Perhaps the most common cause of water color is the presence minerals. Red and
brown colors are due to iron; black to manganese or organic matter; and yellow to
dissolved organic matter such as tannins.
Iron and manganese are common, at least in small amounts, in most rocks and
sediments. In groundwater that contains abundant dissolved oxygen, iron and
manganese form solid mineral phases and cannot be dissolved to any extent. In some
groundwater, however, there is a limited amount or no oxygen present. Under that
condition, iron and manganese dissolve in the water. When this water is pumped from
the well, it encounters oxygen from the atmosphere, and these elements precipitate as
very small particles of iron hydroxide and/or manganese hydroxide minerals.
The yellow color associated with natural dissolved organic matter (e.g., tannins) may
result when rainwater or runoff leaches this organic matter from leaves, roots, and other
vegetative matter, and flushes it down to the aquifer supplying a well. The coloration
both from iron and manganese and from dissolved organic carbon may occur
seasonally. Insoluble oxidized iron (rust) can give water a red tint, manganese oxide
causes a black discoloration, and a combination of the two can yield a yellow-brown hue.