Rainfall-recharge relationships within a karstic terrain in the Eastern Mediterranean semi-arid region, Israel: δ 18O and δD characteristics

Abstract

Annual rainfall variations and processes in the upper vadose zone exert a profound influence on the chemical and isotopic compositions of waters of carbonate aquifers in semi-arid climatic zones. In order to define these processes we have studied the surface temperatures during rainfall events, the isotopic composition of rain, infiltrating and groundwaters. 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 climate is typical of the Eastern Mediterranean semi-arid conditions, with a rainy winter and dry summer and the average annual rainfall in the area is ∼500 mm. Close monitoring indicates that the δD and δ 18O values of individual rainstorm events decrease with increasing rainfall. Annual average isotopic values of years with rainfall of 500–600 mm do not vary systematically. Years with extreme rainfall values define a negative covariation between the δ 18O and rainfall. The δD-δ 18O relationship of all rain events of more than 20 mm fall on the Mediterranean Meteoric Water Line (MMWL) with a slope of ∼8 and d-excesses of 20–30%. These rain events occur when mid-winter surface temperatures are 5 to 10°C. Rainfall events of less than 20 mm, mainly occurring at above 10°C, have slopes of less than 8 and smaller d-excess as a consequence of evaporation processes beneath the clouds. Two main water-types infiltrating into the cave are recognized: slow- and fast-drip. Slow-drip occurs from the tips of stalactites and takes place throughout the year; these waters represent seepage water that remains in the upper vadose zone for up to several decades. Fast-drip emanates from fissures in the cave roof during the winter seasons; these waters represent vadose flow 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 the rock cover and on the intensity of the rainstorms. In the inner parts of the cave, where the rock cover is thick, fast-drip starts only after several massive (>20 mm) rainstorms, whereas below a thin roof the response time of fast-drip is a few hours to a few days after intensive rainstorms. Both fast- and slow-drip water fall on the MMWL, indicating that they are derived mainly from the relatively intensive rainstorms that compose about 2 3 of the annual rainfall. Although fast-dri water represents massive rainstorms, their δ 18O and δD values are nonetheless higher by ∼1% and 10% respectively, relative to the average rainwater compisition. This indicates that the fast-drip water consists predominantly of water derived from intensive rainstorms admixed with a minor component of isotopically 18 O- and D-enriched water, which is represented by the slow-drip waters. The fast-drip waters are derived mainly from massive rain events at surface temperatures of 5 to 10°C, which thermally equilibrate with the host rock and upon reaching the cave they have already acquired the mean temperature (∼20°C) of the cave.