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Sources, distribution and composition of the suspended sediments, Kuwait Bay, Northern Arabian Gulf
ARTICLE IN PRESS Journal of Arid Environments
Journal of Arid Environments 60 (2005) 647–661 www.elsevier.com/locate/jnlabr/yjare
Sources, distribution and composition of the suspended sediments, Kuwait Bay, Northern Arabian Gulf A.N. Al-Ghadbana, A. El-Sammaka,b, a
Environmental Sciences Department, Kuwait Institute for Scientific Research, P.O. Box 24885, 1 3109 Safat, Kuwait b University of Alexandria, Oceanography Department, Alexandria, Egypt Received 14 August 2003; received in revised form 3 June 2004; accepted 28 July 2004 Available online 1 October 2004
Abstract Qualitative and quantitative assessment of suspended sediment levels and transport pathways in Kuwait’s northern territorial waters are described, with special reference to Kuwait Bay. Near-surface water samples were collected from 12 stations, covering the whole of the embayment. Suspended sediment concentrations (SSCs) were derived. Heavy metal concentrations, mineralogical composition and grain size constituents were derived for the suspended sediments retained on the filter papers. The distribution pattern of the suspended sediments indicates the possibility of tidal current activity, dust fallout contribution and to a lesser extent, the effect of the Shatt Al-Arab (especially in northeastern Kuwait Bay). The distribution of various types of sediments and the variations of their textural, compositional and concentrations are controlled largely by the hydrodynamic conditions, nature of sediments. The results demonstrate clearly the potential impact of the Gulf War as manifested by the increase of SSC in 1992, compared to 1989/1990. Such an input may be attributed also to drainage processes from the Iraqi marshes. More studies are required to
Corresponding author. Environmental Sciences Department, Kuwait Institute for Scientific Research, P.O. Box 24885, 13109 Safat, Kuwait. Tel.: +965-4836100X4424; fax: +965-4845350. E-mail address: [email protected] (A. El-Sammak).
0140-1963/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaridenv.2004.07.017
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understand the environmental impacts of the suspended matter and the associated pollutants. r 2004 Elsevier Ltd. All rights reserved. Keywords: Suspended sediments; Arabian gulf; Gulf war; Kuwait bay; Shatt Al Arab
1. Introduction Most of the northern part of the Gulf is characterized by a plume of suspended sediments (Fig. 1). This suspended sediment phenomenon usually results from natural physical processes, for example, flushing of material carried by rivers and receiving bodies of water, and/or contribution of dust storms. It may also be caused by artificial or man-made activities, such as the indiscriminate dredging and implementation of development activities, i.e. excavation and filling around coastal and offshore areas. Another potential source of suspended sediment is the southern area of Iraq, where an area of about 9000 km2 (southern marshes) was drained, resulting in the occurrence of dry lands that are vulnerable to the dominant NW–SE wind direction creating dust storms that settle over the northern part of the Arabian Gulf ( Al-Ghadban et al., 1999).
Fig. 1. Sources of sediments in the Northern Region of the Arabian Gulf.
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These suspended sediments present acute environmental problems, due to adverse effects on the economy and the quality of marine life. The inputs and losses of suspended materials are complex and varied. Sources of suspended materials may be external, internal and/or marginal. Human activities also may contribute suspended matter to the marine environments, through development, excavations, dredging, reclamations and other activities. Suspended sediments are known to accommodate a variety of pollutants thus they play an important role in the augmentation and distribution of such pollutants. The sediments can absorb toxic metals and hydrocarbon; they can also release them to the marine environments. The present paper aims at qualitative and quantitative assessments of suspended sediments and transport pathways in Kuwait’s northern territorial waters, with special reference to Kuwait Bay. Kuwait Bay is an elliptically shaped embayment that protrudes from the Arabian Gulf in the westward direction at its northwestern corner (Fig. 2). It covers an area of about 720 km2 (Al-Shemmari et al., 2002). The bathymetry of Kuwait Bay comprises three physiographic divisions: a submerged estuarine flat located in the northern sector; the Kuwait Bay trough, located in the central part; and the steep shelf slope in the southern sector (Fig. 2). In general, the average depth of Kuwait Bay is about 5 m. Maximum depth, encountered near Ras al Ardh, was less than 30 m (Khalaf et al., 1982; Dames and Moore, 1983). It hosts a major port, namely Shuwaikh Port, three major power-generating stations (Doha east, Doha west and Subiya). Its southern shorelines are highly developed as commercial and metropolitan attractions. In contrast, the northern shorelines remain pristine (AlGhadban and Salman, 1993). The Kuwait waters are nutrient rich (Al-Yamani, 2001). Kuwait Bay presents a unique ecosystem and a significant nursery ground for many fishes and shrimp species (Abou-Seedo et al., 1990). Intertidal mudflats are prominent features in the Sulaibikhat Bay and in the north Kuwait Bay. The Bay is characterized by relatively shallow water throughout. Tidal currents are the major source of water movement in Kuwait Bay (Anderlini et al., 1982). Vertical differences of up to 4 m and horizontal differences of up to 8 km were reported in the springs between high and low waters (Al-Shemmari et al., 2002). Surface waters consistently flow northwesterly into the bay, then curve to the west in the center and flow southwest into Kadima Bay during flood tides, reversing during ebb tides. Dames and Moore (1983) showed that higher current velocities (1.0 m/s) exist on the central part of the bay.
2. Materials and methods Near-surface water samples (about 50 cm below water surface) were collected from 12 stations covering the whole of the embayment (Fig. 3), and at varying depths (1, 3, 5 and 9 m) for Station 2 at the entrance and Station 9 at the western part of the bay. These two stations were selected in order to study the current at the entrance and at the deepest middle zone of the bay. Suspended sediment concentrations (SSCs) were calculated by filtering 1 l of seawater, through 0.45 mm Millipore membrane filter paper.
