Iron Precipitates in a Seep, White Mountain Wilderness, New Mexico
Some springs and seeps have rust colored deposits, why?
Water seeping into the ground holds a small amount of dissolved oxygen. As the water seeps through the ground (groundwater) dissolved and buried organic matter slowly decompose and remove all the oxygen. At other locations oxidation of mineral deposits (e.g., pyrite) cause the oxygen to be depleted. In the absence of dissolved oxygen, the solubility of iron in water usually increases dramatically. Since iron is one of the most common elements on the earth it is usually present in the groundwater system.
At some point the groundwater reaches back to the surface and forms a seep or spring. When the water in the spring contacts the air it once again has a supply of oxygen. The oxygen in the air oxidizes the iron in the water from the ferrous (+2) state to the ferric (+3) state where it is insoluble. The insoluble iron minerals are rust colored and sometimes very pretty. Iron deposits in streams can be benign and they can also be an indication of acid mine drainage. If one does not know the difference it is best not to drink from waters with iron stains.
Figure 1. Iron oxide precipitation in a footprint at a seep near Kenosha, Wisconsin
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Copyright 2008 John Walton