The invention is directed to a water purification system to remove pollutants from surface run-off water that has flowed into storm water drainage channels. A tank having an open reservoir region is positioned in the drainage channel or adjacent to the channel so as to permit water to flow through an upstream port of the tank and pool within the open reservoir region. The tank has a downstream port through which water pooled within the reservoir region may flow if the level of the pooled water sufficiently increases as a result of a rise in the flow level of the water in the drainage channel. The open reservoir region contains bioremediation plants having the physical property of removing pollutants from the pooled run-off water contained in the reservoir region.