Water Quality Monitoring: Turbidity

Turbidity

Turbidity is a measure of cloudiness in water. The more turbid the water, the murkier it is.

Turbidity can be caused by soil erosion, waste discharge, urban runoff, bottom feeders like carp that stir up sediments, household pets playing in the water, and algal growth.

Turbid waters become warmer as suspended particles absorb heat from sunlight, causing oxygen levels to fall. (Warm water holds less oxygen than cooler water.) Photosynthesis decreases with lesser light, resulting in even lower oxygen levels.

Suspended solids in turbid water can clog fish gills, reduce growth rates, decrease resistance to disease, and prevent egg and larval development. Settled particles smother eggs of fish and aquatic insects.

Test Equipment

LaMotte Turbidity in Water Test Kit (Code 7519)
Second stirring rod
Gloves
Wash bottle containing demineralized water
Waste container

Procedure

Put on gloves. Hold one turbidity tube (0835) face down and perpendicular to the creek at arm's length. Submerge it straight down into the water, rotate it to point upstream, then tilt it slightly up until it fills to the 50 ml line with water.
Look down from the top of the tube. If you can see the black dot on the bottom, proceed to step 4.
If the black dot is not visible when looking from the top, pour out half the sample so that the tube is filled to the 25 ml line.
Fill the second turbidity tube with demineralized water to the level that matches the creek water sample.
Place the two tubes side by side and looking down at the dots from the top, notice the difference in sharpness or clarity of the black dots. Ignore any differences in color of the two samples, but note only the difference in clarity.
If the dot in the sample tube with creek water is fuzzier, add turbidity reagent (7520) to the tube with clean water. Shake the reagent bottle vigorously before each use. Fill the dropper from the reagent bottle and add a small number of drops to the clean water tube. Keep count of the number of drops added. Use the stirring rods (1114) to stir contents of both tubes after each addition. Look down from the top of both tubes and note if the dot in the bottom of the clean water tube is as fuzzy as the dot at the bottom of the creek water tube. If the creek water tube is still fuzzier, continue to add turbidity reagent until the dots are equally fuzzy. When the black dots are equally fuzzy, record the number of drops added.

Clean-up

Pour the contents of both columns into the waste container.
Return all glassware and chemicals to their boxes.
When safe to do so, remove gloves.

Water Quality Index

Note: If turbidity is greater than 100 jtu,
the quality index equals 5.

Calculations

If a 50 ml sample was used, multiply the number of drops by 5/8 (0.625) to derive the turbidity (jtu). If a 25 ml sample was used, multiply by 10/8 (1.25).

Convert turbidity (jtu) to water quality index.

          Turbidity:  (jtu)

                     

Water quality index: