Preface: advances in hypolimnetic oxygenation

Abstract

Wagner KJ. 2019. Preface: advances in hypolimnetic oxygenation. Lake Reserv Manage. 35:225–228.

Hypolimnetic oxygenation systems (HOS, here taken to mean techniques that add oxygen to the bottom layer of a lake or reservoir without disrupting stratification) have been in use for over half a century. However, issues of theoretical understanding, equipment, implementation, monitoring, and cost have limited widespread application of what is a potentially very effective means of improving water quality. Advances of the last two decades have pushed HOS to the forefront of water supply management, with some application in recreational lakes as well. Key advances include demonstration of long-term improvement of water quality, including higher concentrations of oxygen and lower concentrations of reduced compounds and algae, better predictability of results, including greater understanding of oxygen demand and its expression, and characterization of capital and operational costs, including likely changes over time as oxygen demand is satisfied. The application of additional management techniques in many cases limits our ability to ascribe all positive results to HOS alone, but the value of HOS as a lake management tool has been clearly demonstrated. More applications with careful monitoring and documentation of results are expected to advance this approach further.

Keywords:

• Diffused oxygen
• hypolimnetic oxygenation
• linear diffuser
• supersaturation chamber

Oxygenation of the hypolimnion of a waterbody to enhance water quality is not a new approach (Cooke et al. 2005, Wagner 2015); oxygenation has been attempted for at least 50 years, and Lorenzen and Fast (1977) describe many different hypolimnetic oxygenation system (HOS) designs. These systems inject air or oxygen either into the hypolimnion or into water withdrawn from it and returned to it. The value of increased oxygen concentrations is well known in the water industry (Wagner 2015). Adequate oxygen prevents the accumulation of many undesirable constituents, including iron, manganese, ammonia, and hydrogen sulfide. Compounds responsible for taste and odor are minimized. Having at least 2 mg/L of oxygen is considered appropriate to maximizing water quality and reducing treatment needs for potable water supply.

Dissolved oxygen is critical to habitat value for a wide range of aquatic organisms (Cooke et al. 2005). Concentrations of at least 4 mg/L are often incorporated into water quality standards at state and provincial levels. Of particular concern is the release of phosphorus from sediments exposed to anoxia and related low redox potential. Internal phosphorus loading has become recognized as a major source of phosphorus to lakes and a primary factor in cyanobacteria blooms (Douglas et al. 2016, Wagner et al. 2017). The relationship between sediment iron dissolution under anoxic conditions and phosphorus release has long been recognized (Mortimer 1941, 1942). More recently, sediment iron release has also been implicated as a factor in cyanobacteria blooms (Molot et al. 2014). Considerable effort has been expended trying to curtail watershed inputs of phosphorus to lakes, with limited success (Osgood 2017). Within lakes, inactivation of phosphorus with compounds that continue to hold phosphorus in the sediment has been used with substantial success (Wagner 2017), but keeping oxygen high enough to avoid phosphorus releases from surficial sediment may provide greater benefits (i.e., the water quality and habitat benefits already noted).

Hypolimnetic oxygenation, here intended to mean oxygen addition to the bottom layer of a stratified lake without disrupting stratification, has a solid theoretical base but has experienced many problems of implementation and documentation (Wagner 2015). Yet over the last two decades considerable experience has been gained and the use of HOS is becoming a more mainstream lake management technique. This issue of Lake and Reservoir Management is devoted to the documentation of that experience.

How oxygen is delivered

There are 3 major ways to get oxygen into the hypolimnion without disrupting stratification (Wagner 2015):

1. Hypolimnetic aeration chambers in which air is introduced, allowing for low-efficiency transfer of oxygen to water that is then released back into the hypolimnion.

2. Pure oxygen diffusers that release small oxygen bubbles that are absorbed into the hypolimnion before they reach the thermocline if the vertical distance is adequate.

3. Supersaturation chambers into which hypolimnetic water is drawn and oxygenated and then distributed within the hypolimnion; the chamber can be in the lake or on shore.

Each of these approaches has advantages and disadvantages. The amount of oxygen supplied and its distribution are both critical features that affect success (Wagner 2015). With increasing power costs hypolimnetic aeration chambers have been less frequently installed in recent years; they can be effective but are not efficient. Where the hypolimnion is at least 6 m thick, diffused pure oxygen represents an efficient approach where the lack of power cost offsets the cost of pure oxygen, but where the hypolimnion is thinner, there may be disruption of thermal stratification or early turnover. Supersaturation chamber systems are designed to have very high oxygen transfer efficiencies and can distribute the oxygenated water near the lake bottom where it is most needed to counter sediment oxygen demand. Shore-based supersaturation is not quite as efficient as in-lake deployment, and there have been issues with increased temperature of return water in the development of this technique. Effervescence in return water has occurred but is less prevalent with in-lake deployment. Maintenance of in-lake equipment can be challenging, however, and having equipment on shore is attractive. This issue of Lake and Reservoir Management focuses on advances in pure oxygen applications.

