Characterization and Dating of Saline Groundwater in the Dead Sea Area

From the 20th International Radiocarbon Conference held in Kona, Hawaii, USA, May 31-June 3, 2009.This work presents an attempt to date brines and determine flow rates of hypersaline groundwater in the extremely dynamic system of the Dead Sea (DS), whose level has dropped in the last 30 yr by ~20 m. The processes that affect the carbon species and isotopes of the groundwater in the DS area were quantified in order to estimate their flow rate based on radiocarbon and tritium methods. In contrast to the conservative behavior of most ions in the groundwater, the carbon system parameters indicate additional processes. The dissolved inorganic carbon (DIC) content of most saline groundwater is close to that of the DS, but its stable isotopic composition (13CDIC) is much lower. The chemical composition and carbon isotope mass balance suggest that the low 13CDIC of the saline groundwater is a result of anaerobic organic matter oxidation by bacterial sulfate reduction (BSR) and methane oxidation. The radiocarbon content (14CDIC) of the saline groundwater ranged from 86 pMC (greater than the ~82 pMC value of the DS in the 2000s) to as low as 14 pMC. The similarity between the 14CDIC value and Na/Cl ratio of the groundwater at the DS shore and that of the 1980s DS brine indicates that the DS penetrated to the aquifer at that time. The low 14CDIC values in some of the saline groundwater suggest the existence of ancient brine in the subaquifer.The Radiocarbon archives are made available by Radiocarbon and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202

Characterization of a carbonate karstic aquifer flow system using multiple radioactive noble gases (3H-3He, 85Kr, 39Ar) and 14C as environmental tracers

International audienceGroundwater 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

Recent seawater intrusion into deep aquifer determined by the radioactive noble-gas isotopes 81Kr and 39Ar

Radioactive noble-gas isotopes tracers 81Kr and 39Ar are used for the first time to measure the residence times of deep (∼1000 m) saline coastal groundwater, and to determine its connection mode with the sea. The average rate of seawater intrusion into the deep aquifer in Israel, located near the Mediterranean Sea, is estimated. 81Kr-ages of the saline water samples, found to be younger than 40 ka, contradict previously estimated ages of up to several million years based on hydrogeological considerations. The new results imply a stronger and more recent connection between the aquifer and the sea, and indicate that the intrusion occurred during the sea-level rise that began about 20 ka ago. These coastal aquifers need to be managed with caution because lowering of the adjacent fresh water level due to over pumping could accelerate seawater intrusion in a relatively short time. This study demonstrates the suitability of these two noble-gas tracers for the examination of hydrogeological systems in general and for the study of seawater intrusion in particular