United States
Environmental Protection
Agency
Office of
Research and
Development
Office of Solid Waste
and Emergency
Response
EPA/540/S-98/500
April 1999
&EPA Ground Water Issue
OF AS TO
OF
David S. Burden* and Judith L. Sims**
INTRODUCTION
The Regional Ground Water Forum is a group of ground-water
technical specialists, representing EPA's Regional Offices,
organized to exchange up-to-date information related to ground-
water remediation at hazardous waste sites. Proper site
characterization has been identified by the EPA Regional Ground
Water Forum as a major issue of concern for decision-makers
at many hazardous waste sites. In order to have a thorough
understanding of the processes involved in remediating the
subsurface, a working knowledge of the major physical,
chemical, and biological factors which affect the fate of
contaminants in the vadose zone must be achieved. This
paper summarizes the basic concepts of soil science as
related to the management of hazardous wastes and serves
as a foundation from which to build a thorough understanding
of soil processes.
For further information contact Dr. David S. Burden, 580-436-
8606, at the Subsurface Protection and Remediation Division
of the National Risk Management Research Laboratory, Ada,
Oklahoma.
Soil as a Waste-Receiver
A thorough soil characterization of a contaminated terrestrial
system is essential to the development of an accurate
assessment of the extent of problems associated with the
specific contamination as well as identification and
Hydroiogist, U.S. Environmental Protection Agency, National Risk
Management Research Laboratory, Subsurface Protection and
Remediation Division
' Soil Scientist, Utah Water Research Laboratory, Utah State University
implementation of remedial actions. Characterization efforts
will define potential means and routes of exposure to human
populations and the environment in orderto develop appropriate
site modification and management procedures for protection
of public health and the environment. The goal of an effective
soil characterization process is the identification and
measurement of specific soil factors that affect the behavior
and fate of specific waste constituents at a specific contaminated
site so that an effective remedial action can be developed and
implemented.
Figure 1 depicts possible degradation and immobilization/
transport processes influencing the behavior and fate of waste
constituents in a soil system. In a soil, residual waste
constituents may pose public and environmental health hazards
through their ability to contaminate the atmosphere through
volatilization or resuspension as airborne particles and the
hydrosphere through leaching and runoff (Dawson and Brown
1981).
Human exposure via atmospheric pathways may occur directly
through dermal contact and inhalation of particles or gas or
indirectly through deposition on crops or bioaccumulation in
grazing game and agricultural animals, either or both of which
may be ingested by humans. Waste constituents may reach
surface waters in runoff, either dissolved or suspended in
water or adsorbed to eroding soil particles. Movement of waste
constituents through the soil may occur as a liquid or gas or
dissolved in soil water. Movement may be in both lateral and
vertical directions to ground and surface waters. Human contact
may occur through ingestion of the contaminated water.
Detoxification of some waste constituents maybe accomplished
by the growth of plants or removed from the site in vegetation.
Superfund Technology Support Center for Ground Water
National Risk Management Research Laboratory
Subsurface Protection and Remediation Division
Robert S. Kerr Environmental Research Center
Ada, Oklahoma
Technology Innovation Office
of and Emergency
US EPA, Washington, DC
Walter W. Jr., Ph.D.
Director
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Figure 1. Processes influencing fate and behavior of waste constituents in the soil environment (Sims et al., 1984).
Behavior and Fate of Waste Constituents in a Soil
System
Degradation and Detoxification
The term degradation may refer to complete mineralization of
hazardous constituents to carbon dioxide, water, inorganic
compounds, and cell mass. In the natural environment, a
constituent may not be completely degraded, but transformed
to intermediate products that may be less, equally, or more
hazardous than the parent compound, as well as less or more
mobile in the environment. The goal of using degradation as a
remedial process is the formation of products that are no
longer hazardous to human health and/or the environment.
Degradation processes in a contaminated soil system may
include biotic and abiotic reactions. The ultimate products of
aerobic metabolism are carbon dioxide and water. Under
anaerobic conditions (i.e., in the absence of oxygen), metabolic
activities result in the formation of incompletely oxidized simple
organic substances such as organic acids as well as other
products such as methane or hydrogen gas.
Both microbial populations and higher plants may be active in
the breakdown of waste constituents. In most bioremediation
management processes at the presenttime, the use of microbial
degradation is the primary focus, though research is being
conducted to evaluate the use of plants to degrade waste
constituents (e.g., Walton and Anderson 1990; Aprill and Sims
1990).
Abiotic degradation reactions in soil systems often do not
result in complete degradation of waste constituents, but may
alter them sufficiently so that they are more susceptible to
further degradation by biotic processes. Abiotic mechanisms
that may account for loss of waste constituents include (Sims
et al., 1984; Dragun, 1988):
(1) hydrolysis- a chemical reaction in which a waste chemical
reacts with water or hydroxide ions;
(2) photochemical degradation- use of incident solar reaction
to accomplish degradation. Two processes may be involved
in photodegradation: (1) direct photodegradation, in which
each particle of light, or quanta excites one substrate
molecule; and (2) sensitized photodegradation, in which a
sensitizing molecule absorbs light in the visible region
and then returns to ground state by transferring its excess
energy to molecular oxygen, forming singlet oxygen, a
highly reactive species of oxygen that readily oxidizes
organic substrates.
(3) substitution and elimination- processes where other
chemicals in the soil react with a waste constituent by
substituting for reactive groups or eliminating reactive
groups;
(4) oxidation- a reaction resulting in the removal of electrons
from a chemical. This removal generally occurs by two
different pathways: (a) heterolytic or polar reactions (an
electrophilic agent attacks a molecule and removes an
electron pair, resulting in the formation of an oxidized
product); or(b) homolytic or free radical reaction (an agent
removes only one electron to form a radical that undergoes
further reaction); and
(5) reduction- a reaction that results in a net gain of electrons.
Immobilization/Transport
Waste constituents may be immobilized in a soil system by
sorption or partitioning to soil particles (e.g., organic materials,
such as humus, or inorganic materials, such as the clays
montmorillonite, vermiculite, or the hydrous oxides). Other
mechanisms of immobilization are chemical precipitation or
polymerization processes. Transport of constituents through
the soil may be as volatile materials, sorbed to mobile soil
particles (i.e., facilitated transport (Huling 1989)), or leached
with soil liquids (water or organic waste liquids). Transport may
also occur as dissolved or sorbed constituents move with
runoff waters or as constituents move into the atmosphere as
volatile materials or are sorbed to suspended airborne soil
particles.
The ultimate fate of waste constituents immobilized in a soil
system is dependent upon the long-term stability of immobilized
waste constituent/soil complexes and reversibility of the
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immobilization reactions. The effectiveness of soil remedial
technologies such as soil flushing will also be dependent
upon the strength and reversibility of immobilization processes.
DESCRIPTION OFTHE SOIL AND VADOSE ZONE
Definition Description of Soils
A soil is defined by the Soil Science Society of America (1987)
as "the unconsolidated mineral matter on the immediate surface
of the earth that (1) serves as a natural medium for the growth
of land plants; and (2) has been subjected to and influenced by
genetic and environmental factors of parent material, climate
(including moisture and temperature effects), macro- and
microorganisms, and topography, all acting over a period of
time and producing a product -soil- that differs from the material
from which it was derived in many physical, chemical, biological,
and morphological properties and characteristics."
The soil is a complex system, consisting of three phases:
(1) soil gases; (2) soil water; (3) organic and inorganic solids.
