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Seasonal dispersion of petroleum contaminants in the Gulf of Thailand
Continental Shelf Research 18 (1998) 641 — 659
Seasonal dispersion of petroleum contaminants in the Gulf of Thailand Gullaya Wattayakorn!,*, Brian King", Eric Wolanski", Pornsri Suthanaruk# !Department of Marine Science, Chulalongkorn University, Bangkok 10330, Thailand "Australian Institute of Marine Science, PMB 3, Townsville MC, Qld, 4810, Australia #Pollution Control Department, Ministry of Science, Technology and Environment, Bangkok 10100, Thailand Received 8 October 1996; received in revised form 16 April 1997; accepted 19 November 1997
Abstract Six oceanographic moorings were maintained in 1993 and 1994 and provided data on the water circulation and the flushing characteristics of the Gulf of Thailand. The concentration of dissolved and dispersed petroleum hydrocarbons (DDPH) was measured at 78 sites in 1994 and 1995 in coastal waters of the Gulf. The water circulation was sluggish and the Gulf was poorly flushed; the mean currents were generally (0.07 m s~1. Under the influence of the South China Sea, an anticyclonic gyre existed in the southeast monsoon, a cyclonic gyre in the northwest monsoon, and sluggish currents the rest of the time. Even in the dry season brackish water was found inshore; this suggests that freshwater, that arrived in the Gulf in the previous wet season, was trapped along the coast and that little mixing occurred between offshore and coastal waters. In coastal waters of the Inner Gulf and the Eastern Sea Board there were occasional acute pollution events (DDPH'40 lg l~1), superimposed 25% of the time upon chronic pollution (DDPH+4 lg l~1) due to limited flushing of the Inner Gulf and the Eastern Sea Board, and the presence of slightly contaminated water elsewhere (DDPH(1.2 lg l~1) 75% of the time. Only the Outer Gulf seems relatively uncontaminated (DDPH+0.01—0.1 lg l~1). The seasonal distribution of the DDPH appeared to be controlled by the water circulation; indeed the highest DDPH values in the Inner Gulf occurred in November—December because of the net currents were weak and variable. The highest DDPH values on the Eastern Sea Board occurred in April—August when the region simultaneously received contaminated coastal water from the Inner Gulf. The smallest DDPH values on the Eastern Sea Board occurred in September—November because strong westward currents prevailed which flushed the contaminants. The observed currents and DDPH data were used to drive an oil spill model which predicted that acute contamination occurs at least once a year everywhere in the Inner Gulf. ( 1998 Elsevier Science Ltd. All rights reserved
*Corresponding author. 0278—4343/98/$19.00 ( 1998 Elsevier Science Ltd. All rights reserved PII S 0 2 78 — 43 4 3( 9 7 ) 00 0 72 - 1
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1. Introduction The Gulf of Thailand (Fig. 1) is a shallow, tropical, semi-enclosed marine embayment of about 350 000 km2 . The Gulf drains parts of Malaysia, Thailand, Cambodia and Viet Nam. It is open only to the South China Sea. It is widely used for recreational activities, as well as for commercial fisheries and mariculture. In recent years, the
Fig. 1. This location map shows the Gulf of Thailand, the bathymetry (depth in m), the major rivers (——) and the international boundaries (- -), and the data collection sites (*"oceanographic mooring sites; f"DDPH sampling sites).
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pressures of both industrial development and rising populations along the coast of Thailand have significantly increased the industrial and domestic waste discharge to the Gulf. Concerns about the extent of pollution in the Gulf prompted the commencement of a series of oceanographic and water quality studies since 1974. Numerical models suggested that the local wind controlled the water circulation and that the tidally-driven net currents were negligible (generally (0.01 m s~1; see a review in Wolanski et al., 1994), however, field data were unavailable for model verification. In particular the influence of the circulation in the South China Sea on the flushing of the Gulf of Thailand was unknown. A number of numerical models have been used by engineers to predict the tide and wind-driven flushing of the Inner Gulf (or the Upper Gulf; see Fig. 1), neglecting the forcing by the South China Sea for which no data were available. Reliable data are needed because of the increasing pollution threats to the Inner Gulf from a number of severely polluted rivers including the Chao Phraya, Bang Pakong, Tha Chin and Mae Klong rivers (Onodera et al., 1987; Ehrhardt et al., 1990). In particular, the Chao Phraya River drains Bangkok and the heavily industrialised surrounding areas and a large number of ships including thousands of small coastal vessels move to/from the docks at Bangkok. There is also concern about the effect of pollutants discharged in coastal waters of the Outer Gulf all along the Eastern Sea Board, an increasingly industrialised coastline just east of the Inner Gulf. Of particular concern is the input of oil from land-based sources such as municipal, refinery and other industrial wastes, and urban run-off (GESAMP, 1992). Wattayakorn (1986, 1987, 1991) has reported chronic petroleum hydrocarbon contamination in coastal waters. Pollution was believed to originate primarily from the discharge of oil from small coastal boats, via urban, industrial, refinery and sewage effluent. Additional oil contamination could also originate from maritime transportation of crude and refined oil through the region, as a result of the discharge of ballast water from tankers. Petpiroon (1988) reported that ship-breaking activities (i.e. where old ships are dismantled and oil discharged on the beach) polluted the seawater and beaches along the Eastern Sea Board. Small but chronic oil discharges as well as accidental spills have increased several folds in the last few years (Wattayakorn, unpublished data). Concern about oil pollution in the Gulf have generated hydrocarbon concentration monitoring studies in 1994 and 1995 at number of sites shown in Fig. 1. Concurrently the National Research Council of Thailand started an oceanographic monitoring study in the Gulf of Thailand in 1991 (the Seawatch Project). The Seawatch Project consisted of a number ()6 in 1993—1994) of long-term moorings at selected sites shown in Fig. 1. Each mooring typically contained instruments for measuring wind speed and direction, near-surface water currents, and vertical profiles of salinity and temperature. These data had never been processed until an opportunity arose to render the data meaningful as result from a grant from the IBM International Foundation. This paper presents the results of the 2-year survey of petroleum hydrocarbon concentrations in coastal waters of the Gulf. It combines these water quality data with the Seawatch oceanographic data in an attempt to understand the influence of the
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water circulation in controlling the seasonal distribution of dissolved hydrocarbons. It also examines the extent of impacts of acute pollution events. 2. Materials and methods 2.1. Oceanographic data From the Seawatch raw data set, hourly data on near-surface wind vectors; currents at 5 m depth; and salinity and temperature at 2.5, 5, 10, 20 and occasionally 30 m depth, were extracted and quality controlled. These data covered the period from April 1993 to August 1994. Up to six (but occasionally as few as two) moorings were maintained at positions shown in Fig. 1. The data were low-pass filtered to obtain monthly means. 2.2. Hydrocarbon data A total of 78 stations in coastal waters were regularly sampled between February 1994 and December 1995. These included 44 stations in the Inner Gulf, 16 stations in the Outer Gulf, and 18 stations along the Eastern Sea Board (see positions in Fig. 1). Water samples were collected at about 500 m from shore for all stations, and additionally at about 3000 m from shore for stations in the Inner Gulf. The samples were collected during low tide at 1 m depth below the sea surface, using a 4 l amber coloured glass bottle mounted on a weighted frame according to the IOC standard procedure (IOC/UNESCO,1984). The samples were treated with 50-ml hexane and stored in a dark, cool place. Subsequent laboratory extraction and analysis of the dissolved and dispersed petroleum hydrocarbons (DDPH) samples were performed according to IOC/UNESCO (1984). Samples were extracted three times in a separating funnel with 3]50 ml of pesticide grade n-hexane. The combined extracts were dried with the addition of anhydrous Na SO and concentrated to 5 ml with a rotary 2 4 evaporator. Ultraviolet fluorescence (UVF) analysis was carried out using a Perkin Elmer model 3000 Spectrofluorometer recording the intensity of fluorescence at 360 nm (excitation at 310 nm). Chrysene was chosen as a compound for calibration of the procedure. The working standards were in the same concentration range as the seawater samples. Blanks and chrysene standards were prepared using the same procedure as that employed for the samples. Statistical analysis followed standard analysis of the Student’s t-test.
