How Is a Secchi Disk Used to Monitor Water Quality?

A secchi disk is a tool for quantifying water clarity, a critical indicator of water quality. It is neither a sophisticated tool to make nor a complicated tool to use. Single observations can provide quick snapshots of water quality conditions; accrued data provide solid information about the long-term health of oceans and lakes. Modified secchi disks can provide information about rivers and streams.

What a Secchi Disk Is

• A secchi disk looks like a white and black plate with a rope through the middle. The rope has marks at even metric intervals. The plate is slowly lowered and the rope is used to measure the depth of the plate. The critical piece of information to collect is the rope measurement when the plate is no longer visible. The deeper the plate, the better the water clarity.

  Visibility through water is limited by the concentration of particles suspended in the water. A secchi disk is most effective in deep bodies of water, such as oceans and lakes. A modified secchi disk is used to measure turbidity of rivers and streams.

  For flowing bodies of water, such as rivers, water is collected and then slowly added to a mounted tube with a secchi disk on the bottom. The tube is marked at even increments called Jackson Turbidity Units (JTU). The user adds water to the tube while watching for the moment the disk is no longer visible. The greater the volume of water necessary to obscure the disk, the less turbid the water.

Using Secchi Disks in Lakes and Oceans

• Sunlight hitting the water is either reflected as light or absorbed as heat.

  Measuring the transparency of water is important because it is indicates how deeply sunlight can penetrate the water. Absorbed sunlight has implications for water temperature and chemistry, and aquatic life.

  The deeper the water clarity, the more light is reflected off the water's surface. The shallower the water clarity, the more light is absorbed by the suspended particles. Absorbed light means warmer water. The temperature of the water affects oxygen levels.

Using Secchi Disks in Rivers and Streams

• Livestock living along the edge of a stream can increase instream turbidity and coliform bacteria levels.

  In flowing rivers and streams, modified secchi disks are used to measure turbidity. Turbidity is another word for the cloudiness of water. The greater the turbidity, the more particles are suspended in the water.

  Anything that washes into the stream and gets carried along with the flow (rather than bonding with the water) is considered a suspended solid. This includes soil from stream bank erosion and fecal matter from livestock, house pets and wildlife.

Using Data for Quick Information

• Muddy, turbid water is a soup of upstream toxins washing over the riverbed and becoming embedded downstream.

  Turbid water is a strong indicator of bacteria, such as fecal coliform and e. coli. A practical rule of thumb is to stay away from flowing, turbid water. It can indicate a strong enough stream flow to prevent solids from settling out, which also indicates a stream flow strong enough to wash heavier contaminants into the stream.

  In a lake situation, shallow water clarity, especially in hot weather conditions, is predictive of algae blooms and fish kills, as well as the re-suspension of heavy metals which may have washed into the lake. Knowledge of this is helpful for optimum treatment of drinking water supplies and pleasant recreation.

  In ocean situations, water transparency might be used to gauge the proximity of large floating bodies of contaminants, such as crude oil.

Using Data for Determining Water Quality Trends

• A strong, healthy water supply depends on how carefully we collect and analyze information over time.

  Single points of observation are of minimal value outside the context of what is seasonally and geologically appropriate. It takes many points of observation, or data, to fully understand long-term trends.

  Watersheds have a natural rate of sediment transfer from upstream to downstream. Changes in the watershed, such as removing woodlands and building parking lots, dramatically accelerate the rate of transfer. Water moves more quickly, builds up a greater velocity and meets with much less ground cover resistance. By tracking turbidity in the rivers and streams and water clarity in the lakes and oceans, we can gauge the impact of land use changes and theoretically adjust development practices.

  Turbidity surges eventually subside and sediment settles out, silting stream bottoms, clogging delicate gills, and smothering eggs. Siltation is a serious threat to our aquatic resources.

  Tracking water transparency in oceans can ensure cleanup efforts are effective.

Monitoring the 2010 Gulf of Mexico Cleanup

• Secchi disks can be used to track crude oil contamination.

  Secchi disks are very effective, simple tools that can be easily obtained and deployed. This makes them ideal for quick, eyewitness observations of water transparency changes following the 2010 Gulf of Mexico oil disaster. Individuals can gather their own information about the places most relevant to their day-to-day lives and can document changes over time.

Where to Get a Secchi Disk

• Biological supply companies such as Forestry Suppliers, Carolina and LaMotte sell secchi disks. Prices range from $30 student disks to $60 professional quality disks. Pay close attention to how long the rope is and how well the construction materials match your needs.