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Trace metal distribution in the sediment cores of mangrove ecosystems along northern Kerala coast, south-west coast of India
Marine Pollution Bulletin 153 (2020) 110946
Contents lists available at ScienceDirect
Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul
Baseline
Trace metal distribution in the sediment cores of mangrove ecosystems along northern Kerala coast, south-west coast of India
T
⁎
M.N. Manjua, C.S. Ratheesh Kumarb, , P. Resmic, T.R. Gireeshkumard, Manju Mary Josepha, P.M. Salasa, N. Chandramohanakumare a
Department of Chemical Oceanography, School of Marine Sciences, Cochin University of Science and Technology, Kochi, PIN-682016, Kerala, India School of Environmental Studies, Cochin University of Science and Technology, Kochi, PIN-682022, Kerala, India c Aquatic Environment and Management, Kerala University of Fisheries and Ocean Studies, Panangad, Kerala PIN-682506, India d National Institute of Oceanography, Regional Centre-Kochi, Dr. Salim Ali Road, Kochi, PIN-682018, India e Inter-University Centre of Marine Biotechnology, School of Marine Sciences, CUSAT, Kochi, PIN-682016, Kerala, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: Trace metals Mangroves Pollution Core sediment
Core sediment samples were collected from five mangrove ecosystems along northern Kerala coast (Kunjimangalam: S1, Pazhayangadi: S2, Pappinissery: S3, Thalassery: S4, and Kadalundi: S5) to assess the status of heavy metal pollution. S1 recorded comparatively lower metal concentration at surface (except Pb) due to low organic content and sandy texture, while the reverse was true for S3. Higher metal contents were recorded at S5 (0–5 cm), which was attributed to its unique biogeochemical behavior. Enrichment factor and geoaccumulation index indicated moderate enrichment for Cd, and the pollution load index revealed progressive deterioration of sediment quality at S5 (0–5 cm). There was no harmful effect of trace metals on biological community (except Ni) according to Sediment Quality Guidelines. Major processes controlling trace metal accumulation in these systems are diagenetic processes, precipitation of heavy metals as sulfides, and the presence of Fe, Mn-oxy hydroxides, which act as adsorption sites for other metals.
Mangrove sediments act as a biogeochemical sink for metals, and trace metal concentration has been found to be increasing because of pollution and urbanization (Agoramoorthy et al., 2008). Heavy metal pollution has been investigated by many authors in Indian estuaries (Ratheesh Kumar et al., 2010; Martin et al., 2011; Dhanakumar et al., 2013). However, scarce information is available on the geochemical characteristics of mangrove ecosystems along northern Kerala coast, even though 83% of the state's mangrove cover is contributed by these systems. The present study is an attempt to understand the trace metal concentration and its distribution in the mangrove sediment cores of northern Kerala coast, which can reveal the effects of anthropogenic and natural processes in these environments. The extent of contamination was evaluated by using indices such as geo-accumulation index, enrichment factor, and pollution load index. Location of sampling stations is presented in Fig. 1. Station 1, Kunjimangalam (S1) in the estuarine environment formed by Pullamcode puzha and Kunjimangalam River, is found to be almost free from anthropogenic inputs. At station 2, Pazhayangadi (S2), domestic waste disposal was the major source of pollution. Station 3, Pappinissery (S3), is formed on the banks of Valapattanam estuary (area of
⁎
about 20 ha), and a plywood manufacturing industry is located on the confluence of S3. Domestic waste disposal and discharges of industrial effluents are the major source of pollution at S3. Station 4, Thalassery (S4), is situated near the barmouth, and domestic waste disposal is the main source of pollution at this station. Mangrove forests in station 5, Kadalundi (S5), are located on the banks of Kadalundi River that meets the Arabian Sea through a permanent barmouth (Manju et al., 2016). Fishing, boat operations, sewage drainage from mainland, industrial and other commercial activities are the potential sources of trace metals at S5. Core sediment samples were collected manually using a PVC pipe (internal diameter: 65 mm and length: 65 cm) from five mangrove ecosystems along the northern Kerala coast during October 2009. First segment of the core was selected as (0–5 cm), and the remaining portions were sliced at 10 cm intervals. General parameters like pH (Eutech, pH Tester), texture (pipette method), total carbon-TC and total sulfur-TS (CHNS Analyser-Vario EL III) and total organic carbon [TOC] (Element Analyser-Vario TOC Select Elementar) were analyzed using standard methods. For the analysis of trace metals (total), 0.2 g of freeze dried and finely powdered sediment samples were digested with a 1:5
Corresponding author. E-mail address: [email protected] (C.S. Ratheesh Kumar).
https://doi.org/10.1016/j.marpolbul.2020.110946 Received 5 August 2019; Received in revised form 27 January 2020; Accepted 28 January 2020 Available online 09 February 2020 0025-326X/ © 2020 Elsevier Ltd. All rights reserved.