3. Results and discussion 3.1. Oceanographic data The oceanographic data showed strong, predictable tidal currents (not shown). These currents were generally shore-parallel. The mean currents (monthly averaged) were weak (peaking at about 0.12 m s~1 but usually they were (0.07 m s~1; see
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Fig. 2. This is a computer-rendered, oblique aerial view of the Gulf of Thailand, looking south towards the South China Sea. This angle of view was necessary to avoid one data set hiding another one behind. The shallow waters are shaded light blue. Some international boundaries on land are shown. At two-monthly intervals, the balls (slightly displaced to the right of the mooring sites) show the mean, colour-coded, salinity
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Fig. 2. (Caption Continued). at the mooring sites from near-surface (top ball) to the bottom of the water column (bottom ball). The vertical interval varied with the local depth, as shown in Fig. 1. Missing data are shown by missing salinity balls. The salinity colour scale is shown on the right-hand side. The circles are centered at the mooring sites, have a 100 km radius, and show the colour-coded, near-surface, salinity. The red arrows show the mean
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Fig. 2. (Caption Continued). wind vectors pointing to the direction the wind blew to. The blue arrows show the net current vectors. The vertical bars show the colour-coded, DDPH values, the length of the bars are proportional to the concentration, the scale is shown on the left hand side. Lowest DDPH are shaded in blue, highest in red.
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Fig. 2) and varied both spatially and seasonally. The data (Fig. 2) revealed a number of previously unreported features. Firstly, the wind field was non-uniform over the Gulf. Secondly, the currents varied seasonally. this is summarised in Fig. 3. Highsalinity South China Sea water entered the Gulf along the Malaysia coast during the southeast monsoon, formed an anticyclonic gyre and left the Gulf along the Vietnam coast. Low-salinity Mekong River water entered the Gulf along the Vietnam coast during the northwest monsoon, formed a cyclonic gyre and left the Gulf along the Malaysia coast. Weak and variable currents prevailed the rest of the time. Thirdly, some of the largest currents generally occurred in offshore waters. Finally, the lowest salinity measured offshore occurred during September to December, this brackish water was presumably the Mekong River water from Vietnam, which was known to be in peak flood ('30 000 m3 s~1) during this period (Nguyen and Vu, 1995). There was always a significant salinity gradient existing between offshore and coastal waters even in the dry season when river runoff was negligible. This suggests that some of the river water that arrived in the Inner Gulf in the previous wet season remained as a brackish water mass for two to three months in the dry season. This finding suggests that the net currents were small and that there was little mixing between offshore and coastal waters. The net currents in January—February were particularly weak and variable throughout the Gulf (Fig. 3). This is a season with practically no advective flushing of the Inner Gulf to the Outer Gulf, though tidal mixing persists (Wolanski et al., 1994). At that time a mass of water took about 2—3 months to exit the Inner Gulf. From March to August an anticyclonic circulation prevailed in the Outer Gulf and this circulation also swept the Inner Gulf. By September the current reversed direction in offshore waters and this generated a cyclonic circulation in the Inner Gulf with no flow separation at Sattahip at the corner between the Inner Gulf and the Eastern Sea Board. Later a clockwise circulation prevailed in the Inner Gulf flow separation occurring at Sattahip. The cyclonic circulation in the Outer Gulf (Fig. 3) in September to November explains also the salinity data with lower salinity offshore (see Fig. 2), as a result of an inflow along the Viet Nam side of the Gulf of brackish water. At that time, the currents along the Eastern Sea Board were oriented against the wind and were strong (about 0.08 m s~1). These currents were westward along the Eastern Sea Board but did not penetrate into the Inner Gulf. Indeed they generated an anti-cyclonic eddy in the Inner Gulf with currents peaking at 0.04 m s~1. Prior to that a cyclonic circulation prevailed in the Inner Gulf. This switch in the flow field, sketched in the inset of Fig. 3, probably reflects a flow separation at the Satthip. Thus the water circulation in the Outer Gulf was largely controlled by that in the South China Sea. In turn the circulation in the Inner Gulf was strongly influenced by the Outer Gulf. 3.2. Hydrocarbon data We distinguish DDPH concentrations into three areas, namely the Inner Gulf, the Eastern Sea Board and the east coast of the Outer Gulf. Fig. 4 shows three peaks in
Fig. 3. Sketch of the water circulation at 5 m below the surface in the Gulf of Thailand, 1993—1994, deduced from the oceanographic data. The numbers indicate the distance in km travelled by a particle of water in 30 days.
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Fig. 4. Frequency distribution of DDPH concentration values (A) for all sites in 1994 and 1995, and (B) for 1995 only for the sites in the Inner Gulf at 500 and 3000 m from the coast and at 500 m in the Outer Gulf (Outer G.) and the Eastern Sea Board (ESB).
the spectrum, the dominant one (75% of the time) for concentrations less than 0.6 lg l~1, a second peak (about 25% of the time) for DDPH values around 4 lg l~1, and a third minor peak (which contains, however, most of the mass of hydrocarbon) at concentrations of 40—80 lg l~1. These three peaks suggest occasional acute pollution events (DDPH'40 lg l~1) superimposed upon chronic pollution (DDPH+ 4 lh l~1; this we argue later is due to the slow flushing of the Gulf coastal waters) and the presence of ‘clean’ water elsewhere (DDPH(0.6 lg l~1). Dissolved petroleum hydrocarbon concentrations for coastal waters of the Gulf of Thailand ranged from 0.05 to 11.84 lg l~1 and 0.01 to 76.2 lg l~1, as chrysene equivalents, in 1994 and 1995, respectively (see Figs. 2 and 4). A summary of the mean, median, standard deviation of the DDPH values is shown in Table 1. The coastal surface waters of the Inner Gulf exhibited concentrations of DDPH of 0.20—8.26 lg l~1 in 1994 and 0.07—76.2 lg l~1 in 1995. The DDPH data were not
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Table 1 Summary of the DDPH concentrations in coastal waters of the Gulf of Thailand (lg l~1 chrysene equivalents) Year 1994
1995
Inner Gulf ESB Overall Inner Gulf —500 m —3000 m ESB Outer Gulf Overall
n
Range
mean
s.d.
