Partially treated sewage seeps into the groundwater from the lagoons
By Heather Sharp
BBC News, Gaza
Gaza's aquifer and only natural freshwater source is "in danger of collapse," the UN is warning.
Engineers have long been battling to keep the densely populated strip's water and sewage system limping along.
But in September the UN Environment Programme warned that damage to the underground aquifer - due to the Israeli and Egyptian blockade, conflict, and years of overuse and underinvestment - could take centuries to reverse if it is not halted now.
Monther Shoblak, director of Gaza's Coastal Municipality Water Utility, sniffs the air at the Beit Lahia water treatment plant and smiles.
"I'm happy when I smell sewage," he jokes, "it means the turbines are working."
Propellers are agitating the frothy sludge in one of the lagoons, aerating it to help bacteria digest it.
He says the machinery sometimes falls silent during the power cuts that plague most of Gaza.
But the mirror-smooth pond next to it is a perpetual concern.
The plant is handling twice its capacity and is only able to partially treat the sewage.
Lagoons designed to allow treated clean water to infiltrate through Gaza's sandy soil back down into the aquifer are instead funnelling sewage straight back into the groundwater
In addition, with several years of drought and the digging of hundreds of illegal, unregulated wells, the UN Environmental Programme says at least three times more water is extracted than is replenished each year.
Up to 80m litres of partially treated sewage are pumped into the sea each day
As the level is dropping in the aquifer, sea water is invading.
With nitrates from the sewage and salt from the sea, only 5-10% of the water in Gaza's wells - and therefore its taps - now meets World Health Organization guidelines, even after it has been chlorinated.
Years of decline
The aquifer has been in decline for years. But Oxfam's Mark Buttle, who co-ordinates international organisations working in the water sector, says the pressures are adding up.
Gaza faces a "pending environmental disaster" he warns.
"Water is life," he says. Action must be taken now, "so that we can prevent future problems with Gaza becoming uninhabitable".
Khader Makdad and his wife find it difficult to pay for water
Next to a school among a dense tangle of crumbling concrete homes, a water pump in Shati refugee camp hums as it sucks water from a borehole close to the sea.
Mr Buttle says it is some of Gaza's worst water.
"It's like sea water," says local resident Shadi Dosh, 27.
"It's not clean. It's only for washing. It has a bad smell," says Nabila Makdad, a mother of six.
Poverty has risen in Gaza as the blockade has ended much economic activity. An estimated 70% of the population lives on less than a dollar a day.
Mrs Makdad and her husband Khader say they make so little from their two street stalls that they have to rely on charity and loans from friends.
They spend 20NIS (about $5) a week buying water for cooking and drinking from private tankers, which bring water from small-scale desalination plants.
"I'd rather buy vegetables or fruit, or put the money towards my children's education, but there's no other way," says Ms Makdad.
Another concern is the blood disorder dubbed "blue baby syndrome", which is associated with nitrate pollution.
It results in low oxygen levels in an infant's blood, which can cause breathing trouble, diarrhoea and vomiting, and in extreme cases, loss of consciousness, convulsions and death.
Some building supplies have arrived, but water and sanitation workers say they need a lot more
The World Health Organization has not discovered any recent, full-blown cases in Gaza.
But in a 2002 study, nearly half the children surveyed had higher than safe levels of methemoglobin, the substance that indicates the condition. Nitrate pollution has increased since then.
Under Israel's crippling blockade, and with the border with Egypt closed, most building materials are refused entry, for fear they could be used to make smuggling tunnels or the rockets that militants fire at Israel.
But limited amounts of materials for sewage and water infrastructure have been allowed in.
Israeli government spokesman Mark Regev says he believes the system for approving shipments is currently "going quite well".
He says Israel "wants to work effectively with the international community" on the issue, which with 50-80m litres of partially treated sewage pumped into the sea each day, is also likely to have an impact on southern Israel's coast.
But Mr Shoblack says, since Israel's military operation in Gaza last winter, only five of 40 orders for building materials have arrived.
Nevertheless, he says, most of the $6m of damage sustained by the water and sewage system during the fighting has been repaired.
The lagoon which burst, killing five people, has now been drained
Thirty kilometres of pipes and 11 wells were damaged, and sewage flooded for up to a kilometre after one waste water plant was hit.
Even some control rooms at a brand new facility - locally referred to as the Tony Blair project as it has been heavily championed by the international Middle East envoy - were damaged.
But Mr Shoblack speaks proudly of his organisation's few achievements - for example, the draining of a lagoon in Beit Lahiya that burst in 2007, killing five people in a flood of sewage.
And he says donors have committed $250m to a master plan including a sea water desalination plant and new sewage treatment facilities - but only if the political and security situation improves.
In the meantime, he shows me a large pipe belching brownish-white sludge into a frothy patch on the beach south of Gaza City.
It pollutes the sea and wastes water that could, if treated, be used to recharge the aquifer.
"We have a vision. We could in a short time change people's lives, if we were allowed to," says Mr Shoblack. "But there is a key phrase - open borders."