Stable and radio-isotope analysis to determine recharge timing and paleoclimate of sandstone aquifers in central and southeast Libya

Stable isotopes of water and C-14 activities (percent modern carbon, pmc) were used to estimate the timing of recharge and paleoclimatic conditions during recharge for three sandstone aquifers in Libya. These aquifers are part of the Nubian Aquifer System, with current and planned groundwater development designed to produce 10(6) m(3)/day of water. C-14 activities from water samples collected at three wellfields (Sarir, Tazerbo and Al Kufra) indicate recharge during late Pleistocene and early Holocene. Stable isotope compositions from all three locations are significantly depleted relative to modern water, indicating that the climate was more humid than today. These results are in line with previous regional groundwater studies. This was the first study of its kind for the aquifers at Tazerbo and Sarir wellfield. The Tazerbo wellfield, which produces from the deepest aquifer, had the oldest estimated recharge ages (late Pleistocene). For Sarir, C-14 activities ranged from 6.30 to 41.42 pmc; the Sarir aquifer is leaky confined and this range is likely due to mixing of shallow younger water with older deeper water. At Al Kufra, the C-14 activity (5.27 pmc) of a well which is part of an agriculture project, is greater than the activity of wells sampled in 1973 (0.5-2.0 pmc). The greater activity is likely due to the lowering of the regional potentiometric surface causing younger water to be captured after four decades of wellfield operation. It is recommended that periodic monitoring be done in large wellfields to identify changes in isotope compositions as production continues.