Handbook of Solvents - George Wypych - ChemTech - Ventech!

1600 Hanadi S. Rifai, Charles J. Newell, Todd H. Wiedemeier
Figure 23.1.6. Conceptual model of Type 1 environment
for chlorinated solvent plumes. [From T.H.
Wiedemeier, H. S. Rifai, C. J. Newell and J.T. Wilson,
Natural Attenuation of Fuels and Chlorinated Solvents
in the Subsurface, after reference 88. Copyright
© 1999 John Wiley & Sons, Inc. Reprinted by permission
of John Wiley & Sons, Inc.]
carbons, landfill leachate, or other
anthropogenic carbon because these
organics exert a tremendous electron-acceptor
demand on the system. This condition is
referred to as a Type 1 environment. In a
Type 1 environment, anthropogenic carbon
is fermented to produce hydrogen which
drives halorespiration.
The geochemistry of groundwater in a
Type 1 environment is typified by strongly
reducing conditions. This environment is
characterized by very low concentrations of
dissolved oxygen, nitrate, and sulfate and elevated
concentrations of Fe(II) and methane
in the source zone (Figure 23.1.6). The presence
of methane is almost always observed
and confirms that fermentation has been occurring
at the site, generating hydrogen. If
measured, hydrogen concentrations are typically
greater than 1 nanomolar. Importantly,
a Type 1 environment results in the rapid and
extensive degradation of the more
highlychlorinated solvents such as PCE,
TCE, and DCE: PCE → TCE → DCE → VC
→ Ethene → Ethane
In this type of plume, cis-1,2-DCE and
VC degrade more slowly than TCE; thus,
they tend to accumulate and form longer
plumes (Figure 23.1.6a). In Figure 23.1.6b,
the PCE declines to zero and is replaced, in
sequence, by a peak in TCE concentrations,
followed by a peak in cis-1,2-DCE, VC, and
ethene. Fermentation constituents (BOD and
acetate) and inorganics are shown in Figure
23.1.6c and 23.1.6d. Figure 23.1.6d illustrates how the fermentation substrate (represented
by BOD) extends beyond the source before being consumed. Both panels show long chloride
and methane plumes extending far downgradient from the plume area, because chloride
is conservative and methane cannot be biodegraded in an anaerobic environment. The acetate
curve indicates where active primary fermentation is occurring; declining acetate concentrations
are due to consumption by methanogens in the plume area.
23.1.5.2.2 Type 2
The classification system of Wiedemeier et a1. 107 recognized that anaerobic conditions may
also result from the fermentation of naturally-occurring organic material in the groundwater
that flows through chlorinated solvent source zones. This Type 2 environment occurs in
hydrogeologic settings that have inherently high organic carbon concentrations, such as
coastal or stream/river deposits with high concentrations of organics, shallow aquifers with