Bottom sediments of the Arabian Gulf—III. Trace metal contents as indicators of pollution and implications for the effect and fate of the Kuwait oil slick

Environmental

PII:

SO269-7491(96)00046-2

Pollution, Vol. 93, No. 3, pp. 285-301, 1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0269-7491/96 $15.00+0.00

ELSEVIER

BOTTOM SEDIMENTS OF THE ARABIAN GULF-III. TRACE METAL CONTENTS AS INDICATORS OF POLLUTION AND IMPLICATIONS FOR THE EFFECT AND FATE OF THE KUWAIT OIL SLICK

F. Al-Abdali,“* M. S. Massoudb & A. N. Al-Ghadban” “Environmental and Earth Sciences Division, Kuwait Institute for Scientific Research, PO Box 24885, 13109, Safat, Kuwait bGeology Department, College of Science, Kuwait University, PO Box 5969, 13060, Safat, Kuwait (Received

3 June 1995; accepted

the Arabian Sea of the Indian Ocean by the Strait of Hormuz. Most river inflow into the Gulf occurs at the northern end, primarily on the northeastern Iranian side, the estuary of major rivers such as the Hendijan, the Hilleh and the Mand, and also on Shatt Al-Arab, the nexus of the Tigris, Euphrates and Karun rivers in the northwest (Fig. 1; Massoud et al., 1996). The bathymetry of the inner sea basin shallows to the northwest and west coasts (Reynolds, 1993), whilst the basin floor is asymetric, with its axis lying close to the Iranian coast. It slopes gradually from the shallow deltaic northern area to deeper waters in the south, with the depth rarely exceeding 100 m (it is only 36 m on average). Between 19 and 30 January 199 1, an estimated 10.8 million barrels (mbbls) of oil were spilled deliberately by Iraqi troops in Gulf waters, mainly from seven abandoned tankers and the Al-Ahmadi Sea Island terminal near the coast of Kuwait, in addition to smaller discharges from the Iraqi Mina Al-Bakr terminal and nearby sunken tankers, and the Saudi Ras Al-Zur refinery at Mina Sa’ud (Tawfiq & Olsen, 1993). It is also estimated that 8 mbbls of oil fallout from the smoke plumes of the 727 oil-well blowouts and fires in the Kuwaiti oil fields, started in late February 1991 by departing Iraqi troops, was deposited in the Gulf marine environment (Literathy, 1993). The airborne fallout contained oil combustion products, particularly ash, which includes all the trace metals present in the burnt crude oil. As part of the efforts exerted by the international community to assess the impact of the Kuwait oil slick, an integrated programme was adopted and sponsored by ROPME (the Regional Organization for the Protection of the Marine Environment), IOC (the Intergovernmental Oceanographic Commission), UNEP (the United Nations Environment Programme) and NOAA (the US National Oceanic and Atmospheric Administration). The programme was partially achieved through the launching of a loo-day cruise in the region from February to June 1992 aboard the NOAA research vessel Mt. Mitchell.

Abstract The trace metal contents of 71 core samples collected in 1992 from the bottom sediments of the Arabian Gulf are used to determine the regional distribution of concentration and pollution levels of these substances in the region. Chronic contamination was recorded in seven locations: the northwestern area (Fe), the northeastern area (Fe, V and Ni), the north-central area (V and Ni), the central area (Fe, Pb, V and Ni), the south-central area (Cu), the eastern area (Cu) and the southeastern area (Fe, V and Ni). Present-day contamination was identljied in three locations only: the north-central area (V), the central area (Pb, V and Ni) and the southeastern area (Fe, V and Ni). Diversljied natural and anthropogenic inputs may have provided the sources of this contamination. The V/Ni ratios of recent marine sediments cannot be used in identlyying oil spillages or in oil-sediment correlation studies. Positive correlations are found between increasing trace metal concentration and decreasing carbonate content and grain size, verifving that adsorption onto muds is the primary mechanism of trace metal concentration in marine sediments. Correlations with TOC (total organic carbon) contents indicate that organic matter is a signiJicant concentrator only in the case of Pb and Cu. With the exception of the Fe contamination in the northwest area due to river transport, all chronic and present-day trace metal concentrations are within the permissible natural background levels in the western ofshore areas, including the two areas thought to be polluted by the Kuwait oil slick, thereby supporting the idea that airborne fallout from oil fires was deposited in a limited coastal area between Kuwait and Bahrain, and verifving that the oil slick had minimal effect on the state of pollution by trace metals in the Arabian Gulf. Copyright 0 1996 Elsevier Science Ltd

INTRODUCTION The Arabian Gulf is a shallow marginal sea oriented from the northwest to the southeast and separated from *To whom correspondence

3 April 1996)

should be addressed. 285

286

F. Al-Abdali

This paper investigates trace metals in the offshore bottom sediments, their chronic (past) and present-day pollution levels, and their regional distributions in the

I

et al. Arabian Gulf, in an attempt to find any relationship between selected contaminating elements and the Kuwait oil slick, particularly the atmospheric oil fallout

II III

“emiiy oil &!Qhted .,-em

IV VI ::. ‘.‘.’ .. I.

TIIB ISLAMIC

Rl.3!Jl~LIC

Sligktlyoilpolluted(TPH 15.sopg/g) Unpollvted .rcm (TPH M -ISwIg)

OFIRAN ,y,“,)

cZtEti~Z(P~~~)

SAUDI ARABIA SULTANATE ot”

OF OMAN

In!

Fig. 1. Regional distribution of chronic total petroleum hydrocarbon (TPH) concentrations @g/g) and trace-metal contaminants bottom sediments of the Arabian Gulf. I-VII refer to chronic oil polluted areas (partly after Massoud et al., 1996).

in

I moderately chronic pdlmted areas cmmtly receivingfresh oil input Emvily

-ot - dsy oil pdlutcd are.* dfectrd by Kuwait oil slick Slightly oil pdtuted areas THE ISLAMIC

SAUDI ARABIA

Fig. 2. Regional distribution of present-day taminants in bottom sediments of the Arabian

total petroleum hydrocarbon (TPH) concentrations (Kg/g) and trace-metal conGulf. A-C refer to present-day polluted areas affected by the Kuwait oil slick (partly after Massoud et al., 1996).

Bottom sediments of the Arabian Gu,f--II

287

containing such elements. Trace metal concentrations, measured on core samples collected during the Mt. Mitchell cruise, are correlated with crude oils, oil discharges and the total petroleum hydrocarbon (TPH), total organic carbon (TOC) and carbonate (CaCOs) contents of these marine sediments.

along the Saudi Arabian coastline during the spring and early summer of 1991 are thought to have provided the source of oil pollution in these three areas.

Regional distribution of sediments

In most aquatic systems, the concentrations of trace metals in both suspended and bottom sediments are much greater than their concentrations in the water column (Horowitz, 1991). Bottom sediments are known to act as a reservoir or sink for many trace metals and some other pollutants. Undisturbed sediment at the bottom of an aquatic environment contains an historical record of chemical conditions, which allows investigators to assess chemical changes over time and, possibly, to establish area baseline levels against which present-day conditions can be compared and contrasted. This history can be traced by analysing sediment cores. Analysis of the top (l-2 cm) surface layer provides data on present-day pollution (Literathy & Foda, 1985).

