Earlier studies have shown that small constructed wetlands are more efficient as sedimentation basins for eroded soil material than expected from calculations based on detention time. It has been suggested that this is caused by sedimentation of aggregates. The present microscopic study of thin sections made from undisturbed samples of wetland sediments confirmed that the fine silt and clay fractions are present in aggregated form. Aggregates from the wetland sediments had the same mineralogical composition as those from the corresponding arable land, but were more rounded, indicating erosion during transport. To prevent breakdown of aggregates, wetlands should therefore be constructed as close to the source of erosion as possible. A correct prediction of particle retention in constructed wetlands has to take into account the presence of aggregates. Textural analysis methods, which require clay dispersion pre-treatment, are not suitable for the calculation of the retention of fine silt and clay
