456 HAZARDOUS WASTE MANAGEMENT
degrade contaminants. Some limitations may include inappro-
priate site hydrogeologic characteristics (including the inability
of the geostrata to transport adapted microorganisms) and con-
taminant toxicity. Monitoring and reporting is required, and a
health-based risk assessment may be required by regulators.
Natural attenuation is frequently enhanced by several
components, such as the creation of a barrier or the addition
of a chemical or biologic additive to assist in the degradation
of contaminants.
The overall economics of this approach can be sig-
nificantly more favorable than the typical pump-and-treat
approach. One must be careful to consider, however, that the
costs of assessment will equal or exceed that necessary for
other methods, and the costs associated with sentinel moni-
toring will be borne for a longer period of time.Barriers This has been used in instances where the over-
all costs of the remedial action is very high, and the geo-
logic features are favorable. It involves the installation of
a physical cut-off wall below grade to divert groundwater.
The barriers can be placed either upgradient of the plume
to limit the movement of clean groundwater through the
contaminated media, or downgradient of the plume with
openings or “gates” to channel the contaminated groundwa-
ter toward a remedial system. This technology has proven
to be more efficient and less costly than traditional pump
and treat methods, but also requires favorable hydrogeologic
conditions. It allows for the return of treated groundwater to
the upgradient end of the plume with a continuous “circu-
lar” flushing of the soil, rather than allowing the dilution by
groundwater moving from the upgradient end of the plume.
The result is greater efficiency, and a shorter treatment time
period. While the cost of the cutoff wall is significant, it is
important to conduct a proper analysis of long-term pump-
and-treat costs, including the operation and maintenance of
a system that would otherwise be designed to accept a much
larger quantity of groundwater.
The creation of a hydraulic barrier to divert upgradient
groundwater from entering the contaminant plume allows
the pumping of groundwater directly from the affected area
and often allows the reinjection of the treated water back
into the soils immediately upgradient of the plume. This
allows for the efficient treatment of the impacted area, with-
out unnecessary dilution of the contaminated groundwater
plume. It does, however, require an accurate assessment of
the groundwater regime during the assessment stage. This
promising concept is not radical, but its use in connection
with natural remediation is growing rapidly.Passive Treatment Walls Passive treatment walls can be
constructed across the flow path of a contaminant plume to
allow the groundwater to move through a placed media, such
as limestone, iron filings, hydrogen peroxide or microbes.
The limestone acts to increase the pH, which can immobi-
lize dissolved metals in the saturated zone. Iron filings can
dechlorinate chlorinated compounds. The contaminants will
be either degraded or retained in concentrated form by the
barrier material.Physical Chemical Soil Washing Soil is composed of a
multitude of substances, with a large variance in size. These
substances range from the very fine silts and clays, to the
larger sand, gravel and rocks. Contaminants tend to adsorb
onto the smallest soil particles, as a result of the larger sur-
face per unit of volume. Although these smaller particles
may represent a small portion of the soil volume, they may
contain as much as 90% of the contamination.
Soil washing involves the physical separation, or clas-
sification, of the soil in order to reduce the volume requiring
treatment or off-side disposal. It is based on the particle size
separation technology used in the mining industry for many
decades. The steps vary, but typically begin with crushing
and screening. It is a water-based process, which involves
the scrubbing of soil in order to cause it to break up into the
smallest particles, and its subsequent screening into various
piles. The fraction of the soil with the highest concentra-
tion of contamination can be treated using technologies fre-
quently used by industry. The goal is to reduce the quantity
of material that must be disposed. The clean soil fractions
can often be returned to the site for use as fill material where
appropriate.
The use of soil washing technology has some limitations,
including a high initial cost for pilot testing and equipment
setup. It will be most useful on large projects (requiring reme-
diation of greater than 10,000 cubic yards of soil). Sites with
a high degree of soil variability, and a significant percentage
of larger particles will show the greatest economic benefit.Soil Vapor Extraction Soil Vapor Extraction (SVE) is an
effective method for the in-situ remediation of soils contain-
ing volatile compounds. Under the appropriate conditions
volatile organic compounds will change from the liquid
phase to the vapor phase, and can be drawn from the subsur-
face using a vacuum pump. There are several factors neces-
sary for the successful use of this technology, including 1)
the appropriate properties of the chemicals of concern (they
must be adequately volatile to move into a vapor phase),
and 2) an appropriate vapor flow rate must be established
through the soils.
Air is drawn into the soils via perimeter wells, and
through the soils to the vapor extraction well. It is drawn to
the surface by a vacuum pump and subsequently through a
series of manifolds to a treatment system such as activated
carbon or catalytic oxidation.
A concentration gradient is formed, whereby in an effort
to reach equilibrium, the liquid phase volatile contaminants
change into the vapor phase and are subsequently transported
through the soils to the treatment system.
This technology is particularly effective for defined spill
areas, with acceptable soils. It is most effective in remediating
the soils in the vadose zone, the area that is in contact with
the fluctuating groundwater table. Groundwater contaminated
with these compounds and similar soil conditions can be reme-
diated using air sparging, a variation of soil vapor extraction.
A variation of this technology is thermal enhanced SVE,
using steam/hot air injection or radio frequency heating to
increase the mobility of certain compounds.C008_001_r03.indd 456C008_001_r03.indd 456 11/18/2005 10:28:44 AM11/18/2005 10:28:44 AM