Natural water softening processes by waterfall effects in karst areas

The reduction of water hardness, which occurs at waterfalls on rivers in ka rst areas, is considered to be a result of the waterfall effects. These con sist of aeration, jet-flow and low-pressure effects. Waterfall effects brin g about two physical changes in river water: an increase in the air-water i nterface and turbulence. A series of experiments was designed and implement ed in order to investigate whether these effects and associated physical ch anges may cause a reduction of water hardness. From an experiment involving the enlargement of interface area, the plot of air-water interface areas a gainst conductivity revealed that the higher the air-water interface, the m ore rapidly conductance declines (and Ca2+ is precipitated). A bubble produ cer was designed and used to simulate bubbles that are produced by aeration and low-pressure effects and a faster decline of water hardness was observ ed at the location with bubbles in this experiment. When a supersaturated s olution was passed through a jet-stream producer, a rapid reduction of wate r hardness and an increase of pH appeared. Field measurements were used to support the laboratory experiments. Work on the Ya He River and at the Dish uiyan Waterfalls revealed that places with aeration had the quickest hardne ss reduction and the highest average rate of calcite deposition.