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When looking out on a lake or stream, sometimes we can
see a brown, funky “foam” in eddies or at a lake’s
edge. The sight of this foam immediately makes one think of soaps, detergents
and pollution in general. These associations understandably lead us
to assume that some manner of human-induced pollution is responsible.
While it is possible that humans are to blame for the foam you see,
it is more than likely just a natural phenomenon.
| A quick test is to smell the foam. If it has a nice
floral or perfume-y smell, it is most likely a detergent. If the
foam smells fishy or “earthy”, it’s probably natural.
Also, natural foam tends to be darker in color than the foam of
detergents, though natural foam can be light in color. By law, the
sudsing agent of detergents produced today must be biodegradable,
meaning that the foam will be short lived if the detergent gets
into a water body. This is in contrast to the late 50’s and
early 60’s, when foam caused by detergents was a big problem. |
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Here’s the nitty-gritty on foam formation. Water
molecules want to cling to each other. At the surface, however, there
are fewer water molecules to cling to since there is air above (thus,
no water molecules). This results in a stronger bond between those molecules
that actually do come in contact with one another, and a “film”
of strongly bonded water (see diagram). This surface film (held together
by surface tension) creates a considerable barrier between the atmosphere
and the water. In fact, other than mercury, water has the greatest surface
tension of any liquid. If the surface tension is reduced, bubbles are
more likely to be formed. This is due to the water’s reduced ability
to “regroup” after agitation. The greater the surface tension,
the easier it is for water to pull itself together and force out trapped
air.
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The surface tension of water varies depending upon
the dissolved stuff in the water and by the temperature. Organic
compounds from decomposing plant or animal matter, or from actively
photosynthesizing plants can reduce the surface tension of lake
or stream water. Both detergent and heat reduce water’s
surface tension, allowing the water to enter smaller pores and
fissures. That is great for forcing dirt out of your clothes.
Hard water increases surface tension. Water softeners reverse
this effect, reduce the surface tension and ultimately give cleaner
clothes and sudsier baths.
On lakes, in bathtubs and in mugs of beer, the cause for foam
is the same. Agitation at the surface causes air to get under
the surface film of the water. Weakened surface tension is unable
to force the air out, but rather keeps water wrapped around a
volume of air, creating a bubble. If this happens for long enough,
foam is created. On lakes, the agitation is usually due to wind,
and the resulting foam will collect on the downwind side of the
lake. In streams you will see the foam in eddies or floating downstream,
and it is created by the flow of the water disturbing the surface
film. |
Tony Thorpe
Back to the Winter
2002 Waterline
Brought to you by the Lakes of
Missouri Volunteer Program |
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