Median
16 162 178
0.20—8.26 0.05—11.8 0.05—11.8
3.07 1.38 1.54
2.99 1.68 2.01
1.73 0.85 0.90
42 40 36 29 147
0.07—76.2 0.12—25.3 0.14—10.0 0.01—12.0 0.01—76.2
3.00 1.97 1.37 1.72 2.07
12.0 5.33 1.72 2.70 7.09
0.30 0.38 0.77 0.35 0.44
Note: ESB"the Eastern Sea Board, n"number of observations, 500 and 3000 m are locations of the sampling points from the shore
normally distributed (Fig. 4); the mean was measurably larger than the median value because of the skewness of the data (see Table 1). The majority of the DDPH data for 1995 samples were within the 0.2—1.0 lg l~1 range, with however a number of samples having concentration '10 lg l~1. The highest concentration (76.2 lg l~1) was found near the Sattahip where a naval base is located. Elsewhere most high DDPH values were found near the river mouths, and/or fish-landing piers and docks in the Inner Gulf under the influence of the Chao Phraya River. Coastal waters around Sichang Island (shown in Fig. 6) were permanently chronically polluted (range: 2.5—11.6 lg l~1, mean "6.6 lg l~1). In the Inner Gulf the DDPH values also showed acute pollution on occasions (DDPH'40 lg l~1; see Fig. 2) and chronic pollution (+4 lg l~1) about 20—25% of the time. This indicated sporadic, local, acute contamination events superimposed on chronic pollution. Seasonal fluctuations were small compared to the standard deviation (due to the skewed nature of these data which made the standard deviation for the DDPH data greater than the mean) and hence may not be significant. For the Eastern Sea Board, the results also showed large spatial and temporal fluctuations of DDPH concentrations with events of acute pollution and chronic pollution occurring about 8% of the time. These were also weak seasonal fluctuations (see Figs. 2 and 5). The DDPH values fluctuated between 0.05—11.84 lg l~1 in 1994 and 0.14—9.96 lg l~1 in 1995. The DDPH values along the east coast of the Outer Gulf showed a similar range as that of the Eastern Sea Board, being 0.01—12.04 lg l~1 with the mean of 1.92 lg l~1, but there were in that area no acute pollution events. Student’s t-test of the data (see Table 2) indicated no significant difference in the overall mean petroleum hydrocarbon concentrations within the coastal water sites in 1995, however a significant difference (P"0.04) was found for the mean hydrocarbon concentrations between the Inner Gulf sites (3.07 lg l~1 ) and the Eastern Sea Board sites (1.38 lg l~1 ) for the 1994 survey.
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Fig. 5. Seasonal distribution of the mean (*) and the standard deviation (error bars) of the DDPH concentration along the Eastern Sea Board of the Gulf of Thailand in 1994—1995. The time on the x-axis is shown as month/year. The standard deviation of the DDPH was at times larger than the mean because of the skewness of the data, no concentration values were negative.
Table 2 Student’s t-test of the DDPH concentration values in the Gulf of Thailand
1994 1995
Gulf of Thailand
mean 1
mean 2
df
P value
Significant difference
Inner Gulf vs. ESB Apr—Aug vs. Oct—Feb 500 m vs. 3000 m Inner Gulf vs. ESB Inner Gulf vs. Outer Gulf ESB vs. Outer Gulf
3.07 1.78 2.99 2.50
1.38 1.00 1.97 1.37
16 108 57 93
0.04 0.01 0.62 0.30
# # ! !
2.50 1.37
1.72 1.72
107 45
0.50 0.55
! !
1994 vs. 1995
1.54
2.07
166
0.38
!
Note: ESB"the Eastern Sea Board, Mean 1 is the mean value of the 1st group of samples, and Mean 2 is the mean value of the 2nd group of samples (e.g. in the 1st line Mean 1 refers to the Inner Gulf and Mean 2 to the Eastern Sea Board).
The histograms of the DDPH concentrations (Fig. 4a) showed highest occurrences in the concentration class of 0.6—1.5 lg l~1 in 1994 and 0.20—0.40 lg l~1 in 1995. The upper quartile was found to be 1.2 lg l~1 for both 1994 and 1995 surveys. 3.3. Risk assessment of impact from acute pollution events Several stations showed evidence of acute oil pollution, with concentrations from 20 to 76 lg l~1. These high values are comparable to those of heavily industrialised regions in the UK (Law, 1981), the severely oil affected area in the northwestern Arabian Gulf (El-Samra et al., 1986), and the Gulf of Oman (Emara, 1990).
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Fig. 6. Map showing regions at risk from two measured acute pollution events near the Sichang and Rayong oceanographic moorings of the Inner Gulf and Eastern Sea Board. f"the oceanographic mooring sites that provided the wind and current meter data used in the oil spill model; ="the sites where high DDPH concentrations were observed in the field, these sites were assumed to be the source of contamination in the model.
To understand how currents within the Gulf of Thailand mix and transport such pollution, and to tentatively identify the extent of the affected area, the oil spill model OILMAP of Spaulding et al. (1993, 1994) was used. The position of two measured high DDPH concentrations were assigned to be a known source in the model (see Fig. 6). The model utilised the local wind and the local current data from the mooring sites at Rayong and Sichang (shown in Fig. 6) to advect and mix these high concentrations (10—26 lg l~1) around the Gulf waters until dilution/degradation reduced the predicted DDPH values to background levels. This reduction was assumed to occur in no more than 10 days because of rapid chemical transformation of oil in tropical conditions. For each site, 100 trajectory calculations were performed by randomly setting the release time to differing parts of the wind/current record to examine where both time-varying wind and currents could advect this pollutant. From such trajectory calculations, a risk assessment to the coastal waters surrounding the acute pollution measurement sites was obtained (Fig. 6).
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For the Inner Gulf in December, (near Sichang) wind and currents can transport acute pollution up to 12 km alongshore from the source within 24 h and expose 30 km of coastal waters to a high risk ('70%) chance of contamination. Overall, some 58 km of coastal waters were predicted to be at risk to this high, measured, DDPH value at the sampling site. For the Eastern Sea Board in March/April, the current can transport acute pollution up to 8 km alongshore from the source within 24 h and expose 12 km of coastal waters to a high risk of contamination. About 50 km of coastline was at some risk of exposure. The oil spill trajectory model cannot be used to contrast with the measurements because the sources of the hydrocarbon are both chronic, common releases of oil as well as accidental, occasional major spills. Little is known about their relative importance and no data on oil discharges are available. The model however suggests that our findings of occasional large pollutant concentration values are probably not localised. Instead this pollutant is likely to be spread within a day over about 12 km, this distance is too small to be resolved by our sampling grid. Thus much of the coastline of the Inner Gulf and some sections of the Eastern Sea Board are occasionally significantly polluted. The exact origin of the hydrocarbon is unknown, since no data are available on oil discharges to the Gulf, but is probably within 50 km of the affected sites we found. 3.4. Chronic pollution The overall mean for all the areas studied was 1.54 lg l~1 in 1994 and 2.1 lg l~1 in 1995. Cripps (1992), proposed the use of the upper quartile as the threshold limit of natural variation if the waters are usually not polluted. If this criterion is used for the Inner Gulf, the background value of DDPH would be 1.2 lg l~1 chrysene equivalents. This value however already reflects permanent contamination of the Inner Gulf. Indeed it is high for inshore water elsewhere (Law, 1981; Levy, 1986; Marchand, 1980; Atwood et al., 1987; Weber and Bicego, 1990), and further it is 100 times larger than the lower quartile value at the sampling points in the unpolluted Outer Gulf. Thus low-level hydrocarbon pollution to some degree occurs all the time in the Inner Gulf and the Eastern Sea Board. In these areas 75% of the time low-level oil contamination occurred at the coastal sampling stations and about 25% of the stations showed evidence of chronic pollution. Additionally there were occasional acute pollution events. The Inner Gulf and the Eastern Sea Board coastal waters are thus measurably affected by oil pollution.
