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Soil in China’s leading rice-producing region shows high levels of heavy metal contamination, in a study that suggests that the proximity of mining and industry to agricultural areas is posing serious threats to the country’s food chain.

In “Cadmium Rice: Heavy metal pollution of China’s rice crops,” researchers for Greenpeace East Asia sampled farmland and uncultivated soil, water and rice grown near a smelter of non-ferrous metals in Hunan Province, China’s top rice producer.

In some locations of the study, the researchers found soil containing cadmium levels more than 200 times the national health standard, adding to a growing body of evidence that parts of the country’s soil are heavily degraded after decades of fast industrialization and high economic growth. All but one of the rice samples exceeded the maximum level of cadmium in rice for human consumption in China.

“Cadmium rice” is a well-known term in China since a 2013 Guangdong Province government report that 44 percent of rice samples had excessive levels of cadmium attracted attention. But the Greenpeace study extended the concept by listing “arsenic rice,” “mercury rice” and “lead rice.” Of those four toxic substances, only mercury levels appeared relatively safe.

With “a fast pace of urbanization, China is struggling to save enough land for farming and keep a high rate of grain self-sufficiency. But apart from quantity, whether it can maintain soil health remains a question,” Tang Damin, a communications officer at Greenpeace, wrote in an email.

The study tried to pinpoint the source of the cadmium — according to another recent government study, 7 percent of soil in China is polluted with the metal.

Greenpeace noted that many provinces where metals mining and smelting was widespread were also major grain-producing provinces. It supplied two maps that showed colored-in provinces representing metal-smelting areas and grain-production areas that mapped closely to each other.

“The main sources of cadmium pollution are emissions from smelting plants,” said Chen Nengchang, a soil expert at the Guangdong Institute of Eco-environment and Soil Sciences, chiming with the study’s findings.

According to the study: “For five metals (arsenic, cadmium, lead, manganese and zinc), strong correlations of concentrations in uncultivated soils indicate a common source, suggesting that emissions from the complex may be a major contributor to elevated concentrations of these five metals in uncultivated soils in this area.”

In rice, “the cadmium levels were most notable, with all but one of the samples exceeding the maximum allowable level of cadmium in rice for human consumption in China (0.2 mg/kg), with the highest level exceeding the limit by 22 times,” it said.

In a telephone interview, Mr. Chen said : “Cadmium intake through food is a chronic intoxication process by small doses. It’s a very slow accumulation, but its excretion and drainage is even slower. Its half life is about 17 to 38 years, that’s to say it would take that long to reduce by half. Basically once inside, it will stay there — if not all your life, at least the better half of it.”

“About a third of the cadmium absorbed by the human body will concentrate in the kidneys, another a quarter in the liver. Its damage to human health is formidable. Generally speaking, one can’t take in more than two grams of cadmium in a lifetime,” he said.

Rice is China’s favorite food, but it is also unusually vulnerable to pollution as it easily absorbs metals from the soil, Deborah Blum reported last week in The Times’ Well blog.

It is “one of the most widely consumed foods in the world,” and “also one of nature’s great scavengers of metallic compounds,” Ms. Blum reported. Rice contamination with cadmium and arsenic was also an issue in the United States, she noted. However the levels found in the study, in Texas, were “not high enough to provoke alarm.”

Last week, the Chinese government disclosed for the first time that one-fifth of China’s farmland was polluted.