United States
Environmental Protection
Agency
Hazardous Waste Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/540/S2-86/002 Mar. 1987
&EPA Project Summary
Systems to Accelerate In Situ
Stabilization of Waste Deposits
In situ systems to accelerate the stabili-
zation of waste deposits have been pre-
sented as alternatives to containment,
isolation or excavation as methods for
remediation of uncontrolled waste sites.
In situ applications involve three essential
elements: selection of a chemical or bio-
logical agent (reactant) which can react
with and stabilize the waste, a method for
delivery of the reactant to the deposit and
a method for recovery of the reaction
products or mobilized waste. The most
promising applications for in situ treatment
methods are for spill or plume types of
contamination, where the contaminants
are relatively evenly distributed and pref-
erably in liquid form. Delivery of reactants
to solid, heterogeneous, low permeability
deposits will be more difficult. In situ
methods may find particular application
when used in combination with other
remedial measures, for example, removal
of the source material and in situ treatment
of the plume.
Four reactant categories have been
examined: biodegradation, surfactant-
assistant flushing, hydrolysis, and oxida-
tion. Of these, biodegradation and
surfactant-assisted flushing appear most
promising as in situ treatment techniques.
For any treatment technique, the potential
toxicity of the applied reactant and any in-
termediate compounds or by-products
must be carefully evaluated. Furthermore,
the potential for undesirable reactions with
other contaminants present must be stud-
ied (e.g., oxidation of phenol with hydro-
gen peroxide may also oxidize chromium
(III) to the more toxic hexavalent
chromium).
Methods of delivery of reactants based
upon gravity include surface flooding,
ponding, surface spraying, ditching, and
subsurface infiltration beds and galleries.
Forced injection (pumping) may also be
used. Permeability is an important con-
sideration in selecting the delivery system.
Gravity delivery methods require a per-
meability of the soil/waste medium in the
range 10~1 cm/sec to 10~3 cm/sec (280
to 2.8 ft/day). Forced injection is most
effective at a permeability in the range of
10~1 cm/sec to 10"" cm/sec (280 to
0.28 ft/day); below this permeability
limit a potential application of forced
injection for reagent delivery coupled
with electro-osmosis for recovery may
exist. Additionally, gravity systems
should be considered only when the
waste deposit lies in the unsaturated
zone and when the depth to the bottom
of the deposit is less than 5 meters (16
feet). Otherwise, forced injection should
be considered.
Recovery systems using gravity include
open ditching and buried drains, and
pumped methods include wellpoint and
deep well systems. Basically, the same
limitations that apply to delivery systems
are also true for recovery systems. Gravity-
induced recovery works best when the
water table is wrtnin 5 meters (16 ft) of the
surface. For depths in the range of 0-8
meters (0-26 ft), well points can also
be considered. Depths greater than the
suction limit (about 8 meters or 26 ft in
practice) will require the use of down-hole
pumps for recovery.
This Project Summary was developed
by EPA's Hazardous Waste Engineering
Research Laboratory, Cincinnati, OH, to
announce key findings of the research pro-
ject that is fully documented in a separate
report of the same title (see Project Report
ordering information at back).
Introduction
This project represents Phase I of a two
phase scope of work to document the
feasibility of engineered approaches to
treating subsurface waste deposits
through the application of in situ methods.
Phase I concentrated on applications of
available technology and examined the
limitations imposed on their use by site
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and waste specific characteristics. A
future phase of the project, Phase II, is
expected to undertake bench scale or pilot
studies to expand the data base available
to potential users of in situ methods.
Procedure
The six part program of investigation
consisted of a literature review, a defini-
tion of the capabilities and limitations of
delivery, recovery and treatment technolo-
gies, visits to sites where remedial activi-
ties were underway, a definition of impor-
tant site and waste characteristics, and an
evaluation of remedial technologies. The
available data were evaluated to determine
classes of organic chemicals amenable to
treatment by various potential in situ treat-
ment methods. Potential delivery and
recovery systems for these treatment
agents were then evaluated with respect
to site hydrogeologic characteristics. The
guidance manual which was developed
identifies combinations of delivery/
recovery technologies and reagents that
have a reasonably high or clearly low
probability of success for in situ treatment
of hazardous waste.