For a soil impacted by wastes, a fourth phase, nonaqueous
phase liquids (NAPLs), may also be present. Gases and water,
which are found in the pore spaces of a soil, together comprise
about 25 to 50 percent (by volume). A waste constituent,
depending upon solubility and tendency to volatilize, may be
found in varying proportions in these two phases. Pore sizes,
continuity, and relative proportions of water and air in the pores
are examples of factors that affect the mobility of contaminants
(both vertically and horizontally) in a specific soil.
Soil solids include both organic and inorganic components.
The inorganic components are comprised of sparingly soluble
constituents known as minerals, which are composed of
primarily quartz, feldspars, iron and aluminum hydrous oxides,
kaolinite, smectites, and micaceous minerals distributed in
sand, silt, and clay size fractions in most soils. The solids may
contain highly reactive charged surfaces that play an important
role in immobilizing waste constituents in a specific soil. Certain
types of clay minerals such as the smectites are especially
high in negative charges, thus exhibiting what is termed as a
high exchange capacity. Clays and hydrous oxides may also
contain positively charged surfaces and act as anion exchange
media for negatively charged constituents.
Soil organic matter also has many highly reactive charged
surfaces and may aid in retaining waste constituents in a soil
system. The term humus refers to the relatively stable portion of
soil organic matter that remains in soil after plant and animal
residues have decomposed. Hydrophobic organic constituents
may be sorbed onto soil organic matter and thus become less
mobile in the soil system. Immobilization of organic waste
constituents may result in additional time for degradation,
reducing bioavailability to microorganisms.
So/7 Profiles the
Surface soils are a portion of the vadose zone, which is the part
of the earth extending from the ground surface to the upper
surface of the principal water-bearing formation (Lehr 1988). In
the vadose zone, water in pore spaces generally coexists with
air, though saturated regions may occur if perched water tables
develop at interfaces of layers with different textures and
permeabilities. Prolonged infiltration may also result in transient
saturated conditions. Topsoils are weathered geological
materials arranged in more or less well-developed layers
(referred to as soil horizons). The depth of a soil horizon is site
specific. Water movement in topsoils is unsaturated, with soil
water held in the soil at less than atmospheric pressure.
Weathered topsoil materials usually gradually merge with
underlying earth materials, which may include residual or
transported solids. Topsoil differs from the material lying below
it in that it is often more weathered, contains organic matter and
biological life associated with organic matter, and is the zone of
maximum plant-root growth. The entire vadose zone may be
hundreds of feet thick, and the transport time of pollutants to the
ground water hundreds or thousands of years, while in other
regions, the vadose zone may be underlain by shallow, potable
aquifers that are especially susceptible to contamination due
to short transport times and presence of soil materials that
have low potential for pollutant attenuation.
Soil horizons are designated A, E, B, and C to represent surface
soil, subsoil, and substratum, respectively. The A and B horizons
are formed by weathering and other soil-forming factors. The
C horizon, relatively unchanged by soil forming processes, is
usually composed of slightly altered parent material or
undifferentiated geological deposits from which the A and
B horizons developed (e.g., sediment from ancient sea and
lake beds, loess), rocks and rock powder released from melting
glaciers, and alluvium from flooding streams). The E horizon is
the horizon of maximum eluviation of silicate clays, iron, and
aluminum oxides. The E horizon is also generally lighter in
color than the A horizon. Not all soils have all four horizons,
while many soils show variations within each master horizon.
Each horizon within a specific soil can have significantly different
characteristics such as depth, texture, bulk density, mineral
composition, and chemical properties that may result in differing
waste transport and attenuative characteristics. Horizon
boundary characteristics can adversely affect vertical movement
(percolation) of soil liquids. Such boundary characteristics
include abrupt changes in texture or structure, such as clay or
sand layers or the presence of hard pans (i.e., a hardened soil
layer in the lower A or in the B horizon caused by cementation
of soil particles with organic matter or with materials such as
silica, sesquioxides, or calcium carbonate and whose hardness
does not change appreciably with changes in water content).
Horizontal flow may also result from such discontinuities,
resulting in contamination of adjacent areas. In general, deeper
soil horizons result in more potential attenuation of waste
constituents and greater protection against transport of
constituents to ground water.
Soil Physical Properties
Soil Texture and Textural Classes
So/7 texture is a term that reflects the relative proportions of the
various particles (commonly referred to as soil separates) in a
soil. Size classes of soil separates, as defined by the U.S.
Department of Agriculture (USDA) and the Unified Soil
Classification System (USCS), are presented in Table 1.
The analytical procedure that is used to separate soil separates
is called particle-size analysis (Brady and Weil 1999). Sieves
are used to mechanically separate the very fine sand and larger
separates from the finer particles. Silt and clay contents are
then determined by measuring the rate of settling of these two
separates from suspension in water. Stones and gravel are
separated from a soil sample before particle-size analysis.
Organic matter is usually also removed, by oxidation reactions,
before analysis. The results of the analysis are used to assign
a soil to a textural class.
Soil textural classes are used to provide information concerning
soil physical properties. The proportions of sand, silt, and clay
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Table 1. U.S. Department of Agriculture (USDA) and Unified Soil Classification System (USCS) Particle Sizes (Fuller 1978)
USDA
Particle
Cobbles
Gravel
Coarse gravel
Fine gravel
Sanc/0.05 - 2.0
Very coarse sand
Coarse sand
Medium sand
Fine sand
Very fine sand
Silt
Clay
Size Range
(mm)
76.2 - 254
2.0 - 76.2
12.7-76.2
2.0- 12.7
1.0-2.0
0.5- 1.0
0.25 - 0.5
0.1 -0.25
0.05 -0.1
0.002 - 0.05
<0.002
USCS
Particle
Cobbles
Gravel
Coarse gravel
Fine gravel
Sand
Very coarse sand
Coarse sand
Medium sand
Fine sand
Very fine sand
Fines'
(Silt and clay)
Size Range
(mm)
>76.2
4.76 - 76.2
19.1 -76.2
4.76 - 19.1
0.074 - 4.76
2.0 - 4.76
0.42 - 2.0
0.074 - 0.42
<0.074
USCS silt and clay designations are determined by response of the soil to manipulation at various water contents
rather than by measurement of size.
in a specific soil are used to define the textural class, using a
textural triangle (Figure 2). The triangle is used by locating the
percentage of one of the soil separates and projecting inward
in the direction of the arrow. The same procedure is used for
one of the other separates. The point at which the two projections
cross identifies the class name.
Three broad groups of textural class names are often used to
classify soil texture (Table 2). These general terms are also
used to provide indications of physical and chemical properties
of soils. Finer-textured soils have a greater surface area and
greater intensities of physical and chemical properties, such
as adsorption and swelling, as shown in Figure 3. Soil textural
classes are not easily subjected to modification in the field.
Texture of a specific soil can only be changed by mixing with soil
of a different textural class.
K. W. Brown and Associates (1980) developed a summary of
the advantages and disadvantages of the use of various textured-
soils for the treatment of hazardous industrial wastes (Table 3).
In general, loam, silt loam, clay loam, sandy clay loam, silty clay
loam, silty clay, and sandy clay soils were identified as best
suited for land treatment of hazardous wastes.
Soil Structure and Aggregation
Soil structure refers to the aggregation of primary soil separates
(sand, silt, and clay) into peds or aggregates. Aggregates,
separated by surfaces of weakness or open planar pores, are
often seen as cracks in the soil. Structure exhibited in the
different horizons of a soil profile are essential characteristics
of the profile, similar to texture or chemical composition. Soils
may also have no structure: coarse soils with no structure are
referred to as possessing single grain structure, while fine
soils are called massive (soil material clings together in large
uniform masses). Red formation is thought to result from the
cementing action of soil colloidal matter (colloidal-sized clay
minerals, colloidal oxides of iron and manganese, and colloidal
organic matter). Peds may vary instability, changing in response
to moisture content, chemical content, chemical composition
of the soil solution, biological activity, and management
practices.
