Tannin is vegetation decay which will turn the water a yellowish color. It is not harmful to humans.

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What is Tannin in Water?

Have you ever turned on your bathtub, and the water is yellow?  It looks gross.  Your first thought is your water is rusty, but is it?  Could it be tannin in the water?  Tannins are not typical in our area, but there are specific pockets where tannins have been found.