Isotope hydrology of groundwaters in the Donana National Park and the associated zone of influence

The authors describe a study, using environmental isotopes, of the groundwaters of a complex hydrological system formed by a free recharge zone with a multi-layer structure, a confined zone with connate waters trapped by the deposition of a thick clay layer in a coastal pool environment, and a series of both recent and ancient highly permeable dune formations. Attempts have been made, using tritium of thermonuclear origin, to determine the approximate average recharge rate during the last 28 years in the free aquifer zone. Despite the difficulties encountered, the value of 78 mm/a obtained, which is 13.2% of the average precipitation, is very similar to that obtained using conventional methods (approximately 84 mm/a). As was expected, there was no tritium in the confined zone. Carbon-14 was used to determine the reduced velocity of the underground stream and to confirm the network of streams deduced from conductivity measurements. The age-correction methods proposed by Tamers, Pearson, Mook and Fontes were compared. The differences in age between the last method and the first two are reasonable and can be explained by the isotopic interchange between the CaCO3 of the rock formation and the gaseous CO2 included in Fontes' model. On the other hand, the differences found with the Mook method are considerable, particularly for sample values below delta13C. The last model is more sensitive to changes in delta13C, and the interval of this parameter for which this model gives reasonable values of age is very small. In addition, the 14C made it possible to determine the sedimentation velocity of clays in the swamp zone. The stable isotopes in the water confirmed the hypothesis that the underground waters in the confinement zone are a mixture of fresh water which has infiltrated into the recharge zone and of connate water trapped by the deposition of the clay layer. (author)