Results and Discussion
Systems to accelerate stabilization of
waste deposits will require (1) selection of
delivery/recovery methods for the treat-
ment technology compatible with site
characteristics and the waste deposit set-
ting, and (2) selection of a suitable treat-
ment technology that will be compatible
with waste composition and site char-
acteristics.
Delivery/Recovery Systems
The selection of the most appropriate
delivery/recovery methods and systems
requires a thorough understanding of the
waste deposit site characteristics. The site
must be defined with respect to the
configuration of the waste deposit (areal
extent and vertical depth), hydrologic
characteristics (surface and subsurface) of
the waste deposit, and surface and sub-
surface geohydrologic characteristics of
the materials surrounding the waste
deposit.
Delivery Methods
The matrix for selection of delivery
methods is presented in Table 1. The table
shows the forced delivery method is
applicable for all conditions. The choice of
a gravity delivery method is more depend-
ent on the listed parameters. The listed
parameters indicate differences in site
characteristics that would warrant selec-
tion of one delivery method(s) over the
2
others. These parameters were selected
for the following reasons:
• Average Permeability of the Waste
Deposit.
Permeability will dictate the flow char-
acteristics of the deposit. If the per-
meability of the deposit is high, then
low net pressure and short time dura-
tions would be required for a solution
to pass through the deposit. Low per-
meability means the deposit is not
easily drainable and would require
higher pressure nnd longer time dura-
tion for a solution to move through the
deposit. Gravity methods are most
effective for highly permeable waste
deposits.
• Depth to the Waste Deposit.
Engineering judgement is the basis for
selecting between gravity and forced
delivery methods at each specific site.
If the depth is too great, then gravity
delivery will lengthen the time for a
solution to travel to the deposit, or
extensive excavation may be required
to obtain effective gravity delivery. A
reasonable maximum depth for gravity
delivery is judged to be about 5 meters
(15 feet).
• Waste Deposit Covered by an Imper-
meable Layer.
For the forced delivery method this
parameter has no bearing, although for
gravity delivery methods it will have a
significant impact. For example,
flooding and spray irrigation cannot be
utilized as delivery methods if the
surface of the deposit is topped by an
impermeable layer of soil or synthetic
material.
• Topography.
Topographic considerations will limit, in
part, the extent of applicability of grav-
ity flow methods. For example, on a
steep slope flooding or ponding delivery
methods cannot be utilized. However,
topograhpy will not affect the forced
delivery methods(s).
• Infiltration Rate.
Gravity delivery at the deposit surface
is most effective for deposits with high
infiltration rates. Infiltration rate has no
bearing in forced delivery systems.
In general, gravity delivery methods are
effective when the waste deposit is
situated in the unsaturated zone with
shallow permeable overburden, and
depth to the deposit is limited to 15 feet
with permeability greater than 10 3
cm/sec.
For waste deposits covered by thick
overburdens of significant depth (more
than 5 meters) the forced delivery
method will be most effective. For
waste deposits having a permeab
lower than 10 4 cm/sec a forcetl
method utilizing electro-osmosis could
be employed for solution injection into
the deposit. In general, the forced
method should be highly effective for
waste deposits within a permeability
range of 10 ' cm/sec to 10 4 cm/sec.
Recovery Methods
Table 2 indicates the applicability of
various recovery methods for different site
characteristics. Only two parameters,
depth to recovery zone and composite
permeability, are considered in the matrix.
Although other parameters such as trans-
mission and storage may play an important
role in designing woll or well point
systems, these two parameters are the
most appropriate guide for the preliminary
selection of recovery methods. It should
be noted that the recovery of injected solu-
tion will be from the saturated zone (water
table aquifer) and normally the recovery
method(s) will be installed beyond the
boundary of the waste deposit. However,
when a recovery method is installed within
the waste deposit, the composite perme-
ability of the waste deposit should be con-
sidered in selecting the recovery method.
