Comparing Unsaturated Colloid Transport through Columns with Differing Sampling Systems

Abstract

Methodological difficulties of colloid sampling in the vadose zone are limiting our knowledge regarding the relevance of colloid transport for groundwater contamination. We compared the colloid sampling efficiency of five different lysimeters in a column experiment (9 cm length, 8 cm diameter) using 59Fe‐labeled goethite: polyester membranes with a pore diameter of (i) 1.2 μm and (ii) 10 μm, (iii) porous glass plates with 16‐μm pore diameter, (iv) wick samplers, and (v) zero‐tension lysimeters. Four replications of each lysimeter type were tested with concentrations of 0.1 and 10 mg L−1 goethite. The irrigation rate was 58 mm h−1, which caused an average transport velocity of 240 mm h−1 Compared with NO3−, a tendency of accelerated transport of goethite in the sand columns was observed. The mean recovery of 59Fe for all lysimeters was 30.7 ± 6.7% for the small and 3.4 ± 3.5% for the large colloid input concentration. For the small goethite concentration, no differences between lysimeter systems were detected. In contrast, the lysimeters performed differently at large concentrations: zero‐tension and 10‐μm membrane lysimeters showed the largest (9.1 and 6.8%), wick lysimeters the smallest colloid recovery (0.7%), which was related to trapping of colloids in the wick. We conclude that membranes of 10‐μm pore size and zero‐tension lysimeters are superior for colloid sampling, but the results of the latter may be biased toward an overestimation of colloid transport because of water saturation at the lysimeter–soil interface.