Elemental geochemistry in acid sulphate soils – A case study from reclaimed islands of Indian Sundarban

Marine Pollution Bulletin 138 (2019) 501–510

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Elemental geochemistry in acid sulphate soils – A case study from reclaimed islands of Indian Sundarban

T

Somdeep Ghosha, Madhurima Bakshia, Shubhro Mitraa, Shouvik Mahantya, Shidharth Sankar Ramb, Shamayita Banerjeec, Anindita Chakrabortyd, M. Sudarshand, ⁎ Subarna Bhattacharyyae, Punarbasu Chaudhuria, a

Department of Environmental Science, University of Calcutta, West Bengal, India Ion Beam Laboratory, Institute of Physics, Odisha, India c Department of Zoology, Charuchandra College, West Bengal, India d UGC-DAE Consortium for Scientific Research, West Bengal, India e School of Environmental Studies, Jadavpur University, Kolkata, India b

A R T I C LE I N FO

A B S T R A C T

Keywords: Acid Sulphate soil Sediment quality Sundarban Trace metals Enrichment Geo accumulation

Sundarban along with its networks of rivers, creeks and magnificent mangroves form a unique ecosystem. Acid sulphate soils have developed in this ecosystem under anoxic reducing conditions. In the present study, we have investigated the distribution of acid sulphate soils along with its elemental characterization and possible sources in four reclaimed islands of Indian Sundarban like Maushuni (I1), Canning (I2), Bally (I3) and Kumirmari (I4). Elements show moderate to strong correlation with each other (P < 0.01; P < 0.05). Except Si, Ca and Pb, a higher enrichment factor was observed for K, Cr, Mn, Fe, Ni, Cu and Zn. Geo-accumulation index values of all sampling locations reveal that Cr, Fe, Cu and Zn are in Igeo class 1. The pollution load index value of the reclaimed islands of Indian Sundarban varies between 1.31 and 1.48. The observation of this study could help to strategize policies to mitigate and manage acid sulphate soils in Indian Sundarban.

Acid sulphate soils (ASS) have developed in coastal and estuarine regions under anoxic reducing conditions beneath the water table due to microbial activities; which consists of iron pyrites (Dent and Pons, 1995; White et al., 1997; Wallin et al., 2015). Globally, ~170,000 sq. km. of ASSs are found in tropics and subtropics (Andriesse and Van Mensvoort, 2002; Huang et al., 2017). They remain stable under anoxic reducing conditions, thereby, causing no harm to the surrounding habitat (Nath et al., 2013a). Due to their potentiality to produce sulphuric acid and extremely low pH in presence of water, they are often described as the nastiest type of soils around the world (Dent and Pons, 1995). Production of any acidity in contact of water can be quickly normalized either by the buffering capacity of soils or by the alkaline water (Sammut et al., 1996). The sulphide minerals (in the form of iron pyrites) present in these soils and/or sediments along with organic matter are rapidly oxidized in presence of air (Boman et al., 2008). Thermodynamic calculations show that pyrites are only stable in absence of air. Sedimentary iron pyrites are formed by following processes: a) microbial reduction of sulphate using organic matter as reducing agent; b) reaction between iron minerals and H2S to form iron mono sulphides; c) reaction between elemental sulphur and iron mono ⁎

sulphides to form iron pyrites (Berner, 1984). Oxidations of sulphide layers in soil, may be due to natural (e.g. drought, draining or irrigation during cultivation) or human induced (e.g. land use change, drainage, groundwater withdrawal) interventions, resulting in the generation of sulphuric acid, and consequently causing soil acidification and leaching of trace metals (Ferreira et al., 2007; Ljung et al., 2009; Boman et al., 2010; Amaral et al., 2011; Wallin et al., 2015). Thus, surface run-off from agricultural fields containing high concentrations of dissolved trace metals with low pH are gradually deteriorating ecological quality of adjacent water bodies e.g. lakes, ponds, streams, river and estuary. Throughout the world, the coastal estuarine habitats serves as important and unique social, economical and cultural centers (Rönnbäck et al., 2007; Birch et al., 2013; Nath et al., 2014) with major ecological significance (Nagelkerken et al., 2000; MacFarlane et al., 2007). Coastal soils consisting mangroves sequester trace metals, which protects coastal marine habitats from pollution (Lacerda et al., 1988; Clark et al., 1998). However, such coastal habitats e.g. Sundarban are susceptible to over utilization from human induced activities and climate change phenomenon (Ghosh et al., 2015). In most region of the world, for thousands of years, paddy cultivation has been traditionally carried

Corresponding author. E-mail address: [email protected] (P. Chaudhuri).

https://doi.org/10.1016/j.marpolbul.2018.11.057 Received 6 September 2018; Received in revised form 19 November 2018; Accepted 23 November 2018 0025-326X/ © 2018 Elsevier Ltd. All rights reserved.

