PAH dominated the composition for both fractions ranging from 34.3%
to 72.7% in all aquatic environments, followed by 4-ringed and 5-ringed groups. The most abundant compound of the 3-ringed group is acenaphthylene. On average, it made up to 30.3%, 28.9%, and 67.7% of the relative individual composition in the sand fraction from the river, the estuary and the coasts, respectively. It made up 28.6%, 32.9%, and 68.9% of the relative composition in the mud fraction for the respective areas.
0 500 1000 1500 2000 2500 3000
0.0 0.1 0.2 0.3 0.4 0.5
PAHs (ng/g d.w.)
PAH vs OC <1%
y = 351.24x - 128.98 R² = 0.78
0 1000 2000 3000 4000 5000 6000
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0
PAHs (ng/g d.w.)
OC (%) PAH vs OC >1%
A
B
80 The other prevalent groups were the 4- and 5-ringed PAHs. The relative composition of both groups greatly varied among the stations in all environments ranging on average from 10.6% to 21.0% for the 4-ringed PAHs, and from 9.12% to 32.7% for the 5-ringed PAHs. These groups had a tendency to concentrate around the urban centres of Pekanbaru and Dumai cities.
However, the Siak tributaries of Tapung Kanan and Tapung Kiri Rivers were dominated by the 5-ringed PAH suggesting a potential repository for the high molecular PAHs.
Dibenzo(a,h)anthracene is the most abundant compound of the 5-ringed PAH in almost all the stations. This compound was particularly pronounced in the Siak tributaries, in the estuary (S143, S138, S134) and the Siak Kecil (S250), and in the coast (S253), where it made up ca.
50% to 90% of the relative composition of the 5-ringed PAH. Since those stations commonly surrounded by large plantation and peatland, we assumed that biomass and peat burnings occurring in the surrounded land could be the source of this compound. If this is true, increased contents of DANTH and other 5-ring PAHs could become signatures of peat-generated PAHs.
However, further investigations (field and laboratory) on profiles of peat-generated PAHs from this area are needed. Widespread distribution of 4- and 5-ringed PAH in the areas strongly suggests invasive pyrogenic non-point sources which were most likely introduced to the aquatic environments through soil washout and atmospheric deposition.
4.3.5. Source Apportionment
It is widely accepted that anthropogenic PAHs stem from two general sources: pyrogenic and petrogenic, which in most cases co-exist in aquatic sediments. In identifying which source is dominant, the apportionment for the PAH in many studies make use of specific characteristics of low over high molecular weight ratio (LMW/HMW), and specific isomeric ratios such as ANTH/(ANTH + PHEN), FLA/(FLA + PYR), BaA/(BaA+CHRY) (Neff 1979; Budzinski et al., 1997; Yunker et al., 2002; De Luca et al., 2005; Soclo et al., 2000). Petrogenic origin from maturation organic matters is typically marked by a high proportion of LMW (2 and 3 ring) over HMW (4 to 6 rings) PAHs. For instance, naphthalene (2 rings) contributes more than half of the total non-alkylated polyaromatic compounds in crude oil, followed by three-ring substances, PHEN and FLU (Requejo et al., 1996).
The specific isomer ratios have often been used due to their molecular stability against the increase temperature during pyrolysis. For example, phenanthrene is known to be thermodynamically more stable than the kinetically-stable isomer anthracene (Budzinski et al., 1997). The proportion of anthracene increases as processes involve higher temperatures. PAHs from combustion sources have typical values for the ratio ANTH / (ANTH + PHEN) > 0.1;
FLA/(FLA-PYR) >0.5, and BaA/(BaA+CHRY) > 0.35. In contrast, PAHs associated with petroleum e.g. crude oil, have typical values of those corresponding isomeric ratios of <0.1,
<0.4, and <0.2 respectively (Yunker et al., 2002). For instance, Alascan Crude Oil had for the
81 respective ratios values of 0.03, 0.26 and 0.10 (Requeojo et al., 1996). However, the boundaries between the assigned values are not clear-cut, and any value falling between those determining values is usually considered as mixtures of petroleum and combustion sources.
These ratios were applied in this study, and the assignment was carried out for both fractions. The values of the isomeric ratios were cross-plotted to get a tendency of the data (Fig.
5). Our results showed that the ratio of LMW/HMW for most of the stations was < 1 clearly indicating combustion sources, except for S104 and S105 in the upstream area of the River, S125 in the river mouth and S251 the mouth of Siak Kecil, and S267 in the northern mouth of the Bengkalis canal. The ratio of ANTH/(ANTH+PHEN) was respectively 0.1 ± 0.05 (mean ± standard deviation, n=9), 0.22 ± 0.18 (n=8), and 0.2 ± 0.2 (n=10) in the River, the estuary and the coast confirming predominance of combustion sources. Likewise, the ratio of FLA/(FLA-PYR) was respectively 0.48 ± 0.24, 0.65 ± 0.13, and 0.69 ± 0.17. High values of this ratio (>
0.5) indicate typical biomass burnings such as grasses, wood, or agricultural debris. The ratio of BaA/(BaA + CHRY) was 0.36 ± 0.13, 0.36 ± 0.15, and 0.47 ± 0.17 for the River, the estuary and the coast. These values were slightly larger than those of petroleum-combustion sources (upper limit 0.35).
All ratios clearly showed that pyrogenic sources were dominant, mainly from biomass (vegetation) with a small contribution from petroleum combustion (Fig. 5). These results could be attributed to widespread and intensive agricultural burnings and forest/peat swamp fires that occurs frequently in those areas particularly since the last severe outbreak of the El Niño.
Accordingly, pyrogenic PAHs, which were primarily formed during dry seasons and especially during burn-episodes, were delivered to the river and the estuary through soil washout, intensified during high rain and flood seasons. The widespread signature of pyrogenic PAHs might also reflect the significance of atmospheric (wet/dry) deposition.
However, the signatures of petrogenic PAHs were also observed in the river, the estuary and the coast. The river sediments are to a great extent contaminated by petrogenic sources, which is particularly observable in the Siak upper tributaries (S101) and around the capital city of Pekanbaru (S35, S116), the Siak Kecil River (S250), Dumai port (S269), and the mouth of Bengkalis canal (S267). The petrogenic PAHs in the upstream river might stem from petroleum discharges from many local small boats which were observable during the sampling. In general, the petroleum contamination observed in the other stations could be attributed to oil discharges from ships operation, regular transportation and cargo activities along the river as the study area is one of the biggest oil-production sites in Sumatra. Strong petrogenic signatures were also reported from estuarine and coastal sediments of the neighboring country, Malaysia (Zakaria et al., 2002).
82 Fig. 5. Cross plot of PAH isomeric ratios of the sand and mud sediment fractions from the Siak River (), the estuary ( ) and the coast ( ).