Geoenvironment Impact Mitigation and Management - Sustainable Practices in Geoenvironmental Engineering - page 362

permeability, exposed surface areas in the void channels, speciic surface area
(SSA), cation exchange capacity (CEC), and surface functional groups associated
with the soil solids.
b. Those that depend on the interactions between the contaminants and the soil sol-
ids and the chemical constituents in the porewater. In this respect, the proper-
ties of the complete soil-water system become more important. These would be
the chemistry of the porewater, the presence and types of inorganic and organic
ligands in the porewater, pH and Eh or pE , exchangeable ions, SSA, and CEC. All
these properties, together with the biological properties of the soil-water system
will deine the initial state of the soil-water system and hence the capability of the
soil to react with incoming waste leachate streams and contaminants.
10.5.1 Physical and Mechanical Properties
In Chapter 2, we pointed out that the two primary types of interactions between the soil par-
ticles (soil solids) and liquid waste streams and contaminants in transport in the soil subsur-
face are physical and chemical in nature. For soil conditions both in the in situ state and in the
prepared state (i.e., engineered soil barriers), we need to have good mechanical and hydraulic
performance characteristics from the soil. The physical, hydraulic, and mechanical proper-
ties useful for mitigation and control of liquid and solid waste substances and contaminants
are those that impede and/or prevent the low or passage of liquid and solid substances.
We need to distinguish between (a) the natural in situ soil condition where the soil is in the
landscape as a surface and subsurface soil, and (b) the situation where human intervention
and manipulation of the soil is possible, i.e., placement of a prepared soil in the ground as
an active mitigation and/or treatment tool. In the case of natural soils in the landscape and
subsurface, transport of liquid waste streams, leachates, and contaminants are controlled by
the in situ physical properties of the subsurface soil. Without human intervention, it becomes
a case of “getting what the natural situation dictates.” Changes in the physical hydraulic, and
mechanical properties of the subsoil that will likely occur because of chemical and physico-
chemical interactions with contaminants will be discussed in the next subsection.
In the case where prepared soil is used as a sole treatment tool or as one of the tools in a
designed mitigation treatment process, control on the soil physical, hydraulic, and mechanical
properties can be exercised. At this stage, the design physical, hydraulic, and mechanical
properties of the soil to be used are important factors in the mitigation and prevention of
contaminant transport. The soil properties of signiicance include physical and hydraulic
properties such as density, permeability, porosity, and mechanical properties such as com-
pactibility, compressibility, consolidation, and strength. All of these properties depend on
the texture, grain (particle) morphology, particle size distribution, and composition. These
all combine to control the packing of the particles and density of the compacted mate-
rial. Figure 10.2 shows the relationship between all of these and the physical, hydraulic,
and mechanical properties obtained in relation to the compactibility of the soil material.
The mechanical properties are of importance in liner, buffer, and barrier systems and the
physical and hydraulic properties feature prominently in luid and gas transport through
the soil. The common assumption that transport of contaminants is halted when luid
transport is stopped is wrong since transport mechanisms for contaminants and pollut-
ants are via diffusive processes. So long as there is water in the soil barrier system, diffu-
sive transport will occur. The water in the soil, even if it is immobile, serves as the carrier
for the contaminants.