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Trimble Juno SD hand-held GPS. ASS samples were dried in a hot air oven at 60 °C and homogenized for physico-chemical and elemental analysis. pH and electrical conductivity (EC) of the ASS samples were measured following Sparks et al. (1996). Organic carbon (OC) content of the ASS was determined with help of weight loss of powdered soil samples by loss on ignition method following Nath et al. (2013a). Total sulphuric acidity (TSA) and salinity was measured following protocols described in Hendro-Prasety and Janssen (1990) and APHA (2005) respectively. All of the analysis was done in triplicate and mean value was considered. Homogenized surface ASS samples were sieved using 63 μm nylon sieves. 150 mg of sieved sample were pelletized using pelletizer (100–130 kg/cm2). All measurement of ASS samples was carried out in vacuum, using a Xenematrix (Ex −3600) Energy dispersive X-ray fluorescence (EDXRF) spectrometer, which comprises of an oil-cooled Rh anode X-ray tube (maximum current 1 mA; voltage 50 kV). The measurements were carried out using different type of filters for optimum detection of examined elements like Titanium Filter for K, Ca, Cr, Mn, Fe, Ni, Cu and Zn, Iron Filter for Pb whereas no filter were used for Al and Si. Certified reference material of estuarine sediment having major elements (e.g. Al, Si, Ca and K) and trace elements (e.g. Fe, Cu, Cr, Mn, Ni, Zn and Pb) from National Institute of Standards and Technology NIST (SRM 1646a) was run in XRF to ensure the quality and accuracy of the experiment. Triplicate measurement of certified reference material was taken to estimate the analytical quality, accuracy and precision. Distribution of major and trace elements along with physico-chemical properties of surface sediments were statistically analyzed. Pearson correlation coefficients between organic carbon content and elemental accumulation were done. Analysis of variance (ANOVA) was carried out to evaluate the variances of elemental distribution in surface sediment of reclaimed islands of Indian Sundarban. The calculated values of F were compared with critical value ‘Fcrit’. If F > Fcrit, hypothesis (H1) of significant statistical deviations within the calculated means are accepted. Otherwise, Null hypotheses (H0) of equality amid the evaluated means were accepted (Ghosh et al., 2016). General statistical analyses were done in Microsoft Excel Program. Cluster analysis (CA) or clustering is a statistical procedure of grouping a set of observations or objects in such a way that the observations or objects will be in the same cluster or group on the basis of their correlation or similarity with each other than to those in other clusters or groups. The most similar data set or observations are linked together to form a group or cluster and the methodology is repeated until all data set or observations belongs to one group or cluster (Birth, 2003). Principal component analysis (PCA) is a multivariate mathematical procedure that used to analyze the differences. It uses an orthogonal transformation model to change a group of data of correlated variables into a group of values of linearly uncorrelated variables, namely, principal components. The total number of principal components is ≤ the number of observations or the smaller of the number of original variables. It estimates and extracts the Eigen characteristics and vectors from the covariance network of unique factor. It can be used as a procedure in analysis of data and for creation of presumptive statistical models. However, PCA has been utilized to implicate more information on connectivity amid correlation between examined elements, sampling locations and accumulation of pollutants in a study (Singh et al., 2004; Yalcin et al., 2008; Watts et al., 2017). CA and PCA were done in statistical software SPSS16. Sediment geochemical background value plays a significant role in proper understanding and scientific interpretation of sediment quality data (Birch, 2017). Evaluation of different sediment quality indices depends on the proper geochemical background values, among 52 different pre-anthropogenic/background/pristine metal concentration values generated from different global projects (Birch, 2017). Due to the absence of any regional geochemical background value of sediment forces, researchers e.g. Banerjee et al. (2012), Chakraborty et al.

