Heavy Metals Fractionation and Acid Volatile Sulfide (AVS) in the Bardawil Lagoon Sediments, Norther

Development in Analytical Chemistry Volume 2, 2015 doi: 10.14355/dac.2015.02.001

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Heavy Metals Fractionation and Acid Volatile Sulfide (AVS) in the Bardawil Lagoon Sediments, Northern Sinai, Egypt Mohamed A. Okbah∗1, Alaa M. Younis2 and Gehan M. El Zokm1 Marine Chemistry, National Institute of Oceanography and Fisheries, Alexandria, Egypt

1

Department of Aquatic Environment, Faculty of Fish Resources, Suez University, Suez, Egypt

2

∗1

m_okbah@yahoo.com; 2ala_den@yahoo.com; 1gehanelzokm@yahoo.com

Abstract Bardawil Lagoon in North Sinai, Egypt, is a unique Mediterranean semi-enclosed coastal water body that is listed among the Ramsar Wetlands of International Importance. The present study is to characterize the distribution of some heavy metals (Fe, Cu Pb and Cd) in Bardawil Lagoon sediments and evaluate the sediment quality based on heavy metals fractionation as well as acid volatile sulfide (AVS) and simultaneously extracted metals (SEM). Grain size analysis, carbonate (CaCO3) and organic matter content have been determined in surface sediments collected at 10 stations from the Bardawil lagoon. The results showed high percentage of CaCO3, ranged from 53.5 to 70.5%. The ratio of ∑SEM / AVS was higher than one in Bardwell lagoon sediments (∑SEM / AVS ranged from 1.28 to 9.19). These data would allow us to classify the sediments as toxic. The extractable concentrations of Fe, Cu, Pb and Cd were evaluated in the sediments using sequential extraction procedure. Metals concentration in the exchangeable and carbonate fractions was found in the following order: Fe > Pb > Cu > Cd, whereas they follow the order of Fe > Cu > Pb > Cd in the oxide fraction. In the organic form, metals had the sequence of Fe > Pb ≈ Cu >Cd. The sequences of metals concentration in the residual fraction were as follow: Fe > Pb > Cu > Cd The results of Pb and Cd fractionation reflect the dangers of these metals which more than 75% are associated with the non-residual fractions.

Keywords Acid Volatile Sulfide; Bardawil Lagoon; Heavy Metals

Introduction Bardawil Lagoon in North Sinai is one of the most important wetland sites in Egypt. It has been clearly demonstrated that large numbers of migratory birds pass through the area in both spring and autumn; it is an important site for wintering water birds (Dunnet and Crick, 1986). The study of heavy metal concentration in different forms in surface lagoon sediment is an important component in understanding the effect of anthropogenic influences on the aquatic environment. The distribution of metals within the aquatic environment is governed by complex processes of material exchange affected by various natural and anthropogenic activities (Ip et al.2007, Raju, et al. 2011). Heavy metals distribution in Bardawil lagoon sediments showed a common feature of increasing levels of Cu, Pb, and Cd in the western part of the lagoon while high levels of Fe, Zn and Mn in the eastern part (Lofty, 2003). In the bottom sediments of Bardawil lagoon high levels concentration were recorded during the hot seasons (Abdo, 2005), the metals can be introduced to coastal and marine environments through a variety of sources, including industries, wastewaters and domestic effluents (Fu and Wang, 2011).

Acid volatile sulfide (AVS) is an operationally defined reactive sulfide fraction that mainly comprises dissolved H2S and mackinawite (FeS), which is considered important for heavy metal fate in reduced sediments (Morse and Rickard, 2004). Heavy metals precipitation with sulfides in reduced sedimentary environments has received considerable attention (DiToro et al., 1992). Sulfides are formed by microbial reduction of SO2_4 when degradation of organic matter takes place under reduced conditions, after O2, NO-3 and Mn- and Fe (II) oxides are depleted (Stumm and Morgan, 1981). Acid volatile sulfide (AVS) has been used to predict the toxicity in sediments of divalent metals, including copper (Cu), cadmium (Cd), nickel (Ni), lead (Pb) and zinc (Zn) (Di Toro et al. 1992; Ankley et al. 1996; Berry et al. 1996). The SEM-AVS concept was developed to predict situations in which toxicity should not occur. Metals in sediment will not be toxic if the molar concentration of AVS is higher than that of SEM