The textural characteristics and regional grain-size distribution of bottom sediments in the Arabian Gulf were studied in detail by Al-Ghadban et al. (1996). The sediments could be divided into seven textural classes: sand, silty sand, muddy sand, sandy silt, sandy mud, silt and mud. Most of the region is covered by muddy sediments, whilst sandy deposits are restricted mainly to the western area off of Bahrain, Qatar and the United Arab Emirates (UAE). Suspension is believed to be the most important process of transportation and deposition, and thus low energy conditions prevail in the area, particularly in the northern area, which can be considered as a low-energy zone with low sediment movement. In contrast, the southern area of the Gulf represents a relatively moderate-to-high energy zone, with higher sediment movement. A north-south sediment transport from the northern area is inferred, with a net movement parallel to the axis of the Arabian Gulf. The above textural classes were simplified and grouped into four main classes, namely sand, muddy sand, sandy mud and mud (Tables 24). They were used with data obtained from the grain-size analysis and lithologic description of collected samples to construct a sketch map showing the regional distribution of bottom sediments in the Arabian Gulf (Fig. 2; Massoud et al., 1996). TPH content of bottom sediments

TPH concentrations in bottom sediments and their regional distribution in the Arabian Gulf were discussed in detail by Massoud et al. (1996). Measurements of these concentrations in the silt-clay fraction of bodyand top-core subsamples were used to determine the chronic and present-day pollution levels in the region, respectively. Seven chronic moderately (TPH 50-89 lg/g) and heavily (TPH 2661448 pg/g) polluted areas are recognized in the Arabian Gulf (see Fig. 1); three in the northern area and four in the southern area. Oil pollution in these areas is ascribed to natural oil seepage, accidental damage to pipelines, accidental spillage from tankers, the Nowruz oil slick and tanker deballasting. Present-day intermediate (TPH 5&144 pg/g) and high (TPH 200-l 122 Kg/g) pollution levels are identified in 10 areas (see Fig. 2). Of these, three polluted areas, namely: area A in the northeastern corner off and along the Iranian coast; area B off Saudi Arabia (opposite Abu Ah Bay); and area C off Bahrain, Qatar and the UAE were probably directly affected by the Kuwait oil slick. Large oil discharges from northern sources, as well as substantial quantities of eroded oiled sediments and oil floating mainly from heavily impacted tidal flats

METHODS

Sampling

A total of 112 sediment core samples (5 cm diameter x 15 cm deep) were collected from the bottom of the Arabian Gulf in order to trace the pollution history of these sediments over the past 60-5000 years (Fig. 3; Massoud et al., 1996). Further details of sampling are mentioned by Al-Ghadban et al. (1996). A subset of 7 1 sediment samples were selected for geochemical analyses (Tables 2 and 3). Each of the selected samples was divided into two subsamples: (I) a surface sediment subsample representing the upper or top 1-2 cm of undisturbed sediments; and (2) a body sediment subsample representing the rest of the core (approximately 13 cm deep). Most of the Arabian Gulf region is covered by finegrained sediments (mainly sandy muds and muds), with an average composition of about 7-26% sand, 3440% silt and 3459% clay, whereas sandy deposits are restricted mainly to the western area (off Bahrain, Qatar and the UAE) and are composed of, on average, about 95% sand, 3.0% silt and 2.0% clay (Al-Ghadban et al., 1996). Therefore, all top- and body-core subsamples containing more than a 10% silt-clay fraction ( < 63 pm) were wet-sieved through a 230-mesh (63 pm) sieve, and only this fraction was freeze-dried and analysed, whilst analysis was made on whole top- and body-core sandy subsamples containing less than a 10% silt-clay fraction (i.e. without sieving the sub-samples). Analytical

procedures

One-hundred-and-twenty-four top- and body-core finely powdered sediment subsamples (of approximately 1 g each) were dissolved in 15 ml of concentrated hydrochloric acid and 5 ml of concentrated nitric acid (3:1 ratio aquaregia). The mixture was digested at 120°C for l-2 h. Upon cooling, the solution (the leachate) was diluted to 30 ml with deionized water and filtered using Whatman No.1 filter paper (EPA, 1979).

288

F. Al-Abdali et al.

V cc+taminants

E ISLAMIC

SAUDI

REPUBLIC

OF IRAN

ARABIA SULTANATE

o&zc

OF

\

OMAN

2pokm ‘4,

Fig. 3. Distribution of chronic vanadium concentrations and contaminants (wg/g) in bottom sediments of the Arabian Gulf,

Concentrations of trace metals were determined in an aliquot using inductively coupled plasma-mass spectrometry (ICP-MS) under the following conditions: reflected power 0 W, incident power 1350 W, coolant flow 1.1 slpm, nebulizer gas flow 0.874 slpm, and nebulizer pressure 2.4 bar. All subsamples were analysed for zinc, lead, cadmium, nickel, manganese, iron, vanadium and copper. Along with the subsamples, system and method blanks were run with standard material for background correction and quality control. TPH and TOC measurements, taken on the same set of top- and body-core subsamples (Massoud et al., 1996) as well as carbonate (CaCOs) measurements made on the whole-core samples (Al-Ghadban & Jacob, 1993) were used to support the trace metal measurements, and to investigate the validity of these techniques as indicators of pollution in the Arabian Gulf.

RESULTS

AND DISCUSSION

Natural background levels of trace metals

Trace metals are natural constituents of all environments and are found in seawater, marine organisms and sediments (Bryan, 1976). Therefore, knowing their natural background levels, or at least their permanent concentrations in a marine environment, is essential for detecting and assessing trace metal pollution (Anderlini et al., 1986). The trace metal concentrations of sediments in different areas of the Arabian Gulf have been studied recently by several authors. Table 1 gives a summary of the concentrations recorded in unpolluted sediments

from these areas. However, all previous studies concentrated on the western nearshore areas only, without giving any indication of the trace metal concentration levels in sediments of the offshore and eastern nearshore areas in the region. Also, the table shows significant differences in the trace metal values recorded by these authors, even for the same area. Differences like these have been noticed by Horowitz (1991) and ascribed mainly to variations in the source of sediments and the procedures used to define the various geochemical substrates. Other factors, which may have led to such discrepancies in the studies made on the Gulf, are the source and grain-size distribution of the sediments in the region. For example, the values recorded for all trace metals in Kuwaiti coastal sediments are much higher than those reported in the nearshore areas of Bahrain, Qatar and the UAE (see Table 1). All northern nearshore areas in the Arabian Gulf, such as the Kuwaiti, Iraqi and Iranian coastlines, are close to the estuaries of major rivers on Shatt Al-Arab and the northeastern Iranian side (see Fig. 1; Massoud et al., 1996). Substantial quantities of trace metals are known to be transported annually by major rivers associated with suspended sediments (Gibbs, 1977; Martin & Meybeck, 1979; Horowitz, 1991). The airborne fallout from dust storms associated with the year-round northwestern Shamal winds may also be a significant source of trace metal input to local waters in these areas (Anderlini et al., 1986), particularly in June and July, when winds are persistent and much stronger and the dust fallout could well cover all northern nearshore areas (Literathy & Foda, 1985). Furthermore, the different types of sediments, whether alluvial or eolian, are

1993

1993

Qatar/Bahrain Basaham and Al-Lihaibi,

Bahrain Basaham

fin thousands ppm &g/g). “Trace metal values are estimated

by the authors

5.5

Northeastern

coast’

8.8 (7.7-9.8)

Oman Fowler et al., 1993

Iranian

2.5 (163.4)

Pb

25

Cd

1.25

in Kuwait

0.38 (0.060.7)

0.96 (0.02-l .9)

0.4 (0.01-0.8)

0.8 1 (0.1-0.25)

3.9 (3.24.5)

1.25 (0.5-2) 0.21

to those recorded

4.11 (1.2-7.2)

2.9 (0.5-5.2)

12.3 (0.5-24)

3.05 (1.74.4)

1.2 (0.7-1.7)

25 (2&30) 4.7

as equivalent

3.1 (2.3-3.8)

26.3 (12.2- -32.2)

12.9 (12.2-13.6)

11 (616) 35.5 (665) 6.5 (3-10)

5.5 (3&80) 60 109 (91-127)

Zn

UAE Fowler et al., 1993

and Al-Lihaibi,

1993

1993

1993

Northeastern Qatar Basaham and Al-Lihaibi,

Saudi-Arabia Sadiq and Zaidi, 1985 Basaham and Al-Lihaibi, Fowler et al., 1993

Kuwait Anderlini et al., 1986 Literathy et al., 1989 Basaham and Al-Lihaibi,

Reference

103

sediments

26 (9.9946)

18.9 (12.8-25)

15 (9-20)

6.5 (0.2-12.8)

5.8 (4.9-6.7)

37 (2450) 60 (4-l 16) 18 (8-28)

by Anderlini

450

200 (89-3 IO)

237 (231)

57 (17-97)

50 (42.8-37.2)

35 (17.7-52.3)

69.5 (l&129) 140.5 (19-262) 89.5 (39-140)

450 (300-600) 432 745 (550-940)

Mn

et al., 1986.

