Environmental Engineering Reference
In-Depth Information
Sampling and equipment Considerations
In contrast to lakes, where DO levels are most likely to vary vertically in
the water column, changes in DO in rivers and streams move horizontally
along the course of the waterway. This is especially true in smaller, shal-
low streams. In larger, deeper rivers, some vertical stratification of dissolved
oxygen might occur. The DO levels in and below riffle areas, waterfalls, or
dam spillways are typically higher than those in pools and slower moving
stretches. If we want to measure the effect of a dam, sampling for DO behind
the dam immediately below the spillway and upstream of the dam would be
important. Because DO levels are critical to fish, a good place to sample is in
the pools that fish tend to favor or in the spawning areas they use.
An hourly time profile of DO levels at a sampling site is a valuable set of
data, because it shows the change in DO levels from the low point (just before
sunrise) to the high point (sometime near midday); however, this might not
be practical for a volunteer monitoring program. Note the time of the DO
sampling to help judge when in the daily cycle the data were collected.
Dissolved oxygen is measured in either milligrams per liter (mg/L) or as
percent saturation. Milligrams per liter is the amount of oxygen in a liter of
water. Percent saturation is the amount of oxygen in a liter of water relative
to the total amount of oxygen that the water can hold at that temperature.
DO samples are collected using a special BOD bottle: a glass bottle with a
“turtleneck” and a ground glass stopper. You can fill the bottle directly in
the stream if the stream is wadeable or boatable, or you can use a sampler
dropped from a bridge or boat into water deep enough to submerse the sam-
pler. Samplers can be made or purchased. Dissolved oxygen is measured
primarily either by using some variation of the Winkler method or by using
a meter and probe.
Winkler Method
The Winkler method involves filling a sample bottle completely with water
(no air is left to bias the test). The dissolved oxygen is then fixed using a series
of reagents that form a titrated acid compound. Titration involves the drop-
by-drop addition of a reagent that neutralizes the acid compound, causing
a change in the color of the solution. The point at which the color changes
is the
endpoint
and is equivalent to the amount of oxygen dissolved in the
sample. The sample is usually fixed and titrated in the field at the sample
site, but preparing the sample in the field and delivering it to a lab for titra-
tion is possible. Dissolved oxygen field kits using the Winkler method are
relatively inexpensive, especially compared to a meter and probe. Field kits
run between $35 and $200, and each kit comes with enough reagents to run
50 to 100 DO tests. Replacement reagents are inexpensive and are available
already measured out for each test in plastic pillows. The reagents can also
be bought in larger quantities in bottles and measured out with a volumetric
