In June, one in every five samples tested by the Brihanmumbai Municipal Corporation (BMC) was found to be unfit for consumption.

The bad news doesn’t end there. The BMC has found 17% of all water samples collected from just outside water reservoirs to be contaminated. This means that water treated in the reservoirs is unfit for consumption even as it is released to households.

Of the 929 samples collected from just outside reservoirs, 160 were unfit for consumption, while four cases of Escherichia coli (E coli), the bacteria that leads to diarrhoea, dehydration, vomiting and nausea, have been reported in water at the city’s purification reservoirs.

In May, the BMC found 7% of water samples from outside reservoirs unfit for consumption and one case of E coli.

The reservoirs are where drinking water undergoes a purification process and where chlorine is added to it. That water remains contaminated just after it is ostensibly made safer to drink has worried officials, who are unable to determine why it’s happening.

This is not the first time that the city’s reservoirs have reported high levels of contamination.

In July 2011, HT had reported on water samples from reservoirs showing alarming levels of E coli.

Back then, the BMC had announced several measures to deal with the problem, but two years later, fresh data shows that the problem still exists.

Rajiv Jalota, additional municipal commissioner, said the BMC was not sure why the water samples collected from outside the reservoirs were contaminated.

“It is almost impossible that water samples after purification can have any kind of contamination,” Jalota said. “We will look into the reason behind these figures immediately.”

Former deputy municipal commissioner Prakash Sanglikar said it was perplexing to see contamination at water reservoirs.

“It’s a scary prospect as it points to large systemic problems,” he said, even as he raised doubts about the BMC’s testing mechanism.

“The BMC must conduct random checks at its water reservoirs and then testing the samples at private laboratories. It should also consider third party audits.”