Hydrogeology Journal

, Volume 14, Issue 8, pp 1556–1568

Chemical characterization of the Neogene Aquifer, Belgium

Report

Abstract

The evolution of groundwater chemistry along the direction of groundwater flow was studied using hydrochemical data from samples collected along a flow line in the Neogene Aquifer, Belgium. Infiltrating water was found to have a very low mineral content and low pH because the sediments are strongly decalcified. Increasing SiO2 and cation concentrations along the groundwater flow line indicate silicate-weathering processes, confirmed with the aid of saturation indices, calculated with PHREEQC, and stability diagrams. A classification system based on redox sensitive species was developed and shows that an extensive redox sequence is present in the aquifer. At a shallow depth, pyrite oxidation has caused an increase in sulphate, while iron is precipitated as hydroxides. Elevated arsenic concentrations are related to the reduction of these iron hydroxides at a relatively shallow depth and to the dissolution of siderite at greater depth. Dissolution of carbonate in the aquifer material, present in deep layers and to the north, has lead to increased Ca2+ and HCO3 concentrations. The Ca2+ from the groundwater is exchanged for Na+, Mg2+ and K+ adsorbed to the clay surfaces at the bottom of the groundwater reservoir. Although the Neogene Aquifer is well flushed, there are still some marine influences present in the deepest parts.

Keywords

Hydrochemistry Groundwater flow Redox reactions Belgium 

Résumé

L'évolution de la chimie des eaux souterraines le long des directions d'écoulement a été étudiée à partir données hydrochimiques issues de prélèvements effectués le long d'une ligne de flux, dans l'Aquifère du Néogène, en Belgique. Les eaux d'infiltration sont apparues faiblement minéralisées et de pH faible, du fait de la décalcification marquée des sédiments. Les augmentations des concentrations en SiO2 et cations en suivant les lignes de flux indiquent la présence de processus d'altération des silicates, confirmée à l'aide des indices de saturation, calculés avec PHREEQC, et des diagrammes d'équilibre. Un système de hiérarchisation basé sur les espèces redox sensibles a été développé, et montre qu'une chaîne d'oxydoréduction développée est présente dans l'aquifère. L'oxydation de la pyrite à faible profondeur entraîne une augmentation des sulfates, tandis que le fer précipite sous forme d'hydroxydes. Les concentrations élevées en arsenic sont liées à la réduction de ces hydroxydes de fer à faible profondeur, ainsi qu'à la dissolution de sidérite plus profondément. La dissolution des carbonates des formations aquifères, présents dans les couches profondes et au nord, entraîne une augmentation des concentrations en Ca2+ et HCO3. Le Ca2+ des eaux souterraines est échangé avec les ions Na+, Mg2+ et K+ adsorbés sur les feuillets d'argile au mur du réservoir. Même si l'Aquifère du Néogène est bien drainé, certaines influences marines rémanentes sont observables dans les secteurs les plus profonds.

Resumen

Se estudió la evolución de la química de aguas subterráneas a lo largo del flujo de estas mediante el uso de datos hidroquímicos de muestras tomadas a lo largo de una línea de flujo en el acuífero Neogeno. Se encontró que el agua que se infiltra tiene un contenido mineral muy bajo y un pH bajo porque los sedimentos están altamente descalcificados. Un incremento en las concentraciones de SiO2 y de cationes a lo largo de la línea de flujo de agua subterránea indica procesos de descomposición de silicatos, lo cual se confirma con el apoyo de índices de saturación calculados con PHREEQC y diagramas de estabilidad. Se desarrolló un sistema de clasificación basado en especies sensitivas redox. Este muestra que hay una secuencia de redox extensa en el acuífero. A poca profundidad la oxidación de pirita ha causado un incremento de sulfatos, mientras que el fierro se precipita como hidróxidos. Las concentraciones elevadas de arsénico se relacionan con la reducción de esos hidróxidos de fierro a nivel poco profundo y a la disolución de siderita a nivel más profundo. La disolución de carbonatos en el material del acuífero, presente a niveles profundos al norte ha llevado a un incremento de concentraciones de Ca2+ y HCO3. El Ca2+ de las agua subterráneas se intercambia por Na+, Mg2+ y K+ adsorbido en las superficies de arcilla al fondo del reservorio de agua subterránea. A pesar de que el acuífero Neogeno tiene buena circulación algunas influencias marinas persisten en las partes más profundas.

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Copyright information

© Springer-Verlag 2006

Authors and Affiliations

  1. 1.Laboratory for Applied Geology and HydrogeologyGhent UniversityGhentBelgium

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