Evolution of groundwater systems at the European coastline

Abstract

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 Rømø, 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 palaeovalley 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.

Transboundary geophysical mapping of geological elements and salinity distribution critical for the assessment of future sea water intrusion in response to sea level rise

“…Since saltwater is not found in the Miocene sediments bordering on the marshland, it is likely that the residual saltwater had been flushed by freshwater during periods where the sea levels were much lower than the present, e.g. during the Last Glacial Maximum where the sea level was more than 120 m lower than now (Edmunds et al, 2001;Clark and Mix, 2002). The origin of the saltwater is therefore expected to be more recent, either due to infiltration and intrusion during the Holocene transgression and/or as a result of seawater intrusion after the drainage of the area.…”

“…Custodio and Bruggeman, 1987;Iribar et al, 1997;Edmunds et al, 2001;Oude Essink, 2001;Oude Essink et al, 2010). Furthermore, a number of recent studies on current saltwater and freshwater interactions in coastal aquifers have demonstrated the increasing global problem of saltwater intrusion (e.g.…”

“…However, real-life geology is never ideal, but it is characterized by heterogeneity to greater or lesser extent, and it strongly controls saltwater intrusion at the European coastline due to sea level rise during the Holocene (Edmunds et al, 2001). According to Carrera et al (2010), seawater intrusion is especially sensitive to the sea-aquifer connection, usually associated with the presence of preferential flow paths, a phenomenon which has been demonstrated also to affect fresh groundwater discharge to the sea Mulligan et al, 2007).…”

Groundwater residence time and paleorecharge conditions in the deep confined aquifers of the coastal watershed, South-East Tanzania

“…We hypothesize that this enrichment is due to sea-water intrusion during the Late Pleistocene-Holocene transition. During such period of marine transgression the sea level was lowered by 115-130 m below its present position (Edmunds et al, 2001b;Loosli et al, 2001). This allowed the deposition of deltaic (sands, silts and gravels) and littoral (sands) sediments.…”

“…14 ka BP, with a rise in sea level occurring in two stages before stabilizing ca. 6 ka BP (De Melo et al, 2001;Edmunds et al, 2001b;Loosli et al, 2001). During the sea-level rise the aquifer sediments were encroached by sea water.…”