A storm over soil pollution and corner-cutting at the site chosen to host the Toyosu wholesale food market has centered on the presence of toxins in water that could be hazardous to human health.

On Sept. 29, the Tokyo Metropolitan Government said it had sampled two carcinogens at levels slightly above the legal limit for drinking water.

The toxins were found in groundwater at three locations at the planned food market in Tokyo’s Koto Ward.

What do the regulated limits mean? Does exceeding them pose a hazard to human health?

Following are questions and answers about conditions at the complex that is slated to replace the renowned Tsukiji fish market in Chuo Ward.

What chemicals were detected and at what levels?

Traces of benzene and arsenic were detected in groundwater at three locations at the 40-hectare site, which is earmarked for sales of fish, fruit and vegetables.

However, the samples were not in the building basements.

The density of benzene was 0.014 mg and 0.011 mg per liter at two locations in the section for fruit and vegetable dealers, slightly exceeding the regulation limit of 0.010 mg for benzene, the standard set for groundwater under the Soil Contamination Countermeasures Act.

Groundwater at one location in the same area was found to have an arsenic level of 0.019 mg per liter, above the 0.010 mg regulation limit.

Legally speaking, the metropolitan government is not obliged to apply those limits to the Toyosu site because the groundwater would not be used for drinking or washing produce.

It has instead conducted water sampling tests voluntarily under a two-year monitoring survey to ease public concerns over a former industrial site that is being repurposed for food.

So far it has conducted eight groundwater tests, and this is the first time chemicals were found to exceed the mandated limit.

The Tokyo government plans to conduct a final round of water sampling next month and to release the results in January.

Do the levels pose a danger to humans?

Experts believe not. However, the report has fueled anxiety and has made it politically more difficult for the metropolitan government to close Tsukiji and shift its operations to Toyosu.

The regulation limits for potentially toxic chemicals for groundwater are as strict as those for tap water.

The levels are set to ensure safety even in the worst case scenario of drinking 2 liters of the contaminated water every day for over 70 years, which would be impossible at the Toyosu site. The scenario is therefore unrealistic.

And even if someone were to exceed the limit over seven decades, it would increase their risk of cancer by only 0.001 percent, according to the Environment Ministry.

Is it excessive to apply tap water standards to Toyosu?

It depends on one’s view of the risk.

“I personally believe the levels do not pose any safety problems, but how people feel about it is another matter,” said environmental safety expert Kohei Urano, a professor emeritus at Yokohama National University.

He said people have become particularly sensitive about the Toyosu site because they know it will handle food.

Tatemasa Hirata, director of the Open University of Japan’s Wakayama Study Center and the chairman of an expert panel on soil contamination set up by Tokyo Gov. Yuriko Koike, released a statement on Oct. 3 saying judgment should not be based on a one-time spike in levels.

Hirata added, it is common for contamination levels of newly scrubbed soil to show ups and downs while continuing on a trend of decline.

Could levels rise in future?

Experts are split on this. Metropolitan government officials insist that it will not happen.

Akio Hata, former president of the Japan Association on Environmental Studies, argues that more contaminated groundwater may seep to the surface, particularly if freed by an event large enough to cause liquefaction, as was seen at some Toyosu locations — a site built on reclaimed land — after the Great East Japan Earthquake in March 2011.

The metropolitan government maintains that liquefaction can be prevented by lowering the groundwater level with pumps and by hardening the soil.

It said this was applied to some parts of the Toyosu site before the 2011 quake, and the tremor caused no observable liquefaction at those locations.

Takeshi Hasegawa, a former member of a technical expert panel for the Toyosu project under the metropolitan government, argued that the groundwater level can be lowered to prevent any contact with humans.

He added, toxins can also be flushed out.

“In Toyosu, if you want to lower the densities of chemicals below the regulation levels, you can pump in lots of tap water and pump out groundwater from wells. That was the method used before construction to lower the soil contamination level,” Hasegawa said.

At the Toyosu site, 19 wells have been dug for pumps to control the groundwater level, and 21 other wells have been drilled for use in monitoring toxin levels.

Using those wells and the pumping system, the groundwater level can be lowered to prevent water from seeping into the basements at the market buildings, Hasegawa said.

The metropolitan government was not pumping the system at its full capacity when assembly members visited the underground space last month and revealed that a thick layer of clean soil — recommended by outside experts — had not been laid as planned.

When will the pumps reach full speed?

The metropolitan government will put the system into full operation in mid-October, with the aim of keeping Toyosu groundwater less than 1.8 meters above the level in Tokyo’s Arakawa River, a standard used by the metro government.

This means the water level will be maintained well below basement floors.

Since Oct. 4, the metropolitan government has been posting data on the recorded groundwater levels at Toyosu, measured twice a day, on its website.

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