Hydrogeological characterisation of karst aquifers in semi-arid environments at the catchment scale – Example of the Western Lower Jordan Valley

The main objective of the study is the characterisation of karstified carbonate aquifers in the semi- arid climate zone, where groundwater resources are frequently threatened by overexploitation and pollution. Despite often limited system knowledge, quantitative and qualitative factors such as groundwater recharge rate, aquifer parameters, flow- and transport dynamics, anthropogenic impacts, and groundwater vulnerability need to be assessed. Apart from the challenges, semi-arid environments provide advantages regarding karst system characterisation due to sharp and intensive recharge input signals alternating with undisturbed recession periods. This is the case for the study region located in the Eastern Mediterranean. Hydrologic system variability was assessed by an extensive high-resolution hydrometric and hydrochemical monitoring network, especially focussing on the karst springs. Additionally, long-term low-frequency data were assembled and analysed. It was aimed at the development and application of sophisticated methods kept as simple as possible in order to facilitate transferability. For 33 springs in the study area, the natural background concentration of chloride in spring water was assessed by an analysis of long-term chloride and nitrate data. Together with chloride concentration data for local precipitation, the long-term mean natural groundwater recharge fraction of precipitation was calculated with the chloride mass balance method. Recharge ranged between 25% and 50% of the precipitation. The proportion of wastewater-borne spring flow was quantified by a chloride end-member mixing model. The larger springs in the area exhibited a wastewater-borne flow fraction between 0% and 20% calculated from recent samples. To quantitatively assess spring aquifer hydraulic/geometric parameters at the catchment scale and groundwater recharge at a high time resolution, a minimally parameterised reservoir model was developed for the Auja spring, the largest freshwater spring in the Lower Jordan Valley. The Auja spring, as well as a number of other springs in the study area, display a discharge maximum, attributed to reaching the finite discharge capacity of the conduit system (flow threshold). Due to system complexity and non-linearity, features such as the flow threshold and a considerable conduit–matrix exchange needed to be incorporated into the model. Nevertheless, it is regarded as relatively simplistic and performs well for the highly variable flow conditions observed. As a number of parameters were calculated by a spring hydrograph recession and event analysis, it requires only six calibration parameters. The effective porosity of the spring aquifer system was estimated via model calibration at ca. 2.4%. During the 45-year simulation period, only five high precipitation years accounted for 33% of the total cumulative recharge. Rapid recharge processes as well as the vulnerability of the groundwater sources (springs) were evaluated by assessing the hydraulic and transport signal time lags due to precipitation events, the duration of the event breakthrough, and the relative fraction of the fast flow discharge component. Focussing of infiltrated precipitation water in the epikarst zone and a subsequent rapid percolation along preferential pathways to the phreatic zone are regarded as important recharge processes in the study area. The transport data enable an evaluation of spring source vulnerability and provide valuable information for water resources management. Furthermore, the coherence of the two artesian Jericho springs Sultan and Duyuk, located 3500 m apart, to a common conduit and hence karst system could be established.