Circulation is one way to oxygenate a lake, but here we consider methods that do not destratify the lake. These are often called hypolimnetic oxygenation, referring to the bottom layer of a stratified lake as the target of the action. In some cases it is important to maintain stratification, either to support a coldwater fishery or to keep separate water layers for some supply function. There are four basic ways to add oxygen to deeper waters without causing mixing and a lot of information to gather and consider when designing an appropriate oxygenation system. The Practical Guide to Lake Management in Massachusetts, available online from the DCR within the Mass.gov website, has a useful review of this technique, and a 2015 publication available from the Water Research Foundation has considerable additional detail.
Hypolimnetic oxygenation is a technique for increasing deep water oxygen and managing algae through control of nutrient levels. The central process is the introduction of more oxygen, intended to limit internal recycling of phosphorus, thereby controlling algae. The extra oxygen also reduces the accumulation of other undesirable substances, such as iron, manganese, ammonia and hydrogen sulfide, which makes this process attractive for water supplies. The four general approaches include hypolimnetic aeration chambers, speece cones, diffused oxygen, and sidestream supersaturation.
Hypolimnetic aeration chambers use an air compressor as with whole lake circulation, but in this case the upward plume is contained in a chamber to avoid mixing with the surface waters; water is pulled in, oxygenated by air contact, and returned to near where it came from initially. Use of air can be effective but is not efficient; transfer is slow and the vertical distance of contact rarely allows more than 30% of the oxygen in air (which is only 21% oxygen to begin with) to be transferred. A lot of water has to be moved through the chamber to reach common oxygen goals, and the cost of power to do that can be substantial.
Speece cones are chambers where deep water is pumped in along with pure oxygen, and the rate of input of each is supposed to balance, such that all the oxygen is dissolved in the water. The oxygenated water is then put back in the deeper part of the lake. Any excess oxygen rises to the surface or is trapped in the chamber; at best this is a waste, and at worst it could cause unintended mixing. Installation of Speece cones in a lake can be challenging, and maintenance can be tedious. This approach can be effective and efficient, but tends to be hard to operate well.
Diffused oxygen is a simple process of releasing small bubbles of pure oxygen into the water column at a deep enough depth to allow complete absorption before the bubbles can rise enough to cause destratification. This generally requires about 20 feet of bubble travel distance, so this approach may not be applicable to lakes with only a thin layer of bottom water. In earlier years, the delivery mechanism had not been perfected, but these days adding diffused oxygen is fairly straightforward and reliable. Liquid oxygen is allowed to convert to gas and moves through delivery lines under the pressure of the gas, resulting in very low power use that offsets the cost of the pure oxygen.
Sidestream supersaturation involves removing water from the targeted deeper water layer, oxygenating it in chambers on land, and putting it back in the deep zone. Speece cones are sometimes used, but rather than placing them in the lake, they are on land and used to supersaturate withdrawn water. As long as the temperature of the withdrawn water has not been increased (which can be a risk when supersaturating), the oxygenated water moves mostly laterally in the target area. This allows thinner layers of bottom water to be oxygenated than would be possible with diffused oxygen. This approach is still being improved, but shows great promise.
Proper application requires the following information:
- An accurate nutrient budget with a detailed analysis of internal sources of phosphorus
- The oxygen demand that must be met by the system; calculations and related interpretation for design purposes are best performed by experienced professionals
- Lake morphometry and stratification data to facilitate choice of system and features for maximum effectiveness
- Location and details of any land based facilties and the power source
Factors favoring the use of this technique include:
- A substantial portion of the P load is associated with anoxic sediment sources within the lake
- Studies have demonstrated the impact of internal loading on the lake.
- External P load has been controlled to the maximum practical extent or is documented to be small; historic loading may have been much greater than current loading
- Hypolimnetic or sediment oxygen demand is high (>500 mg/m2/day)
- In addition to phosphorus management, control of other reduced compounds such as hydrogen sulfide, ammonia, manganese and iron, is desired
- Adequate phosphorus inactivators are present for reaction upon addition of oxygen
- Shoreline space for a compressor, pump and/or oxygen storage is available where access is sufficient, power is available, and noise impacts will be small
- The lake is bowl shaped, or at least not highly irregular in bathymetry (few separate basins and isolated coves)
- Long-term application of the technique is accepted
- Coldwater fishery habitat is abundant or an important goal
Hypolimnetic oxygenation is fairly common in drinking water reservoirs but less often found in recreational lakes, as ongoing management and related costs can be substantial. Permits are required, and potential applicants should consult with their local and state environmental agencies.
Four methods of hypolimnetic oxygenation