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(SEM/AVS ratio smaller than 1) or if the difference between the molar concentrations of SEM and AVS (SEM-AVS) is used. Field studies on the validity of relations between SEM- AVS or SEM/AVS and ecological endpoints still are scarce but seem promising for its use in ecological risk assessment. Hare et al. (2001) found a good relationship of SEM/AVS and pore water trace metal concentrations.

In unpolluted sediments they are mainly bound to silicates and primary minerals. Such species are relatively immobile and usually not available for living organisms. In polluted sediments heavy metals are more mobile and bound to different phases of the sediment. The biogeochemical and ecotoxicological significance of heavy metals in bottom sediments is usually determined on the basis of the presence of particular species bound to specific phases of the bottom sediment, which determines their reactivity, and not on the basis of the coefficient of metal accumulation in the sediment (Helios-rybicka et al, 2000).

The aim of this study is to (1) characterize the distribution of heavy metals in sediments Bardawil Lagoon and (2) evaluate the sediment quality based on trace metals fractionation as well as acid volatile sulfide (AVS) and simultaneously extracted metals (SEM).

Material and Methods Study Area and Analyses of Sampling Bardawil Lagoon in North Sinai, Egypt, is a unique Mediterranean semi-enclosed coastal water body that is listed among the Ramsar Wetlands of International Importance. It is situated between W 32°40` and E 33°30` and between N 31°19` and S 31°03` (FIG.1). Bardawil Lagoon is a large, very saline lagoon in Egypt (49-65 PSU). The lagoon is extremely shallow; it extends for about 90 km and has a maximal width of 22 km the wet area about 650 Km2, the wet area about 650 Km2. The lake is very shallow (from 0.5 to 2 m deep), and thus warms up very quickly during summers and cools easily during winters (Howayda, et al., 2012).

Surface sediment samples from 10 sites were collected from Bardwell Lagoon (FIG. 1). Samples were taken from the central part of the grab sampler to avoid any metallic contamination from the metallic sampler and were frozen at −4◦C until further analysis. Sub-sample was taken to determine chemical and physical characteristics of the sediments such as grain size distribution, total organic matter and calcium carbonate content according to the methods described by Ingram (1970), Loring and Rantala (1992) and Gaudette et al. (1974),respectively. The total concentrations of heavy metals were determined according to Oregioni and Aston (1984) and measured using A.A.S (Perkin Elmer model 373). The sequential extraction procedures for different heavy metals fractions were adopted as shown in TABLE1.

The wet content of the sediment samples, necessary to calculate AVS concentrations on a dry weight basis, the wet content was determined by measuring the weight loss after drying at 105 ℃ for 24 h. The analysis of AVS and SEM were performed according to the method described by Allen et al. (1993).

TABLE 1 EXTRACTION PROCEDURES FOR DIFFERENT FRACTIONS OF HEAVY METALS (TESSIER ET AL., 1979).

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FIG. 1 MAP OF SAMPLING SITES (BARDAWIL LAGOON).

Quality Control Studies Standard Reference Material (IAEA-405): estuarine sediment (International Atomic Energy Agency, Vienna, Austria). Certified values of Fe, Zn, Cu, Ni, Pb and Cd are 37400, 279, 47.7, 32.5, 74.8, and 0.73 μgg−1, and their measured values are 38334, 252, 47, 31, 77, and 0.70 μgg−1, respectively. The recovery of the selected metals ranged from 90 to 104%. The precision was determined by three replicate analyses of one sample and expressed as a coefficient of variation, the result of the precision agreed within 10%. Duplicate measurements of AVS concentrations were done with an analytical precision around 10%.

