We have many tools that we use in the world of stormwater, many of them utilizing high tech instruments and current technology. From water sampling using high tech lab equipment, to digital turbidity meters, all the way to erosion controls equipment designed by engineers. There are so many amazing advancements in technology and equipment that are designed to make our lives – and protecting the environment – easier. Of course, all these tools come with a price. Often a hefty one. There are ways to get basic, useful information using simpler and often cheaper methods. One such way is the use of the Secchi Disk when figuring out turbidity.
What is a Secchi Disk? It’s a super simple device that can easily be made by attaching a simple black and white 8-inch diameter disk to the end of a measuring tape. A Secchi disk is a piece of plastic (or plexiglass, or other material) separated into quadrants with alternating black and white sections. The black and white disk is attached to a metal piece of the same shape. A cord or measuring tape is attached securely to the center of the disk, and there you have it. Although a Secchi disk can be purchased, it can also be made easily in the home with the right equipment.
We now have our equipment. Now what? The Secchi Disk specifically measures the Secchi depth. This is the depth at which the disk disappears as it is lowered into the water, giving us the simplest measure of turbidity, or transparency of the water. To get an accurate measurement, you can lower the disc several times and determine an average. There are things to consider when using this method:
- The Secchi disk data is user-dependent, as results are impacted by vision.
- Secchi depth can be impacted by wave movement, sunlight, and other external factors.
- Water bodies are vastly different, a clear lake is not necessarily a healthy lake. Sometimes sampling or a more accurate method is required.
Overall, the Secchi disk is a way of getting a rough idea of how turbid your water is using a simple, inexpensive tool. They are even used worldwide as a way to measure phytoplankton levels! There are situations in which the answer will require extra care and precision. In issues of compliance, you may be required to sample for specific things or take an accurate turbidity reading (in NTUs, or Nephelometric Turbidity Units) using a precisely calibrated meter that measures light penetration through water.
If you find yourself in a bind, keep in mind that there are stormwater professionals at KCI on the job that can easily assist with turbidity testing or other compliance issues whenever you need!