4.4.6.4   Reduction of Toxicity, Mobility, or Volume through
Treatment -- Alternative 6

This evaluation criterion addresses the statutory preference for
selecting remedial actions that employ treatment technologies
that permanently and significantly reduce the toxicity, mobility,
or volume of hazardous substances.  This preference is satisfied
when treatment reduces the principal threats at a site through
destruction of toxic contaminants, reduction of the total mass of
toxic contaminants, irreversible reduction in contaminant
mobility, or reduction of the total volume of contaminated media. 
The ability of bioremediation to reduce the toxicity or volume of
contamination at a site was assessed based on the following
considerations:

          Reduction in toxicity of contaminated material
          Reduction in mobility of contaminants
          Reduction in volume of contaminated materials
          Irreversibility of treatment

Each of these considerations is addressed below.

Reduction in Toxicity of Contaminated Material

This alternative would reduce the toxicity of the contaminants
during both the anaerobic and aerobic phases of biological
treatment.  Anaerobic dechlorination has been observed to occur
primarily from the meta and para positions, with corresponding
increases in less-chlorinated, ortho-substituted congeners in
both naturally-occurring sediments (Brown and others 1987) and in
the laboratory (Abramowicz and others 1993; GECRD 1994; and
Quensen, Tiedje, and Boyd 1988).  The less-chlorinated congeners
that result from anaerobic dechlorination are both less toxic and
more readily degraded by aerobic bacteria than the initial
highly-chlorinated congeners (Quensen, Tiedje, and Boyd 1988; and
PRC 1995e).  The toxicity of treatment residuals from anaerobic
dechlorination would be further reduced during aerobic
degradation.  Aerobic degradation is a destructive process in
which aerobic bacteria ultimately convert PCBs to nontoxic end
products including carbon dioxide, water, chloride salts, and
cell mass.  

Reduction in Mobility of Contaminants

Bioremediation reduces the mobility of PCBs.  Only the resistant
fraction of PCBs would remain after biological treatment, and the
same mechanisms that cause this resistant fraction to lack
bioavailability, such as sorption to organic material, would
prevent the PCBs from partitioning into solution and becoming
mobile.  

Reduction in Volume of Contaminated Material

This alternative could possibly increase the volume of materials. 
During CSPB, bulking agents may be added to provide organic
substrate and increase the porosity of the material, which would
further increase the volume of the material requiring treatment
at the CTF.  However, depending on the organic content of the
contaminated materials, biological treatment could possibly cause
a reduction in volume as the biodegradable organic matter is
converted to methane and carbon dioxide, which would be released
as gases from the materials during anaerobic and aerobic
treatment, respectively.

Irreversibility of Treatment

Anaerobic dechlorination and aerobic destruction are both
irreversible processes under the conditions that would be present
in the treatment residuals.  Dechlorinated PCBs would not
recombine with chloride ions, and PCBs would not be reformed by
the end products of aerobic PCB destruction.