4. Conclusions 4.1. Water circulation The oceanographic data presented here showed that in addition to the tidal and local wind influences which previous models have incorporated, in the future models of circulation must also include (a) the non-uniform wind fields over the Gulf; (b) the
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influences of the Mekong River plume in the wet season; and (c) the circulation in the South China Sea. Contrary to previous models the net circulation is not forced mainly by the local wind, if this was the case there would be a wind-driven outflow/inflow along the coast and a weak return flow (speed(0.2 m s~1) in the centre of the Gulf In fact the largest currents were observed near the centre of the Gulf. Instead the forcing from the South China Sea largely drives the net circulation. The circulation is characterised by a cyclonic gyre during the northwest monsoon, an anticyclonic eddy during the southeast monsoon, and weak and variable currents the rest of the time. In turn this circulation in the outer Gulf drives a weaker circulation in the Inner Gulf. The water circulation in the Outer Gulf (Fig. 3) was thus largely controlled by that in the South China Sea. In turn the circulation in the Inner Gulf was strongly influenced by the Outer Gulf. Some of the river water that arrived in the previous wet season remains for 2—3 months in the Inner Gulf in the dry season, a finding suggesting that the net currents were small and that little mixing occurred between offshore and coastal waters in the Gulf. It takes at least two months to flush the Inner Gulf. This finding also explains the observations of high rates of mud accumulation in this area (Srisuksawad et al., 1997). 4.2. Oil pollution The DDPH data showed three peaks in the distribution, these were occasional acute pollution events (DDPH'40 lg l~1), superimposed upon chronic pollution (DDPH+4 lg l~1) due to slow flushing of the Gulf, and slightly contaminated water elsewhere in the Inner Gulf and the Eastern Sea Board (DDPH+1.2 lg l~1). Only in the Outer Gulf was the water most of time uncontaminated (DDPH+ 0.01—0.1 lg l~1). The bulk of the pollutants is believed to reach the Gulf from polluted rivers and industrial and shipping discharges in the Inner Gulf and the Eastern Sea Board; indeed these two areas were the most polluted. The inner Gulf coastal waters appeared to be more contaminated by petroleum hydrocarbons than the other parts of the Gulf. The hydrocarbon concentration values were particularly elevated, up to the level of acute contamination, in areas of industrial and shipping activity. In these areas, flushing of ballast, wash and bilge waters from ships is commonly practiced. Oil spills from tankers and thousands of small, dirty coastal vessels, in addition to the waste derived from the oil refinery and associated industries, are additional causes for these high concentrations. Land-based sources are probably important contributors to the steady, chronic contamination of coastal waters in the Inner Gulf and the Eastern Sea Board. 4.3. Influence of the water circulation The DDPH concentration values were highly variable. Some of this variability can is due to the unknown history of local oil discharges nevertheless the circulation in the Gulf apparently explains some key features of this variability.
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The seasonal changes of the currents in the Inner Gulf (Figs. 2 and 3) appears responsible for the elevated DDPH values. Given that the Upper Gulf and the Eastern Sea Board are known origins of acute events, the long term trapping of the pollutants apparently controls the build-up of DDPH to chronic pollution levels. For example, during the months of January and February, there are weak and variable net currents and the coastal waters are resident for a few months. During the remaining months of the year, there exists a clockwise circulation within the Gulf followed by an anticlockwise circulation. In the Upper Gulf and along the Eastern Seaboard, these net drift currents will flush the water from west to east, then to have it return later. Essentially the polluted water sloshes back and forth and this may explain the frequent occurrence (25% of the time) of chronic pollution (DDPH+4 lg l~1). This pool of contaminated water moves back and forth seasonally between the Eastern Sea Board and the Inner Gulf, and this may explain the significant relation between their mean DDPH values in 1995 (Table 2). The seasonal behaviour of the currents also explains the significant seasonal difference (P"0.01) which was observed for the hydrocarbon concentrations along the Eastern Sea Board in 1994. A maximum in DDPH values was obtained in April—August (mean"1.78 lg l~1), when net currents were flushing the Upper Gulf waters to the Eastern Seaboard. A minimum DDPH value occurred in October—February (mean"1.0 lg l~1), when net drift current were bringing in cleaner water from the South China Sea to the area. The pollution in the Inner Gulf was particularly significant in November and December when the currents were sluggish. Along the Eastern Sea Board, the highest DDPH concentrations occurred in April to August when the wind blew from offshore and when net currents were negligible. In contrast, from September to November, the DDPH values were smaller probably because strong net westward currents prevailed and flushed the area. Further, using the oil spill model driven by the local wind and the observed currents, it was possible to estimate the risk to surrounding biological habitats of impact of acute pollution events observed at the sampling sites. The model suggests that within 10 days up to 58 km of coastal waters in the Inner Gulf and 50 km along the Eastern Sea Board be at some risk of contamination from the acute pollution observed at the sampling sites. Since about 20% of our sampling points show acute contamination in the Inner Gulf, this implies that much of the Inner Gulf is acutely polluted at least once a year. 4.4. Implications for management One cannot assume that the currents simply flush away pollutants and maintain a viable marine ecosystem in the Gulf of Thailand. The oceanographic data reveal that the coastal waters of the Inner Gulf and the Eastern Sea Board take two to three months to be flushed. Because of slow flushing hydrocarbon pollution is a serious threat. Chronic pollution is already occurring at some sites. These findings suggest that a more stringent management plan should be implemented for the control of wastes in the Gulf of Thailand.
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Acknowledgements The ASEAN—Canada Cooperative Programme on Marine Sciences, Chulalongkorn University, the Australian Institute of Marine Science and the IBM International Foundation supported this study. The National Research Council of Thailand provided the Seawatch oceanographic data. The generous help provided by Patrick Collins, Duncan Galloway, Felicity McAllister and Kris Summerhayes is gratefully acknowledged. Thanks also go to Kathryn Burns and the reviewers for constructive criticism.
References Atwood, D.K., Burton, F.J., Corredor, J.E., Harvey, G.R., Mata-Jimenez, A.J., Vasquez-Botello, A., Wade, B.A., 1987. Results of the CARIPOL Petroleum Pollution Monitoring Project in the wider Caribbean. Marine Pollution Bulletin 18, 540—548. Cripps, G.C., 1992. Baseline levels of hydrocarbons in seawater of the Southern Ocean: natural variability and regional patterns. Marine Pollution Bulletin 24, 109—114. Ehrhardt, M.G., Wattayakorn, G., Dawson, R., 1990. GC/MS based analyses of individual organic constituents of Chao Phraya river water and estimated discharge rates into the Upper Gulf of Thailand. Estuarine Coastal and Shelf Science 30, 439—451. Ehrhardt, M.G., Burns, K.A., 1993. Hydrocarbons and related photo-oxidation products in Saudi Arabian Gulf coastal waters and hydrocarbons in underlying sediments and bioindicator bivalves. Marine Pollution Bulletin 27, 187—197. El-Samra, M.I., Emara, H.I., Shunbo, E., 1986. Dissolved petroleum hydrocarbon in the northwestern Arabian Gulf. Marine Pollution Bulletin 17, 65—68. Emara, H.I., 1990. Oil pollution in the southern Arabian Gulf and Gulf of Oman. Marine Pollution Bulletin 21, 399-401. GESAMP, 1992. Report of the GESAMP Working Group on the impact of oil, individual hydrocarbons, and related chemicals on the marine environment, including used lubricating oils, oil spill control agents and chemical used offshore. London, IMO. IOC/UNESCO, 1984. Manual for Monitoring Oil and Dissolved/Dispersed Petroleum Hydrocarbons in Marine Waters and on Beaches, Manuals and Guides No. 13. UNESCO. Paris, France. 30 pp. Law, R.J., 1981. Hydrocarbon concentrations in water and sediments from UK marine waters, determined by fluorescence spectroscopy. Marine Pollution Bulletin 12, 153—157. Levy, E.M., 1986. Background levels of petroleum residues in the waters and surficial sediments of the Labrador shelf and Hudson Strait/Foxe Basin regions. Canada Journal of Fisheries and Aquatic Science 43, 536—547. Marchand, M.H., 1980. The Amoco Cadiz oil spill distribution and evaluation of hydrocarbon concentration in seawater and marine sediments. Environment International 4, 421—429. Nguyen, V.P., Vu, V.T., 1995. Natural Conditions of Lower Mekong Delta Basin. Agriculture Press, Hanoi, Vietnam. 151 pp. Onodera, S., Chatkittikunron, W., Saito, K., Phongbetchara, R., Tabucanon, M., 1987. Characterization and determination of lipophilic hydrocarbons in the Chao Phraya, Bang Pakong, and Tha-Chin rivers and the upper Gulf of Thailand. Journal of Chromatography 392, 295—308. Petpiroon, P., 1988. Dissolved petroleum hydrocarbons in seawater along the eastern coastal areas from Pattaya to Trat. Technical paper No.5, Eastern Marine Fisheries Development Center, Marine Fisheries Division, Department of Fisheries, Ministry of Agriculture and Cooperatives. 20 pp. Spaulding, M.L., Anderson, E.L., Isaji, T., Howlett, E., 1993. Simulation of the oil trajectory and fate in the Arabian Gulf from the Mina Al Ahmadi spill. Marine Environmental Research 36, 79—115.
