Collaborative Research: Managing Oxygen Demand in Lakes and Reservoirs - A Competition Between Natural and Artificial Forcing
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Abstract 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.