Introduction to Field Sampling - Field Sampling: Principles and Practices in Environmental Analysis - page 15

1.12. TRAPS
There are many traps into which the inexperienced or unknowledgeable person doing
sampling can stumble in the field. The scenario below is an example of what can happen.
Scenario
An agricultural field planted to sugarcane and having a 5% slope is sampled
by two people. No instructions are given to the persons doing the sampling.
The first person's samples are found to have phosphate levels equivalent to
200 kg per ha, and remediation is recommended to prevent contamination of
adjacent water supplies. The second person's samples have phosphate levels
equivalent to only 50 kg per ha, and no remediation is recommended.
From this scenario the question arises as to how such different results could be obtained
from the same field. The answer is both in the depth to which the soil was sampled and
the place from which the sample was taken. A sample of the top 4 cm of a soil in a
depressional area will usually show a high level of whatever you are looking for. On the
other hand, a sample of the top 30 cm of soil at the highest point in the field will show a
lower level of what you are looking for. The first step in solving this riddle is to make
sure that all samples are taken to the same depth. The second is to make sure that random
samples are taken from the whole field. A third point is to know the history of the site to
make sure that previous usage is not contributing to the differences in the results. The
fourth point is to know where phosphate fertilizer was applied and account for this during
sampling.
When thought about, this makes sense. Contaminants, fertilizer elements, pesticides,
and toxic substances will erode from the high places in the field and collect in the low
areas. In the middle between the two extremes the results will be closer to the average.
Samples taken from throughout the field are required in order to have a representative
indication of the level of phosphate, in this case, in the soil.
1.13. ESSENTIAL UNITS
A common unit used to express the concentration of components or contaminants in the
environment is parts per million (ppm). This is a base unit of measurement and can be
thought of as 1 part per 1,000,000 parts. Usually we use grams or kilograms, so 1 g in
1,000,000 g would be 1 ppm. It may also be expressed as µg/L, which assumes that 1 ml
of water weights 1 g. It is then valid, because a liter is equal to 1,000,000 µL.
Today you will also find units such as ppb and ppt (parts per billion and parts per
trillion) being used. (See Table 1.2.) The amount of a component or contaminant in the
field is easily calculated using these units