out in acid sulphate soils and serves as a major source of staple food for local inhabitants (Lin and Melville, 1994; Lin et al., 1995). The oxidation of sulphides and consequent soil acidification may be regulated by high levels of groundwater in paddy fields, while extensive and efficient irrigation facilities accelerate the loss of acid and trace metals in adjacent water bodies or streams (Bronswijk et al., 1995; Huang et al., 2017). Sundarban mangrove wetland is located at the Hooghly Matla estuarine region in the lower deltaic Bengal and belongs geographically between the imaginary Dampier and Hodges line in the northwest, Bay of Bengal in the south, river Hooghly in the west and river Icchamati in the east, Raimangal, Kalindi and Harinbari forming the International boundary between India and Bangladesh. Indian part of Sundarban consists of 102 islands in which ~30% are virgin and ~70% are almost reclaimed (Mandal et al., 2012). This highly vulnerable and eco-sensitive region in the lower stretch of Bengal is typically sustained by a complex system of tidal creeks (Bakshi et al., 2018a, 2018b). There is a knowledge gap regarding accumulation, speciation and behavior pattern of trace metals in estuarine region, affected by ASSs (Faltmarsch et al., 2008; Nystrand et al., 2012). Therefore, evaluation of effects of ASSs on estuaries is necessary, as their water chemistry e.g. distribution of toxic trace metals, pH, salinity level directly varies with micro environmental conditions e.g. rainfall, tidal amplitude. Extensive export of acidic surface run-off with elevated trace metal concentrations from paddy fields may have detrimental effects on aquatic biodiversity (Faltmarsch et al., 2008). Exposure to low pH and elevated trace metal concentrations impairs normal growth and reproduction of aquatic species (Hudd, 2000) and accumulation of trace metals in marine and estuarine life forms e.g. vegetation, fish, crustaceans, microbes which in turn have deleterious effects on aquatic flora and fauna (Yusof et al., 1994; Sammut et al., 1996; Mitra et al., 2012; Ghosh et al., 2016). The present works aims to understand the elemental geochemistry of ASSs in Indian Sundarbans which has never been reported yet, as far our knowledge and objectives of present study are (i) to quantify and reveal the distribution and variation of toxic trace metals in the ASSs of reclaimed islands Indian Sundarban; (ii) to evaluate the degree of trace metal contamination by using different geochemical indices, and (iii) to assess the effect of human induced and natural interventions on the distribution of trace metals in the ASSs of vulnerable islands of Indian Sundarban. Sundarban (latitude: 21° 31′ to 22° 30′ N and longitude: 88° 10′ to 89° 51′ E), UNESCO-world heritage site, is a fragile and vulnerable ecosystem susceptible to climate change phenomenon e.g. cyclone, flood, sea level rise and anthropogenic activities e.g. cultivation and aquaculture (Ghosh et al., 2015). The geological development of the Sundarban is of recent. Since the Tertiary period, several geomorphological events have occurred in the region leading to the development of Bengal Basin and formation of the Sundarban Delta (Wadia, 1961; Gopal and Chauhan, 2006). Four reclaimed islands covering varied riverine network and eastern to western border of Indian Sundarban namely Maushuni (I1) at the confluence of river Hooghly and Bay of Bengal, Canning (I2) at the river Matla, Bally (I3) at river Bidya and Kumirmari (I4) at river Raimangal have been chosen to identify the distribution of acid sulphate soil associated trace metals in paddy cultivating agricultural lands (Fig. 1 & Table 1). All of the islands represent a wide range of floral and faunal diversity. Avicennia sp., Sonneratia sp., Exocoecaria sp., are the dominant mangrove vegetation found mostly along the periphery of the islands. Four reclaimed islands were thoroughly surveyed to identify the ASSs in agricultural fields. Surface ASS samples (0–5 cm) were collected in triplicates from agricultural fields at seven sampling locations each in four reclaimed islands of Indian Sundarban viz. Mausuni (I1), Canning (I2), Bally (I3), and Kumirmari (I4) in May 2016, in order to assess distribution of toxic trace metal. Collected sediment samples were kept in an ice box and transferred to laboratory and stored at 4 °C. The geographical co-ordinates of study sites were taken with the help of 502

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Fig. 1. Study area. Table 1 Geo chemical background value in upper continental crust (UCC).a

Table 2 Physico chemical properties of soil (n = 21 in each sampling locations).

Geo chemical background value in upper continental crust Elements Al Si K Ca Cr Mn Fe Ni Cu Zn Pb a

Sample

pH

Concentration (mg/kg) 150,500 658,900 31,900 42,400 85 700 40,900 44 25 71 17

(2014), Ghosh et al. (2016) and Bakshi et al. (2017) to apply shale values (Turekian and Wedepohl, 1961; Wedepohl, 1995) and/or upper continental crust (UCC) values (Taylor and McLennan, 1985) as geochemical background values of the region. In our study, we have considered the UCC values (Taylor and McLennan, 1985) as geochemical background value (Table 1) of major elements and trace metals. Enrichment factors (EF) of elements are geochemical index utilized to evaluate the degree of anthropogenic changes in surface sediments of reclaimed islands of Indian Sundarban. The index is based on hypothesis that under uninterrupted natural conditions there is a linear 503

Total organic matter (g/ Kg)

Salinity (g/Kg)

Total sulphuric acidity (meq/ 100 g)

