EPA
United States
Environmental Protection
Agency
Office of Research and
Development
Washington, DC 20460
EPA/600/R-93/164
August t993
Bioremediation Using the
Land Tre4tment Concept
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EPA/600/R-93/164
Environmental Regulations
and Technology
Bioremediation Using the
Land Treatment Concept
Daniel F. Pope and John E. Matthews
August 1993
This report was developed by the
Robert S. Kerr Environmental Research Laboratory
U.S. EPA, ORD
Ada, Oklahoma 74820
Printed on Recycled Paper
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DISCLAIMER
The information in this document has been funded in part by the United States Environmental Protection Agency under
Contract No. 68-C8-0058 to Dynamac Corporation. It has been subjected to the Agency's peer and administrative review, and
it has been approved for publication as an EPA document. Mention of trade names or commercial products does not
constitute endorsement or recommendation for use.
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FOREWORD
EPA is charged by Congress to protect the nation's land, air and water systems. Under a mandate of national environmental
laws focused on air and water quality, solid waste management and the control of toxic substances, pesticides, noise and
radiation, the Agency strives to formulate and implement actions which lead to a compatible balance between human activities
and the ability of natural systems to support and nurture life. |
The Robert S. Kerr Environmental Research Laboratory is the Agency's center of expertise for investigation of the soil and
subsurface environment. Personnel at the Laboratory are responsible for management of research programs to: (a)
determine the fate, transport and transformation rates of pollutants in the soil, the unsaturated and saturated zones of the
subsurface environment; (b) define the processes to be used in characterizing the soil and subsurface environment as a
receptor of pollutants; (c) develop techniques for predicting the effect of pollutants on ground water, soil, and indigenous
organisms; and (d) define and demonstrate the applicability arjd limitations of using natural processes., indigenous to the soil
and subsurface environment, for the protection of this resource
Bioremediation processes using the land treatment concept, whereby contaminated soil is treated in place or excavated and
treated in prepared-bed treatment units, are common soil remediation technologies proposed for hazardous waste sites.
However, RSKERL and other research and demonstration studies have identified complex biological, chemical and physical
interactions within contaminated subsurface media which may| impose limitations on the overall effectiveness of bioremediation
processes utilizing the land treatment concept. This report was developed to summarize and discuss basic considerations
necessary to implement and manage these types of bioremediation systems to improve their efficiency and effectiveness in
reclaiming contaminated soils.
Clinton W. Hall
Director
Robert S. Kerr Environmental Research Laboratory
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CONTENTS
INTRODUCTION 1
Definition of Land Treatment [ 1
Microorganisms and Bioremediation . 1
LAND TREATMENT TECHNOLOGY 3
In-Situand Ex-Situ Land Treatment • 3
Lift Application and Tilling 3
Nutrients, Carbon Sources, and Other Additives 4
Bioaugmentation • 5
Soil Moisture Control — •• • • 5
Types and Concentrations of Contaminants Remediable by Land Treatment 6
Petroleum Derived Contaminants 1 6
Wood Preserving Contaminants [ 7
Levels of Contamination Susceptible to Land Treatment 7
BIBLIOGRAPHY I • • 8
Land Treatment Concept References j. 8
Soil Properties References -i - • 8
Monitoring References 8
Petroleum Contaminant References • • 9
Wood Preserving Contaminant References .] 9
APPENDIX A-
SOIL PROPERTIES ; 10
Soil Horizons • 10
Depth .., - 10
Texture , • 10
Bulk Density J 10
Porosity, Hydraulic Conductivity, and Permeability , 10
Soil Moisture and Water Holding Capacity J 11
Tilth , 12
Sorptive and Exchange Capacity , 12
Organic Matter J 12
pH - 12
Nutrients • 12
Salinity , - 13
Redox Potential 13
Color - 13
Biological Activity . • - 13
Metals in Soils 13
APPENDIX B-
MONITORING 14
Waste Transformation 14
Parent Compound Loss .' 14
Breakdown Products 14
Toxicity Reduction 14
Microorganisms 15
Soil Moisture 15
Nutrients 15
SAMPLING STRATEGIES ! , 15
Measuring Transformation Rates j 16
Volatilization, Leachateand Runoff i — 17
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INTRODUCTION
This document is designed to be used by those who are
involved with the use of land treatment technologies for the
remediation of contaminated solid phase materials. In
addition to a discussion of the basic processes which drive
land treatment applications, the parameters involved in these
processes are examined with respect to the efficiency as
well as the failure of such systems. Design and operation
criteria are suggested in areas ranging from pH control to
tilling practices and moisture and nutrient requirements.
Contaminants commonly related to the wood preserving and
petroleum industries are addressed with respect to their
applicability to land treatment in terms of treatability, loading
rates, and cleanup levels. A bibliography containing
references for further information is provided along with
appendices covering soil properties important in land
treatment and a discussion of monitoring procedures.
Bioremediation of contaminated soils using the land
treatment concept is currently under consideration for
implementation at a large number of Superfund, UST, and
RCRA sites. The ultimate success of these remediations will
depend on systematic design and operation of each specific
land treatment unit. This document is designed to be used
by those who have responsibility for design and day-to-day
operation of a land treatment facility, or who are responsible
for overseeing design and operation of the facility. The
document provides a short discussion of fundamental
processes involved in land treatment, design and operation
of the land treatment unit (LTD), and a bibliography
containing references for further study. Also included are
appendices covering soil properties important in land
treatment and a short discussion of monitoring procedures.
Definition of Land Treatment
Land treatment involves use of natural biological, chemical
and physical processes in the soil to transform organic
contaminants of concern. Biological activity apparently
accounts for most of the transformation of organic
contaminants in soil, although physical and chemical
mechanisms may provide significant loss pathways for some
compounds under some conditions. Degradation by
ultraviolet light may serve as a loss pathway for certain
hydrophobic compounds at the soil surface. Volatilization of
low molecular weight compounds also takes place at the soil
surface and provides a significant loss pathway for such
compounds. Certain chemical reactions such as hydrolysis
can play an important role in transformation of some
compounds. Humification, the addition of compounds to the
humic materials in soil, can be important routes of
transformation for some polycyclic aromatic compounds.
The relative importance of these processes varies widely for
different compounds under different circumstances. The
land treatment concept serves as the basis for design and
operation of soil bioremediation technologies at a large
number of waste sites requiring cleanup.
Microorganisms and Bioremediation
Bioremediation is carried out by microorganisms. Both
bacteria and fungi have been shown to be important in
bioremediation processes. Most research in bioremediation
has centered on bacteria, but some investigators have found
that fungi can play an important role in bioremediation
processes, especially with halogenated compounds (e.g.,
pentachlorophenol, a wood preservative). It is important to
realize, however, that in almost all cases bioremediation
relies on communities of microorganism species, rather than
one or a few species.
Bioremediation consists of utilizing techniques for enhancing
development of large populations of microorganisms that
can transform the pollutants of interest, and bringing these
microorganisms into intimate contact with the pollutants. In
order to do this efficiently, necessary provisions for microbial
growth and reproduction must be maintained.
Life processes for all known living creatures are carried out
in water. Some organisms, such as human beings, can
maintain an internal water environment while moving about
in a relatively dry outer environment. Many microorganisms,
however, cannot maintain an appropriate inner environment
without being in a relatively wet outer environment. Most
microorganisms that are active in bioremediation must live in
water. This water may be in tank reactors or an aquifer, or it
may be a thin film of water on a soil particle or oil droplet.
Microorganisms are sensitive to the osmotic potential of the
solution in which they function. The osmotic potential affects
the ability of the microorganism to maintain itself with a
desirable amount of water internally. If the environment is
too dry, or if the water in the microorganism's environment
contains excessive concentrations of solutes, the
microorganism cannot maintain the proper amount of water
internally. This factor can be a problem for bioremediation
schemes where, for example, process waters or
contaminated soils have high levels of dissolved salts.
Sudden changes in osmotic potential can inhibit microbial
activity, often resulting in lysis (disintegration of cell walls).
Microorganisms can adapt to environmental changes within
limits if such changes are not induced rapidly.
Specific microorganisms are active within a relatively narrow
range of temperatures. Most bacteria that carry out
bioremediation processes are mesophiles ("middle lovers")
and are most active from about 18 to 30 degrees centigrade.
Significantly higher or lower temperatures will limit their
activity. Within this range, activity will usually be greater at
the higher temperatures. Activity decreases as the
temperature moves further outside these limits. At lower
temperatures, activity does not usually stop completely until
the freezing point is reached.
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Most microorganisms active in bioremediation
processes are aerobic, that is, they require free
(uncombined) oxygen. Some treatment processes
make use of anaerobic microorganisms that do not
require free oxygen; however, these processes are not
yet widely used in environmental cleanups.
Microorganisms living in aqueous reactors, aquifers, or
in the subsoil may be supplied with oxygen by pumping
air or oxygen-supplying compounds (e.g., hydrogen
peroxide) into the environmental system.
Microorganisms growing in surface soil are usually
supplied with oxygen by tilling the soil to facilitate air
entry. In many remediation situations the essential
problem is the balance between water and oxygen: the
more water, the less oxygen, and vice versa. In the soil
environment, the oxygen supply and the water supply to
microorganisms are essentially inversely related, since
the pore space in soil is occupied by either air or water.
Microorganisms are active within a relatively broad pH
range. The pH is a measure of the acidity or basicity of
the environment. However, many microorganisms are
inhibited below pH 5 or above pH 9. Although many
microorganisms can adapt to pH levels within that
range, it is thought that fungal species tend to be the
more active members of the microorganism community
below pH 6, and bacteria tend to dominate above pH 7.
The pH range within which bioremediation processes
are considered to operate most efficiently is 6 to 8. The
optimum pH range for a particular situation, however, is
influenced by a complex relationship between the
microorganisms, pollutant chemistry and external
environment, and thus is site-specific. The pH can be
adjusted to the desired range by the addition of acidic
or basic substances (i.e., sulphur or lime).
