Estimation of transit times in karst aquifers using environmental tracers (tritium, helium, chlorofluorocarbons, and sulfur hexafluoride) : application on the Jeita aquifer system-Lebanon

Abstract

In this study, groundwater residence times in the Jeita spring aquifer (Lebanon) were estimated using several environmental tracers such as Tritium (3H), Tritium-Helium (3H- 3He) Chlorofluorocarbons (CFCs), and Sulfur Hexafluoride (SF6). Additional stable isotope and major ion analyses were performed to characterize water types and recharge elevations. Groundwater samples were collected in two sampling campaigns under different flow periods, from six different wells in the Jeita catchment area (Jurassic Kesrouane aquifer) as well as from the cave itself and its new branch, and three samples from wells outside the Jeita catchment area. Tritium in all groundwater samples shows nearly the same concentration with slight variations (2.12 TU to 2.97 TU). The latter makes the Tritium method not accurate to provide reliable age results, thus it was complemented by the Helium method. The results are reproducible for the Tritium-Helium method giving absolute mean residence times that ranged from 7 to 24 years except for the Jeita cave sample where a zero age was determined due to the re-equilibration and degassing of light noble gas with the atmosphere. The youngest age is attributed to a newly discovered branch in the Jeita system (JN) while the oldest ages were obtained in wells tapping the fissured matrix (Ain delbe (AD), Ouata el Joz (OJ), and Ain errihane (AR)). Some samples showed extreme excess air (ΔNe is greater than 70 percent) while the remaining samples have ΔNe in the expected range between (10-35percent). Moreover Tritium-Helium analysis has showed some radiogenic Helium (⁴Herad) in some samples implying a mixture with old groundwater (older than 50 years). Correlations between CFC, SF6 and Tritium indicate that none of the samples fit to a piston flow model except for Ain errihane (AR) well, however the latter is highly dependent on the accuracy and variability of the input concentration of Tritium, CFCs, and SF6 in air in the region for the last 20 years, especially in the case of CFCs and SF6 atmos