Results and Discussion Physicochemical Characterization of the Sediments The results of physicochemical characterization of the sediments are listed in TABLE 2. The distribution of grain size in the sediments of the study area revealed the dominance of the sand fraction at four stations II, IV, VIII and IX and revealed sandy mud at the other stations except station I, which was covered mainly by mud (97% silt clay).

The results showed differences in the distribution of the total organic carbon content; it's varied from 0.51% at station VIII to 4.41% at station I. The carbonate content revealed its maximum value (89.4%) at station I. A significant value (57%) was recorded at station VI. In the other sites, the carbonate content revealed lower values ranging from 8.2 to 29.3%. The variation in the carbonate content was owed to the variability of the mineralogical components.

Heavy Metal Distribution The spatial distribution of heavy metals (Fe, Cu, Pb and Cd) in Bardwell Lagoon sediments are shown in Figure 2.

The metals content in the sediments decreased in the order of Fe > Pb > Cu > Cd. Descriptive statistics including average, maximum, minimum, and standard deviation are recorded in TABLE 2 and FIG. 2). The standard deviation is incorporated to reflect the degree of dispersion distribution of different metals and indirectly indicate the activity of the selected metals in the examined environment (Suresh et al., 2012). The measured heavy metal content varied greatly as follows: Fe, 3541-8103 μgg−1 with an average of 5036 ± 1618 μgg−1; Pb, 22.54 – 54.32μgg−1 with an average of 33.08 ± 10.95 μgg−1; Cu, 7.88 – 37.94μgg−1 with an average of 18.52 ± 11.43 μgg−1; Cd, 2.57 – 5.44 μgg−1 with an average of 3.99 ± 1.05 μgg−1.

Spatial Distribution of AVS and SEM Acid volatile sulfide (AVS) is an operationally defined reactive sulfide fraction that mainly comprises dissolved

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H2S and mackinawite (FeS), which is considered important for trace metal fate in reduced sediments (Morse and Rickard, 2004). A common notion is that the association of metals with acid volatile sulfides (AVS) affords a mechanism for partitioning metals from water to solid phase, thereby reducing biological availability. However, variation in environmental conditions can mobilize the sediment-bound metal and result in adverse environmental impacts (Tao et al., 2005). The concentrations of AVS and simultaneously extracted metals (SEM) in Bardawil Lagoon sediments are shown in TABLE 3 and FIG. 3. The results of AVS concentration showed irregular distribution in Bardawil Lagoon sediment. The high level was recorded at station I (2.175 µmoleg-1) and decrease to 0.128 µmoleg-1 at station VIII. Heavy metals in the sediments can be release to the overlying water as process of AVS oxidation in lagoon sediment, also related to Org-C and Fe–Mn oxides oxidation. Understanding AVS formation rates is important for the management of metal polluted sediment. In the present study, AVS concentrations were highly variable and may be related to surface water temperature and O2 concentrations as well as to sediment composition (Van Griethuysen et al., 2006).

TABLE 2 PHYSICOCHEMICAL CHARACTERIZATION AND TOTAL TRAC METALS CONCENTRATION OF BARDWELL LAGOON SEDIMENTS.

St.

Latitude

Longitude

I II III IV V VI VII VIII IX X Min Max Aver.

SD

31004\37\\ 31005\58\\ 31007\03\\ 31012\15\\ 31008\35\\ 31011\47\\ 31011\26\\ 31003\50\\ 31006\28\\ 31008\01\\

33013\36\\ 33015\03\\ 33016\51\\ 33015\41\\ 33015\40\\ 33009\20\\ 33005\54\\ 33000\02\\ 32056\49\\ 32055\47\\

Fe

Cu Pb Cd µgg-1 37.94 32.14 12.41 22.54 35.22 40.25 8.74 54.32 22.41 25.47 9.13 26.89 11.78 30.22 13.47 24.28 7.88 25.78 17.41 38.24 7.88 22.54 37.94 54.32 18.52 33.08 11.43 10.95

