RAINFALL-RECHARGE RELATIONSHIPS WITHIN A KARSTIC TERRAIN IN THE EASTERN MEDITERRANEAN SEMIARID REGION, ISRAEL - DELTA-O-18 AND DELTA-D CHARACTERISTICS

Annual rainfall variations and processes in the upper vadose zone exer t a profound influence on the chemical and isotopic compositions of wa ters of carbonate aquifers in semi-arid climatic zones. In order to de fine these processes we have studied the surface temperatures during r ainfall events, the isotopic composition of rain, infiltrating and gro undwaters. This study was carried out within a karstic terrain (Soreq Cave), Israel, located in Cenomanian dolomitic rocks, approximately 40 km inland of the Mediterranean Sea, and 400 m above sea level. The cl imate is typical of the Eastern Mediterranean semi-arid conditions, wi th a rainy winter and dry summer and the average annual rainfall in th e area is similar to 500 mm. Close monitoring indicates that the delta D and delta(18)O values of individual rainstorm events decrease with increasing rainfall. Annual average isotopic values of years with rain fall of 500-600 mm do not vary systematically. Years with extreme rain fall values define a negative covariation between the delta(18)O and r ainfall. The delta D-delta(18)O relationship of all rain events of mor e than 20 mm fall on the Mediterranean Meteoric Water Line (MMWL) with a slope of similar to 8 and d-excesses of 20-30%. These rain events o ccur when mid-winter surface temperatures are 5 to 10 degrees C. Rainf all events of less than 20 mm, mainly occurring at above 10 degrees C, have slopes of less than 8 and smaller d-excess as a consequence of e vaporation processes beneath the clouds. Two main water-types infiltra ting into the cave are recognized: slow- and fast-drip. Slow-drip occu rs from the tips of stalactites and takes place throughout the year; t hese waters represent seepage water that remains in the upper vadose z one for up to several decades. Fast-drip emanates from fissures in the cave roof during the winter seasons; these waters represent vadose ho w with a short residence time of less than 1 year. The infiltration of the fast-drip water into the cave depends on the fracture system of t he rock cover and on the intensity of the rainstorms. In the inner par ts of the cave, where the rock cover is thick, fast-drip starts only a fter several massive (>20 mm) rainstorms, whereas below a thin roof th e response time of fast-drip is a few hours to a few days after intens ive rainstorms. Both fast- and slow-drip water fall on the MMWL, indic ating that they are derived mainly from the relatively intensive rains torms that compose about 2/3 of the annual rainfall. Although fast-dri p water represents massive rainstorms, their delta(18)O and delta D va lues are nonetheless higher by similar to 1% and 10% respectively, rel ative to the average rainwater composition. This indicates that the fa st-drip water consists predominantly of water derived from intensive r ainstorms admired with a minor component of isotopically O-18- and D-e nriched water, which is represented by the slow-drip waters. The fast- drip waters are derived mainly from massive rain events at surface tem peratures of 5 to 10 degrees C, which thermally equilibrate with the h ost rock and upon reaching the cave they have already acquired the mea n temperature (similar to 20 degrees C) of the cave. (C) 1998 Elsevier Science B.V. All rights reserved.