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Fig. 2. Bathymetry of Kuwait Bay (After Al-Ghadban and Salman, 1993).
The weight of materials retained on the filter paper was calculated and referred to the volume of water used for filtration. Grain size of the suspended matter was determined using a centrifugal particle size analyser. Heavy metal analysis was performed using atomic absorption spectrometry. The major sediments’ constituents were studied using scanning electron microscope (SEM). XRD was used for semi-quantitative determination of the different minerals in the suspended material.
3. Results and discussion According to Al-Ghadban and Salman (1993), Kuwait Bay can be divided generally into two zones, the eastern and western zones. The Eastern zone is characterized by fairly high current velocity (41.0 cm/s) and is affected generally by
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Fig. 3. Study Area, showing the sampling stations.
the Shatt Al Arab, this is marked by lower temperature and salinity, compared to the western zone. The eastern zone includes also a greater area of deep water than the western zone (Fig. 4). Muds in the north and sandy deposits in the south dominate the surficial sediments of both zones. The prevailing northwesterly (shamal) wind (about 7 months/year) is more likely to affect the western zone of the study area. It results eventually in the resuspension of bottom sediments and, as such, contributes to higher turbidity (suspended sediment) of overlaying waters in the western part of Kuwait Bay (Al-Ghadban, 1990). The southeasterly wind (second dominant) may have a greater effect on the eastern part of the bay. SSCs near the surface for all stations are shown in Fig. 5. The concentrations of suspended sediments vary in locations, with the seasons. A general trend of high concentrations at the entrance and in the northwestern sectors can be observed. The generally low suspended sediment load is found in the northeastern and southwestern areas. The most logical interpretation is that the high current speed in this area can cause a higher rate of agitation of the bottom sediments. The entrance of Kuwait Bay is a critical location, where it is affected by the movement of water during high and low tides. It is also possible to attribute the high-SSCs in this area to the influx from Shatt Al Arab and Khor Al-Sabbiya. Likewise, the effect of the drainage of the Iraqi marshes might affect the eastern sector of the bay at its entrance. Consequently, the high concentrations of suspended sediments in the western and northwestern sectors can be explained by the eddy current in this area, as indicated by Dames and Moors
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Fig. 4. Direction of surface ebb-current and near-bed current (Lee and Samuel, 1992) and inferred zones of current speeds (Dames and More, 1983).
(1983). In fact, changes in the locations of the maximum suspended load is explained by the changes in the tidal conditions, eddy current and the sources of suspended load entering the bay. Fig. 6 illustrates the descriptive statistics of all measurements for the whole bay during different seasons (Maximum, Minimum and average values). The lowest SCC mean value is observed during December 1989 (2.85 mg l 1). Some sampling periods exhibit a wide range of variation, indicating disparity in the suspended sediment sources. The average SSCs at the different stations, before the invasion (1989–1990) and after the liberation of Kuwait (1992) are illustrated in Fig. 7. Very high SSC levels are observed during 1992, i.e. after the liberation. The SCC levels before the invasion (1989–1990) were less than 20 mg l 1; however during 1992 the values were almost more than 30 mg l 1, reaching maximum during April 1992 (i.e. 52.2 mg l 1). According to Sadiq and McCain (1993), the Kuwait oil fires emitted large quantities of soot particles and other inorganic compounds. It is believed that, during 1992 dust fallout resulting from burning oil fields was the main source for suspended sediments in the water. This could be deduced from the uniformity in the high values for all of the stations (except station 1, see Fig. 5) during 1992. The other sources for suspended sediments are point sources which can affect one part of the bay more than the other parts. This pattern indicated the importance of dust-fallout as a source for suspended matter. Likewise, the negative environmental impacts are
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Fig. 5. Histogram showing the suspended sediment concentrations at each station (for location see Fig. 3).
either due to the fires or the efforts to control these fires and the oil spills. Another possible source is the increase of the input of suspended sediments, as a result of the drainage of Iraqi marshes (Al-Ghadban et al., 1999).
3.1. Suspended sediments grain size analysis Fig. 8 represents the grain size compositions of the suspended material for the different stations. The slit fractions are the dominate particles, representing (on average) about 66.9%727.6, followed by the clay-size fractions (31.2%727.99). Sand was found only at two stations, that is, Stations 1 and 8, with a maximum content of 18.5%. Generally, the size range between 1 and 10 mm constitutes about 2/ 3 of the suspended particles. The sand fractions at Stations 1 and 8 were in the very fine sand range. Station 9 is composed mostly of clay-size fractions. The dominance of the fine-grained fractions indicates that a considerable part of the suspended load in Kuwait Bay is due to the resuspension processes of the bottom sediments. Kuwait Bay is covered mostly with muddy sediments. Sandy sediments are restricted generally to the relatively narrow southern offshore flat and to the western and most eastern part of the central flat (Khalaf and Al-Hashash, 1983).
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Fig. 6. Average, maximum and minimum values of all measurements, over different periods for the whole bay.
Fig. 7. SCC at the different stations (see Fig. 3 for locations) before the invasion and after the liberation of Kuwait.
