Toxic materials up to 23 times safety levels have been found in construction materials used in the relocation site for residents to be evicted for the construction of the Yanba Dam in Gunma Prefecture, the Mainichi Shimbun has learned.

The finding may affect the dam construction and the relocation of residents in part of the prefectural town of Naganohara that will be submerged for the controversial project. The Ministry of Land, Infrastructure, Transport and Tourism is investigating the matter.

According to sources close to the dam construction, materials containing toxic substances were used without permission for filling the land and building roads in the residential area for locals to be evicted from Naganohara. The Mainichi Shimbun collected crushed stones used as construction materials from three locations in the relocation site after obtaining permission from landowners and asked a third-party institution designated by the Ministry of the Environment to examine the materials.

As a result, fluorine at levels between 5 to 23 times government-set environmental standards were found in the crushed stones, prompting experts to call for the removal of the construction materials. The crushed stones were also found to have a strong alkaline content, which could affect plants, but are not subject to environmental standards.

The construction materials in question are believed to originate from iron and steel slag — a by-product of iron refining in the shape of stone and sand. Such slag often contains residue from various chemical materials that were added in the production process. Usage of iron and steel slag is partially permitted in road construction and other materials as long as their toxic levels are below environmental standards.

In January, it was found that Daido Steel Co., a major steel maker in Nagoya, effectively paid a construction company in the Gunma Prefecture city of Shibukawa for accepting toxic slag produced at Daido Steel’s Shibukawa plant, in what is called an inverse onerous contract. The same construction firm was also involved in the development of the relocation site for Naganohara residents. The firm has thus far participated in more than 10 public works projects related to the Yanba dam, using slag from Daido Steel and mixing it with natural crushed stones.

The Infrastructure Ministry has obliged companies participating in the construction of the residential site to use natural crushed stones and received a certificate from the Shibukawa construction firm that said the company was using natural crushed stones. However, after receiving information similar to that obtained by the Mainichi, the ministry collected crushed stones used in the relocation site and confirmed that stones apparently emanating from slag were mixed into them. The ministry is questioning companies involved in the project.

As of 2007, 134 households in five districts that will be submerged due to the Yanba dam construction were hoping to move into the relocation site. Among them, 84 households moved into the area by the end of March this year. The main construction of the Yanba dam is slated to begin in October.

Under the Waste Management and Public Cleansing Act, slag is defined as industrial waste and costs 20,000 to 30,000 yen per metric ton for final disposal. However, a company can drastically hold down the cost by employing an inverse onerous contract, which would only cost the seller several hundred yen per ton of slag when handing it over to the buyer.

“There may have been cases in which slag was mistakenly mixed (in crushed stones), but we never laced slag deliberately,” the president of the construction firm in Shibukawa told the Mainichi. “Even if slag was found in crushed stones, there would be no damage if the mixed amount was small.”

A representative of Daido Steel Co. said, “We are not aware that slag was used in landfill (in the relocation site). We will conduct a study to find out the facts.”