Paddy Water Environ DOI 10.1007/s10333-008-0133-y
ARTICLE
Geochemical characteristics of sediments from a reservoir (tank) ecosystem in Sri Lanka Rohana Chandrajith Æ Kushani Mahatantila Æ H. A. H. Jayasena Æ H. J. Tobschall
Received: 15 August 2007 / Accepted: 18 January 2008 Ó Springer-Verlag 2008
Abstract Major, trace and selected high field strength element geochemistry of fresh-water sediments from the Malagane tank, Deduru Oya basin, Sri Lanka, has been investigated in this study. Sediment samples were collected from 13 locations and analyzed for their elemental distribution using the X-ray fluorescence technique. The sediments were characterized by relatively low organic matter, ranging from 4.8 to 16.9%. The elemental distributions were compared with those of the average upper continental curst, and it was found that, with a few exceptions, most of the studied elements are either comparable or depleted. Correlation and principal component analyses were applied to identify the relationships among studied elements. Major elements, most trace and light rare earth elemental distribution show strong positive correlation with Al2O3 and TiO2, which indicate that the phyllosilicates and heavy minerals in the sediments are the probable hosts for these elements. The results also indicate that the sediments in the Malagane tank are representative of the materials from the metamorphic rocks in the watershed and were subjected to changes within the tank ecosystem. The results obtained from this study are vital for future pollution management of tank ecosystems in Sri Lanka, since information on elemental distribution within the sediments of tank ecosystem is lacking.
R. Chandrajith (&) K. Mahatantila H. A. H. Jayasena Department of Geology, University of Peradeniya, Peradeniya, Sri Lanka e-mail:
[email protected] R. Chandrajith H. J. Tobschall Institute of Geology and Mineralogy, University of Erlangen-Nu¨rnberg, Schlossgarten 5, 91054 Erlangen, Germany
Keywords Tank sediments High field strength elements Intermediate climatic zone Principal component analysis Enrichment factors
Introduction Aquatic ecosystems have received significant amounts of contaminants recently due to rapid industrial growth and urbanization. Once released into the aquatic system, contaminants are transferred to the sediments by adsorption onto suspended matter and subsequent sedimentation (Ha˚kanson and Jansson 1983). Sediments formed during the weathering processes could be modified markedly during transportation and subsequent deposition by chemicals of anthropogenic origin (Chapman 1992). Therefore, sediments not only provide a record of catchment inputs, but also information on the watershed pollution and other anthropogenic influences with high local representativity. Studying the composition of surface sediments of the aquatic environment allowed us to understand the fate of the terrestrial materials transported into a basin and the factors responsible for controlling the distribution and geochemistry of sediments (Fo¨rstner and Wittmann 1981). From among natural aquatic ecosystems, geochemistry of lake sediments has been used effectively and widely to evaluate the catchment weathering (e.g. Jin et al. 2006), environmental changes (Last and Smol 2001) and paleoclimatic reconstruction (e.g. Krishnamurthy et al. 1986; Das 2002). Lake sediments from the industrialized areas and densely urbanized regions, as well as from the rural unpolluted areas, have been extensively researched in the recent years, since they act both as a sink and a source of metals in the aquatic ecosystems. Previous studies have suggested that the anthropogenic inputs of metals could be
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significant in recently deposited sediments in addition to the natural enrichment of metals in the aquatic systems (Chandrajith et al. 1995). In Sri Lanka, except for few coastal lakes, natural inland lakes are not available. However, the dry zone of Sri Lanka is covered with thousands of manmade reservoirs, known as ‘tanks’ (after ‘tanque’, the Portuguese word for ‘reservoir’), which were developed during the period from 300 BC to 1,200 AD. Sediments in these tanks provide a unique opportunity to observe recent changes associated with the surrounding ecosystem. Sri Lanka is a tropical island in which the climate is determined by northeast and southwest monsoons rains. The southwest monsoon brings significant amount of rainfall to the western and southern part of the island, while northeast monsoon brings much less rain to the north, east and northwestern part of the country. Hence, the island has been divided into two broad climatic zones known as the wet zone and dry zone with an intermediate zone inbetween them (Fig. 1). The wet zone receives approximately 2,500 mm mean annual rainfall, whereas the dry zone only receives 1,000 mm of mean annual rainfall. Limited water availability and high ambient temperature
Fig. 1 Deduru Oya river basin and the study region with respect to the climatic zones of Sri Lanka
