Sampling - Field Sampling: Principles and Practices in Environmental Analysis - page 134

during the entire sampling period. If the sorbent is placed in a river and the river level
alternately is above and below the sorbent, a representative sample for the time of
sampling will not be obtained. This same question would arise if the sorbent is alternately
in one layer and then in another layer of the media. Unless the time spent in each layer is
known, the average amount of contaminant in the layers would not be accurately known.
A similar argument can be made where the sorbent is in a medium that is moving. An
air-sampling sorbent that is alternately upwind and downwind from the contamination
source will produce an analytical result that is difficult or impossible to interpret. The
same thing would occur if the sorbent is placed in water having different currents or if the
water is moving over it in different directions at different times.
In soil there is always some question about the contact between the adsorbent and soil.
Coarse-textured and sandy soils have less contact than silty soils, while clay soils would
have the most contact between the soil and the adsorbent. The question of contact thus
must be adequately answered for both the absorbent being used or proposed for use and
the soil it will be used in.
This type of sampling can be very effective where there is an episodic nature to the
type of contamination being studied (i.e., contamination occurs at one time or over a
limited time during the year). It is also important in cases in which the time of the
occurrence of the contamination is not known. Placing absorbents in the media at various
times and removing and analyzing them after a certain time interval allows for
determination of the time of contamination [17, 18].
5.12.5. Underwater Samples
Often sampling stops at or above the water table. Sometimes it is necessary to sample the
soil or sediment at the bottom of a body of water, however. In this case, two additional
complications often occur. First, the material is loose and tends to disperse while the
sample is being taken. Second, the physical and chemical environment is very different at
the bottom of a body of water from the environment into which it is to be placed,
particularly in the analytical laboratory. These samples come from a high-pressure,
anaerobic, reducing environment and are put into a relatively low-pressure, aerobic, and
oxidizing environment. Because of these differences significant changes in components
of interest can occur if precautions are not taken in obtaining and handling the sample
(e.g., a sample containing large amounts of iron in an anaerobic, reducing environment is
black; exposed to air, the sample becomes yellow, then red, and the iron precipitates).
A special sampler is needed when sampling the bottom of streams, rivers, ponds, and
lakes. The sampler must retain the soil and its associated water and prevent
contamination by overlying water. As with other sampling, a grid pattern can be used
along with random sampling. The type of sampler needed can be constructed or obtained
from commercial sources (see Appendix B) [20].
5.12.6. Other Sampling
To a large extent, sampling water and air is similar to what has been described for soil.
Transect sampling should be carried out, including areas known not to be contaminated.