Microorganisms are sensitive to the presence of a wide
variety of compounds and elements. High
concentrations of heavy metals, certain highly
halogenated organics, some pesticides and other
exogenous materials can inhibit bioremediation. Effects
of these inhibitors vary with concentration,
environmental factors, rapidity of contact with the
inhibitors, and time of contact such that it is difficult to
set any definite concentration limits above which
bioremediation is precluded. Laboratory treatability
studies often can be used to provide data necessary for
management decisions regarding the impact of a given
inhibitor at field scale.
Metals often are present in soils contaminated with
organic wastes. These metals will not be treated
(transformed or degraded) in the same sense as the
organic materials. However, valence states may be
changed and chemical bonds may be broken so as to
change the toxicity or mobility of the metals. Addition of
manures and other complex organic materials often
used in land treatment may reduce the mobility of many
metals by increasing the ion exchange capacity or
adsorption capacity of the soil.
Microorganisms must have carbon sources and mineral nutrients
(nitrogen and phosphorus, for example) in order to live and
reproduce. In many cases, the pollutants themselves will supply
the carbon source and some nutrients; however, mineral nutrients
and a supplemental carbon source may be supplied if needed.
Mineral nutrients are usually supplied as soluble salts (fertilizers).
If necessary, carbon may be supplied as animal manures (which
will also supply many mineral nutrients), molasses, glucose, wood
chips, corn cobs or a variety of other carbon containing materials.
There must be a balance between the various mineral nutrients
and the carbon source or the microorganisms will not be able to
make optimum use of the carbon source. For most bioremedi-
ation situations, it is supposed that biodegradation is optimal at
carbon/nitrogen ratios in the range of 10-30 to 1 > and nitrogen/
phosphorus ratios of about 10 to 1 by weight. Research and field
experience indicates that these ratios may vary widely depending
on the type of carbonaceous materials present.
There are 15 or more other mineral nutrients that must be
available in appropriate amounts. With the exception of some
process waters and ground waters, these minor nutrients are
usually present in the environment in sufficient amounts. The
amounts of foods and nutrients to be added should be based on
results from laboratory and site treatability studies.
The availability of nutrients to microorganisms is strongly
influenced by pH since soil pH generally is maintained or adjusted
to the 6-8 pH range in a land treatment scenario. Limited
availability of nutrients caused by pH is rarely a problem.
Since microorganisms are responsible for transformation of
pollutants during bioremediation, it would seem reasonable to
assume that the greater the number of microorganisms present,
the faster the transformation would occur. However, results of
population counts and analyses for parent compound
disappearance or transformation are often not closely correlated.
Instead, the rate of transformation is more closely correlated to
the rate at which oxygen and electron acceptor can be
transported to the system.
Several techniques have been devised to determine the
microorganism population. One commonly used method is
standard plate counting. Samples of soil, water, or other matrices
in which microorganisms are growing are applied to Petri plates
containing a nutrient media. The plates are incubated for several
days to allow microorganisms to grow, after which the number of
microorganism colonies growing on the plates are counted. These
counts can then be related to the numbers of microorganisms
present in the original matrix; however, the relationship between
population counts and pollutant transformation rates is not well
defined. Often, there are many types of microorganisms in the
bioremediation environment that will be counted in plate counts,
but only a few of these types may actually be involved in
transformation of the contaminants of concern. The nutrient
media may be spiked with waste compounds of interest;
microorganisms that grow on such media are considered to be
tolerant to the spiked compounds. The use of spiked media *
yields an indication of population levels of tolerant micro-
organisms in the soil tested. Note that this procedure does not
give an indication of the microorganisms present that can degrade
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the spiked compound, only those that can survive and grow
in the presence of the spiked levels of the compounds.
There are several physical constraints on the use of
microorganisms in remediation of soil contaminants. These
are generally related to the problem of getting contaminants
and microorganisms in close contact under environmental
conditions conducive to microbial activity. Generally, a
contaminant must be able to move through the waste/soil
matrix and pass through the microorganism's cell membrane
in order for transformation to occur. In some cases
contaminants can be transformed by extracellular enzymes
(cooxidation or cometabolism) without entering into the
microorganisms.
Waste compounds that have low solubilities in water (for
example, 4, 5, and 6 ring polycyclic aromatic hydrocarbons
[PAHs]) move slowly from soil adsorption sites or free phase
droplets into the soil water and from there into the
microorganism. Wastes in solid matrices (soil) will have less
solvent (water) in which to be dissolved, will be more likely to
have highly variable concentrations throughout the matrix,
will be harder to mix thoroughly for even distribution
throughout the matrix, and often will have a relatively high
tendency to be adsorbed onto matrix solids.
All of these factors tend to limit accessibility of contaminant
compounds to the microorganisms; therefore it is often
easier to achieve biodegradation of a given contaminant in
water than in soil. Also, soil treatment processes where soil
is suspended in water and constantly mixed (soil slurry
bioremediation) will usually have faster biotransformation
rates than simple solid phase soil bioremediation processes.
LAND TREATMENT
TECHNOLOGY
In-Situ and Ex-Situ Land Treatment
Land treatment techniques for bioremediation purposes most
often are used for treatment of contaminated soil, but certain
petroleum waste sludges have long been applied to soil for
treatment. Ideally, the contaminated soil can be treated in
place (in-situ). Often, however, the soil must be excavated
and moved to a location better suited to control of the land
treatment process (ex-situ).
In-situ land treatment is limited by the depth of soil that can
be effectively treated. In most soils, effective oxygen
diffusion sufficient for desirable rates of bioremediation
extends to a range of only a few inches to about 12 inches
into the soil. Usually when it is desired to treat soil in-situ to
depths greater than 12 inches, the surface layer of soil is first
treated to the desired contaminant levels, and then removed,
or tilled so that lower layers are moved to the surface for
treatment. Most tractor mounted tilling devices can till only
to a depth of about 12 inches. Large tractors with •
specialized equipment that can till to depths of 3 feet or
more have been used for in-situ land treatment. Large
augers are now available that can move soil from 50-100
feet depths to the surface, but the practicality of this
technique for in-situ land treatment has not been
demonstrated.
Ex-situ treatment generally involves applications of lifts of
contaminated soil to a prepared bed reactor. This reactor is
usually lined with clay and/or plastic liners, provided with
irrigation, drainage and soil water monitoring systems, and
surrounded with a berm. The lifts of contaminated soil are
usually placed on a bed of relatively porous non-
contaminated soil.
Whether practiced in-situ or ex-situ, effective land treatment
is generally limited to the top 6 to 24 inches of soil, with 12
inches or less being the preferred working depth. At depths
below 12 inches, the oxygen supply is generally inadequate
for useful rates of bioremediation using standard land
treatment practices. Tilling is used to mix the soil and
increase the oxygen levels, but is usually limited to 12 inches
or less unless specialized equipment is available.
The land treatment process may be severely limited in
clayey soils, especially in areas of high rainfall. This
limitation is primarily related to oxygen transfer limitations
and substrate availability to the microorganisms. Clayey
soils should be applied in shallower lifts than sandy soils,
preferably no more than 9 inches in depth. Tilth
("workability" of the soil) can often be improved by adding
organic matter or other bulking agents to the soil. If high
sodium content causes the soil to have poor tilth, gypsum
(calcium suifate) can be added.
The soil should be screened before application to remove
any debris greater than one inch in diameter, especially if
significant amounts of debris or rocks are present. Any large
debris that may adsorb the waste compounds (i.e., wood),
should be removed if possible and treated separately. Small
rocks and other relatively nonadsorptive wastes can be
treated if they do not interfere with tillage operations.
Lift Application and Tilling
After application to the land treatment unit, each lift should
be tilled at intervals to enhance oxygen infiltration and
contaminant mixing with the microorganisms. The soil
should be near the lower end of the recommended soil
moisture percentage range before tilling. Tilling very wet
or saturated soil tends to destroy the soil structure, reduce
oxygen and water intake, and cause reduced microbial
activity. Tilling should not begin until at least 24 hours
after irrigation or a significant rainfall event. Tilling more
than is necessary for enhanced oxygen infiltration and
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contaminant mixing may be counter-productive since it
tends to destroy the soil structure and compact soil below
the tilling zone. Tillers tend to mix soil only along the
tractor's line of travel, so tillage should be carried out in
varying directions, i.e., lengthwise of the LTU, crosswise,
and on the diagonal.
A tractor mounted rotary tiller is recommended. Occasion-
ally a subsoil plow or chisel plow should be used to break up
any hardpan or zone of compaction created by passage of
equipment across the LTU. If the tiller does not operate
deeply enough to mix soil from the last lift into the top few
Inches of soil from the preceding lift, a turning plow may be
used to achieve desired mixing of the two lifts. Use of the
turning plow should be followed immediately by tilling to
complete the mixing action.
Each subsequent lift, usually 6-12 inches in depth, should be
tilled into the top two or three inches of the previous lift. This
will mix populations of well acclimated microorganisms from
the treated lift into the newly applied lift, and help reduce the
length of time for high populations of active degraders to be
built up in the new lift.
Timing of application of succeeding lifts should be based on
reduction to defined levels of particular compounds or
categories of compounds in the preceding lift. For instance,
the goal might be to reduce total petroleum hydrocarbons
(TPH) to less than a regulatory or risk calculated limit in the
current lift before application of a new lift.
Once desired target levels of compounds of interest are
established, data obtained from the LTU monitoring activities
can be statistically analyzed to determine if and when
desired levels are reached and the LTU is ready for
application of another lift.
Nutrients, Carbon Sources, and Other
Additives
Microorganisms in land treatment units require carbon
sources and nutrients. Fertilizers can be used to supply the
nutrients, while wood chips, sawdust or straw can supply
carbon. Various animal manures are often used to supply
both carbon sources and nutrients. High organic levels in
manures, wood chips and the other organic amendments
increase sorptive properties of soil, thereby decreasing
mobility of organic contaminants.
Organic amendments will also increase the water holding
capacity of soil, which can be desirable in sandy soils, but
can cause difficulty when land treatment is conducted in
areas of high rainfall and poor drainage. In an excessively
wet soil the oxygen supply is reduced, which may essentially
stop or severely limit transformation of waste constituents.
Drainage must be carefully designed and managed in these
situations.
Animal manures can provide desirable organic supplements.
