Estimation of sediment pollution caused by trace metal- A case study from Hooghly-Matla estuarine region
Keywords: trace metal, sediment contamination, sequential extraction, Hooghly Matla estuary, pollution load
Unplanned industrial development along with rapidly expanding human settlements, tourism activities, intense fishing, deforestation and aquaculture activities lead to severe environmental degradation due to the accumulation of trace metals in the coastal – estuarine ecosystem (Ghosh et al., 2016). Coastal sediment act as sink for toxic trace metals and record of anthropogenic pollutants inputs into aquatic ecosystem (Fig. 1) (Zwolsman et al., 1997; Chapman et al., 1998; Caccia et al., 2003; Pan et al., 2014). Fe–Mn oxyhydroxides, total organic carbon (TOC), and fine-grained sediments control the geochemical distribution of trace metals in the sediments (Guo et al., 1997; Dong et al., 2000; Mounier et al., 2001; Banerjee et al., 2012). However, if the changes occur in water and sediment conditions [eg, redox, pH], trace metal may be remobilized and even diffused back to the water column. Remobilized trace metal shows adverse effect on surrounding environment (Merhaby et al., 2018). Trace metals level increase in the environment when these metals are released from the rocks. These releases can occur through natural processes or through human activities. Natural processes are like breakdown of rocks, spreading of mid-ocean ridges, and volcanic activity. Human activities include mining, smelting, burning of coal and waste water disposal (Gamberg et al., 2005). Due to human induced activities potentially toxic trace metals are gradually accumulating in the estuarine environment which increasing the threat for coastal estuarine habitats (Lichtfouse et al., 2005; Ghosh et al., 2016).
Bio-Magnification of Trace Metals
Several important ecological, biological, economical, social and cultural functions are supported by the coastal estuarine ecosystem (Primavera, 1997; Costanza, 1999; Nagelkerken et al., 2000; MacFarlane et al., 2007; Rönnbäck et al., 2007; Birch et al., 2013; Nath et al., 2014). The chemical composition of seawater plays an important role in the bioaccumulation and discrimination of trace elements in marine biota as whole life integrators. Trace metals bioaccumulation and biomagnifications in living organisms and food chains causes degradation of marine ecosystem. Trace metal contamination along coastline varies with the local economic development pollution source and geographical conditions (Song et al., 2019). In coastal estuarine environments, mangroves play an essential role in protection of the ecosystem (Bayen, 2012). Sulfides and organic matter in estuarine habitats plays a vital role in trace metal sequestration, which leads to the formation of insoluble metal sulfides and organo-metallic complexes (Foster & Charlesworth, 1996; Clark et al., 1998; Zhou et al., 2011). A trace metal can bind to any molecule with an affinity for that metal. Trace metals have an affinity with sulphur and nitrogen. These affinities make all trace metals potentially toxic (Rainbow et al., 2002). Estuarine sediments favor the accumulation of waterborne macro and toxic trace elements due to their anaerobic and reduced nature (Lacerda & Abrao, 1984). Most of the elements were produced by the biogeochemical cycle but they accumulated in the environment from human induced sources like industrial effluents and domestic sewage containing toxic metals (Amman et al., 2002), urban storm-water runoff (Lantzy & Mackenzie, 1979; Nriagu, 1979; Bhattacharya et al., 2014) and navigation activities (Ghosh et al., 2016).
It has been usually reported that fine-grained sediments of coastal regions control the geochemical distribution of trace metals in the sediments (Tam & Wong, 1997; Lewis et al., 2011). Trace metal are group of pollutant of high ecological significance due to their toxicity, wide source, non-biodegradable properties and accumulative behavior (Liqin Duan et al., 2019). Toxicity of trace metal is difficult to determine. Toxicity depends on the level of the trace metal in the environment and also where it is found in the environment (Naeth et al., 2008). Geo-chemical cycles of toxic elements were highly dependent on climate during pre-industrial phase. Trace metals are non degradable, thus they may be either accumulated in the sediments or get carried out through rivers (Marchand et al., 2006). Hence, sediments qualities in coastal ecosystem are widely used to assess the water pollution caused by trace metals (Dima Merhaby et al., 2018).
