2.3 Field Methods
59
Streams are generally well mixed with regard to dissolved substances if flow is
turbulent and there are no close upstream tributaries or nearby point sources of
contamination. To verify that the stream is uniformly mixed, sampling can be
done for measurement of specific conductance (e.g., using a meter in the field)
and perhaps other parameters at several points along the cross section and at
different depths. If the measurements do not vary beyond the expected analytical variability, sampling at a single point can be done thereafter. If the required
sampling location is not well mixed along the cross section, depth-integrated
samples could be collected at multiple points along the cross section or the sampling site might be moved to a different location. In general, stream sampling
to determine the effects of atmospheric deposition involves sampling of relatively fast-flowing small streams free of point source impacts. The water in such
streams should generally be well mixed. Therefore, we recommend sampling at a
single point in the main area of flow across the stream cross section unless local
conditions suggest the likelihood of incomplete mixing of water in the stream.
2.3.2.1.1 Manual Sampling
At many site locations, the sample bottle may serve as the collection device by
simply dipping the sample bottle into the stream by hand. This avoids the need
for a collecting device, thereby reducing equipment needs and the chance for
unnecessary sample contamination. At stream sites that are hazardous to access
because of steep banks or high flows, a sampling pole (long pole that holds a
bottle on the end) or a weighted bottle holder can be used so that the collection
bottle can be extended out to the stream or lowered into the stream from above.
With the weighted bottle approach, an open sample bottle is placed within
the weighted holder and lowered into the water with a handline. Discrete-volume
samplers (such as a Van Dorn sampler) can also be used to collect the water sample
but are usually not necessary in relatively small streams having well-mixed flows
when the objective is measurement of dissolved constituents in stream water.
There are many acceptable methods of collecting water samples. Some,
such as flow integrated stream samples, are complex and beyond the scope
of the approaches described here; studies needing such sampling should consult appropriate references (e.g., USGS 2006). Methods likely to be used to collect samples for most stream studies of atmospheric deposition effects are
described next. The information in this section is taken largely from the recommendations of Turk (2001).
2.3.2.1.1.1 Grab/Hand Samples
Once the sampling site has been selected, bottles should be assembled and necessary information added to the labels to unambiguously identify the sample.
If the bottles contain DIW, this should be discarded away from the shore so it
does not disturb the sampling site.
59
Streams are generally well mixed with regard to dissolved substances if flow is
turbulent and there are no close upstream tributaries or nearby point sources of
contamination. To verify that the stream is uniformly mixed, sampling can be
done for measurement of specific conductance (e.g., using a meter in the field)
and perhaps other parameters at several points along the cross section and at
different depths. If the measurements do not vary beyond the expected analytical variability, sampling at a single point can be done thereafter. If the required
sampling location is not well mixed along the cross section, depth-integrated
samples could be collected at multiple points along the cross section or the sampling site might be moved to a different location. In general, stream sampling
to determine the effects of atmospheric deposition involves sampling of relatively fast-flowing small streams free of point source impacts. The water in such
streams should generally be well mixed. Therefore, we recommend sampling at a
single point in the main area of flow across the stream cross section unless local
conditions suggest the likelihood of incomplete mixing of water in the stream.
2.3.2.1.1 Manual Sampling
At many site locations, the sample bottle may serve as the collection device by
simply dipping the sample bottle into the stream by hand. This avoids the need
for a collecting device, thereby reducing equipment needs and the chance for
unnecessary sample contamination. At stream sites that are hazardous to access
because of steep banks or high flows, a sampling pole (long pole that holds a
bottle on the end) or a weighted bottle holder can be used so that the collection
bottle can be extended out to the stream or lowered into the stream from above.
With the weighted bottle approach, an open sample bottle is placed within
the weighted holder and lowered into the water with a handline. Discrete-volume
samplers (such as a Van Dorn sampler) can also be used to collect the water sample
but are usually not necessary in relatively small streams having well-mixed flows
when the objective is measurement of dissolved constituents in stream water.
There are many acceptable methods of collecting water samples. Some,
such as flow integrated stream samples, are complex and beyond the scope
of the approaches described here; studies needing such sampling should consult appropriate references (e.g., USGS 2006). Methods likely to be used to collect samples for most stream studies of atmospheric deposition effects are
described next. The information in this section is taken largely from the recommendations of Turk (2001).
2.3.2.1.1.1 Grab/Hand Samples
Once the sampling site has been selected, bottles should be assembled and necessary information added to the labels to unambiguously identify the sample.
If the bottles contain DIW, this should be discarded away from the shore so it
does not disturb the sampling site.