Historical changes of heavy metals content and sequential extraction in a sediment core from the Gorgan Bay, Southeastern Caspian Sea

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1 Indian Journal of Marine Sciences Vol. 37(3), September 2008, pp Historical changes of heavy metals content and sequential extraction in a sediment core from the Gorgan Bay, Southeastern Caspian Sea Abdolreza Karbassi 1, Mohsen Saeedi 2* & Reza Amirnejad 3 1 Faculty of Environment, University of Tehran, P.O.Box , Tehran, Iran 2 Environmental Research Laboratory, Department of Hydraulics and Environment, College of Civil Engineering, Iran University of Science and Technology, P.O.Box , Narmak, Tehran, Iran 3 Faculty of Energy and Environment, Science and Research Branch, Azad University, Tehran, Iran Received 7 August 2006; revised 16 August 2007 In the present investigation sedimentation rate, contents of heavy metals (Zn, Co and Ni), enrichment factors and speciation of Zn, Co and Ni of a sediment core from Gorgan Bay have been studied. Sedimentation rate in the study area has given an ample opportunity to track contents of Zn, Co and Ni with different sedimentary phases for the past 500 years (1500 t0 2002). Sedimentation rate of 1.4 mm/yr was obtained based on 210 Pb activity study of sediment core. A very low content of Al and Fe in the core compared to those of the mean crust was observed. Heavy metal contents increase towards the top of the sediment core. Chemical partitioning studies revealed that percentiles and amounts of Zn, Ni and Co in nonlithogenous phases increase slightly towards the top of the core sediment sample. There seems to be a slight increasing trend in pollution level of the sediments of the study area over the last 70 years. Further, despite many research reports, application of Enrichment Factors (EFs) for determination of origin of heavy metals in sediments and pollution detection may lead to incorrect results due to naturally lower concentration of Al than the mean crust and/or higher contents of heavy metals in sediments than those in the crust, as the EFs in the present investigation were high even in 17th century when there were no significant sources of pollution. The results of Igeo values, on the other hand, show that minor elements fall within non-polluted classification which can obviously show the error of the enrichment factor calculations with the use of Al as earth reference element. [Key words: Caspian Sea, Gorgan Bay, heavy metals, sediment core, chemical partitioning, enrichment factor,] Introduction Minor element pollution in lake, estuary and bay sediments caused by industrialization has been reported earlier 1-3. Geochemical studies of sediment cores are helpful in the assessment of pollution, changes in climate conditions and rate of sedimentation in aquatic environments 4. Most minor metals tend to enrich in the modern organic sediments rather than inorganic sediments 2,5,6. Sediment cores have been used to study the behavior of metals over time 1,7-10. Similar studies are scanty in the Caspian Sea region. In recent years, oil exploration and exploitation have been of main concern in The Caspian Sea region that may lead to environmental pollution. Thus, study of sediment cores from the adjacent bays may provide useful information for any Corresponding author: Mohsen Saeedi, Department of Hydraulics and Iran University of Science and Technology, Tel: , Fax: , [ msaeedi@iust.ac.ir]. future activity that may cause pollution. In the present investigation, sedimentation rate, enrichment factors and sequential extraction of Zn, Co and Ni of a sediment core from Gorgan Bay has been studied, with the objective of knowing past and present conditions. The sedimentation rate in the study area was determined to take an opportunity to track contents of Zn, Co and Ni within different sedimentary phases over the past years. Materials and Methods To the east of Caucasus lies the largest salt lake in the world: the Caspian. It is a rather long narrow body of water stretching about 1200 km from north to south and having average width of 300 km and covers an area of about 389,400 km 2. On a geological time scale, the Caspian Sea used to connect to the world oceans via the Sea of Azov, the Black Sea and the Mediterranean, but today it is landlocked. The ecology of the Caspian is under pressure due to poor management of the marine environment and pollution sources. Many rivers flowing into the Caspian sea at

