GROUNDWATER AGE AND HELIUM FLUXES IN THREE SANDSTONE AQUIFERS IN LIBYA

Libya is rich in non-renewable groundwater resources beneath the Sahara. Six groundwater basins have been targeted by the Libyan government for the development of large well fields to produce water for transport by pipeline to the northern cities. With planned extraction rates of one million cubic meters per day or greater it is necessary to estimate groundwater ages to aid in proper management of aquifers for 50 year design lifetimes.

In this study, we estimate groundwater ages using ¹⁴C and ⁴He for three sandstone aquifers in central and southern Libya (Sarir, Tazerbo and Al Kufra). 14-carbon ages suggest that recharge to the aquifers occurred during the late Pleistocene and early Holocene. Based on in-situ production rates of ⁴He, the estimated groundwater ages are 1 to 2 orders of magnitude greater than those from ¹⁴C. ³He to ⁴He ratios indicate that much of the He in the aquifers is acquired from external fluxes (aquitards, crust, and mantle). Using average ratios for the crustal, mantle and in-situ production we were able to estimate the relative contributions of each source to the He in the aquifers. In addition, by using the ¹⁴C ages to estimate the total helium accumulation rate we can estimate the total helium flux through the aquifers. Results for the total fluxes are comparable to other sandstone aquifers throughout the world and to the average helium flux for the crust. The estimates of helium fluxes to the aquifers can then be used to aid in calibration of numerical flow models developed for the aquifers.