Eighty percent of sewage in India is untreated and flows directly into the nation's rivers, polluting the main sources of drinking water, a study by an environment watchdog showed Tuesday.

Indian cities produce nearly 40,000 million litres of sewage every day and barely 20 percent of it is treated, according to "Excreta Does Matter", a new report released by the Centre for Science and Environment (CSE).

"The untreated waste dumped into rivers seeps into groundwater, thereby creating a ticking health bomb in India," concludes the report.

Weak or non-existent enforcement of environmental laws, rapid urban development and a lack of awareness about the dangers of sewage are all blamed for water pollution.

"Untreated sewage is killing Indian rivers," the report stated.

A 2011 survey by the Central Pollution Control Board revealed only 160 out of nearly 8,000 towns had both sewerage systems and a sewage treatment plant.

Scientists who worked on the CSE report found that thousands of small factories were dumping untreated sewage into rivers and toxic waste was being mixed with fresh water.

Laboratory tests by the team revealed that almost the entire country has nitrate levels higher than the prescribed levels -- a result of sewage leaching into groundwater supplies.

Environmentalists blamed the government for failing to regulate the use of water, with the country's annual consumption expected almost to double by 2050.

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  • A flood affected child watches from inside her submerged house at Boramari village about 75 kilometers (47 miles) east of Guahati, India, Friday, June 29, 2012. (AP Photo/Anupam Nath)

  • Villagers row past a flooded house at Boramari, about 75 kilometers (47 miles) east of Guahati, India, Friday, June 29, 2012. (AP Photo/Anupam Nath)

  • A young Indian boy rows a raft made of banana plant as he takes his family to a safer place in the flooded village of Boramari, about 75 kilometers (47 miles) east of Guahati, India, Friday, June 29, 2012. (AP Photo/Anupam Nath)

  • Shukur Ali, 75, catches fish in flooded waters at Burhaburhi village about 65 kilometers (40 miles) east of Guahati, India, Friday, June 29, 2012. (AP Photo/Anupam Nath)

  • An Indian man takes a break before making his way through flood waters at Burhaburhi village, about 65 kilometers (40 miles) east of Guahati, India, Friday, June 29, 2012. (AP Photo/Anupam Nath)

  • A young Indian girl feeds her brother at a makeshift hut built above flood waters in Burhaburhi village, about 65 kilometers (40 miles) east of Guahati, India, Friday, June 29, 2012. (AP Photo/Anupam Nath)

  • A family rides a raft made of banana plant as they search for dry land in the flooded village of Burhaburhi village, about 65 kilometers (40 miles) east of Guahati, India, Friday, June 29, 2012. (AP Photo/Anupam Nath)

  • Flood affected villagers wait take shelter in a school at Boramari village, about 75 kilometers (47 miles) east of Guahati, India, Friday, June 29, 2012. (AP Photo/Anupam Nath)

  • Young Indian children use a raft, made of banana plants, to wade through flood waters at Burhaburhi village, about 65 kilometers (40 miles) east of Guahati, India, Friday, June 29, 2012. (AP Photo/Anupam Nath)

  • A flood affected Indian man puts his daughter in a raft as they leave for safer place, in Gauhati, India, Tuesday, June 26, 2012. (AP Photo/Anupam Nath)

  • Flood affected people carry a gas cylinder on a makeshift banana raft as flood water is being pumped across a protective boom, in Gauhati, India, Tuesday, June 26, 2012. (AP Photo/Anupam Nath)

  • People prepare to leave their house following an inundation of flood water in Gauhati, India, Tuesday, June 26, 2012. (AP Photo/Anupam Nath)

  • Flood victims sit in a raft as rescue services members paddles through flood waters in Gauhati, India, Tuesday, June 26, 2012. (AP Photo/Anupam Nath)