Maushani (I1) Max 4.89 Min 3.42 Mean 4.05 SD 0.57

1250 1000 1112.9 81.6

3.07 0.72 1.36 0.87

0.28 0.19 0.25 0.03

16.52 7.03 9.90 3.43

Bally (I2) Max Min Mean SD

4.69 3.60 4.17 0.38

1550 970 1195.7 206.8

37.80 17.70 28.39 8.03

1.61 0.12 0.58 0.68

42.20 9.06 25.34 14.02

(I3) 4.74 2.18 3.68 0.80

1390 1010 1212.9 150.4

77.50 1.86 39.76 36.05

0.79 0.23 0.36 0.20

41.40 0.50 16.22 15.82

Kumirmari (I4) Max 4.35 Min 3.37 Mean 3.87 SD 0.36

1650 865 1216.4 283.7

70.50 19.81 46.55 16.99

1.76 0.05 0.97 0.63

16.80 0.53 6.16 5.86

Canning Max Min Mean SD

Taylor and McLennan, 1985.

Electrical conductivity (μS/cm)

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Fig. 2. Box-Whisker plots of concentrations of macro elements found in ASS's of reclaimed islands of Indian Sundarbans. All the boxes represent the 25th percentile and the 75th percentile of elemental concentration, and the whiskers express the minimum and the maximum coefficients (5% and 95%), while the line within the boxes shows the median. Outliers are represents with ‘x’ mark.

of toxic elemental accumulation in surface sediments of reclaimed islands of Indian Sundarban (Hakanson, 1980; Chakraborty et al., 2014). It can be estimated as a ratio between examined element in sediment and its geochemical background value:

association between reference element and other element. Fe and Al are majorly used as reference element, whereas, we have utilized Al as reference element to normalize other examined elements. EF in sediments were evaluated as follows:

EF = EE x × Alb/EEb × Alx

Cf =

where EEx and Alx are the concentrations of the examined element and Al in surface sediment sample respectively, whereas EEb and Alb are their corresponding concentrations in a geochemical background value (Abrahim and Parker, 2008; Ghosh et al., 2016). Another geochemical indices, namely, Geo-accumulation index (Igeo) has been widely used to evaluated the magnitude of toxic elemental contamination in surface sediments of reclaimed islands of Indian Sundarban in seven enrichment class (Müller, 1969; Forstner et al., 1990). Igeo can be estimated as below

EE x EEb

where EEx is the concentration of the examined element and EEb is the respective element content in UCC value (Taylor and McLennan, 1985). According to Hakanson (1980) the surface sediments of the reclaimed islands of Indian Sundarban can be classified as follows Cf < 1, Low Contamination; 1 < Cf < 3, Moderate Contamination; 3 < Cf < 6, Considerable Contamination; Cf > 6, High Contamination. The pollution Load Index (PLI) was used to estimate the magnitude of elemental accumulation and pollution in surface sediments of the reclaimed islands of Indian Sundarban (Tomlinson et al., 1980). It can be evaluated as nth root of the product of Cf of n elements examined. It can be estimated as follows:

Igeo = Log 2 (Cn /1.5 × Bn) where Cn is the concentration of the examined element and Bn is the respective element content in UCC value (Taylor and McLennan, 1985). The factor 1.5 is introduced to correct background matrix which may results due to lithogenic changes. The surface sediments of reclaimed islands of Indian Sundarban was classified based on the Igeo value. According to Müller (1969) the degree of elemental contamination may be classified in a scale ranging between 0 and 6 (Igeo ≤0, unpolluted, class 0; 0 > Igeo < 1, unpolluted to moderately polluted, class 1; 1 < Igeo > 2, moderately polluted, class 2; 2 < Igeo > 3, moderately to strongly polluted, class 3; 3 > Igeo > 4, strongly polluted, class 4; 4 < Igeo > 5, strongly to extremely polluted, class 5; Igeo > 5, extremely polluted, class 6). The Contamination factor (Cf) was used to evaluate the magnitude

PLI = (Cf1 X Cf2 × ……….×Cfn )1/n

Cf =

EE x EEb

where EEx is the concentration of the examined element and EEb is the respective element content in UCC value (Taylor and McLennan, 1985). According to Tomlinson et al. (1980) based on PLI values the surface sediments of the Hooghly estuary can be categorized as follows PLI = 0, No Pollution; 0 < PLI < 1, Baseline level of Pollution; 1 < PLI < 6, Progressive deterioration of estuarine quality; PLI > 6, High Contamination. 504

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Fig. 3. Box-Whisker plots of concentrations of trace metals found in ASS's of reclaimed islands of Indian Sundarbans. All the boxes represent the 25th percentile and the 75th percentile of elemental concentration, and the whiskers express the minimum and the maximum coefficients (5% and 95%), while the line within the boxes shows the median. Outliers are represents with ‘x’ mark. Table 3 Correlation analysis of macro elements, trace metals and organic carbon in ASSs of reclaimed islands of Indian Sundarban.