2 268 INDIAN J. MAR. SCI., VOL. 37, NO. 3, SEPTEMBER 2008 its southern coast are used for the disposal of industrial, agricultural and urban wastes 11,12. As a result of exploitation of energy sources and other mentioned factors, the Caspian Sea has been subjected to environmental pollution. The Bay of Gorgan that is located on the south eastern coast of the sea is considered as the largest bay of the Caspian Sea (Fig. 1). The area of the bay is about 400 km 2 with a maximum length and depth of 70 km and 5 m respectively. The bay is influenced both by water intrusion from the Caspian Sea and to a lesser extent by river freshwater. Agricultural activities in the bay catchment area and ports along the coasts, as well as fisheries and oil sector activities in the Caspian Sea seem to have potential negative impacts on the Gorgan Bay environment. A sediment core (70 cm long) was collected with a Phleger type corer at a water depth of 5 m in the central part of the Gorgan Bay (Fig. 1) during October The core was sliced into sections at 1 cm intervals at the top 10 cm and the rest of the core was sub-sampled at 5 cm intervals. Samples were wet sieved (< 63 µm) and dried at < 40ºC to constant weight. Sequential extraction studies were carried out in four sequential steps: a) acetic acid 25% v/v in about 60 minutes contact time, at room temperature ; Fig. 1 Map showing location of the study area Table 1 Published and obtained data for MESS-3 Element Published data 17 Obtained data Co 14.4 ± Mn 324 ± Ni 46.9 ± Zn 159 ± Fe 4.34 ± Al 8.59 ± Values for trace metals are in mg/kg and for major elements are in %. b) acetic acid 25% v/v 0.1 M hydroxylamine hydrochloride, ph 2, 96±3 C, 6h, occasional agitation; c) 30% H 2 O 2 extraction with 1 M ammonium acetate, ph 2, 85±2 C, 3h, intermittent agitation, and d) hot 50% HCl at 125ºC. The residue was digested with HF-HNO 3 -HClO 4 mixture. Elemental contents were determined by atomic absorption spectroscopy (Varian AA-30 model) following U.S.EPA 7000s series. An international reference standard 17 (MESS-3) was analyzed in the same manner to check the accuracy of the analysis (Table 1). Results showed that the errors in the analysis were ±7%. Sediment dating was done using 210 Pb isotope method. The activity of 210 Pb in sections from sediment core was used to determine the sediment accumulation rate over time. In this method, the activity of the 210 Pb grand daughter, 210 Po was actually measured as 210 Pb is a weak beta emitter and is not readily detected. 210 Po is the alpha emitting grand daughter of 210 Pb, and can be used to the actual 210 Pb concentration in the sample because the two isotopes are assumed to be in equilibrium. The daughter was used because it then can be counted on a high resolution alpha spectrometry system. A yield monitor 208 Po was added to each sample so that the exact activity of 210 Po could be determined 18. Briefly wet chemistry steps included treatment of 0.5 g of freeze dried sediment sample with HCl and HNO 3. The final solution was heated to 70ºC and from this solution Po was spontaneously deposited on a silver plate to produce a thin film of sample solution for a- spectrometry using a Si surface barrier detector. For the estimation of supported 210 Pb, the 226 Ra activity was determined by α-spectrometry. The concentration of 210 Pb was calculated from the count rates corrected for counting background, growth and decay, counting efficiency and recovery of the 208 Po yield monitor. Enrichment factors, (EFs), for each element were calculated from the formula 19 : EF= (C sample/al sample)/(c crust/al crust). Geochemical accumulation index (Igeo) was calculated from the formulae 20 : Igeo=log 2 [Cn/(1.5 Bn)], where Cn is the concentration of metal in sediments and Bn is concentration of metal in the shale. Seven categories of sediment pollution are defined based on the calculated Igeo (see Table 2). Results and Discussion Activity of 210 Pb (Fig. 2) has yield an average sedimentation rate of 1.4 mm/yr and sediment core represents 500 years history. Therefore historical

