When rainwater ought to be filtered through a layer of humus but the filtration area is limited, (as with soakaway ditches, infiltration ponds, grass pavers etc.) activated carbon sheets can be added between the surface layer containing the humus and the soil below, thus taking on the function of a last chance filter.
When this filter combination is employed, it is normally the case that the bulk of contaminants are absorbed or broken down in the humus layer because at normal temperatures the microorganisms found in the humus can decompose the contaminants. In cases of low temperature, a surge in pollutant levels, presence of desiccation cracks or in groundwater protection zones, the activated carbon mats then serve as a safeguard against contaminants. This system of using the mats solely as a backup prolongs their service life considerably.
With low-level pollution one layer of mats is sufficient to serve as an effective
last chance filter; two to three layers are recommended for heavy-duty use.