Evolution of groundwater systems at the European coastline

An overview is given of the status and origin of fresh and saline groundwaters in the sedimentary aquifers at or near the present European coastline. Results are presented as six regional maps summarizing. as far as possible, the conditions likely to have existed at the end of the Pleistocene, after the impact of glaciation, when groundwaters might be expected to have reached their maximum offshore evolution prior to the encroachment of sea water during the Holocene marine transgression. In the eastern Baltic, the groundwater evolution was different to other European regions in that freshwater heads were higher than the present day, promoting recharge during much of the Late Pleistocene. Near the North Sea coasts, there is generally evidence of freshwater movement to depths of 100-150 m but the absence of deeper freshwater (palaeowater) storage may relate to the low hydraulic gradients in the North Sea Basin. In the southeastern part of the North Sea brackish palaeowater is found between Tertiary marine sediments c. 300 m below the island of Romo, 10 km off the Danish west coast. Freshwater of Pleistocene and Holocene ages is found in aquifers at the English Channel coast in several areas, to depths in excess of -300 m: Offshore flow in the Late Pleistocene took place towards the central palacovalley and some of this groundwater may be preserved off the present coastline. In the two Atlantic coastal areas of France and Portugal-Spain a contrast exists due to the proximity of the continental margin and different hydraulic gradients. In Portugal, freshwaters are found at the coastline, and probably offshore, that contain evidence of recharge during the lowered sea levels. In most of the southwestern Atlantic coast of Spain, fresh recent groundwater discharges along and beyond the coastline, while in the old estuary of the Guadalquivir River, saline Holocene water still encroaches the sediments. On the Mediterranean coast of France and Spain some salinity encroachment took place during sea-level rise. In most of the Spanish aquifers fresh recent groundwater has substituted for palaeowater, except for the low-lying areas (Ebro Delta, Inca-Sa Pobla Plain) where brackish Holocene water is still present.