Relief and climate influence on isotopic composition of Atlantic-Mediterranean karst spring waters (Andalusia, southern Spain)

The geographic location of Andalusia, bordered by the Atlantic Ocean and the Mediterranean Sea, and its great topographic and climate diversity enable the analysis of different factors that condition the isotopic composition of precipitation and groundwater. A total of 42 karst springs representative of the main carbonate aquifers were selected in order to sample the base flow waters during the summer of 2020. Isotopic analyses revealed that groundwater delta O-18 was between -5.04 and -9.85 parts per thousand while that of delta H-2 was between -26.39 and -62.69 parts per thousand. Weighted mean isotopic data for precipitation, from six IAEA stations in the study area, was between -4.67 and -9.06 parts per thousand for delta O-18, and between -24.93 and -58.99 parts per thousand for delta H-2. The results showed that diffuse flow groundwaters are lighter and have higher d-excess than meteoric waters, which may be due to preferential recharge during winter months and the sub-cloud evaporation effect. The spatial variation of d-excess, with average values around 14 parts per thousand in the Atlantic watershed and 17 parts per thousand in the Mediterranean watershed, has showed for the first time the effect of the Atlantic-Mediterranean divide on the isotopic composition of spring waters. Furthermore, a progressive decrease in delta O-18 towards the east of the study area seems to be related to the effects of continentality and altitude. This also allowed us to conclude that, in the areas with highest rainfall, springs drain waters with a relatively higher content of heavy isotopes than expected from the regional altitudinal gradient, while springs in drier areas discharge lighter waters. These differences can be attributed to a longer recharge period in the former, which extends in the warmer months or autumn and spring.

Automated summary

This study investigated the isotopic composition of precipitation and groundwater in Andalusia. The results showed that groundwater is lighter and has higher d-excess than meteoric waters. The study also revealed the effects of the Atlantic-Mediterranean divide, continentality, and altitude on the isotopic composition of spring waters.