Structure may modify the influence oftexture in regard to moisture
and air relationships. Interpedal pores are often larger and
more continuous compared to pores among the primary
particles within the peds. For example, a soil with a content of
shrink-swell clays would exhibit only limited permeability if it did
not have a well-developed structure to facilitate water and air
movement. Aggregation in coarse soils stabilizes the soil
surfaces and increases water retention in the soil, and can also
restrict infiltration.
The type of structure also determines the dominant direction of
the pores and thus the direction of water movement. Platy
structures restrict vertical percolation, prismatic and columnar
100
"o0 % % ~o % "S 5
^ Percent Sand
Figure 2. USDA Soil Textural Triangle.
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Table 2. General Terms Used to Describe Soil Texture in
Relation to the Basic Soil Textural Class Names
U.S. Department of Agriculture Classification System
General Terms
Common names Texture Basic soil textural class
names
Sandy soils Coarse
Moderately coarse
Loamy Soils Medium
Moderately fine
Clayey soils
Fine
Sands
Loamy sands
Sandy loam
Fine sandy loam
Very fine sandy loam
Loam
Silt loam
Silt
Sandy clay loam
Silty clay loam
Clay loam
Sandy clay
Silty clay
Clay
structures enhance vertical percolation, and blocky and granular
structures enhance percolation both vertically and horizontally
(Otis 1983). Structural units that can withstand mechanical
disturbance without disintegrating provide better soil hydraulic
properties.
Soil Consistence
So/7 consistence is a term used to describe the resistance of
a soil to deformation or rupture, which is determined by the
degree of cohesion and adhesion of the soil mass. Structure
reflects the shape, size, and distinctness of soil peds, while
consistence reflects the strength and nature of the forces
between particles. Consistence is measured by feeling and
manipulating a soil or by pulling a tillage instrument through it
(Brady and Weil 1999).
Particle Density, Bulk Density, and Porosity
Two methods are commonly used to express soil mass:
particle density and bulk density (Figure 4). Particle density is
defined as the mass of a unit volume of soil solids. Particle
density is not affected by the size of soil particles, the
arrangement of the soil solids, or the pore space, but is
dependent on the chemical composition and crystal structure
of the mineral particles.
Particle densities for most mineral soils range from 2.60 to
2.75g/cm3, since the major portion of soils is usually comprised
of quartz, feldspar, micas, and colloidal silicate clays, with
particle densities in the same range. If a soil contains minerals
with high particle densities (e.g., magnetite, garnet, or
hornblende), the particle density may exceed 2.75 g/cm3.
Organic matter has a lower particle density, ranging from 1.1
to 1.4 g/cm3. Surface soils, with higher contents of organic
matter, usually have lower particle densities than subsurface
soils. However, for general calculations, the average soil with
3 to 5 percent organic matter may be considered to have a
particle density of 2.65 g/cm3 (Brady and Weil 1999).
Bulk density is the mass of oven dry soil per unit bulk volume.
This volume includes both solids and pores. Soils with a high
proportion of pore space to solids have lower bulk densities
than those that have less pore space. Density and distribution
of pore sizes determine the ease and amount of air and water
stored in and moving through pore spaces. Of soils with the
same texture, those with higher bulk densities are more dense,
have less pore volume and are less permeable. Soil bulk
densities usually increase with depth due to less organic
matter, less aggregation, and compression from the weight of
the overlying soil (K.W. Brown and Associates 1980). Sandy
soils, with particles lying close together and low contents of
organic matter, exhibit high bulk densities, ranging from 1.2 to
1.8 g/cm3. Finer textured soils have more pore space, organic
matter, and lower bulk densities, generally ranging from 1.0 to
1.6 g/cm3 (Brady and Weil 1999). Root growth is impaired at
bulk densities greater than 1.6 g/cm3.
A useful soil characteristic utilizing soil bulk density for
determining such values as the mass of a waste constituent
present at a contaminated site is calculation of the mass of a
unit volume of soil. A volume commonly used is an acre-furrow
slice (AFS); i.e., the volume of soil over one acre to a plow depth
of 6 to 7 inches. If an average acre-furrow slice is considered to
have a bulk density of 1.3 g/cm3, the soil to a depth of 7 inches
would weigh approximately 2,000,000 pounds. In metric units,
the weight of a hectare of soil 20 cm deep and with a bulk
density of 1.3 g/cm3 would weigh approximately
2,600,000 kg/HFS. On this basis, a soil containing 1 percent
(10,000 ppm) of a waste constituent would contain
20,000 pounds of the constituent per AFS.
The ratio of bulk density to particle density is a measure of the
soil volume occupied by solids. For example, a soil with a bulk
density of 1.56 g/cm3 and a particle density of 2.6 g/cm3 would
contain 60 percent solids and 40 percent pore space.
Soil Color
Color and color patterns in soil can often be used as indicators
of soil drainage characteristics of soil. Soil colors can be
described in general terms (e.g., brown, gray, yellow, etc.),
although the use of the Munsell color system, which
characterizes soil color in quantitative descriptive terms of hue
(the dominant wavelength or color of the light), value (brilliance
of the light), and chroma (purity of the dominant wavelength of
the light), is preferred (Foth 1984). Soil colors are determined
by comparing a soil sample to the Munsell Color Chart, which
Surface area
Adsorbing power
Swelling
Plasticity and cohesion
Heat of wetting
Colloidal
Clay
Silt
Sand
Figure 3. Relationship between soil texture and selected
soil physical properties. (Reprinted from Nature
and Properties of Soils, 12/E, 1999, by N. C. Brady
and R. R. Weil with permission of Prentice-Hall,
Inc. New Jersey.)
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Table 3. Suitability of Various Textured Soils for Land Treatment of
Associates 1980)
Hazardous Industrial Wastes (K.W. Brown and
Texture
Advantages
Disadvantages
Sand
Loamy sand
Loam
Silt loam
Silt
Silty clay loam
Silty clay
Clay loam
Clay
Sandy clay
Sandy clay loam
Very rapid infiltration; usually oxidized
& dry; low runoff potential
High infiltration; low to medium runoff
Moderate infiltration; fair oxidation;
moderate runoff potential; generally
accessible; good cation exchange
capacity
Moderate infiltration; fair oxidation;
moderate runoff potential;
generally accessible; good cation
exchange capacity
Low infiltration; fair to poor oxidation;
good cation exchange capacity;
good available water
Medium to low percolation; fair structure;
high cation
Good to high available water
Medium-low percolation; good structure;
medium to poor aeration; high cation
exchange capacity; high available water
Low percolation; high cation exchange
capacity; high available water
Medium to low percolation; medium to
high cation exchange capacity
Medium to high available water;
good aeration
Very low cation exchange capacity; very high
hydraulic conductivity rate; low available water; little soil
structure
Low cation exchange capacity; moderate to high
hydraulic conductivity rate; low to medium available water
Fair structure
Some crusting; fair to poor structure
High crusting potential; poor structure;
high runoff
Medium to low infiltration; some crusting
potential
Moderate runoff; often wet; fair oxidation
Medium to low infiltration; moderate to
high runoff; often wet
Low infiltration; often massive structure;
high runoff; sometimes low aeration
Fair structure; moderate to high runoff
Medium infiltration
In the field,
one cubic meter
of a certain soil
appears as
Solids and
pore spaces
1.33Mg
If all the solids
were compressed to
the bottom, the cube
would look like
1/2 pore spaces
1/2 solids
133 Mg
To calculate bulk density of the soil:
Volume = 1 m3 Weight = 1.33 Mg
(solids + pores) (solids only)
To calculate solid particle density :
Volume = 0.5 m3 Weight = 1.33 Mg
(solids only) (solids only)
Bulk density =
Weight of oven dry soil
Volume of soil
(solids + pores)
Solid particle density = Weight of solids
Volume of solids
Therefore
Bulk density, DJ-, =
1.33
= 1.33Mg/m3
Therefore
Bulk density, Dp = 133 = 2.66 Mg/m3
Figure 4. Calculation of soil bulk density and particle density. (Reprinted from Nature and Properties of Soils, 12/E, 1999, by
