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A combined hydrogeological, geochemical and isotopic investigation was performed in the Aveiro Cretaceous coastal aquifer in order to investigate the rock-water interaction and the evolution of palaeowaters. The results indicate an aquifer with well-defined freshwater (<50 mg l−1 Cl) throughout most of the aquifer as far as the coastline and with very slow chemical kinetics for the water–rock interactions. The low mineralization may be explained by the mainly siliciclastic composition of the aquifer sediments, while calcite dissolution and cation exchange were also considered to be involved in the groundwater chemical evolution. This interpretation is in agreement with lithology and facies changes, and was validated by the PHREEQC-2 modelling results. Radiocarbon ages indicate a smooth gradient across the aquifer, implying continuous flow during the late Pleistocene and Holocene. Noble gas ratios indicate that the mean annual air temperatures were lower by 5–6°C at the last glacial maximum (LGM). In contrast to most areas with continental palaeowaters, the environmental isotopes indicate enrichment (0.8–1.0‰ in 18O), interpreted as reflecting the composition of the oceans at the time of the LGM at this maritime site, as well as the constancy of the air mass circulations over the period to the present day. An outlet for the aquifer is inferred offshore to account for the observed geochemical data, with the possibility that freshwater could still also be found offshore.

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