3.1 Assessment of permanganate index and oil and grease in water of Suez Bay
PI is a good indication of the chemical oxygen demand (COD) representing almost 40% of the total COD in water including PAH [8]. The main components of O&G values are the non-polar materials, which were also known as the petroleum-based hydrocarbons, and fatty compounds of bio-origin [9]. Figure (1) shows the distribution of PI and O&G in the SB coastal seawater, which are detailed in Table (S3). The highest observed PI value (16.0 mg O2 L-1) was in the site (1) whereas the lowest value (9.6 mg O2 L-1) in the site (2). The highest value of PI in the site (1) may be due harbor activities in the naval base. The average value was 12.4 mg O2 L-1, which is slightly lower than the oxydisable organic matter values reported for SB in 2013 (17.1 – 18.87 mg O2 L-1)[10], but it is remarkably lower than COD values reported in 2013 in Suez Gulf (36.8 – 296 mg O2 L-1)[11] and that reported for seawater near Damietta Port (22.5 – 585 mg O2 L-1)[12]. This findings indicates the improvement of water quality criteria in SB.
The highest O&G value (37.0 mg L-1) was observed in the site (6) then sites (12) and (2) whereas the lowest value (17.0 mg L-1) in the site (1). The highest value of O&G may be attributed to the harbor activities of Port Tawfik and Attaka and the poorly treated waste effluents of Nasr Petroleum Company (NPC). The NPC was reported to discharge oily wastewater into the Gulf of Suez with a discharge rate of 360000 m3/day. Suez Petroleum Manufacturing Co. (SPMC) was reported to have the water quality of their effluent almost in conformity with the restrictions of effluent standards in Egypt. However, 6060 kg of O&G flows daily in the receiving waters [13]. The average value was 25.6 mg L-1, which is lower than values obtained in 2016 (81.7 mg L-1) for the region from Port Tawfik to El Ain Sokhna [14]. However, the present values of O&G are within those obtained in spring and summer of 2013-2014 (14.7-28.0 mg L-1) in SB [10]. These fluctuation along the period from 2013-2018 may indicate the increasing efforts of water treatment compared with the increasing marine activities within the same period 2001-2019 [15]. However, more efforts should be given to reduce the O&G values to lower than the standard value stated for oily mixtures discharged from ships and unclean ballast water discharged from oil tankers, specified by the Law No. 4 [13].
3.2 Assessment of polycyclic aromatic hydrocarbon
The investigated PAH were 17 compounds; naphthalene (NAP), acenaphthene (ACE), acenaphthylene (ACEL), fluorine (FLU), phenanthrene (PHE), anthracene (ANT), carbazole (CAR), fluoranthene (FLA), pyrene (PYR), benzo(a)anthracene (ANT), chrysene (CHR), benzo(b)fluoranthene (bbFLA), benzo(k)fluoranthene (bkFLA), benzo[a]pyrene (bPYR), dibenzo(a,b)anthracene (dbANT)), indeno(1,2,3-c,d)pyrene (inPYR) and benzo(g,h,i)perylene (bPRL). In water the PAH values (Figure 3) ranges from 0 (below detection limit) in sites (5) Pelgrim Village and (10) NIOF to 114.0 ng L-1 (47.11% of the total PAH in the studied area) in the site (12) Attaka due to fisher boating. The 17 PAHs were distributed in water with the highest existence for PYR (34.52%), FLU (28.19%) and CHR (11.46%) whereas the CAR was totally absent.
The present highest values of NAP, ACE, ACEL, FLU, PHE, ANT, CAR, FLA, PYR, bANT, CHR, bbFLA, bkFLA, bPYR, dbANT, inPYR and bPRL in water; 1.41, 0.42, 1.80, 3.37, 2.95, 1.47, 0, 43.5, 42.3, 10.2, 9.89, 5.00, 1.96, 1.83, 2.36, 1.63, and 0.82 ng L-1 and their mean values 0.27, 0.03, 0.41, 0.48, 0.28, 0.11, 0, 5.24, 6.43, 1.45, 2.13, 0.88, 0.15, 0.15, 0.33, 0.21 and 0.06 ng L-1 in the water of the studied SB sites were lower than reported values such as 5.74, 2.73, 2.79, 4.85, 7.54, 3.45, 22.50, 16.47, 32.63, 8.82, 7.68, 10.58, 5.10, 16.61, 6.64 and 19.06 ng L-1 for NAP, ACEL, ACE, FLU, ANT, PHE, FLU, PYR, bANT, CHR, bbFLU, bkFLU, bPYR, dbANT, bPRL and inPYR, respectively, in Suez Canal within Suez region [16]. The present total PAHs ND – 114.00 ng L-1 were also remarkably lower than the reported values in Suez Canal (992.56 ng L-1) and Alexandria port (1364.59 ng L-1) but are still 10 times higher than that of the Sea of Japan [17].
