Abstract
Abstract. Despite being the main drinking water resource for over 5 million people, the water balance of the Eastern Mountain Aquifer system on the western side of the Dead Sea is poorly understood. The regional aquifer consists of fractured and karstified limestone – aquifers of Cretaceous age, and it can be separated into a Cenomanian aquifer (upper aquifer) and Albian aquifer (lower aquifer). Both aquifers are exposed along the mountain ridge around Jerusalem, which is the main recharge area. From here, the recharged groundwater flows in a highly karstified aquifer system towards the east and discharges in springs in the lower Jordan Valley and Dead Sea region. We investigated the Eastern Mountain Aquifer system for groundwater flow, groundwater age and potential mixtures, and groundwater recharge. We combined 36Cl ∕ Cl, tritium, and the anthropogenic gases SF6, CFC-12 (chlorofluorocarbon) and CFC-11, while using CFC-113 as “dating” tracers to estimate the young water components inside the Eastern Mountain Aquifer system. By application of lumped parameter models, we verified young groundwater components from the last 10 to 30 years and an admixture of a groundwater component older than about 70 years. Concentrations of nitrate, simazine (pesticide), acesulfame K (ACE-K; artificial sweetener) and naproxen (NAP; drug) in the groundwater were further indications of infiltration during the last 30 years. The combination of multiple environmental tracers and lumped parameter modelling helped to understand the groundwater age distribution and to estimate recharge despite scarce data in this very complex hydrogeological setting. Our groundwater recharge rates support groundwater management of this politically difficult area and can be used to inform and calibrate ongoing groundwater flow models.
Highlights
About 20 % of the Earth’s land surface is covered by carbonate karst or sulfate aquifers and serves as the primary water resource for at least 25 % of the world’s population
The present study aims to estimate travel times in the thick unsaturated zone (Cook and Solomon, 1995; Plummer et al, 2006) by estimating the time lag as the relative difference in mean residence time between gas and water-bound tracers, applying both, water-bound tracers (3H and 36Cl) and gas tracers (CFCs and SF6)
The 36Cl / Cl ratios in precipitation, which fall in the recharge area, are assumed to have been stable since the 1980s, as indicated by rainwater samples collected during the winter of 2014–2015, which show 36Cl / Cl ratios of 1.3 × 10−14 to 5.5 × 10−14, resembling results from the early 1980s (Herut et al, 1992)
Summary
About 20 % of the Earth’s land surface is covered by carbonate karst or sulfate aquifers and serves as the primary water resource for at least 25 % of the world’s population. A time lag is possible for the water bound tracers tritium and 36Cl, since the advection through the unsaturated zone may take decades (Suckow et al, 1993; Lin and Wei, 2006), and in infiltration areas dominated by sand or clay, water-bound tracers are generally slower than gas tracers (Solomon et al, 1992; Cook et al, 1995). This can be very different in karst systems, where preferential flow in karst “tubes” allows for a fast recharge to the groundwater table, and fluctuations of groundwater level may allow for further gas exchange thereafter
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