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Hydrogeology Journal

, Volume 27, Issue 1, pp 409–421 | Cite as

Why mixed groundwater at the outlet of open flowing wells in unconfined-aquifer basins can represent deep groundwater: implications for sampling in long-screen wells

  • Zhi-Yuan Zhang
  • Xiao-Wei JiangEmail author
  • Xu-Sheng Wang
  • Li Wan
  • Jun-Zhi Wang
Paper
  • 57 Downloads

Abstract

Groundwater sampled at the outlets of uncased flowing wells in a thick unconfined aquifer, which has undergone mixing, has been found to have hydrochemistry similar to deep groundwater in discharge areas. To identify the hydrodynamic causes, transient models of groundwater flow and age in a three-dimensional homogeneous unit basin with flowing wells are constructed to obtain flow rates in wells and groundwater mean age around wells. Inflow of groundwater to the well in the deep part leads to mixing of groundwater from different sources, and the finally mixed groundwater is found to have the same age as groundwater in the aquifer at a specific depth, termed the equivalent position (EP). EP is always found in the lower half of the flowing well, indicating that a mixed sample at the outlet could represent deep groundwater. Outflow from the well to the unconfined aquifer in the shallow part results in aging of groundwater around the well. For fully penetrating flowing wells in confined aquifers, EP is found in the upper half of the aquifer. The different relative depths of EP to the screen interval in the two types of flowing wells are mainly due to the profiles of horizontal velocity in the inflow segment, which is basically uniform in a confined aquifer but increases from zero to a maximum value in unconfined aquifers. Thus, groundwater at the outlets of topography-controlled flowing wells is a window of the deep part of a basin, and existing long-screen wells could have the potential for groundwater sampling.

Keywords

Discharge area Sample representativeness Groundwater age Groundwater hydraulics Intraborehole flow 

Pourquoi les eaux souterraines mixtes à la sortie de puits jaillissants dans des bassins d’aquifères libres peuvent représenter des eaux souterraines profondes: implications pour l’échantillonnage dans des forages crépinés

Résumé

Le prélèvement d’échantillons d’eaux souterraines à l’exutoire de forages jaillissants en trou nu dans d’épais aquifères libres, qui ont subi un mélange, a mis en évidence une hydrochimie similaire aux eaux souterraines profondes, dans les zones d’émergence. Pour identifier les mécanismes hydrodynamiques, des modèles en régime transitoire des écoulements des eaux souterraines et de l’âge ont été élaborés dans une unité de bassin homogène, en 3D, incluant les puits jaillissants, afin d’obtenir les débits aux forages et l’âge moyen des eaux souterraines à proximité des ouvrages. L’afflux d’eaux souterraines dans les forages, dans leur partie profonde, conduit à un mélange d’eaux souterraines de différentes origines, et le mélange final des eaux souterraines présente un âge identique à celui des eaux souterraines de l’aquifère à une profondeur spécifique, nommée position équivalente (PE). La PE est toujours rencontrée dans la moitié inférieure du forage jaillissant, indiquant qu’un échantillon prélevé à l’exutoire peut représenter les eaux souterraines profondes. Le débit sortant de forages atteignant l’aquifère à nappe libre dans sa partie peu profonde conduit au vieillissement des eaux souterraines autour de l’ouvrage. Pour des forages artésiens complets au sein d’aquifères captifs, la PE est rencontrée dans la moitié supérieure de l’aquifère. Les différentes profondeurs relatives de PE à l’intervalle de la crépine dans les deux types de puits jaillissants sont principalement dues aux profils de vitesse horizontale au droit du segment d’afflux, qui est essentiellement uniforme dans un aquifère captif, mais augmente de 0 à une valeur maximale dans des aquifères libres. Ainsi, l’eau souterraine à l’exutoire des forages jaillissants, contrôlés par la topographie, est une fenêtre sur la partie profonde du bassin, et les forages crépinés existants présentent un bon potentiel pour l’échantillonnage des eaux souterraines.

Por qué aguas subterráneas mezcladas en la salida de pozos surgentes en cuencas de acuíferos no confinados pueden representar aguas subterráneas profundas: implicancias para el muestreo en pozos con filtros largos