The parameter, depth to recovery zone, is
chosen because gravity methods are
practical beyond a 5 meter depth
vacuum well points are also effective to
a 5 to 8 meter depth. The permeability will
dictate the drainage characteristics and
thereby control the selection of recovery
(dewatering) methods.
In general, gravity recovery methods are
suitable for a shallow recovery zone (depth
to water table from the surface should not
be more than 5 meters). For a deeper
recovery zone, forced recovery methods
must be employed.
The two basic treatment concepts eval-
uated are in situ waste destruction by
biodegradation, hydrolysis or chemical
oxidation, and surfactant-assisted flushing
to mobilize the contaminants and facilitate
further in situ treatment or recovery fol-
lowed by above-ground treatment. Poten-
tial applications of these methods to
various classes of organic contaminants
are presented in Table 3.
Biological Renovation of Waste
Deposits
Aerobic and anaerobic bacteria, fungi,
actinomycetes, algae and cyanophytes
(blue-green algae) have all been shown
capable of degrading many classes of
organic chemicals. These microbes include
natural microbial populations, ada
microbial cultures and potentially bi
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\ible J.
Delivery
Methods
Matrix for Delivery Methods
Location of the deposit Thickness of
in relation to existing Contamination overlying
groundwater table starts at impermeable
layer
Unsatu-
rated
GRAVITY
1. Flooding
2. Ponding
3. Surface
Spraying
4. Ditches
5. Infiltration
Galleries
6. Infiltration
Bed
FORCED
1. Injection
Pipes
X
X
X
X
X
X
X
Partially
Saturated
if
if
NA
if
if
if
X
Sur- Sub-
Saturated face surface 0
NA
NA
NA
NA
NA
NA
X
X
X
X
NA
NA
NA
X"'
X
X
X
X
X
X
X
X
X
X
X
X
X
X
<1.5m
OS ft!
NA
X
NA
X
X
X
X
>1.5m
OS ftl
NA
NA
NA
NA
X
X
X
Topography
{Slope/
flat
X
X
X
X
X
X
X
0-3%
X
X
X
X
X
X
X
Infiltration
Rate
cm/hr
(inches/hrl
Hydraulic Conductivity
cm/sec Ift/dayi
.1-.2 .06-. 7 <06 10~' -10~3
>3% I.3-S>I.15-.3I<0.15 1280-2.81
NA
NA
if
X
X
NA
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
NA
if
NA
X
X
X
X
if
X
if
X
X
X
X
12.8-0.28)
NA
if
NA
if
if
if
X
Depth to Bottom
of the
Waste Deposit
Meters /ft)
10.28-0.0003) K16I
NA
NA
NA
NA
NA
NA
X<2>
X
X
X
X
X
X
X
5-12
116-40)
if
if
if
if
X
X
X
O40I
NA
NA
NA
NA
NA
NA
X
X = Applicable
LE = Less Effective
NA = Not Applicable
(J) = May need combined gravity and forced del/very.
(2) = Applicable with electro-osmosis.
Table 2. Matrix for Recovery Methods
Recovery
Methods Depth To Groundwater
0-5 m
(0-16 ft)
h.
CAVITY:
Open Ditches X
and Trenches
Porous Drains X
FORCED:
Wellpoint X
Deep Well NA
Vacuum Well X
Point
Electro- X
osmosis
X = Applicable
LE = iess Effective
NA - Not Applicable
5 -12m
(16-40 ft)
NA
NA
X
X
X
X
>12 m
O40 ft)
NA
NA
NA
X
NA
X
Hydraulic Conductivity
>10'1 -10~3
cm/sec
O280-2.8
ft/day)
X
X
X
X
NA
NA
10-3-10~4
cm/sec
(2.8-0.28
ft/day)
LE
LE
LE
LE
X
NA
10-4-10~7
cm/sec
(0.28-0.003
ft/day)
NA
NA
NA
NA
LE
X
gineered microbial strains. Once the extent
of the contamination and its chemical
characterization have been determined,
the proper microorganisms (or groups of
microbes) may be identified and deve-
loped. The identification of the proper
agents for waste site renovation is based
upon past experience, laboratory screen^
ing, and onsite pilot-scale tests.
