A total of 29 surface sediment samples from the study area have been analysed and the result of organic matter, textural class and heavy metal concentrations are given in table 1and 8.
3.1 Sediment properties
The textural class of the sediments of the study area is shown in the Table 1. There are three types of sediments- sand, slightly muddy sand, and muddy sand. Textural analysis indicates a good correlation between depth and grain size (Table 1). The surface sediments are dominated by coarse grains in the shallower part and finer sediments in the deeper part, whereas the transect 2 (station number 10) and transect 3 (station number 14, 15 and 16) do not show the above observations. The sandy sediments occur in the stations 3,4,13 and 28 while slightly muddy sand occurs at stations 2, 5, 7, 8, 12, 16, 18, 19, 20, 22, 23, 24, 25 and 26. Samples from the station 1, 9, 17, 27, 11 and 6 are muddy sand and 10, 14, 15 and 21 occur as sandy mud (Fig. 3).
3.2. Heavy metal Distribution
Heavy metal analysis of surface sediment samples from the Bay of Bengal and their perceptive values and crustal average (Taylor, 1964) are shown in table 2. In the present study area, Corg content in coarse sediments was particularly lower than the finer sediments. The concentration of Cu, Co, Fe, Mn, Pb, Zn, Cr and Ni ranged from 4.89 to 79.23 ppm, 6.38 to 146.28 ppm, 9004.49 to 44483.12ppm, 215.47 to 1036.13 ppm, 134.50 to 15569.84ppm, 324.90 ppm to 1958.14 ppm, 46.41ppm to 582.35 ppm respectively. The mean values of concentration of different heavy metal in the present area are as follows in the descending order: Fe (29712.16 ppm) >Zn (6693.86 ppm) >Pb (4443.13ppm) >Cr (966.98 ppm) >Mn (526.01 ppm) >Ni (258.13) >Co (66.46 ppm) >Cu (30.26 ppm). The higher concentration of Cu, Co and Mn are observed in station no.6 (Transect 1) although Pb, Zn and Cr are encountered in station no.29 (Transect 6) and Fe and Ni is observed in station no.15 (Transect 3) and 18 (Transect 3) respectively (Fig. 4 and 5). The Mn concentrations in the present study area are higher than that of off Karaikal coast surface sediments in the Bay of Bengal; Cu and Zn concentrations are higher than that of, off Cuddalore coast, off Ennore, off Pichavaram, off Tuticorin coast, shelf sediments of Gulf of Mannar in the East Coast of India (table 2).
3.3 Organic matter
Organic contents vary between 0.23% and 2.40% with an average of 0.88%. The high values of organic matter are associated in the deeper part samples and low values occur in the shallower part (Fig. 3). High organic matter concentration in the present study is enriched in muddy sediments and low in sandy type sediments except for the transects 1 and 3 (station number 1 and 14.
3.4. Statistical Analysis
3.4.1 Principle Component Analysis
The PCA was performed to group the pollutants and identify the influencing factors for the distribution of heavy metals in the study area. The PCA analysis was applied for the organic matter, grain size (sand and mud) and heavy metals. The Kaiser-Meyer-Olkin normalisation technique was used to extract maximum factors that influence the distribution of heavy metals. The technique takes into account only those factors with eigenvalues greater than 1, for each procedure. The Varimax rotation yielded 5 factors. Additionally, factor loading communalities for the first three factors were taken as the percentage of variance and the cumulative percentage of variance was derived (Table 6 and Fig. 6).
First principle component shows maximum loadings of Cr (.915), Ni (.814), Zn (.752) and sand (.652) (Fig. 6). The second PC analysis shows significant loadings of Fe (.87), Mn (0.83), Cu (0.63), mud (.61), Co (0.42). Fe and Mn has the highest positive loadings, Cu, mud and Co show medium positive loadings (Fig. 6). The third PC has significant loadings of Organic matter (0.86), Cu (0.48), Ni (0.34) and Cr (0.31). Organic matter exhibits the highest positive loadings. Cu exhibits low positive loadings while Ni and Cr exhibits very low positive loadings (Fig. 6).
3.4.2 Pearson correlation
The correlation analysis was performed on the normalized data set to test the relationship between the environmental parameters (table 7). According to the Pearson correlation analysis, Fe shows a strong positive correlation with Mn (r=.732) whereas Zn shows a strong positive correlation with Cr (r=.784) and Ni (r=605) and Cr show strong positive correlation with Ni (r=.700). The heavy metals viz. Cu (-.394) and Mn (-.361) exhibit negative correlation with sand while Zn (.283), Cr (.448), Ni (.466), Pb (.101) show low to moderate correlation with sand. Mud has significant correlation with Fe (.495). Weak positive correlation with Cu (.394) and Mn (.361) while negative moderate correlation with Cr (-.448) and Ni (-.283) and negative weak correlation with Pb (-.283) and Zn (-.101).
3.4.3 Q mode cluster
The consequent dendrogram of Q-mode hierarchical cluster analysis provides the grouping of samples according to the heavy metal, organic matter, grain size and depth. The dendrogram exhibits four groups, cluster 1(12, 24, 20, 10, 13, 18, 19, 9, 11, 5, 22 and 7) cluster 2(16, 25, 6, 21, 26, 27, 17, 8) cluster 3(14, 29, 23 and 15) cluster 4 (4, 28, 1, 2 and 3) (Fig. 7).
3.5 Enrichment factor
The mean values of EF are as follows Mn > Pb > Cr > Zn > Cu>Co > Ni. Mn is moderately enriched followed by Cu, Pb, Ni, Co and Zn. According to the Muller (1969) Sutherland (2000) classification, the majority of the metals show minimal enrichment to significant enrichment in the sediment sample (Table 3).
3.6 Contamination factor
The mean values of CF for the metals in the shelf sediments are shown in table 4. CF in the present study area is as follows Pb<Cu<Co<Ni<Cr<Zn<Mn<Fe. The calculated CF value indicates that all the sediment samples have been very highly contaminated by Fe. There is also significant Zn, Pb and Cr contamination in most of the sampling stations in the study area.
3.7 Geo accumulation index
Geo accumulation index shows that most of the samples are extremely contaminated in Pb and Zn. Certain samples are moderate to strongly contaminated in Cr and are uncontaminated to moderately contaminated by Ni and Co. The study area is found to be uncontaminated by Cu and Mn (table 5).