Environmental Engineering Reference
In-Depth Information
mixing. You will want to select a tracer that can be released in small quantities but
that will not be masked by the background concentration in the stream. The tracer
also needs to be one that is not regulated by any stream-management authorities, or
one for which you have a permit to release.
It may be convenient to use electrical conductivity (EC) as a surrogate for tracer
concentration if a sufficient amount of tracer can be released to create an easily
measured increase of the EC of the stream water. In streams that have very low
background EC, a strong correlation between tracer concentration (e.g., chloride)
and EC can be pre-established, and concentration can be estimated from the
measurements of EC. If this is not feasible, water samples will need to be collected
and analyzed with a field analyzer or in the laboratory. This will require a large
number of samples for slug injection tests.
After the location for tracer release is selected, a choice must be made between
the constant-rate injection (CRI) and the slug injection (SI) method. The CRI
method requires a device for injecting tracer solution at a constant rate, but only
three values of concentration are required (see Eq.
3.23
). The SI method does not
require a special device, but many concentration values are required to establish the
time-concentration curve shown in Fig.
3.17
. Here we describe the use of the CRI
method. It is assumed that the background concentration is small enough that the
tracer concentration can be estimated from the measurement of EC. To establish the
relation between EC and tracer concentration, prepare a set of standard solutions
from the tracer chemical and the stream water; for example, solutions of 0, 5,
10, 20,
1,000 mg of sodium chloride in 1 L of stream water. The EC values of
these solutions are plotted against concentration values to establish a calibration
curve.
For successful application of the CRI method, the tracer solution should be
released at an appropriate rate and concentration to ensure that concentration at the
measurement section can be accurately measured relative to the stream background
concentration, and that a sufficient volume of tracer solution exists in the tracer-
injection reservoir to achieve steady state at the sampling location. The constant
release rate of tracer solution can be maintained using a Mariotte bottle or a field-
portable pump with controlled flow rate (see Moore
2004
for construction of a
simple Mariotte bottle from readily available materials). Once a steady value of EC
is established at the sampling location and tracer concentrations are determined, the
observer can calculate discharge using Eq.
3.23
.
In summary, the suggested field activities for stream gauging are the following:
...
1. Determine stream discharge using the area-velocity method. If time permits,
determine the discharge at multiple locations and assess the errors and uncer-
tainty of this method.
2. Estimate stream discharge using the float method at the same location, and
compare the accuracy of this method with the area-velocity method.
3. Determine stream discharge using the tracer dilution method.
4. Compare the values of discharge obtained by all three methods and discuss their
advantages and disadvantages for application at this particular location, as well
as other possible locations and situations.
