China's first national census of pollution has revealed such a dire picture that the government has mooted a new tax on its sources, according to reports.

The level of water pollution countrywide was found to be more than twice government estimates. Previously, the Chinese government said, it had excluded agricultural sources of pollution.

Farms are revealed by the report to be the biggest single source of the main water pollutants.

In 2007, the year covered by the census, more than 209bn tonnes of waste water were also discharged.

Preventing and controlling agricultural pollution would be a higher priority in future, said Zhang Lijun, vice minister of environmental protection, announcing the results of the census on Tuesday.

Agricultural waste and the residues and “run-off” of pesticides and fertilisers are common sources of agricultural pollution. Farmers can reduce amounts of run-off, and save money, by targeting the use of chemicals rather than by spraying large areas indiscriminately.

The census found more than 63.7 trillion cubic metres of “waste gases” were emitted that year.

These included emissions of 23m tonnes of sulphur dioxide, which causes acid rain and is commonly the result of burning coal in power stations without “scrubbers” to remove it.

Wen Jiabao, the Chinese premier, admitted last month the country's pollution was “grim”. Beijing is understood to be concerned that high levels of pollution and other forms of environmental degradation could provoke social unrest.

The census will be used as the basis for policy. The data were much more comprehensive and broken down into greater detail than any information on pollutants China has published previously. But green campaigners complained the data gathered were not published in full.

The census took two years, as about 6m sources of pollution – factories, farms, residential sources and others – had to be examined.

Mr Zhang said: “The census of pollution sources for the first time in the country is a significant survey on the national situation. Its operations went smoothly and its main tasks were basically completed.”

Mr Zhang painted a relatively optimistic picture, forecasting pollution would soon peak, and that the government could take a more targeted approach to combat pollution in future.

He said discharges of industrial pollutants were concentrated in very few industries and regions. There were “no major surprises”.

China's State Council ordered the census in 2006 and $100m (£64m) was allocated to the project, which involved more than 570,000 people countrywide.