text: Stormwater management occurs on a watershed scale to prevent downstream impacts on urban water quality. A watershed is maintained through the cyclical accumulation, storage, and flow of groundwater. Naturally occurring watersheds are damaged when they are sealed by an impervious surface, which diverts pollutant-carrying stormwater runoff into streams. Urban watersheds are affected by greater quantities of pollutants due to the consequences of anthropogenic activities within urban environments. Rainfall on impermeable surfaces accumulates surface runoff containing oil, bacteria, and sediment that eventually makes its way to streams and groundwater. Stormwater control strategies such as infiltration gardens treat contaminated surface runoff and return processed water to the underlying soil, helping to restore the watershed system. The effectiveness of stormwater control systems is measured by the reduction of the amount of rainfall that becomes runoff (retention), and the lag time (rate of depletion) of the runoff. Even rain gardens with small capacities for daily infiltration can create a positive cumulative impact on mitigating urban runoff. Increasing the number of permeable surfaces by designing rain gardens reduces the amount of polluted stormwater that reaches natural bodies of water and recharges groundwater at a higher rate. Additionally, adding a rain garden to a site that experiences excessive rainwater runoff mitigates the water quantity load on public stormwater systems.
  The bioretention approach to water treatment, and specifically rain gardens in this context, is two-fold: to utilize the natural processes within landscapes and soils to transport, store, and filter stormwater before it becomes runoff, and to reduce the overall amount of impervious surface covering the ground that allow for contaminated urban runoff. Rain gardens perform most effectively when they interact with the greater system of stormwater control. This integrated approach to water treatment is called the "stormwater chain", which consists of all associated techniques to prevent surface run-off, retain run-off for infiltration or evaporation, detain run-off and release it at a predetermined rate, and convey rainfall from where it lands to detention or retention facilities. Rain gardens have many reverberating effects on the greater hydrological system. In a bioretention system such as a rain garden, water filters through layers of soil and vegetation media, which treat the water before it enters the groundwater system or an underdrain. Any remaining runoff from a rain garden will have a lower temperature than runoff from an impervious 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.