Accumulation and distribution of heavy metals in surface sediments from the continental shelf adjacent to Abu Qir Bay, Egypt, as a function of grain size

Estuarine-deposited sediment is an essential environmental medium that is widely used for characterizing anthropogenic contaminant levels in urban systems. It has significant implications for human health, and can directly impact aquatic biota. Assessment of Cr, Fe, and Cd contamination and their distribution in different grain size fractions of the surface sediments of Abu Qir Bay, one of the Mediterranean coastal bays, as an industrial development area has been investigated. Since the metals pose serious health risks, this research concentrated on investigating the concentrations and spatial distribution of Cr, Fe, and Cd in Abu Qir Bay sediments. Surface sediment samples were sieved, and six grain size fractions (1.0, 0.5, 0.25, 0.125, 0.063, and <0.063 mm) from each sample were selected for the analysis of studied metals. The data indicates a considerable contamination in all samples by Cr, Fe, and Cd. The finest sediment fractions (0.063 and <0.063 mm) contained the highest concentrations of Cr and Fe. The concentration of these metals was four times greater than in coarse-grained fractions. Silt plus clay (<0.063 mm) was the most critical mass component consisting of more than 40% of the total sediment stored in this fraction. Otherwise, the distribution of Cd in the sediment samples reveals high concentrations associated with the large fractions (1.0 and 0.5 mm) at all sediment samples.


Introduction
Marine sediments can be sensitive indicators for monitoring contaminants in aquatic environments. (Atgin et al. 2000;Pekey et al. 2004). Heavy metals in marine sediments have natural and anthropogenic origin: distribution and accumulation are influenced by sediment texture, mineralogical composition, reduction/oxidation state, and desorption processes and physical transport. Moreover, metals can be absorbed from the water column onto fine particles surfaces and move thereafter towards sediments; metals participate in various biogeochemical mechanisms, have significant mobility, can affect the ecosystems through bio-accumulation process, and are potentially toxic to the environment and for human life (Manahan 2000).
During the last few decades, industrial and urban activities have contributed to the increase of metal contamination in the marine environment and have directly influenced the coastal ecosystems (Buccolieri et al. 2006). Several studies have demonstrated marine sediments from industrialized coastal areas that are greatly contaminated by heavy metals; therefore, the evaluation of trace metals concentration in surface sediment is important for assessing the extent to which the marine environment is contaminated. Furthermore, the sediments reflect a record of past contamination. Usually, in recently polluted areas, concentrations of trace metals in the surface layer are higher than in deeper layers of sediment (Bellucci et al. 2002;Zonta et al. 1994).
The traditional concept of the relationship between metal content and grain size assumes that the fine fraction carries most of the metals in natural sediments. This concept is supported in many cases by strong, significant linear relationships between total-sediment metal concentrations and percentages of various fine-size fractions. Such observations have led to development of methods to correct for the effects of grain size in order to accurately document geographical and temporal variations and identify trends in metal concentrations away from a particular source (Pekey et al. 2004).
The Mediterranean Sea is an area where sediments have different geochemical composition: metal concentrations vary according to the area and different inputs from the coastal environment (Buccolieri et al. 2006). Since Abu Qir Bay is a dynamic area, the several land based effluents cause continuous changes in its ecological characteristics, which are tightly related to the variability (seasonal and/or inter-annual) of the volume and quality of the discharged wastes. These changes undoubtedly affect sediment characterization. Therefore, the present study explores the relationship between the concentration of studied metals in sediments and the grain size composition. The study was also supposed to reveal which fractions are mostly responsible for sediment pollution.

Study area
Abu Qir Bay is one of the Mediterranean coastal bays, and also is a part of the Nile Delta cone, a semicircular inlet of the Mediterranean Sea, located in the southeastern Mediterranean Sea (Fig. 1) latitudes 31° 16′ and 31° 21′ N, longitudes 30° 5′ and 30° 22′ E. The bay has a maximum depth of 20 m and a surface area of 360 km 2 with a shoreline of about 50 km long (Al-Hogaraty et al. 2005;Abdel-Moati 1997). The topography of Abu Qir Bay is characterized by a smooth bottom nature in most parts, but with many subsurface ridges in the extreme west (Faragallah 2004;Abdallah and Mohamed 2015). The bay receives about 5.12 × 10 6 m 3 day −1 mixed wastes from three land-based sources. The southwestern region of the bay receives about 1.52 × 10 6 m 3 day −1 of a mixture of industrial wastes from four main activities, fertilizer industry, textile manufacturing, food processing, and canning wastes as well as paper mill effluents in addition to untreated sewage discharged from Abu Qir drain and dumped into the bay through the "Tapia pump station." In addition, about 2 × 10 6 m 3 day −1 of agricultural drainage waters were discharged into the bay through the opening of Lake Edku (brackish water) directly to the bay and finally at the far east side of Abu Qir Bay. The third land-based source is Rosetta Branch mouth (is rather a continuous source of poor-quality water, rate 1.2 × 10 6 m 3 day −1 ) but with an occasional pulse of the Nile water (Abdel-Moati 1997).