Marine Pollution Bulletin 153 (2020) 110946
M.N. Manju, et al.
Fig. 1. Map showing sampling locations.
PLI = (CF1 ∗ CF2 ∗ …CFn )1/n
mixture of HClO4-HNO3 (Loring and Rantala, 1992) in a microwave reaction system (Perkin Elmer, Anton Paar — Multiwave 3000). The residue was dissolved in 0.5 M HCl and made up to known volume by adding Milli Q water. Trace metals analyzed were Cu, Cd, Co, Pb, Ni, Fe, Mn, and Zn using Atomic Absorption Spectrometer (Perkin Elmer 3110) after calibration with suitable elemental standards (Merck, Germany). The accuracy of trace metal determination was validated using BCSS-1 (standard reference material for marine and estuarine sediments) from the National Research Council, Canada. Triplicate analysis of BCSS-1 showed a good accuracy, and the recovery rates were 91.8% (Mn), 97.1% (Cu), 98.5% (Cd), 95.1% (Co), 92.4% (Pb), and 98.5% (Zn). To assess the contamination status of the mangrove ecosystems, pollution indices such as contamination factor (CF), enrichment factor (EF), pollution load index (PLI), and geo-accumulation index (Igeo) were estimated. The enrichment factor was calculated for each metal by using iron as a normalizing element using the equation
where CF is the contamination factor, n represents number of metals and CF = concentration of metal in sediment/ world shale value of the metal. The concentration of various sedimentary parameters in the study area is provided in Fig. 2. The variation in general sediment characteristics is well documented by Manju et al. (2016). The surface samples (0–5 cm) recorded higher TOC content at all stations. TOC for S2, S3, and S4 exhibited a sharp decrease from top to bottom, which suggested intense mineralization (Bouillon et al., 2004). S1 and S5 revealed a decrease in TOC up to 15 cm and then remained uniform throughout the core (Fig. 2), which indicated preservation of organic matter under anoxic conditions (Cowie and Hedges, 1992; Keil et al., 1994; Hedges and Keil, 1995). Lower organic content and sandy texture resulted in lower metal concentration in sediments at S1, while the reverse was true for S2 and S3 (Fig. 3). In addition to mangrove input, the runoff from the land by tributary rivers as well as the location with respect to the Arabian Sea also resulted in higher accumulation of heavy metals at S5. The maximum content of Cu, Co, Mn, Ni, Pb, and Fe and elevated levels of Cd and Zn was observed at S5 (0–5 cm). However, the peak concentrations of Cd and Zn were recorded at S2 (0–5 cm) and S3 (25–35 cm) respectively. Enhanced build-up of the heavy metals at S5 (0–5 cm) can be attributed to the unique biogeochemical behavior of this mangrove ecosystem. The oxidation of organic matter yields acid volatile sulfide that acts as an important ligand for metals. The differences in the solubilities of metal sulfides account for the distribution pattern of metals. A general decrease in the concentration of Cu from top to bottom of the core was observed in all the mangrove forests. In lower parts of the core, the metals generally precipitate as sulfides or remained bound with organic
EF = (metal/Fe)sediment /(metal/Fe)crust . The geo-accumulation Index (Igeo), (Müller (1979), was calculated according to the equation:
Igeo = log2 (Cn/1.5 Bn) where Cn = measured concentration of heavy metal in the mangrove sediment, Bn = geochemical background value in average shale (Wedepohl, 1995) of element n, where 1.5 is the background matrix correction in factor due to lithogenic effects. The extent of contamination on biological community was assessed by determining the pollution load index (Tomlinson et al., 1980), and was evaluated using the equation, 2
Marine Pollution Bulletin 153 (2020) 110946
M.N. Manju, et al.