The level of polyaromatic hydrocarbon in sediments (Figure 4) ranged from zero (below detection limit) in sites (10) NIOF and (13) Adabiya to 17669.34 ng g-1 (40.46% of the total PAH in the studied area) in the site (9) influenced by discharges of Attaqa Electrical Station. The major component of PAHs in the sediments was from CHR representing 91.93% (40141.42 ng g-1) of the 17 PAHs, which is one of the natural components in coal tar. Chrysene is a 4-ring member produced as smoke during partial combustion of coal, gasoline, junk, animal, and plant resources that may explain its presence near Attaqa Electrical Station. The concentrations of PAHs in sediment have been formerly classified as low, moderate, high and very high polluted (0–100, 100–1000, 1000–5000, >5000 ng g-1 [18]. Therefore, the surface sediments of Suez Bay can be classified to be low to be very highly contaminated in sites (8), (9) and (12).
For those sites where the concentrations were lower, it could be attributed to be the far distance from the point source occasioned with continuing dilution of the pollutant. The upstream sites were reported to record low concentrations of these pollutants probably due to dilution in the flow of water toward that direction [19]. On contrary, the high concentrations recorded in some sites could be ascribed to the closeness of such sites to point source such as heavy shipment of oil tanker and oil drilling activities concentrated at these regions.
The present mean values of NAP, ACE, ACEL, FLU, PHE, ANT, CAR, FLA, PYR, bANT, CHR, bbFLU, bkFLU, bPYR, dbANT. inPYR and bPRL in sediments; ND, 1.36, ND, 1.61, 7.58, 12.49, 19.43, 2.85, 116.91, 17.67, 3087.80, 12.80, 59.15, 11.05, 2.27, 1.76, 4.12 ng g-1, respectively, in the sediment samples of the studied SB sites were lower than reported values such as 14.03, 28.93, 38.69, 21.94, 87.99, 15.54, 125.08, 68.32, 104.08, 62.99, 35.05, 21.60, 37.43, 18.56, 25.89 and 12.89 µg L-1 for NAP, ACE, ACEL, FLU, PHE, ANT, FLU, PYR, CHR, bANT, bPYR, dbANT, bbFLU, bkFLU, bPRL and inPYR, respectively, in Suez Gulf [20]. The present total PAHs ND – 17669.00 ng g-1 were also remarkably lower than the reported values in Suez Gulf (195.53 – 1189.3 µg g-1) [20] and Alexandria port (88 to 6338 ng g−1) [21].
Comparing the major PAHs found in water and sediments, CHR was observed to retreat to the third highest concentration in water beyond PYR and FLU compared with sediment in which CHR with the major PAH. This may be attributed to the very low solubility of CHR in water (0.002 mg L-1) compared with PYR (0.135 mg L-1) and FLU (0.260 mg L-1) [22].
3.2 Source identification of PAHs in SB
The most important sources of PAHs in the marine environment are either petrogenic or pyrogenic. Petrogenic source originates from natural drain of petroleum or petroleum products into the environment, while pyrogenic originates from imperfect combustion of fossil fuels [23]. PAHs in literature were divided into the low molecular weight PAHs (LPAHs) / high molecular weight (HPAHs) ratio. The LPAHs include two to three rings whereas the HPAHs comprise four to six rings and are extremely carcinogenic [24]. With LPAHs/HPAHs > 1 indicates petrogenic while less than one indicates pyrogenic [25]. In the present seawater results for all 13 sites the ratios were less than one which is suggesting a pyrogenic origin prevalence of PAHs except for sites (2) and (4) where no existence was recorded for HPAHs indicating petrogenic origin whereas in sediment all 13 sites show LPAHs/HPAHs ratios less than one indicating a pyrogenic origin of PAHs as detailed in Table (2).
Furthermore, some molecular indices play a major role in establishing the origin of PAHs, e.g. PHE/ANT ratio among three-ring isomers and flourantherene/PYR ratio among four-ring isomers. They were chosen according to their thermodynamic stability; PHE and flouanthene are thermodynamically more stable. Thus, the PHE/ANT ratio of pyrogenic PAH assemblages usually are less than five and the petrogenic ratios usually are greater than five. The FLU/PYR ratio usually approaches or exceeds a value of one in pyrogenic assemblages and usually is substantially less than a value of one in petrogenic PAH assemblages [26]. According to the results in Table (S4), the PHE/ANT ratios are mainly lower than five in all sites, except for in water of site 12, which indicates a dominant pyrogenic nature of PAHs in sediments and water of SB, due industrial as well as shipment activities, which is in agreement with the conclusion taken from the above LPAHs/HPAHs ratios. On contrary, the flourantherene/PYR ratios exceeded one only at the sites 6, 8 and 11 that confirm the pyrogenic origin of PAHs in these sites, whereas the differences may be due to the continuous counter clockwise circulation of water in the SB [5].
3.4 Statistical evaluation of results
The present PAHs contents in water and sediments of SB were statistically compared with the water quality parameters recently published for the same samples [4]. Significant correlation was interestingly obtained between the TPAH in sediments and temperature of water that may indicate that the source of PAHs is influenced with a heat source such as cooling water. PI was also found negatively correlated with the chlorophyll (a) in water. This is an important indication that the presence of oxydisable matter is the cause of decrease in chlorophyll production [27]. The detailed statistical evaluation can be accessed in supplementary data as S4.