Geoenvironment Impact Mitigation and Management - Sustainable Practices in Geoenvironmental Engineering - page 390

• Miscellaneous compounds , including oxygen-containing organic compounds such
as phenol and methanol and nitrogen-containing organic compounds such as
TNT (trinitrotoluene)
As shown in Chapter 2, the density of these compounds in comparison to that of water
has direct control on their transport in the subsoil. We classify nonaqueous phase liq-
uids (NAPL) into the light NAPLs identiied as LNAPLs, and the dense ones called the
DNAPLS. LNAPLs include gasoline, heating oil, kerosene, and aviation gas. DNAPLs
include the organohalide and oxygen-containing organic compounds such as 1,1,1-tri-
chloroethane, chlorinated solvents such as tetrachloroethylene (PCE), trichloroethylene
(TCE) and carbon tetrachloride (CT), PCBs, PCPs (pentachlorophenols) and TCPs (tet-
rachlorophenols). As shown in Figure 2.7 in Chapter 2, since LNAPLs are lighter than
water and the DNAPLs are heavier than water, NAPLs will likely stay above the water
table, and DNAPLs tend to sink through the water table and come to rest at an imperme-
able bottom (bedrock).
The various results of transformations and biodegradation of organic chemicals have
been discussed in various forms in the earlier part of this chapter. The signiicant out-
come of NA as a tool for mitigation of impact is the evidence of occurrence of biodegrada-
tion and transformation of the target organic chemicals in the NA process. The indicators
that need to be prescribed in the LOE relate to speciic decreases in concentration of the
contaminants and transformations (conversions and biodegradation) of organic chemical
contaminants. Determination of the nature and composition of the transformed products
of the original organic chemical contaminants is required. Knowledge of the products
obtained via abiotic and biotic processes is essential. A good example of this is, for exam-
ple, recognizing that abiotic transformation products are generally other kinds of organic
chemical compounds, whereas transformation products resulting from biotic processes
are mostly seen as stages (intermediate products) toward mineralization of organic chemi-
cal compounds. Procedures for the identiication and characterization of abiotic processes
for chlorinated hydrocarbons have been reviewed by the USEPA (2009). Biologically medi-
ated transformation processes are the only types of processes that can lead to mineraliza-
tion of the subject organic chemical compound. Complete conversion to CO 2 and H 2 O (i.e.,
mineralization) does not always occur. However, intermediate products can be formed
during the mineralization. New techniques are being developed to study the sources of
the contaminants and the biodegradation process such as Compound Speciic Isotope
Analysis (CSIA) on dissolved organic contaminants such as chlorinated solvents, aromatic
petroleum hydrocarbons, and fuel oxygenates isotope (USEPA 2008b).
10.8.2 Metals
At the very least, prescription of the indicators for the lines of evidence in respect to heavy
metals requires determination of (a) the nature and concentration of sorbed metal ions,
(b) porewater chemistry including pH and Eh , and (c) the environmental mobility of heavy
metals. The environmental mobility of heavy metals is dependent to a very large extent
upon whether they are in the porewater as free ions, complexed ions, or sorbed onto the
soil particles. Prescription of indicators for LOE should take into account the assimilative
capacity of the subsoil and the nature and fate of the heavy metals in the subsoil. So long
as the full assimilative potential of the soil for HM is not reached, attenuation of the HMs
will continue. Metals that are sorbed onto the soil particles are held by different sets of