Bioswales work to remove pollutants through vegetation and the soil. As the storm water runoff flows through the bioswale, the pollutants are captured and settled by the leaves and stems of the plants. The pollutants then enter the soil where they decompose or can be broken down by bacteria in healthy soil.
There are several classes of water pollutants that may be collected or arrested with bioswales. These fall into the categories of silt, inorganic contaminants, organic chemicals and pathogens.
* Silt. How bioswales and plants are constructed slow the conveyance of silt and reduce the turbidity of receiving waters. Filters can be established to capture debris and silt during the process.
* Organics. Many organic contaminants including Polycyclic aromatic hydrocarbons will volatilize or degrade over time and Bioswales slow the conveyance of these materials into waterways, and before they can affect aquatic life. Although not all organic material will be captured, the concentration of organic material is greatly reduced by bioswales.
* Pathogens are deprived of a host or from a nutrient supply long enough for them to become the target of a heterotroph.
* Common inorganic compounds are macronutrients such as phosphates and nitrates.  Principal sources of these nutrients comes from agricultural runoff attributed to excess fertilization. Excess phosphates and nitrates can cause eutrophication in disposal zones and receiving waters. Specific bioswale plants absorb these excess nutrients.
* Metallic compounds such as mercury, lead, chromium, cadmium and other heavy metals are concentrated in the structures. Unfortunately, these metals slowly poison the surrounding soil.   Regular soil removal is required in order to prevent metals from dissolving and releasing back into the environment. Some bioswales are designed to include hyperaccumulator plant species. These plants absorb but do not transform the metals. Cuttings from these plants often decompose back into the pond or are pruned by gardening services that do not know the compost they are collecting is poisonous.