Introduction to Field Sampling - Field Sampling: Principles and Practices in Environmental Analysis - page 4

Along with this information, careful consideration must be made of the safety
measures needed during sampling. The largest part of safety is to prevent contamination
of workers during the sampling procedure. For example, fields heavily contaminated with
petroleum may be flammable. In this case there can be a fire hazard caused by smoking
or sparks in the field, and this is a safety concern. It is also important, however, that
persons doing the sampling or other persons present not inadvertently remove
contamination from the field. In the above case samples taken off the field may be a fire
hazard if not stored and transported properly.
Considering what happens to the sample once it is taken, including during transport, is
also important. It is possible that the analyte (component or contaminant) of interest
easily undergoes rapid changes both in the field and in sample containers. This may
dictate the use of a rapid sampling and analysis procedure rather than slower, more
accurate, and more precise procedures. A commonly cited example is nitrogen
compounds. Under oxidizing conditions and moderate temperatures ammonia is rapidly
oxidized to nitrite and nitrate. Under anaerobic conditions nitrate can rapidly be reduced
to nitrogen gas. A two-week lag time between collection of a sample containing nitrate
and its analysis would mean that the analytical results would not represent the level of
nitrate in the field [2].
1.2. SOIL
Soil could be considered the loose material, derived from stone, on the Earth's surface.
There are two distinct ways soil is used, however, for agriculture (growing crops and
raising animals) and for engineering (supporting structures). There are thus two distinct
ways of describing soil, and any persons involved in sampling must be familiar with both.
1.2.1. Soil Scientist Description
Normally a field will contain one or several soils. (See Figure 1.2.) It may also have rock
outcrops and water moving through it (or during a flood, over it). Soil will certainly have
water moving under it. If it is a fully developed soil, there will be several subsurface
horizons. (See Figure 1.3; also Figure 2.4 in Chapter 2). These horizontal layers will have
significantly different physical, chemical, and biological characteristics. They develop as
a result of the action of the soil-forming factors, time, topography, parent material, biota,
and climate, the effects of which extend 1.5 to 2 meters deep. This volume of active soil
development is called the solum.
In addition to the surface A horizon there may be several subsurface horizons, the most
common being B and C horizons. The A horizon is distinctive in that it contains more
organic matter and is darker than the underlying horizons. The B horizons are distinctive
in that they contain more clay than over- or underlying horizons and are usually divided
into several subdivisions, depending on changes in horizon characteristics. The C horizon
is the material from which the soil is forming and is considered the soil parent material.
Although the parent material has not been acted upon by the soil-forming factors it may
contain horizons that are a result of the original deposition or formation of this material.