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Forty percent of the farmland in Israel is irrigated with treated wastewater of a high enough quality to safely water the fields, but still containing pollutants that threaten the quality of groundwater, according to a recent water-quality survey.

The Water Authority and the Israel Nature and Parks Authority presented the findings of the survey yesterday at the Israeli Water Association's annual conference in the Kfar Maccabiah convention center in Ramat Gan.

The survey found that Israel uses 75 percent of its wastewater for agriculture. But although that water undergoes purification, all but the water treated at the Shefdan purification plant is still considered polluted by European standards.

The wastewater that is not used for agriculture is diverted to streams or the Meditteranean Sea and does not meet new Environmental Protection Ministry standards.

The main problem cited by the survey is the seepage of wastewater into the aquifer after agricultural use. The survey found that the wastewater, a good deal of which reaches the water table, contains 145,000 tons of salt per year. Testing in agricultural reservoirs revealed high levels of nitrogen, the main component that disqualifies wells as sources of drinking water.

Ariel Cohen of the Israel Nature and Park's Authority's environmental unit, said the quality of most wastewater was "medium or low," but added that increased use of desalinated water could be expected to improve the quality in the near future.