From the UN Report: Gaza in 2020

With no perennial streams and low rainfall, Gaza relies almost completely on the underlying coastal aquifer, which is partly replenished by rainfall and runoff from the Hebron hills to the east, with the recharge estimated at 50 to 60 million cubic metres (MCM) annually. Current abstraction of water from the aquifer, at an estimated 160 MCM per year to meet current overall demand, is well beyond that.

As groundwater levels subsequently decline, sea water infiltrates from the nearby Mediterranean Sea. Salinity levels have thus risen well beyond guidelines by the World Health Organization (WHO) for safe drinking water. This pollution is compounded by contamination of the aquifer by nitrates from uncontrolled sewage, and fertilizers from irrigation of farmlands.

Today 90% of water from the aquifer is not safe for drinking without treatment. Availability of clean water is thus limited for most Gazans with average consumption of 70 to 90 litres per person per day (depending on the season), below the global WHO standard of 100 litres per person per day. The aquifer could become unusable as early as 2016, with the damage irreversible by 2020.