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Hydrogeology Journal

, Volume 18, Issue 6, pp 1425–1436 | Cite as

Changes of freshwater-lens thickness in basaltic island aquifers overlain by thick coastal sediments

  • Kolja RotzollEmail author
  • Delwyn S. Oki
  • Aly I. El-Kadi
Report

Abstract

Freshwater-lens thickness and long-term changes in freshwater volume in coastal aquifers are commonly assessed through repeated measurement of salinity profiles from monitor wells that penetrate into underlying salt water. In Hawaii, the thickest measured freshwater lens is currently 262 m in dike-free, volcanic-rock aquifers that are overlain by thick coastal sediments. The midpoint depth (depth where salinity is 50% salt water) between freshwater and salt water can serve as an indicator for freshwater thickness. Most measured midpoints have risen over the past 40 years, indicating a shrinking lens. The mean rate of rise of the midpoint from 1999–2009 varied locally, with faster rates in highly developed areas (1.0 m/year) and slower rates in less developed areas (0.5  m/year). The thinning of the freshwater lenses is the result of long-term groundwater withdrawal and reduced recharge. Freshwater/salt-water interface locations predicted from measured water levels and the Ghyben-Herzberg principle may be deeper than measured midpoints during some periods and shallower during other periods, although depths may differ up to 100 m in some cases. Moreover, changes in the midpoint are slower than changes in water level. Thus, water levels may not be a reliable indicator of the amount of freshwater in a coastal aquifer.

Keywords

Salt-water/freshwater relations Coastal aquifers Groundwater monitoring Basaltic island hydrology Pacific Islands 

Modifications de l’épaisseur d’une lentille d’eau douce dans un aquifère d’île basaltique surmonté d’épais sédiments côtiers

Résumé

L’épaisseur des lentilles d’eau douce et les changements à long terme du volume d’eau douce dans des aquifères côtiers sont généralement appréhendés au moyen de mesures répétées de profils de salinité sur des puits de surveillance qui pénètrent jusque dans les eaux salées sous-jacentes. A Hawaï, la lentille d’eau douce la plus épaisse qui ait été mesurée est actuellement de 262 m dans des aquifères de roches volcaniques dépourvues de dyke et qui sont couverts par d’épais sédiments côtiers. La profondeur du point central (profondeur à laquelle la salinité est 50% de celle de l’eau salée) entre eau douce et eau salée peut servir d’indicateur de l’épaisseur d’eau douce. La plupart des points centraux mesurés sont remontés au cours des 40 dernières années, signe que les lentilles s’amenuisent. Le taux moyen de remontée des points centraux entre 1999 et 2009 varie localement, avec des taux plus élevés dans des zones très développées (1.0 m/an) et des taux plus bas dans des zones moins développées (0.5 m/an). L’amincissement des lentilles d’eau douce est le résultat du prélèvement d’eau souterraine sur le long terme et d’une recharge réduite. Les positions des interfaces eau douce/eau salée prédites à partir des mesures des niveaux d’eau et du principe de Ghyben-Herzberg peuvent, pour certaines périodes, être plus profondes que les points centraux mesurés et, pour d’autres, moins profondes, même si la différence des profondeurs peut atteindre 100 m dans quelques cas. De plus, les variations de la position des points centraux sont plus lentes que celles des niveaux d’eau. Ainsi, les niveaux d’eau pourraient ne pas être un indicateur fiable de la quantité d’eau douce disponible dans un aquifère côtier.

Cambios en el espesor de las lentes de agua dulce en acuíferos de islas basálticas que están cubiertos por espesos sedimentos costeros

Resumen

El espesor de las lentes de agua dulce y los cambios a largo plazo en el volumen de agua dulce en los acuíferos costeros son comúnmente evaluados a través de mediciones repetidas en los perfiles de salinidad a partir de pozos de monitoreo que penetran en el agua salada subyacente. En Hawaii, las lentes de agua dulce de mayor espesor medido tienen actualmente 262 m, en un acuífero de rocas volcánica libre de dique que está cubierto por espesos sedimentos costeros. La profundidad del punto medio (profundidad donde la salinidad es 50% del agua salina) entre el agua dulce y el agua salada puede servir como un indicador para el espesor del agua dulce. La mayoría de los puntos medios medidos se han elevado durante los últimos 40 años, indicando una reducción de las lentes. El ritmo medio de elevación del punto medio de 1999–2009 varió localmente, con ritmos más rápidos en áreas altamente desarrolladas (1.0 m/año) y ritmos más lentos en áreas menos desarrolladas (0.5 m/año). El adelgazamiento de las lentes de agua dulce es el resultado de una prolongada extracción de aguas subterráneas y de una reducción en la recarga. La localización de la interfase agua dulce – agua salada predicha a partir de los niveles de agua medidos y del principio de Ghyben – Herzberg pueden ser más profundas que los puntos medios medidos durante algunos períodos y más someras durante otros períodos, aunque las profundidades pueden diferir hasta 100 m en algunos casos. Más aún, los cambios en el punto medio son más lentos que el cambio en el nivel de agua. Así, los niveles de agua pueden no ser un indicador confiable de la cantidad de agua dulce disponible en un acuífero costero.

