Tanzania's national environment agency has warned that most of the country's water supply will become dangerously toxic unless drastic anti-pollution measures are taken.
By Daniel Dickinson
BBC, Dar es Salaam
Untreated industrial waste is being pumped into the country's rivers, creating a potentially devastating future crisis, the National Environment Management Council (NEMC) has said.
Most industrial areas are already contaminated
"If this pollution continues unattended for another five or so years, it might end up in situation where perhaps most of the water sources are contaminated," Bonaventure Baya of the NEMC told BBC World Service's Health Matters programme.
"The people handling these obsolete chemicals will probably also be facing serious health impacts - we have cases reported already."
Mr Baya added that the situation was already very worrying.
"I would say most of the towns in Tanzania where we found industries located do have the problem of toxic pollutants," Mr Baya said.
"Most of those industries do not have treatment facilities, and discharge their effluent down from the processes untreated into the environment.
"So the population of this country is at risk due to toxic pollutants."
One place where such pollution is happening is Keko Mwanga B - a very heavily populated area of Dar es Salaam.
The area is dominated by a huge industrial oil terminal, which concerns those who live there - they fear the stream which supplies much of their water is being contaminated.
Chemical analysis has shown the stream is laden with metals like mercury and lead - mostly from small industrial premises, rather than the oil terminal.
The problem has been exacerbated by a water shortage, because the rains are yet to arrive - so increasing people in the area are using the water from the stream in their everyday life.
Water is contaminated with both chemicals and fertilizer
"My worry about this water is that it is being infected from different sources - waste from the drains, probably chemicals from the industry around us," said Shameji Ibrahim the chairman of the area's Street Government.
"People are eating food that is grown using this water - so the chemicals are entering people's bodies.
"It may take a long time for the effects to become apparent, but I think this will happen in my community a lot in the future."
Community activists in Keko Mwanga B are already anticipating this - a Water Health committee has been established, and the 8,000 or so inhabitants are being taught about the potential dangers of drinking contaminated water.
"The sorts of problems we are expecting include cancer, and many different types of poisoning, leading to skin and respiratory diseases," local council health officer Juhmidi Nyambuka told Health Matters.
"These are long-term problems which may not become apparent fro years to come.
"The inhabitants of Keko Mwanga B now understand the dangers, but they probably carry on using this water as they are so poor, and cannot afford water from other sources."
And it is not only the urban areas that are risk from the dangers of toxic pollution.
In addition to the industrial waste, it is also thought Tanzania has 500 tonnes of obsolete fertilizers stored in rural areas, which is slowly leaking into the water table.