Tannins are formed upon the decomposition of vegetation, often found in shallow wells and surface water. These compounds are large molecular weight organics that vary in different water supplies. Tannins can cause a yellow to brown (sometimes pinkish) color in the water and may also impart a bitter taste and odor.
If tannins are found to be present it can indicate that your water is significantly influenced by surface water. This means that if tannins can travel easily from surface water to your groundwater source, pathogens can easily travel through as well. Therefore, when testing for tannins, it is also recommended to test for bacteria.
Tannins are difficult to remove from water. One treatment method may be effective in one area, but may be totally ineffective 10 miles down the road. It is all dependent on the vegetation in a given area. It is usually recommended that the water be treated by a softener prior to the tannin removal unit.