Modeling - Field Sampling: Principles and Practices in Environmental Analysis - page 179

soil.
FIGURE 7.3 An example of the movement of ethers through the column
shown in Figure 7.2. The y axis is in arbitrary units. The x axis is in
column apparent void volumes. MTBE is methyl tertiary butyl ether.
(Data from senior research project of Dan Tener, Chemistry
Department, Wilmington College, Wilmington, Ohio.)
field. The bottom of the soil is retained by a metal grill supported by sand and gravel
underlain by a slanting, water-impervious layer of plastic, cement, or metal. This allows
water percolating through the soil to be collected. The big advantage of lysimeters is that
they contain a largely undisturbed column of soil. Because it is undisturbed it is expected
to more faithfully model the action and interactions of the soil as a whole. A diagram of a
typical arrangement of a lysimeter is shown in Figure 7.4.
Water collected at the bottom of the lysimeter can be pumped to the surface for
sampling and analysis. Alternately, an access tunnel can be constructed to allow samples
to be collected without pumping. There are too many different designs of lysimeters to
describe them all here. In some cases a soil water sampling methodology that collects soil
water at some depth may be called a lysimeter even when it does not involve a physical
lysimeter as such.
Movement of water and contaminants through a lysimeter can be followed under
natural rainfall or irrigation. The amount of water percolating through a soil versus the
amount lost through evapotranspiration can be measured and correlated to environmental
conditions. Knowing the amount of percolating water is essential when modeling soil
phenomena.