Retention ponds are developed to serve two functions, limit flooding and pollutant removal. Retention and detention ponds are often developed in areas with significant construction and non-porous areas. Buildings, roadways, parking lots, drive ways, etc. are all impermeable surfaces, or material water cannot soak into during rainfall. Since the rain cannot penetrate these surfaces, it has no choice but to runoff to areas of lower elevation. Therefore, the more buildings and pavement in a given area, the more chances of flooding because the rain cannot soak into the ground.
Retention and detention ponds are developed in these areas as both temporary and permanent runoff basins. The rain runoff from the impermeable surfaces will find its way to the retention and detention ponds directly or through storm sewers. The ponds are built to handle a quick influx of water and slowly release it into another pond, stream, wetland, or slowly back into the ground. Either way, the ponds catch the large amounts of runoff water that occurs quickly and slows its release. The lag time from input to output eliminates or minimizes the effects of large rains and flooding.
Retention and detention ponds also serve to remove pollutants and trash. Since retention and detention ponds are the drainage basin for an area, they are a magnet for items like garbage. Trash and debris are washed into these areas often after heavy rains or wind. Plastic bags and other waste also enters from storm sewers that typically drain into a retention or detention pond. The trash can create an unsightly pond and harm the fish or other inhabitants of the retention pond.
Retention and detention ponds also catch other pollutants from runoff such as petroleum products from roadways, fertilizers from lawns and fields, sediments, bacteria, suspended solids, and metals. These pollutants can have negative effects on the overall water quality such as pH, turbidity, nutrients, and hardness. The retention and detention ponds collect the runoff pollutants and allow them to settle out of the water and also get used up through biological processes.
When the pollutants enter the pond during a rain event, the pond slows the water movement, allowing the heavier pollutants such as suspended solids, sediments, and metals to settle out of the water column and come to rest in the bottom sediments. This greatly improves the overall clarity or turbidity of the water. Other pollutants such as the fertilizers are used in aquatic plant growth. Many of these nutrients are taken out of the water and used in plant growth. Bacteria can also be dealt with within the ponds biological processes. By retaining the water from runoff and allowing the pollutants to settle out or be used up, the water that is then returned to the ground or to neighboring streams, wetlands, etc. is of much better overall quality.