Natural water softening processes associated with waterfall effects in Karst areas

Abstract

The reduction of water hardness, which occurs at waterfalls on rivers in karst areas, is considered to be a result of the waterfall effects. These consist of aeration, jet-flow and low-pressure effects. Waterfall effects bring about two physical changes in river water: an increase in the air-water interface and turbulence. A series of experiments was designed and implemented in order to investigate whether these effects and associated physical changes may cause a reduction of water hardness. From an experiment involving the enlargement of interface area, the plot of air-water interface areas against conductivity revealed that the higher the air-water interface, the more rapidly conductance declines (and Ca2+ is precipitated). A bubble producer was designed and used to simulate bubbles that are produced by aeration and low-pressure effects and a faster decline of water hardness was observed at the location with bubbles in this experiment. When a supersaturated solution was passed through a jet-stream producer, a rapid reduction of water 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 Dishuiyan Water-falls revealed that places with aeration had the quickest hardness reduction and the highest average rate of calcite deposition.

在喀斯特地区,水流过瀑布后硬度会发生降低,这种现象一般称为瀑布效应,其中包括汽化,射流和低压作用。瀑布效应会引起两种河水物理性质的变化,即气-水界面面积和涡流的增加。为了弄清瀑布效应和这些物理性质变化是否会减少水的硬度,本研究设计并进行了一系列的试验。通过改变气-水界面面积试验,发现界面面积越大,导电率降低的速率越快,由此造成Ca2+ 的沉淀。利用自行设计的气泡生成器进行试验,发现有气泡发生的地方,水的硬度降低很快。试验还发现,从射流器喷出的超饱和溶液的硬度快速减小,而pH 值增大。鸭河和滴水岩瀑布的实际观测证实,有汽化现象发生的地方,水的硬度降低很快,钙沉淀的平均速率也最高。