Fig. 2. Variation of general sedimentary parameters in the sediment core. (a to e) Grain size and TOC at each stations, (f) pH, (g) TN, (h) TS.
precipitation of insoluble nickel sulfides. The concentration of Ni and Fe in the sediments was higher than world shale average, and the possible reason can be erosion of laterite soils (Marchand et al., 2016). This observation was found to be more pronounced at S5 (0–5cm). The mobility of Zn is influenced by redox conditions and availability of organic matter (Ranjan et al., 2013). Positive correlations of Zn with TS (R2 = 0.57, n = 30) and highly significant negative correlation with Eh (R2 = 0.51, n = 30) supported the formation of ZnS (Alloway, 1990; Andrade et al., 2012). The metal concentrations in the present investigation were in good agreement with previous reports from mangrove ecosystems of Cochin and Pichavaram (Ranjan et al., 2008; Ratheesh et al., 2010). The exceptionally higher concentration of Zn at S3 (Fig. 3) can be attributed to pyrite formation. Estimated EF values reflected moderate enrichment for Cd at S1 (25–35 cm, 35–45 cm, 45–55 cm), S2 (0–5 cm and 25–35 cm) and S4 (35–45 cm), and S5 (0–5 cm, 35–45 cm); while moderately severe enrichment for Cd was noted at S3 (0–5 cm), S5 (45–55 cm); minor enrichment was observed for Co, Pb, and Ni. Estimated Igeo values implied
matter (Ranjan et al., 2013). Higher concentration of Cu at S3 was due to its association with clay minerals. The positive correlation between Cu and organic matter (R2 = 0.79, n = 30) indicated the complexation of Cu with humic substances (Alberts and Filip, 1998). Concentration of Pb was higher in sediment core collected from S5 (0–5cm). Increase in Pb content in the sediments can be ascribed to external sources, mainly from aquaculture effluents, agricultural runoff, domestic sewage, vehicular traffic and mechanised boats. Possible landbased sources of Cd pollution in mangrove sediments are fertilizers, pesticides and paint industries (Ranjan et al., 2008). Trapping of Cdrich marine sediments in deeper parts of the sediment cores from Pichavaram mangroves of south India has already been reported (Ranjan et al., 2013). Increased levels of Cd recorded in deeper parts of the core suggested the occurrence of trapping of Cd-rich marine sediments in the mangrove sediments from the deep oceans in the past. Cadmium is also precipitated as sulfides under anoxic conditions (Lapp, 1991). Ni can be found along with pyrite and sulfidic minerals. Fairly higher concentration of Ni accompanied with higher TS at S3, indicated
Fig. 3. Variation of trace metals in the sediment core. 3
Marine Pollution Bulletin 153 (2020) 110946
M.N. Manju, et al.
Funding
Table 1 Factor loading from Principal Component Analysis with equamax rotation.
The study was funded by the Ministry of Earth Sciences (Grant No: MoES/11-MRDF/1/37/P/08), India, Department of Science and Technology (Grant No: SR/S4/ES-290/2007), India, and Kerala State Council for Science, Technology and Environment (Grant No: 404/ 2011/KSCSTE), Kerala, India.
Component
pH Eh Sand Silt Clay TOC TN TS Cd Co Cu Fe Mn Ni Pb Zn
1
2
3
0.071 −0.170 −0.561 0.669 0.442 0.262 0.250 0.183 0.259 0.889 0.582 0.885 0.675 0.888 0.702 0.775
0.816 −0.461 −0.677 0.428 0.724 0.897 0.892 0.677 0.218 0.237 0.734 0.309 −0.269 0.148 0.366 0.546
0.100 0.716 0.151 −0.044 −0.187 −0.172 0.015 −0.291 0.812 0.017 −0.082 0.145 0.538 0.133 0.272 −0.151
CRediT authorship contribution statement M.N. Manju: Investigation, Writing - original draft, Resources. C.S. Ratheesh Kumar: Investigation, Writing - original draft, Writing - review & editing, Resources, Conceptualization. P. Resmi: Writing - review & editing, Formal analysis, Visualization, Resources. T.R. Gireeshkumar: Investigation, Resources. Manju Mary Joseph: Investigation. P.M. Salas: Investigation. N. Chandramohanakumar: Conceptualization, Writing - review & editing, Supervision, Project administration, Funding acquisition. Acknowledgment
moderately polluted condition due to Cd at S2 (25–35 cm), S3 (0–5 cm), and S5 (0–5 cm, 35–45 cm, 45–55 cm). PLI > 1 indicate progressive deterioration of sediment quality (Tomlinson et al., 1980), and the samples from S5 (0–5 cm) showed PLI > 1. According to Sediment Quality Guidelines (SQGs) (Long et al., 1995), there was no or rare biological effects for almost all metals in the study region except Ni. Ni content exceeded ERM (effect range medium) at S2 and S5, S3 (0–5 and 5–15 cm), and S4 (35–45 cm and 45–55 cm), which suggest its serious effects on benthic organisms in the mangrove forests. Higher than ERL (effects range low) was observed for Cu at S2 (5–15 cm, 35–45 cm); Cd (S2: 25–35 cm, S3: 0–5 cm, S5: 0–5 and 45–55 cm) and Ni at S1 and S3. Principal Component Analysis was used to deduce the geochemical processes in the mangrove forest ecosystems (Table 1). Three factors were generated that explained a total of 78.95% variance. The first component accounted for 34.2% of total variance and showed positive loadings on silt, clay, Cu, Fe, Mn Co, Pb, Ni, and Zn and negative loading on sand, which explained the granulometric dependence of trace metals and also indicated scavenging