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Spaulding, M.L., Kolluru, V.S., Anderson, E.L., Howlett, E., 1994. Application of three-dimensional oil spill model (WOSM/OILMAP) to hindcast the Braer spill. Spill Science and Technology Bulletin 1 (1), 23—35. Wattayakorn, G., 1986. Petroleum hydrocarbons in water and sediments from the Gulf of Thailand. In: Proceedings of the 3rd National Seminar on Marine Science, Bangkok, Thailand, 6—8 August, 1986. Wattayakorn, G., 1987. Dissolved/dispersed petroleum hydrocarbons in the Gulf of Thailand. In: Proceedings of the 4th Seminar on the Thai Waters Quality of Living Resources in Thai Waters. Surat Thani, Thailand. 7—9 July 1987. Wattayakorn, G., 1991. Petroleum pollution in the Gulf of Thailand. In: Proceedings Regional Seminar on Coastal and Offshore Engineering: Prospects of the Coastal and Offshore Engineering in the 21st Century. Kuala Terengganu, Malaysia. 9-11 December, 1991. Weber, R.R., Bicego, M.C., 1990. Petroleum aromatic hydrocarbons in surface waters around Elephant Island, Antarctic Peninsula. Marine Pollution Bulletin 21, 448—449. Wolanski, E., Wattayakorn, G., King, B., 1994. Water circulation and pollutant transport in the Gulf of Thailand. In: Proceedings IOC-WESTPAC Symposium, Bali, Indonesia, Indonesia, IOC, pp. 392—410.
Seasonal dispersion of petroleum contaminants in the Gulf of Thailand Gullaya Wattayakorn!,*, Brian King", Eric Wolanski", Pornsri Suthanaruk# !Department of Marine Science, Chulalongkorn University, Bangkok 10330, Thailand "Australian Institute of Marine Science, PMB 3, Townsville MC, Qld, 4810, Australia #Pollution Control Department, Ministry of Science, Technology and Environment, Bangkok 10100, Thailand Received 8 October 1996; received in revised form 16 April 1997; accepted 19 November 1997
Abstract Six oceanographic moorings were maintained in 1993 and 1994 and provided data on the water circulation and the flushing characteristics of the Gulf of Thailand. The concentration of dissolved and dispersed petroleum hydrocarbons (DDPH) was measured at 78 sites in 1994 and 1995 in coastal waters of the Gulf. The water circulation was sluggish and the Gulf was poorly flushed; the mean currents were generally (0.07 m s~1. Under the influence of the South China Sea, an anticyclonic gyre existed in the southeast monsoon, a cyclonic gyre in the northwest monsoon, and sluggish currents the rest of the time. Even in the dry season brackish water was found inshore; this suggests that freshwater, that arrived in the Gulf in the previous wet season, was trapped along the coast and that little mixing occurred between offshore and coastal waters. In coastal waters of the Inner Gulf and the Eastern Sea Board there were occasional acute pollution events (DDPH'40 lg l~1), superimposed 25% of the time upon chronic pollution (DDPH+4 lg l~1) due to limited flushing of the Inner Gulf and the Eastern Sea Board, and the presence of slightly contaminated water elsewhere (DDPH(1.2 lg l~1) 75% of the time. Only the Outer Gulf seems relatively uncontaminated (DDPH+0.01—0.1 lg l~1). The seasonal distribution of the DDPH appeared to be controlled by the water circulation; indeed the highest DDPH values in the Inner Gulf occurred in November—December because of the net currents were weak and variable. The highest DDPH values on the Eastern Sea Board occurred in April—August when the region simultaneously received contaminated coastal water from the Inner Gulf. The smallest DDPH values on the Eastern Sea Board occurred in September—November because strong westward currents prevailed which flushed the contaminants. The observed currents and DDPH data were used to drive an oil spill model which predicted that acute contamination occurs at least once a year everywhere in the Inner Gulf. ( 1998 Elsevier Science Ltd. All rights reserved
*Corresponding author. 0278—4343/98/$19.00 ( 1998 Elsevier Science Ltd. All rights reserved PII S 0 2 78 — 43 4 3( 9 7 ) 00 0 72 - 1
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1. Introduction The Gulf of Thailand (Fig. 1) is a shallow, tropical, semi-enclosed marine embayment of about 350 000 km2 . The Gulf drains parts of Malaysia, Thailand, Cambodia and Viet Nam. It is open only to the South China Sea. It is widely used for recreational activities, as well as for commercial fisheries and mariculture. In recent years, the
Fig. 1. This location map shows the Gulf of Thailand, the bathymetry (depth in m), the major rivers (——) and the international boundaries (- -), and the data collection sites (*"oceanographic mooring sites; f"DDPH sampling sites).
