This is due to the presence of tannin
, also known as fulvic or humic acid, a natural organic material that is created as water passes through peaty soil and decaying vegetation. This can cause water to have a faint yellow to tea-like color, and can cause yellow staining on fabrics, fixtures, china, and laundry. High levels of tannin can give water a tangy or tart aftertaste, as well as a musty or earthy odor. Tannins are more common in surface water supplies and shallow wells than in deep wells. Water in marshy, low-lying, or coastal areas are also more susceptible to tannins.