He called in a statement for "the necessary measures to be taken to end the problem of salinity in Gaza water supplies, a problem that is getting worse."
Shiblak called the water situation "critical."
He said the amount taken from underground aquifers last year to supply 1.5 million people with drinking water and for agriculture was 160 million cubic metres, but that natural replenishment was 80-90 million cubic metres.
"The ground water deficit rose to more than 80 million cubic metres last year, and if this situation continues reserves could collapse in the next few years," Shiblak said.
In September the UN Environment Programme also said Gaza's underground water supplies are "in danger of collapse" following years of overuse and the devastating war Israel waged in the territory at the turn of the year.
"Unless the trend is reversed now, damage could take centuries to reverse. Since the aquifer is a continuum with Israel and Egypt, such action must be coordinated with these countries," UNEP said in a report.
Israel and Egypt have sealed off the impoverished enclave to all but basic goods since the Islamist Hamas movement seized control in June 2007, severely hampering the upkeep of basic infrastructure.
The sewage system has been particularly hard-hit, as Israel does not allow the import of virtually any pipes or other metal equipment that it fears could be used by Palestinian militants to make improvised rockets.
UNEP estimated that restoring the aquifer beneath Gaza could require 1.5 billion dollars (a billion euros) over 20 years, including the construction of desalination plants to ease the pressure on underground sources.
The report said over-extraction was causing seawater to seep into the freshwater aquifer, sending salinity levels above the 250 milligrammes per litre considered safe by the World Health Organisation.
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Image: AFP/File/Said Khatib.