The rivers of Mandovi and Zuari have been polluted due to a high content of manganese, iron, colbalt and other trace elements leaching from iron ore dumps on their banks during the monsoon season, National Institute of Oceanography (NIO) scientist V P Rao said. Speaking during a symposium on Indian estuaries at the 76th annual meeting of Indian Academy of Sciences, Bangalore at NIO conference hall, Dona Paula on Friday, he said that copper, chromium, zinc and lead are the other elements, which are also found in high concentrations, but termed the pollution overall as moderate due to the tidal flushing.
The mining activity along Zuari banks is more than 40 km away and the river is impacted by the mining pollution to a lesser extent. “But at least two-third of Goa’s mining activity is concentrated along the banks of the Mandovi,” he said.
Rao explained that the funnel-shaped Chicalim bay enhances the physical processes, pushing sea water into the channel. “The wind-induced currents and tidal flushing force the sediment settled at the bottom to rise and mix with the river-borne suspended particulate matter from upper streams,” Rao said. But siltation in the river is much higher in the channel.
While strong mixing process in Zuari results in dilution of the pollutants, the Aguada bay being much smaller, is not easier to flush out in the Mandovi. “The pollutants are seen more in Mandovi’s water column” Rao said.
The NIO scientists had taken samples of suspended particulate matter in both estuaries of Zuari and Mandovi on the same day during monsoon for analysis.
The pollution in the estuaries before and after the monsoon season is less as the sediment settles down due to tidal flushing in the channel, Rao said.
The turbidity in the rivers influences primary productivity and results in fish mortality or migration and siltation in channels.