Tannins are plant-based polyphenols—think of the same compounds that give red wine its astringent kick and dark tea its rich color. When rainwater filters through decomposing vegetation, it picks up these molecules like a sponge picking up pigment.
This is especially common in well water from shallow sources, near swamps, or in regions with heavy forest coverage. If you’ve ever noticed your water turning darker after a rainstorm, tannins are likely to blame.
Fun fact: tannins are also why some old wooden barrels—like those used for whiskey—impart color and flavor. So if your water tastes a bit like a forest floor, you’re basically drinking terroir.
