With recent, heavy rains, all eyes are on the metro area’s rivers. With the water so high and visible, you may have recently noticed the foam floating on top of the Raccoon River. It may look like there was an upstream truck spill carrying dish detergent, but in fact, it is not soap causing the foam you are seeing on the river.
Detergents can produce foam, but usually the foam caused by detergents is white. The light tan foam recently seen in the Raccoon River typically occurs when decaying organic matter enters the water or is washed into the rivers and streams and begins to decay. This forms soap-like molecules that are attracted to water on one end and oily substances on the other end. The attraction of these substances to water reduces the surface tension on water. Surface tension of water creates the “skin” on the surface of water that allows water strider insects to skate across the surface of the water and not sink. When this skin becomes weaker, wind and turbulent water can easily break this skin. The soap-like molecules (surfactants) hold onto fats and oils on one side and water on the other with air trapped inside. The stronger the soap and water layer, the larger and more stable the bubbles. Eventually, bacteria break down these substances so they can no longer form bubbles.
When living things die and decay, cells breakup. This occurs in the alimentary tract (the tubular passage extending from the mouth to the anus, through which food is passed and digested) of animals and is eliminated with the fecal matter. Therefore, a high concentration of this waste contributes to the formation of the foam you are seeing on the Raccoon River right now. This can come from poorly operated waste treatment facilities and untreated animal waste.
Testing at Des Moines Water Works’ laboratory shows low phosphorus concentrations, indicating the foam to be from the decay of natural vegetation and waste products, rather than from direct human activity. Des Moines Water Works monitors its source waters daily for contaminants to determine which source to use and how to best treat the water in order to provide safe and clean drinking water to its customers.