To date, aerobic bacteria such as pseu-
domonas have been most commonly used
for in situ biodegradation of contaminants.
These organisms can potentially com-
pletely convert the organic compounds to
C02 and water, and do not produce H2S
-at methane as reaction products. However,
lerobic bacteria are important for the
biodegradation of pesticides and halogen-
ated organics. Organic contaminants that
have been successfully treated by biode-
gradation include phenols, gasoline and
other petroleum products, methylene chlo-
ride, alcohols and acetone.
In the process of designing the microbial
waste treatment system, one must deter-
mine the oxygen, emulsifier (if the wastes
are insoluble) and fertilizer requirements
for optimum waste treatment rates. Micro-
bial agents require the maintenance of
sufficient concentrations of nitrogen,
phosphorus and trace elements, and a pH
range that will support their growth. The
levels of these factors at the site should
be determined during the site investiga-
tion; the need for additional fertilizers or
buffers required to support microbial
growth can then be identified.
Biological renovation of subsurface
waste deposits poses problems relating to
oxygen supply, temperature, permeability
and accessibility not encountered with
surface disposal sites. Injection wells may
be established into and below the waste
site to deliver a fertilizer and oxygen
supply. Oxygen sources would include
injectable solutions of peroxides, oxygen-
charged water produced by ozonation, or
direct sparging of air into the ground
water. Recovery wells or trenches should
be situated at points peripheral to or
downgradient of the waste deposit. Flow
patterns established between injection
and recovery wells should be planned to
aid in confining the waste during the
renovation process. In this way ground
water plumes that may be migrating from
the site can be renovated as well.
Application of Hydrolysis to Waste
Deposit Stabilization
Hydrolysis is a chemical reaction involv-
ing the cleavage of a molecular bond by
reaction with water. The rates of hydroly-
sis for some compounds can be acceler-
ated by altering the solution pH, tempera-
ture, solvent composition, or by introduc-
ing catalysts. For in situ treatment,
alternation of pH, particularly raising the
pH (base-catalyzed hydrolysis), is the most
promising approach. The range of chem-
ical classes potentially treated by base-
catalyzed hydrolysis includes amides,
esters, carbamates, organo-phosphorus
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Table 3.
Potential Applications of Treatment Methods to Waste Contaminants
Chemical Bio-
Class
degradation
Hydrolysis
1'1
Oxidation121
Water
Flushing131
Surfactant
Flushing131
Aliphatic Hydrocarbons
Alky I Ha /ides
Ethers
Halogenated Ethers
and Epoxides
Alcohols
Glycols/Epox/des
Aldehydes, Ketones
Carboxylic Acids
Amides
Esters
Nitriles
Amines
Azo Compounds,
Hydrazine Derivatives
Nitrosamines
Thiols
Sulfides, Disulfides
Sulfonic Acids, Sulfoxides
Benzene & Substituted Benzene
Halogenated Aromatic
Compounds
Aromatic Nitro Compounds
Phenols
Halogenated Phenolic
Compounds
Nitrophenolic Compounds
Fused Polycyclic Hydrocarbons
Fused Non-Aromatic Polycyclics
Heterocyclic Nitrogen
Compounds
Heterocyclic Oxygen
Compounds
Heterocyclic Sulfur
Compounds
Organophosphorus Compounds
Carbamates
Pesticides
111
121
131
Based upon calculated half-lives for base catalyzed at pH 9 to 11.
Based on oxidation of chemicals in water and wastewater by H202-
Based upon aqueous solubility and octanol/water partition coefficient IKOW).
