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Ionic-Equilibrium Time inside Ceramic Cups in Unsaturated Porous Media

Sciences du Sol et des Substrats, ENSH-ENITHP, 2 rue Le Nôtre, 49045 Angers, Cedex 01, France

*Corresponding author (riga{at}angers.inra.fr).

ABSTRACT

In order to improve the methodology of collecting soil solutions with porous ceramic cups without suction, we developed a method to estimate an ionic-equilibrium time (t_C/C) inside cups first filled with distilled water. Characterization of t_C/C was used to select the minimum interval time between two extractions with different water contents () or water pressures (). The method is based on an analytical solution of the general equation of cylindrical diffusion. The model is used for approximating the molecular diffusion coefficient (D) vs. or and predicting the evolution toward ionic equilibrium inside the cups. It was satisfactorily validated, using KNO₃ as a solute, with a fine sand, a silty clay soil, and a peat-perlite mixture. The results showed that t_C/C is (i) dependent on molecular diffusion, which has a particular relationship with for each medium, and (ii) proportional to the squared radius of the cup. The best results obtained near saturation showed that the times necessary to obtain a concentration close to equilibrium with standard cups (22.6-mm diam.) were 4 d with the peat-perlite mixture and 2 wk for the sand and the silty clay soil. Furthermore, t_C/C can take up to 1 mo when diffusivity in sand decreases drastically with low water content. The length of time needed to reach equilibration limits the usefulness of this sampling method for conditions where changes in solute concentration with time are weak, such as diffusion phenomena with slow or nonexistent displacement of solute. Nevertheless, the equilibration time may be reduced to 30 h by using smaller cups (9.6-mm diam.) with high-water-content media.

Received for publication September 20, 1996.

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