The two primary chemicals responsible for many taste and odor events in drinking water reservoirs are 2-methylisoborneol (MIB), which gives a “musty” odor, and geosmin, which gives an “earthy” taste and odor. First identified in the mid 1960s, it was not until the mid 1970s before cyanobacteria (blue-green algae) were identified as the major source of MIB and geosmin in lakes and reservoirs. MIB and geosmin are also produced by actinomycetes, a gram-positive filamentous bacteria common in soils and associated with suspended sediments in water, but cyanobacteria are typically the major source in aquatic systems. Blue-green algae bloom control through epilimnetic circulation has shown to be very effective at resolving taste and odor problems in over 100 municipal raw water supply reservoirs up to 12,000 acres (about 4,800 hectares) in size since 2000. Hypolimnetic circulation can resolve taste and odor issues related to the accumulation of hydrogen sulfide, manganese and iron in stratified bottom waters.