United States
Environmental Protection
Agency
Rober S .Kerr Environmental
Research Laboratory
Ada OK 74820
Research and Development
EPA/600/S8-87/013 July 1987
SEPA Project Summary
State- of-the-Art Report:
Injection of Hazardous Wastes
Into Deep Wells
Arden Strycker and A. Gene Collins
About 11 percent of all
hazardous wastes are disposed of
by Injection wells Into deep
subsurface environments. Some 250
of these Class I wells are in the
United States, and their record of
performance is good.
Provisions of the Resource
Conservation and Recovery Act
(RCRA) require that by 1988 the
Environmental Protection Agency
(EPA) must show that the disposal
of specified wastes is safe to human
health and the environment, or
discontinue the practice of deep-
well injection of hazardous wastes.
These provisions necessitate
knowing the long-term fate of
these wastes in the injection zones.
A survey of the literature shows
that some information is available
on nearly all potential chemical and
biological transformation processes
of hazardous wastes. The literature
survey also indicates that additional
research is needed in all areas of
abiotic and biotic waste interactions.
before definitive explanations can
be given on the long-term fate of
hazardous wastes.
Usually, the first experimental
test is the fluid-fluid test of the
waste's compatibility with the
formation fluids. However, research
shows that this simple test is not
always adequate for determining the
interaction of injected wastes with
the subsurface environment
Among the many factors
affecting the ultimate fate of
injected wastes are the pH-Eh of
the waste and reservoir fluids, brine
concentration of the waste fluids,
clay type and amount In the
reservoir, presence or absence of
iron oxides, presence or absence of
organic complexlng agents,
molecular characteristics of organic
materials, and the anaerobic or
aerobic nature of the environment
Since all of these factors are
interrelated, any mixing of different
types of hazardous wastes in the
reservoir further complicates the
situation, making it difficult to
predict exactly the action or fate of
wastes after their injection. Only
limited relevant research has been
conducted to date, and the results
are insufficient to adequately
address this problem.
The National Institute for
Petroleum and Energy Research
staff conducted research in partial
fulfillment of Contract Number
DW89931947-01-0 under the
sponsorship of the U.S.
Environmental Protection Agency.
The report covers the contract
period from May 1, 1986, to
December 15, 1986, and the work
was completed as of December 15,
1986.
This Project Summary was
developed by EPA's Robert S. Kerr
Environmental Research Laboratory
Ada, OK, to announce key findings of
the research project that is fully
documented In a separate report of
the same title (see Project Report
ordering information at back.)
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Introduction
The Environmental Protection
Agency (EPA) estimates that some 250
Class I wells account for disposal of
about 11 percent of all liquid hazardous
wastes produced annually in the United
States on about 11.5 billion gallons of
injected hazardous wastes. Although
these wells are located in sedimentary
basins throughout this country, most of
them are in the Great Lakes region and
along the Gulf Coast.
Increased concern of the fate of
wastes after disposal has led to
changes in chemical processes.
Indeed, under the Resource
Conservation and Recovery Act
(RCRA), the EPA is required to make
some decision concerning the safety of
underground injection of hazardous
wastes by August 8, 1988. Although a
study prepared for the Underground
Injection Practices Council (UIPC)
showed that to date only a few
malfunctions have resulted in
contamination of water supplies, future
practices will have to ensure that no
additional contaminations will occur.
The literature survey discussed in
this report was initiated to determine
what knowledge is available concerning
ultimate fate of injected hazardous
wastes. The present discussion is
limited to the hazardous wastes listed
in the Hazardous Wastes and Solids
Amendments of 1984. Also, "ultimate
fate" of injected wastes are limited to a
time determined by available laboratory
and field pilot techniques. Since
knowledge of the interactions of
complex waste mixtures is limited,
much of the discussion relates to
particular chemicals or chemical
groups. Because of the limited
research conducted specifically for
deep-well formations, literature
reports that address the interaction of
wastes by any method that has useful
applications to deep-well formations
also were considered in the survey.
Demonstrating the fate of injected
wastes requires knowledge of the
behavior of each waste after it enters
the subsurface environment. The
interaction of a particular waste with
other injected wastes, with reservoir
fluids, and with reservoir solid
materials, such as clays, silicates, and
carbonates must be known.
Unfortunately, hazardous wastes are
complex mixtures of materials, and
when combined with other mixed waste
streams, the potential number of
interactions increase factorially.
