BEIJING » 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 was shown in official figures that had long omitted agricultural waste.

The first 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 as measured by chemical oxygen demand—the main gauge of water pollution, which measures chemical compounds in the water by checking how much oxygen they use. 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 had counted 13.2 million tons of agricultural effluents for the first time, and 324,600 tons of discharges from landfills.

The census keepers had also used updated methodologies and reached many more parts of the countryside and industrial sites than had official statistics, which helped account for the much larger figure in the census, Zhang said. Were it not for the vastly expanded scope of the survey, the chemical oxygen demand level in 2007 would stand at only 5.3 percent higher than previously calculated, he said.

Ma Jun, director of the Institute of Public and Environmental Affairs, a nonprofit research group in Beijing, said that government planners had estimated that the country's rivers and lakes could handle only 7.4 million tons a year of chemical oxygen demand. The scale and significance of agricultural effluent was seldom recognized in previous government planning, which focused on bringing down mainly industrial emissions to around 7 million tons a year from 13.8 million tons, said Ma, a leading expert on water pollution in China.

The new total of more than 30 million tons suggests a much bigger problem. “;We believed we needed to cut our emissions in half, but today's data means a lot more work needs to be done,”; Ma said.

The extent of agricultural waste could prove a more intractable problem than that of the many factories dumping effluent into China's rivers and lakes. “;When it's millions of farmers, it's more difficult to bring it under control,”; Ma said.

Steven Ma, of the Beijing office of Greenpeace, said that the government's decision to calculate and release figures for agriculture would have an effect on the policy debate over water pollution in China. “;Everybody knew there was a problem with agricultural pollution in China, but now there are numbers,”; he said.

Jonathan Ansfield reported from Beijing, and Keith Bradsher from Hong Kong. Zhang Jing contributed research.