Water Chemistry Field Sampling
60
Avoiding disturbance that can affect the water being sampled is especially
important. For grab samples, the most likely disturbances are stirring up sediment or incorporating surface debris into the sample; each can contribute significant amounts of chemicals to the analytical results. Falling into the stream
not only is a major disturbance of sediment but also can pose safety problems;
thus, selection of a stable place to wade or a shore location from which to reach
the sample location is critical. Otherwise, suitable sampling sites often are slippery because of water, ice, algae, or mud, or they contain unstable substrate
such as loose boulders or poorly supported logs. If the sampler tries to use both
hands for handling bottles while leaning over the water, sudden loss of balance
can occur. The sample should not be collected where the sampler has waded
or fallen. If the sampler is holding the bottles in hand, powder-free gloves can
minimize contamination from sweat and the like. Laboratory gloves generally
cover to the wrist, but longer gauntlet-style gloves cover to the elbow and should
be used if the sample is collected by hand at depth greater than several centimeters. In addition to salts in sweat, common contaminants are sunscreen
and insect repellent. All of these potential contaminants can be minimized by
rinsing the hands and arms before collection, and rinsing the gloves, at a site
far enough away (and downgradient) such that the sampling site itself is not
contaminated by the rinsing.
Bottles are individually uncapped, partially filled with stream water, capped
and shaken, and the rinse water discarded away from where the samples are
to be collected (e.g., onshore, downstream, or where the sampler has waded).
Rinse water should be poured over the cap as it is being discarded. Three rinses
for each bottle are needed unless protocols otherwise indicate (e.g., bottles for
total organic carbon [TOC] samples might not be rinsed). The bottles then are
individually filled completely and capped. The bottles are capped underwater if
septum caps are used for aliquots intended for pH and DIC analysis.
At sites where surface films contain significant pollen, insect casings, or
organic film, the bottle should be kept capped until it is submerged below the
surface and then uncapped to fill. The cap should then be replaced before raising the bottle back through the surface film.
The sampling depth should be consistent and documented. In general, for
streams that are less than about 2-m deep, it is recommended, where possible,
to sample at a depth of about 0.3 m or midway between the water surface and
the water/sediment interface, whichever is closest to the surface. It often is
impractical or unsafe to collect samples deeper than about 0.3 m without a
sampling device of some kind.
2.3.2.1.2 Shallow Samples
If the water at the site is very shallow, which may be the case for many small
streams and the outflow of some lakes, it may not be possible to sample very
60
Avoiding disturbance that can affect the water being sampled is especially
important. For grab samples, the most likely disturbances are stirring up sediment or incorporating surface debris into the sample; each can contribute significant amounts of chemicals to the analytical results. Falling into the stream
not only is a major disturbance of sediment but also can pose safety problems;
thus, selection of a stable place to wade or a shore location from which to reach
the sample location is critical. Otherwise, suitable sampling sites often are slippery because of water, ice, algae, or mud, or they contain unstable substrate
such as loose boulders or poorly supported logs. If the sampler tries to use both
hands for handling bottles while leaning over the water, sudden loss of balance
can occur. The sample should not be collected where the sampler has waded
or fallen. If the sampler is holding the bottles in hand, powder-free gloves can
minimize contamination from sweat and the like. Laboratory gloves generally
cover to the wrist, but longer gauntlet-style gloves cover to the elbow and should
be used if the sample is collected by hand at depth greater than several centimeters. In addition to salts in sweat, common contaminants are sunscreen
and insect repellent. All of these potential contaminants can be minimized by
rinsing the hands and arms before collection, and rinsing the gloves, at a site
far enough away (and downgradient) such that the sampling site itself is not
contaminated by the rinsing.
Bottles are individually uncapped, partially filled with stream water, capped
and shaken, and the rinse water discarded away from where the samples are
to be collected (e.g., onshore, downstream, or where the sampler has waded).
Rinse water should be poured over the cap as it is being discarded. Three rinses
for each bottle are needed unless protocols otherwise indicate (e.g., bottles for
total organic carbon [TOC] samples might not be rinsed). The bottles then are
individually filled completely and capped. The bottles are capped underwater if
septum caps are used for aliquots intended for pH and DIC analysis.
At sites where surface films contain significant pollen, insect casings, or
organic film, the bottle should be kept capped until it is submerged below the
surface and then uncapped to fill. The cap should then be replaced before raising the bottle back through the surface film.
The sampling depth should be consistent and documented. In general, for
streams that are less than about 2-m deep, it is recommended, where possible,
to sample at a depth of about 0.3 m or midway between the water surface and
the water/sediment interface, whichever is closest to the surface. It often is
impractical or unsafe to collect samples deeper than about 0.3 m without a
sampling device of some kind.
2.3.2.1.2 Shallow Samples
If the water at the site is very shallow, which may be the case for many small
streams and the outflow of some lakes, it may not be possible to sample very