16

8 (5.&l

1)

4.8 (3.66)

4.6 (3.2-6)

0.008 (0.0060.01)

0.006 (0.0040.007)

9.8 (0.02-19.5) 7.2 (3.4-l 1)

20 (12-28)

16 (12-20)

Fe*

29.2 (10.448)

20.7 (7.3-36)

23 (9-36.6)

4.6 (2.7-7.4)

3.2 (2.7-3.6)

12.5 (0.9-24) 25.5 (249) 19 (19)

40 (30-50) 49 109.5 (85-134)

V

cu

22.5

7.9 (1.7-14)

4.2 (1.3-7.0)

9.6 (1.5-17.6)

3.9 (3.8-4)

3.2 (2.7-3.6)

10.3 (6.8-13.8) 14.5 (2-27) 4.5 (3-6)

22.5 (15-30) 27.5 42 (3450)

in unpolluted marine sediments of different areas in the Arabian Gulf

103 (86120) 115.9 179.5 (150-209)

Ni

(pg/g)

coastline

Table 1. Mean and range values (in parentheses) of trace metal concentrations

290

F. Al-Abdali

Table 2. Trace metal concentrations

Station ~_l. 9 IO II 13 14 I8 20 21 22 24 2.5 26 21 28 33 34 35 36 31 38 39 40 41 42 43 44 45 46 49 51 52 53 58 59 60 61 62 63 65 66 61 68 70 12 73 14 75 78 79 80 85 86 87 88 89 90 91 92 93 94 96 97 99 100 103

Textural class SM M MS SM MS MS +s MS MS +s MS MS SM MS M M SM MS +C +s +s +s +s SM SM SM SM MS +s MS MS MS +s +s SM MS SM MS SM SM SM SM M SM M M M M SM SM MS MS SM M SM SM SM SM SM SM SM M M SM SM

Zn

in body-core

Pb

Cd

8.00 12.30 2.60 5.60 6.40 5.50 ND? 6.60 7.00 6.40 ND2 ND3 5.90 7.90 5.40 12.90 II.50 12.00 3.60 ND3 ND3 ND’ II.60 6.70 8.50 7.70 13.00 9.80 9.50 12.50 14.00 II.30 4.10 2.80 8.40 13.30 15.60 13.03 I I .80 14.62 18.48 28.80 11.30 12.54 I I .44 37.60 20.40 11.10 11.60 15.80 2.54 0.64 13.50 18.80 20.20 28.30 8.26 1.64 7.90 20.40 26.40 16.40 10.98 8.75 14.80

ND* ND2 ND2 ND2 ND2 ND2 ND2 ND2 ND’ ND2 ND2 ND2 ND2 ND* ND* ND2 ND* ND2 ND2 ND2 ND* ND2 ND* ND2 ND2 ND2 ND* ND2 ND2 ND2 ND2 ND2 ND* ND* ND2 ND? ND* 0.10 ND4 0.10 0.11 0.11 ND4 ND4 ND4 0.10 0.07 ND4 ND4 0.15 ND ND 0.08 0.08 0.06 0.06 ND ND 0.09 0.11 0.12 0.15 ND ND 0.15

et al.

subsamples (kg/g dry, silt-clay Arabian Gulf Ni

Mn

Fe

fraction)

v

collected

from bottom sediments

cu

TPH*

TOC (%)*

15.0 21.9 16.0 13.5 15.3 14.6 ND2 4.0 7.7 16.9 ND2 3.6 10.8 12.0 44.0 14.0 21.2 16.5 3.6 3.2 5.1 3.7 9.0 6.2 11.5 17.1 17.4 13.5 11.5 12.0 17.0 56.5 7.2 4.2 16.7 15.0 15.0 10.6 12.9 14.1 12.5 16.0 11.9 10.8 10.9 31.8 18.3 13.9 9.6 14.2 8.8 10.0 10.8 11.3 14.0 19.2 14.5 22.7 8.8 14.2 20.0 13.9 16.5 17.2 23.5

84.2 88.9 17.7 50.4 22.0 17.2 4.2 0.36 20.8 10.2 9.1 32.7 23.8 14.9 16.1 24.7 23.9 24.9 3.3 3.7 4.1 3.4 0.6 4.2 1448 25.4 14.5 16.6 5.1 34.4 ND 783 4.2 4.4 36.2 32.8 54.6 1.7 38.1 31.3 ND 60.0 0.9 I.0 983 ND 55.4 7.1 ND 23.4 0.1 ND 8.1 544 ND 337.0 0.6 ND 36.7 22.4 ND 18.7 24.9 21.9 11.3

1.30 0.82 1.52 1.22 0.54 1.66 0.58 1.52 2.40 0.21 0.79 1.60 1.17 2.65 2.56 1.51 1.90 1.30 0.63 1.55 0.80 0.80 2.80 0.73 1.10

- -__ 35.80 50.20 4.30 23.30 21.10 28.60 ND2 7.90 15.00 6.40 3.80 5.00 14.90 15.00 14.30 25.80 34.00 20.50 8.00 2.50 4.00 4.10 20.90 9.00 17.60 23.80 27.90 25.40 6.90 27.50 36.20 35.50 4.10 8.40 26.10 49.50 42.30 23.17 22.47 26.72 32.10 42.60 15.23 23.30 15.92 60.80 35.92 17.32 17.60 29.00 I I .44 4.63 23.60 25.41 36.70 47.10 29.71 27.51 17.13 30.30 36.40 34.70 38.79 32.32 36.70

71.00 96.00 9.10 48.00 50.00 55.20 4.90 12.30 24.40 2.50 3.80 7.20 25.80 23.80 23.20 45.50 59.00 35.20 5.50 2.10 2.80 4.90 39.00 II.80 29.70 41.50 49.50 42.50 5.30 47.80 69.60 51.40 ND* 2.80 45.20 61.20 77.80 38.61 44.94 48.91 63.62 81.50 36.41 45.97 31.34 54.24 68.31 40.86 36.78 60.49 19.37 6.48 47.50 79.62 71.59 93.65 60.20 58.69 35.13 55.59 69.29 57.51 43.30 62.00 59.19

301.0 411.0 95.3 242.3 243.0 301.0 3.0 68. I 143.1 21.2 29.3 28.0 122.0 145.0 89.2 190.1 230.0 176.0 29.1 18.5 15.8 36.2 31.5 72.8 170.0 232.0 295.0 243.0 48.2 239.0 70.0 257.0 12.3 27.6 230.0 256.0 350.1 217.2 235.9 267.3 288.2 426.0 145.7 274.6 248.7 579.0 309.9 187.0 212.0 279.6 121.8 71.6 254.8 215.0 306.0 358.0 247.4 215.7 237.0 309.0 307.0 268.0 322.8 323.2 327.0

23 354 32 150 5 970 12577 12894 15251 3 059 5 663 11374 885 1099 1215 9 303 8 488 7 667 12215 18070 12490 1198 600 672 1502 16291 2 849 9281 14041 11905 I4 597 2271 17661 19807 16289 3 530 1072 16702 18342 20 678 12936 I 339 13514 17290 23 429 8210 I3 373 IO 147 32 037 19 578 9 835 11085 15 655 6 604 2 900 I4 167 13878 I7 132 24 577 22681 19119 10452 14700 18000 21914 26 555 22 062 I7 145

33.0 95.2 12.1 20.5 22.0 25.0 7.0 5.3 ::s” 3.0 3.6 11.3 11.0 13.8 17.0 22.4 16.5 4.4 2.8 2.8 4.1 21.8 7.3 15.2 21.6 25.3 19.2 8.5 21.3 28.6 24.2 I.5 2.8 20.4 25.6 36.0 17.4 16.9 21.9 29.6 40.5 13.2 18.5 15.4 22.0 34.0 16.0 16.1 25.4 10.8 7.1 22.4 32.3 29.8 45.4 13.8 10.9 16.7 24. I 30.0 26.8 11.7 17.9 27.6

