But, workers blame it on 'composite' particles, rather than effective (in-situ/natural) or absolute (ultimate/chemically dispersed) particles. Composite particle is any suspended particulate matter that comprises of two or more particles irrespective of its mode of formation. The attached articles may help to understand the subject better...
Pl see also: http://link.springer.com/article/10.1007/BF02406845
Since water pollutants are chemical in nature and very minute too, their transfer through sediments will have a lot to do with porosity and drainage. Silt has a porosity of about 25% while clay has a porosity of about 30%, slightly more porous than silt. The higher the porosity the higher its water retention ability and the lower its drainage. It follows that silt has a higher drainage ability compared to clay and is likely to transfer water pollutants more. As to which one of them being more important, it depends on your point of view. From my point of view, both are important since clay retains more pollutants while silt drains more pollutants.