Lake Management Techniques – Nutrient Control
By David Ernes – Lake Management Committee
In a previous article, we discussed the use of algaecides to control algae blooms. One other option is to control the nutrients, primarily phosphorous, the major food source for the algae. One source of these nutrients in the lake is the legacy sediment, which has accumulated over the years. During rain events, high wind conditions, and other activities, the sediment in the shallow regions of the lake can be stirred up and release nutrients. There is little that can be done to control the weather conditions, but we can obey the no-wake buoys where excess speed can stir up this sediment. Dredging is one option being evaluated that can remove this shallow sediment, albeit at a rather significant cost.
In-lake sediment is also found in the deeper areas of the lake where it is difficult to remove. As we know, below about 12-15 feet, the lake is anoxic, meaning the oxygen level is very low. This allows the phosphorous in the sediment to be released into the water, which can mix with the lake during the spring and fall turnovers, or other intense mixing conditions. Two methods available to control this source of phosphorous are aeration and alum treatment.
Alum, or aluminum sulfate, can be applied to a lake, where it is converted to aluminum hydroxide, an active ingredient in several over-the-counter antacids. This forms a floc, which traps phosphorous in the water, tying it up and ‘dragging’ it to the bottom sediment layer. There, it forms a “crust”, trapping the phosphorous in the sediment and preventing it from being released in the water. As the floc sinks, it removes dissolved phosphorus and also traps sediment fines, helping to clarify the water. The effectiveness of this application can range from a few years up to fifteen or twenty years, depending on a number of factors. While there are some potential issues surrounding the use of alum in lakes, modern companies ensure that the application poses minimal risks.
A full inactivation treatment described above can be quite expensive, but another option is to use a ‘stripping’ dose of alum. This is a lower dose and traps the phosphorous in the lake as well as the fine sediment but does not fully cap the sediment in the lake bottom. Its longevity depends on the rate at which new phosphorous enters the lake.
Therefore, application of alum is just one of multiple methods that can be used to control the nutrients and ultimately algal blooms. Each of these methods must be evaluated in terms of effectiveness, longevity, safety and of course cost. If you have any questions concerning the various options being discussed in these articles, contact us at email@example.com. In the meantime, remember to…
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