Bioretention controls the stormwater quantity through interception, infiltration, evaporation, and transpiration. First, rainfall is captured by plant tissue (leaves and stems) and in the soil micropores. Then, water performs infiltration - the downward movement of water through soil - and is stored in the soil until the substrate reaches its moisture capacity, when it begins to pool at the top of the bioretention feature. The pooled water and water from plant and soil surfaces is then evaporated into the atmosphere. Optimal design of bioretention sites aim for shallow pooled water to reach a higher rate of evaporation. Water also evaporates through the leaves of the plants in the feature and back to the atmosphere, which is a process known as evapotranspiration.
Stormwater quality can be controlled by bioretention through settling, filtration, assimilation, adsorption, degradation, and decomposition. When water pools on top of a bioretention feature, suspended solids and large particles will settle out. Dust particles, soil particles, and other small debris are filtered out of the water as it moves downward through the soil and interspersed plant roots. Plants take up some of the nutrients for use in their growth processes, or for mineral storage. Dissolved chemical substances from the water also bind to the surfaces of plant roots, soil particles, and other organic matter in the substrate and are rendered ineffective. Soil microorganisms break down remaining chemicals and small organic matter and effectively decompose the pollutants into a saturated soil matter.
Even though natural water purification is based on the design of planted areas, the key components of bioremediation are the soil quality and microorganism activity. These features are supported by plants, which create secondary pore space to increase soil permeability, prevent soil compaction through complex root structure growth, provide habitats for the microorganisms on the surfaces of their roots, and transport oxygen to the soil.