Introduction to Field Sampling - Field Sampling: Principles and Practices in Environmental Analysis - page 16

TABLE 1.2 Common Units Used to Express the Concentration of a Component in a
Field

Prefix Value Abbreviation

0.00000 1 a

Micro ppm

Nano 0.000000001 ppb

Pico 0.000000000001 ppt

Fento 0.000000000000001 ppq

Atto 0.000000000000000001 ppa

a Equals 1/1,000,000.

when used in conjunction with a media's bulk density. These units also allow for easy
conversion between the unit, the mass of material, often soil, and the amount of
component or contaminant present. Other units of importance include bulk density and
what is called a hectare furrow slice.

1.13.1. Bulk Density and Particle Density
In field sampling a sampler is inserted into the soil and a sample extracted. When using a
core or bucket sampler a volume of soil is obtained. To understand the relationship
between sample volume and the mass of soil it is essential to know the bulk density of the
soil. As noted above, soil is composed of both solids and void space filled with water and
air. Because of this the term bulk density is used rather than density.
The bulk density of a soil is the amount of soil per unit volume. The official units for
bulk density are Mg/m 3 . However, the older units of g/cc will commonly be encountered.
Both units are the same in that 1.24 Mg/ m 3 =1.24 g/cc (1 g/cm 3 ). Most soils have bulk
densities between 1.00 and 1.70 Mg/m 3 . Lower or higher bulk densities indicate an
unusual situation, such as an organic soil or a compacted layer.
Knowing the bulk density the mass of soil taken can be calculated if its volume is
known. On the other hand, knowing the mass the volume can be calculated. The bulk
density will disclose the occurrence of compacted zones in soil, which can affect
sampling. Often it is also important to know how much of the sample is solid and how
much is void space. For this determination the particle density of the soil particles is
needed.
Soil scientists assume that the individual particles in soil have an average density of
2.65 Mg/m 3 . From this it is obvious that soil must contain a significant amount of void
volume. As noted above, a soil sample is assumed to be half solid and half void space.
The void space is assumed to contain water and soil air. The percentage of the soil
sample with this void space can be calculated by dividing the bulk density by the particle
density. More information about the calculation of void space is given in Chapter 2.
Other more specific examples of how bulk density is used in sampling will be given in
later chapters [14].