Urban Water Quality 101: Turbidity, Part 1

The first water quality topic I'd like to raise is turbidity - the cloudiness of the water.  It is one of the simplest measures of water quality to perform and it tells us a lot about our streams if we measure it often enough.

Just eye-balling the water to see if it is clear or cloudy is not enough.  Factors like water depth and lighting conditions can make a big difference in how clear water appears when you are standing streamside.  Take the following photos for example.  Which of these sites would you say has clearer water?  I'll share the answer in Part 2, so check back in a couple of days...

Two views of relatively clear water from Deer Creek in St. Louis County.  But which has clearer water?

The Missouri Stream Team Program provides Level 1 (and above) water quality monitors with turbidity tubes to measure the cloudiness of stream water.  These tubes function using the same general principle as the Secchi disk, a patterned platter that is lowered into the water on a rope.  This technique has been used for the past 145 years to measure water clarity in lakes and oceans.  The disk is lowered until the pattern can no longer be seen and then raised until the pattern is once again visible.  The average of the distance of the disk from the water surface for the two measurements is called Secchi depth.

A turbidity tube (top) and a Secchi disk  (bottom) share
the same black and white pattern.

A turbidity tube, on the other hand, is filled with stream water and the water is released slowly through a hole near the bottom.  When the pattern becomes visible as viewed through the water column from the top, the hole is covered and the clarity is determined based on the amount of water that remains in the tube.  The markings on the side indicate a turbidity value which is measured (on Stream Team equipment) in Nephelometric Turbidity Units (NTUs).  The lowest (clearest) value that can be measured using this equipment is 10 NTU.  The highest (cloudiest) value on the tube is 400 NTU, which corresponds to visibility to a depth of approximately 1 inch.

So, what makes water turbid?  There are really two things that contribute to turbidity: total dissolved solids and total suspended solids.  Many dissolved solids like road salt and nutrients like nitrate are clear and do not add to the turbidity; however, colored dissolved solids like humic acid or tannins can contribute to turbidity.  Suspended solids include any particles that are held in the water column, including soil particles and small algae.