USING IMPELLERS TO DISTRIBUTE LOCAL NUTRIENT LOADINGS IN A STRATIFIED LAKE: LAKE COMO, ITALY

The feasibility of using a vertically downward pointing impeller to remove polluted water from a coastal margin was investigated with a pilot experiment in Lake Como, Italy. The vertical jet induced by the impeller entrained surface water downward to depth, where upon reaching neutral buoyancy it intruded horizontally into the lake proper. A three-dimensional numerical hydrodynamic model was validated with results from an intensive eight-day field study during which rhodamine WT dye was introduced in the lee of the impeller and then tracked as it was transported into the lake. One of the major sources of pollution to the lake is the Cosia River underflow that drains a catchment adjacent to the City of Como. The numerical model, once validated, was used to show that six impellers, with the correct thrust, could be used to reduce the pollutants adjacent to the City of Como by half within two weeks; this was achieved by injecting the surface water to a depth that would intensify the Cosia River intrusion.