Healthy Neighborhood Ponds

Mulch, grass and other yard clippings; they release nutrients and consume oxygen from the water as they decompose.

Grit can have undissolved petroleum-based chemicals, and landscape and household chemicals attached to it. Grass or other rooted ground covers are very effective for trapping debris and sediment from stormwater runoff. They can be used in swales and ditches, and as buffers around grated storm inlets and around the edge of ponds.

Malfunctioning septic systems leaking under-treated wastewater can be a significant source of nutrients and bacteria that may leach into groundwater and nearby ponds.

Leaking, broken or overloaded sanitary sewer lines in an area served by a municipal sewer system can be sources of nutrients and bacteria if they are close enough to a pond. If you notice stormwater inlets with a pungent odor, or a roadside swale that never seems to dry up in the dry season (December-April), it's possibly due to a sanitary sewer line and should be reported to the operator of the sanitary sewer system.

The concentration of nutrients dissolved in water controls the amount of plant growth that takes place in the pond. In South Florida, ponds fill in with organic material grown in the pond itself rather than from sediment and debris carried into it. Algae are the most immediate to respond to any dissolved nutrients in the pond. Exploding algae growth, or "blooms" will make the water look murky green or brown, and will continue until the dissolved oxygen or limiting nutrient in the water is depleted. During this process fish can suffocate in large numbers ("fish kill") and algae die and drop to the bottom.

As aquatic plants complete their growth cycle and die, they sink to the pond bottom along with the algae and form layers of biomass. These layers of decomposing vegetation produce hydrogen sulfide gas that smells like rotten egg and causes chunks of biomass to float to the surface. People sometimes think they are seeing and smelling raw sewage in the pond.

In time, this biomass fills the pond and the pond becomes a marsh. Because of the tremendous amount of fertilizer used in urban landscapes, and our sandy soil, ponds in these settings usually experience very rapid plant and algae growth, thus speeding up the process of turning it into a marsh.

Not only does the pond become unpleasant to live next to as the years go by, it also loses its capacity to remove stormwater pollutants and control floodwaters during large rainfall events. Eventually, the pond must be dredged out to restore its functionality, a costly operation.

Preventive measures to keep pollutants from entering a pond is the most efficient way to preserve pond quality, but some amount of nutrients will find their to it. There are ways to enhance pond quality by treating the water in the pond.

Littoral zones are areas in a pond where nearly flat shelves are constructed to produce relatively shallow water that supports varieties of plants. They are usually formed around the edge of the pond and serve to stop debris from flushing into the pond, and to create habitat for wildlife. The plants remove some nutrients from the water, but should be mechanically removed from the pond before dying to prevent nutrient release and oxygen consumption during decomposition.

Aquatic plants also grow in deeper water, away from the littoral areas as stormwater ponds become shallow enough for sunlight to penetrate to a depth sufficient for plants to root and grow from the pond bottom. Others float on the surface, unsecured to the pond bottom. These plants improve water quality as they draw nutrients from the water, thus reducing algae production and improving water clarity. They also produce oxygen and are a food source for some fish. Given enough nutrients dissolved in the pond water however, many species of these plants will overgrow, blocking water flow and reducing the capacity of the pond for storing floodwater. Like littoral plants, they should be mechanically removed from the pond before dying to prevent nutrient release and oxygen consumption during decomposition. An aquatic plant maintenance plan that only involves controlling the vegetation with herbicides is not a strategy for success. Only mechanical removal of vegetation will give the short term benefits of improved water quality and aesthetics, and the long term benefits of a pond that does not turn into a swamp that must be dredged out. Aquatic plant control with herbicides is a cheaper short term solution, but does not give quality results or long term savings.

Triploid grass carp are large consumers of aquatic plants and are sometimes introduced to control the overgrowth of vegetation. Ideally, this completes the natural cycle by removing aquatic plants after nutrient uptake, oxygenation and water clarification afforded by the plants. Unfortunately, algae is not part of the diet.  

Fountains provide some near-surface water circulation and help oxygenate pond water. Higher oxygen levels promote biologic growth and aerobic decomposition. Reservoir circulators provide large volume water circulation and mixing from all pond depths, increasing oxygen levels and pH levels throughout the water column. They have been used successfully for many years in industrial and domestic wastewater treatment ponds for improving water quality. There are solar powered reservoir circulators that operate independently once deployed. More recently they are proving useful in stormwater treatment ponds due to increased environmental regulations for stormwater quality.

Recirculating irrigation systems draw water from the stormwater pond instead of from a well or municipal water supply. Municipal water supplies are too valuable to be used for irrigation and should be avoided if possible (with exception of reclaimed wastewater). Well water can have high concentrations of dissolved calcium, magnesium, sodium and iron. Dissolved iron concentrations over about 1 part per million, in combination with the scale forming minerals of calcium and magnesium, form brown stains on vegetation and building surfaces. They can inhibit plant growth if concentrations are high enough. Scale can form on leaf surfaces and soil pH can rise to undesirable levels. Well water has very little dissolved oxygen, which is beneficial for plant growth.

Pond water on the other hand has relatively low pH, higher concentration of dissolved oxygen and low levels of dissolved calcium, magnesium and iron. In most cases, urban pond water has elevated levels of nutrients that will be recycled onto the landscape where it is beneficial. In addition, pumping water from the pond induces some circulation, which is good for pond water quality.