Handbook of Solvents - George Wypych - ChemTech - Ventech!

23.1 Natural attenuation of chlorinated solvents 1601
recharge zones in organic-rich environments (such as swamps), or zones impacted by natural
oil seeps. A Type 2 environment generally results in slower biodegradation of the highly
chlorinated solvents compared to a Type 1 environment. However, given sufficient organic
loading, this environment can also result in rapid degradation of these compounds. A Type
2 environment typically will not occur in crystalline igneous and metamorphic rock (see
discussion of likely hydrogeologic settings for Type 3 environments).
23.1.5.2.3 Type 3
A Type 3 environment is characterized by a well-oxygenated groundwater system with little
or no organic matter. Concentrations of dissolved oxygen typically are greater than 1.0
mg/L. In such an environment, halorespiration will not occur and chlorinated solvents such
as PCE, TCE, TCA, and CT will not biodegrade. In this environment, very long dissolved-phase
plumes are likely to form. The most significant natural attenuation mechanisms
for PCE and TCE will be advection, dispersion, and sorption. However, VC (and
possibly DCE) can be rapidly oxidized under these conditions. A Type 3 environment is often
found in crystalline igneous and metamorphic rock (fractured or unfractured) such as
basalt, granite, schist, phyllite, glacial outwash deposits, eolian deposits, thick deposits of
well-sorted, clean, beach sand with no associated peat or other organic carbon deposits, or
any other type of deposit with inherently low organic carbon content if no anthropogenic
carbon has been released.
Two conceptual models are provided for environments in which Type 3 behavior occurs.
For sources with PCE and TCE, the major natural attenuation processes are dilution
and dispersion alone (no biodegradation). As shown in 23.1.7, the PCE and TCE plumes extend
from the source zone and concentrations are slowly reduced by abiotic processes.
Chloride concentrations and oxidation-reduction potential will not change as groundwater
passes through the source zone and forms the chlorinated ethene plume. If TCA is the solvent
of interest, significant abiotic hydrolysis may occur, resulting in a more rapid decrease
in TCA concentrations and an increase in chloride concentrations.
In Figure 23.1.7, a source releases VC and 1,2-DCA into the groundwater at a Type 3
site (an unlikely occurrence as more highly chlorinated solvents are typically released at
sites). Because the VC and 1,2-DCA can be degraded aerobically, these constituents decline
in concentration at a significant rate. Chloride is produced, and a depression in dissolved
oxygen concentration similar to that occurring at fuel sites, is observed.
23.1.5.2.4 Mixed environments
As mentioned above, a single chlorinated solvent plume can exhibit different types of behavior
in different portions of the plume. This can be beneficial for natural biodegradation
of chlorinated solvent plumes. For natural attenuation, this may be the best scenario. PCE,
TCE, and DCE are reductively dechlorinated with accumulation of VC near the source area
(Type 1); then, VC is oxidized (Type 3) to carbon dioxide, either aerobically or via Fe(III)
reduction further downgradient and does not accumulate. Vinyl chloride is removed from
the system much faster under these conditions than under reducing conditions.
A less ideal variation of the mixed Type 1 and Type 3 environments is shown in the
conceptual model in Figure 23.1.8. An extended TCE and 1,2-DCE plume results because
insufficient fermentable carbon results in an anaerobic zone which is too short for complete
biodegradation. Therefore, TCE extends well into the aerobic zone where no
biodegradation occurs. A long DCE plume also extends into the aerobic zone, indicating in-