Environmental Sampling Techniques - Fundamentals of Environmental Sampling and Analysis - page 70

There are other topics that are equally important, such as sample custody, sample
labeling and tracking, sample packaging, and shipment (see Popek 2003 for details).
Note that ignorance of any one of these steps will likely invalidate the data acquired.
4.1.1 Sequence of Sampling Matrices and Analytes
When a project deals with multimedia and/or multiple parameters, collect samples
according to the following sequence. Remember to use a commonsense approach
whenever in doubt. A list of tips is as follows:
Collect from the least to the most contaminated sampling locations within
the site whenever possible. This applies to sampling water, groundwater, soil,
and biological samples. For example, if the goal is to investigate a ground-
water contaminant plume as a result of known gasoline leaks from an
underground storage tank, start from down gradient and work on to up
gradient. This will reduce potential cross-contamination between samples
and minimize decontamination efforts.
If you collect both sediment and water at the same site, collect water first and
then sediment to minimize effects from suspended bed materials.
For shallow surface water in a stream, start sampling downstream and work
upstream to minimize the effects of sediment due to sampling disturbance. If
you collect a sediment sample while standing in the water, be sure to stand
downstream of the collecting point. For deep waters, the collection sequence
from downstream to upstream is less important.
If sampling at different water depths is needed, collect surface water samples
first and then proceed to a deeper interval.
Always collect VOCs first, followed by SVOCs such as extractable organics
(PCBs, pesticide), oil and grease, and total petroleum hydrocarbons (TPHs).
Then proceed to other parameters in the order of total metals, dissolved
metals, microbiological samples, and inorganic nonmetals.
4.1.2 Sample Amount
The minimally required sample amount depends primarily on the concentration of
the analytes present in sample matrices. The sample volume should be sufficient to
perform all required laboratory analyses with an additional amount remaining to
provide for analysis of QA/QC samples including duplicates and spikes (Chapter 5).
Another determining factor is the representativeness associated with the sample
amount. For heterogeneous samples, a larger portion is generally required to be a
representative of the actual sample variations such that possible biased results can be
minimized. This larger portion of the collected sample is then homogenized
thoroughly followed by subsampling. Taking too large or too many samples should
be avoided because storage (cold room and freezer), transportation, and disposal
costs can become a burden to a big project.