UNDERSTANDING TURBIDITY
From The Urban Watershed Project - San Francisco, CA.

Definition:

Turbidity is a unit of measurement which indicates how cloudy or muddy the water is or, by quantifying the degree to which light traveling through a water column is scattered by the suspended particles. (The greater the scattering, the higher the turbidity)

Units and standards

Turbidity is commonly measured in Nephelometric Turbidity Units (NTU), but may also be measured in Jackson Turbidity Units (JTU). Waters with a turbidity level of > 5 NTU are not safe for recreational use or human consumption. Levels > 25 NTU cannot sustain Aquatic life.

Although we often think that clean water is clear, even unpolluted water can have suspended particles that may lessen its clarity but do not diminish its quality.

The velocity of the water resource largely determines the composition of the suspended load. Suspended loads are carried in both the gentle currents of lake waters and the fast currents of flowing waters. Even in flowing waters, the suspended load usually consists of grains less than 0.5 mm in diameter Suspended loads in lake waters usually consist of the smallest sediment fractions, such as silt and clay. (Clay particles are less than .002 mm in diameter. Silt ranges from 0.005 - 0.05 mm. Sand ranges from 0.1 - 1.5 mm.) Particulates can be carried by overland flow, or resuspended from the substrate by changes of speed or direction of the water current. Overland flow may also transport dissolved nutrients that contribute to phytoplankton growth}

Materials which increase turbidity levels

• Suspended silt or soil particles
• Phytoplankton and zooplankton (tiny floating organisms)
• Minute fragments of dead plants

Natural causes

• Natural runoff
• Water turbulence from rain storms
• Wave action
• Seasonal turnovers (releases nutrients from sediment which increase phytoplankton production)

Anthropogenic sources

• Runoff from agricultural fields
• Wash from construction sites and urban areas
• Shoreline erosion from heavy boat traffic
• Dissolved nutrients released in treated wastewater
• Organics discharged by sewage treatment plants

Environmental effects of high turbidity levels

High levels of turbidity over long periods of time can greatly diminish the health and productivity of the estuarine ecosystem.

• Turbid waters decrease light penetration into the water, thereby lowering photosynthetic activity & reducing the area available for submerged aquatic plants to grow. (Algae greatly limits light penetration and can limit primary production to the uppermost layers of water.)
• Invertebrate population decline (caused by fewer photosynthetic organisms available for food)
• Fish population decline (caused by fewer Invertebrates available for food)
• Suspended material in large quantities can foul the filter-feeding systems of certain estuarine animals.
• Particles may accumulate on the gills of fish and inhibit breathing.
• Hinders aquatic predators from spotting and tracking down prey.
• Dissolved oxygen depletion (If turbidity is largely due to organic particles)

Recreational effects

• Turbidity may obscure potentially dangerous obstructions such as boulders and logs from swimmers & boaters.
• The organic constituents of turbid waters may harbor high concentrations of bacteria, viruses, and protozoans.

Health effects

• Turbid conditions caused by organic constituents may increase the possibility for waterborne disease (since organic particulates may harbor microorganisms.)

Industrial effects

• Certain industrial processes may be unable to use turbid water since the abundance of suspended solids may clog or scour pipes and machinery.

Measuring turbidity

Although the most accurate means of assessing turbidity is with a nephelometer which electronically measures light scatter, many programs rely on the Secchi disk. Easy to use, inexpensive, and consistently accurate, this simple weighted disk is used to measure the water depth at which the disk just disappears from view the Secchi depth. Most programs find that the Secchi disk gives sufficiently good clarity readings.

The Secchi disk is 20 centimeters in diameter and divided into alternating black and white quadrants to enhance visibility and contrast (although some are totally white). Attach a white nylon rope with black marks at tenth-of-a-meter increments to the center of the black and white surface of the disk. Mark every one meter increment on the rope with a red line to simplify reading the depth measurement. Make sure the disk hangs horizontally when suspended.

Optimal conditions for recording Secchi Disk readings

• clear sky;
• sun directly overhead but disk should be in shade or shadow;
• protected side of boat with minimal waves or ripples.

If the conditions vary from this ideal situation, record any differences on the data sheet. Additionally, make sure the sun is at your back if it is not directly overhead and do not wear sunglasses while viewing the Secchi disk in the water.

Taking a Secchi Disk reading

• Tie a wrist loop at the end of the rope so the rope end does not accidentally drop into the water when the disk is lowered.
• Lower the disk into the water until the disk just disappears from sight.
• Record the amount of rope submerged to the nearest one -tenth meter (i.e., note the point where the rope and water line meet).
• Raise the disk slowly until the disk just becomes visible and record this depth as well.
• Average the two recorded values. This average is called the limit of visibility.

If the disk hits the bottom before dropping out of sight, note this observation and record the bottom depth. Secchi readings will vary with the specific estuary, location in the estuary, and season. Water clouded with sediment after a storm or with high levels of phytoplankton during a warm spell will have low Secchi readings (poor water clarity). Low productivity winter waters or estuarine water located near the ocean will generally register higher Secchi depths.