Resumen

Se ha encontrado que el agua subterránea muestreada en las salidas de pozos surgentes no entubados en acuíferos espesos no confinados, que ha experimentado una mezcla, tiene una hidroquímica similar a las aguas subterráneas profundas en las áreas de descarga. Para identificar las causas hidrodinámicas, se construyeron modelos transitorios del flujo y de la edad del agua subterránea en una cuenca unitaria homogénea tridimensional para obtener los caudales en los pozos surgentes y la edad media del agua subterránea alrededor de los pozos. La entrada de agua subterránea al pozo en la parte profunda conduce a la mezcla de aguas subterráneas de diferentes fuentes, y finalmente el agua subterránea mezclada tiene la misma edad que el agua subterránea en el acuífero a una profundidad específica, denominada posición equivalente (EP). La EP siempre se encuentra en la mitad inferior del pozo surgente, lo que indica que una muestra mezclada en la salida podría representar aguas subterráneas profundas. El flujo de salida del pozo en un acuífero no confinado en la parte poco profunda da como resultado el envejecimiento del agua subterránea alrededor del pozo. Para pozos surgentes completamente en acuíferos confinados, la EP se encuentra en la mitad superior del acuífero. Las diferentes profundidades relativas de la EP en el intervalo de los filtros en los dos tipos de pozos surgentes se deben principalmente a los perfiles de velocidad horizontal en el segmento de entrada, que es básicamente uniforme en un acuífero confinado, pero aumenta de cero a un valor máximo en acuíferos no confinados. Por lo tanto, el agua subterránea en las salidas de los pozos surgentes controlados por la topografía es una ventana de la parte profunda de una cuenca, y los pozos existentes con filtros extensos tienen posibilidades para el muestreo de las aguas subterráneas.

为什么潜水含水层盆地内无套管自流井出口的混合地下水可以代表深层地下水:对长滤管水井采样的启示

摘要

发现在巨厚潜水含水层无套管自流井出口采集的地下水样虽然经历过混合作用, 但是与排泄区深层地下水具有类似的水化学特征。为了确定该现象的水动力原因,在发育有自流井的三维均质单元盆地建立了地下水流和年龄非稳定流模型,以获取自流井的流量和井周围的地下水平均年龄。在深部,地下水流入到井中导致不同水源的地下水混合,发现最终混合的地下水与含水层内特定深度的地下水具有相同的年龄,该位置称为等效位置。发现等效位置总是在自流井的下半部,表明出口处的水样可以代表深层地下水。从井中流入到潜水含水层浅部的水导致井周围的地下水年龄老化。对于承压含水层中的完整自流井,发现等效位置位于承压含水层的上部。两类含水层中自流井出口水样等效位置的差异 主要是由于流入段水平速度剖面决定的,水平速度在承压含水层中基本一致,但在潜水含水层中呈单调增加。因此,地形控制自流井出口地下水是盆地深部的一个窗口,现有的长滤管水井是可以用于采集地下水样品的。

Por que as águas subterrâneas misturadas na saída de poços de fluxo aberto em bacias aquíferas não confinadas podem representar águas subterrâneas profundas: implicações para a amostragem em poços de telas longas

Resumo

Observou-se que as águas subterrâneas amostradas nas saídas de poços sem revestimento em aquíferos espessos e não confinados, que foram submetidos à mistura, possuem uma hidroquímica semelhante à das águas subterrâneas profundas nas áreas de descarga. Para identificar as causas hidrodinâmicas, modelos transitórios de fluxo de águas subterrâneas e datação em uma bacia tridimensional de unidade homogênea com poços fluindo são construídos para obter taxas de fluxo em poços e idade média das águas subterrâneas ao redor dos poços. O fluxo das águas subterrâneas para o poço na parte profunda leva à mistura de águas subterrâneas de diferentes fontes, e a água subterrânea finalmente misturada tem a mesma idade que a água subterrânea no aquífero em uma profundidade específica, denominada posição equivalente (PE). A PE é sempre encontrada na metade inferior do poço fluindo, indicando que uma amostra mista na saída pode representar águas subterrâneas profundas. O escoamento do poço para o aquífero não confinado na parte rasa resulta no envelhecimento das águas subterrâneas ao redor do poço. Para poços fluindo totalmente penetrantes em aquíferos confinados, a PE é encontrada na metade superior do aquífero. As diferentes profundidades relativas da PE para o intervalo de filtro nos dois tipos de poços fluentes são devidas principalmente aos perfis de velocidade horizontal no segmento de influxo, que é basicamente uniforme em um aquífero confinado, mas aumenta de zero a um valor máximo em aquíferos não confinados.. Assim, a água subterrânea nas saídas de poços fluiindo controlados por topografia é uma janela da parte mais profunda de uma bacia, e os poços de filtro longo existentes têm o potencial de amostragem de água subterrânea.

Notes

Acknowledgements

The authors acknowledge W. P. Gardner and one anonymous reviewer, as well as the associate editor T. Cui, for their constructive suggestions.

Funding information

This study was supported by the National Natural Science Foundation of China (41522205), the National Program for Support of Top-notch Young Professionals, and the Foundation for the Author of National Excellent Doctoral Dissertation (201457).

Supplementary material

10040_2018_1842_MOESM1_ESM.pdf (446 kb)
ESM 1 (PDF 446 kb)

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Copyright information

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

Authors and Affiliations

  • Zhi-Yuan Zhang
    • 1
  • Xiao-Wei Jiang
    • 1
    Email author
  • Xu-Sheng Wang
    • 1
  • Li Wan
    • 1
  • Jun-Zhi Wang
    • 2
  1. 1.MOE (Ministry of Education) Key Laboratory of Groundwater Circulation and EvolutionBeijingChina
  2. 2.Yellow River Engineering Consulting Co., Ltd. (YREC)ZhengzhouChina

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