1.80 1.61 1.81 0.75 1.30 ND 2.2 0.2 0.44 1.51 0.97 0.87 I.16 1.30 0.89 0.99 0.98 1.4 0.99 I .27 1.5 1.26 I.54 1.10 0.92 0.79 1.40 0.73 0.65 1.17 0.78 1.15 0.79 1.03 1.18 1.19 0.84 1.10 0.6 0.99

of the

CaC03 (%)+J 48.2 27.6 94.3 65.2 65.1 56.6 96.7 88.6 75.2 96.3 94.8 91.7 80.7 81.1 83.2 71.1 60.5 75.8 93.9 98.1 97.9 95.9 57.8 90.6 75.9 65.4 70.9 77.7 96 67.3 58.3 65.5 96.4 97.9 62.7 57.1 46.3 75 70.8 63.1 59.6 48.8 83.9 70.1 69.5 61.6 58.6 74.1 56.6 60.8 50.9 94. I 60.2 64.9 57.5 52.0 45.9 50.8 39.4 56.9 57.4 57.6 43.4 50.6 58.2 contd

Bottom sediments

291

of the Arabian Gulf-III

Table 2-contd 104 105 106

107 110 111 Range

M M M

43.40 40.00 39.93

20.70 17.70 11.89

MS MS MS

21.63 27.70 36.90 2.5& 50.80

6.78 10.20 27.40 2.6Ck 37.60

Regression analysis of TOC vs metals 0.004088 0.034964 r2 6.43 2.17 P

x lo-‘0

x IO-9

0.69 0.16

73.68 75.50

303.0 291.0

29 335 19391

37.2 33.3

20.7 15.1

ND ND 0.11 0.71 0.06-0.71

46.20 40.70 47.77 63.64 2.1096.00

384.7 289.4 193.0 32.7 3-579

11791 17 103 13953 21768 35332 150

16.9 28.1 27.3 22.1 1.5095.20

23.6 7.7 13.7 15.5 3.6& 56.50

0.056746

0.04482

0.023759

0.06813

0.001025

0.029128

0.001724

2.07 x IO-”

2.93 x10-‘0

5.58 xlO-‘2

5.8 x10-7

266 307

3.18 XIOP’O

18.5 2.5 13.6 5.8 -

1.34 0.99

48.2 56.5

0.66 1.61 0.46 0.85

50.3 48.6 64.5 57.2

-

-

ND: not detected, ND’: < 0.005 ppm, ND2: < 0.05 ppm, ND3: < 0.1 ppm, ND4: < 0.001 ppm. C: coral, S: sand, MS: muddy sand, SM: sandy mud, M: mud. *Massoud et al. (1996). +Al-Ghadban and Jacob (1993). fAnalysis made on whole sample.

transported in a southerly direction (see Section 1; AlGhadban et al., 1996). Consequently, the northern nearshore areas, such as the Kuwaiti and Iranian coastlines, would receive much higher trace metal inputs than the southern nearshore areas. It is well established that trace metals tend to be concentrated in the finer grain sizes of bottom sediments (Jenne et al., 1980; Solomons & Forstner, 1984; Horowitz & Elrick, 1987). Therefore, trace metal concentrations in the fine-grained muddy sediments of the Kuwaiti coastline would be much higher than those in the coarse sandy deposits covering the nearshore areas of Bahrain, Qatar and the UAE (see Fig. 2; Massoud et al., 1996). These differences emphasize some of the difficulties inherent in comparing and interpreting trace metal data which come from different areas or environments. These difficulties, which have been highlighted by other authors (e.g. Literathy & Foda, 198.5; Horowitz, 1991; Literathy, 1993) limit the ability of the technique as a whole. Therefore, we strongly recommend that trace metal concentrations in sediments must not be used solely as indicators of pollution in aquatic environments such as the Arabian Gulf, but rather in combination with other geochemical techniques. Taking into account the differences in the source and grain-size distribution of sediments in the Gulf region, the environmental conditions controlling the accumulation of trace metals in northern nearshore sediments and the above literature survey, the trace metal concentrations in unpolluted sediments of the northeastern Iranian coast were assumed to be equivalent to those reported by Anderlini et al. (1986) for Kuwaiti coastal sediments. Then, the average of the trace metal levels in nearshore sediments of northeastern Iran, Kuwait (Anderlini et al., 1986), Saudi Arabia (Sadiq & Zaidi, 1985) and Bahrain, Qatar and the UAE (Fowler et al., 1993) was calculated and compared with the results obtained from the present study of bottom sediments in the offshore areas of the Gulf. These computations led to the following estimates which may be considered as guidelines for the natural background levels (upper

limits) of trace metals in the dry, silt-clay unpolluted bottom sediments in the Arabian Zinc (Zn) Lead (Pb)

fraction Gulf.

of

30-60 cLg/s

15-30 pglg 1.2-2.0 /Lg/g Cadmium (Cd) Nickel (Ni) 70-80 cLg/g Manganese (Mn) 300600 Kg/g Iron (Fe) 10 00&20 000 #ug/g Vanadium (V) 20-30 pg/g Copper (Cu) 15-30 pglg

Chronic and present-day pollution levels of trace metals metal measurements in the silt-clay fractions (< 63 pm) of body- and top-core sediment subsamples (Tables 2 and 3) are used in this study to determine the historical and current concentration levels of these metals, respectively, and their regional distribution in the Arabian Gulf. These data are used, in turn, to delineate chronic and present-day pollution levels in the sediments, and to locate areas suspected to be polluted by each individual trace metal in the Gulf region. Trace

Zinc, cadmium and manganese

The concentrations of Zn (0.2-57.0 kg/g), Cd (0.06 1.0 pg/g) and Mn (3.G-579 pg/g) are grouped together because all three metals have attained their natural background levels, whether in the past or currently, throughout the Arabian Gulf. This conclusion implies that Zn, Cd, and Mn are natural constituents of the Gulf marine environment, and not elements derived from pollutant sources. Iron

The highest concentrations of Fe (> 20000 @g/g) in body-core subsamples (Table 2) which may be regarded as chronically polluted levels, were recorded in four main locations in the Arabian Gulf: the northwestern and northeastern areas (represented by subsamples 90, 91, 97, 99, 100, 104 and 111) a part of the central area which coincides nearly with the chronic oil-polluted

F. Al-Abdali et al.

292

Table 3. Trace metal concentrations in top-core subsamples @g/g dry, silt-clay fraction) collected from bottom sediments of the Arabian Gulf Station

9 10 14 18 21 22 25 21 28 34 35 36 31 38 39 40 41 42 44 46 41 51 52 53 60 61 62 63 6.5 66 61 68 70 72 73 74 75 78 19 80 87 88 89 90 93 94 96 91 103 104 105 III 112 Range

Textural class SM M MS MS MS MS MS MS MS M SM MS +c +s +s +s +s SM SM MS MS MS MS MS SM MS SM MS SM SM SM SM M SM M M M M SM SM SM M SM SM SM SM SM M SM M M MS SM

Zn

-~ 24. I 37.0 14.6 18.0 ND? 7.4 ND? 8.0 13.0 23.2 22.8 18.5 0.7 ND’ ND’ 2.3 8.7 3.5 29.4 3.3 14.5 18.0 28.2 14.1 22.3 23.9 40.0 16.2 22.1 21.5 38.4 31.7 17.8 13.9 12.6 15.0 29.3 15.4 11.4 12.0 13.1 30.13 29. I 29.3 9.4 13.4 26.3 IS.13 16.4 25.8 30. I 16.9 28.2 0.740

Pb

___8.6 5.1 3.0 5.3 ND2 ND2 ND2 ND’ ND’ 1.9 3.6 1.4 ND’ ND’ ND’ 0.2 ND’ ND2 ND’ I.8 ND’ I.0 2.4 ND’ 2.0 ND2 2.0 3.4 0.7 3.1 64.0 4.6 ND’ 4.4 3.2 2.9 9.3 ND’ 4.5 ND2 4.0 15.1 8.8 ND’ 2.1 ND’ ND’ 8.7 4.8 5.2 5.4 3.0 ND2 0.2-64

Regression anal.vsis qf TOC versus metals YZ 0.007186 0.101395 6.09 0.853847 P x IO-‘0

Cd

Mn

Ni

Fe

v

CU

TPH*

TOC (%)*

-.

