Hydrogeology Journal

, Volume 16, Issue 4, pp 629–639 | Cite as

In situ estimation of the effective chemical diffusion coefficient of a rock matrix in a fractured aquifer

  • Robel A. Gebrekristos
  • Allen M. Shapiro
  • Brent H. Usher
Paper

Abstract

An in situ method of estimating the effective diffusion coefficient for a chemical constituent that diffuses into the primary porosity of a rock is developed by abruptly changing the concentration of the dissolved constituent in a borehole in contact with the rock matrix and monitoring the time-varying concentration. The experiment was conducted in a borehole completed in mudstone on the campus of the University of the Free State in Bloemfontein, South Africa. Numerous tracer tests were conducted at this site, which left a residual concentration of sodium chloride in boreholes that diffused into the rock matrix over a period of years. Fresh water was introduced into a borehole in contact with the mudstone, and the time-varying increase of chloride was observed by monitoring the electrical conductivity (EC) at various depths in the borehole. Estimates of the effective diffusion coefficient were obtained by interpreting measurements of EC over 34 d. The effective diffusion coefficient at a depth of 36 m was approximately 7.8×10−6 m2/d, but was sensitive to the assumed matrix porosity. The formation factor and mass flux for the mudstone were also estimated from the experiment.

Keywords

Diffusion Fractured rocks Groundwater flow Matrix diffusion South Africa 

Résumé

Une méthode in situ d’estimation du coefficient effectif de diffusion pour un composé chimique qui diffuse dans la porosité primaire d’une roche est développée, en modifiant brusquement la concentration du composé dissous dans un forage en contact avec la matrice rocheuse et en suivant l’évolution de sa concentration dans le temps. L’expérimentation a été réalisée dans un forage équipé dans des argilites sur le campus de l’Université de l’Etat Libre de Bloemfontein (Afrique du Sud). Plusieurs essais de traçage ont été réalisés sur ce site, laissant des concentrations résiduelles en hypochlorite de sodium dans les ouvrages, qui ont diffusé dans la matrice rocheuse au fil des années. De l’eau douce a été injectée dans un forage en contact avec les argilites, et l’augmentation temporelle de la concentration en chlorures a été observée par des mesures étagées de conductivité. L’estimation du coefficient effectif de diffusion est issue de l’interprétation des mesures de conductivité sur une période de 34 jours. Le coefficient effectif de diffusion à 36 m de profondeur est estimé autour de 7.8×10−6 m2/j, mais il apparaît sensible à la porosité évaluée. Le facteur de formation et le flux de masse issu des argilites ont également été estimés à partir de ce test.

Resumen

Se ha desarrollado un método in situ para estimar el coeficiente de difusión efectivo para un constituyente químico que sufre difusión en la porosidad primaria de una roca mediante un cambio abrupto en la concentración del constituyente disuelto en un sondeo en contacto con la matriz de la roca y monitorizando la variación de la concentración en el tiempo. El experimento se desarrolló en un sondeo completo en arcillas en el campus de la Universidad de Free State en Bloemfontein, Sudáfrica. Se han llevado a cabo numerosos ensayos de trazadores en este punto, que ha dejado una concentración residual de cloruro sódico en sondeos que han sufrido difusión en la matriz de la roca durante un periodo de años. El agua dulce se introdujo en el sondeo en contacto con las arcillas, y se observó el incremento variable en el tiempo del cloruro mediante la monitorización de la Conductividad Eléctrica (EC) a varias profundidades en el sondeo. La estimación del coeficiente de difusión efectiva se obtuvo interpretando las medidas de EC durante 34 días. El coeficiente de difusión efectiva a una profundidad de 36 m fue aproximadamente de 7.8×10−6 m2/d, pero fue sensible a la porosidad de la matriz asumida. El factor de formación y el flujo de masa para las arcillas también se estimaron a partir del experimento.

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Copyright information

© Springer-Verlag 2007

Authors and Affiliations

  • Robel A. Gebrekristos
    • 1
    • 3
  • Allen M. Shapiro
    • 2
  • Brent H. Usher
    • 1
  1. 1.Institute for Groundwater StudiesUniversity of the Free StateBloemfonteinSouth Africa
  2. 2.US Geological SurveyMS 431 RestonUSA
  3. 3.Knight Piésold (Pty) LtdRivoniaSouth Africa

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