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Hydrogeology Journal

, Volume 15, Issue 8, pp 1591–1604 | Cite as

Hydrostratigraphy and geochemistry at a coastal sandfill in Singapore

  • Lloyd H. C. Chua
  • Edmond Y. M. Lo
  • David L. Freyberg
  • E. B. Shuy
  • T. T. Lim
  • S. K. Tan
  • Mzila Ngonidzashe
Report

Abstract

A characterization study was carried out in a 10-m-thick sandfill, formed by hydraulic filling with marine sand, in Singapore. Placement methods and compaction were found to influence hydrostratigraphy. The deepest part of the sandfill consists of a loose sand layer and is overlain by a medium sand layer extending to mean sea level (MSL). At certain locations, a thin silty-sand layer was found. The different layers within the saturated zone were found to have different values for hydraulic conductivity (K) and groundwater flow velocity. Estimates for K increase according to the following sequence of methods: repacked sand column, step-pumping test, grain-size analysis and slug test. Slug tests and grain-size analysis yielded comparable estimates of K. The freshwater lens in the older part of the sandfill is about 2 m thicker than in a recently completed area. Comparisons of Ca2+/Cl, Mg2+/Cl, K +/Cl and \({\text{Cl}}^{{\text{ - }}} {\text{/}}{\left( {{\text{Cl}}^{{\text{ - }}} {\text{ + HCO}}^{{\text{ - }}}_{{\text{3}}} } \right)}\) ratios indicate that the chemical composition of the groundwater at shallower depths has probably been altered by mineral dissolution. Weathering of carbonate minerals was found to be a major contributor to the major ions at these depths. The molar ratios approach the value for seawater at greater depths. The groundwater is close to equilibrium with calcite.

Keywords

Sandfill Hydrostratigraphy Heterogeneity Mineral dissolution Hydraulic properties 

Résumé

Une étude de caractérisation à Singapour a été réalisée sur un remblayage de 10 mètres d’épaisseur, formé de remblais hydrauliques composés de sable marin. Il est apparu que les méthodes de positionnement et de compaction influencent l’hydrostratigraphie. La partie la plus profonde du remblais est composée d’une couche de sable meuble recouverte par une couche de sable intermédiaire qui s’étend jusqu’au niveau moyen de la mer (MSL en anglais). Une mince couche silto-sableuse a été trouvée au niveau de certaines localisations,. Il est apparu que les différentes couches dans la zone saturée possèdent différentes valeurs de conductivité hydraulique (K) et différentes vitesses d’écoulement de l’eau souterraine. Les estimations de K augmentent suivant la séquence des méthodes suivantes : colonne de sable recompactée, test de pompage par paliers, analyse granulométrique et slug-test. Les slug-tests et l’analyse granulométrique conduirent à des estimations comparables de K. La lentille d’eau douce dans la partie la plus ancienne du remblais est plus épaisse d’environ 2 mètres que dans une zone réalisée récemment. Les comparaisons des rapports Ca2+/Cl, Mg2+/Cl, K +/Cl et \({\text{Cl}}^{{\text{ - }}} {\text{/}}{\left( {{\text{Cl}}^{{\text{ - }}} {\text{ + HCO}}^{{\text{ - }}}_{{\text{3}}} } \right)}\) indiquent que la composition chimique de l’eau souterraine à faibles profondeurs a probablement été altérée par la dissolution de minéraux. L’altération des minéraux carbonatés serait un processus majeur apportant les ions majeurs à ces profondeurs. Les rapports molaires sont proches de la valeur rencontrée pour l’eau de mer à des profondeurs plus importantes. L’eau souterraine est proche de l’équilibre avec la calcite.

Resumen

Un estudio de caracterización se llevó a cabo en un relleno arenoso de 10 m de espesor, formado por un relleno hidráulico con arena marina, en Singapur. Se encontró que los métodos de colocación y consolidación influyen en la hidroestratigrafía. La parte más profunda del relleno de arena consiste en una capa de arena suelta y está cubierto por una capa de arena media que se extiende hasta el nivel medio del mar (MSL). En ciertos lugares, se encontró una capa delgada de arena limosa. Se encontró que las capas diferentes dentro de la zona saturada tienen valores diferentes de conductividad hidráulica (K) y de velocidad del agua subterránea. Los estimativos para K aumentan según la sucesión siguiente de métodos: columna de arena reempacada, prueba de bombeo escalonada, análisis de tamaño de grano y prueba slug. Las pruebas slug y los análisis de tamaño de grano obtuvieron estimativos comparables de K. El lente de agua dulce en la parte más antigua del relleno de arena, es aproximadamente 2 m más espeso que en una área recientemente completada. Las comparaciones entre las relaciones, Ca2+/Cl, Mg2+/Cl, K +/Cl y \({{\text{Cl}}^{{\text{ - }}} } \mathord{\left/ {\vphantom {{{\text{Cl}}^{{\text{ - }}} } {{\left( {{\text{Cl}}^{{\text{ - }}} {\text{ + HCO}}^{{\text{ - }}}_{{\text{3}}} } \right)}}}} \right. \kern-\nulldelimiterspace} {{\left( {{\text{Cl}}^{{\text{ - }}} {\text{ + HCO}}^{{\text{ - }}}_{{\text{3}}} } \right)}}\) indican que la composición química del agua subterránea a las profundidades más someras probablemente se ha alterado por la disolución mineral. Se encontró que la meteorización de minerales del carbonato es un contribuyente mayor para los iones mayores en estas profundidades. Las proporciones de molaridad se acercan el valor para agua de mar a profundidades mayores. El agua subterránea está cerca del equilibrio con la calcita.

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Copyright information

© Springer-Verlag 2007

Authors and Affiliations

  • Lloyd H. C. Chua
    • 1
  • Edmond Y. M. Lo
    • 1
  • David L. Freyberg
    • 2
  • E. B. Shuy
    • 1
  • T. T. Lim
    • 1
  • S. K. Tan
    • 1
  • Mzila Ngonidzashe
    • 1
  1. 1.Division of Environmental and Water Resources Engineering, School of Civil and Environmental EngineeringNanyang Technological UniversitySingaporeSingapore
  2. 2.Department of Civil and Environmental EngineeringStanford UniversityStanfordUSA

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