Al Si K Ca Cr Mn Fe Ni Cu Zn Pb OC ⁎⁎ ⁎

Al

Si

K

Ca

Cr

Mn

Fe

Ni

Cu

Zn

Pb

1 0.699⁎⁎ ⁎⁎ 0.682 0.513⁎⁎ 0.299⁎⁎ −0.053 0.192 0.094 0.233⁎ 0.190 0.362⁎⁎ −0.188

1 0.538⁎⁎ ⁎⁎ 0.615 −0.059 0.168 −0.054 0.039 0.047 0.114 0.012 0.124

1 0.703⁎⁎ 0.216⁎ 0.066 0.403⁎⁎ 0.106 0.343⁎⁎ 0.347⁎⁎ 0.364⁎⁎ −0.064

1 −0.010 0.396⁎⁎ 0.204 0.090 0.235⁎ 0.353⁎⁎ 0.109 0.101

1 −0.170 0.495⁎⁎ 0.041 0.230⁎ 0.152 0.672⁎⁎ −0.537⁎⁎

1 0.127 0.153 0.342⁎⁎ 0.467⁎⁎ −0.216⁎ −0.069

1 0.234⁎ ⁎⁎ 0.441 0.450⁎⁎ 0.418⁎⁎ −0.207

1 0.145 ⁎⁎ 0.282 −0.031 −0.033

1 0.891⁎⁎ ⁎⁎ 0.337 −0.126

1 0.205 −0.032

1 −0.432⁎⁎

OC

1

Correlation is significant at the 0.01 level. Correlation is significant at the 0.05 level; n = 84.

100 g). Low pH value along with high TSA value is evident in all of the sampling location. Maximum TSA value and lowest pH value has been observed in Canning (S2) respectively. Value of pH is lower in the ASS's of reclaimed islands of Indian Sundarban might be result of oxidation of pyrites (FeS and FeS2 to Fe2SO4) and release of organic acids due to

Mean values of pH, EC, OC and TSA are shown in Table 2. Irrespective of the sampling stations of different islands, mean values of pH, EC, OC and TSA in surface sediments of reclaimed islands of Indian Sundarban varied between pH (2.18–4.89), EC (865–1650 μS/cm), Salinity (0.05–1.76 g/kg), OC (0.67–4.30%) and TSA (0.5–42.2 meq/ 505

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evolution of coastal sedimentary processes and tidal estuarine environments. The highest variation in distribution of elements was observed in Al, Si, K and Ca. In acid sulphate soils of reclaimed islands of Indian Sundarban, irrespective of sampling stations the distribution of major elements were found to be in the increasing order Ca < K < Al < Si and that of the trace and toxic metals were Pb < Ni < Cu < Zn < Cr < Mn < Fe. The most abundant elements in all the sediment samples of reclaimed islands were Si, Fe and Al which might be due to presence of lateritic and basaltic trappean rocks in the soil. The high and elevated concentrations of K in the soil are mainly due to the Archean gneissic rocks of granites and granodiorites (Sarkar et al., 2004; Ghosh et al., 2016). The highest concentration of most of the observed elements such as Al, Si, K, Cr, Fe, Cu, Zn and Pb were found in Bally Island (I3), Ca and Mn in Canning (I2) and elevated concentrations of Ni was observed in Kumirmari Island (I4) respectively. High concentrations of Fe along with low pH at all the sampling sites indicates oxidation of ASSs at reclaimed islands of Indian Sundarban due to specific hydrological variations (e.g. alternating periods of wetting and drying) in paddy cultivating agricultural lands (Huang et al., 2017). The discharge of acids during oxidation of ASSs mobilizes both major elements and trace metals, which in turn may have detrimental to the wetland ecosystem (Rosicky et al., 2004; Nath et al., 2013b). As evident from the data presented in Table 3, in ASS's of reclaimed islands of Indian Sundarban, the distribution of Pb was strongly correlated with Al, K, Cr, Fe and Cu (P < 0.01); Zn was strongly correlated with K, Ca, Mn, Fe, Ni and Cu (P < 0.01), Cu with K, Mn and Fe (P < 0.01); Fe had a strong correlation with K and Mn; Mn with Ca (P < 0.01); Cr with Al (P < 0.01), Ca with Al, Si and K (P < 0.01); K with Al and Si (P < 0.01); Si with Al (P < 0.01). As far as spatial distribution is concerned, Pb was strongly correlated with Mn (P < 0.05); Cu with Al, Ca and Cr (P > 0.05); Ni with Fe (P < 0.05); Cr with K (P < 0.05). The strong correlation among these elements suggests similar source of origin like natural (weathering and erosion) and human induced activities (agricultural or aquacultural run-off, industrial emission, domestic and municipal sewage) (Ladislao et al., 2015; Ghosh et al., 2016; Bakshi et al., 2017, 2018a). Due to relative low OC content, of it does not acts as a carrier or transporter of macro elements and trace metals, thereby, shows insignificant correlation with most of the elements and negative correlation with Cr and Pb (P < 0.01) (Banerjee et al., 2012; Rogers et al. (2013)). The ANOVA results showed statistically significant island specific variation in