of heavy metals by Fe hydroxides (Sarika and Chandramohanakumar, 2008). This factor also revealed the key control of Fe over the linkage of these metals with organic matrix by association as Fe-oxy-hydroxides (Rubio et al., 2000). Metals (except Cd) are strongly related with each other, reflected their identical behaviour and origin from common source. Component 2 (described 33.56% of total variance) composed of positive loadings on pH, clay, TOC, TN, TP, TS, Zn, and Cu and negative loadings on sand, and Eh suggested digenetic processes. It supports the formation of CuS and ZnS, but higher loadings of Cu than Zn indicated that formation of more stable complex of CuS (Besser et al., 1996). Component 3 accounted for 11.18% of total variance and consisted of significant positive loadings on Eh, Cd and Mn, which provided the idea that Mn oxides serve as adsorption site for Cd. Correlation analysis of Cd and Mn also supported the above inference (R2 = 0.47). Accumulation of heavy metals was enhanced by the fine sediment granulometry and enriched organic carbon content. The cycling of Fe and Mn is mainly controlled by redox conditions and the organic matter decay, which also affects the concentrations and associations of other heavy metals. Precipitation of heavy metals as sulfides is one of the important mechanisms controlling the dispersal of Cu, Ni, and Zn. The environmental conditions and the processes evaluated through this study is a baseline information, which could be useful while proposing conservation strategies for the sustainable management of the mangrove forests.
We express our sincere gratitude to Dean and Director, School of Marine Sciences, Cochin University of Science and Technology for providing the facilities. We also acknowledge the laboratory and instrumental facilities provided by the Head, Department of Chemical Oceanography and the Hon. Director, Inter-University Centre for Development of Marine Biotechnology, School of Marine Sciences, CUSAT. References Agoramoorthy, G., Chen, F.A., Hsu, M.J., 2008. Threat of heavy metal pollution in halophytic and mangrove plants of Tamil Nadu, India. Environ. Pollut. 155, 320–326. Alberts, J.J., Filip, Z., 1998. Metal binding in estuarine humic and fulvic acids: FTIR analysis of humic acid-metal complexes. Environ. Technol. 19, 923–931. Alloway, B.J., 1990. Soil processes and the behaviour of metals. In: Alloway, B.J. (Ed.), Heavy Metals in Soils. J Wiley and Sons, Inc, New York, Blackie, pp. 7–28. Andrade, R.A., Sanders, C.J., Boaventura, G., Patchineelam, S.R., 2012. Pyritization of trace metals in mangrove sediments. Environ. Earth Sci. https://doi.org/10.1007/ s12665-012-1620-4. Besser, J.M., Ingersoll, C.G., Giesty, J.P., 1996. Effects of spatial and temporal variation of acid-volatile sulfide on the bioavailability of copper and zinc in freshwater sediments. Environmental Toxicology and Chemistry: An International Journal 15, 286–293. Bouillon, S., Moens, T., Dehairs, F., 2004. Carbon sources supporting benthic mineralization in mangrove and adjacent seagrass sediments (Gazi Bay, Kenya). Biogeosciences 1, 71–78. Cowie, G.L., Hedges, J.I., 1992. The role of anoxia in organic matter preservation in coastal sediments: relative stabilities of the major biochemical under oxic and anoxic depositional conditions. Org. Geochem. 19, 229–234. Dhanakumar, S., Murthy, K.R., Solaraj, G., Mohanraj, R., 2013. Heavy-metal fractionation in surface sediments of the Cauvery River Estuarine Region, Southeastern coast of India. Arch. Environ. Contam. Toxicol. 65, 14–23. Hedges, J.I., Keil, R.G., 1995. Sedimentary organic matter preservation: an assessment and speculative synthesis. Mar. Chem. 49, 81–115. Keil, R.G., Montlucon, D.B., Prahl, F.G., Hedges, J.I., 1994. Sorptive preservation of labile organic matter in marine sediments. Nature 370, 549–552. Lapp, B., 1991. Metallmobilität in marinen Sedimenten der Kieler Bucht. Berichte aus dem, Christian-Albrechts-Universität. Long, E.R., MacDonald, D.D., Smith, S.L., Calder, F.D., 1995. Incidence of adverse biological effects within ranges of chemical concentrations in marine and estuarine sediments. Environ. Manag. 19, 81–97. Loring, D.H., Rantala, R.T.T., 1992. Manual for the geochemical analyses of marine sediments and suspended particulate matter. Earth Science Review 32, 235–283. Manju, M.N., Resmi, P., Ratheesh Kumar, C.S., Gireesh Kumar, T.R., Chandramohanakumar, N., Joseph, M.M., 2016. Biochemical and stable carbon isotope records of mangrove derived organic matter in the sediment cores. Environmental Earth Science 75, 1–15. Marchand, C., Fernandez, J.M., Moreton, B., 2016. Trace metal geochemistry in mangrove sediments and their transfer to mangrove plants (New Caledonia). Sci. Total Environ. 562, 216–227. Martin, G.D., Nisha, P.A., Balachandran, K.K., Madhu, N.V., Nair, M., Shaiju, P., Joseph, T., Srinivas, K., Gupta, G.V.M., 2011. Eutrophication induced changes in benthic community structure of a flow-restricted tropical estuary (Cochin backwaters), India. Environ. Monit. Assess. 176, 427–438. Müller, G., 1979. Schwermetalle in den Sedimenten des Rheins-Veränderungen seit 1971. Umschau 79, 778–783.