N. C. Brady and R. R. Weil with permission of Prentice-Hall, Inc. New Jersey.)
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consists of color chips arranged systematically on charts
according to hue, value, and chroma. The Munsell notation of
color is a numerical and letter designation of each of the three
properties of color. For example, in the Munsell notation
10YR 6/4, 10YR refers to the hue, 6 refers to the value, and 4
refers to the chroma.
Uniform red, yellow, or brown colors indicate that a soil is well-
drained and is seldom or never saturated with water (Foth,
1984). Organic matter is an important coloring agent in soils
and is most often highest in content in surface soil layers.
Organic matter is usually brown to black, depending upon the
degree of decomposition. In areas where conditions are
unfavorable for plant growth and accumulation of organic matter,
soils may be light gray or nearly white, colors inherited from
parent materials such as quartz or marl. In arid regions, surface
soils may be white due to the accumulation of soluble salts
after the evaporation of water, while subsoils may be white due
to limited leaching and the accumulation of calcium carbonate.
A red color in soil is generally produced by unhydrated and
oxidized iron oxide, while yellow soil colors are produced by
hydrated iron oxide. A yellow color in the subsoil is an indication
of imperfect drainage but not of water saturation.
Gray or blue colors indicate that the soil is saturated continuously
or for extended periods. Soils with spots or streaks (mottles) of
red, yellow, or black in a gray matrix (referred to as "mottled"
soils) are usually periodically saturated with water. Mottles
result from chemical and biological reactions when saturated
conditions, organic matter, and temperatures above 4° C occur
together in the soil (Otis 1983). Bacteria utilizing organic matter
deplete oxygen present in the soil. Other bacteria continue
organic decomposition using insoluble oxidized iron and
manganese compounds in their metabolism. The iron and
manganese are reduced to soluble forms, causing a soil to
lose its red, yellow, and brown colors and to turn it gray. When
water moves through the soil, the soluble iron and manganese
are transported to larger pores in the soil. When they contact air-
filled pores, they are re-oxidized, forming insoluble compounds
that accumulate as red, yellow or black mottles near pore
surfaces. The soil from which the compounds were removed
remains gray. Examination of a soil profile for the presence of
mottles may provide an indication of the depth of the seasonal
high water table at that location.
OF CLAYS, COLLOIDAL
CLAYS, AND
Colloidal in Soils
The colloidal fraction of a soil is of primary importance in
sorption and immobilization of both organic and inorganic
waste constituents in a soil system. The colloidal fraction,
composed of organic and inorganic particles with a maximum
size of 0.001 mm, is the most chemically active portion of a soil.
These particles are characterized by large exposed surfaces
(bothouterandinnersurfaces), a capacity to sorb and immobilize
solids, gases, ions, and polar compounds, and a tendency to
hasten (catalyze) or retard chemical reactions (Anderson et al.,
1982). Of the mineral (inorganic) portion of soils, only clay
particles are colloidal in size, and even some clay particles are
too large to be classified as colloidal. However, recognizing the
exceptions, the term "clay" is often used to refer to the inorganic
colloidal portion of soils. The organic colloidal fraction is
composed of amorphous humus, derived from organic
materials during their breakdown by microorganisms.
Clay in
Silicate Clay Minerals
The layer silicate clays have a planar, layer-like, crystalline
geometry, large specific surface areas, and high residual
negative charge densities that are neutralized by a large external
swarm of cations, thus resulting in a capacity for strong sorption
of and catalytic action towards waste constituents (Allrichs
1972).
Two basic sheet-like molecules compose the structure of
silicate clays. A tetrahedral sheet is comprised of a series of
tetrahedrons with four oxygen atoms surrounding a central
cation, which is usually silicon (Si4+), but may also be aluminum
(AP+), in a close-packed arrangement (Figure 5a). An octahedral
sheet is comprised of a series of octahedrons, with six oxygen
atoms forming the corners around a cation, which is usually AI3+
(referred to as a dioctahedral sheet), but may be magnesium
Silicon
Oxygen
(a)
Aluminum
or magnesium
O Oxygen or
hydroxy
(b)
Figure 5. Basic molecular and structural components of silicate clays: (a) a tetrahedron, and (b) and octahedron. (Reprinted
from Nature and Properties of Soils, 12/E, 1999, by N. C. Brady and R. R. Weil with permission of Prentice-Hall, Inc.
New Jersey.)
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(Mg2+) (referred to as a trioctahedral sheet) or iron (Fe2+ or Fe3+).
The sheets are formed by the sharing of corner oxygens
(Figure 5b).
The sheets may be joined in two ways, either in a 1:1 or 2:1
arrangement. In 1:1 arrangements, one tetrahedral layer is
connected to one octahedral layer by sharing a common oxygen.
Layers of these 1:1 units produce clays such as kaolinite. The
2:1 arrangement has single tetrahedral layers joined to each
side of the octahedral layer by sharing of oxygen atoms. This 2:1
unit produces the basic layer that when formed into multi-layers
forms such clays as montmorillonite, vermiculite, and mica.
Source of Charge in Clay Minerals
Some clays exhibit high negative charges due to substitution of
lower valency cations for higher valency cations within normally
neutral crystals. This process is referred to as isomorphous
substitution. During formation, ions of similar radii, such as AI3+
for some Si4+ in the tetrahedral layer, and Mg2+ or Fe2+ for AI3+ or
Fe3+ in the octahedral layer, may be substituted. The lower
valency of the substituting cation results in a residual negative
charge, which must be balanced by a cation external to the layer
unit, either on its edge or in the interlayer surfaces.
Additional negatively charged adsorptive sites occur on the
edges of clay minerals, as well as on humus, allophane, and
iron and aluminum hydroxides. Because these charges are
dependent on soil pH, they are termed variable or pH-dependent,
compared to the more permanent charges resulting from
isomorphous substitution (Brady and Weil 1999).
One mechanism of pH-dependent charge is associated with
the hydroxyl (OH") groups on the edges and surfaces of the
inorganic and organic colloids. Inorganic colloids with pH
dependent charge include 1:1 clays and Fe and Al hydrous
oxides. Under moderately acid conditions, there is little or no
charge on these colloids, but as the pH increases, the hydrogen
dissociates, reacting with OH" ions in the soil solution to form
water.and resulting in a negative charge. These reactions are
reversible. As the pH increases, the negative charge on the
particle surfaces increases, as the pH is lowered, the negative
charge is reduced.
A second source of increased negative charge with increasing
pH is the removal of positively charged aluminum hydroxy ions,
such as AI(OH)2+, from silicate clays. As pH is raised, AI(OH)2+
ions react with OH~ ions in the soil solution to form insoluble
AI(OH)3, thus freeing the negatively charged exchange site.
