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Groundwater flow in the transition zone between freshwater and saltwater: a field-based study and analysis of measurement errors

  • Vincent E. A. Post
  • Eddie Banks
  • Miriam Brunke
Paper

Abstract

The quantification of groundwater flow near the freshwater–saltwater transition zone at the coast is difficult because of variable-density effects and tidal dynamics. Head measurements were collected along a transect perpendicular to the shoreline at a site south of the city of Adelaide, South Australia, to determine the transient flow pattern. This paper presents a detailed overview of the measurement procedure, data post-processing methods and uncertainty analysis in order to assess how measurement errors affect the accuracy of the inferred flow patterns. A particular difficulty encountered was that some of the piezometers were leaky, which necessitated regular measurements of the electrical conductivity and temperature of the water inside the wells to correct for density effects. Other difficulties included failure of pressure transducers, data logger clock drift and operator error. The data obtained were sufficiently accurate to show that there is net seaward horizontal flow of freshwater in the top part of the aquifer, and a net landward flow of saltwater in the lower part. The vertical flow direction alternated with the tide, but due to the large uncertainty of the head gradients and density terms, no net flow could be established with any degree of confidence. While the measurement problems were amplified under the prevailing conditions at the site, similar errors can lead to large uncertainties everywhere. The methodology outlined acknowledges the inherent uncertainty involved in measuring groundwater flow. It can also assist to establish the accuracy requirements of the experimental setup.

Keywords

Groundwater density/viscosity Equipment/field techniques Groundwater flow Salt-water/fresh-water relations Australia 

Ecoulement des eaux souterraines dans la zone de transition eau douce-eau salée: étude sur le terrain et analyse des erreurs de mesure

Résumé

La quantification de l’écoulement d’eaux souterraines dans la zone de transition eau douce–eau salée en zone côtière est. difficile à cause des effets d’une densité variable et de la dynamique des marées. Les mesures de charge hydraulique ont été collectées le long d’un transect perpendiculaire au trait de côte dans un site au Sud de la ville d’Adélaïde, en Australie du Sud, afin de déterminer le modèle d’écoulement transitoire. Cet article présente un aperçu détaillé de la procédure de mesures, des méthodes de post-traitement des données et de l’analyse de l’incertitude, afin d’évaluer la façon dont les erreurs de mesure affectent l’exactitude des schémas d’écoulement présumés. Une difficulté particulière rencontrée était que certains piézomètres avaient des fuites, ce qui a nécessité des mesures régulières de la conductivité électrique et de la température de l’eau dans les forages dans le but de corriger les effets de la densité. D’autres difficultés comprenaient la défaillance des capteurs de pression, la dérive de l’horloge de l’enregistreur de données et les erreurs de l’opérateur. Les données obtenues ont été suffisamment précises pour montrer qu’il y a un net écoulement horizontal de l’eau douce vers la mer dans la partie supérieure de l’aquifère et un net écoulement d’eau salée vers la terre dans la partie inférieure. La direction de l’écoulement vertical alternait avec la marée, mais en raison de la grande incertitude sur les gradients hydrauliques et le terme de densité, aucun écoulement net n’a pu être établi avec un quelconque degré de confiance. Bien que les problèmes de mesures aient été amplifiés par les conditions prévalant sur le site, des erreurs similaires peuvent conduire à de grandes incertitudes partout ailleurs. La méthodologie décrite reconnaît l’incertitude inhérente à la mesure de l’écoulement d’eaux souterraines. Elle peut aussi aider à établir les conditions de fiabilité de la mise en œuvre expérimentale.

Flujo de agua subterránea en la zona de transición entre agua dulce y agua salada: un estudio de campo y análisis de errores de medición

Resumen

La cuantificación del flujo de agua subterránea cerca de la zona de transición agua dulce–agua salada en la costa es difícil debido a los efectos de densidad variable y la dinámica de las mareas. Las mediciones de la carga hidráulica se tomaron a lo largo de una transecta perpendicular a la costa en un sitio al sur de la ciudad de Adelaide, en el sur de Australia, para determinar el patrón del flujo transitorio. Este trabajo presenta una descripción detallada del procedimiento de medición, los métodos de post-procesamiento de datos y el análisis de incertidumbre para evaluar cómo los errores de medición afectan la precisión de los patrones de los flujos inferidos. Una dificultad particular encontrada fue que algunos de los piezómetros tenían filtraciones, lo que requería mediciones regulares de la conductividad eléctrica y la temperatura del agua dentro de los pozos para corregir los efectos de la densidad. Otras dificultades incluyen la falla de los transductores de presión, la deriva del reloj del registrador de datos y el error del operador. Los datos obtenidos fueron lo suficientemente precisos como para mostrar que hay un flujo horizontal neto de agua dulce hacia el mar en la parte superior del acuífero, y un flujo neto hacia la tierra de agua salada en la parte inferior. La dirección del flujo vertical alternó con la marea, pero debido a la gran incertidumbre de los gradientes de la carga hidráulica y los términos de densidad, no se pudo establecer un flujo neto con ningún grado de confianza. Si bien los problemas de medición se amplificaron bajo las condiciones imperantes en el sitio, errores similares pueden conducir a grandes incertidumbres en todas partes. La metodología descripta reconoce la incertidumbre inherente involucrada en la medición del flujo del agua subterránea. También puede ayudar a establecer los requisitos de precisión para la configuración experimental.