Sampling
Surface sediment samples (0-5 cm) were collected during two sampling campaigns from ten stations in Abu Qir Bay. The selected ten stations were representing the southwestern area of the Bay (20 km 2 ) including five stations (1, 2, 3, 5, and 9) close to the inshore and the other five stations (4, 6, 7, 8, and 11) collected in a parallel line from the offshore (Fig. 1). Samples were collected with a van Veen grab. The samples were then well mixed to form a composite sample for each station. A sample was taken from the center with a polyethylene spoon to avoid contamination by the metallic parts of the sampler. The grain size was analyzed using a mechanical sieves methods, Samples were then stored in plastic cups that were cleaned by 1:1 HCl and 1:1 HNO 3 and were stored frozen at 4 °C until analysis (American Society for Testing and Materials (ASTM) 1991).

Fractionation
Sediment subsamples were wet fractionated using sieves (monofilament polyester) of different pore sizes. Before sieving, large calcareous debris and rock fragments were carefully removed by plastic tweezers. The following sediment fractions were prepared and analyzed: A: 1.0 mm; B: 0.5 mm; C: 0.25 mm; D: 0.125 mm; E: 0.063 mm, and F: <0.063 mm. Each fraction was dried at 90 °C, over 8 h, until a constant weight was reached.

Chemical analysis
About 0.3 g of dried sediment sample (from each of the sediment fractions) was decomposed using a mixture of concentrated nitric acid (HNO 3 ) hydrofluoric acid (HF) and perchloric acid (HClO 4 ) (10:8:4) in a Teflon vessel heated at 160 °C over 24 h (Loring and Rantala 1992) and brought into solution in 0.5M HCl (25 ml) using deionized double distilled water (DDDW). All chemicals were supplied by MERK and Prolabo Central Drug House (P) Ltd., Egypt, and were of analytical reagent grade.
Samples were analyzed using an air-acetylene flame atomic absorption spectrophotometer Shimadzu Model AA-6800, Duisburg, Germany, with D (subscript 2) background correction and an autosampler. The reagent blanks were monitored throughout the analysis, and they were used to correct the analytical results. Calibration standards were regularly performed to evaluate the accuracy of the analytical method. Results are reported in μg/g on a dry weight basis. Total organic carbon (TOC) was determined by titration with FAS (ferrous ammonium sulphate) after digestion with K 2 Cr 2 O 7 -H 2 SO 4 solution (Lu 2000). The accuracy of the analytical procedure was checked using a triplicate analysis of certified reference material (BCSS-1) from the National Research Council of Canada. Analytical results indicate a good agreement between the certified and measured values and metal recovery being practically complete for most of them (Table 1).

Statistical analysis
Descriptive statistics (means and standard deviations) and correlation analyses were performed using the SPSS 12 statistical software (IBM, Armonk, NY, USA). The standard deviations of pooled samples (sediments) refer to the variability within different replicates. A p value of less than 0.05 (p < 0.05) was considered to indicate statistical significance. The detection limits of Cd, Cr, and Fe in sediment were 0.01, 0.2, and 0.005 μg/g, respectively.

