Sens communPollution of rainwater. If rainwater is naturally polluted (drops of water can only reach a sufficient size to fall to the ground when there are solid particles in the atmosphere to initiate the nucleation process, part of the urban air pollutants is therefore driven to the ground during rainy periods), the pollutant concentrations are however extremely low, and in most situations, rainwater is of drinking water quality when it reaches the ground. The most common limiting factor is pH (acid rain), but this acidity is very quickly buffered by the materials on which water flows or passes through.
according to Greater Lyon Urban Community
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