REAL-TIME MONITORING OF INFILTRATION PROCESSES AND GROUNDWATER RECHARGE IN AN UNCONFINED AQUIFER

The impact of different land uses on the recharge rates of the Coastal Plain Aquifer, Israel, is being evaluated using a novel technique for deep vadose-zone monitoring. The study is being carried out in the city of Ashdod, the growth of which has probably been the fastest in Israel. The concomitant dramatic changes in land use from undeveloped sand dunes to intensive agriculture and, recently, to urbanized areas may have had a significant impact on the quantities and quality of the recharging water. A determination of infiltration rates is essential for estimating the annual water volumes available for pumping, and evaluating the migration rate of various contaminants from land surface into the water table. Infiltration rates under different urban land uses are being compared to those beneath nearby cultivated areas and the undeveloped sand dunes. The 10- to 30-m thick vadose zone consists of unconsolidated sand and loamy clay interbeds. The infiltration is being monitored using a novel methodology for water-content measurement in deep vadose zones, based on time domain reflectometry (TDR). The TDR sensors are installed in slanted boreholes, drilled throughout the entire unsaturated zone, providing real-time information on temporal variations of water content at various depths. Consequently, wetting-front propagation of each rain event is tracked all along the unsaturated cross section from land surface to the water table.

Preliminary results have shed light on the infiltration process and provide information about questions such as: (1) the amount of rain needed to activate percolation, (2) the impact of clay layers in moderating the percolation rates, (3) the travel time of an infiltration pulse across the entire vadose zone, (4) the possibility of distinguishing infiltration pulses generated by individual events at various depths, and (5) the impact (if any) of a single rain event on water-level fluctuations.