Environmental Engineering Reference
In-Depth Information
and the disk will not reach a vanishing point. Deeper, slower moving riv-
ers are the most appropriate places for Secchi disk measurement, although
the current might require that the disk be weighted so it does not sway and
make measurement difficult. Secchi disks cost about $50 but can be home-
made. The line attached to the Secchi disk must be marked in waterproof ink
according to units designated by the sampling program. Many programs
require samplers to measure to the nearest 1/10 meter. Meter intervals can
be tagged (e.g., with duct tape) for ease of use. To measure water clarity with
a Secchi disk:
1. Check to make sure that the Secchi disk is securely attached to the
measured line.
2. Lean over the side of the boat and lower the Secchi disk into the
water, keeping your back to the sun to block glare.
3. Lower the disk until it disappears from view. Lower it one-third of a
meter and then slowly raise the disk until it just reappears. Move the
disk up and down until you find the exact vanishing point.
4. Attach a clothespin to the line at the point where the line enters the
water. Record the measurement on your data sheet. Repeating the
measurement provides you with a quality control check.
The key to consistent results is to train samplers to follow standard sampling
procedures and, if possible, have the same individual take the reading at the
same site throughout the season.
Transparency Tube
Pioneered by Australia's Department of Conservation, the transparency tube
is a clear, narrow plastic tube marked in units with a dark pattern painted on
the bottom. Water is poured into the tube until the pattern disappears. Some
U.S. volunteer monitoring programs, such as the Tennessee Valley Authority
(TVA) Clean Water Initiative and the Minnesota Pollution Control Agency
(MPCA), are testing the transparency tube in streams and rivers. The MPCA
uses tubes marked in centimeters and has found tube readings to relate fairly
well to lab measurements of turbidity and total suspended solids, although
they do not recommend the transparency tube for applications where pre-
cise and accurate measurement is required or in highly colored waters. The
TVA and MPCA have suggested the following sampling considerations:
1. Collect the sample in a bottle or bucket in midstream and at mid-
depth if possible. Avoid stagnant water, and sample as far from the
shoreline as is safe. Avoid collecting sediment from the bottom of
the stream.
2. Face upstream as you fill the bottle or bucket.