Manure should be applied to each lift at the rate of about
3% - 4% by weight of soil. The manure should be analyzed
for nitrogen and phosphorus to determine if any additional
amounts of these nutrients need be applied. The manure
should be in small particles and should be thoroughly tilled
into the soil lift. Initial contaminant levels should be
measured after incorporation of organic supplements to
avoid incorporating dilution effects into calculation of waste
transformation.
Manures are often mixed with sawdust or straw since
these materials are used as bedding in stock facilities.
This is acceptable and even desirable since they act as
bulking agents in soil. However, the high percentage of
cellulosic material will usually exert a high nitrogen
demand, thereby reducing the amount of nitrogen available
to microorganisms for transforming contaminants of
concern. If necessary, available nitrogen can be increased
with addition of appropriate inorganic fertilizers, including
fertilizer grade ammonium nitrate (for nitrogen), triple
superphosphate (for phosphorus), or diarrimonium
phosphate (for both nitrogen and phosphorus). Nitrogen
fertilizers can cause soil pH to be lowered due to formation
and leaching of the nitrate ion coupled with soil cations.
Agricultural fertilizer is usually supplied in prilled or pelleted
form (the fertilizer compounds formed into pellets with a clay
binder) suitable for easy application over large areas.
Technical grade, unformulated fertilizer compounds (for
instance, ammonium chloride crystals) are difficult to spread
evenly over the land surface. The pelleted fertilizers may be
applied with a hand-operated or tractor-operated cyclone
spreader. Completely water-soluble fertilizers can be
applied through irrigation systems, allowing application rates
to be closely controlled, applications to be made as often as
irrigation water is applied, and immediate availability to the
microorganisms. Equipment is available to meter
concentrated nutrient solutions into the irrigation system on a
demand basis. :
Nutrient requirements for biodegradation in the field have not
been thoroughly studied, and detailed information is not
available to indicate the optimal levels of particular
nutrients. The amount of nitrogen and phosphorus needed
may be estimated by having representative soil samples
analyzed by the state agriculture department or university
laboratory, or by a private agricultural soil testing
laboratory. General fertilizer recommendations for
vegetable gardens should be followed. The analytical
laboratory should be made aware that the soil contains
hazardous materials.
Sometimes inorganic micronutrients, microbial carbon
sources, or complex growth factors may be needed to
enhance microbial activity. Animal manures generally will
supply these factors. Proprietary mixtures of these
ingredients are sometimes offered for sale to enhance
microbial activity. Proof of the efficacy/cost effectiveness of
these mixtures is lacking in most cases.
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Bioaugmentation
Microorganism cultures are often proposed for addition to
bioremediation units. Two factors limit use of these added
microbial cultures in LTUs: (1) nonindigenous
microorganisms rarely compete well enough with indigenous
populations to develop and sustain useful population levels,
and (2) most soils with long-term exposure to biodegradable
wastes have indigenous microorganisms that are effective
degraders if the LTU is managed properly. If the use of
proprietary additives is proposed, results of well-designed
treatability studies with appropriate controls should be
provided by the vendor to support such use.
Certain soil factors may interfere with microbiological activity
in the LTU soil. High salt levels, indicated by high electrical
conductivity (EC) readings, may reduce or stop useful
microbiological activity. If levels are too high, it may be
necessary to leach the soil with water to remove excess
salts before biodegradation can occur. High levels of
sodium may be detrimental to soil structure. 'Sodium levels
may be reduced by applying calcium supplements (usually
gypsum, CaSO4) and leaching. Leaching of contaminants
may also occur at the same time.
Soil Moisture Control
Historically, it has been recommended that soil moisture be
maintained in the range of 40% - 70% of field capacity;
however, recent experience indicates that 70% - 80% of field
capacity may be optimum. A soil is at field capacity when
soil micropores are filled with water and soil macropores are
filled with air. This condition allows soil microorganisms to
get air and water, both of which are necessary for aerobic
biodegradation to occur. Maintaining soil at somewhat less
than 100% of field capacity allows more rapid movement of
air into the soil, thus facilitating aerobic metabolism without
seriously reducing the supply of water to microorganisms. If
soils are allowed to dry excessively, microbial activity can be
inhibited or stopped; if the wilting point is reached, cells may
lyse or rupture. ,
Field capacity of a soil may be determined by saturating a
soil sample with water and allowing it to drain freely for 24
hours. The soil is weighed, oven-dried at 105° C to constant
weight, then weighed again. The difference between the
weight of drained soil and the oven-dried soil gives the
weight of water in that amount of soil at field capacity. The
weight of water divided by the dry weight of the soil gives the
percentage of water in the soil at field capacity. A sandy soil
might hold as little as 5% of its dry weight in water at field
capacity as compared to 30% for a clay soil.
Continuous maintenance of soil moisture at adequate levels
is of utmost importance. Either too little or too much soil
moisture is deleterious to microbial activity. Monitoring soil
moisture and scheduling irrigation is important, requires
constant attention, and is perhaps one of the most neglected
areas of LTU operations.
Moisture enhancement can be accomplished by using
traveling gun or similar irrigation systems that can be
removed to allow easy application of lifts. Hand-moved
sprinkler irrigation systems are more often used, although
they are usually more expensive. Sprinkler systems can be
designed with quick detach couplings to facilitate movement
when placing lifts of contaminated soil. Permanently
installed sprinkler systems with buried laterals and mains
may be used, but the sprinkler uprights must be avoided
when placing soil lifts and during other LTU operations. The
uprights may need to be lengthened if many lifts are placed
during the operating life of the LTU.
If a permanently installed, buried line system is used, the
uprights should be connected to the buried lateral lines with
a short piece of plastic pipe. Some of the uprights will be hit
by field equipment during operations, and the plastic pipe will
break before the lateral line or other parts of the piping
system. The plastic pipe can be easily repaired, while a bent
or broken lateral line or upright can be difficult to repair.
The operating pressure for most sprinklers ranges from 30 to
50 Ib/in2. Sprinklers may have two nozzles, one to apply
water at a distance from the sprinkler (range nozzle) and one
to cover the area near the'sprinkler (spreader nozzle).
Sprinklers may be static or rotating, with a hammer or other
device to cause the sprinkler to rotate. Since one sprinkler
will not apply water uniformly over an area, sprinkler patterns
should be overlapped to provide more uniform coverage.
The usual overlap is around 50%; that is, the area covered
by one sprinkler reaches to the next sprinkler. Highly
uniform coverage is difficult to achieve in the field, especially
in areas where winds of more than 5 mph are common.
Small LTUs can be covered with sprinklers set only on the
sides of the LTU. Sprinklers can cover full, half or quarter
circles so that sprinklers on the sides or in the corners of the
LTU will cover only the LTU and not the berm or areas
outside the LTU.
The irrigation system should be sized to allow application of
at least one inch of water in 10-12 hours. The rate of water
application should never be more than the soil can absorb
with little or no runoff since LTUs consist of bare soil and
excessive runoff can rapidly cause significant erosion.
Generally, coarser (sandy or loamy) soils can take up water
at a faster rate than finer textured clay or clay loam soils.
Usually, application rates of more than 0.5 inches of water
per hour are not recommended; clayey soils with slopes
greater than 0.2% - 0.3% will require lower rates of water
application. A water meter to measure the volume of water
applied is helpful in controlling application.
Surface drainage of the LTU can be critical in high rainfall
areas. If soil is saturated more than an hour or two, aerobic
microbial action is reduced. The LTU surface should be
sloped no more than 0.5% - 1.0%, as greater slopes will
allow large amounts of soil to be washed into the drainage
system during heavy rains. Even a slope of 0.5% - 1.0% will
allow soil to be eroded; therefore the drainage system
•should be designed to allow collection and return of eroded
soil to the treatment unit.
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Underdrainage is generally provided by a sand layer or a
geotextile/drainage net layer under the LTD. The system
should be designed so that excess water in soil pores over
field capacity will be quickly drained away so microbial
activity will not be inhibited. The lifts of contaminated soil
are usually placed on a bed of sand or other porous soil,
which results in a "perched" water table. In this event the
contaminated soil lift will take up water from irrigation or rain
until the soil nears saturation, at which point excess water
will be discharged into the treatment unit drainage system.
The interface between the lift and the coarse layer
underneath should be composed of well graded materials so
that the transition from the (usually) relatively fine soil texture
of the lift to the coarse texture of the drainage layer is
gradual rather than sudden. Grading of the materials
reduces the tendency for the soil lift to become saturated
before drainage occurs, which inhibits aerobic biological
activity. The change in texture at the interface can be made
more gradual by tilling the lift into the top few inches of the
drainage layer.
Some storage capacity should be provided so that runoff and
leachate water can be recycled onto the LTD. A one-inch
rain might give a combined runoff and leachate of 10,000 to
27,000 gallons per acre if the LTU is being maintained at the
proper (relatively high) moisture content. Therefore, it may
not be practical to provide storage capacity for large rainfall/
runoff events.
In many cases leachate/runoff water cannot be discharged
without treatment. Biological reactors are commonly used to
treat this water prior to discharge. Alternatively, effluent from
the biological treatment unit may be applied to the LTU
through the irrigation system. Nutrients and microorganisms
from the biological treatment system may enhance the
microbial activity within the LTU.
Types and Concentrations of
Contaminants Remediable by Land
Treatment
The types of contaminants most commonly treated in land
treatment units are petroleum compounds and organic wood
preservatives. Historically, petroleum refineries have used
land treatment to dispose of waste sludges. Although waste
petroleum sludges currently are not often applied to soil for
treatment, the technology has been applied to remediation of
soil contaminated with many types of petroleum products,
including fuel, lubricating oil and used petroleum products.
Land treatment has historically been used to remediate
contaminated process waters from wood preserving
operations. This technology currently is not used for this
purpose, but is currently used to remediate soil
contaminated with wood preserving wastes.
Other applications for land treatment technology include
remediation of soil contaminated with coal tar wastes,
pesticides and explosives. Since coal tar wastes are similar
to creosote wastes (wood preserving creosote is made from
coal tar), such wastes are considered amenable to land
treatment. Land treatment appears to be potentially useful
for certain pesticides, but the evidence for applicability of this
technology to explosives contaminated soil is inconclusive.