Table 4 ANOVA coefficient of macro elements and trace metals of ASSs. Elements

F

P-value

F crit

Al Si K Ca Cr Mn Fe Ni Cu Zn Pb Degree of freedom

4.744793 6.604019 4.298893 6.903995 18.49966 63.72592 4.935819 6.257934 20.88741 49.39487 8.840367 Between groups 27

4.43E − 07 1.49E − 09 2.04E − 06 6.52E − 10 3.49E − 19 6.2E − 33 2.35E − 07 3.99E − 09 1.85E − 20 5.39E − 30 5.11E − 12 Within groups 56

1.685604 1.685604 1.685604 1.685604 1.685604 1.685604 1.685604 1.685604 1.685604 1.685604 1.685604 Total 83

decomposition of organic content (Baggie et al., 2018; Chatterjee et al., 2009) in agricultural fields. OC content estimated in the surface sediments of reclaimed islands are moderately low with maximum at Canning (S2) whereas salinity of surface sediments is maximum at Kumirmari (S4). The distribution of major and trace metals found in the surface sediments of reclaimed islands varied considerably with respect to their concentration and sampling locations. Such variations may be ascribed to (i) several physico-chemical factors e.g. grain size, OC content, Fe and/or Mn oxy-hydroxides, ligands and chelating agents (Banerjee et al., 2012; Guo et al., 1997); (ii) or to variation in source substrata undergone natural weathering process (Zhang and Gao, 2015) and (iii) diverse magnitude of pollutants released from anthropogenic interventions (Ghosh et al., 2016, 2018). The distribution of major elements in the reclaimed islands of Indian Sundarban varied between (51,419–115,411) mg/kg for Al, (121,860–229,807) mg/kg for Si, (16,529–30,030) mg/kg for K and (3497–9221) mg/kg for Ca respectively (Fig. 2). The concentrations for trace metals and toxic heavy metals ranged between (54.0–134.7) mg/kg for Cu, (23497–94,701) mg/kg for Fe, (400.8–952.1) mg/kg for Mn, (142.3–312.4) mg/kg for Cr, (32.2–111.8) mg/kg for Ni, (1.7–9.4) mg/ kg for Pb and (102.0–352.3) mg/kg for Zn respectively (Figs. 2 and 3). Studies by Lin (1995) and Smith et al. (2003) indicate that the spatial pattern of distribution of ASS often reflects the image of

Fig. 4. Enrichment factor of examined macro elements and trace metals at reclaimed islands of Indian Sundarbans (n = 21). 506

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Fig. 5. Geo- accumulation index (Igeo) of examined macro elements and trace metals at reclaimed islands of Indian Sundarbans (n = 21).

The significant correlation between Cu, Pb, Mn and Zn in ASS's indicates the application of pesticides and fertilizers in the adjacent agricultural fields as possible source (Raven et al., 1998; Majumdar et al., 2009; Wuana and Okieimen, 2011), which may flow into ASS as surface run-off. In addition, association of Mn, Fe, Cu, Pb, Ni and Zn in the ASS's might also be due to use of several biosolids like municipal sewage sludge, livestock manures and composts (Basta et al., 2005; Wuana and Okieimen, 2011). Attributed signature of Cu, Ni, Pb, Fe, Mn and V in ASS's suggests air borne industrial sources like thermal power plants, iron and steel industries (Chandra Mouli et al., 2006; Stafilov et al., 2010; Wuana and Okieimen, 2011; Bačeva et al., 2012). Hence, multiple sources such as local brick kilns, thermal power plants, iron and steel smelters, use of fertilizers, manures and pesticides at or near by and surrounding the sampling locations can be ascribed for the accumulation of trace metals observed in the present study. In our study, we have used various sediment quality indices (EF, Igeo, CF and PLI) for the assessment of soil contamination and to distinguish between the multiple sources of contamination of ASSs (discharge or emission or deposition from anthropogenic sources such as storm-water and agricultural and/or aquaculture run-off; industrial or mining activities; domestic and municipal wastes, and natural phenomenon like weathering and erosion) (Bastami et al., 2012; Ghosh et al., 2016). Mean EF values of major elements and trace metals for the ASS of four reclaimed islands of Indian Sundarban are ranged between Si (0.52–0.54), K (1.32–1.47), Ca (0.23–27), Cr (4.16–4.5), Mn (1.56–1.88), Fe (3.36–3.75), Ni (2.52–3.15), Cu (4.96–5.90), Zn (4.71–3.87) and Pb (0.47–0.58) (Fig. 4). The values shows following sequential orders Cu > Cr > Zn > Fe > Ni > Mn > K > Si > Pb > Ca in sampling locations I1 and I2; Cu > Zn > Cr > Fe > Ni > Mn > K > Pb > Si > Ca in I3 and Cu > Zn > Cr > Fe > Ni > Mn > K > Si > Pb > Ca in I4 respectively (Fig. 3). 0 > EF < 1 indicates that the enrichment of elements in soils are due to natural geogenic processes like weathering or erosion, whereas EF > 1 suggested that enrichment of elements are majorly due to anthropogenic process or activities (Sakan et al., 2009; Watts et al., 2017). Our study indicates the enrichment of all major elements and trace metal in all sampling locations (except Si, Ca and Pb) shows greater influence of anthropogenic activities along with natural geogenic process. We have also used Müller's (1969) Geo – accumulation index (Igeo) for further classification of contamination of ASS's of reclaimed islands of Indian Sundarban. The mean Igeo value for all the examined elements varied between −1.52 to −1.45 for Al, −2.45 to −2.38 for Si, −1.09

