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Taking Turbidity Measurements

One way to estimate turbidity (a measure of the clarity of the water) is to determine through how many centimeters of water you can see a white and black object (a 4.5 cm diameter secchi disk) using a turbidity tube. This is useful for water bodies that are too shallow for the conventional Secchi depth measurement used around the world for estimating lake water clarity (a 20 cm disk; see below) and is now being used in many states for volunteer stream monitoring.

The method shown here has researchers flooding a long clear tube with lake water while looking down at a white and black disk at the tube's bottom. The researcher first fills the tube so that the white and black disk can't be seen while looking down the tube, and then releases water by controlling a valve with his toes until the disk becomes visible. The length of the water column in the tube is then recorded using a tape measure attached to the tube - the longer the measurement, the more transparent the water.

(See a detailed drawing of a Turbidity Tube on Water on the Web ).

The secchi disk depth provides another "low tech" method for assessing the clarity of a pond or lake. A Secchi disk is a circular plate divided into quarters painted alternately black and white. The disk is attached to a rope and lowered into the water until it is no longer visible. Secchi disk depth, then, is a measure of water clarity. Higher Secchi readings mean more rope was let out before the disk disappeared from sight and indicates clearer water. Lower readings indicate more turbid or colored water. Clear water lets light penetrate more deeply into the lake than does murky water. This light allows photosynthesis to occur and oxygen to be produced. The rule of thumb is that light can penetrate to a depth of about 2 - 3 times the Secchi disk depth (see a close-up of a secchi disk on Water on the Web ).

As part of the GLEI study, we are comparing the two simple methods above to two analytical laboratory methods.

The first determines the turbidity of the water using an optical instrument called a turbidimeter (more technically a nephelometer) that measures light scattering from suspended particles.

The second method determines the absolute amount (weight) of particulate material suspended in the water sample, the total suspended solids concentration (TSS), by weighing this material with an analytical balance after carefully filtering the water sample through a fine filter woven from glass fibers.

The filter on the left is clean and unused, the middle filter shows particulate matter from a moderate amount of algae in the water, and the right filter was used to filter water from a location which had both suspended sediment particles and algae in the water.

Additional discussion of turbidity and its effects on aquatic organisms as well as other routine field measurements may be found on Water on the Web .