Petroleum Derived Contaminants
Crude oil is refined into petroleum products including the
following general groups: gasolines, middle distillates (diesel
fuel, kerosene, jet fuel, lighter petroleum oils), heavier fuel
oils and lubricating oils, and asphalts and tars. Certain
individual compounds may also be produced from crude oil,
including benzene, toluene, hexane, and many others.
Gasoline, a mixture of C4 to C12 hydrocarbons, includes
paraffins (CNH2N+2), olefins (alkenes), naphthenes (5 and 6
carbon cycloparaffins and their alkyl derivatives, sometimes
including polycyclic members), and aromatics (12% to 20%
by weight of gasoline). Jet fuel includes avgas (for propeller-
driven aircraft — similar to gasoline), Jet A & A1 fuel
(commercial aircraft fuel, with a heavy kerosene base), JP4
(Air Force jet fuel, naphtha based), and JP5 (Army jet fuel-
kerosene base). Diesel and kerosene are similar in
composition, being largely composed of chains of 9-17
carbons in length. Heating oil and bunker C are heavy fuel '
oils. Lubricating oils contain long chain hydrocarbons (20+ C
chain length). Tars and asphalts are mixtures of long chain,
high molecular weight hydrocarbons including bitumens,
waxes, resins and pitch.
Specific compounds found in petroleum products and
refinery wastes include: aliphatics, olefins, naphthenes, and
asphaltenes; single ring aromatics benzene, toluene, ethyl
benzene and o,m,p-xylene; multiple ring aromatics
(polycyclic aromatic hydrocarbons—PAHs), phenols and
cresols; bitumens, waxes, resins and pitch; and metals
including lead, chromium and cadmium. Caustics (alkali
metal hydroxides) may also be found.
Refinery waste sources include production wastes (refinery
effluents, slop oil-emulsion solids, leaded tank bottoms, heat
exchanger bundle cleaning sludge); incidental wastes
(runoff, equipment washdown, spills, ballast tank water); and
treatment wastes (API separator sludge, float from air
flotation units, sludge from biotreatment).
Since the majority of gasoline components have significant
volatility, land treatment is not usually considered for
treatment of gasoline contaminated soil due to the high
losses of volatiles expected with routine land treatment
operations. Diesel and kerosene type fuels have a
significant portion of volatile components, but "weathered"
wastes that have lost most of the volatile components
usually are suitable candidates for land treatment. Most of
the heavier petroleum products (fuel oils, lubricating oils,
waste sludges and oils) are susceptible to land treatment.
However, the long chain hydrocarbons (20+ carbons) and 5-
6 ring PAHs that may be found in these materials
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biodegrade slowly if at all. Petroleum products that consist
mostly of long chain hydrocarbons and other high molecular
weight compounds (asphalts, tars) are not suitable
candidates for land treatment because of the resistance of
such compounds to biodegradation and the physical difficulty
of mixing these products with the soil.
Wood Preserving Contaminants
The major portion of the wood preserving industry in the
United States treats wood under pressure in cylinders with
one of four types of preservatives:
1) pentachlorophenol (penta, PGP) in petroleum, or other
solvents;
2) creosote;
3) water solutions of copper, chromium, and arsenic
(CCA); ammoniacal solutions of copper and arsenic
(ACA); ammoniacal solutions of copper, zinc and
arsenic (ACZA); and
4) fire retardants, which include various combinations of
phosphates, borates or boric acid, and zinc compounds.
A relatively minor portion of the wood treating industry
uses nonpressure treatment of wood with similar
preservatives applied in a variety of ways.
Technical grade pentachlorophenol used for treating wood
contains 85% to 90% pentachlorophenol, remaining
materials being 2,3,4,6-tetrachlorophenol (4% to 8%),
"higher chlorophenols" (2% to 6%), and dioxins and furans
(0.1%). The principal chlorodibenzodioxin and
chlorodibenzofuran contaminants are those containing six to
eight chlorines. Pentachlorophenol is mixed with a carrier
(usually a fuel oil similar to kerosene or diesel fuel) at 4% -
5% pentachlorophenol by weight in the carrier, in order to
produce the solution used for treating wood.
The other major organic wood preservative used in the
United States is coal tar creosote. Creosote is used either
full strength or diluted with petroleum oil or coal tar. Wood
preserving creosote contains approximately 85% polynuclear
aromatic hydrocarbons (PAHs), 10% phenolic compounds,
and 5% nitrogen, sulfur or oxygen containing heterocycles.
The carrier oils for pentachlorophenol and creosote are
similar in biodegradability to the petroleum diesel fuel and
kerosene products. Pentachlorophenol and the associated
phenolics in pentachlorophenol and creosote treating
solutions are biodegradable, though levels of
pentachlorophenol in soil of about 1000 mg/kg are difficult to
bioremediate. Higher pentachlorophenol levels are usually
lowered by mixing with less contaminated soil before land
treatment. The compounds of most interest in creosote are
the PAHs, which vary in susceptibility to bioremediation
according to the number of rings. The two-ring PAHs are
readily biodegradable, the three-ring PAHs are more difficult,
and biodegradation becomes increasingly more difficult for
the four- and five-ring PAHs.
The inorganic wood preservatives will be discussed briefly
since they are not usually remediated by biological means.
Their main impact on bioremediation comes from the
possible toxicity of the inorganic preservatives to
microorganisms, and the necessity for providing means
other than land treatment for remediation of inorganic
contaminated soil. Generally, if soil is contaminated with
organic and inorganic wood preservatives, it is first
bioremediated to treat the organic contaminants, and then
solidified/stabilized to treat the inorganics. There has been
little research on concentrations of the inorganic wood
preservatives that would be problematic in soil
bioremediation.
Levels of Contamination Susceptible to Land
Treatment
The levels of petroleum product contamination amenable to
land treatment vary by waste type and site conditions. In
many cases, soils with higher levels of contaminants than
are recommended for land treatment can be mixed with less
contaminated soil to bring contamination levels down to
recommended starting levels for treatment. Levels of
petroleum product contamination as high as 25% by weight
of soil have been reported as treatable, although experience
indicates that levels 5% to 8% by weight or less are more
readily treated. Long chain hydrocarbons (20 or more
carbons in the chain) are more resistant to biological
treatment, so petroleum products containing excessively
high percentages of these compounds (bunker C, asphalt,
tars, etc.) are not good candidates for land treatment under
most commonly established cleanup standards for soil.
Soil contaminated with 15,000 to 20,000 mg/kg dry weight
creosote wastes have been treated in soil systems, although
more usual starting levels are in the 5,000 to 10,000 mg/kg
range. Pentachlorophenol wastes are rarely treated at more
than 1000 mg/kg starting levels since pentachlorophenol is
quite toxic to microorganisms at the higher levels.
The final levels attainable also vary by waste and site
conditions. Generally, once total contaminant levels are
below 50-200 mg/kg PAH, remediation by land treatment is
slow, and further treatment by conventional land treatment
techniques may be ineffective. For instance, land treatment
of creosote wastes is generally considered successful if total
carcinogenic polynuclear aromatic hydrocarbons are
reduced to below 50-100 mg/kg, and specific components
are reduced to their "land ban" levels (for instance, pyrene to
7 mg/kg). Laboratory treatability studies may be used to
assess the "best case" potential for final contaminant levels,
with the assumption that actual final levels in the field would
rarely be lower than those found in the laboratory study.
-------
BIBLIOGRAPHY
Land Treatment Concept References
Bulman, T.L., S. Lesage, P.J.A. Fowlie, and M.D. Webber.
November 1985. The Persistence of Polynuclear Aromatic
Hydrocarbons in Soil. PACE Report No. 85-2. Petroleum
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Loehr, R. 1989. Treatability Potential for EPA Listed
Hazardous Chemicals in Soil. U. S. Environmental
Protection Agency, Robert S. Kerr Environmental Research
Laboratory, Ada, OK, EPA/600/2-89/011.
Loehr, R.C., and J.F. Malina, Jr., Ed. 1986. Land
Treatment: A Hazardous Waste Management Alternative.
Water Resources Symposium Number Thirteen. Center for
Research In Water Resources, Bureau of Engineering
Research, College of Engineering, The University of Texas
at Austin. Austin, Texas.
Lynch, J., and B.R. Genes. 1989. Land Treatment of
Hydrocarbon Contaminated Soils. In: Petroleum
Contaminated Soils, Vol. 1: Remediation Techniques,
Environmental Fate, and Risk Assessment, P. T. Kostecki
and E. J. Calabrese, Eds., Lewis Publishers, Chelsea, Ml, p.
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Park, K.S., R.C. Sims, R.R. Dupont, W.J. Doucette, and J.E.
Matthews. 1990. Fate of PAH Compounds in Two Soil
Types: Influence of Volatilization, Abiotic Loss and
Biological Activity. Environ. Toxicol. Chem., 9:187.
Rochkind, M.L., J.W. Blackburn, and G.S. Sayler. 1986.
Microbial Decomposition of Chlorinated Aromatic
Compounds. EPA/600/2-86/090, Hazardous Waste
Engineering Research Laboratory, U. S. Environmental
Protection Agency, Cincinnati, OH.
Ross, D., T.P. Marziarz, and A.L. Bourquin. 1988.
Bioremediation of Hazardous Waste Sites in the USA: Case
Histories. In: Superfund '88, Proc. 9th National Conf.,
Hazardous Materials Control Research Institute, Silver
Spring, MD, p. 395.
Sims, J.L., R.C. Sims, and J.E. Matthews. Bioremediation of
Contaminated Surface Soils. August 1989. U.S.
Environmental Protection Agency, Robert S. Kerr
Environmental Research Laboratory, Ada, OK, EPA-600/9-
89/073.
Sims, R.C., D. L. Sorensen, J.L. Sims, J. E. McLean, R.
Mahmood, and R. R. Dupont. 1984. Review of In-Place
Treatment Technologies for Contaminated Surface Soils—
Volume 2: Background Information for In-situ Treatment.
U.S. Environmental Protection Agency, Risk Reduction
Research Laboratory, Cincinnati, OH, EPA-540/2-84-003b.