淡水和海水之间过渡带中的地下水流:基于野外的研究及测量误差的分析

摘要

由于可变密度效应和潮汐动力学,量化沿海淡水和海水过渡带附近的地下水流非常困难。收集了澳大利亚南部Adelaide市以南研究区沿垂直于海岸线断面的水头测量数据,以瞬时水流模式。本文详细论述了测量程序,数据后处理方法、不确定性分析,目的就是评价测量误差是怎样影响推测的水流模式精确度的。遇到的特别困难就是一些测压计有漏洞,迫使进行井内的电导率和水温度定期测量,针对密度效应进行纠正。其它困难包括压力传感器失效、数据记录仪移动以及操作者的误差。所获取的数据足够精确,足以显示出在含水层顶部存在着一个纯粹的向海水平淡水水流,在含水层下部存在着一个纯粹的向陆地的咸水水流。垂直水流随着潮汐变化改变方向,但是由于水头梯度和密度方面很大的不确定性,任何程度的信心都无法建立纯粹的水流。当测量问题在研究区普遍条件下被放大时,类似的误差可导致处处更大的不确定性。概述的方法认可测量地下水流中涉及到的固有不确定性。该方法也可支持建立实验机构的精确度标准。

Fluxo das águas subterrâneas na zona de transição entre agua doce e salgada: um estudo baseado em campo e análise de erros de medição

Resumo

A quantificação do fluxo das águas subterrâneas próximo a zonas de transição entre água doce e salgada na costa é difícil devido aos efeitos de densidade variável e as dinâmicas das marés. Medições de carga foram coletadas ao longo de um transecto perpendicular a costa em um local ao sul da cidade de Adelaide, no sul da Austrália, para determinar o padrão de fluxo transitório. Este artigo apresenta uma visão geral detalhada do procedimento de medição, métodos de pós-processamento dos dados e análise de incerteza, para avaliar como os erros de medição afetam a acurácia dos padrões de fluxo inferidos. Uma dificuldade particular encontrada foi de que alguns dos piezômetros estavam vazando, o que exigia medições regulares da condutividade elétrica e da temperatura da água dentro dos poços para corrigir os efeitos de densidade. Outras dificuldades incluem a falha nos transdutores de pressão, desvio do relógio do datalloger e erro do operador. Os dados obtidos foram suficientemente acurados para demonstrar que há fluxo horizontal liquido de água doce em direção ao mar na parte superior do aquífero e um fluxo líquido de água salgada em direção ao continente na parte inferior. A direção do fluxo vertical alternou com a maré, mas devido à grande incerteza dos gradientes de carga e dos termos de densidade, nenhum fluxo líquido poderia ser estabelecido com qualquer grau de confiança. Enquanto os problemas de medição foram amplificados nas condições prevalecentes no local, erros similares podem levar a grandes incertezas em todos os lugares. A metodologia descrita reconhece a incerteza inerente envolvida na medição do fluxo de águas subterrâneas. Ela também pode ajudar a estabelecer os requisitos de acurácia da configuração experimental.

Notes

Acknowledgements

Michael Teubner is thanked for his advice on the uncertainty analysis. Scott Prinos and an anonymous reviewer are thanked for their comments, which helped to improve the manuscript.

Funding Information

This study was supported by funding from the Australian National Collaborative Research Infrastructure Scheme.

Supplementary material

10040_2018_1725_MOESM1_ESM.mpg (2.3 mb)
ESM 1 (MPG 2376 kb)

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  • Vincent E. A. Post
    • 1
    • 2
  • Eddie Banks
    • 2
  • Miriam Brunke
    • 3
  1. 1.Federal Institute for Geosciences and Natural Resources (BGR)HannoverGermany
  2. 2.School of the EnvironmentFlinders UniversityAdelaideAustralia
  3. 3.AmsterdamThe Netherlands

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