~. ND2 ND2 ND2 ND2 ND2 ND2 ND2 ND’ ND’ 0.95 ND2 ND’ ND’ ND’ ND’ 0.6 ND’ ND2 ND’ ND2 ND’ 1.0 ND’ ND’ 0.7 ND2 ND’ ND2 0.4 ND’ ND’ ND’ ND’ ND’ ND’ ND’ ND2 ND’ 0.1 ND2 ND2 ND2 ND2 ND’ ND2 ND’ ND’ ND2 ND2 ND2 ND2 ND2 ND2 0.4-l .o

59.3 77.0 38.3 51.2 6.2 16.5 2.3 2.4 32.0 48.8 51.0 41.6 9.9 3.3 2.9 3.3 42.5 II.3 70.0 12.0 46.8 43.9 64.8 35.7 37.1 44.1 89.0 26.0 48.0 49.5 72.9 68.8 44.8 31.2 54.4 45.1 56.4 43.8 36.5 28.4 31.1 28.0 44.0 70.4 23.3 31.9 60. I 31.2 41.0 43.5 59. I 33.0 63.5 2.3-89

258.0 333.0 278.0 267.0 49.6 131.0 17.1 115.0 186.0 232.0 228.0 238.0 66.0 31.0 18.0 27.0 236.0 63.3 328.0 84.0 222.0 233.0 308.0 232.0 230.0 257.0 405.0 139.4 246.0 259.0 321.0 300.0 185.0 212.0 315.0 252.0 250.0 208.0 240.0 152.0 176.0 133.0 203.0 280.0 176.0 202.0 301.0 156.0 222.0 214.0 269.0 223.0 289.0 17405

0.003795 0.217125

0.000257 5.6 x IO-‘2

0.005903 2.43 x IO-‘5

ND: not detected, ND’: < 0.005 ppm, ND’: < 0.05 ppm. C: coral, S: sand, MS: muddy sand, SM: sandy mud, M: mud. *Massoud et al. (1996).

38.7 20 277 39.0 25 979 23.0 13915 25.0 I5 144 3 970 ND2 7 527 8.7 ND2 898 II.5 5 033 16.1 7 093 19.4 9 705 14511 25.0 9 900 18.7 2 499 1.9 I 082 4.9 779 2.6 I II6 3.5 8 979 20.5 3 368 6.3 21.5 I4 189 3 930 6.6 IO 129 22.2 10630 21.6 13484 26.7 1837 16.8 9445 16.7 24.0 12482 19573 41.5 10201 17.8 10454 22.7 10423 21.0 I 5 052 33.3 I 5 650 35.0 8 532 13.1 1946 15.5 I 2 453 26.0 I I 541 32.8 I 1564 32.1 8 834 Il.1 8 277 16.0 8 476 22.8 9981 24. I 9 866 33.8 12831 26.0 15846 36.6 8019 15.2 7 857 24.0 13599 28.6 8 645 19.1 13504 27.0 15316 20.0 16647 33.3 13231 28.0 14950 25.6 898-25 979 2.6-41.5

0.005437 1.4 x IO-‘3

0.005217 9.11 x IO-‘4

16.30 17.10 9.10 I I .70 3.70 5.10 2.30 7.30 7.80 Il.80 18.00 10.30 0.70 0.30 0.30 0.20 10.20 4.90 15.90 4.00 Il.10 10.00 13.30 7.10 10.50 13.80 14.80 7.80 13.80 9.30 12.20 10.80 16.40 7.20 II.00 10.20 17.90 10.40 8.20 1.20 7.50 7.60 12.00 14.60 6.10 6.90 I I .30 15.60 13.50 12.00 15.04 7.20 15.2 0.2-18.0

0.015681 1.65 ._ x IO-”

50. I 51.7 18.1 ND 57.0 19.1 33.2 77.1 95.0 64.2 ND 24.9 5.9 7.0 9.4 10.6 57.5 34.7 79.9 ND ND 176 ND 53.4 74.8 37.3 ND ND ND 76.0 225.0 21.10 201.0 ND 453.1 ND 38.8 :122 ND ND 40.8 65.9 ND 144.0 ND 27.8 ND II.4 17.0 14.1 ND 29.8 68.9

I.10 0.58 1.23 ND 0.83 0.68 1.53 1.07 0.78 0.93 ND 1.23 0.76 0.95 0.64 0.49 0.86 1.27 1.24 ND ND 2.08 ND 0.47 0.65 0.82 ND ND ND 0.54 2.4 0.6 2.1 ND I.35 ND 0.96 I .I3 ND ND 0.98 1.33 ND 0.76 ND 0.67 ND I.19 0.78 0.48 ND 1.5 1.68

-

-

--

Bottom sediments of the Arabian Gulf-III

293

Copper and lead

1986) have concluded that Cu antifouling coatings used by ships and fishing boats are a major source of Cu pollution. Other studies on Kuwait coastal sediments (Forstner & Wittmann, 1979; Anderlini et al., 1986) suggest that Cu contamination is due to ‘industrial effluents’ from Doha power station, the United Fisheries of Kuwait dry dock in Khor Subiya, Urn Al-Kasr Naval Base, and various industries around the Shuaiba industrial area, including fertilizer production and oilrefining facilities, where concentrations greater than 30 pug/g were recorded. Industrial effluents from the extensive oil production, loading and transport facilities off the coast of the UAE could act as a source of Cu pollution (44 pg/g) in area V. On the other hand, effluents from industrial and agricultural centers may be entering the Gulfs waters and may provide a source of Cu contaminations (56.5 pg/g) in area IV near the eastern Iranian coast. Also, fishing boats painted with Cu antifouling coatings could be another source of Cu pollution in the area. What is causing concern is the vicinity of this area to a known prawn nursery (fig. 1 of Mathews et al., 1993, p. 252). Therefore, further urgent investigations are required to study in detail the effect of Cu pollutants on fish and prawn populations in this area of the eastern Iranian coast. Chronic and present-day lead pollution levels (from 37.6 to 64 wg/g; Tables 2 and 3) were recorded in the bottom sediments of the oil-polluted area III in the central area of the Gulf (Figs 1 and 2). Petroleum hydrocarbon contamination in this offshore area is thought to be derived from diversified sources, including natural oil seepage, the Nowruz oil slick and accidental damage to pipelines and spillage from tankers (Massoud et al., 1996). Crude oil is known to contain minor amounts of Pb, so it seems likely that the higher concentrations of Pb were derived from one, or probably more than one, of the above oil pollutant sources.

The regional distributions of Cu and Pb concentrations in bottom sediments show some similarities. With the exception of only one or two locations recording high (polluted) levels of Cu and Pb, most areas in the Arabian Gulf are covered with unpolluted sediments containing natural background concentration levels of the two trace metals. Also, the highest concentrations of both metals are found in sediments with elevated organic carbon content (TOC > 2.0%; see below), indicating that they are derived from pollutant sources. Copper is recorded in high concentrations (from 44 to 56.5 pg/g; see Table 2) in two areas already contaminated with chronic petroleum hydrocarbon pollutants: area IV near the eastern Iranian coast and area V off the coast of the UAE (Fig. 1). The presence of Cu in high concentrations causes great danger to all marine organisms, including fish, crustacea, phyto- and zoo-plankton, and filter feeders (Cuchlaine, 1975). Studies of marine sediments contaminated with Cu in several parts of the world, such as San Francisco Bay (Hilderbrand, 1976) and the Suez Canal and eastern shores of the Nile Delta (Ghanem,

Vanadium and nickel are by far the largest trace metal constituents of crude oil (see Table 5) and hence their presence in high concentration in marine sediments may indicate direct input from oil pollutants. Vanadium and nickel concentrations in body-core sediment subsamples (Table 2) were in the ranges 1.595.2 and 2.1-96.0 pg/g, respectively. These values have been used in this study to construct two maps (Figs 3 and 4) showing the regional distributions of the chronic concentration levels of the two metals in the Arabian Gulf. The two maps reveal that both V and Ni pollution (high concentration) levels are recorded in the sediments from two locations in the northeastern and central areas, which nearly coincide with the chronic petroleum hydrocarbon (oil) polluted areas I, II and III, indicating that V and Ni contaminations in the two locations are mainly derived from the chronic oil pollutants in these areas. Oil pollution in these three areas is attributed mainly to natural seepage, the Nowruz oil slick and accidental damage to pipelines (Massoud et al., 1996).