Fig. 6. Contamination factor (CF) of examined macro elements and trace metals at reclaimed islands of Indian Sundarbans (n = 21).

Fig. 7. Pollution Load Index (PLI) at reclaimed islands of Indian Sundarbans (n = 21).

distribution of major elements and trace metals at 99.99% confidence level (Table 4). Hence, our present study affirm the heterogeneously distribution of major elements and trace metals at all the sampling locations as evident from Figs. 2 & 3 and Table 4. 507

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(b)

(a)

Fig. 8. Principal Component Analysis of examined macro elements and trace metals. (a) Component Plot of Factors 1, 2,3, (b) Component matrix.

Fig. 9. Cluster Analysis of examined macro elements and trace metals.

to −0.91 for K, −3.67 to −3.37 for Ca, 0.51 to 0.67 for Cr, −0.89 to −0.60 for Mn, 0.26 to 0.42 for Fe, −0.17 to 0.04 for Ni, 0.46 to 0.69 for Zn and −2.63 to −2.28 for Pb (Fig. 5). Igeo value suggests that the ASS's of reclaimed islands of Indian Sundarban are uncontaminated to moderately contaminated. As per Müller's (1969) classification major elements and trace metals e.g. Al, Si, K, Ca, Ni, Mn, and Pb fall under Igeo class 0 i.e. uncontaminated in all of the sampling stations in reclaimed islands whereas trace metals Cr, Fe, Cu and Zn falls in Igeo class 1 i.e. uncontaminated to moderately contaminated. Our study indicates that the enrichment of major elements and trace metals (Al, Si, K, Ca, Ni, Mn, and Pb) in the surface soils of reclaimed islands of Indian Sundarban are not geological in origin but may be due to anthropogenic processes like domestic and municipal sewage, industrial and mining activities, aquaculture and agricultural. Enrichment of Cr, Fe, Cu and Zn in the surface soils of reclaimed islands may be due to both natural geogenic process like extensive weathering of parent rocks and anthropogenic inputs from multiple sources e.g. iron and steel plants, thermal power plants, dyes and tanning products (Achyuthan et al., 2002; Chandra Mouli et al., 2006; Karar and Gupta, 2006; Banerjee et al., 2012; Ghosh et al., 2016; Bakshi et al., 2017, 2018a, 2018b; Ghosh et al., 2018). The island specific variations of contamination factor (Cf) of trace metals detected in ASS collected from reclaimed islands of Indian