Geochemical characterization of the bay sediment
Profile mean size values (ф) distribution and TOC % in the sediments from Abu Qir Bay are given in Table 2. The average grain size ranges from 2.69 ф (fine sand) to 3.72ф (very fine sand) with average of 3.25 ф (very fine sand)). It is well established that grain size is one of the controlling factors affecting natural concentrations of trace metals in sediments (Zhang et al. 2001). The results reveal narrow variation in textural composition observed in the sediments of all sites; the sediments were predominately comprised with sand silty clay at all locations (Table 2). A slightly high content of the fine sediments particles were shown in the front of Tapia outfall due to the mixing between sediments with fine precipitation mostly cellulose fiber materials mainly from the paper industrial effluent brought with Tapia discharge. Horowitz and Elrick (Horowitz and Elrick 1987) reported that fine to very fine-grained sediments tend to have relatively high metal concentrations due in part to the high specific surface area of the smaller particles. As TOC plays an important role in controlling the availability of inorganic and organic contaminants and the toxicity of sediments (McGrath et al. 2002), its concentrations ranged from 0.14 to 0.85%, the average TOC being 0.48% (Table 2). The highest TOC value was observed at sampling site 3 due to anoxic conditions occurred in sediments consequent The skewness (SK) values reveal that the dominated is symmetrical in nature and represented by 60% followed by both coarse skewed and fine skewed which is represented by 20% for both. The near-symmetrical nature zone is attributed to the absence of extreme conditions like tidal variations, wave breaking, and seasonal supply of detrital materials. On the other hand, the fine skewed zones resulting in such a high-energy environment indicate excessive riverine inputs represented by El Tabia huge drain in the study area. All samples were kurtosis classified as very platykurtic ( Table 2). The extremely low values of kurtosis suggest that part of the sediment is sorted elsewhere in a high-energy environment (Friedman 1967). The dominant very platykurtic nature of sediment is likely due to the continuous addition of finer/coarser materials in varying proportions (Prabhakara et al. 2001).

