Palaeowater in coastal aquifers of Spain

Abstract

Abstract In the framework of the PALAEAUX project, three coastal aquifers have been studied in Spain, looking at the possible existence of Pleistocene or Early Holocene fresh groundwater, in addition to the formerly studied Llobregat Delta. They constitute representative situations of the most common coastal aquifers. The main characteristics of these four areas are: The Llobregat Delta system, studied since 1965, is placed to the southwest of Barcelona (NE Mediterranean coast of the Iberian Peninsula). It is a sedimentary fluvial formation of Pleistocene to recent age, which is representative of small size coastal alluvial formations with the deep confined aquifer open to the sea offshore. Marine sea water penetrated the deep Pleistocene layer during the Flandrian sea-level rise but afterwards relatively high freshwater heads upstream allowed its progressive flushing to the sea through the submarine outlet since 6000–8000 a ago. Freshwater turnover time is shorter than the Holocene duration and it has been renovated due to natural discharge of the aquifer to the sea. The renovation process has recently been accelerated due to intensive groundwater exploitation during the past century. Nowadays, only freshwater a few centuries old remains in low permeability areas of the confined Pleistocene aquifer. The Doñana sedimentary aquifer system corresponds to the ancient estuary of the Guadalquivir River (SW Spain). It contains old saline groundwater that has not been flushed away due to the low freshwater head. In this aquifer 39 At, 85 Kr, 3 H, 14 C and 13 C measurements have been used to ascertain the age structure of several mixed groundwater samples. After correction, the oldest fresh groundwater 14 C age is c. 12–15 ka but 39 Ar ages point to somewhat younger water. Neither the stable isotope contents nor the recharge temperature calculated from noble gases show a climatic signature for these waters. The Inca-Sa Pobla carbonate system is the northeast sector of the island of Mallorca. Brackish and salt groundwater in the coastal area has been found along the coastal strip of S’Albufera, displaying what seems a convective-like flow pattern inside the up to 200 m thick aquifer. The most consistent estimated average 14 C ages of the mixed water vary between 9 and 13 ka, i.e. Early Holocene-Late Pleistocene. Water 18 O and 2 H contents do not show a clear climatic signature. The Amurga volcanic Massif is on the southeast of the island of Gran Canaria. Brackish groundwater is found in the thick phonolite rock sequence. The unsaturated zone can be up to 200 m thick. Groundwater salinity is due to recharge under arid conditions and the influence of marine airborne salts. Groundwater 14 C age is possibly c. 11 ka, indicating a Late Pleistocene age water mixture. The small recharge due to aridity and the thick unsaturated zone explains this. Stable-water isotopes show no clear climatic signature