Sundarban are shown in Fig. 6. Throughout the region Cf values ranged between 0.52 and 0.57 for Al; 0.27–0.29 for Si; 0.71–0.82 for K, 0.12–0.15 for Ca, 2.12–2.46 for Cr; 0.79–1.00 for Mn; 1.81–2.05 for Fe; 1.36–1.59 for Ni; 2.65–3.16 for Cu; 2.07–2.53 for Zn; 0.25–0.33 for Pb. The Cf values showed an increasing trend in the following orders Pb < Mn < Ni < Fe < Zn < Cr < Cu for sampling location I1 and I2; Pb < Mn < Ni < Fe < Cr < Zn < Cu for I3 and I4 respectively. As presented in Fig. 6 the mean Cf value of Fe, Ni, Cu, Cr and Zn is in the range of 1–3 suggesting moderate contamination in all of the sampling locations except I3 where the Cf value of Cu is in the range of 3–6 suggesting considerable contaminations. The mean Cf values of Mn and Pb < 1, indicates low contamination of the ASS's in terms of Mn and Pb in all of the studied islands of Indian Sundarban. A significant reduction in the Pb concentration was evident in all significant estuarine region of India might be due to use of unleaded petrol in the last three decades (Chakraborty et al., 2014; Ghosh et al., 2016). Reduction of Mn may be due to flushing out of the ASS system in the dissolved phase as surface run-off (Keene et al., 2010) due to slow oxidation kinetics (Otero et al., 2009) under specific hydrological conditions e.g. rain or occasional connection with adjacent water body at ASS sites. The pollution load in the ASS's of reclaimed islands of Indian Sundarban was estimated by calculating pollution load index (PLI) following Tomlinson et al. (1980). The PLI value of ASS's of reclaimed 508

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CSR-KC/CRS/13/TE- 01/0804), for funding this research in University of Calcutta. We are also thankful to the University of Calcutta and UGCDAE, Kolkata for providing instrumental and infrastructural facilities.

islands of Indian Sunadarbans ranged between 1.31 and 1.48 (Fig. 7); maximum at Bally Island (I3) and minimum at Kumirmari Island (I4). The PLI value > 1 in all of the sampling locations suggests gradual deterioration of sediment quality in reclaimed islands of Indian Sundarban. Further the entry of such toxic metals into biogeochemical cycles may pose severe antagonistic effects on the living organisms. Mitra et al. (2012) have already reported that aquatic and sediment dwellers accumulated toxic trace metals in many folds higher than their surrounding concentration. It has also been suggested elsewhere that if these toxic trace metals accumulated in the crustaceans and fish, then it may lead to potential risk to human society (Mitra et al., 2012; Ghosh et al., 2016). The output of PCA has revealed that, Eigen values that were > 1 represented 70.5% of the total variance in elements detected in the ASS from reclaimed islands of Indian Sundarban. This suggests that distinctive controlling sources or components were responsible for the distributions of major elements and trace metals in ASS. The PCA analysis of major elements and trace metals (Fig. 8) have shown three different components of which the 1st principal component showed 34.9% of variability among the elements Al, Si, K, Ca, Fe, Cu, and Zn. The 2nd principal component extracted cumulatively represents for 53.5% of variability among the group of major elements e.g. Cu, Fe, Pb, Cr and Zn though 3rd principal component showed 70.5% of cumulative variability among Mn, Ni, Cu and Zn. Cluster analysis was applied to the ASS's data set to group major elements and trace metals on the basis of their association with each other. Cluster analysis rendered a dendrogram (Fig. 9) where all the major elements and trace metals were grouped with each other. 3 broad sub clusters were observed; Cluster- I consists of the Al, Si, K and Ca, cluster-II consists of Cu, Fe, Pb, Cr and Zn, cluster – III consists of Mn and Ni. In our study, we have observed that the major elements and trace metals grouped together in a cluster might have similar sources of contamination like anthropogenic activities e.g. domestic and industrial effluent, agricultural and aquaculture run – off, tanning and dyeing product (Sarkar et al., 2004; Karar and Gupta, 2006; Banerjee et al., 2012; Ghosh et al., 2016, 2018; Bakshi et al., 2018b) and natural geogenic processes e.g. extensive chemical weathering of parent rock (Achyuthan et al., 2002; Ghosh et al., 2016, 2018). The work has evaluated the distribution of major elements and trace metals in ASS's in the reclaimed islands of Indian Sundarban. Agricultural and aquaculture run-off, domestic and municipal sewage and atmospheric dispersion from multifarious industries are all potential sources of contamination. However, a more detailed and extensive survey of surface soils are required to identify the spatial distribution of ASS and to determine other potential sources of contamination. PCA and CA have clubbed major elements and trace metals into different groups suggesting similar source for them which was also complemented by co-relation analysis. The PLI value of reclaimed islands of Indian Sundarban varied 1.31–1.48; minimum at Kumirmari Island (I4) and maximum at Bally Island (I3) suggesting gradual deterioration of soil quality. The coastal region of West Bengal especially Sundarban are in a stage of rapid development and urbanizations with the construction of tourism based local business and recreational centers resulting in the change in land use pattern. Therefore, gradual changes in physico – chemical properties of soil along with the distribution of major elements and trace metals are expected in near future. If the major elements and trace metals accumulated in ASS's of reclaimed islands of Indian Sundarban contaminate adjacent water bodies due to surface and storm water run-off, thereby exposing the aquatic and sediment dwellers to chronic contaminants, then the aquatic environment may be impacted, causing a threat to human health and economical hazards.

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