Environmental Aspects of Artificial Aeration and Oxygenation of Reservoirs: A Review of Theory, Techniques, and Experiences
Thomas Ginn, Marc Lorenzen, Robert Pastorok
Artificial circulation and hypolimnetic aeration have been used successfully in management of eutrophic reservoirs to alleviate water quality problems, control algal blooms, and improve fish habitat. This report includes: 1) a comprehensive review of aeration/circulation techniques and past experiences encompassing literature from January 1972 through December 1982; 2) statistical analysis of artifical circulation experiences to examine the causes of alternative responses to treatment; 3) a summary of morphometric and water quality data for 107 reservoirs managed by the U.S. Army Corps of Engineers; and 4) a generic evaluation procedure for alternative management applications in reservoirs.
Artificial destratification by mechanically pumping or diffuse-air mixing usually elevates dissolved oxygen content of the lake by bringing anoxic bottom waters to the lake surface where aeration occurs though contact with the atmosphere. Oxygenation may cause precipitation of phosphate compounds and inhibition of nutrient release from sediments, but invasion of benthic macroinvertebrates into the profundal zone may play a role in maintenance of high phosphorus release rates from oxygenated surficial sediments. Water quality generally improves after treatment, but undersizing of water pumps or improper timing of destratification relative to occurrence of algal blooms can aggravate existing oxygen deficits
When the mixed depth is increased, models of algal production predict a decline in the ratio of photosynthetic rate to respiration rate and a consequent decline in algal biomass per unit area of lake surface. With sufficient mixing, a drop in pH of the upper waters is observed, followed by a shift from nuisance blue-green algae to a mixed assemblage of green algal species. Zooplankton and benthic macroinvertebrates often increase during artifical circulation as a result of habitat expansion and possible enhancement of food resources. Although short-term increases in fish growth and yield have been attributed to improvement of food and habitat, long-term observations are unavailable.
Hypolimnetic aeration improves water quality without disrupting thermal stratification. Although the potential benefits of hypolimnetic treatment in controlling algal blooms are more limited than those realized with whole lake mixing, the risk of adverse impacts appears to be lower for hypolimnetic aeration. Oxygenation of downstream reaches can be achieved by hypolimnetic aeration or oxygenation, localized mixing, aeration in the outlet works, and tailwaters aeration.
An annotated biobliography of aeration/circulation experiences is included as an appendix to this report.