The surface geometry of the silicate clays is important in the
attenuation of waste constituents. The interlayer surfaces
provide most of their surface area (Allrichs 1972). The source
of the charge deficit in a clay is important in determining the
bonding action between layers and consequently the amount
of swelling. Marshall (1964) suggested that tetrahedral sources
of charge deficit provide larger bonding energies than octahedral
sources, since the distance from the cause of charge deficiency
is closer to the planar surface in the tetrahedra.
The size of the interlayer spacing may determine which
compounds may enter the interlayer of a clay mineral. For
example, montmorillonite will allow larger size molecules or
compounds to enter if there is an attraction for them, while the
interlayer surfaces of vermiculite with a smaller swelling
potential than montmorillonite are small enough to prevent the
entrance of many compounds.
Anion exchange on colloidal materials (especially on iron and
aluminum oxides, some 1:1-type clays, and allophane) also
occurs, but to a much lesser extent than that of cation exchange
because it occurs at pH values less than 3. Anions may replace
hydroxy groups and other anions in colloidal materials. Some
anions, such as phosphates, molybdates, and sulfates, react
with protonated hydroxy groups and are held tightly by the soil
solids.
Soil Chemical Properties Associated with Charged
Soil Solids
Cation exchange capacity (CEC) is a measure of the number of
cation sorption sites per unit weight of soil, which is a result of
both the negative charges from isomorphous substitution and
from pH-dependent edge charges. CEC is defined as the sum
total of exchangeable cations adsorbed, expressed in SI units
as centimoles of positive charge per kilogram of soil (cmol/kg).
If a soil has a CEC of 12 cmol/kg, one kg of this soil can adsorb
12 cmol of K+, and can exchange the K* with 12 cmol of another
monovalent cation, such as H+ or Na+, or with 6 cmol of a
divalent cation such as Ca2+ or Mg2+. The 12 cmol of negative
charge associated with one kg of soil is attracting 12 cmol of
positive charges (Brady and Weil 1999).
CEC was formerly expressed in milliequivalents per 100 grams
of oven dry soil (meq/100 g). One milliequivalent per 100 grams
of soil is equal to one centimole per kilogram of soil, so soil data
using either of these methods of expression are comparable.
CEC values of typical soils range from 1-10 cmol/kg for loam
soils, 12-20 cmol/kg for clay loam soils, and greater than
20 cmol/kg for clay and organic soils.
The proportion of the cation exchange capacity of a specific soil
satisfied by a given cation is referred to as the percentage
saturation for that cation. For example, if 25 percent of the CEC
were satisfied by Mg2+ ions, the exchange complex has a
percentage magnesium saturation of 25. The percentage
saturation with hf and AI3+ provides an indication of the acidity
in a soil (i.e., exchangeable acidity), while the percentage
saturation due to nonacid cations (e.g., Ca2+, Mg2+, K+, Na+),
referred as percentage base saturation, provides an indication
of alkalinity and neutrality in a soil.
Physical Characteristics of Soil Colloidal Clays
Dispersion of clay particles in a soil can occur due to the
repulsion of negatively charged particles for each other. Sodium
in its hydrated state is much larger than other common cations
and is not tightly held, and does not reduce the electronegativity
of clay particles; thus the presence of large quantities of sodium
in a soil tends to keep clay particles dispersed, thus reducing
water infiltration and perculation into the soil. If sufficient salts
are added, the effective electronegativity of the clay particles will
be reduced so that the clay particles can flocculate. However,
the required salt concentration may be so high as to restrict
microbial activity and degradative processes in the soil. The
sodium absorption ratio (SAR) is a measurement of the
relationship between soluble sodium and soluble divalent
cations, which can be used to predict the exchangeable sodium
percentage of soil equilibrated with a given solution. SAR is
defined as follows:
SAR = (Na)/[(Ca + Mg)/2]1/2
where concentrations, denoted by parentheses, are expressed
in meq per liter.
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As a general rule, an SAR of 15 or greater is considered
unacceptable and may result in reductions in permeability.
However, there is a difference in acceptable SAR values,
depending on the dominant clay type in a particular soil. For
example, with kaolinite, the SAR may be as high as 20 before
serious swelling problems occur. However, in soils in which
montmorillonite is dominant, SAR values of 8-10 may cause
serious swelling problems and reduction in permeability. In
terms of exchangeable sodium percentage (ESP), an ESP of
less than 5 percent is satisfactory, greater than 10 percent can
result in reduced permeability in fine-textured soils, and greater
than 20 percent can result in reduced permeability in coarse-
textured soils.
The tendency of some silicate clays to swell and shrink can
result in soils with wide, deep cracks when the soils are dry, but
the soils can become impervious due to swelling when water
is added. Swelling results from the penetration of water between
the crystal layers, from water attracted to both the clay colloids
and to the ions adsorbed by them, and from air bubbles
entrapped as water moves into the small pores of these soils.
Soils high in smectite clays are especially susceptible to shrink-
swell conditions.
Types of Silicate Clays and Other Clay Minerals
There are a number of silicate clays, but most are variations of
afewmajortypes. Kaolinite is the most common type of 1:1 type
clay. The 1:1 units are bonded together by hydrogen bonding
between hydroxyl groups, thus there is no interlayer surface
area, and kaolinite does not swell with the addition of water.
There is also little substitution within the tetrahedral or octahedral
layers, resulting in a low charge deficit. Kaolinite, therefore, has
low reactivity and sorptive capacity for cations. Kaolinite contains
more surface hydroxyl groups than other silicate clays, and is
considered to be primarily responsible for anion exchange in
temperate or arid region soils.
Montmorillonite, a member of the smectite group of clays (a
group noted for interlayer expansion), is a common 2:1 type clay
that has a negative charge of 80 to 100 cmol/kg and swells and
shrinks as water moves freely between the weakly bonded 2:1
units. Water is attracted to the oxygen surface of the clay and to
the neutralizing cations in the interlayers. The swelling caused
by the water exposes the large surface area and the large
charge deficit, which is available for cation adsorption. Bentonite,
commonly used to construct clay liners at waste sites, is a type
of clay composed of primarily smectite minerals produced by
the alteration of volcanic ash in situ.
Fine-grained mica possesses the same 2:1 structure as
montmorillonite but differs in that adjacent 2:1 units are tightly
bonded by potassium bridges. Fine-grained mica, therefore,
does not swell in water, and most of its charge is neutralized by
potassium ions. Fine-grained mica does not exhibit large
potentials for shrinking, swelling, and plasticity. Fine-grained
micas are expected to exhibit minimal interactions with waste
constituents present in soil systems (Allrichs 1972).
Vermiculite, considered a limited-expansion clay mineral, is
similar to the fine-grained micas, except that potassium is not
present. Vermiculite usually has hydrated magnesium (Mg2+)
or calcium (Ca2+) in the interlayers neutralizing the charges and
acting as bridges to hold the units together. The degree of
swelling is less than that of the smectites but more than
kaolinite. The CEC of Vermiculite is the highest of the silicate
clays, due to the high negative charge in the tetrahedral sheet.
Soils also contain colloidal clays that are amorphous (lacking
crystalline structure) that can be detected by x-ray diffraction.
Allophane is an amorphous silicate colloidal clay that is
developed from volcanic ash and is commonly found in the
northwestern part of the U.S. Soils containing allophane lack
discrete mineral particles and are like a gel (Foth 1984). Features
of soils high in allophane content include high organic matter
content, large amounts of surface area, and high water-holding
capacity. Allophane also has a high cation exchange capacity
and can adsorb anions.