3.2. Heavy metal analysis Samples from three stations (2, 7 and 12) were selected for heavy metal analysis. These stations were selected in order to (a) provide reasonable coverage of the area from east to west and (b) to elucidate the relationship between the suspended sediment distribution and the water circulation in the study area. Heavy metals were
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Fig. 8. Sand–silt–clay ratio at different stations.
analysed for samples collected only during 1992, i.e. following the liberation of Kuwait. Fig. 9 illustrates the concentrations of the different heavy metals associated with the suspended sediments. Pb and Cr show higher concentrations at Station 2, decreasing westward. On the other hand, Cd, Ni, Zn, and Fe concentrations are highest at Station 7. These results infer that the middle part of the bay is characterized by low currents, some eddies. This area is receiving also land runoff. Moreover, this might be the result of sunken vessels. These sunken vessels are considered as a source for heavy metals, particularly Fe. According to Al-Ghadban and Salman (1993) several sunken vessels in this area produce considerable amounts of Fe together with other heavy metals. 3.3. Mineralogical composition of suspended sediments The semi-quantitative mineralogical analysis undertaken indicates that terrigenous minerals are the dominating minerals in the eastern sector. Quartz represents more than 55% of the total identified minerals, followed by feldspars (430%). Clay minerals, mainly koalinite, represent about 5% of the minerals. In the northwestern sector, both terrigenous and carbonate fractions are equal in their contents. Calcite and dolomite represent about 50% of the total minerals. The calcite content reached 42% of the sample (Station 8), which suggests the possible effect of agitation of the
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Fig. 9. Concentrations of different heavy metals at the selected stations (for locations see Fig. 3).
sea bed in the northwestern part of the bay; the supply of dust storms into Kuwait Bay; or the direct precipitation of inorganic carbonate. It is more likely that the latter is more dominant, since dolomite was recorded at Station 8 and not detected at Station 2. The abundance of dolomite was recognized also in the dust-fallout sediments and the recent surficial deposits of Kuwait (Al-Bakri et al., 1984). The higher quartz and feldspar, together with the lower carbonate contents towards the eastern part of the bay, might indicate that the Shatt Al-Arab is a major supplier of suspended load into this part of the bay. It has been stated that the Shatt Al-Arab may be a major supplier of suspended sediments in the northern Arabian Gulf (AlBakri et al., 1985; Al-Ghadban et al., 1999; Khan, 2002). Dust-fallout sediments of Kuwait are mainly in the range of silt size and are composed mainly of calcareous grains (about 56%) and quartz (about 20%) (Khalaf and Al-Hashash, 1983); they are characterized by the abundance of dolomite. Vinagradov (1973) studied the dust storm structure over the Mesopotamian lowland and the Arabian Gulf. These investigations found that the dust storm axis is mainly in a NW direction, passing over the southeastern part of the Shatt Al-Arab flood plain. A considerable proportion of the offshore sediments of Kuwait Bay have been deposited from the airborne suspended dust, brought to Kuwait from southern Iraq by northwesterly winds (Khalaf et al., 1985). SEM analysis indicates that suspended sediments collected from the study area are composed of several types of grains, mainly biogenic silica, authegenic calcite, quartz and feldspars. The organic substances were also observed. These types of grains are related mainly to the different sources of suspended sediments; these can highlight also the variability in
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the sources of the suspended sediments. According to Khalaf et al. (1985), the area of Kuwait is affected by the sand and dust storms, over about 19% of the time. Dust storms and rising sand (wind-generating sand) in Kuwait are more frequent in summer, especially in June and July. The maximum dust storm was found to be in May while the minimum was in February (Khalaf et al., 1985). 3.4. Suspended sediments distribution The suspended sediment distribution patterns constructed for several sets of tidal conditions indicate the following: (1) the existence of high concentrations of suspended sediments at the entrance to the bay (Station 2) and the northwestern part (Station 8 and/or Station 9); (2) the persistence of a high zone of SSC at the western part during the flooding and ebbing stages of the tide, inferring the presence of counter-clockwise eddy current; (3) the area characterized by low SSC in the winter season, despite the presence of higher tidal currents; (4) higher concentrations of SSC were recorded, under spring tidal conditions; (5) summer conditions show the existence of a high turbidity zone in the northeastern, as well as the central part, of the bay; (6) at the mid- to last stages of ebbing (low water), higher concentrations of suspended sediment were recorded inferring local resuspension, especially within shallow water depths; and (7) the effect of Khor Al-Subiya and/or Shatt Al-Arab inflow can be observed clearly during the ebbing stage of the tide (Al-Ghadban and Salman, 1993). Vertical profiles were established at Stations 2 (at the entrance) and 9 (western part), where current speed, direction, temperature and conductivity measurements were taken at several water depths, together with water sampling for SSC (AlGhadban and Salman, 1993). Higher SSCs were recorded with increasing water depths, which demonstrate the effect of the bottom currents in agitating the bottom sediments, particularly on spring tide (43.5 m tidal range). This results in entrapping much of the sediment, resuspended and newly introduced, within this segment of Kuwait Bay. The directions and speeds of the near-surface and near-bed tidal currents imply a rectilinear type of flow, which infers that the sediments should have a unidirectional movement (into or away from the study area). The temperature/ depth relationship demonstrates clearly the presence of stratification. Three degrees of temperature differences between surface and near-bed water were observed. A similar stratification was deduced also in an early study by Mathews et al. (1980). Vertical profiles of current velocities and directions were measured also at Station 2, and two tidal cycles, to record the effectiveness of the tidal currents in the resuspension of the bottom sediments. The data demonstrate the effect of the tidal currents, in agitating the bottom sediments, especially during periods of higher current velocities, about 2 h before flood and ebb periods. A strong relationship to current velocity and phase was observed. Maximum concentrations occurred near the maximum tidal ebb and flood velocities, and minimum concentrations occurred shortly after slack water. The mass transport of suspended solids was compared with tidal phase to obtain the residual movement of the suspended material. The inferred transport at Station 2
