What causes tannins in well water?

Tannins are caused by vegetation decaying and leaching into the water supply. To know if you have tannins or if it is insoluble iron run the well for a little while and fill up a white cup, if the yellowish color never disappears then you probably have tannins in your water. You are more likely to have tannins if you are located near a body of water or in a forest area. Hope this helps.