Accumulation of macro and trace metals in the coastal estuarine environment is from geogenic source like physical and chemical weathering of rocks, and human induced sources like wastewater discharge (Callander, 2005). Natural elemental compositions of sediments were regulated by mineral composition of local and upstream lithology (Arhin et al., 2016). It has been extensively recognized that trace metals content in estuarine and coastal sediments act as a useful tool for the assessment of sediment quality (Burton and Scott, 1992; Caccia et al., 2003). However, it is not possible to predict the exact toxicity and bioavailability of macro and trace metals using their total concentration in sediment. For the better understanding of trace metals impacts in environment proper knowledge about speciation and distribution is needed (Chakraborty et al., 2014).
AIMS AND OBJECTIVES
The broad aim of this study is to investigate the accumulations of trace metals in Hooghly Matla estuarine region along with the mobility and speciation of some selected trace metals. Special emphasis has been given to the estimation of sediment pollution.
The main objectives of the study are as follows:
- To determine physico-chemical parameter of estuarine sediments.
- To evaluate distribution of trace metals in sediments of Hooghly Matla estuarine region.
- To assess the mobility of selected trace metals in acid soluble, exchangeable and carbonate bound fraction of the sediment.
The Sundarbans mangrove wetland is situated at the mouth of the Hooghly–Matla estuarine region (Ghosh et al. 2019). The region is crisscrossed by complex networks of narrow tidal channels and creeks, which surround numerous islands at different levels of elevation at semidiurnal tides. Hooghly estuary (87⁰55’ N–88⁰ 48’ N latitude and 21⁰29’ E to 22⁰ 9’ E longitude) is the first deltaic distributary of the river Ganga (Fig. 4). This estuary along with the magnificent and luxurious mangroves of Indian Sundarbans forms one of the most ecologically significant and diverse ecosystems in the world (Bhattacharya and Das, 2002). The river Hooghly transverse through the densely populated cities like Kolkata and Howrah, and it has a centuries-old port on its banks. It is a globally significant, unique, ecologically sensitive and UNESCO world heritage estuary with a catchment area of approximately 69,104 km2. The Hooghly estuary is well mixed due to strong tidal influx and low depth (6 m) (Sadhuram et al. 2005). Tides are semidiurnal in nature with amplitude variation between 1.8 to 5.5 m (Banerjee et al. 2012). The region experiences a tropical wet and dry climate. The ambient temperature ranges between 9⁰C to 35⁰C. January is the coldest month and May is the hottest month. South west monsoon provide a major portion of the annual rainfall. Minimum rainfall is 2 mm and maximum is 80 mm (Barui, 2011). The tide is semidiurnal in nature and depth is between ~10 to 15 m. The freshwater discharges from Farakka barrage to Bhagirathi channel ranged between ~900 m3/s to 4000 m3/s (Mukhopadhyay et al. 2006).
The present study was carried out in some selected reclaimed and virgin islands of Indian Sundarbans and in the mangrove habitats of Hooghly estuary covering Hooghly Matla estuarine region. Ten samples were collected from five sampling stations like Falta (S1), Nayachar (S2), Chemaguri (S3), Tapoban (S4) and Petuaghat (S5) (Fig. 4) were chosen in the Hooghly estuary, as contaminants generated either by human-induced pathways or by natural weathering process are distributed and deposited in the downstream region of the estuary. Eighteen samples were collected from six reclaimed islands of Indian Sundarbans, i.e., Maushuni (S6) and Canning (S7), Basanti (S8), Kumirmari (S9), Rakhaskhali (S10) and Jharkhali (S11) (Fig. 4 and Table 1), which are claimed from nature for human settlements and occupational purposes, e.g., agriculture, aquaculture. Sediment samples were also obtained from thirty spots of Virgin mangrove dominated Islands of Indian Sundarbans (S12 – S40). All sampling stations including the reclaimed island of Indian Sundarbans and Hooghly estuary belong to different tidal and environmental regimes and have different land use pattern, e.g., agriculture, aquaculture, hotel and resort, fishing harbor, forest land, with a variable degree of exposures to trace metals.