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was onshore. Mass sediment transport for the complete tidal cycle, demonstrated onshore sediment transport. This onshore transport was identified for surface and bottom flows. The results infer that Kuwait Bay is a depositional basin and that the suspended sediments are transported generally via tidal currents, eventually settling out into the bay. In order to achieve a meaningful understanding regarding the spatial and cyclic distribution of suspended sediments, Cluster (Factor) Analysis was performed on a data matrix comprising 14 variables (sampling periods) and 12 cases (sampling locations) (Fig. 10). The resulting dendrogram indicates that two groups of similar periods can be recognized: Group 1, which includes the sampling periods during 1992 (i.e. after the liberation of Kuwait, and Group 2, which comprises the period from 1989 to 1990. The latter group can be divided into three subgroups. Subgroup A include October 1989 and June 1990. Subgroup B includes sampling periods November and December 1989. Subgroup C includes March, May and June 1989 (the spring season). May and February 1990 cluster with a subgroup, but at a lower level of similarity. This dendrogram specifies the effect of seasons on the SSC. The data reveal also the increase in SSC, due to the environmental crisis caused by burning the oil-well fields during the Iraqi invasion of Kuwait. For the spatial distribution of suspended sediments, the Q-mode cluster factor analysis was performed on the same data matrix, showing the similarities between Stations. The resulting tree diagram illustrates that there are three groups of Stations. The first group clusters Stations 10, 11, 9 and 7 (the western sector). The second group includes Stations 6, 8, 5, 12 and 4 (the north and the central provinces). The third group, on the other hand, includes Stations 1, 2 and 3, which are located at the entrance of the bay (the eastern sector). This discrimination is in accordance with the bathymetry of Kuwait Bay. This pattern reflects also the effect of the inconsistency in sediment sources to the bay, i.e. the change in the magnitudes of different sources of suspended sediments entering the study basin. In fact, the cluster analysis produces a clear discrimination to the distribution of suspended sediments, in terms of time and location.
Fig. 10. Tree diagram clustering the similar sampling stations (L) and periods (R).
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4. Conclusions Five major sources for suspended sediments in the waters of Kuwait Bay can be identified: (1) dust fallout; (2) biogenic components; (3) Shatt Al-Arab and Khor AlZubair alluvial sediments; (4) agitation of bottom sediments; and (5) particles contributed from construction and engineering works. Based upon the suspended sediment concentration patterns, the prevailing hydrodynamic conditions and mechanisms controlling the sediment transportation, a conceptual model has been constructed in Fig. 11. Fig. 11 shows also the variation in sediment sources as well as the areas of deposition and erosion. On the basis of the nature of the sediment and the tidal current patterns, it can be suggested that suspended sediment transport in Kuwait Bay, especially within the bottom water layers is slightly flood dominant; therefore, the net pattern of movement is onshore. Some areas in the bay are more active than others, in terms of suspended sediment transport. Some areas include the tidal channel, where tidal currents are relatively more active, e.g. Sulaibikhat Bay and the Ras Al-Ardh channel. The distribution pattern of the suspended sediments indicates the possibility of tidally induced transport, dust fallout and, to a lesser extent, the effect of the Shatt Al-Arab (especially in the northeastern part of Kuwait Bay). The distribution of various types of sediments and the variations of their textural, compositional and
Fig. 11. Conceptual model for the suspended sediment transport pattern in Kuwait Bay.
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concentrations are controlled largely by the hydrodynamic conditions combined with the nature of the sediments. The results demonstrate clearly the potential impact of the Gulf War as manifested by the increase of SSC in 1992 compared to 1989/1990. Such an input may be attributed also to the drainage process from the Iraqi marshes. Finally it is recommended that more studies be undertaken to understand the environmental impacts of the suspended matter and the associated pollutants.
Acknowledgments This study was a part of an intensive study that was funded by the Kuwait Institute for Scientific Research (KISR) and Kuwait Foundation for Advancement of Sciences (KFAS). The authors acknowledge their support. Thanks are also due to the anonymous reviewers for their valuable comments and critical revision of the manuscript.
References Abou-Seedo, F., Clayton, D.A., Wright, J.M., 1990. Tidal and turbidity effects on the shallow-water fish assemblage of Kuwait Bay. Marine Ecological Progress Series 65, 213–223. Al-Bakri, D., Khalaf, F., Al-Ghahdban, A.N., 1984. Minerology, genesis and sources of surficial sediments in Kuwait marine environment, northern Arabian Gulf. Journal of Sedimentary Petrology 54 (4), 1266–1279. Al-Ghadban, A.N., Salman A.S., 1993. Preliminary assessment of the suspended sediment and its associated pollutants in Kuwait Bay. Kuwait Institute for Scientific Research, Report No. EES-115, Kuwait. Al-Ghadban, A.N., Saeed, T., Al-Dousari, A.M., Al-Shemmari, H., Al-Mutairi, M., 1999. Preliminary assessment of the impact of drainage of Iraqi marches on Kuwait’s northern marine environment. Part I. Physical manipulation. Water Science Technology 40 (4), 75–87. Al-Shemmari, H., Al Senafy, M., Al-Fayad, K., 2002. Effects of seasonal variations on the water quality in Kuwait bay. Proceedings of the International Conference On Coastal Zone Management and Development, EPA, Kuwait, V1-V4. Al-Yamani, F., 2001. A Strategic Plan for Sustainable Utilization of Kuwait’s Marine Environment. Kuwait Foundation for Advancement of Sciences, 140pp. Anderlini, V.C., Jacob, J.C., Lee, J.W., 1982. Atlas of physical and chemical oceanographic characteristics of Kuwait Bay. Kuwait Institute for Scientific Research, Report No. KISR704. Dames, More, 1983. Studies for Subiya area Kuwait Bay and development of electrical networks. Part 1. Hydraulic studies and aquatic biology. Ministry of Electricity and Water, Kuwait. Khalaf, F.I., Al-Hashash, M.K., 1983. Aeolian sedimentation in the northwestern part of the Arabian Gulf. Journal of Arid Environment 6, 319–332. Khalaf, F.I., Al-Ghadban, A.N., Al-Saleh, S., Al-Omran, L., 1982. Sedimentology and mineralogy of Kuwait Bay bottom sediments, Kuwait Arabian Gulf. Marine Geology 46, 71–99. Khalaf, F.I., Al-Kadi, A., Al-Saleh, S., 1985. Mineralogical composition and potential sources of dust fallout deposits in Kuwait, northern Arabian Gulf. Sedimentary Geology 42, 255–278. Khan, N.Y., 2002. Human and physical geography (Chapter 1). In: Khan, N.T., Munawar, M., Price, A.R.G. (Eds.), The Gulf Ecosystem: Health and Sustainability. Backhuys Publishers, Leiden, The Netherlands.