Furthermore, since subsurface
environments often take many years to
reach chemical and biological
equilibrium, predicting exactly what will
happen a priori may be nearly
impossible. For example, some toxic
metals may adsorb onto clays as they
are injected, making the migrating
fluids immediately nonhazardous. This
adsorption process does not guarantee
that some waste injected into the
reservoir in the future might lead to
desorption of the sams- metals, thus
rendering the fluids hazardous again.
The first section following the
introduction of this report discusses
what is known about nonorganic
hazardous materials. Such major
processes as neutralization, hydrolysis,
ion exchange, precipitation,
complexation, and adsorption are
included in the discussion. As might be
expected, organic {materials not
considered in the iRCRA list of
hazardous wastes may strongly
influence these processes and will be
discussed as appropriate.
The second fol owing section
discusses what is known about organic
hazardous materials. Such processes
as thermal degradation, adsorption,
oxidation, reduction, i hydrolysis, and
microbial degradation are included in
this discussion. Microbial degradation
actually is a general term for many
biological processes and will be
discussed at some length.
Discussion
PROCESSES INVOLVING
NONORGANIC HAZARDOUS
WASTES
The most likeily reactions or
transformations lor nonorganic
hazardous wastes include precipitation,
adsorption/desorption, ion exchange,
hydrolysis, complexation, oxidation/
reduction, acid/base reaction, and
mineral dissolution. Only a small
percent of total wast ?s injected actually
contain toxic metal:
Included are arsenic
chromium, lead, mercury, nickel, and
selenium. Most o
1.0 percent).
barium, cadmium,
the nonorganic
wastes injected are classified as
nonhazardous and contain various alkali
metal salts, such as lithium, sodium,
and potassium. Tie presence and
concentration of these nonhazardous
metals are important considerations,
however, when evaluating the fate of
hazardous wastes- The particular
hazardous - materials addressed as
hazardous in this section include free
metal and associated salts of arsenic,
cadmium, chromium (VI), lead,
mercury, nickel, selenium, thorium,
inorganic salts of cyanide, and any
solution with a pH lower than 2.0
derived from mineral acids.
Subsurface reservoir fluids have
equilibrated with reservoir minerals and
clays during geologic time. All of the
minerals, rocks, hydrocarbons, and
gases are interrelated and contribute to
the final stable solute/solvent matrix that
exists in the reservoir. On the other
hand, waste solutions considered for
deep-well injection were generated in
a different environment and have
attained a thermodynamic equilibrium
under different circumstances.
Consequently, upon injection into the
formation, adjustments must occur
before a new solute/solution equilibrium
is reached. Some injected wastes result
in immediate precipitation of solids
from solution and potentially may plug
the formation from further injection.
Other wastes may result in less
immediate problems, but still may
reduce the life of the injection well
significantly. Thus, injection well
operators usually conduct laboratory
tests on injected hazardous wastes and
native reservoir fluids to determine their
compatibility.
Some research studies in the
literature show that the type oi
precipitate produced from incompatible
fluids is important in determining the
resulting degree of permeability
reduction. For example, ferric hydroxide
appreciably blocks the flow of fluid:
through a porous matrix, but bariun
sulfate and calcium sulfate do nol
Ferric hydroxide is gelatinous in natur
while barium sulfate and calcium sulfat
are finely crystalline. The type c
precipitate seems to determine th
effectiveness of blocking in the porou
matrix. One method of preventin
injection wells from becoming plugge
due to incompatible waters has bee
the injection of a buffer zone i
nonreactive water. The effectiveness
this method has been demonstrated.
Other problems with incompatib
fluids may result from extreme p
values. High pH solutions tend
dissolve silica and release fines th
migrate and plug the pores, resulting
permeability damage to the formatic
Additional formation damage may occ
as the dissolved silica reprecipitates
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another section of the reservoir.1
Alternatively, low pH solutions may
lead to silica gels or the dissolution of
some clays and carbonate (either
matrix or cements). All of these
problems are not as evident in
carbonate formations. However, later
deposition of materials with changes in
pH may also be a problem in carbonate
formations.
Studies of compatible and
incompatible fluids with reservoir fluids
also show that simply mixing the fluids
together in a flask is inadequate to treat
the many complexities that often occur.
Examples of apparently incompatible
fluids causing little trouble in contrast to
apparently compatible fluids plugging
the injection wells were noted and
further illustrate the inadequacy of this
simple test.
