Hydrochemical study of a cross-section through the coastal plain and its surroundings near the French-Belgian border
Lebbe, L.; Walraevens, K. (1989). Hydrochemical study of a cross-section through the coastal plain and its surroundings near the French-Belgian border. Natuurwet. Tijdschr. 70(1-4): 56-64
In: Natuurwetenschappelijk Tijdschrift. L. Walschot/Natuur- en Geneeskundige Vennootschap: Gent. ISSN 0770-1748, meer
Ook verschenen in:De Breuck, W.; Walschot, L. (Ed.) (1989). Proceedings of the 10th Salt-Water Intrusion Meeting Ghent (Belgium), 16-20 May 1988. Natuurwetenschappelijk Tijdschrift, 70(1-4). Natuurwetenschappelijk Tijdschrift: Gent. 408 pp., meer
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Motion > Water motion > Water currents
Polders
Topographic features > Beach features > Dunes
Topographic features > Landforms > Coasts
Water > Ground water
België, West [Marine Regions]; France, Nord [Marine Regions]
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| Abstract |
The area studied is an almost rectangular area of 3 km wide and 9 km long. The northern boundary is the low-low-waterline of the North Sea. The western boundary is the French-Belgian border. In that area 127 groundwater samples have been taken from the phreatic aquifer which ranges in thickness from 35 m in the north to only a few metres in the south. Every sample has undergone a total chemical analysis, the results of which are classified according to STUYFZAND (1986). For explanation of the presente hydrochemical distribution, knowledge of the geological evolution and of the mean water and/or drainage levels is all-important. From north to south one distinguishes the shore, the young dunes (partly covering the outmost old-dune ridge, partly overlying old tidal-flat deposits), a strip of polderland, the inmost old dunes and De Moeren. The southern boundary of the survey area coincides with the boundary between De Moeren and the surrounding polders. De Moeren are low-lying polders, which were reclaimed much later than the surrounding polders. Formerly, the area of De Moeren was a brackish lake, mostly isolated from the sea. This brackish lake developed due to the formation of the inmost old dunes. The captured seawater was gradually diluted by the addition of fresh surface water from the surroundings. The mean water level of the lake was slightly higher than the mean groundwater level in the old tidal flat between both old-dune ridges. As a consequence the salt, and later brackish lake water infiltrated, flowing toward the tidal with a deep water cycle, under the fresh-water pocket of the inmost old-dune ridge. This groundwater movement was increased as the old tidal flat in between both old-dune ridges was reclaimed as polderland. As De Moeren were reclaimed as low polders, the considered groundwater flow was reversed, and the old salt to brackish waters that once infiltrated De Moeren, return to them now by an upward flow. The fresh-water pocket under the inmost old-dune ridge between the old tidal flat (now polders) and the former lake (now low polders) changed shape only slightly. The development of the young dunes, partly on the outmost old dunes and partly on the old tidal flat, caused the fresh-water pocket under the outmost old-dune ridge to expand toward the south. Beneath the shore this fresh-water pocket ends up in a fresh-water tongue, which is covered by a salt-water lens. This salt water is infiltrating the higher parts of the shore and is seeping out on to the lower part of the shore and under the sea. |
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