Inter-estuarine and seasonal to decadal variations of heavy metal pollution in the Gulf of Cambay, India.

Toxic heavy metals adsorbed preferentially onto suspended sediments enter our food chain by bio-assimilation in coastal ocean organisms. To decipher metal pollution status in the Gulf of Cambay (food hub of India) under rising anthropogenic pressure, we present seasonal abundances of Ti, Cr, Co, Ni, Cu, Zn, Cd, and Pb in > 150 samples of suspended sediments (> 0.45 µm) collected in four seasons (2016-2017) from two large estuaries (Narmada and Tapi). The suspended sediments of both the estuaries generally show low heavy metal pollution (Igeo < 1). The hotspots of moderate-to-high pollution of Pb (Igeo < 4), Cd (Igeo < 3), and Zn (Igeo < 2) are found at salinity < 2, and those of Co (Igeo < 2) at salinity ~ 20-30 in the Tapi estuary during the non-monsoon seasons indicating their decoupled sources in Surat. The PLI values show no or little seasonality in the overall metal pollution status of both the estuaries. A comparison with the literature data suggests that suspended sediments efficiently capture active metal pollution in Indian estuaries. Furthermore, a recent decline (2004-2017) in estuarine metal pollution in the Gulf of Cambay found in this study could be due to enhanced organic matter supplies by enhanced urban sewage discharge and/or more trapping of contaminated riverine sediments upstream of the newly built large dam reservoirs counteracting the growth of anthropogenic metal inputs in the Narmada and Tapi watersheds. The data scarcity of heavy metal concentrations in suspended sediments limits reporting unambiguously the current pollution status of other major Indian estuaries.