Fig. 9. Results of the KCl advection experiment for the Darcy configuration (reservoir levels at 20 cm, hydraulic gradient 7.72%). (a) Upstream reservoir conductivity, exponentially decreasing due to advection of salt to the sand and dilution with fluid from the recharge reservoir; (b) downstream reservoir conductivity; (c) electric potential difference between the z = 11 cm electrodes and the reference (see Fig. 3). All electric potential differences decrease as soon as salt is added, and become positive after the salt front passes the electrode. The vertical lines correspond to the injection time (I), the theoretical travel times between the upstream reservoir and the electrodes located at 5, 12, 19, and 26 cm from the upstream reservoir (5, 12, 19, 26), and the theoretical travel time between the upstream and downstream reservoir (A). (d) Corresponding local electric potential differences (i.e., between adjacent electrodes).