Sims, R.C., W.J. Doucette, J.E. McLean, W.J. Grenney, and
R.R. Dupont. 1988. Treatment Potential for 56 EPA Listed
Hazardous Chemicals in Soil. U.S. Environmental Protection
Agency, Robert S. Kerr Environmental Research Laboratory,
Ada, OK, EPA/600/6-86/001, April.
St. John, W.D., and D.J. Sikes. 1988. Complex Industrial
Waste Sites. In.: Environmental Biotechnology—
Reducing Risks from Environmental Chemicals through
Biotechnology, G.S. Omenn, Ed., Plenum Press, New
York, NY, p. 163. . . ,
U.S. EPA. 1989. Guide for Conducting Treatability Studies
under CERCLA. U.S. Environmental Protection Agency,
Office of Solid and Emergency Response and Office of
Research and Development, Washington, DC, Contract No.
68-03-3413, November.
U.S. EPA. 1990. Handbook on In Situ Treatment of
Hazardous Waste-Contaminated Soils. U.S. Environmental
Protection Agency, Risk Reduction Research Laboratory,
Cincinnati, OH, EPA/540/2-90-002, January.
U.S. EPA. 1986. Permit Guidance Manual on Hazardous
Waste Land Treatment Demonstrations. EPA-530/SW-86-
032, Office of Solid Waste and Emergency Response, U.S.
Environmental Protection Agency, Washington, DC.
U.S. EPA. 1991. On-Site Treatment of Creosote and
Pentachlorophenol Sludges and Contaminated Soil. EPA/
600/2-91/019. Extramural Activities and Assistance
Division, Robert S. Kerr Environmental Research Laboratory,
Ada, OK. May.
Soil Properties References
Dragun, J. The Soil Chemistry of Hazardous Materials.
Hazardous Materials Control Institute, Silver Spring, MD.
Foth, H.D. 1990. Fundamentals of Soil Science, Eighth
Edition. John Wiley & Sons, New York, NY.
McLean, Joan C., and Bert E. Biedsoe. Behavior of Metals
in Soils. Ground Water Issue. EPA/540/S-92/018. October
1992. Superfund Technology Support Center For Ground
Water, Robert S. Kerr Environmental Research Laboratory,
Ada, OK. . \
Paul, E.A., and F.F. Clark. 1989. Soil Microbiology and
Biochemistry. Chapter 2—Soil as a Habitat for Organisms
and Their Reactions. Academic Press, San Diego, CA.
Monitoring References
Blackwood, Larry G. Assurance Levels of Standard Sample
Size Formulas: Implications for Data Quality Planning.
Environmental Science and Technology, Vol. 25, No. 8,
1991.
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Eklund, Bart. Practical Guidance for Flux Chamber
Measurements of Fugitive Volatile Organic Emission Rates.
J. Air Waste Management Association, 42:1583-1591.
December 1992.
Englund, E.J., Weber, D.D., and N. Leviant. EPA/600/J-92/
166. NTIS PB92-180314. 1992. The Effects of Sampling
Design Parameters on Block Selection. USEPA
Environmental Monitoring Systems Laboratory. Las Vegas,
NV.
Hawley-Fedder, Ruth and Brian D. Andresen. Sampling and
Extraction Techniques for Organic Analysis of Soil Samples.
UCRL-ID-106599. February 12,1991. Lawrence Livermore
National Laboratory.
Gilbert, R.O. Statistical Methods for Environmental Pollution
Monitoring. 1987. Van Nostrand Reinhold. ISBN 0-442-
23050-8.
Gilbert, R.O., and J.C. Simpson. An Approach for Testing
Attainment of Soil Background Standards at Superfund
Sites. (American Statistical Association 1990, Joint
Statistical Meetings, Anaheim, CA. August 6-9,1990.)
Pacific Northwest Laboratory, Richland, WA 99352.
Keith, Lawrence H., Ed. Principles of Environmental
Sampling. 1988. American Chemical Society. Washington,
DC.
T.E. Lewis, A.B. Crockett, R.L. Siegrist, and K. Zarrabi. Soil
Sampling and Analysis for Volatile Organic Compounds.
Ground-Water Issue. EPA/540/4-91/001. February 1991.
USEPA Environmental Monitoring Systems Laboratory. Las
Vegas, NV.
U.S. EPA. A Guide: Methods for Evaluating the Attainment
of Cleanup Standards for Soils and Solid Media. Quick
Reference Fact Sheet. Publication: 9355.4-04FS. July
1991. Office of Emergency and Remedial Response,
Hazardous Site Control Division OS-220W.
U.S. EPA. Permit Guidance Manual on Unsaturated Zone
Monitoring for Hazardous Waste Land Treatment Units.
EPA/530-SW-86-040. October. USEPA Environmental
Monitoring Systems Laboratory, Las Vegas, NV 89114.
U.S. EPA. 1991. Handbook of Suggested Practices for the
Design and Installation of Ground-Water Monitoring Wells.
EPA/600/4-89/034. March. USEPA Environmental
Monitoring Systems Laboratory, Las Vegas, NV 89193-
3478.
Petroleum Contaminant References
Drews, A.W., Ed. Manual on Hydrocarbon Analysis: 4th
Edition. ASTM Manual Series: MNL 3. ASTM, 1916 Race
Street, Philadelphia, PA. 19103.
Hoffman, H.L. Petroleum—Petroleum Products. Kirk-
Othmer Encyclopedia of Chemical Technology. 3rd Ed. Vol
17. Gulf Publishing Company.
Miller, Michael W. and Dennis M. Stainken. An Analytical
Manual for Petroleum and Gasoline Products for New
Jersey's Environmental Program, in: Petroleum
Contaminated Soils. Volume 3. Paul T. Kostecki and
Edward J. Calabrese. Technical Editor Charles E. Bell.
Lewis Publishers. 1990.
Wood Preserving Contaminant
References
USDA. 1980. The Biologic and Economic Assessment of
Pentachlorophenol, Inorganic Arsenicals, Creosote. USDA,
Number 1658-11. Washington, DC.
A Technology Overview of Existing and Emerging
Environmental Solutions for Wood Treating Chemicals.
December 1990. National Environmental Technology
Applications Corporation. University of Pittsburgh Applied
Research Center.
Becker, G. 1977. Experience and Experiments with
Creosote for Crossties. Proc. Am. Wood-Pres. Assoc.
73:16-25.
Bevenue, A. and H. Beckman. 1967. Pentachlorophenol:
A Discussion of Its Properties and Its Occurrence as a
Residue in Human and Animal Tissues. Residue Rev.
19:83.
Buser, H.R. 1975. Polychlorinated Dibenzo-p-dioxins,
Separation and Identification of Ispmers by Gas
Chromatography-Mass Spectrometry. J. Chromatog.
114:95-108.
Buser, H.R. 1976. High Resolution Gas Chromatography of
Polychlorinated Dibenzo-p-dioxins and Dibenzofurans. Anal.
Chem. 48:1553.
Crosby, D.G. 1981. Environmental Chemistry of
Pentachlorophenol. Pure Appl. Chem. 53:1052-1080.
DaRos, B., R. Merrill, H.K. Willard, and C.D. Wolbach.
Emissions and Residue Values from Waste Disposal During
Wood Preserving. Project Summary. EPA-600/S2-82-062.
August 1982.
USEPA. 1978. Report of the Ad Hoc Study Group on
Pentachlorophenol Contaminants. Environmental Health
Advisory Committee. Science Advisory Board, Washington,
DC.
Hoffman, R.E., and S.E. Hrudley. Evaluation of the
Reclamation of Decommissioned Wood Preserving Plant
Sites in Alberta. Waste and Chemicals Division and
H.E.L.P. Project, Alberta Environment.
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JRB Associates, Inc. Wood Preserving: Preliminary Report
of Plants and Processes. 1981. National Institute for
Occupational Safety and Health.
Lorenz, LR. and LR. Gjovik. 1972. Analyzing Creosote by
Gas Chromatography: Relationship to Creosote
Specifications. Proc., Amer. Wood Pres. Assoc. 68:32-42.
Micklewright, James T. Wood Preservation Statistics, 1988.
A Report to the Wood-Preserving Industry in the United
States. 1990 Proceedings of the American Wood
Preservers' Association.
Nicholas, Darrel D., Ed. Wood Deterioration and Its
Prevention by Preservative Treatments. Volume II:
Preservatives and Preservative Systems. Syracuse
University Press. 1973.
Thompson, Warren S., and Peter Koch. Preservative
Treatment of Hardwoods: A Review. USDA Forest Service.
General Technical Report SO-35. 1981.
almost pure organic matter is called the "O" horizon. As soils
mature, clay particles are moved downward along with
water. The clay tends to accumulate in lower soil layers.
The zone of clay accumulation is called the B horizon. The
C horizons comprise the relatively undifferentiated material,
often distinguished from the parent rock only by the lack of
consolidation.
Depth
Soil depth to bedrock or ground water affects the volume of
soil that may be contaminated, potential directions of
contaminant movement, and the difficulty of access to the
contaminated volumes of soil. Land treatment uses the
concept of a "treatment zone," meaning a zone in which
migrating contaminants are adsorbed and degraded. The
depth and activity of this zone affects the potential for
migration of contaminants to ground water.
Texture
APPENDIX A
SOIL PROPERTIES
Soils are composed of organic matter, inorganic solids
(sand, silt, clay, and larger fragments), air, and water.
Organic matter may range from less than one percent in
many soils, especially those in hot or cold desert climates, to
50% or more in the peat soils found in peat bogs. Soils are
generally classified according to their sand, silt and clay
content; the ratios of these components may vary in almost
any proportion. Air and water occupy the pore spaces
among the sand, silt and clay particles. Pore space
occupies about 20% - 60% of most uncompacted soils.
Soil parameters important in land treatment include: soil
horizons, depth, texture (grain size distribution—sand, silt,
clay proportions), bulk density, porosity (effective, total),
hydraulic conductivity, permeability, tilth, cation and anion
exchange capacity, organic matter content, pH, water
content and water holding capacity, nutrient content, salinity,
redox potential, color, and biological activity.
