Tannin staining in ground water
Here we can see water from a shallow bore just north of Mandurah Western Australia. The client was keen to fit a StainStopper to stop the water staining. A quick test with Aquatek Liquid shows about half the colour is from dissolved iron, the rest sadly is from tannins. The water fresh from the bore has a dark colour to it as you can see in the right hand side sample. After several days there was still nothing dropping out of solution.
A good source of information on tannin is from Water &Wastes Digest.
Tannins are large molecular weight organic compounds. They are formed through the decomposition of plants and, to a much lesser degree, animals. These organics are generally found in surface waters or shallow wells. There are two primary categories in which tannin fall: Humic acid and Fulvic acid. These are simplified structures of very large molecules. Gallic acid, guaiacyl, syringyl and cresylic moieties are the building blocks of these organic molecules. The structure of the tannin varies from location to location, depending on the vegetation in the area. Tannins can cause a yellow to dark tea color in water and may impart taste and odor. Tannins are not directly regulated by a governmental agency, however there is a secondary drinking water standard of 15 alpha units for color. A secondary drinking water standard is not federally enforced. Tannins are not a health issue; however, they are aesthetically displeasing.
As many of you know first hand, tannins can be 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 dependent on the vegetation in a given area. Styrene-based macroporous anion resin has long been used to remove tannins from water. This type of resin worked well in some areas but poorly in other areas. More recently acrylic-based resin have emerged on the market and are producing excellent results in most cases. They can be manufactured as a gel or with macroporosity. There are also low cross linked/high water retention styrene-based resins that are being used successfully.