Distribution of Groundwater Recharge in Fractured/Karst Aquifers

Accurate estimation of groundwater recharge is important to evaluate aquifer's water balance. Spatial and temporal distribution of the recharge depends on variability of hydraulic properties of rock. This variability is larger for sedimentary fractured environments where fast and slow flow paths exist. Therefore, quantifying groundwater fluxes through various rock sections within the vadose zone and into the aquifer is of critical importance for determining aquifers storage deviation. The research objective was to investigate how the lithology changes influence flow in the unsaturated zone and the spatial and temporal variation of groundwater recharge. To assess these variations for the Western Mountain Aquifer (Yarkon-Taninim) of Israel, we considered a range of parameters values for both fractures/karst and porous media, namely; permeability, block size, spacing between fractures, karst volumetric volume, etc. A conceptual model was built for nine monitoring wells sites in the study area. A one-dimensional, dual permeability mathematical model of water flow in a variably saturated, fractured/karst-porous media was applied to simulate water flow in each location. Model parameters were determined by solving the inverse problem using data of groundwater level observations. The results of simulation show that the highest groundwater replenishment occurs in locations where fast flow paths conditions are expected, while the lowest recharges took place in locations containing low permeability layers. The spatial differences in recharge are larger during years with high precipitation which can reach a few hundred percent. Additionally, fast and slow flow paths conditions also influence annual cumulative groundwater recharge dynamic. In areas where fast flow paths exist, most of the groundwater recharge occurs during the rainy season (60-80% of the total recharge for the tested years), while in locations with slow flow path conditions the recharge rate stays relatively constant with a near linear pattern and continues during summer.