China’s government on Tuesday unveiled its most detailed survey ever of the pollution plaguing the country, revealing that water pollution in 2007 was more than twice as severe as official figures that had long omitted agricultural waste.

The first-ever national pollution census, environmentalists said, represented a small step forward for China in terms of transparency. But the results also raised serious questions about the shortcomings of China’s previous pollution data and suggested that even with limited progress in some areas, the country still had a long way to go to clean its waterways and air.

The pollution census, scheduled to be repeated in 2020, took more than two years to complete. It involved 570,000 people, and included 1.1 billion pieces of data from nearly 6 million sources of pollution, including factories, farms, homes and pollution-treatment facilities, the government announced at a news conference.

But the comprehensiveness of the survey also resulted in stark discrepancies between some of the calculations and annual figures that the government has published in the past.

By far the biggest of these involved China’s total discharge of chemical oxygen demand — the main gauge of water pollution. These discharges totaled 30.3 million tons in 2007, the census showed.

In recent years the Ministry of Environmental Protection has done a much narrower calculation of these discharges, excluding agricultural effluents like fertilizers and pesticides as well as fluids leaking from landfills. By that narrower measure, discharges came to only 13.8 million tons in 2007, which officials described at the time as a decline of more than 3 percent from 2006 and a “turning point.”

Zhang Lijun, the vice minister of environmental protection, sought to play down the differences with previous data. He noted that the census counted 13.2 million tons of agricultural effluents for the first time, and another 324,600 tons of discharges from landfills.