NDLI: Controls on the chemical composition of groundwater from alluvial aquifers in the Wanaka and Wakatipu basins, Central Otago, New Zealand

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Controls on the chemical composition of groundwater from alluvial aquifers in the Wanaka and Wakatipu basins, Central Otago, New Zealand

Content Provider	Springer Nature Link
Author	Rosen, Michael Jones, Sarah
Copyright Year	1998
Abstract	Groundwater in alluvial aquifers of the Wakatipu and Wanaka basins, Central Otago, New Zealand, has a composition expressed in equivalent units of Ca$^{2+}$≫Mg$^{2+}$≅Na$^{+}$>K$^{+}$ for cations, and HCO$_{3}$ $^{–}$≫SO$_{4}$ $^{2-}$>NO$_{3}$ $^{–}$≅Cl$^{–}$ for anions. Ca$^{2+}$ and HCO$_{3}$ $^{–}$ occur on a 1 : 1 equivalent basis and account for >80% of the ions in solution. However, some groundwater has increased proportions of Na$^{+}$ and SO$_{4}$ $^{2-}$, reflecting a different source for this water.The rock material of the alluvial aquifers of both basins is derived from the erosion and weathering of metamorphic Otago Schist (grey and green schists). Calcite is an accessory mineral in both the grey and green schists at <5% of the rock. Geological mapping of both basins indicates that dissolution of calcite from the schist is the only likely mechanism for producing groundwater with such a constant composition dominated by Ca$^{2+}$ and HCO$_{3}$ $^{–}$ on a 1 : 1 equivalent basis. Groundwater with higher proportions of Na$^{+}$ and SO$_{4}$ $^{2-}$ occurs near areas where the schist crops out at the surface, and this groundwater represents deeper and possibly older water derived from basement fluids. Anomalously high K$^{+}$ in the Wakatipu basin and high NO$_{3}$ $^{–}$ concentrations in the Wanaka basin cannot be accounted for by interaction with basement lithologies, and these concentrations probably represent the influence of anthropogenic sources on groundwater composition. Dans les aquifères alluviaux des bassins de Wakatipu et de Wanaka (Otago central, Nouvelle-Zélande), la composition chimique des eaux souterraines peut être résumée ainsi, en équivalents: Ca$^{2+}$≫Mg$^{2+}$≅Na$^{+}$ >K$^{+}$ pour les cations, et HCO$_{3}$ $^{–}$≫SO$_{4}$ $^{2-}$>NO$_{3}$ $^{–}$ et Cl$^{–}$ pour les anions. En équivalents, le rapport entre Ca$^{2+}$ et HCO$_{3}$ $^{–}$ est de l'ordre de 1; ces ions représentent plus de 80% des ions présents dans la solution. Cependant, une partie de ces eaux souterraines possède des teneurs plus élevées en Na$^{+}$ et SO$_{4}$ $^{2-}$, ce qui traduit une origine différente.Les matériaux constituant les aquifères alluviaux de ces deux bassins proviennent de l'érosion et de l'altération des schistes métamorphiques d'Otago (schistes gris et verts). La calcite est un minéral accessoire présent pour moins de 5% aussi bien dans les schistes gris que dans les verts. La cartographie géologique des deux bassins montre que la dissolution de la calcite des schistes est le seul mécanisme possible pour donner à l'eau souterraine une composition chimique aussi constante et caractérisée par un rapport 1 entre Ca$^{2+}$ et HCO$_{3}$ $^{–}$. L'eau souterraine possédant les plus fortes teneurs en Na$^{+}$ et en SO$_{4}$ $^{2-}$ est présente près des zones où affleurent les schistes; cette eau souterraine est une eau d'origine plus profonde et probablement plus ancienne, provenant du substratum. Les concentrations anormalement fortes en K$^{+}$ dans le bassin de Wakatipu et les concentrations élevées en NO$_{3}$ $^{–}$ dans celui de Wanaka ne peuvent pas être attribuées à une interaction avec les roches du substratum; de telles teneurs correspondent probablement à l'influence anthropique sur la composition de l'eau souterraine. Las aguas subterráneas en los acuíferos aluviales de las cuencas de Wakatipu y Wanaka, en Central Otago, Nueva Zelanda, tienen una composición química de manera que, expresándolos en términos de en equivalentes, los iones se pueden ordenar como Ca$^{2+}$≫Mg$^{2+}$≅Na$^{+}$>K$^{+ }$para los cationes, y HCO$_{3}$ $^{–}$≫SO$_{4}$ $^{2-}$>NO$_{3}$ $^{–}$≅Cl$^{–}$para los aniones. Ca$^{2+ }$y HCO$_{3}$ $^{–}$ presentan una equivalencia 1 : 1, y entre los dos suponen más del 80% de los iones en disolución. Sin embargo, algunas aguas subterráneas presentan cantidades mayores de Na$^{+}$ y SO$_{4}$ $^{2-}$, lo que refleja una procedencia diferente.El material rocoso de los acuíferos aluviales en ambas cuencas procede de la erosión y la alteración del Esquisto de Otago (esquistos grises y verdes). La calcita es un mineral presente en ambos esquistos, aunque con proporciones menores del 5%. Los mapas geológicos de ambas cuencas indican que la disolución de calcita procedente de los esquistos es el único mecanismo probable para producir unas aguas subterráneas con una composición tan constante de carbonato cálcico. Aguas subterráneas con mayores contenidos de Na$^{+}$ y SO$_{4}$ $^{2-}$ tienen lugar en zonas donde los esquistos afloran en superficie, por lo que estas aguas son más profundas y más viejas y provienen de circulación profunda, por la base del acuífero. Valores anormalmente altos de K$^{+}$ en la cuenca de Wakatipu y de NO$_{3}$ $^{–}$ en la de Wanaka no pueden justificarse por la interacción con los materiales de la base del acuífero, por lo que estas concentraciones están probablemente influenciadas por fuentes antrópicas.
Starting Page	264
Ending Page	281
Page Count	18
File Format	PDF
ISSN	14312174
Journal	Hydrogeology Journal
Volume Number	6
Issue Number	2
e-ISSN	14350157
Language	English
Publisher	Springer-Verlag
Publisher Date	1998-08-17
Publisher Institution	International Association of Hydrogeologists
Publisher Place	Berlin, Heidelberg
Access Restriction	One Nation One Subscription (ONOS)
Content Type	Text
Resource Type	Article
Subject	Earth and Planetary Sciences Water Science and Technology

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