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are characterized in the dry zone, where the monsoons are confined to 2 or 3 months of the year. Since water is scarce in the dry zone of Sri Lanka, a large number of rural communities depend on the water storage reservoirs (tanks) for their irrigation and domestic uses. These sophisticated irrigation and water management systems, developed over thousands of years, are now considered as one of the greatest water management systems in the ancient world. Tanks were built by damming the stream or rivulet across at many places along linear watersheds to collect, reserve, regulate and to use water in a sustainable manner. A tank is not a single component in the ecosystem, but several components integrated into a well planned water supply system. Madduma Bandara (1985) used the term ‘cascades’ to identify this pattern, where water from upstream tanks was successively stored in those downstream. These small cascades are linked to a tail end large reservoirs or giant feeder canals diverting water from major streams to form a network of extremely complex large water management system. The paddy (rice) fields are located downstream to the tank and lie parallel to the stream or to the artificial channels. A tank has three basic components: the bund, tank body (wewa) and the upper periphery, which is a gently sloping land locally termed ‘‘thaulla.’’ All waters discharging from the upper tank and the paddy fields drain through the thaulla into the downstream tank. The tank cascade system (TCS) in Sri Lanka is not only considered as a water management system but also supports a better environmental management. These systems admirably capture the harmony between local and regional hydrological characteristics while preserving the natural environment. TCSs now cannot be eliminated from the dry zone ecosystem of Sri Lanka, since it is deeply rooted and integrated with the natural environment. The system was faced with an imminent decline from the 13th century due to numerous reasons such as malaria epidemic, foreign inventions and political instability. However, the system did not completely collapse since the British revamped it by the 19th century (Panabokke 1999). Since sediments to the tank are supplied from the terrestrial material in the surrounding environment, the chemical structure of a given sediment is a function of characteristics of the catchment. Therefore, the study of sediments in a tank could reflect the chemical inputs and anthropogenic influence from within the ecosystem. As part of an extensive study on the ancient TCS in Sri Lanka, a detailed hydrochemical analysis of water and bottom sediments of the Malagane tank cascade system in the Deduru Oya river basin (Fig. 1), located in the intermediate climatic zone of Sri Lanka, was carried out. This system was selected, since the tank does not dry up even during the extreme dry conditions. The hydrogeochemistry of the Malagane tank cascade has been discussed elsewhere
Paddy Water Environ
(Mahatantila et al. 2008). This paper discusses the geochemical characteristics of sediments of the Malagane tank and the surrounding ecosystem.
Materials and methods Malagane tank is the tail-end tank located in the Malagane cascade within the Deduru Oya basin, which is the fourth largest river basin in Sri Lanka having the highest density of small tanks (one tank per 1.2 km2) (Panabokke et al. 2002). From the hydrological point of view, Malagane tank is considered as a small and shallow tank with maximum depth of 5 m (mean 1.9 m) at its full capacity. The tank is situated in the Maguru Oya sub-basin with a catchment area of 231 km2 that receives an annual average rainfall of 1,890 mm (Somarathne et al. 2003). Sediment samples were collected when the water level reached its minimum level in the month of August 2005. A stainless steel auger was used to collect sediments from the upper 30 cm sediment layer from 13 predefined locations well distributed over the tank bed (Fig. 2). Sediments were then scooped and transferred into precleaned polythene bags and frozen. The sediments were dried at 105°C and sieved to obtain the granulometric parameters. Subsamples of sediments were finely ground using an agate swing mill to a powdered form and passed through a 63 lm aperture sieve and used for the chemical analysis. The following were measured during this study: pH; nutrients such as leachable N, leachable P and organic matter (OM); major oxides such as SiO2, Al2O3, Fe2O3, etc.; trace elements such as As, Ba, Cr, Cu, etc.; high field strength elements such as Ce, Hf, La, Nb, etc. (Table 1). The pH of sediments was measured using 1:1 w/v sediment/water extractions. The percentage of organic matter in
the sediments was determined as percentage loss on ignition (LOI) after drying the \63 lm grain-size fraction at 550°C (Ha˚kanson and Jansson 1983). Available fraction of nitrogen and phosphorous in sediments was extracted using 1 M KCl and 0.5 M NaHCO3 (pH = 8.5) extracts, respectively. The nitrogen content was determined as ammonium and nitrate, whereas the phosphorus was measured as phosphate using spectrophotometric methods and subsequently the elemental concentrations were calculated. Total elemental content of sediment samples were determined with Phillips PW-2400 X-ray fluorescence spectrometer using fused glass discs. The precision and accuracy of all analyses were checked by replicate analysis. International reference samples, SARM-52 and JSd-1 were prepared and analyzed with samples as unknowns. The analytical precision determined by replicate analyses and reference materials were better than 8% at the 95% confidence level. The agreement between reference values of international standard samples and this measurement was excellent.