Precipitation is only one of many
interactions possible. Although
reactions between hazardous wastes
and silica, dolomite, etc., are relatively
straightforward and reactions between
waste and reservoir fluids can often be
easily characterized, the interactions
between waste materials and formation
clays have been difficult to
characterize. Obvious interactions such
as clay swelling and clay particle
migration are possible with any injected
fluid. In secondary and/or tertiary
petroleum recovery operations,
petroleum engineers usually avoid
injecting alkaline solutions, and
sometimes all aqueous solutions when
water-sensitive clays are present.
Damage to the clays can result in
drastically reduced permeabilities, and
often specialized products that stabilize
clays are used in treating fluids to
prevent damage to reservoirs.
Adsorption of organic and
nonorganic materials varies, depending
upon the amount and type of clay
present in the formation, because
different clays have different amounts
of surface area and different charge
densities. Since clays possess an
overall negative charge, cations such
as moderately soluble metal wastes are
attracted to these clays. The more
soluble ions previously attached to the
clays may resolubilize when other less
soluble ions replace them on the clay
surface. This process is termed ion
exchange. Some of the metals
associated with the clay, may bond so
tightly that they may be considered
immobile, or permanently adsorbed.
Heavy metals are particularly
susceptible to this behavior.
Champlin conducted flow
experiments through cores and
measured the ion and particle
concentrations throughout.2 He found
that when the salinity of the effluent
was low, both the ions and particles
were strongly retained by the sand in
the core. When the salinity of the water
was high, both the ions and suspended
particles passed through the core and
were not retained by the sand. Most
importantly, isotopic labeling led him to
further postulate that when the ions and
particles passed through the core, the
ions were still attached to the migrating
particles. Thus, a mechanism was
proposed where salinity affected the
attractive forces between the
suspended particles and the sand
grains. The physical and chemical
forces of the metal ions were
sufficiently strong to remain attached to
the clay particles.
Certainly, clays provide a surface
capable of attracting certain nonorganic
hazardous wastes. The adsorption of
these wastes onto the clays in the
injection zone is desirable, since the
mobile fluids would be rendered less
hazardous, and the clays may be
protected from additional swelling and
migration. Of course, the adsorption of
materials is not desirable when such
processes result in the plugging of the
well. Predicting how much waste will be
adsorbed, how long the waste will
remain immobile, and under what
circumstances the waste will be
desorbed is difficult.
Processes Involving Organic
Hazardous Wastes
Unlike that of most nonorganic
hazardous materials, the organic
hazardous wastes can be made non-
hazardous by molecular
transformations of the compounds.
Such processes as ion exchange,
oxidation, reduction, hydrolysis,
cyclization, and biological
transformation are all possible means
of rendering the wastes nonhazardous.
Alternatively, these same processes
can lead to increased toxicity for
certain wastes. Because the number of
potential products is almost infinite,
assessing the "ultimate fate" of these
wastes is even more difficult than for
the nonorganic hazardous wastes.
A brief discussion of the various
processes of degradation available to
organic wastes will be presented in this
section. The details of the discussions
are shorter than the corresponding
discussions for nonorganic materials for
two reasons: (1) less definitive work has
been done, and (2) the number of
possible interactions is much greater
than for that of nonorganic materials,
making definitive fate predictions more
difficult to determine.
As with nonorganic hazardous
wastes, adsorption is a major
mechanism of fluid-solid interactions
that affect the mobility of organic
wastes. Adsorption has been found to
be dependent upon the particular clays
present, that is, montmorillonite and
vermiculite have very high adsorption
capacities, while kaolinite has a very
low adsorption capacity. Illite and
chlorite are intermediate in their ability
to adsorb. These properties have been
attributed to the available surface area
for the respective clays.
Many factors influence the degree
of adsorption of various chemicals,
including chemical shape and
configutation, acidity, water solubility,
charge distribution, polarity, molecular
size, and polarizability. Most organic
compounds that adsorb on clay
materials have contribution from all of
these properties. Thus, predicting the
degree of adsorption according to
relative acidity, may lead to incorrect
predictions if resonance stabilization is
not considered. Molecular shape may
increase or decrease the adsorption
energies of any particular compound,
even though the other chemical
properties may be very similar. All of
these listed properties have been
demonstrated to be important, and are
found to be interdependent.
Another important factor that
influences adsorption is temperature.