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sediments of the Ria de Vigo (NW Spain), an assessment of metal pollution. Mar. Pollut. Bull. 40, 968–980. Sarika, P.R., Chandramohanakumar, N., 2008. Distribution of heavy metals in mangrove sediments of Cochin estuary. Research Journal of Chemistry and Environment 12, 37–44. Tomlinson, D.L., Wilson, J.G., Harris, C.R., Jeffrey, D.W., 1980. Problems in the assessment of heavy-metal levels in estuaries and the formation of pollution. Helgoländer Meeresuntersuchungen 33, 566–575. Wedepohl, K.H., 1995. The composition of the continental crust. Geochimica et Cosmochimic Acta 59, 217–239.
Ranjan, R.K., Ramanathan, A.L., Singh, G., Chidambaram, S., 2008. Assessment of metal enrichments in tsunami genic sediments of Pichavaram mangroves, southeast coast of India. Environ. Monit. Assess. 147, 389–411. Ranjan, R.K., Singh, G., Routh, J., Ramanathan, A.L., 2013. Trace metal fractionation in the Pichavaram mangrove-estuarine sediments in southeast India after the tsunami of 2004. Environ. Monit. Assess. 185, 8197–8213. Ratheesh Kumar, C.S., Joseph, M.M., Gireeshkumar, T.R., Renjith, K.R., Manju, M.N., Chandramohanakumar, N., 2010. Spatial variability and contamination of heavy metals in the inter-tidal systems of a tropical environment. International Journal of Environmental Research 4, 91–700. Rubio, B., Nombela, M.A., Vilas, F., 2000. Geochemistry of major and trace metals in
5
Contents lists available at ScienceDirect
Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul
Baseline
Trace metal distribution in the sediment cores of mangrove ecosystems along northern Kerala coast, south-west coast of India
T
⁎
M.N. Manjua, C.S. Ratheesh Kumarb, , P. Resmic, T.R. Gireeshkumard, Manju Mary Josepha, P.M. Salasa, N. Chandramohanakumare a
Department of Chemical Oceanography, School of Marine Sciences, Cochin University of Science and Technology, Kochi, PIN-682016, Kerala, India School of Environmental Studies, Cochin University of Science and Technology, Kochi, PIN-682022, Kerala, India c Aquatic Environment and Management, Kerala University of Fisheries and Ocean Studies, Panangad, Kerala PIN-682506, India d National Institute of Oceanography, Regional Centre-Kochi, Dr. Salim Ali Road, Kochi, PIN-682018, India e Inter-University Centre of Marine Biotechnology, School of Marine Sciences, CUSAT, Kochi, PIN-682016, Kerala, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: Trace metals Mangroves Pollution Core sediment
Core sediment samples were collected from five mangrove ecosystems along northern Kerala coast (Kunjimangalam: S1, Pazhayangadi: S2, Pappinissery: S3, Thalassery: S4, and Kadalundi: S5) to assess the status of heavy metal pollution. S1 recorded comparatively lower metal concentration at surface (except Pb) due to low organic content and sandy texture, while the reverse was true for S3. Higher metal contents were recorded at S5 (0–5 cm), which was attributed to its unique biogeochemical behavior. Enrichment factor and geoaccumulation index indicated moderate enrichment for Cd, and the pollution load index revealed progressive deterioration of sediment quality at S5 (0–5 cm). There was no harmful effect of trace metals on biological community (except Ni) according to Sediment Quality Guidelines. Major processes controlling trace metal accumulation in these systems are diagenetic processes, precipitation of heavy metals as sulfides, and the presence of Fe, Mn-oxy hydroxides, which act as adsorption sites for other metals.