Results and discussion The TCS in Sri Lanka consists of three main geochemical domains, namely, thaulla, tank body and the bund region. These are characterized by a high degree of heterogeneity in space and time (Mahatantila 2007). The geochemical composition of sediments in such systems depends not only on natural processes such as erosion from the watersheds, but also on anthropogenic influences such as the use of agrochemicals, deforestation and waste dumping. Mahatantila et al. (2008) showed the differences in the hydrogeochemistry of the Malagane TCS. In this study, we mainly discuss the geochemical composition of sediments in a man-made
Fig. 2 Map showing the Malagane Tank Cascade and location of sediment samples
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Paddy Water Environ Table 1 Major and trace element data for the sediment samples collected from the Malagane tank Sample number
H1
H2
H3
H4
H5
H6
H7
H8
H9
H10
H11
H12
H13
MGS (mm)
0.063
0.50
0.603
0.088
0.163
0.063
0.361
0.063
0.388
0.063
0.063
0.088
0.125
pH
5.00
5.02
5.42
5.28
6.25
5.70
6.34
4.98
5.19
5.25
6.23
6.55
4.87
LN (mg/kg)
104.3
117.8
147
68.9
21.5
25.8
18
40.7
19.7
47.2
17.3
21.5
23.2
LP (mg/kg)
0.52
0.7
0.13
0.37
0.52
0.59
0.71
0.85
0.65
0.03
0.45
OM%
16.4
6.06
8.11
16.9
6.21
12.12
6.30
16.03
11.23
9.25
5.10
7.04
SiO2%
44.7
70.2
70.4
49.1
67.2
54.7
67.9
51.5
61.7
58.5
66.7
62.8
74.7
TiO2%
0.86
0.63
0.15
0.95
0.57
0.9
0.49
0.9
0.55
0.84
0.86
0.76
0.47
Al2O3%
20.49
12.6
9.56
21.27
9.12
18.36
11.7
20.19
12.86
17.88
13.42
15.78
10.79
Fe2O3%
6.17
2.11
0.9
6.35
3.17
6.46
1.39
5.29
2.81
5.23
4.87
4.34
2.51
MnO%
0.071
0.039
0.022
0.118
0.048
0.309
0.032
0.055
0.048
0.06
0.066
0.057
0.046
MgO% CaO%
0.83 1.12
0.27 1.9
0.12 1.54
0.77 0.95
1.04 2.19
0.77 1.07
0.13 1.55
0.78 1.03
0.4 1.09
0.63 0.9
0.77 1.71
0.57 1.04
0.34 0.73
0.58 4.81
Na2O%
0.78
3.24
2.87
0.71
1.66
1.02
2.96
0.8
1.66
0.88
1.49
1.39
1.02
K2O%
0.83
0.78
0.42
1.08
0.47
1.21
0.51
1.24
1.44
1.29
0.81
1.46
1.27 0.023
P2O5%
0.177
0.029
0.025
0.047
0.012
0.035
0.009
0.084
0.022
0.028
0.023
0.024
LOI%
22.86
6.98
12.96
17.68
13.17
15.07
13.01
17.62
16.67
12.35
8.54
11.68
7.46
As (mg/kg)
7
15
0.5
14
24
18
0.5
8
2
7
6
4
2
Ba (mg/kg)
544
499
276
741
256
1184
364
711
804
719
489
706
637
Ce (mg/kg)
76
37
0.5
99
44
120
41
115
65
91
66
87
63
Co (mg/kg)
24
10
6
26
14
29
5
19
10
20
17
17
11
Cr (mg/kg)
117
68
50
120
93
108
52
118
75
103
96
90
70
Cu (mg/kg)
12
0.5
0.5
23
0.5
15
0.5
18
0.5
15
15
5
1
Ga (mg/kg)
24
16
10
24
10
22
13
24
14
21
16
19
11
Hf (mg/kg)
12
13
3
7
3
6
8
13
9
9
7
13
3
La (mg/kg)
49
20
6
44
8
27
12
56
28
45
19
40
15
Nb (mg/kg)
14
7
3
18
6
17
6
17
8
14
10
13
8
Ni (mg/kg) Pb (mg/kg)
40 23
14 18
1 11
55 31
23 10
51 33
5 14
44 33
16 24
45 30
29 20
36 31
22 19
Rb (mg/kg)
50
15
4
57
8
62
4
64
35
52
24
45
34
Sr (mg/kg)
189
513
492
186
206
227
504
202
360
213
240
292
248
Th (mg/kg)
14
13
9
19
17
18
10
24
19
19
21
23
21
V (mg/kg)
131
44
18
130
84
116
26
114
60
93
105
74
45
Y (mg/kg)
21
10
8
29
13
36
9
27
14
25
18
22
14
Zn (mg/kg)
72
31
16
69
31
56
21
77
36
57
44
48
30
Zr (mg/kg)
174
359
125
218
328
275
543
255
480
310
434
394
297
MGS mean grain size, LN leachable nitrogen, LP leachable phosphorus, OM organic matter