The hydrous oxide clays are oxides containing associated
water molecules. They are formed by intense weathering, ater
silicon has been removed from the silicate clays, leaving iron
and aluminum hydroxides in a highly colloidal state. Aluminum
and iron hydroxides are protonated in the acid environments
that are common in highly weathered tropical soils and thus are
important in anion exchange reactions.
Organic Colloids
Soil contains many organic
decomposition. Soil organic
material; (2) animal matter;
and dead; (4) synthesized
plants and microorganisms;
of organic debris (Anderson
compounds in various stages of
matter is derived from: (1) plant
(3) microorganisms, both living
and secreted products of living
and (5) decomposition products
1982).
Schnitzer (1978) estimated that 65 to 75 percent of organic
matter in mineral soils consists of humic materials; i.e., humic
acid (HA), fulvic acid (FA), and humin. They are amorphous,
dark-colored, hydrophilic, acidic, partly aromatic, chemically
complex substances ranging in molecular weight from
hundreds to several thousand, with large surface areas
(500-800 m2/g) and high cation exchange capacities
(200-500 cmol/kg)(primarily pH-dependent). These colloidal-
sized humus particles are composed of carbon, hydrogen, and
oxygen, likely in the form of polyphenols, polyquinones, and
polysaccharides (Brady and Weil 1999). Humus is involved in
the formation of structural aggregation in soils and imparts a
characteristic black or brown color to soils. It also reduces
physical properties due to the presence of silicate clay minerals,
such as plasticity and cohesion.
The remainder of the organic matter is composed primarily of
polysaccharides and protein-like substances (Flaig et al. 1975).
These include substances with still recognizable physical and
chemical characteristics, such as carbohydrates, proteins,
peptides, amino acids, fats, waxes, alkanes, and low molecular
weight organic acids (Schnitzer 1978). They are readily
decomposed by microorganisms and have a short lifetime in
soils. Schnitzer (1978) identified the following important
characteristics of all humic materials:
(1) ability to form water-soluble and water-insoluble complexes
with metal ions and hydrous oxides; and
(2) ability to interact with minerals and a variety of organic
compounds, including alkanes, fatty acids, dialkyl
phthalat.es, pesticides, herbicides, carbohydrates, amino
acids, peptides, and proteins.
The formation of water soluble complexes with metals and
organics can increase concentrations of these constituents in
soil solutions and natural waters to levels greater than their
calculated solubilities.
The oxygen-containing functional groups are important for the
reactions of humic materials with metals, minerals, and organic
-------
compounds. Humic matter is somewhat organophilic, which
may be important for sorption of some nonionic organic waste
constituents. The oxygen-containing functional groups
contribute to the high cation exchange capacity, thus acting
similarly to clays in preventing cations from leaching from a
soil. Humus can account for 20-90 percent of the adsorbing
power of mineral soils (Brady and Weil 1999). Research has
been conducted to determine if waste constituents bound to
organic matter through abiotic sorption processes or through
biological incorporation into soil organic matter (i.e.,
humification) through oxidative coupling and polymerization
reactions will result in long-term detoxification and binding of
the constituents (Bollag and Bollag 1990).
Humus also increases the ability of a soil to hold and retain
water, since it swells when wet and can adsorb 2-6 times its
own weight in water (which is 4-5 times the capacity of silicate
clays). However, it only rewets slowly if thoroughly dried.
Humus and other organic materials, because of their chemical
composition, can add to the nutrient status of a soil, thus
increasing microbial activity that may be responsible for
degradation of waste constituents present in the soil. Nitrogen,
phosphorus, and minor nutrients such as sulfur, zinc, and
boron can be supplied by organic matter.
The amount of nitrogen relative to the amount of carbon (referred
to as the C:N ratio) in decomposing organic matter is an
especially important characteristic, for insufficient nitrogen may
limit rates of degradation of organic waste constituents. The
C:N ratio in the upper 15 cm of agricultural soils ranges from 8:1
to 15:1, with an average value of 10:1 to 12:1. Organic matter
with a low nitrogen content (or wide C:N ratio) is often associated
with a slow rate of decomposition. Materials that contain more
than 1.5 to 1.7 percent nitrogen probably do not need additional
fertilizer or soil nitrogen to meet the requirements of
microorganisms during degradative processes. This
corresponds to a threshold carbon-nitrogen ratio of 25 to 30
(Taylor et al. 1980). However, C:N ratios should be used with
caution, for the ratio does not indicate availability of the carbon
or nitrogen to microorganisms.
In summary, humified soil organic material, because of its
surface area, surface properties, and functional groups, can
serve as a buffer, an ion exchanger, a surfactant, a chelating
agent, and a general sorbent, all of which are important in the
attenuation of waste constituents in soil systems (Allrichs
1972).
Additional Soil Chemical Properties
Soil pH: Acidity and Alkalinity
Soil pH determines in part the degree of surface charge on
colloidal-sized soil particles. At high pH values, negatively
charged surfaces develop, while at low pH values, positively
charged surfaces occur. The tendency for sorption of anions or
cations is thus dependent on the soil water pH.
Soil pH also has major effects on biological activity in a soil.
Some organisms have small tolerances to variations in pH
while others can tolerate a wide pH range. The optimum range
for degradation of most waste constituents is pH 6.5 to 8.5
Bacteria and actinomycetes have pH optima near neutrality and
do not compete effectively with fungi under acidic conditions
(Taylor et al. 1980). Soil pH also affects availability of nutrients.
There are several sources of hydrogen (H+) and hydroxyl (OH")
ions in soil solutions. The hydrolysis of exchangeable bases
(Ca2+, Mg2+, Na+, and K+) that dissociate from cation exchange
sites contribute OH' ions. Exchangeable H+ in moderately acid
and acid soils that has been released from exchange sites
contributes H* to the soil solution. When the pH of the soil is
below 5.5, aluminum becomes soluble and is adsorbed as
exchangeable aluminum. Adsorbed aluminum is in equilibrium
with aluminum ions in the soil solution. Upon hydrolysis, each
aluminum ion becomes the source of H+ ions. At pH levels less
than 5.0 (strongly acid soils), aluminum ions are hydrolyzed,
thus releasing H+ ions:
AI3+ + H2O->AI(OH)2+ + H+
At pH levels of 5.0 to 6.5 (moderately acid soils), aluminum is
converted to hydroxy ions:
AI3+ + OH-->AI(OH)2+
AI(OH)2+ + OH-->AI(OH)2+
These hydroxy ions are hydrolyzed to release H+ ions:
AI3+ + 2H2O -> AI(OH)2+ + 2H+
AI(OH)2+ + H2O -> AI(OH)3 + H+
In neutral to alkaline soils, with pH values 7.0 and above,
neither hydrogen nor aluminum dominate the exchange sites.
The exchange sites are occupied by calcium, magnesium, and
other base-forming cations.
Total soil acidity can be characterized into three types. Active
acidity is a measure of the H+ activity in the soil solution. The
quantity of H+ due to active acidity is very low, but this is the
concentration of H+ in the environment to which biological life in
the soil is exposed.
Exchangeable acidity is associated with exchangeable
aluminum and hydrogen ions, present in large quantities in
acid soils and in lesser quantities in moderately acid soils.
However, even in moderately acid soils, the amount of limestone
required to neutralize this type of acidity is 100 times that
required for active acidity. At a given pH, exchangeable acidity is
highest for smectites, intermediate for vermiculites, and lowest
for kaolinite.
