Not all retention or detention ponds are created equal. Some retention ponds are much more effective at catching pollutants, removing or using up those pollutants, and discharging cleaner water than other ponds. There are some things you can do to ensure your retention or detention pond is functioning efficiently, though.
Planning - Planning the proper size and location is the first step to creating a functional retention pond. If you have an existing pond, this step may not be of much use to you since you already have the pond. The first thing to remember is water runs down hill. As simple as that is, it can often be overlooked. A retention or detention pond needs to be created in an area that will allow it to catch as many of the runoff and pollutants as possible. The area needs to be low lying and the surrounding areas need to be sloped toward it so runoff from parking lots, road ways, etc. can drain into the pond. It is also crucial to make the pond the proper size and depth to accommodate the runoff water and allow the pollutants to settle out.
Aquatic Plants - Aquatic plants can add to the functionality, beauty, and diversity of your retention or detention pond. Emergent vegetation is great at slowing runoff before it enters the pond. Slowing the runoff allows suspended solids, sediments, and trash or debris to get blocked in the vegetation before it even reaches the pond. The trash can then be picked up and the rest will settle into the soil. Emergent vegetation also serves as a food source and hiding places for various wildlife species. Floating and submerged aquatic vegetation (and emergent vegetation, for that matter) serve to use up the nutrients that are added to the water from runoff. They serve to filter the nutrients out of the water. They also can serve as a food source and hiding places for fish and wildlife. All forms of aquatic vegetation can add to the beauty of the pond and its efficiency, but it is important to have a balance and plant species that will be of the most benefit. It is important to talk to a local lake management or nursery professional to get info on the best types of aquatic vegetation for your pond and its purpose.
Bacteria - Beneficial bacteria can be added to a retention pond to assist in decomposing and breaking down organic materials such as petroleum products and dead plants, fish, etc. The bacteria will serve to assist in the overall water quality by reducing the pollutants that are added to the pond and also help with the existing decomposition. The added plants that were discussed above will need to be decomposed after they die and the added bacteria will speed up the decomposition process and help ensure those nutrients don't get back into the water system. This step is another important one to consult a professional before you start.
Aeration - Oxygen is the key ingredient in any healthy marine environment. Dissolved oxygen in the water is essential for aquatic life, such as fish, insects, bacteria, aquatic plants, etc. Without proper levels of oxygen, a pond will not be able to support life and serve as a healthy ecosystem. This is even more important when discussing a retention pond. As discussed, retention ponds are collecting runoff pollutants and debris from the surrounding areas, holding that extra water, cleaning the water, and releasing back into the ground, streams, wetlands, etc. Proper aeration is vital for the retention pond to settle, filter, and use up the pollutants that enter during runoff.
Oxygen is essential for a retention pond to be productive. The pollutants that enter the pond, as discussed, either settle out or get used up in biological processes. The presence of oxygen helps oxidize certain elements that are suspended in the water. The oxidation process causes some materials to become heavier and they will settle out of the water column quicker.
The dissolved oxygen is also needed for decomposition of organic matter. The bacteria that are in the pond or that are added can decompose material aerobically (with oxygen) or anaerobically (without oxygen). Aerobic decomposition is a better process in your pond because it is quicker and produces less harmful byproducts. Without proper aeration, the bacteria will not be able to decompose the organic matter in the pond or that that is washed into it quickly or efficiently.
Agitation at the surface that is caused by an aeration device is also beneficial because it helps vent certain gases and elements from the water. Carbon dioxide is produced during aerobic decomposition and the large amounts of the gas can be detrimental to the pond and its inhabitants. By splashing the water and exposing a larger surface area to the air, the higher concentration of carbon dioxide in the water can vent into the atmosphere. Other volatile pollutants can evaporate into the atmosphere when the water is exposed to the air.