Tests show that colorful sewage pits found on farmland in Langfang, Hebei province, are severely polluted and must be treated quickly, according to an investigative report released on Wednesday.

A cluster of red and yellow pits occupy a strip of arable land in Dacheng county, of which the largest covers more than 170,000 square meters, equal to 23 standard soccer fields.

Surrounding the pits is black sludge, and piles of dark-red waste residue were discovered at a nearby industrial zone, according to aerial photographs provided by Chongqing Liangjiang Voluntary Service Center, an NGO, which went viral online on Wednesday.

The Ministry of Environmental Protection launched an investigation with the Hebei government on the pits on Wednesday morning and confirmed by the afternoon that they were polluted.

The Dacheng county government said two of the largest pits formed gradually over years and had been contaminated by the illegally dumped acid waste in 2013, and the suspects have been detained.

The pollution control measures are ongoing, and have been listed as major tasks with a cost of 38.48 million yuan ($5.6 million), the authority said.

The county also will conduct thorough surveys of all the pits and treat those with pollution quickly, officials said.

That move comes as the ministry on Wednesday ordered the county to monitor sewage in the pits, the soil and underground water and to speed up pollution treatment.

Preliminary tests showed the sewage had strong acidic qualities, which may be from waste produced by acid-washing－which is done at steel and iron plants and electroplating factories－said Xiang Chun of the service center, headquartered in Chongqing.

"The pollution could not only affect the crops but also contaminate the underground water," he said.

In addition to the pits in Langfang, the volunteer group found similar pits in farmland in Tianjin's Jinghai district, of which the largest covers 140,000 square meters.

Xiang said they did not know the sewage sources, but guessed they were dumped by tanker trucks because no pipes were found near the site.

The ministry has started joint investigation on these pits in Tianjin with the municipal government and has said it will release the results soon.

Tianjin solved the pollution in 14 similar pits in Jinghai district since 2014, and plans to deal with the four pits, according to the ministry.