|Sl. No.||Station Code||Station Name||Latitude||Longitude|
|2||S2||Nayachar||22° 1'15.40"N||88° 8'5.23"E|
|12||S12||Mangrove dominated islands of Indian Sundarban||21°45'36.8"N||88°56'10.2”N|
Materials and Methods
Sampling and preservation
During 2017 April–June, sediment samples were collected from intertidal regions of the sampling station during low tide. The samples were air dried at room temperature, grounded and stored in a moisture fee plastic container for analysis. These previously collected sediment samples were used for analysis.
pH and conductivity of the surface sediments was measured with Thermo scientific orion star (Model No.: A329) where ratio of soil : water is 1:5 and 1:10 respectively. Total percentage of organic carbon in the sediment sample was determined by slight modification of the method suggested by Walkey and Black (1934). Thermo scientific orion star (Fig. 5) contains two separate cells for two different characters like pH and conductivity. The cell used for pH measurement was kept in a buffer solution and it was washed with distilled water and cleaned with tissue paper before placing it into the sample solution. Clean the cell after use and again keep in buffer solution. The same process was repeated for remaining samples.
For measuring the electric conductivity same equipment was use but the cell use for conductivity is different. Add 20 ml of distilled water to 1gm of sediment sample in a small beaker. Wash the cell with distilled water and cleaned with tissue paper. This cell now kept in the sample solution to measure the conductivity. After using the cell clean it well for further use. Repeat the same procedure for remaining samples.
Sequential extraction process for trace metal analysis
Glass ware: Conical flasks, beakers, pipette, measuring cylinder, volumetric flask, funnel, test tubes and other non glass ware like centrifuge tubes, axiva syringe filter, marker, cello tape, spatula tissue paper and butter paper.
Instruments: Weighing machine, THERMO SCIENTIFIC ORION STAR, inductively coupled plasma optical emission spectrometer, agate motor pistil, water bath and freeze.
Chemicals and solutions
- Water: Throughout the work glass-distilled water should be used. Alternatively, filtered water (e.g., Milli-Q or equivalent) may be used. Since it may contain organically complex metal ions, de-ionised water should not be used.
- Solution A (acetic acid, 0.11 mol /l]: In a fume cupboard add 25±0.2 ml of glacial acetic acid to 0.5 l of distilled water in a 1 l graduated polypropylene or polyethylene and make up to volume with distilled water. To obtain an acetic acid solution of 0.11 mol /l take 250 ml of this solution (acetic acid, 0.43 mol /l) and dilute to 1 l with distilled water.
- Solution B [Hydroxylammonium chloride (hydroxylamine hydrochloride), 0.5 mol /l]: In a 400 ml of distilled water dissolve 34.75 g of hydroxylammonium chloride. Transfer the solution into a 1 l calibrated flask and add 25 ml of 2 mol /l HNO3 by means of a calibrated pipette. Make up the total volume to 1 l with distilled water. This solution was prepared on the same day the extraction is carried out.
- Solution C (hydrogen peroxide, 300 mg g−1, 8.8 mol /l): Hydrogen peroxide was used as supplied by the manufacturer, i.e., acid-stabilised to pH 2–3.
- Solution D (ammonium acetate, 1.0 mol /l): In 900 ml of distilled water dissolve 77.08 g of ammonium acetate. With concentrated HNO3 adjust the pH to 2.0±0.1 and make up to 1 l with distilled water.
