Stressors and Soil Contamination - Sustainable Practices in Geoenvironmental Engineering - page 58

The two types of interactions involved in contaminant transport processes are:
1. Physical interactions: These interactions involve luid and contaminant movement
through the soil fabric and structure. As such, the nature of the void spaces and
the amount of surface area presented by the soil particles to the liquid waste and
leachate stream are important factors. The principal factors include:
a. Void spaces: The size distribution of voids and the continuity of voids are
important factors in determining the rate of transport of the leachate stream
through the soil. The nature of these voids, their continuity, and their distribu-
tion are all dependent on the density and structure of the soil. These in turn are
functions of soil composition and manner in which the soil was formed in situ.
b. Surface area and microstructure: The interactions of the various contaminants
in the leachate stream with the surfaces of the soil particles will be dictated by
the amount of surfaces exposed to the contaminants. Because of the existence
of soil microstructural units (i.e., packets of soil particles grouped together
to form aggregate groups or peds or clusters of particles), not every single
particle will have its total surface area exposed to the leachate stream. The
sizes and types of microstructural units that comprise the soil in question will
determine the amount of surface areas exposed to the inluent leachate. The
detailed discussion on soil microstructure and hydraulic conductivity will be
found in Section 10.5 in Chapter 10 in the discussion on mitigation of impacts
from contaminant transport.
2. Chemical interactions: This grouping of interactions includes all the types of
chemical reactions that occur when two chemically reactive participants interact
with each other. The surface properties, and especially the surface chemistry of
the soil solids, are very important factors. The surface chemically reactive groups
for the soil solids and for the contaminants are identiied as functional groups.
These will be discussed in a later section in this chapter.
From the schematics shown in Figures 2.9 and 2.10, we obtain an appreciation of the
constituents of a typical soil unit and have learned that the different soil solids are known
as soil fractions. Although the types of soil fractions can range from sands to clay minerals
to soil organics, it is the soil fractions with reactive surfaces that are of interest in the study
of transport and fate of contaminants. Reactive surfaces , in the present soil unit context, are
deined as those surfaces that can react chemically with dissolved solutes in the porewater
of the soil. We should also note that contaminants in the ground will also have reactive
surfaces, and in this case, these reactive surfaces can react chemically with other dissolved
solutes in the porewater and also with the soil solids.
2.5.4.1 Speciic Surface Area and Cation Exchange Capacity
The soil fractions that have more particles with signiicant reactive surfaces are the clay
minerals, oxides and hydrous oxides, soil organics, and carbonates. Table 2.1 gives the
surface charge characteristics, SSA, and cation exchange capacity (CEC) for some clay
minerals. We deine the SSA as the total surface area of all the soil solids or particles per
unit volume. Since theoretical calculations for SSA can become both complex and tedious,
because of the irregular shapes and sizes of the soil particles, laboratory techniques are
often used. A popular procedure is to determine the amount of gas or liquid (adsorbate)