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pressures of both industrial development and rising populations along the coast of Thailand have significantly increased the industrial and domestic waste discharge to the Gulf. Concerns about the extent of pollution in the Gulf prompted the commencement of a series of oceanographic and water quality studies since 1974. Numerical models suggested that the local wind controlled the water circulation and that the tidally-driven net currents were negligible (generally (0.01 m s~1; see a review in Wolanski et al., 1994), however, field data were unavailable for model verification. In particular the influence of the circulation in the South China Sea on the flushing of the Gulf of Thailand was unknown. A number of numerical models have been used by engineers to predict the tide and wind-driven flushing of the Inner Gulf (or the Upper Gulf; see Fig. 1), neglecting the forcing by the South China Sea for which no data were available. Reliable data are needed because of the increasing pollution threats to the Inner Gulf from a number of severely polluted rivers including the Chao Phraya, Bang Pakong, Tha Chin and Mae Klong rivers (Onodera et al., 1987; Ehrhardt et al., 1990). In particular, the Chao Phraya River drains Bangkok and the heavily industrialised surrounding areas and a large number of ships including thousands of small coastal vessels move to/from the docks at Bangkok. There is also concern about the effect of pollutants discharged in coastal waters of the Outer Gulf all along the Eastern Sea Board, an increasingly industrialised coastline just east of the Inner Gulf. Of particular concern is the input of oil from land-based sources such as municipal, refinery and other industrial wastes, and urban run-off (GESAMP, 1992). Wattayakorn (1986, 1987, 1991) has reported chronic petroleum hydrocarbon contamination in coastal waters. Pollution was believed to originate primarily from the discharge of oil from small coastal boats, via urban, industrial, refinery and sewage effluent. Additional oil contamination could also originate from maritime transportation of crude and refined oil through the region, as a result of the discharge of ballast water from tankers. Petpiroon (1988) reported that ship-breaking activities (i.e. where old ships are dismantled and oil discharged on the beach) polluted the seawater and beaches along the Eastern Sea Board. Small but chronic oil discharges as well as accidental spills have increased several folds in the last few years (Wattayakorn, unpublished data). Concern about oil pollution in the Gulf have generated hydrocarbon concentration monitoring studies in 1994 and 1995 at number of sites shown in Fig. 1. Concurrently the National Research Council of Thailand started an oceanographic monitoring study in the Gulf of Thailand in 1991 (the Seawatch Project). The Seawatch Project consisted of a number ()6 in 1993—1994) of long-term moorings at selected sites shown in Fig. 1. Each mooring typically contained instruments for measuring wind speed and direction, near-surface water currents, and vertical profiles of salinity and temperature. These data had never been processed until an opportunity arose to render the data meaningful as result from a grant from the IBM International Foundation. This paper presents the results of the 2-year survey of petroleum hydrocarbon concentrations in coastal waters of the Gulf. It combines these water quality data with the Seawatch oceanographic data in an attempt to understand the influence of the
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water circulation in controlling the seasonal distribution of dissolved hydrocarbons. It also examines the extent of impacts of acute pollution events. 2. Materials and methods 2.1. Oceanographic data From the Seawatch raw data set, hourly data on near-surface wind vectors; currents at 5 m depth; and salinity and temperature at 2.5, 5, 10, 20 and occasionally 30 m depth, were extracted and quality controlled. These data covered the period from April 1993 to August 1994. Up to six (but occasionally as few as two) moorings were maintained at positions shown in Fig. 1. The data were low-pass filtered to obtain monthly means. 2.2. Hydrocarbon data A total of 78 stations in coastal waters were regularly sampled between February 1994 and December 1995. These included 44 stations in the Inner Gulf, 16 stations in the Outer Gulf, and 18 stations along the Eastern Sea Board (see positions in Fig. 1). Water samples were collected at about 500 m from shore for all stations, and additionally at about 3000 m from shore for stations in the Inner Gulf. The samples were collected during low tide at 1 m depth below the sea surface, using a 4 l amber coloured glass bottle mounted on a weighted frame according to the IOC standard procedure (IOC/UNESCO,1984). The samples were treated with 50-ml hexane and stored in a dark, cool place. Subsequent laboratory extraction and analysis of the dissolved and dispersed petroleum hydrocarbons (DDPH) samples were performed according to IOC/UNESCO (1984). Samples were extracted three times in a separating funnel with 3]50 ml of pesticide grade n-hexane. The combined extracts were dried with the addition of anhydrous Na SO and concentrated to 5 ml with a rotary 2 4 evaporator. Ultraviolet fluorescence (UVF) analysis was carried out using a Perkin Elmer model 3000 Spectrofluorometer recording the intensity of fluorescence at 360 nm (excitation at 310 nm). Chrysene was chosen as a compound for calibration of the procedure. The working standards were in the same concentration range as the seawater samples. Blanks and chrysene standards were prepared using the same procedure as that employed for the samples. Statistical analysis followed standard analysis of the Student’s t-test.
3. Results and discussion 3.1. Oceanographic data The oceanographic data showed strong, predictable tidal currents (not shown). These currents were generally shore-parallel. The mean currents (monthly averaged) were weak (peaking at about 0.12 m s~1 but usually they were (0.07 m s~1; see
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Fig. 2. This is a computer-rendered, oblique aerial view of the Gulf of Thailand, looking south towards the South China Sea. This angle of view was necessary to avoid one data set hiding another one behind. The shallow waters are shaded light blue. Some international boundaries on land are shown. At two-monthly intervals, the balls (slightly displaced to the right of the mooring sites) show the mean, colour-coded, salinity
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Fig. 2. (Caption Continued). at the mooring sites from near-surface (top ball) to the bottom of the water column (bottom ball). The vertical interval varied with the local depth, as shown in Fig. 1. Missing data are shown by missing salinity balls. The salinity colour scale is shown on the right-hand side. The circles are centered at the mooring sites, have a 100 km radius, and show the colour-coded, near-surface, salinity. The red arrows show the mean
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Fig. 2. (Caption Continued). wind vectors pointing to the direction the wind blew to. The blue arrows show the net current vectors. The vertical bars show the colour-coded, DDPH values, the length of the bars are proportional to the concentration, the scale is shown on the left hand side. Lowest DDPH are shaded in blue, highest in red.
.
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Fig. 2) and varied both spatially and seasonally. The data (Fig. 2) revealed a number of previously unreported features. Firstly, the wind field was non-uniform over the Gulf. Secondly, the currents varied seasonally. this is summarised in Fig. 3. Highsalinity South China Sea water entered the Gulf along the Malaysia coast during the southeast monsoon, formed an anticyclonic gyre and left the Gulf along the Vietnam coast. Low-salinity Mekong River water entered the Gulf along the Vietnam coast during the northwest monsoon, formed a cyclonic gyre and left the Gulf along the Malaysia coast. Weak and variable currents prevailed the rest of the time. Thirdly, some of the largest currents generally occurred in offshore waters. Finally, the lowest salinity measured offshore occurred during September to December, this brackish water was presumably the Mekong River water from Vietnam, which was known to be in peak flood ('30 000 m3 s~1) during this period (Nguyen and Vu, 1995). There was always a significant salinity gradient existing between offshore and coastal waters even in the dry season when river runoff was negligible. This suggests that some of the river water that arrived in the Inner Gulf in the previous wet season remained as a brackish water mass for two to three months in the dry season. This finding suggests that the net currents were small and that there was little mixing between offshore and coastal waters. The net currents in January—February were particularly weak and variable throughout the Gulf (Fig. 3). This is a season with practically no advective flushing of the Inner Gulf to the Outer Gulf, though tidal mixing persists (Wolanski et al., 1994). At that time a mass of water took about 2—3 months to exit the Inner Gulf. From March to August an anticyclonic circulation prevailed in the Outer Gulf and this circulation also swept the Inner Gulf. By September the current reversed direction in offshore waters and this generated a cyclonic circulation in the Inner Gulf with no flow separation at Sattahip at the corner between the Inner Gulf and the Eastern Sea Board. Later a clockwise circulation prevailed in the Inner Gulf flow separation occurring at Sattahip. The cyclonic circulation in the Outer Gulf (Fig. 3) in September to November explains also the salinity data with lower salinity offshore (see Fig. 2), as a result of an inflow along the Viet Nam side of the Gulf of brackish water. At that time, the currents along the Eastern Sea Board were oriented against the wind and were strong (about 0.08 m s~1). These currents were westward along the Eastern Sea Board but did not penetrate into the Inner Gulf. Indeed they generated an anti-cyclonic eddy in the Inner Gulf with currents peaking at 0.04 m s~1. Prior to that a cyclonic circulation prevailed in the Inner Gulf. This switch in the flow field, sketched in the inset of Fig. 3, probably reflects a flow separation at the Satthip. Thus the water circulation in the Outer Gulf was largely controlled by that in the South China Sea. In turn the circulation in the Inner Gulf was strongly influenced by the Outer Gulf. 3.2. Hydrocarbon data We distinguish DDPH concentrations into three areas, namely the Inner Gulf, the Eastern Sea Board and the east coast of the Outer Gulf. Fig. 4 shows three peaks in
Fig. 3. Sketch of the water circulation at 5 m below the surface in the Gulf of Thailand, 1993—1994, deduced from the oceanographic data. The numbers indicate the distance in km travelled by a particle of water in 30 days.