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Mathews, C.P., Al-Attar, M., Samuel, M., 1980. The physical and chemical oceanography of Kuwait Bay and its influence on the fisheries and development of the area. Annual Research Report, Kuwait Institute for Scientific Research, pp. 99–103. Sadiq, M., McCain, J.C., 1993. The Gulf War Aftermath. An Environmental Tragedy. Kluwer Academic Publisher, Dordrecht, Boston, London. Vinagradov, V., 1973. Structure of dust storms from ITOS-I TV images obtained over Iraq and the Gulf of Persia. Moscow Institute of Aerospace Method, Moscow.
Journal of Arid Environments 60 (2005) 647–661 www.elsevier.com/locate/jnlabr/yjare
Sources, distribution and composition of the suspended sediments, Kuwait Bay, Northern Arabian Gulf A.N. Al-Ghadbana, A. El-Sammaka,b, a
Environmental Sciences Department, Kuwait Institute for Scientific Research, P.O. Box 24885, 1 3109 Safat, Kuwait b University of Alexandria, Oceanography Department, Alexandria, Egypt Received 14 August 2003; received in revised form 3 June 2004; accepted 28 July 2004 Available online 1 October 2004
Abstract Qualitative and quantitative assessment of suspended sediment levels and transport pathways in Kuwait’s northern territorial waters are described, with special reference to Kuwait Bay. Near-surface water samples were collected from 12 stations, covering the whole of the embayment. Suspended sediment concentrations (SSCs) were derived. Heavy metal concentrations, mineralogical composition and grain size constituents were derived for the suspended sediments retained on the filter papers. The distribution pattern of the suspended sediments indicates the possibility of tidal current activity, dust fallout contribution and to a lesser extent, the effect of the Shatt Al-Arab (especially in northeastern Kuwait Bay). The distribution of various types of sediments and the variations of their textural, compositional and concentrations are controlled largely by the hydrodynamic conditions, nature of sediments. The results demonstrate clearly the potential impact of the Gulf War as manifested by the increase of SSC in 1992, compared to 1989/1990. Such an input may be attributed also to drainage processes from the Iraqi marshes. More studies are required to
Corresponding author. Environmental Sciences Department, Kuwait Institute for Scientific Research, P.O. Box 24885, 13109 Safat, Kuwait. Tel.: +965-4836100X4424; fax: +965-4845350. E-mail address: [email protected] (A. El-Sammak).
0140-1963/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaridenv.2004.07.017
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understand the environmental impacts of the suspended matter and the associated pollutants. r 2004 Elsevier Ltd. All rights reserved. Keywords: Suspended sediments; Arabian gulf; Gulf war; Kuwait bay; Shatt Al Arab
1. Introduction Most of the northern part of the Gulf is characterized by a plume of suspended sediments (Fig. 1). This suspended sediment phenomenon usually results from natural physical processes, for example, flushing of material carried by rivers and receiving bodies of water, and/or contribution of dust storms. It may also be caused by artificial or man-made activities, such as the indiscriminate dredging and implementation of development activities, i.e. excavation and filling around coastal and offshore areas. Another potential source of suspended sediment is the southern area of Iraq, where an area of about 9000 km2 (southern marshes) was drained, resulting in the occurrence of dry lands that are vulnerable to the dominant NW–SE wind direction creating dust storms that settle over the northern part of the Arabian Gulf ( Al-Ghadban et al., 1999).
Fig. 1. Sources of sediments in the Northern Region of the Arabian Gulf.