irrigation system, which is now intergraded with the natural ecosystem. Figure 3 illustrates the spatial variation of mean grain size in the sediments of the Malgane tank that shows a gradational decrease toward the center of the tank from the inflow region. Medium to coarse sands have been deposited in the thaulla area where the inflow is passing into the tank. However, a gradational increase of the grain size has also been observed from the middle of the tank toward the tank bund. Cyclothemic sequences of fine sand and sandy clay have been observed while silty clay is predominant in the
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middle of the tank. Coarse sand layers are observed toward the sluice gate, which may be indicative of scouring effects due to hydraulic conditions associated with water released to the downstream paddy fields. The pH of the sediments of Malagane tank varies from 4.87 to 6.55 (mean = 5.5) indicating moderate to slightly acidic conditions. Sediments become acidic due to leaching of base-forming cations such as Ca2+ and Mg2+ or due to decomposing of organic matter. The leachable nitrogen in the sediments varies from 147 mg/kg (near the bund) to 17 mg/kg (in the tank bed), while the leachable
Paddy Water Environ
Fig. 3 Distribution of mean grain size in the sediments of Malagane tank
phosphorous varies from 0.03 to 0.9 mg/kg, respectively. The sediments from the thaulla show the lowest organic matter content (4.8%), whereas the highest (16.9%) were found close to the bund. Phosphorus (P) is one of the most important elements in the freshwater ecosystem. The mean leachable P concentration of the sediment of Malagane tank is 1.44 mg/kg. However, abrupt decrease of P in sediments can be observed toward the center of the tank from the thaulla. Biological processes and inorganic fertilizers are responsible for nitrogen in most aquatic sediments. The mean nitrogen content of sediments of the Malagane tank is 51.8 mg/kg. The highest leachable N values were obtained closer to the tank bund and the content does not depend on the grain size. In thaulla, where the biological activities are prominent, the denitrifying bacteria could covert the sediment nitrates into gaseous nitrogen and escape from the system or let the plants take up nitrates for their metabolism. The bulk geochemistry of the major and trace elements of surface sediment samples from the Malagane tank are listed in Table 1. Figures 4 and 5 illustrate the box and whisker plots of major and trace elements distributions in sediments of the Malagane tank, respectively. To understand the provenance for the sediments, enrichment factors were calculated. The sediment samples were divided into three groups, viz.: (1) close to the bund, (2) main tank body and (3) the inflow (thaulla region), since the hydrodynamics of the flow varies within the tank system (Fig. 2). Since the background geochemical values for sediments in man-made tanks in Sri Lanka is not available, the bulk chemical composition of the sediments of Malagane tank was compared with the composition of the upper continental crust (UCC) proposed by Rudnick and Gao (2003) (Fig. 6). With few exceptions, the major and trace element
Fig. 4 Box and whisker plots for studied major elements from Malagane tank, Sri Lanka
Fig. 5 Box and whisker plots for studied trace elements from Malagane tank, Sri Lanka