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Fig. 4. Frequency distribution of DDPH concentration values (A) for all sites in 1994 and 1995, and (B) for 1995 only for the sites in the Inner Gulf at 500 and 3000 m from the coast and at 500 m in the Outer Gulf (Outer G.) and the Eastern Sea Board (ESB).
the spectrum, the dominant one (75% of the time) for concentrations less than 0.6 lg l~1, a second peak (about 25% of the time) for DDPH values around 4 lg l~1, and a third minor peak (which contains, however, most of the mass of hydrocarbon) at concentrations of 40—80 lg l~1. These three peaks suggest occasional acute pollution events (DDPH'40 lg l~1) superimposed upon chronic pollution (DDPH+ 4 lh l~1; this we argue later is due to the slow flushing of the Gulf coastal waters) and the presence of ‘clean’ water elsewhere (DDPH(0.6 lg l~1). Dissolved petroleum hydrocarbon concentrations for coastal waters of the Gulf of Thailand ranged from 0.05 to 11.84 lg l~1 and 0.01 to 76.2 lg l~1, as chrysene equivalents, in 1994 and 1995, respectively (see Figs. 2 and 4). A summary of the mean, median, standard deviation of the DDPH values is shown in Table 1. The coastal surface waters of the Inner Gulf exhibited concentrations of DDPH of 0.20—8.26 lg l~1 in 1994 and 0.07—76.2 lg l~1 in 1995. The DDPH data were not
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Table 1 Summary of the DDPH concentrations in coastal waters of the Gulf of Thailand (lg l~1 chrysene equivalents) Year 1994
1995
Inner Gulf ESB Overall Inner Gulf —500 m —3000 m ESB Outer Gulf Overall
n
Range
mean
s.d.
Median
16 162 178
0.20—8.26 0.05—11.8 0.05—11.8
3.07 1.38 1.54
2.99 1.68 2.01
1.73 0.85 0.90
42 40 36 29 147
0.07—76.2 0.12—25.3 0.14—10.0 0.01—12.0 0.01—76.2
3.00 1.97 1.37 1.72 2.07
12.0 5.33 1.72 2.70 7.09
0.30 0.38 0.77 0.35 0.44
Note: ESB"the Eastern Sea Board, n"number of observations, 500 and 3000 m are locations of the sampling points from the shore
normally distributed (Fig. 4); the mean was measurably larger than the median value because of the skewness of the data (see Table 1). The majority of the DDPH data for 1995 samples were within the 0.2—1.0 lg l~1 range, with however a number of samples having concentration '10 lg l~1. The highest concentration (76.2 lg l~1) was found near the Sattahip where a naval base is located. Elsewhere most high DDPH values were found near the river mouths, and/or fish-landing piers and docks in the Inner Gulf under the influence of the Chao Phraya River. Coastal waters around Sichang Island (shown in Fig. 6) were permanently chronically polluted (range: 2.5—11.6 lg l~1, mean "6.6 lg l~1). In the Inner Gulf the DDPH values also showed acute pollution on occasions (DDPH'40 lg l~1; see Fig. 2) and chronic pollution (+4 lg l~1) about 20—25% of the time. This indicated sporadic, local, acute contamination events superimposed on chronic pollution. Seasonal fluctuations were small compared to the standard deviation (due to the skewed nature of these data which made the standard deviation for the DDPH data greater than the mean) and hence may not be significant. For the Eastern Sea Board, the results also showed large spatial and temporal fluctuations of DDPH concentrations with events of acute pollution and chronic pollution occurring about 8% of the time. These were also weak seasonal fluctuations (see Figs. 2 and 5). The DDPH values fluctuated between 0.05—11.84 lg l~1 in 1994 and 0.14—9.96 lg l~1 in 1995. The DDPH values along the east coast of the Outer Gulf showed a similar range as that of the Eastern Sea Board, being 0.01—12.04 lg l~1 with the mean of 1.92 lg l~1, but there were in that area no acute pollution events. Student’s t-test of the data (see Table 2) indicated no significant difference in the overall mean petroleum hydrocarbon concentrations within the coastal water sites in 1995, however a significant difference (P"0.04) was found for the mean hydrocarbon concentrations between the Inner Gulf sites (3.07 lg l~1 ) and the Eastern Sea Board sites (1.38 lg l~1 ) for the 1994 survey.
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Fig. 5. Seasonal distribution of the mean (*) and the standard deviation (error bars) of the DDPH concentration along the Eastern Sea Board of the Gulf of Thailand in 1994—1995. The time on the x-axis is shown as month/year. The standard deviation of the DDPH was at times larger than the mean because of the skewness of the data, no concentration values were negative.
Table 2 Student’s t-test of the DDPH concentration values in the Gulf of Thailand
1994 1995
Gulf of Thailand
mean 1
mean 2
df
P value
Significant difference
Inner Gulf vs. ESB Apr—Aug vs. Oct—Feb 500 m vs. 3000 m Inner Gulf vs. ESB Inner Gulf vs. Outer Gulf ESB vs. Outer Gulf
3.07 1.78 2.99 2.50
1.38 1.00 1.97 1.37
16 108 57 93
0.04 0.01 0.62 0.30
# # ! !
2.50 1.37
1.72 1.72
107 45
0.50 0.55
! !
1994 vs. 1995
1.54
2.07
166
0.38
!
Note: ESB"the Eastern Sea Board, Mean 1 is the mean value of the 1st group of samples, and Mean 2 is the mean value of the 2nd group of samples (e.g. in the 1st line Mean 1 refers to the Inner Gulf and Mean 2 to the Eastern Sea Board).
The histograms of the DDPH concentrations (Fig. 4a) showed highest occurrences in the concentration class of 0.6—1.5 lg l~1 in 1994 and 0.20—0.40 lg l~1 in 1995. The upper quartile was found to be 1.2 lg l~1 for both 1994 and 1995 surveys. 3.3. Risk assessment of impact from acute pollution events Several stations showed evidence of acute oil pollution, with concentrations from 20 to 76 lg l~1. These high values are comparable to those of heavily industrialised regions in the UK (Law, 1981), the severely oil affected area in the northwestern Arabian Gulf (El-Samra et al., 1986), and the Gulf of Oman (Emara, 1990).
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Fig. 6. Map showing regions at risk from two measured acute pollution events near the Sichang and Rayong oceanographic moorings of the Inner Gulf and Eastern Sea Board. f"the oceanographic mooring sites that provided the wind and current meter data used in the oil spill model; ="the sites where high DDPH concentrations were observed in the field, these sites were assumed to be the source of contamination in the model.
To understand how currents within the Gulf of Thailand mix and transport such pollution, and to tentatively identify the extent of the affected area, the oil spill model OILMAP of Spaulding et al. (1993, 1994) was used. The position of two measured high DDPH concentrations were assigned to be a known source in the model (see Fig. 6). The model utilised the local wind and the local current data from the mooring sites at Rayong and Sichang (shown in Fig. 6) to advect and mix these high concentrations (10—26 lg l~1) around the Gulf waters until dilution/degradation reduced the predicted DDPH values to background levels. This reduction was assumed to occur in no more than 10 days because of rapid chemical transformation of oil in tropical conditions. For each site, 100 trajectory calculations were performed by randomly setting the release time to differing parts of the wind/current record to examine where both time-varying wind and currents could advect this pollutant. From such trajectory calculations, a risk assessment to the coastal waters surrounding the acute pollution measurement sites was obtained (Fig. 6).
