Sampling - Field Sampling: Principles and Practices in Environmental Analysis - page 108

to data obtained from other sources, such as aerial maps and soil surveys.
5.1. GENERAL SAMPLING CONSIDERATIONS
Analytical and instrumental methods of analysis are very precise and accurate, but the
results of analyses of environmental samples are not. This means that the variability
comes either from the sampling procedures or during sample handling and storage prior
to analysis. Sampling is the most important source of variability, and both the sampling
and handling process must be carried out with great attention to detail.
Sampling is the act of isolating a portion of a larger entity, analyzing it, and using the
analytical results to describe the characteristics of the whole entity. In statistical terms the
sample is an individual, which is a member of a family of similar individuals. Analysis
shows that even environmental samples taken from sites close together may not be
individuals or members of a family. When samples are taken at progressively increasing
distances from a specific location they change in one of several ways. Characteristics can
change gradually until a very different individual is identified, or they may change in a
repetitive manner over long distances.
On the other hand, samples may show gradual or repetitive changes and all of a sudden
show a dramatic change in a completely different direction. For example, a soil at the top
of a hill might be well developed, with distinct A, E, and B horizons. As expected, the B
horizon is high in clay, but there is good internal drainage. The hillside profile might
have similar horizons except that they would be thinner and less well developed. At the
bottom of the hill next to a stream the soil has a thick, dark, high organic matter A
horizon underlain by a sandy B horizon with a small amount of clay in it. There is thus a
gradual change down the hill and an abrupt change at the bottom.
It is essential that such changes in the environment be kept in mind both when
designing a sampling plan and during noninvasive and invasive sampling. If unexpected
changes occur, they are recorded in the project notebook and accounted for in the
sampling procedures, sample analysis, and the interpretation of sample analysis.
In sampling, air and water variability is not as great as with soil. The atmosphere is
particularly uniform, but the hydrosphere is more variable in that the salt content of water
varies with location. It is important to remember, however, that there are layers and
definite direction in movement in both of these spheres. Downwind or downstream from
a source of contamination it is dramatically diluted, and thus changes in location, either
horizontally or vertically, may lead to sampling different layers, and this may result in
differing and perhaps confusing analytical results. These sources of possible variation and
error must be kept in mind when designing sampling plans for air and water as well as
soil [1].
In the United States sampling of hazardous fields and sites falls under the purview of
the United Stated Environmental Protection Agency (USEPA). This organization has a
publication, Test Methods for Evaluating Solid Waste Physical/Chemical Methods
(USEPA SW-846 3rd ed., November 1990). In addition, Standard Methods for the
Examination of Water and Wastewater, published by the American Public Health
Association, American Water Works Association, and Water Environment Federation