A highly efficient impeller apparatus for mixing concentrated oxygen in a liquid-filled pond, pool or lagoon. The downward and outward direction of liquid flow, coupled with optimally small bubbles produced through injection holes in tubes mounted on the impeller blades, optimizes the absorption of oxygen by the liquid. A rigid shroud surrounding the impeller constrains the liquid motion to axial, rather than radial, flow to ensure efficient mixing and minimize the electrical load requirements. The impeller drive may powered by a solar panel and battery combination, or by electrical utility service, or by a combination thereof.