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TECHNICAL NOTES

Design and Testing of a Drop Counter for Use in Vadose Zone Water Samplers

^a Soil and Water Management, Katholieke Univ. Leuven, Celestijnenlaan 200E, B-3001 Heverlee, Belgium
^b Agrosphere, ICG-IV, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany
^c current address: Laborelec, Rodestraat 125, 1630 Linkebeek, Belgium
^* Corresponding author (mertensja{at}yahoo.co.nz).

Received 18 May 2007.

Measuring leachate mobility in the vadose zone is necessary to understand the processes controlling groundwater and river contamination, and requires recording leachate volumes with time. Many field studies manually measure the volume of leachate with a daily, weekly, or even biweekly resolution; however, measurement of leachate at higher temporal resolution is needed for the calibration of solute transport models and is useful in identifying the contribution of preferential flow to solute transport. We designed a simple, robust, and low-cost drop counter for measuring leachate volumes at a high temporal resolution. Laboratory experiments showed a nearly perfect linear relationship between the applied flux and the number of drops per unit time, and indicate an average drop size of 35 µL. Due to manual manufacturing, variability between drop counters is not negligible and a one-point calibration is necessary. The drop counter consists of an upper (guiding) tube from which a drop forms and a lower (capillary) tube that "sucks" the drop down before it falls. The presence of the lower tube makes the counter less vulnerable to temperature and ionic strength effects of the leachate. Counting of the drops can be easily achieved using any type of datalogger capable of logging electrical pulses.