Organic matter - A key factor in controlling mercury distribution in estuarine sediment

Organic matter (OM) was found to play an important role in controlling mercury (Hg) distribution and speciation in estuarine sediment of the Vembanad Lake. The sedimentary organic carbon (OC) from the northern part of the lake was influenced mainly by marine in situ biological production. However, the sedimentary OC was mainly of terrestrial origin in the southern part of the lake. The average concentrations of Hg in the sediments were relatively higher in the northern part (ranging from ~ 6.5 to 172.0 μg/kg, with an average concentration of 59.6 μg/kg) than the southern part of the lake (ranging from ~ 6.8 to 114.0 μg/kg with an average concentration of 49.7 μg/kg). This study suggests that nature and source of sedimentary OC probably control Hg speciation in the Vembanad Lake system. Speciation study of Hg suggests that a major part (55-90%) of the total Hg was mainly associated with organic phases in the sediment. The variation in concentrations of organic matter bound Hg with the increasing Hg/TOC ratio suggests that more Hg binding sites were available in the sediment from the northern part of the lake but not in the southern part of the lake. Anthropogenic activities were responsible for the higher Hg content in the sediment collected from the northern part of the lake. It is suggested that the strong affinity of sedimentary total nitrogen (TN) for Hg is independent of its source. The sedimentary OM was found to control Hg distribution in different size fraction of the estuarine sediment.