Distribution of heavy metals in different fractions of sediments
The metal content in different grain size fractions of surficial sediment collected from Abu Qir Bay from 10 sampling sites is given in Table 3 and Fig. 2. In general, the study sediments with grain size diameter from 1.0 to 0.250 mm represent only 10% of the whole sediment spectrum, while the fine sand (0.125 mm) representing about 23% of the bay sediments. It is worth mentioning that the highest percentages (55%) of grains in most of the samples are those having a diameter 0.063 mm (very fine sand) of the sediments. On the other hand, the sediment grains of <0.063 mm (silt and clay) represents 12% of the whole bay sediments.
The distribution pattern of TOC with the different grain size fractions in Abu Qir Bay is quite reverse to that mentioned for mud (silt and clay fractions) percentages (Fig. 3). Under natural conditions, the muddy sediments (<0.063 mm) are often of high percentage of TOC and vice versa. This fact however is not clearly evident here, because of the contamination of the relatively coarser sediments in the front of Tapia pumping station outlet with the allochthonous organic matter (Faragallah 2004).
The percentages of grains higher than 1.0 mm is negligible; they are absent in most of the samples. Generally, samples collected from the near shore sediments consisted mostly of the fine grained 0.063 and <0.063 mm (silt and mud fractions) fractions, while off shore sediment samples consisted mostly of coarse and very fine sand grained fractions (1.0-0.125 mm) due to very strong recent impact of untreated urban wastewater depositing a high amounts of organic matter accompanied by silt and clay (Al-Hogaraty et al. 2005).
The results indicated that the grain size was the main control factor in the distribution of the studied heavy metals in bottom sediment samples collected from Abu Qir Bay. The results indicated the highest content of total Cr and Fe in the silt/clay fraction (<0.063 mm); the lowest concentrations were found in the Bay sediments of >0.063 mm fraction in most sites (Table 3). This is a common concentration pattern documented in the literature, and is usually explained by increased sorption capacity as specific surface area increases with decreasing grain size (Förstner 1989;Vdović et al. 1991;Abdallah 2013). Heavy metal concentrations in bottom sediments increased along with decreasing the particle size: the highest contents were observed in silty fractions. Sediments containing higher percentages of fine grained fractions have higher specific surface area, and surface processes such as adsorption and adhesion of dissolved and colloidal species are more intensive in this type of sediment compared to sediments of coarse-grained particles (Wardas 1998). The finest silt/clay fraction (< 0.063 mm) of the studied sediment is believed to be the main potential mobile carrier of Cr and Fe pollutants of Abu Qir Bay. This may be attributed to an anthropogenic source since the studied stations are in the area of the discharge of untreated urban and industrial wastewaters.
The results of the present study related to the analysis of combined fraction which consisting of grains < 0.250 mm (very fine sand, silt, i.e., 0.125, 0.063 mm) that is dominant in this area. Therefore, high values of correlation coefficients were observed between the total metal of Cr and Fe in the combined sediment fraction (< 0.250 mm) and the amount of the finest fraction in this combined fraction (Fig. 4), with correlation coefficients 0.658 and 0.740, respectively. The correlations are significant at (p < 0.05) level for Cr and Fe. Comparing the concentrations of Cr and Fe in the sediment size fractions at all stations, it is evident that the sediment from the most contaminated area (Station 9) at the front of Tapia pump station accumulate Cr and Fe to a much higher level than sediment from other offshore  , where surface wave impact on the sediment resuspension is considered significant (Figs. 5 and 6).
Otherwise, the distribution of Cd in the sediment samples reveals high concentrations associated with the large fractions (1.0 and 0.5 mm) at all sediment samples from Abu Qir Bay, while the lowest concentrations were determined at sediment fractions <0.5 mm, as well (Table 3). The concentration of Cd over size fractions shows increases with increasing grain size in all stations, and increases with decreasing grain size in site 1 (Fig. 7). This may be attributed to an anthropogenic source since this station is in the area of the discharge of untreated urban and industrial waste waters (Abdel-Moati 1997). Rohatgi and Chen (Rohatg andChen 1975 andStevenson 1986) indicated that Cd has the strongest affinity, among other metals, to be released from the adsorption sites by the formation of soluble inorganic and organic complexes. This may explain the same behavior of cadmium and organic carbon in terms of the lack of the smallest grain size fractions.
Since the different sediment fractions have various motilities in the marine environment, and trace metal distribution on various sediment fractions can vary for different metals, it is possible to expect that trace metal distribution can vary from fraction to fraction; i.e., it is inhomogeneously distributed between different size fractions of particles (Krumgalz et al. 1992). The present data reveals that Cr and Fe are preferentially associated with smaller particles, but Cd favors larger ones; this is in agreement with a previous study by Faisst, W.K. (Faisst 1976).
Assessing the condition of a coastal bay environment the information on metal mobility cannot be obtained from its total content in bottom sediments. The labile part of metals mostly has anthropogenic and biogeochemical origins. From the environmental point of view, this part of metals is very important because due to metal movement, it can be desorbed from bottom sediments to water and then accumulated in benthic organisms (Dembska et al. 2001). The average content of heavy metals in the sum of labile fractions (0.063 and <0.063 mm) occurred in the following order: Fe > Cr > Cd. The highest concentrations of labile forms of the examined elements, similar to the total forms, were observed in the fraction < 0.063 for Cr and Fe, but in fraction >0.5 for Cd. The ratio of a labile to total form concentrations in the studied bottom sediments, expressed in percent, followed the order Cr > Fe > Cd. It reflects the mobility of the studied metals as well as their anthropogenic origin (Carral et al. 1994). The research revealed a relatively low content of Cd labile form in its total concentration, which in most of the studied samples reached from 7 to 46%. In turn, Cr is one of the most mobile metals and it was confirmed by this study; the percentage of a labile form in its total content in most of the studied sediments varied between 39 and 85%. The labile forms of Fe constituted on average from 30 to 50% of its total concentrations. Those increased contents were mostly caused by anthropogenic influences, runoff from inappropriately fertilized agricultural areas (Maadia outfall and Abu Qir factory for fertilizers), or discharges from sewage treatment plants (El Tapia pump station).

Conclusion
About 78% of the grain size fractions in all studied samples belonged to grains between 0.125 and 0.063 mm. Sediments with diameter of 1.0 to 0.250 mm represent about 10% of the whole sediment spectrum, while the fine sands (0.125 mm) represent about 23% of the Bay sediments. It is worth mentioning that the highest percentages of grains in most of the samples are those having a diameter of 0.063 mm (very fine sand) accounting for about 55% of the sediments. On the other hand, the sediment grains of <0.063 mm (very fine sand) represents 12% of the whole bay sediments. The finest silt/clay fraction (< 0.063 mm) of the studied sediment is believed to be the main potential mobile carrier of Cr and Fe pollutants of Abu Qir Bay. Otherwise, the distribution of Cd in the sediment samples reveals high concentrations associated with the large fractions (1.0 and 0.5 mm) in all sediment samples from Abu Qir Bay. The research revealed relatively low content of Cd labile form in its total concentration, which in most of