Soil Horizons
Soil horizons are the various layers present in most soil
columns. Physical and chemical differences between soils
in the layers affect movement of contaminants through the
soil profile. Organic matter from dead plants and animals
accumulates in the upper, "A" horizons. A top layer of
Soil texture (as defined by the proportions of sand, silt, clay)
influences porosity, hydraulic conductivity, permeability, tilth,
cation exchange capacity (CEC), and sorption capacity for
contaminants. Finer textured soils have greater surface
areas per unit volume. The differences between the
chemical and physical properties of the various sands and
silts is largely due to the different particle sizes. Clays are
not only much smaller than sands and silts but also are quite
different in chemical composition. Clay particles have
negatively charged surfaces that attract and hold cations
(Ca++, Na+, NH4+, H*. etc.) or other materials with a positively
charged portion, giving rise to a cation exchange capacity.
The edges of the clay particles may also have a positive
charge, giving rise to an anion exchange capacity. Clay
particles are flat, platelike structures, with.a very high surface
area. Clay particles have interior layers that can separate
enough to allow water and many ions to enter and be held.
"Shrink-swell" clays allow much water to enter these interior
areas, causing the clay particles to change greatly in volume
as the moisture content changes.
Bulk Density
The soil bulk density is the mass of dry soil per unit bulk
volume. The bulk volume is determined before drying the
soil to obtain the mass. Bulk density is used in most soil
transport and fate models.
Porosity, Hydraulic Conductivity, and
Permeability
Porosity, hydraulic conductivity, and permeability are three
parameters that are closely related and commonly confused.
The terms describe the soil characteristics and rate of water
movement through the soil.
10
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Porosity is the ratio of the volume of void spaces in a soil to
the total volume of the soil. The void spaces may be
occupied by air, water, or other fluids, such as contaminants.
The effective porosity represents the interconnection
between the void spaces and is defined as the volume of
void spaces through which water or other fluids can travel,
divided by the total volume of the soil.
Primary porosity is a characteristic of the original soil or rock
matrix; secondary porosity is caused by weathering or
fracturing processes occurring after the soil or rock was
emplaced. Secondary porosity can greatly enhance the
effective porosity of the soil or rock.
Typically, more rounded particles such as gravel, sand, and
silt have lower porosities than soils rich in the platy clay
minerals. Soils containing a mixture of grain sizes will also
exhibit lower porosities. The smaller particles tend to fill in
void spaces between the larger ones.
Porosity can be an important controlling influence on
hydraulic conductivity, which is a proportionality constant
describing the rate at which water can move through the soil.
Hydraulic conductivity is a function of the properties of both
the porous medium and the fluid passing through it.
Typically, the hydraulic conductivity has higher values for
gravel and sand and lower values for clay. Thus, even
though clay-rich soils usually have higher porosities than
sandy or gravelly soils, they usually have lower hydraulic
conductivities, because the pores in clay-rich soil are much
smaller.
The hydraulic conductivity can vary over 13 orders of
magnitude, depending on the type of material and whether
the measurement was made in the field or in the laboratory.
The methods of measurement differ significantly, and
interpretations placed on the values may be dependent on
the type of measurement. In practical terms, this implies that
an order-of-magnitude knowledge of hydraulic conductivity
may be all that is attainable, and that decimal places beyond
the second probably have little significance.
Pore spaces may be classified according to size as
micropores and macropores. Porosity of sandy soils largely
consists of macropores, while porosity of clay soils is largely
micropores. The ratio of micropores to macropores
influences the movement of soil gases and water in the soil
and is of particular importance for bioremediation, since the
ratios and interactions of soil gas and water greatly influence
microbial activity.
Permeability describes the conductive properties of a porous
medium independently of the fluid flowing through it. It
includes the influence of media properties that affect flow,
including the grain size.distribution and roundness, and the
nature of their arrangement. Permeability is widely used in
situations where multiphase flow systems (vapor, water, and
nonaqueous phase liquids) are present.
These conductive properties determine the feasibility of
adding or removing materials such as water, air, and
nutrients to the soil. Soil hydraulic conductivities of about
1.0 X 1 Cr4 to 1.0 X 10-6 cm/sec are favorable for adding or
removing materials. Soils with conductivities above this
range may require careful management to prevent excessive
drainage or contaminant mobility for some remediation
technologies: in soils with conductivities below this range it
may be difficult to add or remove materials for remediation.
The hydraulic conductivity of saturated soils is a function of
the grain size and sorting of the particulate materials, and
therefore, is somewhat stable over time. Hydraulic
conductivity in unsaturated soil is not only influenced by
grain size and sorting but also is strongly influenced by water
content of the soil. At low soil water content, soil water
moves largely in response to adhesive and cohesive forces
in the soil, which are measured as matric potential. Soluble
contaminants in unsaturated soil move in the thin films of
water surrounding the soil particles. The thicker the film of
water (e.g., the wetter the soil), the larger the conduit for
contaminant movement, and more of the contaminant that
can move in a given period of time.
Movement of contaminants in the vadose zone is usually in
the soil gas, pore water or as nonaqueous phase liquids
(NAPLs). Soil gases may move into the atmosphere, ground
water, soil pore water, be adsorbed on soil particles or
undergo chemical/biological transformation. Dissolved
contaminants in soil water undergo similar changes. NAPLs
move in response to gravity and changes in soil
permeability.
Soil Moisture and Water Holding Capacity
Soil moisture holding capacity is determined by the
proportion of clay and organic matter in the soil. Clays and
organic matter tend to hold larger amounts of water relative
to their volume than do the coarser grained silts and sands.
When a soil is saturated with water, then allowed to drain
freely for 24 hours, the soil is said to be at field capacity.
Essentially this means that the soil micropores are filled with
water, and the macropores are filled with air.
The ratio of air and water in the soil strongly influences many
important processes in the soil. Aerobic microbial activity is
usually optimum when soil moisture is about 70% - 80% of
field capacity; the higher end of the range is more desirable
for coarser soils. Relatively dry soils tend to adsorb many
contaminants more strongly than wetter soils, since water
competes with the contaminants for adsorption sites. When
the soil is not saturated, water and water-soluble compounds
may move in any direction in the soil in response to matric
potential, whereas water in saturated soils moves largely in
response to gravity. Water and water-soluble compounds
move faster through wetter soils than drier soils. Very dry
soils, especially soils with high organic matter content, may
be very difficult to wet since dry organic matter tends to be
hydrophobia NAPLs may move through moderately wet
soils faster than either dry or very wet soils, since dry soils
tend to adsorb much of the NAPL and the pores full of water
in wet soils inhibit NAPL movement.
11
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Tilth
Tilth refers to ability of the soil to undergo manipulation
(plowing, tilling) and retain a desirable loose, friable structure
that promotes ready movement of water and air. Structure
refers to the tendency of soil to agglomerate into units called
peds, granules or aggregates. In surface soils, this
agglomeration is influenced by the microbial secretion of
polymeric materials that cement soil materials together into
small particles. High levels of sodium in the soil (measured
as exchangeable sodium percentage (ESP) or sodium
absorption ratio (SAR)) may disperse the soil particles
causing a loss of structure. Sodium absorption ratios higher
than 15 may indicate a problem, as do ESP values greater
than 10% of the cation exchange capacity (CEC) in fine
textured soils and greater than 20% of the CEC in coarse
textured soils.
Sorptive and Exchange Capacity
Clay materials in soils generally have a high adsorptive
capacity for many organic and inorganic materials. Coarse,
sandy soils may allow rapid movement of relatively small
amounts of contaminants into lower soil layers and aquifers,
while soils high in clay may significantly retard movement of
many contaminants. Inorganics and some organics may be
influenced by the CEC, which denotes the capacity of the
clay particle for adsorption of positively charged materials on
the negatively charged surfaces of the clay. Mobility of
metals in the soil may be greatly affected by the CEC. Clays
may have an anion exchange capacity due to the positive
charge on the edges of some clay particles. The anion
exchange capacity is usually less than the cation exchange
capacity. Clays also will adsorb uncharged molecules due to
Van der Waals interactions of the uncharged materials with
clay particles. .
Organic Matter
Soil organic matter is generally composed of 25% - 35%
polysaccharides and protein-like compounds which are
readily decomposed by microorganisms and therefore have
a short halflife in soils. About 65% - 75% of the soil organic
matter is composed of humic materials, which are complex
mixtures of high molecular weight organics and are resistant
to degradation. These percentages do not include those
organic compounds that may be present as contaminants;
i.e., oil and grease, volatile organics, etc. Soils high in
organic matter will adsorb significant quantities of organic
contaminants, since organic compounds have a strong
tendency to adsorb onto soil organic matter, thereby slowing
movement. Soil organic matter usually has a relatively high
CEC and may have a significant anion exchange capacity,
although anion exchange capacity is usually much less than
the CEC. Increased soil organic levels are generally
favorable to microbial activity, due to increased CEC, tilth,
water holding capacity, and available carbon. Soil organic
matter levels tend to be lower in warm, moist climates, since
these conditions allow rapid microbial oxidation of the
organic matter. Soil organic matter may be increased by
addition of straw, hay, sawdust or wood chips, manures, and
many other organic materials. Addition of easily transformed
organic materials may cause shortages of nutrients
(particularly nitrogen and phosphorus) due to the increased
microbial population feeding on the added organic matter.
pH
The pH of the soil affects microbial activity; availability of
nutrients, plant growth, immobilization of metals, rates of
abiotic transformation of organic waste constituents, and soil
structure. A pH range of 6-8 is considered optimum for
bioremediation in most cases. Most metals tend to be less
mobile in high pH soils (arsenic is an exception), but acidic
organics such as pentachiorophenol are more mobile. Soils
with high sodium levels and high pH (most often found in dry
climates) tend to deflocculate and crust, limiting oxygen
diffusion and water uptake. Soil pH may be lowered by
addition of ferrous or aluminum sulfate, elemental sulfur or
sulfuric acid; soil pH may be raised by addition of agricultural
lime.