Mangrove sediments act as a biogeochemical sink for metals, and trace metal concentration has been found to be increasing because of pollution and urbanization (Agoramoorthy et al., 2008). Heavy metal pollution has been investigated by many authors in Indian estuaries (Ratheesh Kumar et al., 2010; Martin et al., 2011; Dhanakumar et al., 2013). However, scarce information is available on the geochemical characteristics of mangrove ecosystems along northern Kerala coast, even though 83% of the state's mangrove cover is contributed by these systems. The present study is an attempt to understand the trace metal concentration and its distribution in the mangrove sediment cores of northern Kerala coast, which can reveal the effects of anthropogenic and natural processes in these environments. The extent of contamination was evaluated by using indices such as geo-accumulation index, enrichment factor, and pollution load index. Location of sampling stations is presented in Fig. 1. Station 1, Kunjimangalam (S1) in the estuarine environment formed by Pullamcode puzha and Kunjimangalam River, is found to be almost free from anthropogenic inputs. At station 2, Pazhayangadi (S2), domestic waste disposal was the major source of pollution. Station 3, Pappinissery (S3), is formed on the banks of Valapattanam estuary (area of
⁎
about 20 ha), and a plywood manufacturing industry is located on the confluence of S3. Domestic waste disposal and discharges of industrial effluents are the major source of pollution at S3. Station 4, Thalassery (S4), is situated near the barmouth, and domestic waste disposal is the main source of pollution at this station. Mangrove forests in station 5, Kadalundi (S5), are located on the banks of Kadalundi River that meets the Arabian Sea through a permanent barmouth (Manju et al., 2016). Fishing, boat operations, sewage drainage from mainland, industrial and other commercial activities are the potential sources of trace metals at S5. Core sediment samples were collected manually using a PVC pipe (internal diameter: 65 mm and length: 65 cm) from five mangrove ecosystems along the northern Kerala coast during October 2009. First segment of the core was selected as (0–5 cm), and the remaining portions were sliced at 10 cm intervals. General parameters like pH (Eutech, pH Tester), texture (pipette method), total carbon-TC and total sulfur-TS (CHNS Analyser-Vario EL III) and total organic carbon [TOC] (Element Analyser-Vario TOC Select Elementar) were analyzed using standard methods. For the analysis of trace metals (total), 0.2 g of freeze dried and finely powdered sediment samples were digested with a 1:5
Corresponding author. E-mail address: [email protected] (C.S. Ratheesh Kumar).
https://doi.org/10.1016/j.marpolbul.2020.110946 Received 5 August 2019; Received in revised form 27 January 2020; Accepted 28 January 2020 Available online 09 February 2020 0025-326X/ © 2020 Elsevier Ltd. All rights reserved.
Marine Pollution Bulletin 153 (2020) 110946
M.N. Manju, et al.
Fig. 1. Map showing sampling locations.
