水污染物的总量控制是建立在天然水体对污染物具有稀释扩散和自净作用理论上，即天然水体对污染物具有一定环境容量的基础上。
Total emission control of water pollutants is based on natural water dilution of pollutants and autopurification theory, in another word, natural water has some environmental capacity on pollutants.
因为污染物除随河水流动方向流动以外，还会向河水流动不同方向扩散。污染物也可能由于水体的自净作用而在流动过程中逐渐消失。
Due to the diffusional effect, the pollutant will flow not only towards the down-stream, but also in other directions, including the direction against the flow stream.
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