Environmental Sampling Design - Fundamentals of Environmental Sampling and Analysis - page 50

accurately and precisely represent a characteristic of a population, a parameter
variation at a sampling point, a process condition, or an environmental condition.'' It
can be further defined in three different levels (Popek, 2003): (1) Ultimate
representativeness is a measure of how well the overall data collection design
(including the sampling design and analysis) represents a characteristic of a
population and the environmental condition at the site; (2) Sampling point
representativeness is a measure of how well a sample collected represents the
characteristic properties of the sampling point; (3) Collected sample represent-
ativeness is a measure of how well subsampling techniques are during analysis and
the use of proper preparation and analytical procedures.
Representative Solids Samples
The representativeness depends largely on the types of sample matrices.
Contaminants accumulated in soils at a certain depth typically do not vary much
temporarily in a short-term. However, significant vertical variations may occur at
different soil depths. In special cases, heterogeneity is even more severe at the
microscopic scale (among soil particles) than the large-scale spatial variations,
which presents a challenge when subsamples are taken. This is the case when the
contaminant of interest is present in a ''nugget'' form commonly found in
contaminated soils, sludge, and other waste materials in solid samples. Uncertainties
associated with the representativeness of these types of heterogeneous samples
frequently far exceed those inherent in sample collection and analysis (Keith, 1990),
making sample preparation process (such as subsampling, mixing, grounding, and
sieving) particularly important for representative samples. Mixing is impossible for
wastes with immiscible phases. In such cases, representative samples should be
collected from each individual phase.
Representative Air Samples
Air is a unique matrix with potentially extreme variations and heterogeneity
compared with soil and water. Atmospheric contaminant concentrations at the same
location may have several orders of magnitude difference within minutes depending
on the changes in local meteorological conditions (wind velocity and direction). It
is thus important to determine whether the air sample collected is representative of
the ''typical'' or ''worst case'' site condition both spatially and temporally.
Meteorological and topographical factors must be incorporated into an air-sampling
plan. These precautions also apply to atmospheric precipitation samples such as
snow, rain, fog, and dew.
Representative Water Samples
Water and groundwater samples may have typical seasonal variations depending on
the water balance due to recent precipitation and water usage. There are some
special techniques for representative groundwater samples, which will be introduced