Almost one-fifth of China’s farmland is polluted, according to a government report released this week. Officials have acknowledged the country’s problems with water and air pollution, but the extent of soil contamination has been a closely guarded “state secret,” for fear of incriminating certain provinces or companies.

About 19.4% of China’s farmland is polluted by cadmium, nickel and arsenic, according to the seven-year study that analyzed a little over half of China’s entire land area. One-fifth of China’s total arable land is about 26 million hectares (64 million acres), the same area as the United Kingdom, by the most recent estimates.

The pollution is concentrated around the Yangtze and Pearl River Deltas—key sources of water in the country and home to millions of people—as well as in parts of the south where much of China’s rice is grown. Last year, half of all samples of rice in Guangzhou were found to have poisonous levels of cadmium, a chemical that can cause kidney failure when ingested. The main causes are agriculture and industry, the report said. (Farmers contribute to soil pollution with their use of fertilizers and pesticides and improper disposal of animal waste.)

Why officials chose to release the results isn’t clear. Authorities have recently admitted environmental mistakes, like the existence of villages near industrial plants where cancer rates have soared, which they had long denied. Still, the soil study results may be optimistic. In December, an official said 3 million hectares of Chinese farmland are now too polluted to even grow crops on. Other estimates of China’s soil pollution are as much as 40-70% of total land, as we’ve noted before.

The worst part may be that the brunt of pollution is borne by villagers, whose farmland and animals are closest to the factories and mines that release contaminants. They are also the most likely to be punished for protesting. Last week, a school teacher in Hunan province was given three years in prison (paywall) for organizing a protest against a local chemical plant. One reason may be that local governments often depend upon these factories or other industrial projects for revenue.