Nutrients
Nutrient content relates to the concentration of nutrients
available for use by microorganisms. Nitrogen and
phosphorus often limit microbial activity in soils. An organic
carbon:nitrogen:phosphorus ratio of 100-300:10:1 is
recommended to stimulate microbial activity, with the lower
C:N ratios recommended when most of the carbon is in a
readily degradable form. The percent base saturation, a
general indicator of soil fertility, is defined as the total of the
four principal exchangeable bases (calcium, magnesium,
sodium, potassium) divided by the total exchange capacity of
the soil. A base saturation of about 80% is desirable, with
calcium comprising about 60% - 70% of the CEC and
potassium about 5% - 10% of the CEC.
Most soils have low levels of nitrogen, although soils with
high levels of organic matter may have significant amounts
of nitrogen as part of the organic matter; this nitrogen is
usually released slowly as the organic matter decomposes.
Inorganic nitrogen in the soil is usually quite water soluble
and therefore readily lost to leaching, which may cause
ground water pollution problems. Since microorganisms
benefit from a steady supply of nitrogen, it is advantageous
to supply nitrogen either in small amounts frequently or in a
form (e.g., as organic fertilizers or "slow-release" inorganic
fertilizers) that supplies nitrogen to the microorganisms
slowly.
Many soils contain significant quantities of phosphorus, but
the phosphorus may be strongly bound in the soil, and little
may be readily available to the microorganisms. Usually
bound phosphorus is in equilibrium with phosphorus
dissolved in the soil water; the equilibrium is heavily
12
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*••«'
weighted toward the bound form. For this reason,
phosphorous fertilizers are often applied to raise the amount
of phosphorus in the soil water.
Salinity
The electrical conductivity (EC) of a soil reflects the soluble
salt content (salinity). An EC of 2 or less indicates that
salinity is not a problem in most instances. Ah EC of 2-4-
may inhibit activity of very salt-sensitive microorganisms,
while an EC of 4-8 may restrict activity of many
microorganisms. An EC greater than 8 will restrict activity of,
most microorganisms.
Redox Potential
The redox potential of the soil (oxidation-reduction potential,
reported as Eh) is controlled by the concentration of O2 in the
gas arid liquid phases. The O2 concentration is a function of
the rate of gas exchange with the atmosphere and the rate
of respiration in the soil. Respiration in the soil may deplete
O2, lowering the redox potential and creating anaerobic
(reducing) conditions. These conditions are unfavorable to
aerobic biotransformation, but may promote anaerobic
processes such as .reductive dehalogenation. Many reduced
forms of polyvalent metal cations aremore soluble (and
mobile) than.their oxidized forms. Well aerated soils have
an Eh of about 0.8 to 0.4 volts; moderately reduced soils are
about 0.4 to 0.1 V; reduced soils are.about 0.1 to -0.1 V; and
highly reduced soils are about-0.1 to-0.3 V. Redox y..
potentials are difficult to measure and are not widely used in
thefield. ... .
Color , .'- v /
•* . •
The color of soils is largely due to chemical changes and
organic matter-content. Dark colors in soil are caused by •
highly decayed organic matter. Reds and yellows are
caused by oxidized and hydrated iron in soil minerals.
Uniform reds, yellows, and browns indicate that a soil is well
drained. Mottled grays or blues may indicate poor drainage.
The location of any mottled layers may indicate the level of
the seasonal high water table.
Biological Activity
Biological activity in the soil is affected by .all of the soil '
characteristics discussed in this Appendix. Biological-activity
apparently accounts for most of the transformation of organic
contaminants in soil. •
.; l •* • '"'• '"
Both bacteria and fungi have been shown to be important in
bioremediation processes: Most research in bioremediation
has centered on bacteria, but some investigators have found
that fungi can play an important role in bioremediation
processes, especially with halogenated compounds (e.g.,
pentachlorophenol). In most cases bioremediation relies on
communities of microorganism species, rather than one or a
few species. .Bioremediation consists of utilizing techniques
for enhancing development of large populations of ' .
microorganisms that can transform pollutants of mterest, and
bringing these microorganisms into intimate contact with the
pollutants. In order to do this efficiently, necessary conditions
for the growth and activity of the microorganisms must be
maintained.
Microbial activity in the soil can be estimated by using plate
counts, most probable number (MPN) counts, direct
microscopic counts, respiration measurements, ATP activity
measurements, and others. Unfortunately, the relationship
of these measurements to practical use of bioremediation
techniques is unclear, at best. Generally use of these
measurements is limited to determining if soil conditions or
waste characteristics are suitable for microbial activity, and
whether particular management techniques have enhanced
microbial activity.
By culturing soil microorganisms on special media, counts of
"specific degraders" can be determined. For instance, if
PAHs are added to a media with no other carbon sources
present, any microorganisms that grow can be assumed to
have the capability of using PAHs as a sole source of
carbon. Again, the relationship of these counts to actual
biodegradation in the field is unclear.
If biodegradable contaminants have be.en present in thejsoil
for more than a few months or years, and microorganism's - .
are able to grow and reproduce in the contaminated soil,
microorganisms that can transform the wastes are likely to
be present. Treatability studies can be used to determine
techniques that might be appropriate to optimize their
transforming activity, as well as determine if'the
microorganisms are capable of transforming the wastes
to acceptable levels of acceptable end products in'an
acceptable time frame. ,
Bioaugmentation commonly takes two forms?'
Microorganisms'may be isolated from the site in question, .
cultured in quantity and added to the site soil, or "•'"
microorganisms isolated from other sites may be cultured
and added to the site soil. It is very difficult to show that .
added microorganisms survive and grow in the soil, and,
even more difficult to show that the added microorganisms
have any significant affect on transformation.
Metals in Soils
The mean concentrations of metals commonly found in
uncontaminated soils are shown in Table A-2. The actual
"background" concentrations at a given site may vary widely
from these numbers. High concentrations of certain metals
(particularly the "heavy" metals lead, mercury, cadmium,
chromium and others) are known to inhibit microorganism
activity in laboratory studies, but the particular levels of
metals that would be of significance in field bioremediation
are ftot known with certainty. The influence of metals '
13
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concentration on bioremediation appears to be site,
contaminant, and microorganism dependent. In cases
where high concentrations of metals appear to be of
concern, treatability studies should be conducted to
determine the influence of metals concentrations on
bioremediation.
TABLE A-2. MEAN CONCENTRATION (MG/KG) OF METALS IN THE EARTH'S CRUST AND SOILS a
Al Fe Mn Be Cu Cr Cd Zn As Se Ba Ni Ag Pb Hg
Soils 72000 26000 550 0.92 25 54 0.35 60 7.2 0.39 580 19 0.05 19 0.09
Crust 82000 41000 350 2.6 50 100 0.11 75 1.5 0.05 500 80 0.07 14 0.05
a McLean and Bledsoe, 1992
APPENDIX B
MONITORING
The land treatment unit should be monitored to determine
the fate of the contaminants in the unit. Generally, success
in land treatment has been determined by measuring the
disappearance of the waste components. Since waste
components may "disappear" from analytical view without
actually being remediated, a mass balance approach should
be taken, monitoring each soil phase (soil solids, gas, water
and nonaqueous phase liquid) to determine how much of
each waste component is in each phase. By this method, it
can be determined whether remediation is actually taking
place or whether the waste components are merely being
moved to different phases. In addition, the toxicity of the
various phases may be monitored to ensure that the
transformation of waste components does not produce more
toxic components, thereby creating a worse problem.
Waste Transformation
Parent Compound Loss
In most cases contaminant monitoring at soil bioremediation
sites is confined to analysis for parent compound loss. This
loss may be due to degradation, fixation, or any other
process that transforms the parent compound or removes it
from the detection ability of the extraction and analytical
method. The power of the extracting solution to remove the
contaminant from the soil matrix is of considerable
importance, since alternating temperature or moisture cycles
can cause waste components to bind so strongly to the soil
that removal is difficult. Parent compound loss is usually
followed even if other monitoring schemes are also used.
Breakdown Products
At some sites analysis for breakdown products may be
conducted, especially if such products are known to have
significant toxicity. Often, the specific breakdown products
are not known; it can be costly to determine the identity of
these products. Usually, breakdown products must be
identified using radiolabeled compounds and gas
chromatography/mass spectrometry (GC/MS) analysis.
Toxicity Reduction
Measures of toxicity may be required to determine if toxicity
of contaminants is actually reduced or if toxic contaminants
are merely transformed to other toxic materials. One assay
commonly used is the Microtox microbial bioassay. Cultures
of phosphorescent (light-emitting) marine bacteria are
exposed to soil extracts, and the decline in light output over
time is measured. The Microtox assay measures general
metabolic inhibition. The major advantages of the assay are
that it is quick, easy, repeatable, inexpensive, and there is a
large amount of published literature about its uses and
results. Its major disadvantage (as for most acute
bioassays) is that results of the assay have no direct
relationship to toxicity of the contaminants of concern to
humans. . '
14
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The Ames test, a measure of the mutagenic potential of a
sample, has been used widely in research, though
somewhat less in field applications. There is a high
correlation between mutagenicity (as measured in the Ames
test) and carcinogenicity. The Ames test takes several days
to complete and is more expensive than the Microtox assay.
Bioassays using many other species have been used for
assessing toxicity of environmental samples. Most of these
tests are time consuming and expensive.
Microorganisms
Microbial activity in the soil can be estimated with several
methods, including plate counts, most probable number
(MPN) counts, direct microscopic counts, respiration
measurements, ATP activity measurements, and others.
Unfortunately, the relationship of these measurements to
practical use of bioremediation techniques is not clear.
Generally, use of these measurements is limited to
determining if soil conditions or waste characteristics are
suitable for microbial activity, and whether particular
management techniques have enhanced microbial activity.
Oxygen and carbon dioxide levels can be useful as a
general index of microbial activity. Monitoring oxygen or
carbon dioxide alone can be deceiving since many soil
components can take up or release oxygen or carbon
dioxide by abiotic processes. Monitoring both yields a more
reliable indication of microbial respiration. Soil gas
concentrations of CO2 and O2 often fluctuate daily due to
microbial activity; therefore, it is desirable to measure CO2
and O2 at the same time of day for each sampling event.
Since the respiration estimated may not result only from
transformation of the compounds of interest, respiration
cannot be used as a direct measure of transformation of
these compounds.
