Modeling - Field Sampling: Principles and Practices in Environmental Analysis - page 177

7.1.3. Quantitative
In many instances quantitative rather than simple qualitative models may be desirable.
Here again several types of physical models are commonly used. The most important and
commonly used among these is the soil column. Columns from several centimeters in
length and diameter to several meters in length and decimeters in diameter are frequently
constructed to model the movement of water and contaminants through soil. Glass,
plastic, and metal have been used to contain the column, with plastic being the material
of choice today. Such columns are often fitted with ports, electrodes, and other
monitoring devices so that the conditions in the column and the movement of
components through the column can be followed.
Columns can be constructed in a number of different ways. A retaining material, screen
covered with vermiculite, or coarse sand covers the bottom. This is often constructed so
that the flow out of the column can be controlled. Air-dried soil is added to the column
and then water is used to saturate it. Alternatively the column can be partially filled with
water, and soil can be slowly added. The latter method produces a more uniform soil
column, although it may be less representative of soil in the environment. Water,
contaminants, or solutions of contaminants in water can be added to the columns and
movement through the column followed. A column used for modeling water and
contaminant through soil is shown in Figure 7.2.
Soil columns constructed and used in this way can be very useful in determining the
fate of contaminants, insecticides, herbicides, and so on in soil. A graph of the movement
of ethers dissolved in water through the soil column shown in Figure 7.2 is shown in
Figure 7.3. The units of the x axis are apparent void volumes. The apparent void volume
is void space in soil. (See Chapter 2.) The more soluble methyl tertiary butyl ether
(MTBE) elutes from the column before the less soluble octyl ether. Note that the more
soluble compound comes out a little before one complete void volume. This is attributed
to preferential movement through large pores in the soil.
The data obtained from the graph in Figure 7.3 can be used to predict where a
contaminant is most likely to be found in a soil profile and how far and fast it will move,
and can be estimated on the basis of the mm of rainfall. From these data the breadth and
depth of needed sampling can be estimated. It should be noted that such column
conductivities are often lower than field conductivities. This can be due to plant cover,
soil structure, cracks, and animal holes. When needed, field studies using lysimeters can
be used to validate column studies.
Lysimeters are another dynamic physical way of modeling the environment. A
lysimeter is constructed by isolating an undisturbed volume of soil in the field. The soil is
isolated by concrete or metal dividers set in the