surface, which reduces the thermal shock on receiving bodies of water. Additionally, increasing the amount of permeable surfaces by designing urban rain gardens reduces the amount of polluted stormwater that reaches natural bodies of water and recharges groundwater at a higher rate. Bioretention The concept of LID (low-impact design) for stormwater management is based on bioretention: a landscape and water design practice that utilizes the chemical, biological, and physical properties of soils, microorganisms, and plants to control the quality and quantity of water flow within a site. Bioretention facilities are primarily designed for water management, and can treat urban runoff, stormwater, groundwater, and in special cases, wastewater. Carefully designed constructed wetlands are necessary for the bioretention of sewage water or grey water, which have greater effects on human health than the implications of treating urban runoff and rainfall. Environmental benefits of bioretention sites include increased wildlife diversity and habitat production and minimized energy use and pollution. Prioritizing water management through natural bioretention sites eliminates the possibility of covering the land with impermeable surfaces. Water treatment process 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