compositions of Malagane tank sediments are either similar or slightly depleted to that of the UCC composition. For instance, alkali and alkaline earth elements, P, Mn and Cu are depleted against the UCC composition. However, As and V are enriched in sediments in the main tank body and sediments close to the bund. Trace elements such as Ga, As and Pb tend to accumulate with clay minerals or get adsorbed onto Fe-oxide particles in the -63 lm fraction of sediments giving a positive anomaly with the UCC values (Chandrajith et al. 2001a). However, sediments in the bund area shows a positive anomaly for As, even though the sediments are coarser compared to the middle of the tank. This is probably due to the availability of iron oxide minerals such as pyrite and arsenopyrite in the sediments or due to absorption of As into iron oxide-coated sand and/or
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Paddy Water Environ Fig. 6 Average upper continental crust normalized elements from the Malagane tank. The EF is calculated as the ratio of the element abundance to the average crustal abundance of Rudnick and Gao (2003)
organic matter in the area. In tank sediments, particularly those which are in the center of the tank and deposited close to the bund, are mostly submerged throughout the year. This condition is favorable for the removal of As species from sediments, since As is stable, exhibiting weak affinity toward clays and gibbsite (Weerasooriya et al. 2003). Therefore, As is mostly bound to iron oxides, which are known to absorb arsenic species effectively (Vithanage et al. 2006). The Zr and Ce contents are enriched in the thaulla and in the tank body, whereas Hf, Pb and Th are enriched throughout the tank compared with that of the UCC values. Sediments from the streams that drain through the high grade metamorphic terrain of Sri Lanka are highly enriched with such trace elements, notably Zr, Hf, Th, U, Ce and La, as compared to the upper crustal abundance (Chandrajith et al. 2001a). These elements are considered to indicate provenance compositions as a consequence of their immobile behavior (Taylor and Mclennan 1985). Presumably these elements are associated with heavy minerals such as zircon, rutile and monazite found in abundance in the stream sediments of Sri Lanka (Chandrajith et al. 2001a, b). The thaulla of the tanks acts as a sediment trap in the cascade system (Mahatantila et al. 2008). Therefore, heavy minerals supplied from the watershed were deposited mainly at thaulla and only fine gains including organic particles flush into the tank body. Multielemental relations In recent years, various multivariate statistical techniques have been widely used to interpret geochemical data. The multivariate statistical techniques yield a better understanding of the geochemical behavior occurring in the natural aquatic systems (e.g. Chandrajith et al. 2001b; Das and Haake 2003; Mwamburi 2003). The multielement geochemical data obtained from the sediments of the Malagane tank was explored using a multivariate statistical approach. Simple correlation and principal component analysis (PCA) were applied to examine the importance of each parameter and to investigate the correlations among them.
