Isotopic evidence of aquifer recharge during the last ice age in Portugal

Abstract

Summary The present paper will focus on the research that has been conducted in the palaeowaters in the South of Portugal, Alentejo region. Three flow directions were identified in Sado sedimentary basin aquifer: S–N, SW–NE and SE–NW; the last two result from Palaeozoic substratum irregularities, corresponding to an impermeable tectonic structure (horst). Two aquifers showing different isotopic composition were identified and analysed: the Eocene and the Plio-Miocene. The Plio-Miocene aquifer has an average isotopic composition of −29.6 ± 1.3‰ in deuterium and −4.65 ± 0.28‰ in oxygen-18, and the Eocene has average values of deuterium of −28.7 ± 2.1‰ and −4.81 ± 0.11‰ for oxygen-18. In single cases show that the range of isotopic values overlap for the two aquifers, probably as a result of the exploitation in some wells by a mixture of waters from both aquifers. The spatial distribution of 3 H and 14 C obtained in the Plio-Miocene waters indicates a flow path from S to N, with recent waters near the southern border of the basin and older waters to the N near the Sado River. The recharge of this aquifer system is diffuse and is dispersed over the entire basin. Groundwater samples from the two aquifers having similar apparent 14 C ages have distinct isotopic compositions, with the Eocene aquifer showing an enriched composition relatively to the Plio-Miocene aquifer. This suggests that recharge of the Eocene waters could have occurred under different climatic conditions.