Tannins are dissolved, which is why straining does nothing.
What it is
Tannins are large organic molecules leached out of decaying vegetation, peat, and marshy ground near a well. They are dissolved rather than suspended, so they are measured as color or as total organic carbon rather than as particles. Shallow wells and wells near wetlands usually carry more of them.
What it usually does
They tint water anywhere from straw to strong tea, give a faint earthy or musty taste, and gray out white laundry over time. Because they are organic and dissolved, they also consume chlorine, which usually means a disinfection system has to be sized larger than the water alone would suggest.
What it does not prove
Color is not a measure of anything harmful. Tannins are mostly an aesthetic problem, and a tea color does not establish that bacteria or metals are present. It does usually indicate surface water reaching the well, though, which is a reason to look at the well construction rather than only at treatment.
When to test
Usually worth measuring before sizing tannin resin, and usually worth running alongside a bacteria test, since water showing surface influence is water worth checking for other surface arrivals. Let a sample stand overnight first: color that settles out was particles, not tannins.
See what this usually means for your water.
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