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These suspended sediments present acute environmental problems, due to adverse effects on the economy and the quality of marine life. The inputs and losses of suspended materials are complex and varied. Sources of suspended materials may be external, internal and/or marginal. Human activities also may contribute suspended matter to the marine environments, through development, excavations, dredging, reclamations and other activities. Suspended sediments are known to accommodate a variety of pollutants thus they play an important role in the augmentation and distribution of such pollutants. The sediments can absorb toxic metals and hydrocarbon; they can also release them to the marine environments. The present paper aims at qualitative and quantitative assessments of suspended sediments and transport pathways in Kuwait’s northern territorial waters, with special reference to Kuwait Bay. Kuwait Bay is an elliptically shaped embayment that protrudes from the Arabian Gulf in the westward direction at its northwestern corner (Fig. 2). It covers an area of about 720 km2 (Al-Shemmari et al., 2002). The bathymetry of Kuwait Bay comprises three physiographic divisions: a submerged estuarine flat located in the northern sector; the Kuwait Bay trough, located in the central part; and the steep shelf slope in the southern sector (Fig. 2). In general, the average depth of Kuwait Bay is about 5 m. Maximum depth, encountered near Ras al Ardh, was less than 30 m (Khalaf et al., 1982; Dames and Moore, 1983). It hosts a major port, namely Shuwaikh Port, three major power-generating stations (Doha east, Doha west and Subiya). Its southern shorelines are highly developed as commercial and metropolitan attractions. In contrast, the northern shorelines remain pristine (AlGhadban and Salman, 1993). The Kuwait waters are nutrient rich (Al-Yamani, 2001). Kuwait Bay presents a unique ecosystem and a significant nursery ground for many fishes and shrimp species (Abou-Seedo et al., 1990). Intertidal mudflats are prominent features in the Sulaibikhat Bay and in the north Kuwait Bay. The Bay is characterized by relatively shallow water throughout. Tidal currents are the major source of water movement in Kuwait Bay (Anderlini et al., 1982). Vertical differences of up to 4 m and horizontal differences of up to 8 km were reported in the springs between high and low waters (Al-Shemmari et al., 2002). Surface waters consistently flow northwesterly into the bay, then curve to the west in the center and flow southwest into Kadima Bay during flood tides, reversing during ebb tides. Dames and Moore (1983) showed that higher current velocities (1.0 m/s) exist on the central part of the bay.
2. Materials and methods Near-surface water samples (about 50 cm below water surface) were collected from 12 stations covering the whole of the embayment (Fig. 3), and at varying depths (1, 3, 5 and 9 m) for Station 2 at the entrance and Station 9 at the western part of the bay. These two stations were selected in order to study the current at the entrance and at the deepest middle zone of the bay. Suspended sediment concentrations (SSCs) were calculated by filtering 1 l of seawater, through 0.45 mm Millipore membrane filter paper.
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Fig. 2. Bathymetry of Kuwait Bay (After Al-Ghadban and Salman, 1993).
The weight of materials retained on the filter paper was calculated and referred to the volume of water used for filtration. Grain size of the suspended matter was determined using a centrifugal particle size analyser. Heavy metal analysis was performed using atomic absorption spectrometry. The major sediments’ constituents were studied using scanning electron microscope (SEM). XRD was used for semi-quantitative determination of the different minerals in the suspended material.
3. Results and discussion According to Al-Ghadban and Salman (1993), Kuwait Bay can be divided generally into two zones, the eastern and western zones. The Eastern zone is characterized by fairly high current velocity (41.0 cm/s) and is affected generally by
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Fig. 3. Study Area, showing the sampling stations.
the Shatt Al Arab, this is marked by lower temperature and salinity, compared to the western zone. The eastern zone includes also a greater area of deep water than the western zone (Fig. 4). Muds in the north and sandy deposits in the south dominate the surficial sediments of both zones. The prevailing northwesterly (shamal) wind (about 7 months/year) is more likely to affect the western zone of the study area. It results eventually in the resuspension of bottom sediments and, as such, contributes to higher turbidity (suspended sediment) of overlaying waters in the western part of Kuwait Bay (Al-Ghadban, 1990). The southeasterly wind (second dominant) may have a greater effect on the eastern part of the bay. SSCs near the surface for all stations are shown in Fig. 5. The concentrations of suspended sediments vary in locations, with the seasons. A general trend of high concentrations at the entrance and in the northwestern sectors can be observed. The generally low suspended sediment load is found in the northeastern and southwestern areas. The most logical interpretation is that the high current speed in this area can cause a higher rate of agitation of the bottom sediments. The entrance of Kuwait Bay is a critical location, where it is affected by the movement of water during high and low tides. It is also possible to attribute the high-SSCs in this area to the influx from Shatt Al Arab and Khor Al-Sabbiya. Likewise, the effect of the drainage of the Iraqi marshes might affect the eastern sector of the bay at its entrance. Consequently, the high concentrations of suspended sediments in the western and northwestern sectors can be explained by the eddy current in this area, as indicated by Dames and Moors
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Fig. 4. Direction of surface ebb-current and near-bed current (Lee and Samuel, 1992) and inferred zones of current speeds (Dames and More, 1983).
(1983). In fact, changes in the locations of the maximum suspended load is explained by the changes in the tidal conditions, eddy current and the sources of suspended load entering the bay. Fig. 6 illustrates the descriptive statistics of all measurements for the whole bay during different seasons (Maximum, Minimum and average values). The lowest SCC mean value is observed during December 1989 (2.85 mg l 1). Some sampling periods exhibit a wide range of variation, indicating disparity in the suspended sediment sources. The average SSCs at the different stations, before the invasion (1989–1990) and after the liberation of Kuwait (1992) are illustrated in Fig. 7. Very high SSC levels are observed during 1992, i.e. after the liberation. The SCC levels before the invasion (1989–1990) were less than 20 mg l 1; however during 1992 the values were almost more than 30 mg l 1, reaching maximum during April 1992 (i.e. 52.2 mg l 1). According to Sadiq and McCain (1993), the Kuwait oil fires emitted large quantities of soot particles and other inorganic compounds. It is believed that, during 1992 dust fallout resulting from burning oil fields was the main source for suspended sediments in the water. This could be deduced from the uniformity in the high values for all of the stations (except station 1, see Fig. 5) during 1992. The other sources for suspended sediments are point sources which can affect one part of the bay more than the other parts. This pattern indicated the importance of dust-fallout as a source for suspended matter. Likewise, the negative environmental impacts are
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Fig. 5. Histogram showing the suspended sediment concentrations at each station (for location see Fig. 3).
either due to the fires or the efforts to control these fires and the oil spills. Another possible source is the increase of the input of suspended sediments, as a result of the drainage of Iraqi marshes (Al-Ghadban et al., 1999).