3 KARABASSI et al.: HEAVY METALS IN SEDIMENT CORE 269 trends of metal contents and their chemical partitioning in the sediments of the area could be studied since about 1500 to Down core variation of metal contents are presented in Table 3 and shown in Fig. 3. Concentrations of Al and Mn increase towards the core top and Fe concentration decreases. A very low concentration of Al and Fe in Table 2 Categories of sediment pollution based on the calculated Igeo 20 Calculated Igeo Igeo Index State of Pollution >5 6 Extremely Polluted >4-5 5 Highly Polluted- Extremely Polluted >3-4 4 Highly Polluted >2-3 3 Moderately Polluted-Highly Polluted >1-2 2 Moderately Polluted >0-1 1 Unpolluted- Moderately Polluted <0 0 Unpolluted sediment core comparing to those of mean crust may be indicative of low detrital input to Gorgan Bay. An increase in the concentration of Al needs further investigation. Heavy metals content increased towards top of the core (Fig. 3). Mean contents of Co and Ni are below the mean continental crust and mean world sediment. Content of Zn at the bottom of the core is very close to mean crust. However, the highest content of Zn (at the top of the sediment core) is almost 1.5-fold higher than mean crust. Increase of metal content particularly Zn, towards core top may be related to recent anthropogenic activities. However, for better understanding of background contents of heavy metals in sediments of the study area and to understand the associations of metals with different phases in sediments, sequential chemical partitioning studies were carried out for sediments of depths 1, 10 and 55 cm of the core sample (years 2002, 1930 and 1609 respectively). Results of sequential chemical extraction (Table 4) of sediment revealed that percentiles and amounts of Zn, Ni and Co in non-lithogenous (fraction of metals that are incorporated in the first three stages of the sequential extraction procedure) phases increase towards top of the core. This may be indicative of the influences of anthropogenic activities on aquatic environment of the Gorgan Bay. Calculated enrichment factors and geochemical accumulation indices (Igeo) of heavy metals over time (Table 5) shows that Zn, Ni and Co have increased about 5.8, 8.6 and 15.8% respectively Fig. 2 Activity of 210 Pb in sediment core Fig. 3 Down core variations of Zn, Ni and Co contents over time Table 3 Metal contents in core sediment samples from the Gorgan Bay Year Zn Ni Co Mn Fe (%) Al (%) Mean St Dev Mean Crust* Mean Sediment** *,** Mean Crust and Mean sediment 26

4 270 INDIAN J. MAR. SCI., VOL. 37, NO. 3, SEPTEMBER 2008 from 1930 to These increases are in agreement with increasing percentile of metals in nonlithogenous fraction from 1930 to 2002 (12%, 14% and 14% for Zn, Ni and Co, respectively (Table 4). The enrichment of metals has not been increased to pollution or even moderately pollution level according to the calculated Igeo indices (Table 5). Fig. 4 Percentile of heavy metals in non-lithogenous fraction of sediments from 1609 to 2002 Table 4 Sequential chemical extraction of heavy metals in core sediment samples Metal Year a Zn Ni Co b c d (37) 21 (22) 21 (22) 18 (19) (34) 15.1(17) 21 (24) 22 (17) (30) 11 (14) 16 (21) 26.7 ( (31) 5.2 (15) 7.1 (20) 11.9 (4) (28) 4.3 (14) 5.5(18) 12.6( (25) 3.1 (13) 3.1 (13) 11.9 ( (26) 2.5 (16) 2.3 (15) 6.9 (43) (24) 1.5 (11) 1.7 (13) 6.8 (52) (23) 1.1 (10) 1.5 (14) 6 (53) Numbers in parentheses represent percentile of metal extracted each phase There seems to be a slight increasing trend in enrichment level of the sediments of the study area over the last 70 years (Tables 4, 5 and Fig. 4). The salinity of Gorgan Bay indicates clearly the seawater intrusion into the Bay. There is a major river (Qarasoo river) flowing into the Gorgan Bay as well as many seasonal streams which drain large agricultural lands that can deliver pollutants into the Bay. In the vicinity of the study area, there are two major sources of pollution; namely agricultural practices as well as oil exploration in the Caspian Sea. Thus, the sources of pollution can be both land and sea based. Since slight increase in the contents of studied metals is noticed 70 years ago where there was no oil extraction in the