3745 3845 4518 4987 6233 8103 3644 4247 3541 5922 3541 8103 5036 1618

3.21 2.84 4.77 2.57 5.21 3.28 5.44 4.12 3.89 4.58 2.57 5.44 3.99 1.05

Concentrations

Concentration

Fe 10000 8000 6000 4000 2000

% TOC 4.41 1.02 1.68 0.6 2.06 1.41 0.68 0.51 0.98 1.16 0.51 4.41 1.62 1.33

I

II

III

IV

V

VI

VII

VIII

IX

% sand 3.00 97.55 15.04 100 34.42 22.93 37.78 97.57 97.50 31.20 3.00 100 53.33 39.54

% Silt+Clay 97.00 2.45 84.96 0.00 65.78 77.07 62.22 2.43 2.50 68.80 30.72 27.80 25.40 22.20

Type Sediment mud sand Sandy mud sand Sandy mud Sandy mud Sandy mud sand sand Sandy mud

60 50 40 30 20 10 0

0 I

% CaCO3 89.40 29.30 27.00 18.70 20.30 57.00 25.60 8.20 23.20 21.70 8.20 89.40 34.83 27.11

II

III

IV

V

VI

VII

VIII

IX

X

Stations

X

Cu

Stations

Pb

Cd

FIG. 2 SPATIAL DISTRIBUTION OF TOTAL TRACE METALS CONCENTRATION, FE, CU, PB AND CD (µGG-1) IN BARDWELL LAGOON SEDIMENTS TABLE 3 CONCENTRATION OF SEM-CU, SEM-ZN, SEM-CD, SEM-PB, SEM-NI, SEM-FE AND AVS (µMOL/G, DRY WEIGHT) IN BARDWELL LAGOON SEDIMENTS

4

St.

SEM-Cu

SEM-Zn

SEM-Cd

SEM-Pb

SEM-Ni

∑SEM

AVS

I II III IV V VI VII VIII IX X Min Max Aver.

SD

0.245 0.088 0.193 0.026 0.176 0.100 0.032 0.065 0.185 0.283 0.026 0.283 0.142 0.094

2.274 1.440 2.307 0.705 0.909 0.713 0.928 0.853 1.400 1.016 0.705 2.307 1.296 0.621

0.016 0.009 0.018 0.004 0.006 0.004 0.026 0.005 0.013 0.014 0.004 0.026 0.012 0.008

0.082 0.041 0.083 0.021 0.030 0.042 0.029 0.040 0.069 0.125 0.021 0.125 0.059 0.036

0.158 0.086 0.157 0.028 0.106 0.076 0.083 0.046 0.114 0.144 0.028 0.158 0.099 0.047

2.775 1.664 2.758 0.784 1.227 0.935 1.098 1.009 1.781 1.582 0.784 2.775 1.598 0.745

2.175 0.455 0.300 0.289 0.516 0.291 0.298 0.128 0.495 0.276 0.128 2.175 0.627 0.702

∑SEM/ AVS 1.276 3.657 9.193 2.713 2.378 3.213 3.685 7.883 3.598 5.732 1.276 9.193 4.483 2.729

∑SEM -AVS 0.600 1.209 2.458 0.495 0.711 0.644 0.800 0.881 1.286 1.306 0.495 2.458 1.112 0.662

SEMFe

68.556 17.238 24.883 1.454 28.292 18.045 27.501 9.687 30.181 38.646 1.454 68.556 27.875 21.168

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Concentration

3.00 2.50 2.00 1.50 1.00 0.50 0.00 I

II

III

IV

V

VI

VII

VIII

IX

X

Stations ∑SEM

AVS

FIG. 3 SPATIAL DISTRIBUTION OF ACID VOLATILE SULFIDE (AVS) A SIMULTANEOUSLY EXTRACTED METALS (SEM) CONCENTRATION (µMLOE G-1) IN BARDWELL LAGOON SEDIMENTS

