Shanghai government officials were scrambling to solve the mystery of nearly 3,000 dead pigs dumped in a suburban river that is an important source of the city's drinking water, the latest environmental crisis to hit a city that is also battling a sudden fall in air quality.

Shanghai residents used social media to air their complaints that the municipal government was slow to respond to the dead pig crisis, which broke on Friday but remained largely unexplained on Monday night.

(Read More: Pollution 'Worst on Record' in Beijing)

"Is this the water we are drinking? And this has happened more than once or twice in the past, I don't know what our 'relevant authorities' are doing!" said one post on Sina Weibo, the popular microblogging site.

Reports of the dead pigs emerged as China's nominal parliament met in Beijing where one of the most politically sensitive topics is air and water pollution. Public opinion polls show rising public dissatisfaction with the quality of China's food supply and environment, with many rich Chinese saying they have considered emigration to avoid the pollution caused by China's rapid economic growth.