+ = can be used ? = futher research needed
- = cannot be used -? = probably cannot be used
+ ? = probably can be used
compounds, pesticides and herbicides.
Base-catalyzed hydrolysis has been suc-
cessfully used for treatment of surface
spills of acrylonitrile and pesticides.
The primary design concern for imple-
mentation of base-catalyzed hydrolysis
within a waste deposit will be the produc-
tion and maintenance of high pH (9 to 11)
conditions with saturation or high mois-
ture content in the waste deposit. For
shallow subsurface or surface deposits,
surface application of lime, sodium car-
bonate or sodium hydroxide followed by
surface application of water may be
appropriate. For deeper deposits, subsur-
face delivery or injection of alkaline solu-
tions may be required.
Potential for In Situ Oxidation of
Waste Deposits
The potential application of three oxi-
dants (ozone, hydrogen peroxide, and
hypochlorites) to waste deposits was
evaluated. Although in widespread use in
surface water treatment applications,
significant problems may preclude their
effective implementation as in situ treat-
ment agents for waste deposits.
Hypochlorite reacts with organic com-
pounds as both a chlorinating agent and
an oxidizing agent. Documentation on the
effectiveness of hypochlorite as an oxidiz-
ing agent for organic materials is extreme-
ly limited. Hypochlorite additions may lead
to production of undesirable chlorinated
by-products (e.g., chloroform) rather than
oxidative degradation products. Therefore
the use of hypochlorite for in situ treat-
ment of organic wastes is not recom-
mended.
While ozone is an effective oxidizing
agent for many organic compounds in
wastewater treatment applications, its rel-
atively rapid decomposition rates in aque-
ous systems, particularly in the presence
of certain chemical contaminants or other
agents which catalyze its decomposition
to oxygen, preclude its effective applica-
tion to subsurface waste deposits. The
half-life of ozone in ground water is less
than one-half hour. Considering that fir""
rates of water through waste deposits I
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1 likely to be on the order of inches/hour or
less, it is unlikely that effective oxidant
doses of ozone can be delivered outside of
the immediate vicinity of the point of appli-
cation. Successful use of ozone for in situ
chemical oxidation is unlikely. However,
ozonation has been used successfully to
supply oxygen for microbial biodegrada-
tion, and to chemically oxidize complex
organics in a surface reactor to simpler
compounds that are more readily biode-
gradable. This use of ozone as a supple-
mentary treatment for biodegradation
seems promising.
Hydrogen peroxide is a weaker ozidizing
agent than ozone; however, its stability in
water is considerably greater. Since de-
composition of hydrogen peroxide to ox-
ygen may be catalyzed by iron or certain
other metals, effective delivery of hydro-
gen peroxide throughout an entire waste
deposit may be difficult or impossible
because of the relatively low transport
velocities achievable in waste deposits.
Prior to consideration of hydrogen perox-
ide as an in situ treatment method, it will
be necessary to investigate the stability (or
rate of decomposition) of hydrogen perox-
ide in the specific waste deposit matrix.
Hydrogen peroxide may also be used as an
oxygen source for microbial biodegrad-
ation.
Surfactant-Assisted Flushing or
Sofubilization of Wastes
Flushing or mobilization of wastes can
serve two purposes: to promote the recov-
ery of wastes from the subsurface for
treatment on the surface, or to solubilize
adsorbed compounds in order to enhance
the rate of other in situ treatment tech-
niques (such as biodegradation or hydro-
lysis). Flushing or mobilization using water
alone may be sufficient for relatively solu-
ble compounds such as phenols; however,
the use of surfactants will be required for
significant solubilization of insoluble (hy-
drophobic) compounds.
Surfactants (surface active agents) are
a class of natural and synthetic chemicals
which promote the wetting, solubilization,
and emulsification of various types of or-
ganic chemicals. A simple approach to
evaluating the potential use of surfactants
in organic waste recovery involves consid-
eration of the aqueous solubility or
octanol-water partition coefficient, K0w
Surfactants would be most effective in
promoting the mobilization of organic
compounds of relatively low water solu-
bility and high Kow values.