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A widely used correlation criterion between two variables is the simple correlation coefficient, which indicates the sufficiency of one variable to predict the other (Davis 1986). Table 2 shows the Pearson’s correlation matrix obtained for the sediments from the Malagane tank in which most elements exhibit high to moderate correlations. Silicon shows significant negative correlations with most of the trace and high field strength (HFS) elements. With few exceptions, Al2O3 significantly and positively correlates with most of the studied trace metals, indicating their significance in association with clay and/or micaceous minerals in the sediments. Among the alkali and alkaline earth elements, Ca negatively correlates with K, Pb and Rb, while Mg positively correlates with As, Co, Cr, Ni and negatively with Sr. Ba shows positive correlations with Mn, K, Ce, Pb, Rb and Th. Sr correlates negatively with most of the studied elements; however, positive correlation is observed with Na. Trace and HFS elements with the exception of Hf, Zr and Th show significant positive correlations with Al-, Ti-, Fe-oxides. These elements usually indicate provenance composition due to their immobile behavior. Phyllosilicate minerals and/or heavy mineral phases very likely host these elements in sediments. However, Zr and Hf do not show any significant correlations with the other elements studied. Although the fine fractions of stream sediments are reservoirs for most soluble elements such as alkali and alkaline earth elements, usually HFS and related trace elements are more enriched in the coarser fraction of sediments of streams that drains the high grade metamorphic rocks (Chandrajith et al. 2001a). Strong positive correlations between Al2O3 and light REE (e.g., La and Ce) suggest that these elements are dominantly associated with clay minerals (Cullers et al. 1987). Strong positive correlation ([+0.8) between REE and major oxides also suggest that these elements originated from a common source (Bea 1996). Phyllosilicates and heavy minerals in the sand-silt-clay fractions of sediments are thus the probable source and host for light REE (Taylor and McLennan 1985). Chandrajith et al. (2001a) showed that many river sediments in Sri Lanka are enriched with HFS elements including light rare earth elements and associated with heavy minerals such as zircon, rutile and monazite.
Paddy Water Environ Table 2 Correlation coefficients (r) from correlation matrix obtained from geochemical data of Malagane tank sediments (n = 13) SiO2 SiO2
TiO2
Al2O3
Fe2O3
MnO
MgO
CaO
1.000 -0.867 -0.982 -0.919 -0.466 -0.593
TiO2
1.000
Al2O3
Na2O
0.529
K2O
P2O5
As
Ba
Ce
Co
Cr
0.729 -0.453 -0.723 -0.220 -0.586 -0.812 -0.883 -0.896
0.847
0.927
0.495
0.721 -0.326 -0.698
0.478
0.413
0.393
0.589
0.873
0.864
0.902
1.000
0.881
0.455
0.497 -0.587 -0.689
0.489
0.636
0.148
0.617
0.828
0.841
0.847
1.000
0.613
0.785 -0.445 -0.837
0.467
0.522
0.381
0.638
0.873
0.975
0.960
1.000
0.383 -0.263 -0.392
0.279
0.065
0.472
0.775
0.620
0.727
0.464
1.000
0.155
0.373
0.660
0.245
0.604
0.761
0.872
Fe2O3 MnO MgO CaO
0.017 -0.721 1.000
0.662 -0.789 -0.275
Na2O
0.410 -0.678 -0.625 -0.418 -0.350
1.000 -0.624 -0.436 -0.161 -0.574 -0.815 -0.791 -0.846
K2O P2O5
1.000
0.041 -0.160 1.000 -0.003
0.800 0.093
1.000
0.133
0.253
0.484
0.467
1.000
0.824
0.669
0.504
1.000
0.835
0.831
1.000
0.921
As Ba
0.726 0.306
Ce Co
0.416 0.482
Cr
1.000 Ga
SiO2 TiO2 Al2O3
0.407 0.562
Hf
La
Nb
Ni
Pb
Rb
Sr
Th
V
Y
Zn
Zr
-0.847 -0.976 -0.528 -0.899 -0.932 -0.886 -0.803 -0.882 0.659 -0.403 -0.885 -0.830 -0.970 0.393 0.843 0.875 0.500 0.767 0.902 0.902 0.785 0.790 -0.710 0.598 0.904 0.831 0.884 -0.033 0.867