PLI = (CF1 ∗ CF2 ∗ …CFn )1/n
mixture of HClO4-HNO3 (Loring and Rantala, 1992) in a microwave reaction system (Perkin Elmer, Anton Paar — Multiwave 3000). The residue was dissolved in 0.5 M HCl and made up to known volume by adding Milli Q water. Trace metals analyzed were Cu, Cd, Co, Pb, Ni, Fe, Mn, and Zn using Atomic Absorption Spectrometer (Perkin Elmer 3110) after calibration with suitable elemental standards (Merck, Germany). The accuracy of trace metal determination was validated using BCSS-1 (standard reference material for marine and estuarine sediments) from the National Research Council, Canada. Triplicate analysis of BCSS-1 showed a good accuracy, and the recovery rates were 91.8% (Mn), 97.1% (Cu), 98.5% (Cd), 95.1% (Co), 92.4% (Pb), and 98.5% (Zn). To assess the contamination status of the mangrove ecosystems, pollution indices such as contamination factor (CF), enrichment factor (EF), pollution load index (PLI), and geo-accumulation index (Igeo) were estimated. The enrichment factor was calculated for each metal by using iron as a normalizing element using the equation
where CF is the contamination factor, n represents number of metals and CF = concentration of metal in sediment/ world shale value of the metal. The concentration of various sedimentary parameters in the study area is provided in Fig. 2. The variation in general sediment characteristics is well documented by Manju et al. (2016). The surface samples (0–5 cm) recorded higher TOC content at all stations. TOC for S2, S3, and S4 exhibited a sharp decrease from top to bottom, which suggested intense mineralization (Bouillon et al., 2004). S1 and S5 revealed a decrease in TOC up to 15 cm and then remained uniform throughout the core (Fig. 2), which indicated preservation of organic matter under anoxic conditions (Cowie and Hedges, 1992; Keil et al., 1994; Hedges and Keil, 1995). Lower organic content and sandy texture resulted in lower metal concentration in sediments at S1, while the reverse was true for S2 and S3 (Fig. 3). In addition to mangrove input, the runoff from the land by tributary rivers as well as the location with respect to the Arabian Sea also resulted in higher accumulation of heavy metals at S5. The maximum content of Cu, Co, Mn, Ni, Pb, and Fe and elevated levels of Cd and Zn was observed at S5 (0–5 cm). However, the peak concentrations of Cd and Zn were recorded at S2 (0–5 cm) and S3 (25–35 cm) respectively. Enhanced build-up of the heavy metals at S5 (0–5 cm) can be attributed to the unique biogeochemical behavior of this mangrove ecosystem. The oxidation of organic matter yields acid volatile sulfide that acts as an important ligand for metals. The differences in the solubilities of metal sulfides account for the distribution pattern of metals. A general decrease in the concentration of Cu from top to bottom of the core was observed in all the mangrove forests. In lower parts of the core, the metals generally precipitate as sulfides or remained bound with organic
EF = (metal/Fe)sediment /(metal/Fe)crust . The geo-accumulation Index (Igeo), (Müller (1979), was calculated according to the equation:
Igeo = log2 (Cn/1.5 Bn) where Cn = measured concentration of heavy metal in the mangrove sediment, Bn = geochemical background value in average shale (Wedepohl, 1995) of element n, where 1.5 is the background matrix correction in factor due to lithogenic effects. The extent of contamination on biological community was assessed by determining the pollution load index (Tomlinson et al., 1980), and was evaluated using the equation, 2
Marine Pollution Bulletin 153 (2020) 110946
M.N. Manju, et al.
Fig. 2. Variation of general sedimentary parameters in the sediment core. (a to e) Grain size and TOC at each stations, (f) pH, (g) TN, (h) TS.
precipitation of insoluble nickel sulfides. The concentration of Ni and Fe in the sediments was higher than world shale average, and the possible reason can be erosion of laterite soils (Marchand et al., 2016). This observation was found to be more pronounced at S5 (0–5cm). The mobility of Zn is influenced by redox conditions and availability of organic matter (Ranjan et al., 2013). Positive correlations of Zn with TS (R2 = 0.57, n = 30) and highly significant negative correlation with Eh (R2 = 0.51, n = 30) supported the formation of ZnS (Alloway, 1990; Andrade et al., 2012). The metal concentrations in the present investigation were in good agreement with previous reports from mangrove ecosystems of Cochin and Pichavaram (Ranjan et al., 2008; Ratheesh et al., 2010). The exceptionally higher concentration of Zn at S3 (Fig. 3) can be attributed to pyrite formation. Estimated EF values reflected moderate enrichment for Cd at S1 (25–35 cm, 35–45 cm, 45–55 cm), S2 (0–5 cm and 25–35 cm) and S4 (35–45 cm), and S5 (0–5 cm, 35–45 cm); while moderately severe enrichment for Cd was noted at S3 (0–5 cm), S5 (45–55 cm); minor enrichment was observed for Co, Pb, and Ni. Estimated Igeo values implied
matter (Ranjan et al., 2013). Higher concentration of Cu at S3 was due to its association with clay minerals. The positive correlation between Cu and organic matter (R2 = 0.79, n = 30) indicated the complexation of Cu with humic substances (Alberts and Filip, 1998). Concentration of Pb was higher in sediment core collected from S5 (0–5cm). Increase in Pb content in the sediments can be ascribed to external sources, mainly from aquaculture effluents, agricultural runoff, domestic sewage, vehicular traffic and mechanised boats. Possible landbased sources of Cd pollution in mangrove sediments are fertilizers, pesticides and paint industries (Ranjan et al., 2008). Trapping of Cdrich marine sediments in deeper parts of the sediment cores from Pichavaram mangroves of south India has already been reported (Ranjan et al., 2013). Increased levels of Cd recorded in deeper parts of the core suggested the occurrence of trapping of Cd-rich marine sediments in the mangrove sediments from the deep oceans in the past. Cadmium is also precipitated as sulfides under anoxic conditions (Lapp, 1991). Ni can be found along with pyrite and sulfidic minerals. Fairly higher concentration of Ni accompanied with higher TS at S3, indicated
Fig. 3. Variation of trace metals in the sediment core. 3
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Funding
Table 1 Factor loading from Principal Component Analysis with equamax rotation.