Potential acidity is the acidity remaining in the soil after active
and exchangeable acidity have been neutralized. Potential
acidity is associated with aluminum hydroxy ions and with
hydrogen and aluminum ions that are held in nonexchangeable
forms by organic matter and silicate clays. Potential acidity is
estimated to be 1000 times greater than the active and
exchangeable acidity in a sandy soil and 50,000 to 100,000
times greater in a clayey soil high in organic matter. The amount
of limestone required to neutralize potential acidity in an average
mineral soil usually ranges from 4 to 8 metric tons per hectare-
furrow slice (1.8 to 3.6 tons per acre-furrow slice), while the
amount required to neutralize the active acidity in a hectare-
furrow slice would be only 2 kilograms.
If a soil hasahighCEC( i.e., has high clay and/or organic matter
contents), more potential acidity must be neutralized in order to
achieve an increase in percentage base saturation; thus the
soil has a high buffering capacity and high resistance to change
in pH.
Neutral and alkaline soils of arid and semiarid regions that are
high in salts (e.g., chlorides and sulfates of calcium,
magnesium, sodium, and potassium) may exhibit detrimental
effects on biological activity in soils (Brady and Weil 1999). If
soils contain sodium salts, additional deleterious effects on
soil physical properties may be present. Salt content of soils is
10
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determined by measuring the electrical conductivity (EC) of a
soil solution; i.e., the ability of the salt in the soil solution to
conduct electricity.
Salt-affected soils are classified as saline, sodic, and saline-
sodic. Saline soils are high in soluble salts. Plant growth and
microbial activity may be impaired due to the osmotic pressure
of the soil solution restricting water uptake. Saline soils have
pH values less than 8.5 because the salts present are neutral.
Saline salts may be reclaimed by leaching out the salts with
water low in sodium.
A soil high in sodium (sodic soil) has a high pH due to the
hydrolysis of sodium carbonate and the formation of the strong
base, sodium hydroxide. A sodic soil does not contain large
quantities of soluble salts. When the cation exchange capacity
of a soil is 15 percent or more saturated with sodium, or a
significant amount of sodium carbonate exists in a soil, pH
values may range from 8.5 to 10 or higher. Sodic soils are
usually in an unsatisfactory physical condition, with reduced
water infiltration and aeration due to the dispersion of clay
minerals.
Saline-sodic soils contain large quantities of both neutral
soluble salts and sodium ions. The pH is usually 8.5 or less
because of the presence of neutral salts. To reclaim these
soils, especially if the soils are low in calcium and magnesium
salt concentrations, irrigation water should be high in calcium
or magnesium to prevent an increase in pH due to the hydrolysis
of exchangeable sodium, which may result in dispersion of soil
colloids.
The pH of a calcareous (containing CaCO3) soil or a calcareous
soil horizon ranges from 6.8 to 8.3. The presence of a calcareous
soil horizon may affect transport of waste constituents by an
abrupt change in soil pH, which may affect solubility or ionization
states of the constituents.
Soil pH can be adjusted by using agricultural management
techniques. To decrease soil acidity, agricultural limes, which
include carbonates, oxides, or hydroxides of calcium and
magnesium, are used. The amount of a particular liming
material required is dependent on: (1) the change in pH required;
(2) the buffer capacity of the soil; (3) the chemical composition
of the liming materials used; and (4) the fineness of the liming
materials (Brady and Weil 1999).
Oxidation-Reduction (Redox) Potential
A measurement of the oxidation-reduction potential, or Eh, of a
soil solution, in conjunction with measurement of soil pH, may
be useful in understanding the form and mobility of waste
constituents in a soil system. Eh is an expression of the
electron density of a system. As a system becomes reduced,
there is a corresponding increase in electron density, resulting
in a progressively increased negative potential (Taylor et al.
1980). With Eh and pH known, Eh-pH diagrams can be
constructed, showing stability fields for major dissolved
chemical species and solid phases.
The maximum rate of decomposition of degradable organic
compounds is correlated with a continuous supply of oxygen.
Excessive levels of degradable materials may lead to depletion
of O2 in soil and the creation of anaerobic conditions, which
usually slows the rate and extent of decomposition and may
produce some reduced compounds that are odorous and toxic
to microorganisms and plants.
However, the degradative pathways for some waste constituents
may involve several essential reductive steps. For example, an
important initial step in the degradation of DDT is a reductive
dechlorination, which involves anaerobiosis (Guenzi and Beard
1967). Parr and Smith (1973,1976) have shown that toxaphene
and trifluralin degrade more rapidly under anaerobic conditions.
Recent research has shown that anaerobic dehalogenation
reactions specifically involving reductive processes can
effectively degrade a wide variety of soil and ground-water
organic halogenated waste constituents, including
polychlorinated biphenyls, and carboxylated, oxygen-
substituted, nitrogen-substituted, cyano-substituted,
methylene-substituted, and chlorinated benzenes (Vogel et al.
1987; Kuhn and Suflita 1989).
An engineering management tool to maximize detoxification
and degradation of compounds subject to reductive
dechlorination may be alternation of aerobic/anaerobic
conditions (Guenther 1975) by adjusting Eh. Anaerobic
conditions may be created and maintained by flooding (with
appropriate runoff and leaching control) or by addition of readily
degradable organic matter. Regular cultivation of soil is a
possible means of maintaining aerobic conditions.
Nutrient Status of Soils
The biological degradation of organic waste constituents
requires the presence of nutrients for optimum biological growth.
Three of the major nutrients, nitrogen, phosphorus, and
potassium, can be supplied in common inorganic fertilizers.
Calcium deficiencies usually occur only in acid soils and can
be corrected by liming. If the soil is deficient in magnesium, the
use of dolomitic lime (i.e., calcium-magnesium carbonate) is
advised. A high level of exchangeable bases (calcium,
magnesium, sodium, and potassium) on the exchange surface
sites of the soil is also desirable for good microbial activity and
to prevent excessively acid conditions.
Though sulfur levels in soils are typically sufficient, sulfur is
usually added as a constituent of most inorganic fertilizers.
Micronutrients also occur in adequate amounts in most soils.
At sites contaminated with waste constituents, a primary danger
may be in overloading the soil with one of these elements that
may have been present in the waste, thus causing toxicity and
leaching problems.
The pH of the soil is also important for some elements. The pH
affects their solubility and biological availability, and thus their
toxicity and leaching potential.
HYDRAULIC PROPERTIES OF THE SOIL
PROFILE
Introduction
Soil/water relationships and associated soil hydraulic properties
affect both the movement of waste constituents through the soil
as well as soil processes acting within the soil to effect
attenuation (i.e., degradation, detoxification, and/or
immobilization) of the waste constituents. Biodegradation
requires water for microbial growth and for diffusion of nutrients
and by-products during the degradation process. Soil hydraulic
properties are those properties whose measurement involves
water flow or retention of water within the soil profile (U.S. EPA
1977).
11
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Soil Water Content Soil Water Energy
The total volume of a soil consists of about 50 percent pore
space and 50 percent solid matter. Water entering the soil (i.e.,
infiltration) fills the pore spaces until they are full. The water
continues to move down into the subsoil, displacing air as it
travels; this flow, when the soil is at maximum retentive capacity,
is said to be saturated. After water input to the soil ceases, the
water drains from the pores, and the soil becomes unsaturated.
Water in the soil below the saturation level in the soil is held
against the force of gravity. The forces that hold the water in the
soil result from cohesion of water molecules to each other and
adhesion of water molecules to soil surfaces, a phenomenon
referred to as capillarity (Figure 6). Capillarity can result in water
movement in the horizontal direction as well as in the vertical
direction.
