Hydrochemical and isotopic evolution in a deep carbonate aquifer in northern Andalusia, Spain

The Loma de Ubeda aquifer is formed by Jurassic carbonates with a mean thickness of about 80 m. The carbonate sequence is covered in the southern part of the study area by a thick sequence of Miocene sediments, and groundwater becomes confined. Groundwater samples in the phreatic zone contain tritium activities similar to present-day precipitation, as well as high nitrate contents, indicating the relevance of recent recharge. Further down-gradient, in the confined part of the aquifer, tritium contents become null. δ18O values in most groundwater samples are similar to those of present-day recharge. The uncorrected 14C-DIC 'ages' increase from recent up to 20-25 ka in the deepest part of the aquifer. Carbon-13 values in the confined part become progressively depleted (up to -21 per mille,) indicating the presence of a source of organic carbon in the aquifer. The δ34S values in dissolved sulphate and the high concentrations of H2S reflect the importance of sulphate reduction. Geochemical models suggest that the residence time of groundwater in the deepest part is up to 10,000 years younger than the uncorrected 14C-DIC ages. (author)