Authorities in Inner Mongolia are investigating reports that illegal industrial waste water is once again being discharged into the southern Tengger Desert.

Experts fear the pollution could permanently damage groundwater sources.

Citing an official at Tengri industrial park, The Beijing News reported the park and the local Alxa League government were investigating.

According to a report in the paper at the weekend, chemical engineering factories at Tengri industrial park had been pumping polluted water into the desert.

The official, surnamed Chen, told the paper local authorities had closed 15 factories in 2012 after state television unveiled a similar case. But a lack of supervision by local authorities might have allowed the problem to reoccur, Chen said.

Despite extensive media coverage of the scandal two years ago, several waste-water ponds were found in the desert, each larger than a soccer pitch, when reporters from the Beijing-based newspaper visited the area.

Sections of pipes linking the ponds to the factories could be seen, while the rest were covered in sand, the paper said.

The inky-coloured water had a pungent smell, local residents told the paper, adding the factories had hired many guards to prevent people from getting close to the ponds.

The factories also buried industrial waste in the sands after the polluted water dried up in the ponds, residents told the paper.

Liu Shurun, an ecologist at Inner Mongolia Normal University, told the paper the discharge from the factories might have caused permanent damage to the groundwater.

The affected areas had rich groundwater resources, he said. "If underground water is contaminated, the damage will be irreparable."

The local groundwater level has dropped more than 40 metres over the past few years, a local resident claimed.

The Ministry of Land and Resources said last month the mainland would launch a new network to monitor groundwater quality in three years, and revisions to existing standards had been drafted.

This article appeared in the South China Morning Post print edition as Factories 'empty waste water in desert'