The study was funded by the Ministry of Earth Sciences (Grant No: MoES/11-MRDF/1/37/P/08), India, Department of Science and Technology (Grant No: SR/S4/ES-290/2007), India, and Kerala State Council for Science, Technology and Environment (Grant No: 404/ 2011/KSCSTE), Kerala, India.
Component
pH Eh Sand Silt Clay TOC TN TS Cd Co Cu Fe Mn Ni Pb Zn
1
2
3
0.071 −0.170 −0.561 0.669 0.442 0.262 0.250 0.183 0.259 0.889 0.582 0.885 0.675 0.888 0.702 0.775
0.816 −0.461 −0.677 0.428 0.724 0.897 0.892 0.677 0.218 0.237 0.734 0.309 −0.269 0.148 0.366 0.546
0.100 0.716 0.151 −0.044 −0.187 −0.172 0.015 −0.291 0.812 0.017 −0.082 0.145 0.538 0.133 0.272 −0.151
CRediT authorship contribution statement M.N. Manju: Investigation, Writing - original draft, Resources. C.S. Ratheesh Kumar: Investigation, Writing - original draft, Writing - review & editing, Resources, Conceptualization. P. Resmi: Writing - review & editing, Formal analysis, Visualization, Resources. T.R. Gireeshkumar: Investigation, Resources. Manju Mary Joseph: Investigation. P.M. Salas: Investigation. N. Chandramohanakumar: Conceptualization, Writing - review & editing, Supervision, Project administration, Funding acquisition. Acknowledgment
moderately polluted condition due to Cd at S2 (25–35 cm), S3 (0–5 cm), and S5 (0–5 cm, 35–45 cm, 45–55 cm). PLI > 1 indicate progressive deterioration of sediment quality (Tomlinson et al., 1980), and the samples from S5 (0–5 cm) showed PLI > 1. According to Sediment Quality Guidelines (SQGs) (Long et al., 1995), there was no or rare biological effects for almost all metals in the study region except Ni. Ni content exceeded ERM (effect range medium) at S2 and S5, S3 (0–5 and 5–15 cm), and S4 (35–45 cm and 45–55 cm), which suggest its serious effects on benthic organisms in the mangrove forests. Higher than ERL (effects range low) was observed for Cu at S2 (5–15 cm, 35–45 cm); Cd (S2: 25–35 cm, S3: 0–5 cm, S5: 0–5 and 45–55 cm) and Ni at S1 and S3. Principal Component Analysis was used to deduce the geochemical processes in the mangrove forest ecosystems (Table 1). Three factors were generated that explained a total of 78.95% variance. The first component accounted for 34.2% of total variance and showed positive loadings on silt, clay, Cu, Fe, Mn Co, Pb, Ni, and Zn and negative loading on sand, which explained the granulometric dependence of trace metals and also indicated scavenging of heavy metals by Fe hydroxides (Sarika and Chandramohanakumar, 2008). This factor also revealed the key control of Fe over the linkage of these metals with organic matrix by association as Fe-oxy-hydroxides (Rubio et al., 2000). Metals (except Cd) are strongly related with each other, reflected their identical behaviour and origin from common source. Component 2 (described 33.56% of total variance) composed of positive loadings on pH, clay, TOC, TN, TP, TS, Zn, and Cu and negative loadings on sand, and Eh suggested digenetic processes. It supports the formation of CuS and ZnS, but higher loadings of Cu than Zn indicated that formation of more stable complex of CuS (Besser et al., 1996). Component 3 accounted for 11.18% of total variance and consisted of significant positive loadings on Eh, Cd and Mn, which provided the idea that Mn oxides serve as adsorption site for Cd. Correlation analysis of Cd and Mn also supported the above inference (R2 = 0.47). Accumulation of heavy metals was enhanced by the fine sediment granulometry and enriched organic carbon content. The cycling of Fe and Mn is mainly controlled by redox conditions and the organic matter decay, which also affects the concentrations and associations of other heavy metals. Precipitation of heavy metals as sulfides is one of the important mechanisms controlling the dispersal of Cu, Ni, and Zn. The environmental conditions and the processes evaluated through this study is a baseline information, which could be useful while proposing conservation strategies for the sustainable management of the mangrove forests.
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