Caspian Sea, it can be inferred that agricultural activities in the catchment may have been the main source of metals delivery into the study area. Arjomandi et al. 21 reported the high contents of Zn and Ni in chemical fertilizers that are used in northern part of Iran. This may justify higher contents of Zn and Ni in the sediment core. Further, despite many research reports 19,22,23, applying EFs for metals pollution detection and may lead incorrect results due to naturally lower concentration of Al and/or higher concentration of heavy metals in sediments. As the EFs in the present investigation area of study were high, even in 17 th century when there were no significant source of pollution. For this reason we used Igeo index too, and the results showed that despite high EFs in the sediments of the area there seems to be not even moderate pollution of metals in the sediments of the study area, and just metal enrichment in the area has been started. As we know, natural concentrations of heavy metals in soils of northern part of Iran and Gorgan Bay watershed soils Table 5 Historical enrichment factors and Igeo of Co, Ni and Zn in core sediment samples Year Zn Ni Co EF Igeo EF Igeo EF Igeo 2002\(1) (Unpolluted) 6.3 0(Unpolluted) 6.6 0(Unpolluted) 1966\(5) (Unpolluted) 6.2 0(Unpolluted) 6.4 0(Unpolluted) 1944\(8) (Unpolluted) 6.3 0(Unpolluted) 5.5 0(Unpolluted) 1930\(10) (Unpolluted) 5.8 0(Unpolluted) 5.7 0(Unpolluted) 1894\(15) (Unpolluted) 5.7 0(Unpolluted) 5.3 0(Unpolluted) 1859\(20) (Unpolluted) 6.6 0(Unpolluted) 5.7 0(Unpolluted) 1787\(30) (Unpolluted) 6.7 0(Unpolluted) 5.0 0(Unpolluted) 1680\(45) (Unpolluted) 6.7 0(Unpolluted) 5.6 0(Unpolluted) 1609\(55) (Unpolluted) 4.4 0(Unpolluted) 4.8 0(Unpolluted) 1502\(70) (Unpolluted) 4.6 0(Unpolluted) 4.8 0(Unpolluted) Numbers in parentheses represent depths in core sample in cm EF= Enrichment factor

5 KARABASSI et al.: HEAVY METALS IN SEDIMENT CORE 271 are relatively high and the contents of some metals like Fe and Al are low 24. For instance, mean contents of Zn, Ni, Co were reported as 87.3, 55.0, 21.3 mg/kg and for Fe and Al 2.6 and 0.85% respectively in soils of Tadjan River watershed in the vicinity of Gorgan Bay 25, which may lead to higher EFs of heavy metals in the soils and sediments of the area. Based on the results of present investigation, chemical partitioning study of sediments and assessing changes of percentile of metals in non-lithogenous fraction and application of Igeo index for the detection of heavy metals enrichment in sediments seems to be better approaches. Finally, baseline background concentrations and lithogenous fraction of Co, Ni and Zn in sediments of the study area were proposed as: Zn; 80.5±8.4, Ni; 31.2±5.7 and Co; 12±1. Conclusion Based on the present investigation it can be concluded that though chemical partitioning studies is a useful tool in detecting the association of metals with different sedimentary phases, it should be carefully used in the assessment of environmental pollution as large amount of metals may naturally occur as anthropogenic fractions (including loosely bonded ions, sulfides ions and metals associated with organics). Our findings indicate clearly that chemical partitioning data can be useful if their down core variations are known. Enrichment factors may be useful when used qualitatively and their quantification may vary highly from place to place. Thus both chemical partitioning studies and EFs need to be used once their concentrations/variations are precisely known in pre-industrial era and present time. Application of the geochemical accumulation index may lead to more realistic results in comparison with EFs in the sediments of the study area as low Al content of sediments. Geochemical accumulation index of metals in sediments of the study area indicated that the sediments have not been contaminated yet, but the present trend shows that metals enrichment in the Bay is increasing. However, more detailed studies on the influence of pollution sources in Gorgan Bay catchment and metals budget in the bay are needed. References 1 Al-Masri M S, Aba A, Khalil H & Al-Hares Z, Sedimentation rates and pollution history of a dried lake: Al- Oteibeh Lake, Sci. 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