area III (subsamples 62, 68 and 74; Fig. I), and an area at the southern entrance (the Strait of Hormuz) along the southeastern Iranian coast (subsamples 9 and lo), coinciding with the oil-polluted area VII. Present-day high concentrations (Table 3), which reach pollution levels, were recorded only in the latter area (Fig. 2) indicating that this area along the southeastern Iranian coast is still receiving fresh input. Studies of the transport of trace metals by major world rivers indicate that substantial quantities of Fe are transported daily, associated with suspended sediments in these rivers (Horowitz, 1991). Since the northwestern and northeastern areas of the Arabian Gulf are close to the discharge sites of major rivers; the Tigris, Euphrates and Karun (Shatt Al Arab-Fig. 1; Massoud et al., 1996) as well as the Hendijan, Hilleh and Mand (Iranian northeastern side), all northern areas of the Gulf may have been enriched with Fe transported by these rivers. Fluvial transport could be also the source of Fe enrichment in the extreme southern area along the southeastern Iranian coast, where the estuaries of a number of small rivers are located. Furthermore, the area is part of the Strait of Hormuz, one of the most important waterways in the world. In peak periods, one ship passes the Strait of Hormuz every 6 min (Reynolds, 1993), and hence local oil spills are frequent in the area (Fowler, 1988). Accordingly, accidental spills of crude oil/fuel oils contaminated with Fe from tankers would act as an additional source of pollution to this area. The area in the central area of the Gulf, which coincides nearly with the chronic oil-polluted area III, is situated in the vicinity of offshore oil production facilities beneath a navigation route (Haslam, 1983). Therefore, accidental damage to pipelines and spillage of oil from tankers could act as source of pollution in this area.

Vanadium and nickel

294

et al.

F. Al-Abdali

Ni contaminants

THE ISLAMIC

SAUDI

REPUBLIC

OF IRAN

ARABIA SULTANATE

Fig. 4. Distribution

of chronic nickel concentrations

and contaminants

\

&g/g) in bottom sediments of the Arabian Gulf.

tamination is still limited to the above four areas, there is a difference in the number of locations and, consequently, the area and bulk of sediments polluted by each metal. Whilst V pollutants cover a larger area at the bottom of the Gulf including areas I, II and III in the northeastern and central areas and the southern area VII along the southeastern Iranian coast (the Strait of Hormuz), Ni pollutants cover a smaller area of the Gulf and are restricted only to area III in the central area and area VII in the Strait of Hormuz. Massoud et al. (1996) noticed that the sediments of areas I and II, which are heavily polluted by high chronic TPH

In addition,

V and Ni contaminations are recorded in the extreme southern oil polluted area VII along the southeastern Iranian coast. As this area is part of the Strait of Hormuz, local oil discharges due to tanker deballasting and accidental spillage from tankers could be the source of these contaminations. Concentrations of V (2.641.5 @g/g) and Ni (2.33 89 pg/g) in the top-core sediment subsamples (Table 3) have been used to draw two maps (Figs 5 and 6) portraying the regional distribution patterns of present-day V and Ni concentration levels in bottom sediments throughout the Arabian Gulf. Although V and Ni con-

Table 4. Average trace metal concentrations (pg/g) in the different grain-size textural classes of bottom sediments in the Arabian Gulf

calculated from the measurements made on top- and body-core subsamples Zn

Pb

2.3 12.6 21.1 22.3

0.05

Textural class

Cd __.

Ni

Mn

Fe

v

cu

1.9 15.8 23.5 25.5

2.3 7.7 11.2 13.2

Top-core subsamples

Sand (0.84 mm), n = 5 Muddy sand (0.105 mm) n = 16 Sandy mud (0.053 mm), n = 21 Mud (0.00012 mm), n = I I

Regression analysis of size versus metals r* 0.85 0.006 P

I .47 4.9 5.4

0.128 0.113 0.10 0.20

0.64 0.035

0.63 0.024

12.4 28.7 45.1 48.4

0.81 0.004

76 180 238 240

0.92 0.0007

2891 8981 11804 13 181

0.91 0.001

0.81 0.003

0.85 0.003

Body core subsamples

Sand (0.84 mm), n = IO Muddy sand (0.105 mm), n = 20 Sandy mud (0.053 mm), n = 27 Mud (0.00012 mm), n= 14 Regression analysis r* P

6.53 20.2 28.7 32.7

3.84 8.2 13.4 15.6

0.05 0.09 0.07 0.11

7.0 33.7 54.4 55.1

31 166 259 282

3 108 7 546 15510 18 193

6.0 16.9 22.2 28.2

6.4 15.0 13.4 16.8

qf size versus metals 0.87

0.003

0.75 0.007

0.64 0.004

0.87 0.003

0.88 0.003

0.68 0.016

0.84 0.013

0.92 0.0004

295

Bottom sediments of the Arabian Gulf-III concentrations (372 ,ug g-’on average; Table 2 and Fig. 1) are currently receiving much Iower intermediate presentday TPH concentrations (104.8 Fgg-’ on average; Table 3 and Fig. 2), indicating a significant decrease in oit input at the present time. Also, elemental analyses of Gulf crude oil shows that it has a very high content of

V, which is three to four times higher than that of Ni (Table 5). Although this relationship is reversed in sediments (see the following section), one should not overlook the possibility that the relatively low present-day Ni content of the sediments in areas f and II is due to the low oil input in these areas at the present time.

Table 5. Published data on V and Ni contents of Gulf oils/residues and the Kuwait oil slick (Literathy & Foda, I985; Al-Arfaj & Alam, 1993) -.--~__ I-._-______.~ V/Ni Origin and type of oil Ni (&g/g) V (l&g) -~ .. . _~____ . _...^_. “^-~_.-.l. Kuwait Export crude 30 8 3.75 + 538 “C residue 116 34 3.41 Hout, crude 28 6 4.67 i- 340 “C residue 58 13 4.46 + 350 “C residue Ifi3 31 3.5 Khafji, crude 55 :: 3.44 + 340 “C residue 95 3.39 + 550 “C residue 170 49 3.47 Wafra Burgan, crude 34 6.8 5.0 120 25 4.8 + 564 “C residue Wafra Ratawi, crude 55 I5 3.67 + 562 “C residue 150 34 4.41 Eocene, crude -” + 562 “C residue 135 67 2.01 Iran 72 202

25 69

2.88 2.93

130

34

93 71 244 23 101 36 66 143 41 70

:2 74 8 36 II 20 44 12 20

3.82 3.00 3.38 3.30 2.88 2.81 3.27 3.30 3.25 3.42 3.50

212 80 175 142

26 22 63 54

-1 8.1 3.64 3.78 2.63

Heavy + 565 “C Residue Light + 565 “C residue Light-Berri

205 116 -

+ 565 “C residue Medium-Khursaniyah f 565 “C residue Medium ~uluf/Marjan + S&5“C residue

9 96 179

64 25 I6 -32 54

3.20 4.64 -I. 1.50

9.7 11.9 13.3

2.9 3.2 3.2

Bahrgansar Nowruz (STRIP) crude + 550 “C residue Nowruz crude ACIP DTAH Aboozar (Ardeshir) crude + 565 “C residue Dorrood (Darius) crude + 565 “C residue Foroozan (Fereidoon) crude + 343 “C residue + 566 “C residue Sirri, crude + 350 “C residue Iraq

Basrah, Heavy + 460 “C residue Light + 525 “C residue Medium + 520 “C residue Kirkuk + 525 “C residue

Saudi Arabia

3.00 -3.31

Saudi Arabia Kuwait oil slick NW Abu Ali Abu Ah Ras At-I&war

27.6 38 42.9

296

F. Al-Abdali et al.

Effect of grain size, organic matter and carbonates Grain size, organic matter, and carbonate content are thought to play an important role in controlling the concentrations of trace metals on/in suspended and bottom sediments (Jenne, 1976; Jenne et al., 1980; Hirner et al., 1990; Horowitz, 1991). In general, positive correlations have been reported

THE ISLAMIC

SAUDI

between increasing trace metal concentrations and decreasing grain size and carbonate content and increasing organic matter content in some marine environments. The effects of the above three factors on the concentrations of trace metals in the bottom sediments of the Arabian Gulf are discussed below.

