East Kolkata Wetlands (EKW), a Ramsar site, receives ~90 0 MLD of urban sewage and is sustaining the aquaculture practices in the wetlands which are locally known as bheries . In the present study, assessed the contamination status of trace metals (Cd, Co, Cr, Cu, Ni, Pb, Zn ) in different fish tissues, and also in the abiotic component (water and sediment) from 30 wetlands. The wetlands differ in the rate of intake of sewage. One group receives sewage once in every 7 days, the second group receives once in every month while the third group receives sewage very irregularly and termed as partially sewage fed .
Ten w etlands were selected from each group. The tissues (gill, liver, and muscle) of 5 fish species ( a total of 30 samples from each fish species) including the 3 Indian Major Carps: rohu (Labeo rohita ), catla (Labeo catla ), mrigal (Cirrhinus mrigala), Minor Carp: bata (Labeo bata ), and tilapia ( Oreochromis niloticus) were assessed . A ll the studied trace metals were recorded at concentrations much below their respective permissible limits set for fish muscle consumption, with the exception of occasional higher Cd levels in m rigal (0.32 µg/g). The findings suggest that, overall, the muscle tissues of the studied fishes are safe for human consumption. The water samples from inlet, mid-pond, and outlet zones of all the 30 representative wetlands were analysed. Results indicated that the concentrations of all the studied trace metals were within the permissible thresholds. Notably, Pb exceeded the permissible limit with detected level of 0.03 mg/L particularly in sewage intake zones, while at the mid-pond and outlet regions the concentrations were significantly low . The findings suggest that the water quality across most wetlands wa s within safe limits for aquaculture.
The sediment analysis of weekly sewage-fed wetlands revealed significantly high trace metal pollution across all sites with respect to the UPSEA threshold limits. The spatial pattern indicated dominance of Cd in the inlet area , while Pb, Zn progressively towards the outlet sediments. The monthly sewage-fed wetlands also revealed significantly high trace metal pollution by Cr and Ni; while Zn and Pb showed moderate to heavy enrichment, particularly at outlet sites. The partially sewage-fed wetlands revealed that most of the trace metals (Cd, Co, Pb, Zn) were within the safe limits, while only Cr and Ni at inlet sites indicated moderate enrichment, likely linked to occasional sewage inflows. Spatially, concentrations decreased from inlet to outlet. Overall, the study highlights that sewage loading intensity strongly governed sediment trace metal accumulation. Weekly sewage fed wetlands are under ecological stress, monthly sewage fed systems are at moderate risk, whereas partially sewage-fed wetlands remains comparatively safe. These findings underscore the need for routine monitoring for management and production sustainability.