THE "cadmium rice" scandal has raised awareness of the extent of heavy metal pollution in China, but the situation in Shanghai is considerably better. Zhang Qian talks to researchers mapping city pollution.

Cadmium discovered in rice from Hunan Province astonished Chinese residents once again, indicating a more pervasive food safety problem and focusing attention on the dangerous levels of heavy metal soil pollution.

The discovery of the toxic, cancer-causing heavy metal in Hunan rice came to light in February in Guangzhou Province. News reports said contaminated batches had been discovered over the years.

Scientists found that no cadmium was part of any chemical additives used after the rice was harvested, thus, leaving heavy metal soil pollution as the likely cause.

Cadmium, a known carcinogen, builds up in the body and damages the kidneys, lungs and bones, causing brittle bones and pain.

It is one of several toxic heavy metals that have leached from Hunan mines, mine tailings and chemical factories into waterways, mainly the Xiangjiang River and tributaries. Water from contaminated rivers, lakes and streams is typically diverted in rice paddies where metals settle into the soil and taint the crops.

Though less obvious than air and water pollution, soil pollution is now getting unprecedented public and official attention, with the revelation of "cadmium rice."

Pollution maps

China's Ministry of Land and Resources is said to be working on a nationwide soil pollution map, with checks on 81 chemical indexes (including 78 chemical elements) in the topsoil and deep soil all over the country.

The Shanghai Environmental Protection Bureau is also launching a soil pollution investigation of key industrial areas in the city.

Lead, mercury, cadmium, arsenic and chromium are the top five heavy metals frequently discovered in polluted soil near industrial areas. Antimony and selenium are also found in some regions. Heavy metals are essential in electronic gadgets and their batteries.

"Soil pollution is not a new issue, the problem has existed for more than a decade. But the polluted regions are expanding at an enormous speed in recent years," says Professor Chen Zhenlou of Resource and Environmental Science School of East China Normal University.

"Generally speaking, heavy metal soil pollution in Shanghai is not as serious as that in Hunan and Jiangxi provinces, and there is no need to worry about 'cadmium rice' with the relatively low content of cadmium in local soil," says Professor Chen. However, there are polluted areas and sources of pollution date back to the days before toxic discharges were banned.

Rice quality report

About one-fifth of China's farmland, more than 20 million hectares, is polluted by heavy metal and farmland polluted by cadmium is found in around 25 regions in 11 provinces, according to a report issued in 2010 by institutes including the Agriculture Ministry's rice quality test center.

The report, titled "Research on China's Rice Quality, the Safety Situation and Development of Countermeasures," indicates that the problem is most serious in regions south of the Yangtze River, including Hunan and Jiangxi provinces.

"The problem is that the soil cannot self-purify itself from heavy metals," says Professor Chen, "Once the heavy metal pollution happens, it stays. Even pesticides degrade in 20-30 years, but heavy metals can never degrade the natural way."

Using irrigation water polluted by domestic sewage and industrial waste was common in the 1970s and 80s.

Irrigating with such polluted water has been banned since the 1990s, but the heavy metals discharged before that period have persisted.

It's also possible that unscrupulous enterprises have continued to discharge toxins that pollute water and soil. The metal mining industrial chains must take responsibility for the serious soil pollution in Hunan and Jiangxi provinces, according to Chen. Their industrial discharge enters farmland, surface water and underground water, eventually leading to heavy metal pollution when elements settle.

The relatively high heavy metal content in Shanghai farmland in Pudong New Area and Songjiang District are typical results of historic wrongdoing, says Chen. He declined to disclose details.

Regardless of the official ban on using industrial polluted water, uninformed farmers may be perpetuating the damage, he says.

"We still need to be alert since lead and mercury content is relatively high in some areas of the city."

Heavy metal soil pollution is also caused by other industries, including power generation, steel and petrochemical manufacturing where heavy metals are often used as catalysts.

Baoshan District is famous for its steel industry and lead pollution is a problem. Jinshan is known for petrochemicals, which use mercury.

Polluting factories have been closed and relocated, though soil pollution persists. Suzhou Creek has been dredged, removing some of the problem.

Industrial discharge pollution, so called "point-source" pollution, can be controlled with strict regulations.

Non-point-source pollution of heavy metals is related to use of chemical fertilizers containing some heavy metals and the power-generation system that relies on burning coal. This is not as severe as direct industrial discharge.

In addition to Pudong New Area, Jinshan, Baoshan and Songjiang districts, heavy metal pollution is also found in some regions in Chongming County, such as Dongtan wetlands, a surprising result of the investigation, says Chen.

Chongming Island is widely regarded and promoted as a "pure land" and ecological island.

"We believe polluted silt and sand brought downstream by the Yangtze River from the upper reaches is the major cause of the situation," says Chen. "Since it is an estuary alluvial island of the Yangtze River, it isn't surprising that polluted soil from the upper reaches piled up here."

Health is seriously threatened by heavy metal pollution of crops, water and aquatic products. Since humans are at the top of food chain, high levels of heavy metals can accumulate in the body.

In the earth, heavy metals are stable, but when they are mined and processed, they become metallic particles in the air and settle on the soil and on water sources. Air pollution can turn into acid rain.

Health threat

Heavy-metal polluted crops in China may total 1.2 million tons a year, according to Professor Huang Qiaoyun of the Central China Agricultural University in Wuhan.

"Heavy metal is damaging to humans and children are more vulnerable," says Dr Yan Chonghuai, director of the Environmental Medicine Laboratory at the Shanghai Institute for Pediatric Research.

Cadmium pollution common in metallurgy, plastics and electronics industries damages the kidneys and bones, where it replaces calcium and results in painful, brittle bones.

In the 1970s hundreds of Japanese villagers were poisoned by rice from cadmium-polluted paddies. They suffered softened skeletons, kidney failure and needle-like pain in the bones. The condition was called "itai, itai," which is Japanese for "It hurts, it hurts."

The condition has not been found in China.

Even if cadmium is no longer ingested, the process of renal failure will continue in severe cases.

According to the World Health Organization, the safe standard of cadmium intake is no more than 7 micrograms per week for each kilogram of body weight. That means no more than 60 micrograms a week for a person weighing 60 kilograms.

Mercury pollution is often related with metallurgy, coal-burning and energy-generation industries that release methyl mercury, damaging brain and kidneys.

Lead, which tends to remain in the blood, poses serious danger to the nervous system, especially for children. Loss of concentration is an early sign; lead also damages the digestive system and the endocrine system. Lead is used in metallurgy, machine building, electroplating, battery production and coal burning.

Positive move

Developing a nation-wide map of soil pollution is a positive move, says Professor Chen, but solving the pollution problem is difficult since the metals don't degrade, and chemical and physical soil remediation is at a research phase.

Remediation by hyper-accumulator plants is the mostly widely used method, though it's not widespread and it takes a long time to be effective. Soil removal and replacement is applied in seriously polluted regions, but this too is limited in scope and is very costly.

A new relatively low-cost soil remediation by using biocharcoal was recently reported to be effective by a team of scientists headed by Professor Cao Xinde of Shanghai Jiao Tong University. The biochar is made from straw, blue-green algae, sawdust and rice husks and put into the soil to absorb heavy metals. It can be used in combination with bioaccumulator plants. It has not been used on a large scale.

"Considering all the difficulties in remediation for heavy metal soil pollution, it is important that we stop further pollution while trying our best to find better solutions," says Professor Chen.