Hydrol. Earth Syst. Sci., 24, 249–267, 2020 https://doi.org/10.5194/hess-24-249-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. A multi-environmental tracer study to determine groundwater residence times and recharge in a structurally complex multi-aquifer system Cornelia Wilske1,2, Axel Suckow2, Ulf Mallast1, Christiane Meier3, Silke Merchel4, Broder Merkel5, Stefan Pavetich4,6, Tino Rödiger7, Georg Rugel4, Agnes Sachse7, Stephan M. Weise1, and Christian Siebert1 1Department of Catchment Hydrology, Helmholtz Centre for Environmental Research, Halle (Saale), 06120, Germany 2CSIRO Land and Water, Urrbrae, South Australia, 5064, Australia 3Umweltbundesamt (UBA), Dessau-Roßlau, 06844, Germany 4Helmholtz-Zentrum Dresden-Rossendorf, Dresden, 01328, Germany 5Technische Universität Bergakademie Freiberg, 09599, Germany 6Department of Nuclear Physics, Research School of Physics and Engineering, Australian National University, Canberra, ACT 2609, Australia 7Department of Computational Hydrosystems, Helmholtz Centre for Environmental Research, Leipzig, 04318, Germany Correspondence: Christian Siebert ([email protected]) Received: 30 August 2019 – Discussion started: 9 September 2019 Revised: 18 November 2019 – Accepted: 11 December 2019 – Published: 16 January 2020 Abstract. Despite being the main drinking water resource the groundwater were further indications of infiltration dur- for over 5 million people, the water balance of the Eastern ing the last 30 years. The combination of multiple environ- Mountain Aquifer system on the western side of the Dead mental tracers and lumped parameter modelling helped to Sea is poorly understood. The regional aquifer consists of understand the groundwater age distribution and to estimate fractured and karstified limestone – aquifers of Cretaceous recharge despite scarce data in this very complex hydro- age, and it can be separated into a Cenomanian aquifer (upper geological setting. Our groundwater recharge rates support aquifer) and Albian aquifer (lower aquifer). Both aquifers are groundwater management of this politically difficult area and exposed along the mountain ridge around Jerusalem, which can be used to inform and calibrate ongoing groundwater is the main recharge area. From here, the recharged ground- flow models. water 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 1 Introduction potential mixtures, and groundwater recharge. We combined 36 Cl=Cl, tritium, and the anthropogenic gases SF6, CFC-12 About 20 % of the Earth’s land surface is covered by carbon- (chlorofluorocarbon) and CFC-11, while using CFC-113 as ate karst or sulfate aquifers and serves as the primary water “dating” tracers to estimate the young water components in- resource for at least 25 % of the world’s population. In addi- side the Eastern Mountain Aquifer system. By application tion, about a fifth of the world’s karst systems are in (semi- of lumped parameter models, we verified young groundwa- )arid areas, whose water scarcity is aggravated by strong pop- ter components from the last 10 to 30 years and an admix- ulation growth (Ford and Williams, 2007). Karst systems rep- ture of a groundwater component older than about 70 years. resent abundant but highly variable water resources whose Concentrations of nitrate, simazine (pesticide), acesulfame K extremely heterogeneous and anisotropic flow behaviour pre- (ACE-K; artificial sweetener) and naproxen (NAP; drug) in vents exact predictions regarding mass transport and the us- able water quantities (Bakalowicz, 2005). Nevertheless, the Published by Copernicus Publications on behalf of the European Geosciences Union. 250 C. Wilske et al.: A multi-environmental tracer study to determine groundwater residence times and recharge water balance meets the basic requirement for sustainable In our study, the Eastern Mountain Aquifer (EMA) sys- management and protection of any water resource. tem in the western Dead Sea catchment is the pivotal water Environmental tracers play an important role in sustain- resource for some million people in the West Bank and Is- able water management strategies because they allow for es- rael. An unequal distribution of borehole information leads timating the groundwater age distribution with depth, and to poor and limited data for studying that aquifer system. Pre- together with simple lumped parameter models they can vious studies considered age dating tracers to quantify water be used to quantify groundwater infiltration rates (Vogel, movement and flow velocity within the EMA and associated 1967; Solomon et al., 1995; IAEA, 2006). Especially in karst aquifers. Paul et al. (1986) and Yechieli et al. (1996) studied