They complain that pieces of pork meat, blood, stomach contents, sewerage and old dishwater has for several months been running into the Vaal River from the jail.
According to witnesses, it ran into the river about 7km downstream from the Vaal Dam.
Alex de Koker from Ecowild, a non-governmental organisation which looked after the interests of about 52 farmers with farms on the edge of the Vaal River, said he didn't know "where to go with his complaints".
Three departments were in charge of matters at Groenpunt. The department of correctional services looked after the prisoners’ wellbeing, water affairs made sure water licences were abided to and public works maintained the infrastructure.
Bokkie Redelinghuys, acting head of the prison, said on Monday that there was "a problem" but that it had been fixed. "One of the pumps of the sewage works broke and some stuff ran into the Vaal River. But correctional services fixed it."
But when De Koker and other reliable sources went to investigate on Sunday, the situation was unchanged.
De Koker said he had been complaining about the matter since May. Groenpunt had its own feeding pen with about 4 000 pigs. They were apparently slaughtered on the premises. Redelinghuys said he was not aware of this as he had only been the acting head of the prison for two months.
Beeld learnt that some of the prisoners and their family members had also complained about unhygienic conditions to Rand Water.
Gregg Mulzack, head of communications at Rand Water, said it was no concern of his what was going on at Groenpunt. When Beeld pointed out to him that drinking water for Gauteng was sometimes (even if it was in small amounts) taken from the Vaal Barrage, he said the reporter "was becoming ridiculous".
In danger of disease
Water tests done by Rand Water on Monday showed no signs of any harmful bacteria or viruses, he said.
A scientist said such unhygienic conditions (like those at Groenpunt) were an ideal breeding ground for viruses like giardia and cryptosporidium.
Water tests sometimes did not immediately show the presence of these viruses. "If the same water is tested two or three days later, it could possibly pick up the viruses."
Annalien Burger, chairperson of Save the Vaal in Parys, said she was shocked to hear of all the rubbish that Groenpunt was dumping in the Vaal River.
"Don't they know it is Parys's drinking water that they are messing with?"
"We don't have all of Rand Water's technology. Ngwathe (the municipality serving Parys) tries to purify its own water, but it is struggling."
The department of water affairs was not available for comment.