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In order to establish the natural processes and geochemical factors responsible for enrichment of trace metal ions (Cu, Co, Ni, Zn and Cr) with respect to textural parameters (sand, silt and clay weight percentages) along with depth, multivariate statistical approach has been carried out for sediments in different water zones of Chilika lake, the largest brackish water lagoon in Asia. The rotated varimax factor results reveal that Cobalt enrichment is controlled by both textural parameters as well as adsorption mechanism. In fresh and saline water region, textural parameters and in mixed water, adsorption phenomenon predominates. Zn in fresh water is related to clay, whereas it is in adsorbed state in mixed water. Cu in fresh water sediments is in absorbed state and in mixed water it is related to depth and Co concentration. Cr does not show any specific association in fresh water, but in both mixed and saline water it is associated with clay minerals. Although both textural parameters and adsorption mechanism play an important role for the enrichment of trace metal ions in these lagoonal sediments, their relative importance varies with specific metal ions as well as the water quality. Sequential extraction technique was used to characterize the various forms of metals in the < 63 micro size sediments of Chilika lake. The concentrations determined indicated selective accumulation of the various metals in the different phases with spatial variability in different water zones. Slightly higher availability of Co and Zn near Balugaon township in exchangeable phase may be related to anthropogenic activities. Among the non-lithogenous (NL) phases, reducible phase associated with higher concentration of Ni, Cu and Cr. Organic bound Zn and Co contributed highest percentages among NL fractions. Residual fraction contributed more than 50% in most of the cases, reflected the predominance of physical weathering, high erosion rate along the drainage basin.

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In the present study, concentration of some selected trace metals (Fe, Mn, Ni, Co, Pb, Zn, Cu, Cr and Cd) are measured in Brahmani, Baitarani river complex along with Dhamara estuary and its near shore. Chemical partitioning has been made to establish association of metals into different geochemical phases. The exchangeable fraction is having high environmental risk among non-lithogeneous phases due to greater potential for mobility into pore water. The metals with highest bio-availability being Cd, Zn and Cr. The metals like Mn, Zn, Cd and Cu represent an appreciable portion in carbonate phase. Fe-Mn oxides act as efficient scavenger for most of the metals playing a prime role in controlling their fate and transport. Among non-lithogeneous phases apart from reducible, Cr showed a significant enrichment in organic phase. Risk assessment code values indicate that all metals except Fe fall under medium-risk zone. In estuarine zone Cd, Zn, Pb and Cr are released to 32.43, 26.10, 21.81 and 20 %, respectively, indicating their significant bio-availability pose high ecological risk. A quantitative approach has been made through the use of different risk indices like enrichment factor, geo-accumulation index and pollution load index. Factor analysis indicates that in riverine zone, Fe-Mn oxides/hydroxides seem to play an important role in scavenging metals, in estuarine zone, organic precipitation and adsorption to the fine silt and clay particles while in coastal zone, co-precipitation with Fe could be the mechanism for the same. Canonical discriminant function indicates that it is highly successful in discriminating the groups as predicted.

The article presents first systematic report on the concentration of selected major elements [iron (Fe) and manganese (Mn)] and minor elements [zinc (Zn), copper (Cu), chromium (Cr), lead (Pb), nickel (Ni), and cobalt (Co)] from the core sediment of Chilika Lake, India. The analyzed samples revealed higher content of Pb than the background levels in the entire study area. The extent of contamination from minor and major elements is expressed by assessing (i) the metal enrichments in the sediment through the calculations of anthropogenic factor (AF), pollution load index (PLI), Enrichment factor (EF), and geoaccumulation index (Igeo) and (ii) potential biological risks by the use of sediment quality guidelines like effect range median (ERM) and effect range low (ERL) benchmarks. The estimated indices indicated that sediment is enriched with Pb, Ni, Cr, Cu and Co. The enrichment of these elements seems to be due to the fine granulometric characteristics of the sediment with Fe and Mn oxyhydroxides being the main metal carriers and fishing boats using low grade paints, fuel, and fishing technology using lead beads fixed to fishing nets. Trace element input to the Chilika lake needs to be monitored with due emphasis on Cr and Pb contaminations since the ERM and ERL benchmarks indicated potential biological risk with these metals.

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