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Hydrogeology Journal

, Volume 11, Issue 5, pp 549–559 | Cite as

Rates of salinization by free convection in high-permeability sediments: insights from numerical modeling and application to the Dutch coastal area

  • Vincent E. A. PostEmail author
  • Henk Kooi
Paper

Abstract

Numerical modeling and dimensional analysis is used to study the salinization of thick, high-permeability aquifers by free convection from a salt source at the surface. Current understanding of this process mainly concerns the initial stages of salinization only (boundary-layer development, break-up into fingers and initial phase of finger descent). In the modeling, special attention is paid to the role of two processes in the long-term salinization rate: (1) the progressive loss of salt from fingers by lateral diffusion, and (2) the coalescence of fingers during their descent. From the numerical simulations a relationship is derived that describes the development of the horizontally averaged salinity with depth and time as a function of permeability and initial-density contrast for aquifer Rayleigh numbers up to Ra =6,000. This relationship is consistent with and provides an extension to previous generalized relationships of the rate of finger descent. Its applicability to real-world aquifers (Ra >105) that include complexities due to anisotropy, heterogeneity, and mechanical dispersion is discussed. Application to the Pleistocene coastal aquifer of the Netherlands (thickness ≈200 m, permeability ≈10-11 m2) suggests that salinization of the aquifer during historic episodes of inundation by seawater occurred within decades.

Keywords

Coastal aquifers Numerical modeling Salinization Salt-water/fresh-water relations The Netherlands 

Résumé

Une modélisation numérique et une analyse dimensionnelle ont été mises en oeuvre pour étudier la salinisation d'aquifères épais et à forte perméabilité par convection libre d'une source de sel en surface. La compréhension habituelle de ce processus concerne principalement les étapes initiales de la salinisation seule (développement d'une couche limite, partition en digitations et phase initiale de développement des digitation). Dans la modélisation, une attention particulière a été portée au rôle de deux processus du taux de salinisation à long terme: (1) la perte progressive de sel dans les digitations par diffusion latérale et (2) la coalescence des digitations au cours de leur développement. À partir de simulations numériques, une relation a été obtenue qui permet de décrire l'extension de la salinité horizontalement en profondeur et au cours du temps en fonction de la perméabilité et du contraste initial de densité pour des nombres de Rayleigh de l'aquifère jusqu'à Ra =6,000. Cette relation est compatible avec ces résultats et fournit une extension des relations précédemment généralisées du taux de développement des digitations. On discute son applicabilité à des aquifères réels (Ra >105) incluant des complexités liées à l'anisotropie, l'hétérogénéité et la dispersion mécanique. L'application à l'aquifère côtier du Pléistocène des Pays-Bas (épaisseur environ 200 m, perméabilité environ10–11 m2) laisse penser que la salinisation de cet aquifère au cours d'épisodes historiques d'inondation par la mer s'est produit durant des décennies.

Resumen

Se utiliza modelos numéricos y un análisis dimensional para estudiar la salinización de acuíferos potentes de alta permeabilidad por convección libre a partir de una fuente salina superficial. El conocimiento acutal de este proceso se limita a las fases iniciales de la salinización (desarrollo de la capa de contorno, creación de digitaciones y fase inicial de la progresión de éstas). En la modelación, se presta atención especial al papel desempeñado por dos procesos de salinización a largo plazo: (1) la pérdida progresiva de sal por difusión lateral desde las digitaciones, y (2) la coalescencia de las digitaciones durante su avance. A partir de las simulaciones numéricas, se obtiene una relación que describe el desarrollo de la salinidad con la profundidad y el tiempo, promediada horizontalmente, el cual depende de la permeabilidad y del contraste inicial de densidad para números de Rayleigh inferiores a 6.000. Esta relación es coherente con índices previos generalizados del avance de las digitaciones, y representa una extensión a estos. Se discute su aplicabilidad a acuíferos reales (con números de Rayleigh superiores a 105), que tienen complejidades asociadas a la anisotropía, la heterogeneidad y la dispersión mecánica. La aplicación al acuífero costero Pleistoceno de los Países Bajos (20 m de potencia y 10-11 m2 de permeabilidad) sugiere que la salinización tuvo lugar en décadas, debido a episodios históricos de inundación por aguas marinas.

Notes

Acknowledgements

This project is funded by the Netherlands Institute of Applied Geoscience TNO. SARA (Academic Computing Services Amsterdam) is acknowledged for their permission to use their IBM SP2 for the numerical calculations. We would also like to thank Toon Leijnse for his constructive comments, Lester Reiniers of the province of Noord-Holland for the chloride data, Prof. Meijer and Dr Van der Plicht of the Centre for Isotope Research in Groningen for the 14C measurements and Victor Bense and Elmer van den Berg for their reading of the manuscript.

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Copyright information

© Springer-Verlag 2003

Authors and Affiliations

  1. 1.Faculty of Earth and Life Sciences, Department of Hydrology and HydrogeologyVrije Universiteit1081 HV AmsterdamThe Netherlands

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