Water Chemistry Field Sampling
68
multiplied by the cross-sectional area of that interval, and the products of all
intervals are summed to provide the overall estimate of stream flow.
The salt dilution gauging method also provides an approach for estimating
stream flow at locations where reliable measurements with flow meters are not
possible. Such conditions occur in streams that are highly turbulent or have
irregular channels, or in small streams or during low flow under conditions
when a large part of the flow is through gravel and rocks in the stream bed.
For such conditions, salt dilution gauging provides a more reliable method for
discharge measurement. Salt dilution gauging involves the addition of a known
quantity of salt upstream of the gauging site, either by a single addition or by
continuous injection. Discharge is computed based on the concentration or
dilution of the salt, determined by conductivity measurements, as it passes the
gauging site. We do not recommend routine use of this method, and it may not
be allowed by land owners or managers.
Making flow measurements during periods of high flow may not be safe, or
it may not be possible to wade into the stream under such conditions. If there is
any doubt about the safety of wading under the existing flow conditions, field
staff should not enter the stream.
If information on flow is needed to aid in the interpretation of stream chemistry measurements, but neither installation of a stream gauge nor collection of
flow measurements are feasible, water level can be manually recorded from a
staff gauge at the time that the water sample is collected. This will provide data
on stage but not discharge. For some research or monitoring project objectives,
relative differences in stream stage may be sufficient in place of the more quantitative discharge data. The staff gauge (not allowed in wilderness settings)
should be located just upstream of an effective control. The staff gauge is usually a pressure-treated post or metal fence post anchored in the stream with a
large ruler attached. The elevation of the ruler should be surveyed in reference
to an object near the bank that would be considered immovable. This enables
future verification that the staff gauge has not moved. If the chemical concentrations that are being measured are statistically related to flow, they are also
related to stage, although the relationships can differ. Changes in chemical
concentrations of flow-dependent constituents can be estimated from stream
stage measurements in a manner similar to that done with flow measurements.
Another approach is to characterize regional flow conditions based on nearby
gauges on similar watersheds. Using this approach, it is possible to obtain a general idea of the likely flow conditions at the sampling site at the time of sample
collection without the need for site-specific measurements. It is important to
note that, although discharge can be a useful parameter in evaluating the effects
of atmospheric deposition, it is not absolutely essential to have site-specific discharge data. One should not choose to avoid sampling a site for water chemistry
simply because it is impractical or impossible to collect parallel data on discharge.
68
multiplied by the cross-sectional area of that interval, and the products of all
intervals are summed to provide the overall estimate of stream flow.
The salt dilution gauging method also provides an approach for estimating
stream flow at locations where reliable measurements with flow meters are not
possible. Such conditions occur in streams that are highly turbulent or have
irregular channels, or in small streams or during low flow under conditions
when a large part of the flow is through gravel and rocks in the stream bed.
For such conditions, salt dilution gauging provides a more reliable method for
discharge measurement. Salt dilution gauging involves the addition of a known
quantity of salt upstream of the gauging site, either by a single addition or by
continuous injection. Discharge is computed based on the concentration or
dilution of the salt, determined by conductivity measurements, as it passes the
gauging site. We do not recommend routine use of this method, and it may not
be allowed by land owners or managers.
Making flow measurements during periods of high flow may not be safe, or
it may not be possible to wade into the stream under such conditions. If there is
any doubt about the safety of wading under the existing flow conditions, field
staff should not enter the stream.
If information on flow is needed to aid in the interpretation of stream chemistry measurements, but neither installation of a stream gauge nor collection of
flow measurements are feasible, water level can be manually recorded from a
staff gauge at the time that the water sample is collected. This will provide data
on stage but not discharge. For some research or monitoring project objectives,
relative differences in stream stage may be sufficient in place of the more quantitative discharge data. The staff gauge (not allowed in wilderness settings)
should be located just upstream of an effective control. The staff gauge is usually a pressure-treated post or metal fence post anchored in the stream with a
large ruler attached. The elevation of the ruler should be surveyed in reference
to an object near the bank that would be considered immovable. This enables
future verification that the staff gauge has not moved. If the chemical concentrations that are being measured are statistically related to flow, they are also
related to stage, although the relationships can differ. Changes in chemical
concentrations of flow-dependent constituents can be estimated from stream
stage measurements in a manner similar to that done with flow measurements.
Another approach is to characterize regional flow conditions based on nearby
gauges on similar watersheds. Using this approach, it is possible to obtain a general idea of the likely flow conditions at the sampling site at the time of sample
collection without the need for site-specific measurements. It is important to
note that, although discharge can be a useful parameter in evaluating the effects
of atmospheric deposition, it is not absolutely essential to have site-specific discharge data. One should not choose to avoid sampling a site for water chemistry
simply because it is impractical or impossible to collect parallel data on discharge.