Would like to know why TDS of harvested rooftop rainwater differs with the different roofing materials

Hello,

I have studied the effect of roofing material on quality of harvested rooftop rainwater.

RCC flat roof of residential building, AC sheet, GI sheet and fibre sheet which are sloping roofs and tiled roof were chosen for study.

My observation is that Total dissolved solids (TDS) found in harvested rooftop rainwater is not same for different roofing materials. Why it is like so?

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