Tannins are generally found in surface water supplies such as dug wells, rivers and lakes.
Tannins in well waters forms from humic acid, which is one of the major components of humic substances (or Natural Organic Mater (NOM). These are the dark brown constituents of soil organic matter. They can also be found in peat, coal, many upland streams and ocean water. The term is widely applied to refer to brown colour in water, caused by decaying organic substances.
Tannins can be removed by self-cleaning automatic tannins filters. These filters remove tannin by ion-exchange, using anion exchange media. The units we use regenerate with rock salt (sodium chloride) in the same way water softeners function. Frequently we see shallow wells under the influence of surface run-off water, achieve high levels of tannins (turning the water brown) during heavy rainy periods. Tannin filters are often an excellent relatively low-cost technology to use for this type of problem, if the actual cause of the problem (such as sealing off the well from the surface) cannot be achieved.