上覆厚层滨海沉积物的玄武岩岛含水层中淡水透镜体的厚度变化

摘要

评价海岸带含水层淡水透镜体的厚度和淡水含量长期变化的常用方法是对下伏咸水层中监测井的盐度剖面进行重复测量。在夏威夷, 当前已测的淡水透镜体厚度最大为262m, 位于某上覆厚海岸带沉积的无岩墙的火山岩含水层中。淡水与咸水之间的深度中位点 (盐度为50%咸水时的深度) 可以作为淡水层厚度的指标。过去40年测得的大多数中位点都在抬升, 说明淡水透镜体在缩小。1999–2009年期间, 不同地区中位点抬升的速率不同, 发达地区速率大些 (1.0 m/年) , 欠发达地区的速率小些 (0.5 m/年)。淡水透镜体变薄是长期地下水开采和补给减少的结果。根据监测到的水位和Ghyben-Herzberg原理预测的咸淡水界面, 在某些时段会高于观测到的深度中位点, 在其它时段则低于深度中位点, 虽然某些情况下深度相差会达到100 m。此外, 中位点的变化要比水位的变化慢。因此, 在海岸带含水层中, 水位也许并不是一个衡量可获淡水量的可靠性指标。

Alterações na espessura da lente de água doce em aquíferos de ilhas basálticas cobertas por espessos sedimentos costeiros

Resumo

A espessura da lente de água doce e as alterações a longo prazo dos volumes de água doce nos aquíferos costeiros avaliam-se geralmente com base em medições repetidas de perfis de salinidade em furos de monitorização que entram na camada de água salgada subjacente. No Havai, a lente de água doce com maior espessura mede actualmente 262 m e localiza-se em aquíferos de rochas vulcânicas sem diques e cobertas por espessos sedimentos costeiros. A profundidade do ponto intermédio entre a água doce e a água salgada (profundidade onde a salinidade é 50% da água salgada) pode servir como um indicador de espessura da água doce. A maioria dos pontos intermédios medidos subiu nos últimos 40 anos, indicando uma lente a diminuir de espessura. A taxa média de subida do ponto intermédio entre 1999 e 2009 variou localmente, com taxas superiores em áreas altamente desenvolvidas (1.0 m/ano) e taxas inferiores em áreas menos desenvolvidas (0.5 m/ano). A diminuição da espessura da lente de água doce resulta da exploração prolongada das águas subterrâneas e da recarga reduzida. As estimativas de localização da interface água doce-água salgada baseadas em medições de níveis piezométricos e no princípio de Ghyben-Herzberg podem resultar em profundidades maiores ou menores do que as obtidas com base nos pontos intermédios, dependendo do período, podendo as diferenças chegar aos 100 m em alguns casos. Para além disso, as deslocações do ponto intermédio são mais lentas do que as oscilações do nível piezométrico. Deste modo, os níveis piezométricos podem não ser indicadores fiáveis da quantidade de água doce disponível num aquífero costeiro.

Notes

Acknowledgements

This work was supported by the US Geological Survey Pacific Islands Water Science Center. The authors thank the Honolulu Board of Water Supply and the Hawaii Department of Land and Natural Resources, Commission on Water Resource Management for providing access to salinity profiles and water levels. Comments from Scot Izuka, Kevin Chiu, and two anonymous reviewers helped improve the manuscript. This study is published as a part of the continuing University of Hawai‘i at Mnoa Water Resources Research Center “Contributed Papers” series. As such, it will be assigned a sequential “WRRC-CP” number upon publication. This number, along with the abstract of the paper, will then be posted at the WRRC website <http://www.wrrc.hawaii.edu/index.html>

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Copyright information

© Springer-Verlag 2010

Authors and Affiliations

  • Kolja Rotzoll
    • 1
    Email author
  • Delwyn S. Oki
    • 2
  • Aly I. El-Kadi
    • 1
  1. 1.Water Resources Research CenterUniversity of HawaiiHonoluluUSA
  2. 2.US Geological Survey, Pacific Islands Water Science CenterHonoluluUSA

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