Abstract
A salt water lens is found above fresh water under the shore between Dunkerque (France) and Nieuwpoort (Belgium). This inverse density distribution is in a dynamic equilibrium. It develops due to the infiltration of salt water on the back shore during high tide. Under this salt water lens, water infiltrated in the adjacent dune area flows towards the sea and discharges at the seabed. This water quality distribution differs from the classic salt water wedge under fresh water described in the literature. Here, the evolution to this water quality distribution is simulated with a density dependent numerical model. A large tidal range, shore morphology and a permeable groundwater reservoir are the main conditions for the observed water quality distribution. By altering these conditions, intermediate water quality distributions between the classic salt water wedge and the one discussed here develop. Based on these simulations, it is expected that similar kinds of inverse density distribution could be present in a number of coastal areas, which have tides, a gently sloping shore and a permeable substratum.
Résumé
On a trouvé dans un aquifère côtier, situé entre Dunkerque (France) et Nieuwpoort (Belgique) une lentille d’eau salée au dessus de l’eau douce. Cette distribution inverse de la densité se trouve dans un équilibre dynamique. Elle se développe à cause des infiltrations d’eau salée pendant la haute marée. Au dessous de ces lentilles d’eau salée, l’eau infiltrée au long de dunes se dirige vers la mer et se décharge au fonds de la mer. Cette distribution de la qualité des eaux diffère de biseau d’eau salée classique présentée dans la littérature. Dans cet article l’évaluation de la distribution de la qualité de l’eau est simulée avec un modèle numérique à densité variable. Les conditions principales considérées pour analyser la distribution observée de la qualité de l’eau ont été un grand intervalle de variation pour la marée, la morphologie de la côte et un réservoir perméable d’eau souterraine. En alternant cettes conditions on a mis en évidence dans la distribution de la qualité de l’eau des stages intermédiaires entre le biseau classique et la situation présentée auparavant. À partir des simulations on peut considérer qu’il est possible d’avoir des distributions inverses de la densité dans des aquifères côtières dans la présence des marées et dans des conditions d’une côte à pente douce et d’une couche perméable.
Resumen
Se encontró un lente de agua salada sobre agua dulce, debajo de la zona litoral entre Dunkerke (Francia) y Nieuwpoort (Bélgica). Esta distribución invertida de la densidad está en equilibrio dinámico. Se desarrolla debido a la infiltración del agua salada sobre la zona emergida de la playa (berma de playa), durante los episodios de marea alta. Bajo este lente de agua salada, el agua infiltrada en el área de dunas adyacente, fluye hacia el mar y descarga en el lecho marino. Esta distribución de calidad de agua, es diferente de la clásica cuña de agua salada bajo el agua dulce descrita en la literatura. La evolución para esta distribución de calidad de agua se simuló con un modelo numérico de densidad dependiente. Las principales condiciones para la distribución observada de calidad de agua son: Un rango amplio de mareas, la morfología de la zona costera y un reservorio permeable de agua subterránea. Cuando se alteran estas condiciones, se desarrollan distribuciones intermedias de calidad de agua, oscilando entre la cuña de agua salada clásica y el modelo aquí discutido. Con base en estas simulaciones se espera que tipos similares de distribución de densidad invertida, podrían estar presentes en un número de áreas costeras, que tengan mareas, una pendiente litoral suave y un sustrato permeable.
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The authors wish to thank dr. Maria Clara Castro, dr. Leonard Konikow and dr. Patrick Goblet for their constructive review of the paper
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Vandenbohede, A., Lebbe, L. Occurrence of salt water above fresh water in dynamic equilibrium in a coastal groundwater flow system near De Panne, Belgium. Hydrogeol J 14, 462–472 (2006). https://doi.org/10.1007/s10040-005-0446-5
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DOI: https://doi.org/10.1007/s10040-005-0446-5