Fig. 5. Depth profiles of colloids in sand, wick, and outflow (second run with large colloid concentrations). The accumulation of ⁵⁹Fe in the top 3 cm of the sand illustrates that unsaturated sand was a good filter for goethite colloids. Note the absence of colloids in the sand directly above the zero-tension lysimeter, which is probably related to saturated conditions above the lysimeter–soil interface. Colloids were trapped in the wick.