Urban Water Quality 101: Turbidity, Part 2

Now that we know how the turbidity tube works and what can contribute to turbidity, lets see how the two sites from the last post measure up:

This site on Deer Creek has a turbidity between 10 and 15 NTU. If I were reporting this data, I would
probably call it 12 NTU. Notice that the black and white pattern is barely visible through the water.

This site on Deer Creek has a turbidity of less than 10 NTU. With as clear as the water is, the pattern is clearly visible;
if a longer tube were available, you could probably see through several more inches. This is the clearer of the two samples.

Surprised by the results?  The image on the top was taken in a shallow location with some suspended sediments.  The image on the bottom is a deeper pool with few suspended particles; the raindrops on the water's surface make the stream bottom appear more difficult to see.

So what does it all mean?  If we would measure turbidity of a local stream, what could that tell us?  Plenty!  If your stream is consistently at 10 NTU or less, you have a fairly clear stream.  If the spikes that you see hit about 50 or 60, then there might be some periodic erosion occurring, a bloom of small algae, or decomposition of leaf litter in a standing pool.

We could learn even more, especially if each turbidity measurement is paired with one or two other bits of data; a rainfall record, height on a stream gauge, or flow measurements (especially if you have a USGS gauge near your stream) would be particularly useful for comparison.  Take, for example, the graph below created using a fictitious data set. 

In this graph, the turbidity (in brown) increases each time the stream rises (in blue).  This indicates that there is either erosion occurring or that something being brought into the stream in runoff is clouding the stream.   Starting in late March, notice that the turbidity rises slowly to somewhere between 10 and 20, rises sharply when the water level rises, and then decreases when the stream falls again.  This might indicate that algae are blooming in the water column and then being washed away with each storm.


For more information on turbidity, check out page 182 of The Streamkeeper's Field Guide by Tom Murdoch and Martha Cheo (1996).  Many thanks to the Stream Team Program for providing this resource to me many moons ago - I've used it often!

You may also want to have a look at the EPA's page on Using a Secchi Disk or Transparancy Tube or their guidance on the Importance of Turbidity.