NOBLE GAS RECHARGE TEMPERATURES AND RESIDENCE TIMES OF THREE AQUIFERS IN LIBYA

Libya suffers from a shortage in water resources due to its arid climate. The annual precipitation in Libya is less than 200 mm in the narrow coastal plain, while the south part of the country receives less than 1mm. On the other hand, Libya has huge resources of good quality groundwater distributed in six basin systems. In 1983, the Libyan government established the Great Manmade River Project in order to transport 6.5 million cubic meters a day of this groundwater to the coastal cities, where over 90% of the population lives.

Noble gases, ¹⁴C and stable isotopes of water were used to estimate groundwater recharge temperatures and residence times for three aquifer systems in two of the groundwater basins in central- and south-east Libya: the Sirt and Al Kufra Basins. The Sarir well field, established in the Sirt Basin, has waters ranging from 10-15 ka with recharge temperatures of 18° to 30°C. The Nubian Aquifer in Al Kufra yielded ages of 12-20 ka and recharge temperatures of 20°C. The Tazerbo well field, located on the northern edge of the Al Kufra basin, show recharge temperatures of 20°-25° for waters recharged 23-27 ka ago. All three locations have present day average annual temperatures of 22°-23°C.

The average stable isotopes compositions of water for Sarir are δD = -73.7‰ and δ¹⁸O = -9.3‰; for the Nubian Aquifer, δD = -84.1‰ and δ¹⁸O = -11.4‰; and for Tazerbo, δD = -84.6‰ and δ¹⁸O = -11.8‰. All three locations are significantly depleted compared to modern values and are consistent with the results of others who have concluded that the groundwaters were recharged with precipitation during the Late Pleistocene.