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For the Inner Gulf in December, (near Sichang) wind and currents can transport acute pollution up to 12 km alongshore from the source within 24 h and expose 30 km of coastal waters to a high risk ('70%) chance of contamination. Overall, some 58 km of coastal waters were predicted to be at risk to this high, measured, DDPH value at the sampling site. For the Eastern Sea Board in March/April, the current can transport acute pollution up to 8 km alongshore from the source within 24 h and expose 12 km of coastal waters to a high risk of contamination. About 50 km of coastline was at some risk of exposure. The oil spill trajectory model cannot be used to contrast with the measurements because the sources of the hydrocarbon are both chronic, common releases of oil as well as accidental, occasional major spills. Little is known about their relative importance and no data on oil discharges are available. The model however suggests that our findings of occasional large pollutant concentration values are probably not localised. Instead this pollutant is likely to be spread within a day over about 12 km, this distance is too small to be resolved by our sampling grid. Thus much of the coastline of the Inner Gulf and some sections of the Eastern Sea Board are occasionally significantly polluted. The exact origin of the hydrocarbon is unknown, since no data are available on oil discharges to the Gulf, but is probably within 50 km of the affected sites we found. 3.4. Chronic pollution The overall mean for all the areas studied was 1.54 lg l~1 in 1994 and 2.1 lg l~1 in 1995. Cripps (1992), proposed the use of the upper quartile as the threshold limit of natural variation if the waters are usually not polluted. If this criterion is used for the Inner Gulf, the background value of DDPH would be 1.2 lg l~1 chrysene equivalents. This value however already reflects permanent contamination of the Inner Gulf. Indeed it is high for inshore water elsewhere (Law, 1981; Levy, 1986; Marchand, 1980; Atwood et al., 1987; Weber and Bicego, 1990), and further it is 100 times larger than the lower quartile value at the sampling points in the unpolluted Outer Gulf. Thus low-level hydrocarbon pollution to some degree occurs all the time in the Inner Gulf and the Eastern Sea Board. In these areas 75% of the time low-level oil contamination occurred at the coastal sampling stations and about 25% of the stations showed evidence of chronic pollution. Additionally there were occasional acute pollution events. The Inner Gulf and the Eastern Sea Board coastal waters are thus measurably affected by oil pollution.
4. Conclusions 4.1. Water circulation The oceanographic data presented here showed that in addition to the tidal and local wind influences which previous models have incorporated, in the future models of circulation must also include (a) the non-uniform wind fields over the Gulf; (b) the
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influences of the Mekong River plume in the wet season; and (c) the circulation in the South China Sea. Contrary to previous models the net circulation is not forced mainly by the local wind, if this was the case there would be a wind-driven outflow/inflow along the coast and a weak return flow (speed(0.2 m s~1) in the centre of the Gulf In fact the largest currents were observed near the centre of the Gulf. Instead the forcing from the South China Sea largely drives the net circulation. The circulation is characterised by a cyclonic gyre during the northwest monsoon, an anticyclonic eddy during the southeast monsoon, and weak and variable currents the rest of the time. In turn this circulation in the outer Gulf drives a weaker circulation in the Inner Gulf. The water circulation in the Outer Gulf (Fig. 3) was thus largely controlled by that in the South China Sea. In turn the circulation in the Inner Gulf was strongly influenced by the Outer Gulf. Some of the river water that arrived in the previous wet season remains for 2—3 months in the Inner Gulf in the dry season, a finding suggesting that the net currents were small and that little mixing occurred between offshore and coastal waters in the Gulf. It takes at least two months to flush the Inner Gulf. This finding also explains the observations of high rates of mud accumulation in this area (Srisuksawad et al., 1997). 4.2. Oil pollution The DDPH data showed three peaks in the distribution, these were occasional acute pollution events (DDPH'40 lg l~1), superimposed upon chronic pollution (DDPH+4 lg l~1) due to slow flushing of the Gulf, and slightly contaminated water elsewhere in the Inner Gulf and the Eastern Sea Board (DDPH+1.2 lg l~1). Only in the Outer Gulf was the water most of time uncontaminated (DDPH+ 0.01—0.1 lg l~1). The bulk of the pollutants is believed to reach the Gulf from polluted rivers and industrial and shipping discharges in the Inner Gulf and the Eastern Sea Board; indeed these two areas were the most polluted. The inner Gulf coastal waters appeared to be more contaminated by petroleum hydrocarbons than the other parts of the Gulf. The hydrocarbon concentration values were particularly elevated, up to the level of acute contamination, in areas of industrial and shipping activity. In these areas, flushing of ballast, wash and bilge waters from ships is commonly practiced. Oil spills from tankers and thousands of small, dirty coastal vessels, in addition to the waste derived from the oil refinery and associated industries, are additional causes for these high concentrations. Land-based sources are probably important contributors to the steady, chronic contamination of coastal waters in the Inner Gulf and the Eastern Sea Board. 4.3. Influence of the water circulation The DDPH concentration values were highly variable. Some of this variability can is due to the unknown history of local oil discharges nevertheless the circulation in the Gulf apparently explains some key features of this variability.
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The seasonal changes of the currents in the Inner Gulf (Figs. 2 and 3) appears responsible for the elevated DDPH values. Given that the Upper Gulf and the Eastern Sea Board are known origins of acute events, the long term trapping of the pollutants apparently controls the build-up of DDPH to chronic pollution levels. For example, during the months of January and February, there are weak and variable net currents and the coastal waters are resident for a few months. During the remaining months of the year, there exists a clockwise circulation within the Gulf followed by an anticlockwise circulation. In the Upper Gulf and along the Eastern Seaboard, these net drift currents will flush the water from west to east, then to have it return later. Essentially the polluted water sloshes back and forth and this may explain the frequent occurrence (25% of the time) of chronic pollution (DDPH+4 lg l~1). This pool of contaminated water moves back and forth seasonally between the Eastern Sea Board and the Inner Gulf, and this may explain the significant relation between their mean DDPH values in 1995 (Table 2). The seasonal behaviour of the currents also explains the significant seasonal difference (P"0.01) which was observed for the hydrocarbon concentrations along the Eastern Sea Board in 1994. A maximum in DDPH values was obtained in April—August (mean"1.78 lg l~1), when net currents were flushing the Upper Gulf waters to the Eastern Seaboard. A minimum DDPH value occurred in October—February (mean"1.0 lg l~1), when net drift current were bringing in cleaner water from the South China Sea to the area. The pollution in the Inner Gulf was particularly significant in November and December when the currents were sluggish. Along the Eastern Sea Board, the highest DDPH concentrations occurred in April to August when the wind blew from offshore and when net currents were negligible. In contrast, from September to November, the DDPH values were smaller probably because strong net westward currents prevailed and flushed the area. Further, using the oil spill model driven by the local wind and the observed currents, it was possible to estimate the risk to surrounding biological habitats of impact of acute pollution events observed at the sampling sites. The model suggests that within 10 days up to 58 km of coastal waters in the Inner Gulf and 50 km along the Eastern Sea Board be at some risk of contamination from the acute pollution observed at the sampling sites. Since about 20% of our sampling points show acute contamination in the Inner Gulf, this implies that much of the Inner Gulf is acutely polluted at least once a year. 4.4. Implications for management One cannot assume that the currents simply flush away pollutants and maintain a viable marine ecosystem in the Gulf of Thailand. The oceanographic data reveal that the coastal waters of the Inner Gulf and the Eastern Sea Board take two to three months to be flushed. Because of slow flushing hydrocarbon pollution is a serious threat. Chronic pollution is already occurring at some sites. These findings suggest that a more stringent management plan should be implemented for the control of wastes in the Gulf of Thailand.
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Acknowledgements The ASEAN—Canada Cooperative Programme on Marine Sciences, Chulalongkorn University, the Australian Institute of Marine Science and the IBM International Foundation supported this study. The National Research Council of Thailand provided the Seawatch oceanographic data. The generous help provided by Patrick Collins, Duncan Galloway, Felicity McAllister and Kris Summerhayes is gratefully acknowledged. Thanks also go to Kathryn Burns and the reviewers for constructive criticism.
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