Chlorine-36 dating of deep groundwater from northern Sahara

The “Continental Intercalaire” from Sahara is one of the largest confined aquifers in the world. In the northern part of the basin, the depth of the aquifer ranges between 400 and 1800 m and the main flow direction is from west to east. Chlorine-36 analyses were performed on groundwater samples mainly collected along this flowpath. They show a wide range of 36Cl contents, from 8 to 99 × 10−15 at at−1 expressed as 36Cl/Cl atomic ratio. In order to evaluate the epigene production, measurements were also performed on chloride extracted by leaching from a soil profile, near the recharge zone. This showed that the contribution of epigene production remains limited. A range of 36Cl/Cl ratios from 116 to 133 × 10−15 was adopted for groundwater recharging the aquifer, including both meteoric production and epigene production. For most of the samples collected along the main flowpath, a significant decay effect was observed. An attempt of dating was made by using an equation that takes into account radioactive decay of the meteoric–epigene input, deep production and chloride dissolution within the aquifer. The calculated 36Cl residence time varies from 16 to 500 ka for the minimum ages, and from 25 to 1200 ka for the maximum ages. An increase of residence time is observed up to about 500 km from the recharge zone. This corresponds to mean flow velocities of 0.2–0.5 m a−1 and to an average permeability of 1.1 ± 0.6 × 10−5 m s−1, at the scale of several hundreds of kilometres.