Coastal cities and towns let untreated sewage flow into the sea, causing pollution of sea-water. Cities of Mumbai, Ahmedabad, Surat and Chennai contribute to about half the pollution in coastal waters, a recent study by the Environment Ministry revealed. "Municipal waste water constitutes the single largest source of coastal pollution. Nine coastal States generate around 7,663 million litres (mld) of sewage each day. Out of this only 1,073 mld (14 per cent) is treated.