3.1. Suspended sediments grain size analysis Fig. 8 represents the grain size compositions of the suspended material for the different stations. The slit fractions are the dominate particles, representing (on average) about 66.9%727.6, followed by the clay-size fractions (31.2%727.99). Sand was found only at two stations, that is, Stations 1 and 8, with a maximum content of 18.5%. Generally, the size range between 1 and 10 mm constitutes about 2/ 3 of the suspended particles. The sand fractions at Stations 1 and 8 were in the very fine sand range. Station 9 is composed mostly of clay-size fractions. The dominance of the fine-grained fractions indicates that a considerable part of the suspended load in Kuwait Bay is due to the resuspension processes of the bottom sediments. Kuwait Bay is covered mostly with muddy sediments. Sandy sediments are restricted generally to the relatively narrow southern offshore flat and to the western and most eastern part of the central flat (Khalaf and Al-Hashash, 1983).
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Fig. 6. Average, maximum and minimum values of all measurements, over different periods for the whole bay.
Fig. 7. SCC at the different stations (see Fig. 3 for locations) before the invasion and after the liberation of Kuwait.
3.2. Heavy metal analysis Samples from three stations (2, 7 and 12) were selected for heavy metal analysis. These stations were selected in order to (a) provide reasonable coverage of the area from east to west and (b) to elucidate the relationship between the suspended sediment distribution and the water circulation in the study area. Heavy metals were
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Fig. 8. Sand–silt–clay ratio at different stations.
analysed for samples collected only during 1992, i.e. following the liberation of Kuwait. Fig. 9 illustrates the concentrations of the different heavy metals associated with the suspended sediments. Pb and Cr show higher concentrations at Station 2, decreasing westward. On the other hand, Cd, Ni, Zn, and Fe concentrations are highest at Station 7. These results infer that the middle part of the bay is characterized by low currents, some eddies. This area is receiving also land runoff. Moreover, this might be the result of sunken vessels. These sunken vessels are considered as a source for heavy metals, particularly Fe. According to Al-Ghadban and Salman (1993) several sunken vessels in this area produce considerable amounts of Fe together with other heavy metals. 3.3. Mineralogical composition of suspended sediments The semi-quantitative mineralogical analysis undertaken indicates that terrigenous minerals are the dominating minerals in the eastern sector. Quartz represents more than 55% of the total identified minerals, followed by feldspars (430%). Clay minerals, mainly koalinite, represent about 5% of the minerals. In the northwestern sector, both terrigenous and carbonate fractions are equal in their contents. Calcite and dolomite represent about 50% of the total minerals. The calcite content reached 42% of the sample (Station 8), which suggests the possible effect of agitation of the
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Fig. 9. Concentrations of different heavy metals at the selected stations (for locations see Fig. 3).
sea bed in the northwestern part of the bay; the supply of dust storms into Kuwait Bay; or the direct precipitation of inorganic carbonate. It is more likely that the latter is more dominant, since dolomite was recorded at Station 8 and not detected at Station 2. The abundance of dolomite was recognized also in the dust-fallout sediments and the recent surficial deposits of Kuwait (Al-Bakri et al., 1984). The higher quartz and feldspar, together with the lower carbonate contents towards the eastern part of the bay, might indicate that the Shatt Al-Arab is a major supplier of suspended load into this part of the bay. It has been stated that the Shatt Al-Arab may be a major supplier of suspended sediments in the northern Arabian Gulf (AlBakri et al., 1985; Al-Ghadban et al., 1999; Khan, 2002). Dust-fallout sediments of Kuwait are mainly in the range of silt size and are composed mainly of calcareous grains (about 56%) and quartz (about 20%) (Khalaf and Al-Hashash, 1983); they are characterized by the abundance of dolomite. Vinagradov (1973) studied the dust storm structure over the Mesopotamian lowland and the Arabian Gulf. These investigations found that the dust storm axis is mainly in a NW direction, passing over the southeastern part of the Shatt Al-Arab flood plain. A considerable proportion of the offshore sediments of Kuwait Bay have been deposited from the airborne suspended dust, brought to Kuwait from southern Iraq by northwesterly winds (Khalaf et al., 1985). SEM analysis indicates that suspended sediments collected from the study area are composed of several types of grains, mainly biogenic silica, authegenic calcite, quartz and feldspars. The organic substances were also observed. These types of grains are related mainly to the different sources of suspended sediments; these can highlight also the variability in
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the sources of the suspended sediments. According to Khalaf et al. (1985), the area of Kuwait is affected by the sand and dust storms, over about 19% of the time. Dust storms and rising sand (wind-generating sand) in Kuwait are more frequent in summer, especially in June and July. The maximum dust storm was found to be in May while the minimum was in February (Khalaf et al., 1985). 3.4. Suspended sediments distribution The suspended sediment distribution patterns constructed for several sets of tidal conditions indicate the following: (1) the existence of high concentrations of suspended sediments at the entrance to the bay (Station 2) and the northwestern part (Station 8 and/or Station 9); (2) the persistence of a high zone of SSC at the western part during the flooding and ebbing stages of the tide, inferring the presence of counter-clockwise eddy current; (3) the area characterized by low SSC in the winter season, despite the presence of higher tidal currents; (4) higher concentrations of SSC were recorded, under spring tidal conditions; (5) summer conditions show the existence of a high turbidity zone in the northeastern, as well as the central part, of the bay; (6) at the mid- to last stages of ebbing (low water), higher concentrations of suspended sediment were recorded inferring local resuspension, especially within shallow water depths; and (7) the effect of Khor Al-Subiya and/or Shatt Al-Arab inflow can be observed clearly during the ebbing stage of the tide (Al-Ghadban and Salman, 1993). Vertical profiles were