0.990
0.544
0.919
0.948
0.874
0.854
0.893 -0.589
0.393
0.825
0.843
0.953 -0.346
Fe2O3
0.887
0.878
0.300
0.761
0.933
0.965
0.787
0.865 -0.831
0.553
0.969
0.918
0.920 -0.308
MnO
0.481
0.465 -0.114
0.167
0.590
0.602
0.531
0.547 -0.373
0.172
0.514
0.768
0.384 -0.186
0.606
0.502
0.451
0.606
0.740
0.380
0.514 -0.875
0.535
0.876
0.629
0.683 -0.236
MgO
0.055
CaO
-0.427 -0.515 -0.168 -0.620 -0.608 -0.530 -0.723 -0.765
0.412 -0.473 -0.292 -0.561 -0.514
0.223
Na2O
-0.707 -0.634 -0.057 -0.690 -0.793 -0.858 -0.695 -0.846
0.953 -0.736 -0.813 -0.780 -0.801
0.322
K2O
0.332
0.476
0.378
0.618
0.603
0.559
0.821
0.767 -0.435
0.774
0.333
0.579
0.495
P2O5
0.401
0.623
0.449
0.629
0.456
0.403
0.273
0.471 -0.396 -0.005
0.575
0.308
0.673 -0.520
0.107
As
0.237
0.192 -0.099
0.002
0.267
0.409
0.066
0.129 -0.368
0.091
0.463
0.357
0.243 -0.196
Ba
0.506
0.614
0.230
0.518
0.737
0.682
0.855
0.822 -0.416
0.525
0.490
0.814
0.549 -0.029
Ce
0.773
0.824
0.372
0.777
0.947
0.915
0.925
0.938 -0.736
0.730
0.793
0.935
0.853 -0.071
Co
0.842
0.838
0.198
0.672
0.908
0.957
0.751
0.839 -0.793
0.463
0.930
0.934
0.858 -0.408
Cr
0.858
0.842
0.316
0.787
0.894
0.940
0.716
0.825 -0.877
0.592
0.983
0.847
0.945 -0.358
Cu Ga
1.000
0.851 1.000
0.229 0.617
0.720 0.912
0.888 0.940
0.876 0.865
0.737 0.856
0.776 -0.685 0.873 -0.550
0.464 0.391
0.863 0.819
0.846 0.838
0.867 -0.351 0.945 -0.308
Hf
1.000
La Nb Ni Pb Rb Sr Th V Y
0.693
0.419
0.272
0.512
0.399
0.028
0.224
0.268
0.219
0.509
1.000
0.856
0.779
0.842
0.868 -0.572
0.529
0.718
0.702
0.926 -0.260
1.000
0.170
0.963
0.921
0.948 -0.717
0.593
0.862
0.951
0.938 -0.277
1.000
0.847
0.907 -0.835
0.626
0.909
0.950
0.906 -0.332
0.947 -0.546
0.668
0.661
0.887
0.813 -0.122
1.000 -0.715
0.659
0.774
0.913
0.899 -0.317
1.000
1.000 -0.705 -0.856 -0.723 -0.753 1.000
0.317
0.525
0.571
0.551
1.000
0.827
0.912 -0.320
1.000
0.099
0.833 -0.300
Zn
1.000 -0.366
Zr
1.000
Significant correlations are highlighted
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Paddy Water Environ Table 3 Factor loadings for sediments in Malagane tank obtained using varimax rotation with Kaiser normalization Component PC-1
PC-2
PC-3
PC-4
PC-5
SiO2%
-0.783
-0.413
-0.236
-0.311
0.242
TiO2%
0.635
0.609
0.165
0.366
0.168
Al2O3% Fe2O3%
0.793 0.536
0.323 0.656
0.302 0.228
0.344 0.427
-0.200 -0.149
MnO%
0.040
0.213
0.100
0.935
-0.110
MgO%
0.215
0.940
-0.109
0.168
-0.045
CaO%
-0.252
0.001
-0.900
-0.146
0.280
Na2O%
-0.236
-0.733
-0.552
-0.135
0.257
K2O%
0.223
0.176
0.870
0.184
0.241
P2O5%
0.716
0.191
-0.035
-0.150
-0.528
LOI%
0.783
0.373
0.229
0.189
-0.376
As
-0.022
0.564
-0.508
0.482
0.080
Ba
0.236
0.133
0.611
0.701
0.086
Ce
0.458
0.516
0.513
0.469
0.099
Co
0.451
0.612
0.172
0.563
-0.234
Cr
0.543
0.764
0.153
0.270
-0.149
Cu
0.533
0.526
0.210
0.379
-0.203
Ga Hf
0.829 0.873
0.311 -0.113
0.242 0.099
0.367 -0.112
-0.120 0.381
La
0.815
0.326
0.434
0.043
-0.074
Nb
0.610
0.479
0.396
0.461
-0.087
Ni
0.472
0.646
0.333
0.455
-0.124
Pb
0.555
0.273
0.620
0.448
0.091
Rb
0.535
0.410
0.600
0.389
-0.146
Sr
-0.158
-0.896
-0.317
-0.113
0.214
Th
0.090
0.645
0.634
0.002
0.317
V
0.526
0.755
0.081
0.309
-0.160
Y
0.403
0.498
0.382
0.641
-0.111
Zn
0.731
0.549
0.284
0.209
-0.177
Zr
-0.143
-0.157
0.035
-0.106
0.872
% Variance
66.773
8.981
8.342
5.102
4.926
Significant correlations are highlighted
R-mode principal component analysis (PCA) is particularly useful when simultaneously considering several related random variables; therefore, identifying a new, smaller set of uncorrelated variables could account for a large proportion of the total variance in the original variables. The PCA was applied in this study to reduce the large geochemical database, organize data into groups of similar characteristics, identify the weight of each parameter and detect correlations among them. The principal components (PCs) were extracted using the correlation matrix and rotated to an orthogonal simple structure using the variance maximizing (Varimax) criterion. In this study, only factors with eigenvalues greater than 1 are considered.
