Level
Sedimentation. McIntyre et al. (1989) and McIntyre and Naney (1990) provide
Sedimentation
an example of using Cesium-137 isotope tracers to determine sedimentation rates
for different periods of land use from 1880 to recently. Historical sedimentation
may serve as a baseline for comparison to measured rates for trend detection.
Monitoring the loading rate is very useful measure for evaluating current
Pollutant Loading Rate
tions, trends in pollutant loading, or evaluating the effect of land treatment.
Monitoring
loading rate or the mass of pollutant exported per unit time (e.g.,
is a basic
measurement for eutrophication studies and pollutant budgets. Loading rates are
directly comparable to one another but they can be vary significantly from year
to
year.
The three major tasks for determining pollutant loads are:
measuring water discharge (cubic feet per second);
2. measuring pollutant concentration (milligrams per liter); and
3. calculating pollutant loads (multiplying discharge times concentration over
a year).
The primary difference between level I and level load monitoring is the type of
sampling gear, time required, and overall cost. Level I load monitoring does not
require continuous stream gaging to measure discharge. Grab sampling is used to
obtain water samples to measure concentration for level I. Level II load monitor-
ing requires continuous stream gaging to measure discharge and an automatic
sampler to take water quality samples.
Where there is a lower variability in discharge and where peak flows are not
Level
Pollutant Loading
extreme, as in the case of PS and irrigation return flows, level load monitoring
Rate Monitoring
may be employed. For other cases with high variability in discharge, large errors
in the loading estimate should be expected.
Several methods may be used to determine Level stream discharge measure-
ments. Sampling sites should have a stable stream bed and a natural downstream
control. A current meter may be used to measure stream velocity using either a
rotating propeller or cup wheel. Because stream velocity varies by depth in the
channel and the location, several measurements must be made to measure
instantaneous velocity and calculate the average velocity. Using a measurement
of the cross-sectional area, and multiplying times average velocity, the total
discharge can then be calculated.
A staff gage or tape measurements (distance from bridge to water level) used to
determine water level elevation may also serve to determine level I stream
discharge. To calculate discharge based on water level elevation, a stage discharge
relationship is developed from detailed measurements of the stream bed and
known discharges for several stream elevations. The resulting stage discharge
relationship or rating curve can be used to estimate discharge based on elevation
of the water surface at the time of sampling.
To sample pollutant concentrations a grab sampling technique may be used. The
concentration sample should be taken at the same location in the stream for each
and every sample. Sampling depth and sample handling protocols should be
developed. Overall a predetermined schedule should be developed for sampling
both discharge and concentration. Wedepohl et al. (1990) provide several methods
to calculate pollutant loads.
3.28
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