- Blanks: Cd, Cr, Cu, Ni, Pb, and Zn should be determined as follows:
Vessel blank: To one vessel from each batch, taken through the cleaning procedure, add 40 ml of solution A. Analyse this blank solution along with the sample solutions from step1 (described below).
Reagent blank: Analyse a sample of each batch of solutions A, B, C and D.
Procedural blank: With each batch of extractions, a blank sample (i.e., a vessel with no sediment) should be carried through the complete procedure and analysed at the end of each extraction step.
For each batch of extractions 1 g of dry sediment sample was used. Extractions can be performed by shaking in a mechanical, end over-end shaker at a speed of 50±5 rpm and a room temperature of 27±2 °C. At the starting and at the ending of each step of the extraction procedure room temperature should be measured. Sequential extraction steps described below.
Step 1. In a 100 ml conical flask 40 ml of solution A was added to 1 g of sediment, then stopper and extract by shaking for 16 h at 27±2 °C (overnight) (Fig. 6 and 7). Addition of the extractant solution and the beginning of the shaking should occur at the same time. Extract should be separated from the solid residue by centrifugation at 3000 g for 20 min and decant the supernatant liquid into a polyethylene container. The extracted samples were immediately stored in a refrigerator at about 4°C for further analysis. By adding 20 ml of distilled water residue should be washed, kept for shaking for 15 min and centrifuging for 20 min at 3000 g. Decant the supernatant and discard, taking care not to discard any of the solid residue.
Step 2. To the residue of step 1 40 ml of freshly prepared solution B was added in the centrifuge tube. Resuspend by manual shaking and then extract by mechanical shaking for 16 h at 22±5 °C (overnight). Addition of the extractant solution and the beginning of the shaking should occur at the same time. As like in the step 1 separate the extract from the solid residue by centrifugation and decantation. Retain the extract in a polyethylene container, as before, for analysis. By adding 20 ml of distilled water residue was washed, kept for shaking for 15 min and centrifuging for 20 min at 3000 g. Decant the supernatant and discard, taking care not to discard any of the solid residue.
Step 3. 10 ml of solution C was added carefully to the residue in the centrifuge tube in small aliquots to avoid losses due to possible violent reaction. With the help of its cap cover the vessel loosely and digest at room temperature for 1 h with occasional manual shaking. Continue the digestion in a water bath for 1 h at 85±2 °C, reduce the volume to less than 3 ml by further heating. Add further 10 ml of solution C. Covered the vessel and heat again at 85±2 °C and digest for 1 h. Reduce the volume of liquid to about 1 ml. Do not take to complete dryness. To the cool moist residue add 50 ml of solution D and shake for 16 h at 27±2 °C (overnight). Addition of the extractant solution and the beginning of the shaking should occur at the same time. As like in the step 1 separate the extract from the solid residue by centrifugation and decantation. Retain the extract in a polyethylene container, as before, and stored at 4⁰ C for analysis by inductively coupled plasma optical emission spectrometry (ICP-OES).
Inductively coupled plasma optical emission spectrometry [ICP OES]: Inductively coupled plasma optical emission spectrometry (Fig. 8 and 9) is also known as inductively coupled plasma atomic emission spectroscopy, used for the detection of trace elements. By using inductively coupled plasma it produces excited atoms and ions that emit electromagnetic radiation at wavelengths characteristics of a particular element. Plasma is a gas which contains a significant number of argon ions. By seeding the argon gas passing through a plasma torch with electrons, plasma is formed. Elements are introduced into the plasma when they are in the form of atoms, a proportion of these atoms will ionized within the plasma. Electrons jump from lower level to higher energy level when atom or ion excited within the plasma. These electrons upon relaxation back to their initial ground state emit energy in the form of photons. These photons possess wavelengths. Photons from different elements possess different wavelengths. This is the characteristics of those respective elements. Calibration standard solutions were prepared by using ICP Multi-elemental Standard Solution IV and IX (Merck, Germany). Correlation coefficients of the calibration curves of toxic elements were in the range of 0.9993–0.9999.
