Depletion of oxygen in eutrophic, stratified waterbodies is a significant global problem, which negatively affects drinking-water treatment and cold-water fisheries. Mitigation is increasingly accomplished using oxygenation with bubble plumes. While bubble plumes are successful at adding oxygen, the added energy may induce large-scale mixing, which alters the thermal structure of the reservoir, increases sediment oxygen demand, and changes other sediment-water biogeochemical fluxes (phosphorus, iron, manganese, hydrogen sulfide, and methane). Recent studies of hypolimnetic oxygen demand have shown that oxygen is correlated with the gas flow rate in aerated lakes and that diffusion across the sediment-water interface is greatly enhanced by turbulent episodes generated by seiche currents. Yet, no models exist to predict the currents induced by bubble plumes in crossflow or to relate the turbulent diffusion at the sediment-water interface to the bulk fluid velocity above the benthic boundary layer. Because both of these tools are needed to develop comprehensive models of oxygen dynamics in oxygenated or aerated reservoirs, the purpose of this project is to elucidate the physical mechanisms by which currents resulting from both natural forcing (e.g., inflows and seiches) and artificial forcing (e.g., bubble plumes) affect oxygen demand in lakes and reservoirs. This goal will be realized through laboratory experiments and field measurements in three different waterbodies. Dye visualization and particle image velocimetry will be used to map the intrusion dynamics for bubble plumes in crossflow. Field experiments will document the intrusion formation and water column dynamics using profiles from conductivity, temperature, and depth (CTD) probes and acoustic Doppler current profilers (ADCP); microstructure at the sediment-water interface will be measured using acoustic Doppler velocimetry (ADV) and temperature and oxygen microsensors. The PIs will collaborate with a multinational, interdisciplinary team of colleagues from Spain and Switzerland and will pursue three primary objectives: (1) to develop a comprehensive near-field model for plume mixing in stratification and crossflow based on the double-plume integral model approach, (2) to formulate models for oxygen flux across hypolimnetic interfaces due to currents and turbulent mixing by adapting models from film renewal theory, and (3) to integrate the plume and oxygen demand models with a 3D hydrodynamic model, validated using the complete laboratory and field-scale data sets. The primary intellectual merit will be development of the first scientifically rigorous model for bubble plumes in variable crossflow that includes oxygen transfer between bubbles and water and the formulation of mechanistic models for the flux of oxygen at the sediment-water interface based on ambient currents and turbulence. As an integral part of the research, field experiments will be conducted in three morphologically different lakes, providing a rich archive of data characterizing the intrusion formation from different bubble plumes, the bulk oxygen demand in the hypolimnion, and the benthic mixing. Models developed for these three components (near-field plume mixing, thermocline mixing, and benthic boundary layer oxygen flux) close the gap necessary to develop a comprehensive numerical lake model capable of predicting oxygen dynamics in the hypolimnion of lakes and reservoirs. With several multi-million dollar bubble-plume diffuser installations being considered in the United States, the availability of the coupled 3D lake model will be valuable during design. The primary broader impact of the proposed activities will be the completed numerical lake model, which will be capable of simulating a wide range of lake oxygen dynamics. The project leverages the expertise and resources of a multinational, interdisciplinary team of researchers who are leaders in limnology and lake and reservoir management. The results of this research will be disseminated to researchers and managers in an international forum through the International Water Association Specialist Group on Lake and Reservoir Management chaired by the co-PI. As an integral part of the proposed activities, an innovative program will be developed to mentor graduate students as they develop skills to manage large projects and supervise undergraduates.

Project Report

Low dissolved oxygen in the bottom waters of lakes and reservoirs can lead to degraded water quality, negatively affecting fish habitat and drinking water treatment processes. Bubble plumes are increasingly used to add oxygen to the bottom waters, and there are now many examples of successful installations in both drinking water and hydropower reservoirs. While bubble plumes are effective at adding oxygen, they also add energy which can generate additional turbulence and influence large-scale currents, increasing the consumption of oxygen by a factor of 5 or more. Given that current methods for designing bubble plume installations cannot reliably predict the changes in turbulence, currents, and increase in oxygen consumption, there remains a need to improve our understanding of how bubble plumes influence these factors. If we can better understand the influence of bubble plumes on a lake or reservoir, we may be able to better predict the impacts of a proposed bubble plume installation and optimize the design. The purpose of this project was to develop a comprehensive model of mixing and oxygen dynamics in oxygenated lakes and reservoirs, accounting for the effect of both natural and plume-induced turbulence and currents on oxygen flux into the sediment. During the course of this project, three intensive field sampling campaigns were conducted, collecting high resolution measurements of water velocity, turbulence, sediment oxygen flux, and dissolved oxygen. Two campaigns were performed on Carvins Cove Reservoir (2011 & 2013), a drinking water reservoir in Virginia, USA, and one campaign on Lake Hallwil (2012), a large lake in Switzerland. Both lakes have bubble plume systems, but differ in size, bubble plume system type, and the strength of natural currents, providing a wide range of conditions to collect detailed observations. The large datasets from these field campaigns were synthesized to determine an appropriate equation which can relate the amount of turbulence near the sediment to the rate of oxygen flux into the sediment. These equations have been incorporated into a 3D lake model of both Carvins Cove Reservoir and Lake Hallwil. This 3D model is able to predict the naturally-occuring currents and turbulence, and has also been previously coupled to a 1D model which simulates the effects of the bubble plume. Most importantly, since the model now includes equations which relate turbulence to sediment oxygen flux, we can now predict the impact of the bubble plume on oxygen concentrations, turbulence, and currents.

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