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

tions, trends in pollutant loading, or evaluating the effect of land treatment.

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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