The culprit is Siang river that originates in the Tibetan Glaciers and meets the Brahmaputra in Assam and there are fears it is getting worse.
A little over ten days after a test of a water sample from the river indicated that its turbidity was way over the safe limits for human consumption and set alarm bells ringing, NDTV got the Siang water samples tested at the prestigious IIT-Guwahati.
The test results found the particle pollutants level, known as turbidity, was 1249 NTU, or Nephelometric Turbidity Unit that measures the concentration of suspended particulates in liquid. The safe limit for potable water is 5 NTU.
That is 250 times more than the safe limit and more than twice the figures recorded earlier. The IIT report said the water also has very high iron and lead content
The findings are scary, particularly since the sample was taken from a remote, sparsely populated, high altitude place.
The IIT report says it could be "inferred without reservation" that the water from the Siang river in Arunachal Pradesh "is not suitable for drinking purposes" unless it is treated and dangerous for human consumption.
This week, a worried Rajya Sabha member from Assam Ripun Bora also flagged the sharp decline in water quality in parliament.
"There is an abnormal change of water in Brahmaputra during the last one month and poisonous, muddy, turbid water is flowing in this river. As a result, a lot of wild animals, aquatic life and fish died.... This catastrophe has jeopardized Brahmaputra valley civilization," Mr Bora told the Rajya Sabha on Tuesday.
Mr Bora blamed the muddy, poisonous water on what he said was China's 1,000 km tunnel being built upstream.
But China last week denied reports of any tunnel being built to divert Brahmaputra water.
Assam Chief Minister Sarbananda Sonowal, however, appeared to play down the risks.
"All the test results have come to us and it is not saying that it is not fit for human consumption," the Chief Minister said, citing a test report from Hyderabad-based Indian Institute of Chemical Technology.
"So, challenges are there but now gradually, it is purifying and I believe measures are being taken up in the highest level. We need not worry," he said.