REPUBLIC

Of IRAN

ARABIA SULTANATE

Fig. 5. Distribution

of present-day vanadium concentrations

and contaminants

@g/g) in bottom sediments of the Arabian Gulf.

“v< SAUDI

ARABIA SULTANATE

Fig. 6. Distribution

of present-day nickel concentrations

and contaminants

\

(pg/g) in bottom sediments of the Arabian Gulf.

Bottom sediments of the Arabian Gulf-III

Table 4 gives a summary of the average present-day and chronic trace metal concentration levels in the different grain-size textural classes of bottom sediments in the Arabian Gulf, which are calculated from the measurements made on top- and body-core subsamples. The strong correlation between the two parameters is evident from the regression analysis of grain size versus trace metals in top-core subsamples (r2 between 0.63 and 0.92; p between 0.0007 and 0.035) and in body-core subsamples (9 between 0.64 and 0.92; p between 0.0004 and 0.016 at 95% CL). The table also shows a positive relationship between increasing trace metal concentrations and decreasing grain size. High concentrations of all trace metals are recorded in muddy sediments (sandy mud and mud), with the highest concentrations associated with muds (Zn 22.3-32.7, Pb 5.4-15.6, Cd 0.1 l-0.20, Ni 48.456.5, Mn 240-282, Fe 13 181-18 193, V 25.5-28.2, and Cu 13.2-16.8 Kg/g on average), indicating that adsorption onto muds is probably the primary mechanism of trace metal accumulation in the bottom sediments of the Arabian Gulf. It is noticed that the average highest concentrations of all trace metals are within the permissible natural background levels, reflecting the overall picture of trace metal contamination which is concentrated in a limited number of relatively small polluted areas, with the rest of the Arabian Gulf covered with unpolluted sediments (see Figs 1 and 2). Statistical data obtained from a regression analysis of TOC percentages versus trace metal contents of all sediment core samples (Tables 2 and 3) show weak a correlation between the two parameters (r2 between 0.004038 and 0.06813 for body-core subsamples and

H

297

between 0.000257 and 0.101395 for top-core subsamples). However, it is noticed that the highest concentrations of Pb (37-64 pg/g) and Cu (44-56.5 /Igig) are associated with the highest TOC levels (2.2-2.65%) recorded in the bottom sediments of the Arabian Gulf, indicating that organic matter is a significant concentrator only in the case of the Pb and Cu trace metals. It should be kept in mind that the positive relationship between the highest TOC levels and the highest Pb and Cu contents is not recognized in the results of the above regression analysis, since sediments exhibiting these elevated trace metal concentrations are recorded only in one or two sampling stations out of the 71 stations covered by the analysis (see previous section). Al-Ghadban et al. (1996) suggested that the sedimentological characteristics of bottom sediments in the Arabian Gulf reflect the interaction between autochthonous calcareous fragments, mostly of biogenic origin, rock fragments derived from beach rocks and submerged reef flats, and allochthonous terrigenous detritus supplied to the area mainly by dust storms and river deltas in the far northern area and along the Iranian side. They used carbonate (CaCO,) measurements made on wholecore sediment samples (Al-Ghadban & Jacob, 1993) to draw a map showing the regional distribution of carbonates in the Gulf (Fig. 7). It is observed, through the plotting of locations recording the highest concentrations (contamination) of the various trace metals on the map, that, with the exception of Cu contaminants recorded in only one location off the coast of the UAE, all trace metals tend to be concentrated in bottom sediments with low carbonate contents throughout the Arabian Gulf.

30-70%CBC03

r

THE ISLAMIC REPUBLIC OFIRAN

SA UDI ARABU

Fig. 7. Map showing the regional distribution of carbonates (CaCOJ various trace metals in bottom sediments of the Arabian

and locations recording the highest concentrations Gulf (partly after Al-Ghadban et al., 1996).

of the

298

et al.

F. Al-Abdali

GENERAL

DISCUSSION

in identifying different types of crude oils, and in oil-oil and oil-source rock correlation studies. Therefore, the technique was suggested by some authors as an aid to the identification of oil spillages, although the usefulness of these ratios could be limited by the accuracy of their determination, possible lack of sufficient differences between suspect samples, and doubtful effects of weathering and exposure (Horowitz et al., 1970). The V/ Ni ratios of the Gulf crude oil/residues and the Kuwait oil slick (from about 2.9 to 5.0; Table 5) do not correlate with those of the sediments (0.418 on average; Table 6), with the exception of one sample collected from area C (station 27; Table 6) which has a V/Ni ratio (4.79) almost identical to that of the Kuwaiti Wafra Burgan crude residue (4.8; Table 5). As the bottom sediments of the Arabian Gulf contain much higher concentrations of Ni, which are nearly twice those of V (see Tables 2, 3 and 6), the relationship between the two metals in the Gulf sediments is completely reversed. These observations are in agreement with earlier conclusions drawn by Literathy and Foda (1985) from the study of the Nowruz oil slick. The two authors believe that the polar photo-oxidative products generated on weathering of the seeped oil can react with complexes of V and Ni metals, thereby solubilizing them and leading to an altered ratio between the concentrations of the two metals. Consequently, it is suggested that V/Ni ratios cannot be used in identifying oil spillages or in oil-recent-marine-sediment correlation studies.

and fate of the Kuwait oil slick As previously stated, an estimated 10.8 mbbls of crude oil and 8 mbbls of oil fallout from the smoke plumes of well blowouts and fires were spilled and deposited in the Gulf marine environment from January to February 1991. Massoud et al. (1996) studied in detail the TPH concentrations in bottom sediments and their regional distribution patterns in the Arabian Gulf to shed light on the effect and fate of the Kuwait oil slick. Three oilpolluted areas were identified which are covered with sediments containing present-day high and intermediate TPH concentrations, thought to be derived directly from the oil slick. These areas are area A in the northeastern corner off and along the Iranian coast, area B off Saudi Arabia (opposite Abu Ali Bay), and area C off Bahrain, Qatar and the UAE (Fig. 2). Since airborne oil fallout could have formed a major source of trace metals, in addition to the spilled oil, it is important to correlate the trace metal contamination in the bottom sediments and their regional distributions in the Arabian Gulf with the trace metal contents of the Gulf crude oil/residues, the 1991 spilled oil and the three oil-polluted offshore areas A, B and C, in order to throw more light on the effect and fate of the Kuwait oil slick. Effect

Vanadium/nickel

ratios

Vanadium/nickel ratios have been used successfully by several authors (e.g. Lewan, 1984; Massoud et al., 1991)

Table 6. Present-day trace metal concentrations (pg/g) in bottom sediments of the three areas in the Arabian Gulf (A, B and C) thought to be polluted by the Kuwait oil slick

-__

Area/station

Areu A corner) 112

< 0.01

co.01

25.6

63.5

0.403

269

< 0.01 < 0.01 2 0.01 0

< 0.01 < 0.01 2 0.01 0

13.1 17.1 2 15.1 2.828

44.8 43.8 2 44.11 0.445

0.292 0.390

185 208 2 196.5 16.263

< 0.05 < 0.01 < 0.01 1.9 < 0.01 -c 0.05 < 0.01 1.0 2.4 2.0 -c 0.05 <: 0.05 I2 0.628 0.936

< 0.05 < 0.01 < 0.01 0.95 < 0.01 < 0.05 < 0.01 I.0 co.01 0.7 < 0.05 < 0.05 I2 0.241 0.393

