SOURCES OF PRECIPITATION OVER SOUTH-EASTERN SPAIN AND GROUNDWATER RECHARGE - AN ISOTOPIC STUDY

The deuterium and oxygen-18 compositions were monitored in several single rain events in south-eastern Spain from November 1989 till April 1990. Large variations in both delta-D and delta-O-18, as well as in the value of deuterium excess (d-value) of up to +27 parts per thousand, were recorded during this time period. The observed variations are clearly related to different source regions of the vapour (Atlantic Ocean or Mediterranean Sea) as well as to different trajectories of precipitating air masses. Whereas the rains of Atlantic origin were characterized by d-values close to +10 parts per thousand, the Mediterranean-derived precipitation showed much higher values, of up to +27 parts per thousand. The apparent correlation between altitude of the sampling site and the d-value of the corresponding rain can be attributed to two mechanisms: the evaporative isotopic enrichment of rain drops below the clouds, especially at initial stages of the given precipitation event, and/or precipitation from air masses with different isotopic signature (d-value) of the water vapour. The range of the d-values observed in samples from local springs and wells (between +10 and +13 parts per thousand) suggests that groundwater in the studied areas is recharged predominantly by the Atlantic-derived precipitation.