The mysterious foot injuries reported this month by joggers who frequent a path along the Tama River in Kawasaki's Nakahara Ward were caused by chemicals leaching from the pavement, the Kanto Regional Development Bureau has determined.
"It's the first time for such a thing to happen. I have never heard of a similar case before," said Takanori Maeda, vice manager of the land ministry's Keihin Kasen Office, which is overseen by the bureau.
The road is popular with residents and is part of the route for the annual Kawasaki International Tamagawa Marathon.
The Keihin Kasen Office had been investigating the injuries since the beginning of the month. It took until Thursday to solve the mystery.
On April 1, a group of three joggers claimed they felt pain and burning in their feet after running on the road. They were later diagnosed with chemical burns caused by contact with strong chemicals.
Maeda explained that the culprit was lime.
He said the road crew accidentally mixed more lime into the pavement than necessary when it was conducting repair work. When it rained, the strongly alkaline substance seeped out and pooled into puddles, eventually making its way into runners' shoes.
Maeda said the ministry ordered the construction firm to mix dirt with the lime, but the crew inadvertently added dirt that already contained lime, resulting in a mixture that was overly concentrated.
Fortunately, Maeda said no one was severely injured. The Keihin Kasen Office is still getting reports of similar injuries.
The ministry has restricted a roughly 250-meter section of the path from public access until repairs are complete. The repairs are estimated to take two weeks.