Quality control: Laboratory-ware including the centrifuge tubes should be of borosilicate glass, polypropylene or PTFE. Vessels was cleaned by soaking in 4 mol l−1 HNO3 (overnight) and rinsed repeatedly with distilled water before use. Extractions were performed by using a mechanical end-over-end shaker, at a speed of 30±10 rpm. Record the speed. Centrifugation should be carried out at 3000 g for 20 min.
RESULTS AND DISCUSSION
Physico-Chemical Properties of Sediment
Irrespective of the sampling stations mean range for physico-chemical properties of intertidal surface sediments of Hooghly Matla estuarine region varied between pH (4.27–8.55), EC (43.55–8277 µS/cm2) and OC (0.28–0.94%) (Fig. 10-12).
Distribution of Trace Metals
Distribution of trace metals varied considerably between elements and sampling locations, which might be the due to the reflection of several key factors such as variability in the weathering products due to varied bed or parent rocks, several physico-chemical processes (hydrodynamic process, the residence time, material source, stokes settling velocity, tidal amplitude, erosion and deposition condition). (Zhang and Gao, 2015) along with the varied magnitude of contaminants released from human induced sources (Ghosh et al. 2016 ; Watts et al. 2017)) and sediment dispersal pattern (Ladislao et al. 2015). The average range of concentrations for trace metals in sediment of Hooghly Matla estuarine region were depicted in table 2. The average range of concentrations for trace metals varied between (7533 – 15481) mg/kg for Fe, (331.5 – 696.2) mg/kg for Mn, (16.5 – 34.1) mg/kg for Cu and (13.2 – 27.5) mg/kg for Zn. Elevated concentration of Fe were evident in all sediment samples of Hooghly Matla estuarine region might be due to presence of laterites and basaltic trappean bed rocks (Sarkar et al. 2004) and/ or due to mineralogical and textural composition of the sediment (Ramanathan et al. 1999). The surfacial aggregation of trace metals might be ascribed to riverine inputs and human induced activities like domestic and urban sewage, storm water, copper smelting and ferro alloys industries, petrochemical industries, steel plants, coal based thermal power plant, captive power plants, pharmaceutical industries, brick kilns, immersion of idols, battery industries, gun and shell factory, agriculture and aquaculture and emission from boats (Stafilov et al. 2010; Bačeva et al. 2012; Magesh et al. 2013; Bawa et al. 2015; Ghosh et al. 2016; Rodríguez-Seijo et al. 2016).
|Values are in mg/kg|
Assessment of Trace Metals in Acid Soluble, Exchangeable and Carbonate Bound Sediment
The second most abundant fraction for Cu and Zn was associated with the reducible fraction (Step II), suggesting adsorption of these metals by the Fe–Mn colloids particles which act as important scavengers of these metals in sediments, thus suggesting the role of Fe–Mn colloid particles in controlling the mobility of Cu in the environment matrices. Fe were found to be dominant in the residual fraction followed by reducible fraction (Step II), associated with Fe–Mn oxyhydroxides and oxidizable fraction (Step III), bound to the sulphides and minimum concentration in the acid-soluble fraction (Step I), associated with exchangeable and carbonate bound. The geochemical relation between Fe and Zn with the residual fraction suggests its non-availability under existing natural conditions. The association with the reducible fraction (Step II) as the second most dominant fraction suggests that it plays an important role in the leaching of metals into the environment. Low abundance of Fe and Mn in the Step III and Step I fractions suggests low release and availability under suboxic and natural conditions (Fig. 13 – 16; Table 3 & 4). The available fraction (Step I) is more in abundance than the oxidizable fraction (Step III), their abundance in these fractions does not pose a threat to the sediment quality and biotic community. A small portion of Cu is also found to be bound with the available fraction (Step I) suggesting its bioavailability while the oxidizable fraction bound to sulphur, and organics is the least abundant fraction and does not act as a significant scavenger for Sundarban sediments.