What we have learned

From a review of 30 varied HOS (Preece et al. 2019) and another review of 8 diffused oxygen HOS (Mobley et al. 2019) it has been documented that HOS increases oxygen in the hypolimnion in a predictable manner (i.e., good agreement between models and field data). Anoxia can be prevented, the accumulation of reduced compounds can be minimized, internal phosphorus loading can be curtailed, taste and odor problems can be nearly eliminated, and treatment costs for potable water can be reduced. Benefits are most evident when HOS is operated year-round or at least throughout the entire stratification period. Further, clear demonstrations of the benefits of HOS alone are rare; in most cases there are other management activities involved, confounding simple cause-and-effect conclusions (Preece et al. 2019). However, Horne (2019) provides a case where the only management application was an HOS that eliminated hydrogen sulfide production that appeared to have caused downstream fish mortality. After HOS operation, not only did the downstream fish situation improve significantly but fish in the reservoir were found at much greater depth, suggesting a major expansion of habitat. As exhibited in this issue, there is little doubt that a properly designed, installed, and operated HOS can greatly enhance hypolimnetic water quality.

Decreased iron and manganese concentrations are often the target of HOS in potable water supplies, and success has been documented in many cases (Austin et al. 2019, Mobley et al. 2019, Munger et al. 2019, Preece et al. 2019, Wagner 2015), but the importance of system hydrology has been emphasized (Munger et al. 2019). Preventing anoxia from occurring is essential to manganese control (Wagner 2015). System hydrology is also very important to phosphorus management. Internal phosphorus loading can be controlled to an acceptable degree with HOS (Preece et al. 2019, Horne and Beutel 2019), but in many applications the watershed is a major contributor of phosphorus and the reduced internal loading associated with HOS may not sufficiently depress phosphorus to prevent algae (especially cyanobacteria) blooms.

One of the problems that has plagued HOS has been induced oxygen demand (Wagner 2015, Gantzer et al. 2019). The addition of air, oxygen, or oxygenated water creates currents that have been found to increase oxygen demand. That is, the process by which oxygen demand is countered seems to create additional demand such that oxygen mass input has had to be raised by as much as 5 times the measured oxygen demand prior to HOS operation. This increased demand is especially evident when an HOS is turned off during stratification (Mobley et al. 2019, Horne et al. 2019). Gantzer et al. (2019) explain that this is not so much an induced demand as satisfaction of pent-up demand that could not be expressed at lower oxygen availability. As that pent-up demand is satisfied, the long-term oxygen demand declines (Gantzer et al. 2019, Horne et al. 2019) and less oxygen is needed through the HOS to maintain desirable conditions.

HOS can also reduce oxygen demand over time by oxidizing organic matter settling to the bottom before it reaches the bottom or an anoxic zone (Gantzer et al. 2019). HOS can break the self-sustaining cycle of low oxygen, leading to increased productivity through internal phosphorus loading, which then increases oxygen demand as algae die and settle to the bottom.

Not all HOS can address oxygen demand equally, however. Austin et al. (2019) provide strong evidence that while a hypolimnetic aeration chamber improved conditions over the pretreatment condition, a diffused pure oxygen system provided much better conditions with approximately the same amount of oxygen input. The difference was attributed to the greater currents created by air addition, which increased oxygen demand more than the air-induced input could address. The pure oxygen system appears to have limited the expression of additional existing oxygen demand but may not have satisfied it over the period of data collection. Longer term studies suggest that pure oxygen HOS will eventually result in lower oxygen demand (Gantzer et al. 2019, Horne et al. 2019).

With more and more HOS in application, a better understanding of costs is developing. Wagner (2015) and Mobley et al. (2019) find a large range of actual capital and operational costs, but a narrower range when put on a volumetric scale. That is, the cost per unit of hypolimnion volume oxygenated is reasonably predictable given local site features and scale factors. Operational costs vary less than capital costs, and oxygen generators will result in lower costs than trucking in oxygen to be stored in tanks until used (Gantzer et al. 2019). However, site factors (e.g., getting power to the site) and institutional constraints (e.g., oxygen delivery may be easier than operating an oxygen generator) often favor tanked storage of liquid oxygen.

The future of oxygenation

The history of HOS spans more than any one individual’s career, but use of HOS is still not a mature science. The articles in this issue of Lake and Reservoir Management demonstrate advances that have made HOS better understood, more successful, and more economical. Yet there is more to learn. Material science has enhanced pure oxygen delivery by providing materials that do not degrade rapidly, but better materials may yet be possible. On-site oxygen generation could be improved to further decrease costs. Smaller scale systems are now on the market that may make HOS accessible for recreational lakes and other waterbodies not used for potable supply and therefore without ratepayers or other steady sources of funding. Pure oxygen diffuser systems seem to be the most efficient and effective way to oxygenate a thick hypolimnion, but application to thin hypolimnia may promote destratification. The ability of supersaturation chamber systems to put a layer of oxygenated water where it is most needed has great potential that has not yet been fully tapped.

While considerable experience is embodied in the articles in this issue, more experience and documentation are needed to fine-tune applications, improve predictability, and demonstrate consistent success. Melding of applications and research is essential to further advancement. Nearly all the examples provided in the articles in this issue were derived from a teaming of academic programs with a practical application. Monitoring is essential, preferably on a continual basis, and the lack of it has led to disastrous consequences (Horne 2019) and limited advancement to date. Water-supply utilities and recreational lake groups with adequate funding should look to research institutions or consultants with a research interest to participate in HOS programs for best results and further advancement of technology and oxygenation approaches.

Acknowledgments

David Austin, Paul Gantzer, Alex Horne, Chris Holdren and Ellen Preece helped improve this preface by providing insightful comments on earlier drafts. The other authors of the articles in this issue of Lake and Reservoir Management and the many NALMS members who discuss oxygenation at each annual symposium are also acknowledged for their contributions to the field of hypolimnetic oxygenation.