Pollution of rainwater is caused by atmospheric pollution (such as emissions from industries and transport) as well as pollutants that are taken up during surface runoff. Among other things paper, oil, remainders of metals and rubbers, organic matter from vegetation and excrements can accumulate in the surface runoff. The main sources of anthropogenic pollution of rainwater is traffic and waste. The constituents vary in their nature and concentration according to local conditions.

Atmospheric rainwater is usually very pure and most contamination of the water occurs after contact with the catchment system. Rainwater from ground catchment systems is not recommended for drinking unless it is first boiled or treated. For the pretreatment of rainwater physical (such as sedimentation and filtration) and biological processes can be used. Biological treatment often occurs in biologically active soil filters or root zones. Please refer to the respective sections of this course for further information about physical and biological treatment processes.