Since adsorption processes are
generally exothermic and desorption
processes are generally endothermic,
an increase in temperature would
normally reduce adsorption processes.
However, a number of exceptions have
been found. The pesticide EPTC is an
example.
Mortland3 discusses some of the
mechanisms by which organic
chemicals are adsorbed. These include
the replacement of metals with cationic
molecules, replacement of metals by
neutral molecules that are protonated to
become cationic, ion exchange with
polyvalent metals attached to the clay,
coordination with metal cations, and by
hydrogen bonding. Many of these
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mechanisms have been studied and
demonstrated using such techiques as
adsorption, isotherms, calorimetry, X--
ray diffraction, UV-visible
spectroscopy, electron spin resonance
spectroscopy, and infrared
spectroscopy.
Another process by which certain
hazardous organic wastes can be
transformed into other compounds is
by oxidation reactions. Such
compounds as phenols, aromatic
amines, olefins, dienes, alkyl sulfides,
and eneamines are particularly
susceptible. The exact species of
different oxygen radicals present in
aqueous and soil environments that
initiate the oxidation process may
include such oxy-radicals as H0«,
RO«, and 102, depending upon the
environmental conditions. The
evaluation of the potential importance
of this process at typical injection well
conditions to hazardous waste injection
has not been done. However, if certain
hazardous wastes containing chromium
(VI) are co-injected either
simultaneously or sequentially with
organic wastes, the oxidation process
most certainly would be important.
Chromium (VI) can be an excellent
oxidizing agent.
Hydrolysis is another process of
waste transformation, and is the
process by which some functional
group attached to a molecule is
replaced by an -OH functional group
originating from a water molecule.
Hydrolysis can be catalyzed by either
an acid or a base, as defined in
chemistry for Lewis acids and bases
Mabey and Mill4 provide a good review
of hydrolysis processes of organic
materials in the environment. Factors
such as pH, temperature, and the
presence of other ions are known to
affect the rate of hydrolysis. Depending
upon whether the pH is high or low,
different mechanisms may apply. At
low pH, the hydronium ion (H30 + )
predominates in hydrolysis, while at
high pH, the hydroxide ion (OH-)
predominates.While the effect of
temperature is known, the magnitude
for the influence temperature has on
different compounds is not always
known. The presence of certain alkaline
earth and heavy metal ions may
catalyze hydrolysis for a variety of
esters.
Many potential processes are
available for biodegradation. These
processes include mineralization,
detoxification, cometabolism, activation,
and defusing. Mineralization is the
conversion of organic wastes to
inorganic wastes. Detoxification is the
conversion of toxic compounds to
nontoxic compounds. Cometabolism is
the conversion of one organic
compound to another without the
microorganism using this process as a
nutrient. Activation is the conversion of
a nontoxic compound to a toxic
compound. Defusing is the process of
converting a compound potentially
capable of becoming hazardous to
another nonhazardous Compound by
circumventing the hazardous
intermediate. Although defusing has
been confirmed in the laboratory, it has
not been identified in the environment.
The first question With respect to
biodegradation most certainly is: Do
any microbes exist in the injection
zone, or can microbes capable of
converting organic wastes to
nonhazardous wastes exist at those
conditions? A number of researchers
have attempted to answer this question.
DiTommaso and Elkan5 analyzed a
saline aquifer at the depths of 850-
1000 ft. Much of the waste injected into
this zone contained organic compounds
such as acetic acid, formic acid, and
methanol. About 3000 organisms/ml
were isolated from the unpolluted
aquifer. Most of the organisms
identified were aerobes typically found
in aquatic environments. The most
common genera include
Agrobacterium, Pbeudomonas,
Proteus. Bacillus, Aerobacter,
Corynebactor, Arthobacter, and
Micrococcus. Conversely, samples
taken after waste injection were found
to contain primarily anaerobic
methanogenic bacteria.
Horvath6 provides a good review of
some of the studies conducted on
biodegradation of wastes in subsurface
environments, and summarizes some
of the processes involved in the
degradation of such compounds as
acetate, formate, methanol,
formaldehyde, and Aromatic acids.
Horvath also developed a laboratory
model to evaluate these processes.
These studies indicated an interesting
observation concerning methanogenic
and sulfate-reducing bacteria.
