Geoenvironment Impact Mitigation and Management - Sustainable Practices in Geoenvironmental Engineering - page 388

Vertical cross-section

Reduced contaminant
concentration and toxicity

Section A-A

Permeable reactive
barrier

Contaminant
plume

Direction of
groundwater flow

Optional addition of
enhanced NA
treatment zone

Plan view of section A-A

FIGURE 10.16
Cross-sectional and plan view of PRB. Plan view shows leachate plume entering the PRB with contaminants
and leaving the PRB with reduced contaminant concentration and toxicity. If required, an enhanced NA treat-
ment zone can be situated in front of the PRB to increase the effectiveness of the contaminant impact mitigation
scheme. (Adapted from Yong, R.N. and Mulligan, C.N., Natural Attenuation of Contaminants in Soils , CRC Press,
Boca Raton, FL, 310 pp., 2004.)

10.8 Lines of Evidence
Lines of evidence (LOE) is a term that is associated with the use of NA as a tool for mitiga-
tion and management of impacts from waste and contaminant discharges to the land envi-
ronment. LOE refers to the requirement to determine whether a soil has capabilities for in
situ attenuation of contaminants. This requirement originates from procedures associated
with the use of monitored natural attenuation (MNA) as a treatment procedure. This is a
prudent course of action since there is need to determine how effective a particular soil
will be in attenuating contaminants. The types of information and analyses required for
LOE indicators are shown in Figure 10.17. Site and problem speciicities will dictate how
many pieces of information and what speciic kinds of analyses will be needed. The type of
information needed to deine the site characteristics is shown in the top right-hand corner
of the diagram. The physical (geological and hydrogeologic) setting sets the parameters
of the problem to be resolved. Whether the natural attenuation capability of the subsoil
is capable of mitigating and managing the contaminant plume anticipated within the site
boundaries will be established by the other two categories; patterns of natural attenuation
and supporting laboratory tests and analyses.
Knowledge of the patterns of natural attenuation identiied in the central box is essen-
tial. What is sought in this category is evidence of previous natural (intrinsic) remediation