Water Chemistry Field Sampling
58
and during analysis of these parameters. Filled syringes should be transported
from the field to the lab in plastic containers that minimize disturbance of the
seal of the syringe.
Throughout the water chemistry sampling process it is important to take
precautions to avoid contaminating the sample. Many surface waters in regions
of the United States considered sensitive to effects of atmospheric deposition
have low ionic strength (i.e., low levels of chemical constituents). Samples from
such waters can be contaminated easily by perspiration from hands, sneezing, smoking, suntan lotion, insect repellent, fumes from gasoline engines, or
chemicals used during sample collection.
For QA, sample collection should be routinely replicated so that the variability
introduced by the collection process can be quantified. Although duplicate collection of samples from a subset of the sampling sites is sometimes done, the collection of three replicate samples from a subset of prespecified sites is an alternative
approach for characterizing variability. The entire collection process should be
repeated for the duplicate pairs or triplicates so that either two or three sample
bottles representing the same sample location and approximately the same sample time are returned to the laboratory. The frequency of replicate sampling is
dependent on the overall structure and requirements of the QA program. Some
studies replicate all samples in the field, but they have only a subset of the replicates analyzed in the laboratory. Some studies do not include collection of field
replicates. We recommend collection and laboratory analysis of duplicate pairs
for 5% to 10% of the field samples in a given study. These can be used to ascertain
the collective level of error or uncertainty introduced by field sampling, sample
handling, short-term (seconds to minutes) temporal variation, short-distance
(centimeters) spatial variation, and laboratory analyses. Thus, replicated samples
provide an indication of the overall confidence that the laboratory results reflect
the actual conditions in the water body at the time of sample collection.
Water temperature should be measured approximately at the location of
sample collection. This can be accomplished by placing the thermometer in
the water at the sampling point and waiting for the reading to stabilize. If this
is not practical, temperature can be measured in a sampling bottle designated
for this purpose and labeled as such. In the latter case, cool the bottle to ambient stream temperature prior to filling it with stream or lake water for immediate temperature measurement.
2.3.2.1 Collection of Stream Water
The collected stream water sample should be representative of stream water at
the location of interest with respect to the measurements of interest. Collection
of water at a single point will provide a representative sample of the channel cross section if the stream is uniformly mixed. Mixing of stream water
increases with increasing flow velocity and roughness of the channel bottom.
58
and during analysis of these parameters. Filled syringes should be transported
from the field to the lab in plastic containers that minimize disturbance of the
seal of the syringe.
Throughout the water chemistry sampling process it is important to take
precautions to avoid contaminating the sample. Many surface waters in regions
of the United States considered sensitive to effects of atmospheric deposition
have low ionic strength (i.e., low levels of chemical constituents). Samples from
such waters can be contaminated easily by perspiration from hands, sneezing, smoking, suntan lotion, insect repellent, fumes from gasoline engines, or
chemicals used during sample collection.
For QA, sample collection should be routinely replicated so that the variability
introduced by the collection process can be quantified. Although duplicate collection of samples from a subset of the sampling sites is sometimes done, the collection of three replicate samples from a subset of prespecified sites is an alternative
approach for characterizing variability. The entire collection process should be
repeated for the duplicate pairs or triplicates so that either two or three sample
bottles representing the same sample location and approximately the same sample time are returned to the laboratory. The frequency of replicate sampling is
dependent on the overall structure and requirements of the QA program. Some
studies replicate all samples in the field, but they have only a subset of the replicates analyzed in the laboratory. Some studies do not include collection of field
replicates. We recommend collection and laboratory analysis of duplicate pairs
for 5% to 10% of the field samples in a given study. These can be used to ascertain
the collective level of error or uncertainty introduced by field sampling, sample
handling, short-term (seconds to minutes) temporal variation, short-distance
(centimeters) spatial variation, and laboratory analyses. Thus, replicated samples
provide an indication of the overall confidence that the laboratory results reflect
the actual conditions in the water body at the time of sample collection.
Water temperature should be measured approximately at the location of
sample collection. This can be accomplished by placing the thermometer in
the water at the sampling point and waiting for the reading to stabilize. If this
is not practical, temperature can be measured in a sampling bottle designated
for this purpose and labeled as such. In the latter case, cool the bottle to ambient stream temperature prior to filling it with stream or lake water for immediate temperature measurement.
2.3.2.1 Collection of Stream Water
The collected stream water sample should be representative of stream water at
the location of interest with respect to the measurements of interest. Collection
of water at a single point will provide a representative sample of the channel cross section if the stream is uniformly mixed. Mixing of stream water
increases with increasing flow velocity and roughness of the channel bottom.