The quality of the rivers is gradually degrading due to the continuously increased population pressure, urban growth, pollution and overexploitation of natural resources. Economic, environmental, and social policies related to the management of estuarine and coastal ecosystems to ensure environmental safety of local inhabitants, forest dwellers are mostly inadequate (Sarkar et al. 2007). Around the globe, especially in third world developing countries, problems related with sediment pollution or contamination or increased pollution load are correlated with deforestation and agriculture, which represents significant concerns for management of river basins (Owens et al. 2005). The existing management strategies prevailing in India are a combination of legislative policy, community awareness, NGO management and sustainable exploitation of resources. Throughout the world exploitation of natural resources for economic and social growth, are among the most enduring and age old legacies. Current policies and practices of environmental management in vulnerable coastal estuarine ecosystem have inherited these tensions and legacies (Randeria, 2007). Country specific institutions and organizations have framed and drafted the several policies, legislation and regulations to assess trace metal contamination by estimating total trace metals without relating them with natural and anthropogenic parameters at the local level. Reporting of trace metal concentrations or increased pollution load in sediment along with their spatio-temporal variation does not bear any importance without considering local geological processes and anthropogenic pressure. Lack of communication between the scientists, policymakers and local people are the main hindrance in effective transfer of knowledge to the local grass root workers and forest dwellers (Mukherjee et al. 2014). However, this situation can be overcome by increased knowledge sharing with the stakeholders and by encouraging stakeholders to get involved in the management process. It should include an efficient policy and framework for water resource management. However, this management policy must be sought through a multidisciplinary and holistic approach, including science, economics, politics, education and awareness (Sarkar et al., 2007).
The work has potentially evaluated the distribution of trace metals in different phases or fractions like acid leachable, exchangeable and carbonate bound, enhancing the information on the status of trace metal contamination and pollution load in surface sediments of the Hooghly Matla estuarine system. The varied accumulation of trace metals in this area might be because of several factors e.g. colossal anthropogenic action, tidal abundancy, hydrodynamics, blending, disintegration and sedimentation. The supply of fresh organic matter from the mangroves and variable hydrodynamic and anoxic conditions may determine its affinity to the acid-soluble fraction. The physico-chemical parameters of Hooghly Matla estuarine region varied between pH (4.27 – 8.55), EC (43.55 – 8277 µS/cm2) and OC (0.28 – 0.94%) while average range of concentrations for trace metals varied between (7533 – 15481) mg/kg for Fe, (331.5 – 696.2) mg/kg for Mn, (16.5 – 34.1) mg/kg for Cu and (13.2 – 27.5) mg/kg for Zn. Cu, Zn and Fe were associated with the particles of Fe–Mn oxides as the second most dominating factor, suggesting the role of Fe–Mn oxyhydroxide as a major scavenger for these trace metals. Moreover, effective and efficient management policies to control the release of trace metals in the Hooghly Matla estuarine habitat and their associated adverse antagonistic influences on the ecosystem must be considered for better management of this vulnerable coastal estuarine habitat.
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First and foremost, I would like to thank my guide Dr. Punarbasu Chaudhuri, assistant professor for guiding me in such a good manner and to provide me with whatever aid I required. I thank him for constant support throughout my tenure. Because of his guidance as well as patience I have been able to complete my report on time.
In the period of this internship I was constantly supported by Somdeep Ghosh, Research Scholar in the department. He helped me with the whole learning process also answered each question of mine patiently.
I would like to thank University of Calcutta, specially the Department of environmental science, Ballygunge Science College for hosting me and for providing me an atmosphere worth staying and learning. I am grateful to IAS-NASI-INSA for providing me such a great research opportunity. This kind of exposure will help me in my future a lot.
I am grateful to my college as they allowed me for this fellowship. I would be not here if it was not my parents support. I think them for showing in me this level of confidence and for supporting me at each and every step of my life.