Sustainable Geoenvironmental Engineering Practice - Sustainable Practices in Geoenvironmental Engineering - page 477

Attenuated leachate leaving
unit of soil mass

SQI data requirements for
soil attributes for
contamination impact
mitigation and
control

Physical
Density
Porosity

Chemical
Cation exchange capacity
CEC
pH and Eh
Sodium adsorption ratio SAR
Electrical conductivity
Organic C

Biological
Potentially
mineralizable N
Microbial biomass C
and N
Soil respiration

Hydraulic
conductivity

Calculated exposed
surface area

FIGURE 13.4
Soil properties pertinent to use of soil as a management tool for mitigation of impact from contaminants dis-
charged in the ground. Data from these attributes serve as input to determination of contamination mitigation
soil quality index (SQI).

attributes. We use the term attribute in the discussion to mean the property or character-
istic being measured.
The data obtained from tests and other kinds of measurements (ield and laboratory) of
the physical, chemical, and biological attributes can be used as (a) input to compare with
individual attribute indicators, thus leading to immediate comparison of the sustaining
capability of each individual attribute, and/or (b) input to statistical and analytical mod-
els developed to produce a lumped (combined or total) index parameter. Prescription of
individual attribute indicators is performed once again on the basis of intended function
of the soil. Take, for example, the set of attributes in the physical properties list in Figure
13.4. Density, porosity, calculated exposed surface area, and hydraulic conductivity have
been chosen as the set of pertinent attributes. Consider two speciic applications for the
soil: (1) use as permeable reactive barrier (PRB) material, as in Figure 13.5, and (2) use as
an engineered clay barrier (ECB) in the liner system shown in Figure 1.11 in Chapter 1.
The primary controlling property in both the PRB and ECB applications is the hydraulic
conductivity. For the PRB, one permits the transporting luid to penetrate the PRB at a rate
that allows for partitioning and transformation processes to occur. Residence time in the
PRB is paramount. This is controlled by an appropriate soil permeability and thickness of
the PRB. In general, one might want to design a wall thickness in conjunction with a Darcy
coeficient of permeability k in the range of 10 −5 to 10 −7 cm/s, depending on the partitioning
processes envisaged. In the case of the ECB application, a k value of considerably less than
10 −7 cm/s is generally sought. The prescription of k indicators for desired objectives can