The 2006 Mumbai "sweet" seawater incident was a strange phenomenon during which residents of Mumbai claimed that the water at Mahim Creek had suddenly turned sweet. Within hours, residents of Gujarat claimed that seawater at Teethal beach had turned sweet as well. This caused a mass hysteria among people who started coming in large numbers to drink the sea water.
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In the aftermath of the incidents, local authorities feared the possibility of a severe outbreak of water-borne diseases, such as gastroenteritis. The Maharashtra Pollution Control Board had warned people not to drink the water, but despite this many people had collected it in bottles, even as plastic and rubbish had drifted by on the current. The Municipal Corporation of Greater Mumbai had ordered a bacteriological report into the "sweet" water, but suspected that "contamination in the water might have been reduced due to the waters from Mithi river flowing into the mouth of Mahim Bay".
Possible explanations
Geologists at the Indian Institute of Technology Bombay offered the explanation that water turning sweet is a natural phenomenon. Continuous rainfall over the preceding few days had caused a large pool of fresh water to accrue in an underground rock formation near to the coast, which then discharged into the sea as a large "plume" as fractures in the rocks widened. Because of the differences in density, the discharged fresh water floated on top of the salt water of the sea and spread along the coast. Over time, the two would mix to become normal sea water once more.
According to Valsad District Collector D Rawal the reason for the water in Gujarat tasting less salty than usual was that because of the monsoon, two rivers Auranga and Banki were in spate and were flowing into the sea in the region.
Similar natural phenomenon is also observed in the case of halocline.