Modeling and characterization of flow processes in the carbonate aquifer vadose zone

Quantification of groundwater recharge is difficult due to inherent rock heterogeneity and multiple space and time scales. The Western Mountain Aquifer (Yarkon-Taninim) of Israel is one of the country's major water resources. In recent years significant differences in water balance were found between actual water assessment and evaluation by different models. These differences could stem from underestimation of the aquifer replenishment, due to lack of knowledge concerning water infiltration through the fractured-porous vadose zone composed of karst limestone and dolomite formations, and its hydrologic connectivity with the underneath phreatic aquifer. The overall aim of this research was to quantify fluxes of deep percolation within the fractured-porous unsaturated zone to assess the natural replenishment of groundwater. Observations and the associated simulations were considered on two different time scales: 1) long-term - multi annual data of groundwater level from 1990 to 2011 with daily precipitation and evapotranspiration - for 9 wells in the study area, and (2) short-time - 30 minute data of groundwater levels from 2011 - 2012 with hourly precipitation and daily evapotranspiration - for 3 wells in the study area. A one-dimensional, dual permeability mathematical model of water flow in a variably saturated, fractured-porous media was applied to simulate water flow. The model was calibrated for each well location using the time series of the monitored groundwater levels. Fair agreement between simulated and observed groundwater levels was obtained. The estimated parameters properly reflect the small and large scale spatial variability in the key flow properties of diverse geological sections. Time lags of groundwater response to precipitation events depend on the thickness and properties of the vadose zone. The observations and numerical model study demonstrated the practical value of this approach for quantifying unsaturated-zone preferential flow in the carbonate aquifer vadose zone with respect to different time scales.