TPH reduction were more responsive to a vari-
tion at sites where it can be easily used.
able that was not well controlled.
Annual reductions in TPH concentrations for
the heavy oil after approximately 1 year were not
significantly different for any of the treatments.
For heavy-oil-contaminated soil, landfarming was
DISCUSSION
more effective than bioventing during the initial
After 362 days of treatment, landfarming was
phase of treatment, and this effect was more pro-
as effective in reducing TPH concentrations in
nounced when nutrients were added. The lag pe-
diesel-contaminated soil as was bioventing. Nu-
riod that occurs prior to rapid microbial growth
trient additions had a significant effect on reduc-
and increased activity appeared to have a greater
ing diesel concentrations for both landfarming and
effect in the heavy-oil-contaminated soil compared
bioventing.
to the diesel-contaminated soil. We hypothesize
For heavy-oil-contaminated soil, the effect of
that the measured increase in TPH concentrations
nutrient additions was less evident and there were
is an analytical artifact of microbial fatty acids and
no significant differences among any treatments
related microbial biomass that are produced dur-
at t362. For both bioventing treatments, measured
ing adaptation of the soil microbiota to new con-
TPH concentrations increased from t238 to t362.
ditions and carbon sources. If this is the cause, it
Several factors may account for this. When using
is reasonable to predict that, given favorable in-
TPH as an indicator of treatment effects, many
cubation conditions that are sufficiently long, there
have observed an apparent increase in TPH con-
may be a period of rapid heavy-oil bioremediation
centrations at the onset of biotreatment. We hy-
following the temporary increase in measured
pothesize that initial increases in measured TPH
TPH values. However, if the favorable incubation
are caused by an increase in microbial biomass that
time is too short owing to a brief summer season,
accompanies a growing microbial population and
the same cycle may occur in subsequent years.
precedes measurable decreases in TPH concentra-
Low-cost strategies that further stimulate the mi-
tions. During the initial phases of degradation, the
crobial population may be needed to effectively
microorganisms use bioavailable forms of native
treat these soils. Recent studies suggest that plant-
based systems may be useful (Nichols et al. 1997).
soil carbon rather than contaminant carbon. Na-
These data demonstrate that, depending on the
tive soil carbon forms are less likely to be included
nature of the contaminant and nutrient status of
in TPH extractions or measurements. However, the
the soil, the brevity of the summer season--when
metabolites produced by the rapidly expanding
temperatures are favorable for microbial activity--
population do appear as part of the TPH measure-
may prevent either unamended or nutrient-
ment. As the microbial population increases, mea-
amended biotreatment from attaining sufficient
sured TPH concentrations increase as bioavailable
microbial numbers and activity to substantially
native soil carbon becomes depleted with no de-
reduce TPH concentrations during that season.
crease in contaminant carbon in the soil. TPH con-
This may result from microbial population cycling,
centrations would not decrease until after
bioavailable carbon has been consumed and con-
from low populations at early summer to high
taminant carbon is used. Subsequent mineraliza-
populations at late summer, with little TPH me-
tion of the contaminant carbon would then even-
tabolized. The persistence of the heavier fraction
tually result in a decrease in soil contaminant TPH
of older oil spills in cold regions suggests that this
concentrations.
may be happening (Collins et al. 1993). For diesel
spills, the effect is not as pronounced and many of
the compounds are more readily degraded; hence,
nutrient amendments appeared to provide suffi-
CONCLUSIONS
cient stimulation to significantly reduce TPH con-
This demonstration showed that landfarming
centrations.
can be a reasonable alternative to bioventing for
Despite both treatments being done ex situ (re-
both heavy-oil- and diesel-contaminated soils at
quiring excavation and transportation of soil),
remote sites typical of cold regions. In diesel-
costs for the landfarming treatment were less than
contaminated soil, reduction in TPH concen-
for bioventing. The reduced requirements for in-
trations was greatest using landfarming with nu-
frastructure also make landfarming attractive in
trient amendments. Landfarming with nutrient
remote sites typical of cold regions. Landfarm in-
additions should be considered as a treatment op-
stallation and operation in a remote area is less
7
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