Millions of tons of chromium residue from chemical manufacturing has been deposited across the nation, posing a grave threat to farmland and groundwater.

About 20 cites in the country are reported to have 6 million tons of chromium slag.

The toxic waste is a by-product from chemical plants, which mushroomed in the 1950s. After the pollution came to light, the central government ordered more than half of them to close in the 1990s.

However, although production ended, the chromium residue remains.

The Dahe Daily in Central China's Henan Province reported that there are six "hazardous fortresses" in Henan, containing 500,000 tons of the toxic chemical sealed inside concrete structures.

In Beisi village of Henan's Gongyi City, there is a stockpile of 50,000 tons of chromium slag that has stood 3 kilometers away from a river for almost 20 years.

The "fortress" is located in a ravine between two hills and the nearest houses are several kilometers away, according to a China Daily reporter at the site on Tuesday.

Local officials said the residue was from a local plant, which began to manufacture the industrial chemical chromic anhydrid in 1976.

After local residents complained about the potential hazards of the residue, in 1990 the factory was forced to store the slag in the more remote Beisi village.

In 1992, the factory was shut down but the residue is still there.

"I didn't know we faced such a potential threat," a herdsman, surnamed Zhuang, told China Daily.