aquifers, wide ranges of residence times are observable due 36Cl=Cl to detect very old groundwater brines in the Dead to the strongly heterogeneous hydraulic system, allowing wa- Sea area. In contrast young-age dating tracers such as tritium ter to rapidly flow through conduits and fractures and very or anthropogenic trace gases (CFCs and SF6) were used to slowly flow through the small pores of the matrix. That leads quantify the duration of water flow from recharge areas to to large heterogeneities in the groundwater age distribution, the springs (Lange, 2011). Environmental tracer investiga- hence requiring the application of multiple tracers to con- tions of the main Cretaceous aquifers (Upper Cenomanian strain the age distribution, especially in karst. We define and Albian) in the western Dead Sea catchment attempted to young groundwater as having measurable concentrations of quantify the duration of water flow from the recharge area to anthropogenic tracers, thus a mixing component recharged the springs in the mountain region uphill of the Dead Sea after about 1950, while in old groundwater these tracers are coast. The young-age dating tracers demonstrated a large not detectable (e.g. Plummer et al., 1993; Cook and Herczeg, young groundwater fraction with a mean residence time of 2000). The atmospheric tracer CFC-11 (chlorofluorocarbon), less than 30 years in the springs of the mountain region and 36 CFC-12, CFC-113, SF6 (sulfur hexafluoride) and Cl=Cl fast connections to the recharge area. All previous studies and tritium from bomb tests or anthropogenic organic trace together show large heterogeneities in the groundwater age pollutants like pesticides, sweeteners or drugs are increas- distribution (Avrahamov et al., 2018). ingly used as tracers of young groundwater (IAEA 2006). In this study we combine for the first time in this area 36 Gas tracers like CFCs and SF6 move through the unsaturated bomb-derived Cl, anthropogenic organic trace substances zone primary by diffusion, leading to a time lag at the water and environmental tracers like tritium, CFCs, SF6 in combi- table compared to the atmosphere (Cook and Solomon, 1995; nation with lumped parameter models to interpret the distri- Cook et al., 1995). A time lag is also possible for the water bution of these tracers to quantify recharge. Małoszewski and bound tracers tritium and 36Cl, since the advection through Zuber (1982, 1993, 1996) have shown that LPMs are a use- the unsaturated zone may take decades (Suckow et al., 1993; ful tool for interpreting tracer data obtained at separate sam- Lin and Wei, 2006), and in infiltration areas dominated by pling sites when it is not possible to use distributed parameter sand or clay, water-bound tracers are generally slower than models, as the latter require more detailed and often unavail- gas tracers (Solomon et al., 1992; Cook et al., 1995). This can able knowledge about distributed parameters for the investi- be very different in karst systems, where preferential flow in gated system. In detail this work aims (i) to validate young karst “tubes” allows for a fast recharge to the groundwater rainwater input and short groundwater travel times via karst table, and fluctuations of groundwater level may allow for conduits, related to rapid flow paths from the recharge area; further gas exchange thereafter. (ii) to quantify the time lag of gas tracers in the unsaturated The deconvolution of measured tracer concentrations into zone; (iii) to quantify groundwater mixing of groundwater recharge rates therefore needs modelling. If the age of water components with different ages via lumped parameter mod- would be known as function of depth, any flow model could els; and (vi) to estimate groundwater recharge and support be directly constrained, and the recharge rates could be de- calculations of future groundwater resource development. duced. However, the “idealized groundwater age”, which is often understood as the time span an imaginary water par- 1.1 Study area cel needs between infiltrating at the groundwater surface and being sampled at a well or spring (Suckow, 2014a) is not The study area, which represents the western surface directly measurable. In addition, groundwater mixes both drainage basin of the Dead Sea, is embedded in a region that along its natural flow through the aquifer and during sam- is morphologically and geologically dominated by the tec- pling in the well. Therefore, simple lumped parameter mod- tonic processes associated with the Jordan–Dead Sea rift, ac- els (LPMs) are used to interpret the measured tracer con- tive since the late Oligocene (Garfunkel et al., 1981; Rosen- centrations as mean residence