< 0.05 II.5 16.1 19.4 20.5 6.3 21.5 21.6 26.6 16.7 24.0 21.0 12 17.112 7.695

6.2 2.4 32.0 48.8 42.5 II.3 70 43.9 64.8 37.1 44.1 49.5 I2 39.716 21.554

0.01 4.79 0.503 0.398 0.482 0.558 0.307 0.492 0.412 0.450 0.544 0.424 I2 0.780 I .270

28.2

17.8 15.4 2

Fe

CU

14950

15.2

of

16.4 deviation

Area C (of the coast of Bahrain, Qatar and the UAE) 21 27 28 34 41 42 44 51 52 60 61 66 n Mean Standard

V

Pb

(northeastern

Area B (off the coast Saudi Arabia) 70 78 n Mean Standard

Cd

Zn

deviation

1.979

< 0.05 8.0 13.0 23.2 8.7 3.5 29.4 18 28.2 22.3 23.9 27.5 12 17.145 IO.179

i.341 0.0692

49.6 II5 I86 232 236 63 328 233 308 230 257 259 I2 208.05 88.994

8 532 8 843 2 8 687.5 219.910

3 970 5033 7 093 9 705 8 979 3 368 14 I89 10630 13484 9445 12482 10423 I2 9 066.75 378.65

16.4 10.4 2 13.4 4.242

3.7 7.3 7.8 II.8 19.2 4.9 15.9 10.0 13.3 10.56 13.8 9.3 I2 10.63 4.491

Bottom sediments of the Arabian Gulf-III Trace metal content of areas polluted by the Kuwait oil slick Trajectory models (Al-Rabeh et al., 1992; Al-Rabeh et al., 1993) and conclusions drawn from the study of data and samples collected during the Mt. Mitchell

cruise (Gerges, 1993) including the study of present-day TPH concentrations in the top layer of bottom sediments in the Arabian Gulf (Massoud et al., 1996), all indicate that with the exception of several hundred thousand barrels of oil which has seeped from sunken tankers in the extreme northeastern corner (area A; see Fig. 2) the main bulk of the Kuwait oil slick spread over a short period from its northwestern sources in a southwesterly direction. It has also been noticed from the present study of the regional distribution of present-day trace metal concentrations in bottom sediments that all the trace metal contamination is present in areas situated in the northeastern, central and southeastern parts of the Gulf (Fig. 2) which are not only distant from the proposed trajectory of the oil slick and the areas impacted by it, but are also contaminated with trace metals derived from other diversified sources. To check this, the values of present-day trace metal concentrations in the bottom sediments of the three areas A, B and C polluted by the Kuwait oil slick were listed separately in Table 6. The table shows clearly that all trace metal concentrations in the sediments of these areas are relatively low and within the natural background permissible limits. Being probably contaminated by the Kuwait oil slick and in the meantime unpolluted by trace metals, the sediments of areas A, B and C provide further evidence that the 1991 oil slick had a minimal contribution to the high concentration levels of these metals and the contamination caused by them in the Arabian Gulf. This conclusion also may imply that the estimated 8 mbbls of airborne fallout from the oil fires did not have a serious impact on the state of pollution by trace metals in the region. Trux (1992) has demonstrated that the smoke plumes from the oil fires spread in a southwesterly direction, impacting the Saudi Arabian coast between Kuwait and Bahrain. A similar conclusion was reached by Literathy (1993) who computed that an area 50 km wide and 500 km long extending from burnt wells in onshore Kuwaiti oilfields (i.e. a 25 000 km2 marine area covering the coastline between Kuwait and Bahrain) could be considered as the maximum area affected by distant airborne fallout. The trace metal content of sediments in the western nearshore areas of the Gulf has been studied by several authors. In their study of the heavily oil impacted Abu Ali area along the Saudi Arabian coast, Al-Arfaj and Alam (1993) reported higher trace metal concentrations in heavily oiled sites than moderately oiled areas, with the lowest concentrations observed in benthic samples in areas of high current. Basaham and Al-Lihaibi (1993) and Fowler et al. (1993) studied the trace metals in coastal sediments of the western Gulf, including the nearshore areas of Kuwait, Saudi Arabia, Bahrain, Qatar and the UAE, and concluded that the effect of

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anthropogenic enrichment upon the absolute concentration of the metals in these areas is minimal. The present study of trace metals in offshore sediments of the western Gulf demonstrates that, with the exception of elevated chronic concentrations of Fe in the northwestern area due to river transport, all chronic and present-day concentrations of trace metals are within the permissible natural background levels in all western offshore areas, including the suspected oil polluted area B off Saudi Arabia (opposite Abu Ali Bay) and area C off Bahrain, Qatar and the UAE. These findings support Literathy’s hypothesis that the airborne fallout from oil fires was deposited in a limited marine area along the coastline between Kuwait and Bahrain, and verify that the Kuwait oil slick as a whole (i.e. the spilled oil plus the oil fallouts) had a minimal effect on the state of pollution by trace metals in the Arabian Gulf.

CONCLUSIONS 1. The following estimates may be considered as guidelines for the natural background levels (upper permissible limits) of trace metal concentrations in the dry, silt-clay fraction of unpolluted bottom sediments in the Arabian Gulf: Zn 3&60, Pb 15-30, Cd 1.2-2.0, Ni 70-80, Mn 30&600, Fe 10 000-20 000, V 20-30, and Cu 15-30 pg/g. 2. The chronic and present-day concentrations of Zn (0.2-57.0 pg/g), Cd (0.061.0 pg/g) and Mn (3.0579 pg/g) in sediments/are all within the permissible natural background levels, indicating that Zn, Cd, and Mn are natural constituents of the Gulf marine environment. 3. Chronic contamination with Fe was recorded in four main locations in the Arabian Gulf: the northwestern and northeastern areas, the central area, and the southern area (the Strait of Hormuz) along the southeastern Iranian coast which is still receiving fresh input. Proposed sources of pollution are river transport and accidental damage to pipelines and crude oil/fuel oil spillages. 4. With the exception of only one or two locations recording contamination with Cu and Pb associated with elevated TOC contents, most areas in the Arabian Gulf are covered with sediments unpolluted by the two metals. Possible sources of Cu pollution are industrial and agricultural effluents and/or the Cu antifouling coatings of boats and ships, whereas Pb contamination could be derived from diversified oil pollutant sources. 5. As V is the major trace metal constituent of crude oils, followed by Ni, but in much smaller amounts, their occurrence in high concentrations in sediments may indicate direct input from oil pollutants. Chronic V and Ni contamination was recorded in four areas; two in the northeastern and north-central areas, and two in the central and southeastern areas, which are already polluted by

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petroleum hydrocarbons, indicating that the contamination with the two metals is derived from chronic oil pollutants in these areas. Whilst all four areas are currently receiving high concentrations of V, the two areas in the northeastern and north-central areas have low presentday Ni contents, probably due to the low oil input in these areas at the present time. 6. The V/Ni ratios of the Gulf crude oil/residues and the Kuwait oil slick do not correlate with those of the sediments due to the alteration of the relationship between the two metals in the sediments. Therefore, this technique cannot be used in identi-

fying oil spillages or in oil-recent-marine-sediment correlation studies. 7. Positive correlations are found between increasing trace metal concentrations and decreasing carbonate content and grain size, with preferential association of high concentrations of all trace metals with fine-grained sediments, verifying that adsorption onto muds is the primary mechanism of trace metal accumulation in bottom sediments throughout the Arabian Gulf. On the other hand, a positive correlation is only found between the highest Pb and Cu concentrations and the highest TOC contents of bottom sediments in the Arabian Gulf, indicating that organic matter is a significant concentrator only in the case of the Pb and Cu trace metals. 8. With the exception of elevated chronic concentrations of Fe in the northwestern area due to river transport, all chronic and present-day concentrations of trace metals are within the permissible natural background levels in all western offshore areas, including the oil-polluted areas B off Saudi Arabia (opposite Abu Ali Bay) and C off Bahrain, Qatar and the UAE, thought to be affected directly by the southwesterly-driven Kuwait oil slick. These findings support the idea that airborne fallout from oil fires was deposited in a limited marine area along the coastline between Kuwait and Bahrain, and verify that the Kuwait oil slick as a whole (i.e. the spilled oil plus the oil fallout) had a minimal effect on the state of pollution by trace metals in the Arabian Gulf.

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