Water supplies on some Indian Ocean islands may have been ruined for years or decades by the salt water that flooded them during the Asian tsunami.
Some islands in the Maldives may be the worst affected
It means communities may be reliant on outside aid for the foreseeable future.
Reports from across the stricken region suggest salt water filled wells and invaded porous rocks which communities depend on for their water supply.
Atolls in the Maldives may be worst affected; islanders may have to wait for rains to flush out water systems.
Here, "lenses" - reservoirs of fresh groundwater in the limestone rocks - provide the islanders with their water.
"Because the islands are limestone, permeable, and don't have much soil, the sea water that washed over them will have infiltrated the rocks quicker than it could retreat to the sea," John Chilton, a hydrogeologist at the British Geological Survey, told the BBC News website.
These lenses sit above more dense salt water that permeates the rocks. They also lie close to the surface - one reason why they were so vulnerable to the tsunami.
"If you have a catastrophic disturbance like this, the freshwater lens may have been severely damaged and it may take a long time for the rain water to restore it," Mr Chilton explained.
"In most cases, they have fairly heavy monsoon climates, so the recovery of the small lenses may be measured in years rather than decades. But in the short term, they're not going to be useable for their drinking water supply."
An expert in water sanitation from the World Health Organization has already arrived in the Maldives to help determine the best way to restore safe water supplies.
A third of the country's 300,000 population were severely affected by the tsunami, according to government officials.
An emergency relief effort to provide islanders with fresh drinking water is now well underway. But the long-term effects of the tsunami may be more difficult to deal with.
"I think they have the most difficulty with fresh water sources to begin with," Alfred Ironside, a spokesman for Unicef, told the New York Times.
"They're small islands, in these atolls, and the wells are not so replenishable if they get salt water in them.
Mr Chilton said that one option for inhabitants in the Maldives and other affected areas of the region is to collect rain water from the roofs of houses. But this is clearly not possible in areas where there has been widespread destruction.
Bottled water has already been sent out to the Maldives
In some cases, small islands will need to be evacuated.
"I would expect the short-term solution where only a small proportion of the population have survived may be to move people off the island," Mr Chilton explained.
"They seem to be doing that on the Indonesian coast already. If you have small numbers of survivors, it may be easier to take them to the aid rather than take the aid to them."
In other areas, such as Sri Lanka, portable desalting machines are being used to decontaminate wells.
Disaster aid group Medair is using a new technique to drill new wells in the worst-affected areas of the island nation.
It is using a high-pressure jet of water to penetrate about six metres below the earth to find clean water.
The technique works only in soft or sandy soil, not rock, but it has worked successfully in Madagascar and Darfur.