Laboratory tests suggest that surfac-
tants may enhance the recovery of sub-
surface gasoline leaks by groundwater
pumping, and promote the mobilization of
crude oil and PCBs from soils. However,
certain environmental factors may reduce
the in situ effectiveness of surfactants.
These include precipitation of the surfac-
tant by groundwater with high TDS or
alkaline earth cation concentrations (Ca,
Mg); reduction of surfactant effectiveness
due to nonoptimal pH or temperature; or
adsorption of the surfactant by soil par-
ticles, negating its solubilizing properties.
Nevertheless, the use of surfactants either
alone (to flush otherwise insoluble organ-
ics) or in combination with other treat-
ments (to solubilize the waste materials
and thereby promote biodegradation) is a
promising avenue for further research.
Selection and Evaluation of Sys-
tems for Treatment of Specific
Waste Problems
Before final selection of delivery/
recovery and treatment methods, the
following steps must be undertaken for
each site:
Initial Site Evaluation -
In this phase the following information
is obtained for each site:
• Extent and nature of the waste deposit;
• Site soil characteristics such as por-
osity and permeability, and uniformity;
• Surface drainage characteristics of the
site;
• Groundwater table location and ground-
water flow direction and velocity;
• Field permeability testing of the waste
deposit and host materials;
• Surface infiltration rate;
• Soil, waste deposit and groundwater
samples collection and laboratory
analyses.
Identification of Feasible Methods
Based on the field investigations and
laboratory testing, feasible treatment and
delivery/recovery methods commensurate
with the treatment requirements, are
identified using the matrices in Tables 1
through 3. These feasible methods are
carefully evaluated based on engineering
judgement to narrow down the choices for
the field demonstration program.
Bench Scale Tests
Bench scale tests may be necessary to
demonstrate the effectiveness of a given
treatment method for a specific combina-
tion of chemical contaminants and waste
deposit matrix.
Field Demonstration Program
A field demonstration program for the
selected feasible methods is undertaken
to evaluate the effectiveness of the
methods and to generate design informa-
tion, such as ditch spacing and well spac-
ing required for proper delivery and re-
covery of the treatment agent.
Design and Economic Evaluation
of the Effective Methods
Based on the field demonstration pro-
gram, alternate delivery/recovery systems
are developed and a cost evaluation is per-
formed. Based on cost and effectiveness
analysis, final selection of a delivery/
recovery system was made for subse-
quent implementation.
Conclusions and
Recommendations
To accelerate stabilization of waste piles
or deposits using a combination of chem-
ical or biological reagents and delivery/
recovery systems involving gravity or
forced methods of injection, a great deal
more information based upon specific field
experimentation must be assembled.
Each in situ application will resemble a
research effort which must be customized
to the site and waste characteristics. The
essence of a successful application of an
in situ method is the performance of a
treatability study designed to account for
the peculiarities of the waste and treat-
ment reagent combination as well as the
unique geohydrological characteristics of
the site. Since treatability studies cannot
exist for the generalized case, almost all
conclusions to be drawn from the liter-
ature survey were necessarily based upon
engineering judgement. Verification of
hypotheses by reference to documented
field experience was not feasible in most
situations.
A major constraint on the feasibility of
in situ treatment is the degree of homo-
geneity of the waste deposit. Subsurface
deposits contained in drums or within non-
uniform formations which impede the flow
of waterborne reagents cannot be con-
sidered as realistic candidates for in situ
treatment. The experience that exists
strongly suggests that the greatest in situ
success will be with a plume or a spill
situation rather than with a source deposit
itself.
The full report was submitted in fulfill-
ment of Contract No. 68-03-3113, Task
37-2 by JRB Associates under the spon-
sorship of the U.S. Environmental Protec-
tion Agency.
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