Soil microorganisms can be cultured on specific media to
determine counts of "specific degraders." If PAHs are added
to a media with no other carbon sources present, any
microorganisms that grow in the media can be assumed to
have the capability of using PAHs as a sole source of
carbon. Again, the relationship of these counts to actual
biodegradation in the field is unclear.
Soil Moisture
Monitoring methods for soil moisture range from "eyeballing"
the soil, to the use of neutron probes. Since soil moisture
appears to be one of the most important determinants of
microbial activity, accurate and reproducible methods of
determining soil moisture are of considerable interest.
Gravimetric methods are very accurate, but somewhat time
consuming, and rarely used in the field. Gypsum block
monitors are often used in research, but are not suited to
LTUs since the blocks require individual calibration, are
permanently installed and would be disturbed by tilling.
Neutron probes are accurate but expensive. The moisture
probes sold in garden supply stores, while inexpensive, are
usually very inaccurate. The moisture monitoring devices
most likely to be useful are those based on the porous cup
tensiometer or soil capacitance. The capacitance based
types are somewhat new, but the porous cup tensiometer
types have been widely used in agriculture, are relatively
simple in concept and use, and are inexpensive.
Nutrients
Soil nutrients are usually determined by a number of
standard tests used in agricultural laboratories. Many land
grant universities have laboratories that analyze soil samples
for farmers, and there are also commercial laboratories
available. In most cases for LTUs, nutrient levels are based
on the ratio of soil carbon to other nutrients. Generally,
carbon to nitrogen to phosphorus (C:N:P) ratios of 100-
300:10:1 in the soil have been used, although some
investigators have found that C:N ratios of 100-120:10 may
be more appropriate where most of the carbon is in a readily
degradable form. Little research has been conducted on the
• specific concentrations of nutrients that would be optimal for
LTUs; however, nutrient concentrations for optimal microbial
activity may be similar to that for optimal growth of crop
plants. There is a large variety of material available
concerning nutrient levels versus growth and yield of crop
plants.
SAMPLING STRATEGIES
Sampling program goals must be delineated in order to
decide how many samples are needed for a monitoring
program. These goals may be formulated as a statement
that "It is necessary to know the average concentration of
this constituent in the LTU soil to +/- 5 ppm." If the variability
of the concentration of this contaminant in the soil is known
(or can be estimated), then statistical formulas can be used
to calculate the number of samples likely to be necessary to
estimate the concentration of that compound to the required
precision. Other goals may also be of interest. For
example, it might be necessary to know the highest
concentration likely to be found, rather than the average. It
also might be desirable to know that the concentration of a
given contaminant does not exceed some given level. The
accuracy needed for monitoring to determine operation and
maintenance practices is usually somewhat less than the
accuracy needed for regulatory monitoring to decide if target
final levels have been achieved. In general, the LTU
monitoring program design is based on the identified data
needs:
1) What are the desired confidence limits for the data? (Is
it sufficient to know the concentration of the contaminant
is some value +/- 10 ppm, or must the concentration be
known to +/- 5 ppm?)
15
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2) Is knowledge of the average concentration sufficient, or
must it be known if the concentration is less than some
value? (Is it sufficient to know that the average
concentration over the LTD is less than some given
value, or is there a need to be confident that there is no
location in the LTU where the maximum value exceeds
some given value?)
For monitoring purposes LTUs larger than about one acre
should be divided into sections approximately equal in size.
In most cases, sections should be no larger than one acre.
If it is known that certain locations in the LTU have
characteristic high or low concentrations ("hot spots" or "cold
spots"), these locations should be segregated into separate
sections when deciding on the size and coverage of the
sampling sections. This procedure, sometimes known as
"stratified random sampling," yields smaller confidence limits
for the concentrations in the sections. Random locations for
sampling should be determined in each section for each
sampling event.
Locations for collecting samples may be determined by
laying out a grid on the section and either choosing sample
locations from the grid points randomly or in some regular
pattern. If variability within the section is low or unknown, a
regular sampling pattern is probably the best choice. In this
case, the first sampling location would be chosen randomly.
All of the following sampling locations would be chosen
according to a pattern starting from the first randomly chosen
location. If variability within the section is known to be high,
and the approximate locations of high and low spots are
known, the section can be subdivided into similar areas and
each area sampled randomly. There are a number of
methods for choosing sample locations under different
scenarios of contaminant distribution; the references in the
Bibliography section of this document cover the more
oommoniy used methods.
Concentrations of contaminants in most LTUs are so
variable that many more samples must be taken to achieve
reasonable confidence limits for deciding if regulatory limits
have been achieved. Obviously, the more samples one
analyzes from a given section, the more one knows about
the range of concentrations of the compounds of interest in
that section. However, since analytical costs are usually a
major portion of project costs, the number of samples one
can afford to analyze is limited. Formulas are available to
determine how many samples are necessary to give any
required degree of precision in estimating the mean
concentration or variability of concentration in the plot. Since
the statistical basis for calculating the number of samples to
be taken is moderately complex and beyond the scope of
this brief review, references are provided in the Bibliography.
Compositing of samples eliminates high and low values,
tending to compress all data toward the mean value for the
LTU soil. Compositing decreases apparent variability of the
data by a factor of the square root of (N-1), where N is equal
to the number of subsamples composited to form the sample
for analysis. If the data are used to determine compliance
with maximum contaminant levels, data from composited
samples may indicate compliance has been achieved when
such may not be the case for significant portions of the LTU.
It is commonly supposed that compositing of samples, by
reducing the apparent variability of the data, allows more
accurate statistical analysis. However, data from sample
analyses are used to estimate concentrations and variability
of contaminants in the LTU; no manipulation of samples or
data can actually change the concentrations and variability
of the soil contaminants in the LTU. Since sample
compositing eliminates high and low values from the data,
data from composited samples should be used only to
estimate the mean concentrations of LTU contaminants and
not the range or variability of contaminant concentrations in
the LTU. If it is desirable to know the range and variability of
contaminant concentrations in the LTU, discrete soil samples
should be taken and analyzed. '.
The amount of sample to be taken (the sample support) is
largely determined by the requirements of, the analytical
procedure and any sample archiving required. The larger
the sample, the more likely it is to be representative of the
mean values in the whole section. Usually only a few grams
are needed for analysis, so an aliquot must be taken for
analysis from samples larger than this amount. This usually
involves a mixing procedure in the field or lab, which may
result in the loss of volatile contaminants.: Sometimes
subsamples are taken from a number of locations within the
section and composited to make one or more samples for
analysis. The object is to increase the sample support,
making the sample more representative of the section while
minimizing analytical costs. The caveats on compositing
mentioned above also apply here. '•
The sampling schedule should be based on timing of lift
applications. A lift should be sampled immediately before
application of a new lift. The latest lift applied should be
sampled immediately after application and tilling. Sampling
should be continued on the latest lift at specified intervals
after application until target levels of contaminants are
reached and sustained; then another lift can be applied.
Measuring Transformation Rates
Contaminant transformation rates may be determined to
estimate the time required for treating a number of lifts of
contaminated soil. At least five time-sequence points are
needed to calculate transformation rates for compounds that
degrade in a nonlinear fashion. This category includes most
compounds of regulatory interest. Sampling time points
should be evenly distributed throughout the time frame for
which rate estimates are desired, although it may be
desirable to have more replicate points for the beginning and
ending time points if starting and ending concentrations are
of particular interest. If the number of samples that can be
taken or analyzed is limited, and the time endpoint of the
experiment is not known at the beginning, ;time spacing
between samples can be increased as the experiment
proceeds. This is common in situations where it is desirable
to reach a given final concentration of a waste component.
16
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Rates should not be extrapolated beyond the time frame of
the data from which the rates were calculated. For instance,
in many treatability studies, data from laboratory experiments
that last six months are used to calculate estimated results
for months seven, eight, nine etc. Also, estimation of the
time required for achieving certain contaminant
concentrations is often extrapolated from experimental data
taken from experiments that never reached the levels
desired. Both procedures often result in estimates that are
not verifiable in the field.
Volatilization, Leachate and Runoff
When monitoring the transformation of waste compounds of
interest in the LTD soil, it is important to try to achieve as
complete a mass balance as possible. Routes of loss other
than transformation must also be monitored. Leaching,
volatilization and runoff are usually the most important
alternate routes of loss to be considered in the LTU.
Methods and equipment for monitoring these routes of loss
are discussed below. The location and frequency of
sampling for these routes of loss are subject to the same
considerations as soil sampling as discussed above.
Volatilization is usually measured by collecting volatiles
released from the soil surface. A canopy is placed over a
defined area of contaminated soil and vapors collecting
under the canopy are swept into an adsorbent for later
extraction and analysis. In some cases the vapors may be
measured directly with various kinds of detectors such as
photoionization detectors (PID). Unless the canopy covers
the entire LTU surface without interruption in time or space,
the measurement must be considered as an approximation
of the overall rate of vapor loss. Volatilization is often much
greater immediately after application of a new lift of
contaminated soil or after tilling.
Leaching from in-situ LTUs can be monitored with
lysimeters. Porous cup and pan lysimeters are commonly
used. Porous cup lysimeters have the advantage that they
can be used to take samples of the soil pore water even
when the soil is relatively dry. On the other hand, pan
lysimeters collect only water that is actively moving down
through the soil. Leachate monitoring for ex-situ LTUs is
relatively straightforward since most ex-situ units have liners
and leachate collection systems to collect leachate that may
be generated. The collected leachate can be sampled
periodically if it is treated separately or disposed. If the
leachate is recycled as irrigation water for the LTU, it should
be sampled at the end of the treatment cycle for each lift to
establish the mass balance for that lift of contaminated soil.
In most cases, installation of monitoring wells downgradient
of the LTU will be required in addition to lysimeters. Usually
at least one monitoring well is placed upgradient of the LTU
to determine if any contaminant detected in a downgradient
well is coming from the LTU or another source of
contamination.
Runoff water from the LTU should be sampled after each
major rainfall event if the runoff water is disposed. If the
runoff water is recycled onto the LTU, it can be sampled as
noted for recycled leachate.
•&U.S. GOVERNMENT PRINTING OFFICE: 1993 - 7SO-002/80268
17
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