Comparison of the metals between the sediments revealed significant variation between the different stations; this may be related to the different content of organic matter, high positive correlation between the concentration of AVS and the content of Total organic carbon (r =0.96, n=10, p<0.05). Zheng et al., (2004) showed that the production of AVS in aquatic systems is mediated by sulfate –reducing bacteria, which reduce inorganic sulfate to sulfide. The averages concentrations of heavy metals (SEM; simultaneously extracted metals) in the investigated area were 0.026-0.283(0.142 ± 0.09) µmoleg-1 dry weight for Cu; 0.705 - 2.307 (1.296 ± 0.621) µmoleg-1 dry weight for Zn; 0.0040.026 (0.012 ± 0.008) µmoleg-1 dry weight for Cd; 0.021 – 0.125 (0.059 ± 0.036) µmoleg-1 dry weight for Pb; 0.028 – 0.158 (0.099 ± 0.047) µmoleg-1 dry weight for Ni. Generally, the concentrations of heavy metals were in the order of Zn > Cu> Ni >Pb > Cd. Among the metals, Zn and Cu were the dominating species, accounting for about 90 % of SEM. To test the toxicity of the sediments, we calculated the SEM / AVS ratio. We compared the ∑SEM of Zn, Cu, Ni, Pb and Cd with the AVS concentration. The ratio of ∑SEM / AVS was higher than one in Bardwell lagoon sediments (∑SEM / AVS ranged from 1.28 to 9.19). These data would allow us to classify the sediments toxic. It is widely recognized that for sediments containing a molar excess of simultaneously-extracted metals (SEM = Σ (Cd, Cu, Ni, Pb, Zn) over acid-volatile sulfide (AVS), the pore water concentrations of these metals will be negligible and acute or chronic effects should not result from these metals (Berry et al., 1996; USEPA, 2005; Simpson and Spadaro, 2011). The metal concentrations in excess of the binding capacity attributed to AVS may be bound by other solid phases, including particulate organic carbon (POC) and iron and manganese oxyhydroxide phases, or partition to the pore waters (Di Toro et al., 2005; Simpson and Batley, 2007).

Fractionation of heavy metals Fractionation of heavy metals concentration (Fe, Cu, Pb and Cd) in Bardwell Lagoon sediments is recorded in TABLE 4 and the relative percentage is illustrated in Figures4. Metals concentration in the exchangeable and the carbonate fractions were found in the following order: Fe > Pb > Cu > Cd, whereas they follow the order of Fe > Cu > Pb > Cd in the oxide fraction. In the organic form, metals had the sequence of Fe > Pb ≈ Cu >Cd. The sequences of metals concentration in the residual fraction were as follow: Fe > Pb > Cu > Cd (TABLE 4).

TABLE 4 MIN., MAX. AVERAGE AND SD OF HEAVY METAL CONCENTRATIONS (µGG−1) IN EACH FRACTION OF BARDWELL LAGOON SEDIMENTS.

Metals Fe

Cu

Pb

Cd

Min Max Aver.

SD Min Max Aver.

SD Min Max Aver.

SD Min Max Aver.

SD

Water Ext. 0.05 8.82 2.81 2.88 0.08 2.10 0.49 0.74 0.74 4.98 2.59 1.58 0.03 0.55 0.23 0.18

Exch 3.11 25.66 9.08 8.23 0.23 4.64 1.40 1.51 1.14 8.21 4.18 2.36 0.17 0.84 0.45 0.24

Carb.

