Assemblages of Total Mercury in the Tropical Macrotidal Bidyadhari Estuarine Stretches of Indian Sundarban Mangrove Eco-Region

The study was conducted to estimate total mercury in water and sediment of Bidyadhari river of Indian Sundarban delta in pre-monsoon, monsoon and post-monsoon period. Bidyadhari river presently serves as a sewage and excess rainwater outlet from the Kolkata metropolitan and adjacent area which ultimately empties at the Bay of Bengal in the course of the Indian Sundarban delta. Four different study sites situated around the course of the river were selected from the outfall of sewage canals at Kulti-Ghushighata (S1) where metropolitan sewages discharged and mixed up into water of Bidyadhari river which ultimately carried through this river via stations Malancha (S2), Kanmari (S3) to Dhamakhali (S4), just before the river confluences with the larger Raimangal river at northern Sundarban delta. Mean mercury concentration in collected water ranged BDL to 0.014 ± 0.001 μg ml-1 and sediment samples ranged BDL to 0.260 ± 0.014 μg g-1. Highest mercury accumulations in river water both high tide and low tide was found at S4 followed by S3 with pronounced seasonal variation. Mercury present in the sediment (0-5 cm) showed a remarkable site and season specific differences with highest concentration in S4. Box whisker plot revealed that one extreme value was found at the S4 along with one outlier was at S3 and five outliers were at S4 during monsoon period. Regarding total mercury assemblages, PCA analysis showed all the sites except S4 are significantly associated. Based on Effective Range Low (ERL) value it is considered that sediment is still low mercury enrichment with less ecotoxicological risk while level often above the requirement desirable limit of drinking water recommended by WHO.