Apparently, even thoMgh the required
nutrients were available, methanogenic
bacteria did not proliferate, whereas the
sulfate-reducing bacteria did in the
zone studied. This behavior may have
occurred because of the pH that
predominated for the test and
supported work done by others who
indicated that the two types of bacteria
are mutually exclusive. Obviously, the
degradation of injected wastes will
depend upon which type of bacteria
prevails. Methane-producing and
sulfate-reducing bacteria do not
degrade the same compounds.
In summary, microbial degradation
often is a very important mechanism by
which certain hazardous wastes may be
transformed to nonhazardous wastes.
Bacteria have been identified in the
subsurface environment. Within certain
reasonable limits, bacteria are capable
of surviving relatively hostile
environments. Studies have also shown
that transformation processes via
biological pathways can be very
complicated and difficult to establish.
Therefore, this method should not be
depended upon for waste detoxification
in the subsurface environment, unless
pilot studies have established the
existence of these processes.
Conclusions
Many conclusions were presented
by the papers reviewed in this report;
some of the more significant ones are
as follows:
The basic compatibility test
conducted by mixing waste fluids
and reservoir fluids does not always
give meaningful results. The test
must be conducted under reservoir
conditions. Precipitates, if formed in
the compatibility test, may or may
not plug the well depending upon
the type of precipitate formed.
For nonorganic wastes, solution pH
is critical for determining the
ultimate fate. The identity of soluble
species, solubility products,
adsorption characteristics, and
chemical interactions are some of
the variables affected by pH.
The brine concentration, even
though not listed as hazardous, is
important in affecting clay stability
and adsorption characteristics.
The presence of organic
complexing agents may or may not
affect the mobility of heavy metals
in the reservoir.
Adsorption of nonorganic wastes is
dependent upon a number of
factors, such as Eh, pH, clay type,
and the presence or absence of iron
oxides and hydroxides.
Mathematical models dc
successfully predict certain reactior
processes. However, some
interactions have not been modeled
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The major mechanism of
degradation for certain halogenated
hydrocarbons is hydrolysis.
Microbial degradation of organic
hazardous wastes has been shown
to be useful in decontamination
after deep-well injection. Wastes
can support growth of certain
bacteria during the injection
process. However, results are not
always predictable, and studies
suggest that each case should be
evaluated individually to determine
the feasibility of the process.
Some case studies corroborate the
interrelationships of degradation
processes discussed in this report.
References
1 Thornton, S.D. and P.B. Lorenz.
Role of Silicate and Aluminate Ions
in the Reaction of Sodium
Hydroxide with Reservoir Minerals.
Paper SPE 16277 pres. SPE 1987
Int. Symp. on Oilfield Chem., San
Antonio, Texas, February 4-6.
2 Champlm, Jerry B. Francis. The
Transport of Radioisotopes by
Fine Pariiculate Matter in Aquifers.
NTIS Report PB-232 179,
December 1969, 187 pp.
3 Mortland, M M. Interaction
Between Organic Molecules and
Mineral Surfaces. Ground Water
Quality, (ed. C.H. Ward, W. Giger,
and P.L. McCarty), publ. John
Wiley & Sons, New York, 1985, pp
370-385.
4 Mabey, W., and T. Mill. Critical
Review of Hydrolysis of Organic
Compounds in Water under
Environmental Conditions. J. Phys.
Chem. Ref Data, v. 7(2), 1978, pp.
383-415
5 DiTommaso, Anthony and Gerald
H. Elkan. Role of Bacteria in
Decomposition of Injected Liquid
Waste at Wilmington, North
Carolina. Underground Waste
Management and Artificial
Recharge Prep., v.1, 1973, pp.
585-599.
6 Horvath, Edward. Interactions of
Aquifer Flora and Industrial Waste
in a Model Deep Well Disposal
System. Ph.D. Thesis, North
Carolina State University, 1977,
111 pp
Arden Strycker and A. Gene Collins are with the National Institute for Petroleum
and Energy Research. Bartlesville, OK 74005.
Lowell £. Leach is the EPA Project Officer (see below).
The complete report, entitled "State-of-the-Art Report: Injection of
Hazardous Wastes into Deep Wells," (Order No. PB 87-170 551; Cost: $13.95,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Robert S. Kerr Environmental Research Laboratory
U.S. Environmental Protection Agency
P.O. Box 1198
Ada, OK 74820
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United States
Environmental Protection
Agency
Official Business
Penalty for Private Use $300
EPA/600/S8-87/013
Center for Environmental Research
Information
Cincinnati OH 45268
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