Suitability of Fiberglass Wicks to Sample Colloids from Vadose Zone Pore Water

Fiberglass wicks are frequently used to sample pore water and determine water fluxes in soils. In this study we evaluated the performance of fiberglass wicks to sample colloids. Different colloids were used for the wick testing: feldspathoids, ferrihydrite, montmorillonite, kaolinite, and a mixture of mineral colloids extracted from a coarse calcareous sediment. The colloids were dispersed in either a buffered Na2CO3–NaHCO3 solution (ionic strength 6.7 mM, pH 10) or deionized water. Colloid breakthrough curves through 77‐cm‐long fiberglass wicks were determined for three different flow rates. Flow rate, pH, and colloid type affected colloid breakthrough. Colloid recovery in the effluent was higher at pH 10 than at pH 7, and increased with increasing flow rate. The mixture of mineral colloids extracted from sediment moved almost conservatively through the wicks; the colloid recoveries ranged from 88 to about 100% for pH 7 and 10, respectively. Ferrihydrite at pH 10 moved conservatively, with recoveries ranging from 95 to about 100%. All other colloids, however, showed lower mass recoveries. At pH 10, colloid recovery ranged from 55% for montmorillonite to about 100% for ferrihydrite and the mixture of mineral colloids, whereas at pH 7, the recovery ranged from <5% for kaolinite and ferrihydrite to approximately 100% for the mixture of mineral colloids. These results suggest that for certain conditions and colloid types, fiberglass wicks can be an acceptable tool for colloid sampling in the vadose zone. However, under many conditions studied here colloids were significantly retained inside the wicks, and consequently, the use of wicks for colloid sampling in the vadose zone must be considered with caution.