10.95 469.20 98.35 166.10 1.11 9.61 4.32 3.40 4.01 9.94 6.60 2.20 0.24 1.17 0.69 0.35

Oxides 26.32 1971.23 899.57 740.96 0.27 8.55 2.59 2.98 1.59 10.41 5.30 2.98 0.67 1.54 1.24 0.42

Organic 220.90 911.54 544.94 260.01 0.41 11.86 4.10 3.94 1.49 12.04 5.19 3.91 0.12 1.10 0.55 0.40

Sum 258.29 3386.45 1553.17 1067.60 2.09 36.76 12.89 11.79 8.96 45.58 23.85 9.15 1.23 5.20 3.15 1.33

Residual 3282.71 4716.55 3482.58 1550.70 5.79 1.18 5.62 3.60 13.58 8.74 9.23 8.33 1.34 0.24 0.84 0.59

Total 3541.00 8103.00 5035.75 1618.05 7.88 37.94 18.52 11.43 22.54 54.32 33.08 10.95 2.57 5.44 3.99 1.05

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Iron (Fe) is mostly concentrated in the residual fraction, it represented >70% of the total concentration in stations I, II, VI, VIII, IX and X, whereas it was represent around 50% in stations III, IV, V and VII. The other important Fe phase is the non-residual form. Associations of Fe with different fractions in the study area are given in TABLE 4 and FIG.4. Fe content in the five fractions (TABLE 4) can be arranged in the following order: residual (3482μgg−1) > Fe–Mn oxides (899 μgg−1) > organic form (545 μgg−1) > carbonate (98 μgg−1) > exchangeable and water soluble (11.9 μgg−1).

Copper (Cu) is concentrated in the non-residual fraction (the exchangeable, carbonate, oxides and organic fractions constitute) rather than in the residual form (FIG. 4 and TABLE 4), indicate that Cu is from an anthropogenic source.

The exchangeable and water soluble fractions were detected in the study area; it ranged from 0.08 to 2.10 μgg−1 with an average (0.49 ±0.74 μgg−1). Average values of Cu in the five fractions of the study area are shown in TABLE 6. They are arranged as follows: residual (5.62 μgg−1) >carbonate (4.32 μgg−1) >organic (4.10 μgg−1) > Fe–Mn oxides (2.59 μgg−1) > exchangeable and water soluble (1.89 μgg−1).

Lead (Pb) in our study area is a major part in the non-residual fraction. The concentration of Pb bound in the nonresidual fraction (labile form) varied from 38% at station IV to 99% at station V and reached more than 75% from the total at stations I, III, VII, VIII, IX and X. Cu

Fe 120

100

100

80 60

80

% 40

% 60

20

40 0 I

20 0 I

II

III

IV

V VI Stations

VII VIII IX

X

II

III

IV

V VI Stations

VII VIII

IX

X

Water Ext.

Exch

Carb.

Oxides

Organic

Residual

Pb

Cd

120

120

100

100 80

80

% 60

% 60

40

40

20 0

20

I

0 I

II

III

IV

V VI Stations

VII VIII

IX

X

II

III

IV

V VI Stations

VII VIII

IX

X

Water Ext.

Exch

Carb.

Oxides

Organic

Residual

FIG. 4 PERCENTAGE OF TRACE METALS (FE, CU, PB AND CD) IN EACH FRACTION TO THE TOTAL CONTENT OF BARDWELL LAGOON SEDIMENTS.

The next important phase of Pb concentration is the residual fraction. The association of Pb with different fractions is shown in FIG. 4 and TABLE 4. As shown in TABLE 4, the average values for the five fractions from the study area are arranged as follows: residual (9.23 μgg−1) > exchangeable and water soluble (6.77 μgg−1) > carbonate (6.60 μgg−1) >Fe–Mn oxides (5.30 μgg−1) > organic form (5.19 μgg−1). Metals which are associated in the residual fraction are at low risk of mobility (Filgueiras, et al., 2004). The data obtained in the exchangeable and water soluble Pb fraction ranged from 1.88 and 13.19 with an average (6.67 ± 3.94 μgg−1). This indicates that Pb is highly distributed in the study area. The large amounts of Pb in the exchangeable fraction (FIG. 4) indicate that Pb is from an anthropogenic source. The average concentration of Pb associated with carbonate is 6.60 ± 2.20 μgg−1, the high concentration is recorded at station VIII (9.94 μgg−1). Pb associated with the exchangeable and carbonate fractions is considered to be weakly bound and may equilibrate with the aqueous phase, becoming more bioavailability (Dong, et al., 1998). These data might reflect the dangers of Pb in the study area. Also, Pb bound to the organic

