Geosmin and 2- MIB
Geosmin is a naturally occurring organic compound produced by bacteria in soil, algae found in surface water, and a variety of other organisms. Geosmin has a very distinct earthy flavor and aroma and is responsible for the earthy taste of beets, spinach, and mushrooms. Geosmin is a contributor to the strong scent in the air when it has rained after a prolonged dry spell or when soil is disturbed. Geosmin is responsible for many taste and odor complaints in drinking water. Most people can detect Geosmin in water at levels as low as 15 parts per trillion (ng/L), with sensitive individuals detecting it as low as 4 parts per trillion (ng/L). Chemically it is a bicyclic alcohol with a formula of C12H22O.
2-Methylisoborneol (MIB) is an organic chemical with strong odor. The odor detection threshold is very low and this chemical can have major influences on water quality aesthetics. Various strains of algae, particularly blue-green algae (such as Anabaena) produce 2-MIB along with other odorous compounds such as Geosmin. This gives a musty or earthy odor that can be quite strong if an algal bloom is present. Subsequent death of the algae will also release any 2-MIB or Geosmin that is trapped in the cells. These compounds are detectable by the average person at very low levels, in the part per trillion range, and are responsible for many taste and odor issues in drinking water treatment and distribution.