BEIJING - China's environmental watchdog fined 88 companies 6.13 million yuan (about $1 million) for violations resulting in underground water pollution in a recent 40-day campaign focused in North China.

The campaign covered 25,875 companies that discharge waste water and a total of 558 violation cases were investigated, the Ministry of Environmental Protection said in a press release on Thursday.

Starting in late February, the campaign was especially organized to investigate companies that discharge industrial waste in Beijing and Tianjin municipalities, as well as Hebei, Shanxi, Shandong and Henan provinces, according to the statement.

Environmental authorities also found 55 companies discharging or storing waste water using wells, sewage pits, ditches and other channels that did not feature leak-proof materials, the release said.

The ministry said nationwide efforts will continue to crack down on violations that lead to underground water pollution.

Environmental pollution scares have popped up across China in recent years.

In February, microblog posts indicated that some factories in the city of Weifang in east China's Shandong Province had disposed of waste water underground, thus polluting the local water supply.

According to figures released by the ministry last year, underground water in 57 percent of monitoring sites across China is polluted or extremely polluted. In addition, 298 million rural residents do not have access to safe drinking water.