established at Stations 2 (at the entrance) and 9 (western part), where current speed, direction, temperature and conductivity measurements were taken at several water depths, together with water sampling for SSC (AlGhadban and Salman, 1993). Higher SSCs were recorded with increasing water depths, which demonstrate the effect of the bottom currents in agitating the bottom sediments, particularly on spring tide (43.5 m tidal range). This results in entrapping much of the sediment, resuspended and newly introduced, within this segment of Kuwait Bay. The directions and speeds of the near-surface and near-bed tidal currents imply a rectilinear type of flow, which infers that the sediments should have a unidirectional movement (into or away from the study area). The temperature/ depth relationship demonstrates clearly the presence of stratification. Three degrees of temperature differences between surface and near-bed water were observed. A similar stratification was deduced also in an early study by Mathews et al. (1980). Vertical profiles of current velocities and directions were measured also at Station 2, and two tidal cycles, to record the effectiveness of the tidal currents in the resuspension of the bottom sediments. The data demonstrate the effect of the tidal currents, in agitating the bottom sediments, especially during periods of higher current velocities, about 2 h before flood and ebb periods. A strong relationship to current velocity and phase was observed. Maximum concentrations occurred near the maximum tidal ebb and flood velocities, and minimum concentrations occurred shortly after slack water. The mass transport of suspended solids was compared with tidal phase to obtain the residual movement of the suspended material. The inferred transport at Station 2
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was onshore. Mass sediment transport for the complete tidal cycle, demonstrated onshore sediment transport. This onshore transport was identified for surface and bottom flows. The results infer that Kuwait Bay is a depositional basin and that the suspended sediments are transported generally via tidal currents, eventually settling out into the bay. In order to achieve a meaningful understanding regarding the spatial and cyclic distribution of suspended sediments, Cluster (Factor) Analysis was performed on a data matrix comprising 14 variables (sampling periods) and 12 cases (sampling locations) (Fig. 10). The resulting dendrogram indicates that two groups of similar periods can be recognized: Group 1, which includes the sampling periods during 1992 (i.e. after the liberation of Kuwait, and Group 2, which comprises the period from 1989 to 1990. The latter group can be divided into three subgroups. Subgroup A include October 1989 and June 1990. Subgroup B includes sampling periods November and December 1989. Subgroup C includes March, May and June 1989 (the spring season). May and February 1990 cluster with a subgroup, but at a lower level of similarity. This dendrogram specifies the effect of seasons on the SSC. The data reveal also the increase in SSC, due to the environmental crisis caused by burning the oil-well fields during the Iraqi invasion of Kuwait. For the spatial distribution of suspended sediments, the Q-mode cluster factor analysis was performed on the same data matrix, showing the similarities between Stations. The resulting tree diagram illustrates that there are three groups of Stations. The first group clusters Stations 10, 11, 9 and 7 (the western sector). The second group includes Stations 6, 8, 5, 12 and 4 (the north and the central provinces). The third group, on the other hand, includes Stations 1, 2 and 3, which are located at the entrance of the bay (the eastern sector). This discrimination is in accordance with the bathymetry of Kuwait Bay. This pattern reflects also the effect of the inconsistency in sediment sources to the bay, i.e. the change in the magnitudes of different sources of suspended sediments entering the study basin. In fact, the cluster analysis produces a clear discrimination to the distribution of suspended sediments, in terms of time and location.
Fig. 10. Tree diagram clustering the similar sampling stations (L) and periods (R).
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4. Conclusions Five major sources for suspended sediments in the waters of Kuwait Bay can be identified: (1) dust fallout; (2) biogenic components; (3) Shatt Al-Arab and Khor AlZubair alluvial sediments; (4) agitation of bottom sediments; and (5) particles contributed from construction and engineering works. Based upon the suspended sediment concentration patterns, the prevailing hydrodynamic conditions and mechanisms controlling the sediment transportation, a conceptual model has been constructed in Fig. 11. Fig. 11 shows also the variation in sediment sources as well as the areas of deposition and erosion. On the basis of the nature of the sediment and the tidal current patterns, it can be suggested that suspended sediment transport in Kuwait Bay, especially within the bottom water layers is slightly flood dominant; therefore, the net pattern of movement is onshore. Some areas in the bay are more active than others, in terms of suspended sediment transport. Some areas include the tidal channel, where tidal currents are relatively more active, e.g. Sulaibikhat Bay and the Ras Al-Ardh channel. The distribution pattern of the suspended sediments indicates the possibility of tidally induced transport, dust fallout and, to a lesser extent, the effect of the Shatt Al-Arab (especially in the northeastern part of Kuwait Bay). The distribution of various types of sediments and the variations of their textural, compositional and
Fig. 11. Conceptual model for the suspended sediment transport pattern in Kuwait Bay.
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concentrations are controlled largely by the hydrodynamic conditions combined with the nature of the sediments. The results demonstrate clearly the potential impact of the Gulf War as manifested by the increase of SSC in 1992 compared to 1989/1990. Such an input may be attributed also to the drainage process from the Iraqi marshes. Finally it is recommended that more studies be undertaken to understand the environmental impacts of the suspended matter and the associated pollutants.
Acknowledgments This study was a part of an intensive study that was funded by the Kuwait Institute for Scientific Research (KISR) and Kuwait Foundation for Advancement of Sciences (KFAS). The authors acknowledge their support. Thanks are also due to the anonymous reviewers for their valuable comments and critical revision of the manuscript.
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