Characterization of a carbonate karstic aquifer flow system using multiple radioactive noble gases (3H-3He, 85Kr, 39Ar) and 14C as environmental tracers
Résumé
Groundwater age in a carbonate karstic aquifer was assessed using a multiple tracer method that enables identification of modern groundwater (recharged after 1955; using 3H-3He, 85Kr CFCs, SF6,), older components (39Ar, 14C) and quantification of the mixing between them. Twelve wells were sampled in the Eastern Mountain Aquifer (EMA) of Israel along two trajectories, from the recharge area in the mountains, to the natural outlets in the Dead Sea area. The concentration of the dissolved 39Ar in the groundwater decreased from 96 to 12% along the trajectories, indicating recent recharge upstream, and groundwater aged more than 800y downstream. Other tracers present a similar general trend of decreasing concentrations with distance from the recharge area at two distinct rates, suggesting two different groundwater flow velocities in the two different groundwater flow paths. In most of the wells, pronounced mixing was observed according to the presence of young (after 1955) and older water components. The fraction of the young water was quantified by tritium (3H) and by the combination of 3H and 85Kr measurements and found to be between 1 and 67%. The wide age distribution is likely caused by the karstic nature of the aquifer with pronounced dispersion and exchange between highly permeable flow channels and stagnant water stored in the rock matrix. Another mixing mechanism is vertical leakage from the upper to the lower sub-aquifer and vice versa according to the groundwater head differences between the two sub-aquifers. Mixing, diffusive exchange and water rock interaction lead to a reduction of 14C in DIC, resulting in an apparent half-life of ∼900 y instead of 5730y for radioactive decay only. This is concluded from the comparison of 14C and 39Ar ages.