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Five factors that represented 95% of the total variance were extracted from the factor analysis. Table 3 provides the results of the PCA factor loading for the sediments from the Malagane tank. The extracted components PC-1, PC-2, PC-3, PC-4 and PC-5 represent 67, 9, 8, 5 and 5% of the total variance of the geochemical data, respectively. The first two components explain the dispersion of most of the studied major and trace elements. These trace and HFS elements are associated with heavy minerals, clay minerals and iron oxides in tank sediments. For instance, Al–Hf–La–Nb–Ga–Ni is positively correlated in PC-1 indicating their association with clay and mica group minerals. PC-2 explains the dispersion of negatively correlated Na–Sr and positively correlated Ti–Fe–Co–Cr–Ni–Th–V. PC-3 indicates mainly the correlation between Ca–Ba–Rb–Th (LILE) and Pb. The fourth component (PC-4) extracted from the tank sediment geochemical analysis represents the Mn–Ba–Y, indicating their association with Mn-oxides in sediments. Na, Ca and Sr indicate negative correlations within extracted components. These cations are selectively leached and removed during weathering, whereas cations with relatively larger ionic radii, such as K, Cs, Rb and Ba, may be fixed by preferential exchange and adsorption on clays (Condie l993). Only Zr shows a higher factor score in the PC-5.
Conclusions Despite the limited number of samples analyzed, it can be concluded that the tank sediments of Sri Lanka are considered to be relatively unpolluted from anthropogenic sources. Therefore, the geochemistry of the sediments is mostly representative of the geochemical characteristics of the surrounding watershed. From a geological point of view, the Malagane basin is underlain by Precambrian supracrustal rocks, which supply mineralogical constituents into the tank sediments. The chemical weathering within the catchment as well as associated geochemical processes occurring within the sediments controls the elemental behavior as illustrated by higher Al2O3, depletion of mobile elements compared to the UCC levels and positive correlation with Al2O3, Fe2O3, MnO and with most trace elements. Correlations among the elements and the PCA indicate that sediments of Malagane tank have not been subjected to intense weathering and recycling. Although extensive paddy cultivations are predominant in the tank watershed, the anthropogenic pollution levels are not alarming. The results obtained through this study are vital for future pollution management of the tank ecosystem in Sri Lanka, since information about metal loadings into the tank ecosystem is lacking.
Paddy Water Environ Acknowledgments This research is supported by the National Science Foundation of Sri Lanka (NSF) through the research grant RG/2004/GMR/01 awarded to RC and HAHJ. RC acknowledges the fellowship received from Alexander von Humboldt Foundation through Georg Forster Fellowship to enable him to do the chemical analysis at Erlanegn. HAHJ acknowledges the financial support given by the Biological and Ecological Engineering Department of the Oregon State University, USA, and the Research Promotion Centre of the UGC, Colombo, Sri Lanka, to complete this research work.
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