Percolation trenches are often used to treat runoff from impervious surfaces, such as sidewalks and parking lots, on sites where there is limited space available for managing stormwater. They are effective at treating stormwater only if the soil has sufficient porosity.  To function properly, a trench must be designed with a pretreatment structure such as a grass channel or swale, in order to capture sediment and avoid clogging the trench. It may not be appropriate for sites where there is a possibility of groundwater contamination, or where there is soil with a high clay content that could clog the trench.