Hydrogeology Journal

, Volume 15, Issue 2, pp 281–296 | Cite as

A radioisotope tracer investigation to determine the direction of groundwater movement adjacent to a tidal creek during spring and neap tides

Paper

Abstract

A radioisotope tracer (82Br) was injected into a sand aquifer adjacent to a tidal creek at Hat Head, New South Wales, Australia. The injection was timed to coincide with the falling limb of a spring tide in August 2001 and was repeated in July 2002 during a period of neap tides. The tracer movement was detected using gamma logging and fixed gamma detectors in a bore 0.9 m from an injection bore and in a line approximately perpendicular to the creek. Movement of the tracer was detected by comparing measured gamma activity with calibrated activities determined under laboratory conditions. Net movement of the tracer indicated approximately 0.001 m/day laterally towards the creek and 0.07 m/day vertically upward during spring tide conditions. This pattern is reversed during neap tide conditions with little net vertical movement but horizontal movement of 0.15 m/day. The measurements indicated an oscillatory motion coinciding with the tide but lagging the tide by approximately 4.5–6.5 h. The vertical flow during spring tides indicates that vertical movement of the saline interface occurs and results in mixing of different water types beneath the banks of the creek.

Keywords

Coastal aquifers Radioisotope tracer Br-82 Australia Hydrochemistry 

Résumé

Un traceur isotopique radioactif (82Br) a été injecté dans un aquifère sableux bordant une crique soumise à l’influence de la marée, à Hat Head (Nouvelles-Galles du Sud, Australie). L’injection fut programmée afin de coïncider avec la phase descendante d’une marée de vive-eau en août 2001, puis répétée en juillet 2002 en période de marée de morte-eau. Les mouvements du traceur étaient suivis à la fois par la réalisation de diagraphies d’activité gamma et par des détecteurs gamma fixes, dans un piézomètre situé à 0.9 m d’un forage d’injection et sur une ligne approximativement perpendiculaire à la crique. Ces mouvements étaient détectés par comparaison entre les activités gamma mesurées et les activités calibrées déterminées en laboratoire. Le mouvement net du traceur en conditions de vives-eaux indique une vitesse latérale de l’ordre de 0.001 m/jour en direction de la crique, et une vitesse verticale ascendante de 0.07 m/jour. Cette tendance s’inverse en conditions de mortes-eaux, avec un faible mouvement vertical, mais un mouvement horizontal de 0.15 m/jour. Les mesures mettent en évidence un mouvement oscillatoire coïncidant avec la marée, avec toutefois un décalage de 4.5 à 6.5 heures environ. Le flux vertical observé au cours des marées de vives-eaux indique l’existence d’un mouvement vertical de l’interface eau douce / eau salée, qui génère un mélange des différents types d’eau sous les rives de la crique.

Resumen

Un radioisótopo trazador (82Br), se inyectó en un acuífero de arena, adyacente a un canal de marea en Hat Head, New South Wales, Australia. La inyección fue sincronizada para coincidir con la parte descendente de una marea viva en agosto de 2001 y se repitió en julio de 2002, durante un periodo de mareas muertas. El movimiento del trazador fue detectado mediante el uso de registros gamma y detectores gamma estáticos, en una perforación a 0.9 m de una perforación de inyección y en una línea aproximadamente perpendicular al canal. El movimiento del trazador se detectó, comparando la actividad gamma medida con las actividades calibradas determinadas bajo condiciones de laboratorio. El movimiento neto del trazador indicó aproximadamente 0.001 m/dia lateralmente hacia el canal y 0.07 m/dia verticalmente ascendente durante las condiciones de mareas vivas. Este modelo se invierte durante las condiciones de marea muerta, con pequeño movimiento vertical neto, pero con un movimiento horizontal de 0.15 m/d. Las mediciones indicaron un movimiento oscilatorio coincidiendo con el tipo de marea, pero retrasándose a la marea por aproximadamente 4.5–6.5 horas. El flujo vertical durante las mareas vivas, indica que el movimiento vertical de la interfase salina ocurre y produce mezclas de tipos de agua diferentes, bajo los bancos del canal.

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Copyright information

© Springer-Verlag 2006

Authors and Affiliations

  1. 1.Water Research Laboratory, School of Civil and Environmental EngineeringUniversity of New South WalesSydneyAustralia
  2. 2.Australian Nuclear Science and Technology OrganisationMenaiAustralia

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