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fraction and the Fe–Mn oxides fraction. Organic matter and Fe–Mn oxide have a high scavenging effect and may provide a sink for Pb (Dong, et al., 1998; Okbah et al., 2011). This indicates a large amount of Pb discharging into the water of the study area Cadmium (Cd) fractionation revealed that a major part is found to be associated with the Fe–Mn oxides compared with the organic, exchangeable and carbonate phases. Cd in the non-residual fraction (the exchangeable, carbonate, oxides and organic fractions were 79% of the total Cd in the study area. Cd bound to the exchangeable and water soluble fractions were high (17% from the total Cd). Associations of Cd with the different fractions are given in TABLE 4 and FIG.4. The average values of Cd in the five fractions are shown in TABLE 4. They are arranged as follows: Fe–Mn oxides (1.24 μgg−1) >residual (0.84 μgg−1) > carbonate (0.69 μgg−1) = exchangeable (0.68 μgg−1) > organic form (0.55 μgg−1).

Risk Assessment Code Risk assessment code has been used to assess environmental risks and estimate possible damage to benthic organisms caused by contaminated sediments (Passos et al., 2011). According to the Risk Assessment Code (RAC) (Perin,et al.,1985; Singh, et al.,2005), the metals in the sediments are bound to the fractions with different strengths.

Because there are differences in the toxic effects of the metals, as well as differences in their concentrations and the length of exposure (Mohammed and Markert 2009), the RAC assesses the availability of metals in sediments by applying a scale to the percentage of exchangeable and carbonate fractions. This is important because fractions introduced by anthropogenic activities were typified by being adsorptive, exchangeable and bound to carbonate fractions, i.e. weakly bonded metals that might equilibrate with the aqueous phase. This classification is given in TABLE 5. This criterion of RAC indicates that sediment which can release <1% of the total metal in exchangeable and carbonate fractions is considered safe to the environment. By contrast, sediment releasing >50% of the total metal in the same sediment is considered highly dangerous and can easily enter the food chain (Jain, 2004). The RAC for Fe showed no to low risk, whereas Cu, Pb and Cd showed medium to high risk for the aquatic environment (FIG.5).

TABLE 5 RISK ASSESSMENT CODE (RAC)

Risk

Criteria (%)

No risk

<1

Low risk

1 - 10

Medium risk

11 - 30

High risk

13 - 50

Very high risk

> 50

This Study % of Exch. + Char.

Fe

Cu

Pb

Cd

0.4 – 6.11

17 – 37.6

23 – 33

16 - 37

Fe

Cu

Pd

Ave.

Min.

Max.

Ave.

Max.

Min.

Ave.

Max.

Min.

Ave.

Min.

40 35 30 25 20 15 10 5 0 Max.

Exch + Carb (%)

Criteria (%) [Perin,et al.,1985; Singh, et al.,2005]

Cd

FIG. 5 RISK ASSESSMENT CODE (RAC)

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Conclusions Acid volatile sulfide (AVS) is usually one of the most important or reactive phases. We report first set of data on AVS contents in Bardwell lagoon sediments The AVS contents ranged from 0.128 to 2.175µmolg-1 (dry sediment weight).The ratio of ∑SEM / AVS was higher than one in the sediments, these allow us to classify the sediments toxic. Trace metals (Cu, Pb and Cd) in our study area are a major part in the non-residual fraction (labile form), these represent more than 60% from the total concentration. The Risk assessment code (RAC) for Fe showed no to low risk, whereas Cu, Pb and Cd showed medium to high risk for the study area.

ACKNOWLEDGMENTS

The authors would like to thank Prof. Dr. Mamdouh Fahmy, National Institute of Oceanography and Fisheries, Alexandrina, for helping us in sampling collection.

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