Soil water content (measured gravimetrically as the amount of
water lost from a soil upon drying to a constant weight at 90° C),
expressed as either the weight of water per unit weight of dry
soil (mass basis) or as the volume of water per unit bulk volume
of soil (volumetric basis), can be determined at saturation or at
any degree of unsaturation. Indirect methods of measuring soil
water content in the field include the resistance method and
neutron scattering.
The movement of soil water is an energy-related process
(Brady and Weil 1999). All materials, including water have a
tendency to move or change from a state of higher energy to a
state of lower energy; i.e., in soils, water will move from an area
where the free energy of the water is high, which is a wet soil,
to an area where its free energy is lower, or to a dry soil. The
differences in energy levels in a soil profile determine the
direction of water movement.
Three forces act upon soil water to affect its free energy level
(Brady and Weil 1999). Adhesion, or the attraction of soil solids
for water, provides a matric force (responsible for adsorption
and capillarity) that reduces the free energy of adsorbed water
molecules and even some of those held by cohesion. The
attraction of ions and other solutes for water, i.e., osmotic
forces, also reduces the free energy of soil water. Osmotic
potential is usually not important in mass flow of water in a soil,
but may be important in the uptake of water into biological cells.
Since both osmotic and matric forces reduce the free energy of
Figure 6. Phenomenon of capillarity in a fine glass tube
placed in water. (Reprinted from Nature and
Properties of Soils, 12/E, 1999, by N. C. Brady and R.
R. Weil with permission of Prentice-Hall, Inc. New
Jersey.)
water, their expression in energy terms, or potential, are negative.
Gravity also acts on soil water, tending to pull water downward.
The free energy of water at a higher elevation in a soil profile is
higher than pure water at a lower elevation, so water flows
downward.
The term soil water potential is used to describe the difference
between the free energy of soil water and that of pure water in
a standard reference state. Soil water potential is comprised of
matric potential, osmotic potential, and gravitational potential.
The free energy level of water at a single point in a soil is not as
important in describing water movement and behavior in a soil
as differences in free energy levels at different points in a soil.
Soil water energy may be described in terms of the height in
centimeters of a unit water column whose weight equals the
potential of the soil water, or in terms of standard atmospheric
pressure at sea level (i.e., 14.7 Mm2, 760 mm Hg, or 1020 cm
of water). A bar is approximately equal to a standard atmosphere,
and 10 bars is equal to the SI unit megapascal (MPa). All of
these terms are negative quantities, since water at less than
saturation is held in soil pores at less than atmospheric
pressure. The term so/7 suction is also used to describe the
energy of soil water, but is reported as a positive quantity. Soil
water potential can be measured in the field using tensiometers,
which are fine porous cups filled with water that is held in
equilibrium with water in the adjacent soil.
The force by which water is held in soil pores is approximately
inversely proportional to the pore diameter. As water evaporates
or drains, the larger pores (macropores) drain first, while the
smaller pores (micropores) are still filled with water. Therefore,
as soil water content decreases, the absolute value of the
matric potential increases. A graphical representation of such
a relationship is referred to as a soil water characteristic curve
and is illustrated in Figure 7 for three different soil textures. The
shape of the soil water characteristic curve is strongly dependent
on soil texture and structure. Soils with primarily large pores,
such as sands, lose nearly all their water at small (absolute
value) soil water potentials. However, soils with a mixture of
pore sizes, such as loamy soils, hold more water at saturation
due to a larger porosity, and lose water more slowly as the
absolute value of the soil water potential increases.
Terms relating to soil water content at different soil water
potentials are illustrated in Figure 8. Gravitational water
movement when a soil is at or near saturation may be important
both in transport of waste constituents through a soil and in
leaching of nutrients, which may result in decreased biological
degradation of organic constituents. If water does not drain
quickly from a soil, saturated conditions may have a harmful
effect on microbial activity due to poor aeration. Saturation may
also affect solubility of waste constituents due to a change in
the oxidation-reduction potential of the soil.
The terms field capacity and permanent wilting point are
qualitative descriptions of soil water content. Field capacity
refers to the percentage of water remaining in a soil after having
been saturated and after free gravitational drainage has ceased.
Field capacity is not a unique value but represents a range of
water contents. In sandy soils, soil water content at field capacity
corresponds to matric potentials in the range of -0.10 to -0.15
bar, while in medium to fine-textured soils, potentials at field
capacity range from-0.3 to-0.5 bar, with-0.3 bar most commonly
used.
Drainage does not cease at field capacity but continues at a
reduced rate due to movement of water through micropores by
12
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TE
o
f
"E
-.001
(bars)
(MPa)
Soil water potential (|)
Figure 7. Soil water characteristic curves for three mineral
soils. (Reprinted from Nature and Properties of
Soils, 12/E, 1999, by N. C. Brady and R. R. Weil with
permission of Prentice-Hall, Inc. New Jersey.)
capillarity. Adhesive attraction between water molecules and
walls of micropores causes water to move through the pores,
pulling along other water due to the cohesive attraction among
water molecules. Capillary water can move in any direction in
the soil, following micropore channels.
When moisture in a soil is no longer in adequate supply to meet
the demands of plants growing in the soil, and plants wilt and
remain wilted even when placed in a humid chamber, the water
content is at the permanent wilting point. This water content
occurs in most soils when the matric potential is in the range of
-15 bars. The amount of water held in a soil between field
capacity and the permanent wilting point is referred to as
available water. This is the water available for plants and for soil
microbial and chemical reactions. Information on optimal and
marginal water potentials for growth, reproduction, and survival
of individual species of microorganisms in soils is limited
(Taylor et al. 1980). Bacterial activity is highest in wetter
conditions, but usually noticeably decreases by about -3 bars
(Clark 1967). Some fungi can grow and survive in soils under
dry conditions. Hygroscopic water is water bound tightly by soil
solids at potential values less than -31 bars. It is considered
nonliquid and can only move in the vapor phase.
Though fine-textured soils have the maximum total water holding
capacity, medium-textured soils have the maximum available
water due to a range of pore sizes. Even at the permanent
wilting point, soils contain a considerable amount of water,
though unavailable for use.
020
Gravitatio
-0.01
(high)
I t Rapidly
available
Optimum
zone
Capi ary
Available J, wafer
water
Slowly
> ' available
Hygroscopic
water
-10/ \ -100 -1,000
-15 -31
Soil moisture potential (bars, log scale)
-10,000
(Low)
Figure 8. Soil water characteristic curve of a loam soil as related to terms used to describe soil water. (Reprinted from
Nature and Properties of Soils, 12/E, 1999, by N. C. Brady and R. R. Weil with permission of Prentice-Hall, Inc. New
Jersey.)
13
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So/7
Permeability
Permeability of a soil describes the ease with which liquids
pass through the soil. Knowledge of permeability is required to
predict the rate of movement of waste constituents through a
soil.
Water moves through soils according to Darcy's Law:
q = K dH/dL
where:
q = flux of water per unit cross sectional area (cm/h);
K = permeability or hydraulic conductivity (cm/h); and
dH/dL = total head (hydraulic gradient) (m/m).
The total head, H, is equal to the sum of the soil water pressure
head (matric potential), h and the head due to gravity, z, or
H = h + z. In most cases, the head due to osmotic effects is so
low as to be negligible. However, at some contaminated sites
where the presence of salts as part of contamination is
significant, osmotic effects on water movement may be
important.
The hydraulic conductivity, K, changes rapidly as a function of
soil water content. Hydraulic conductivity decreases greatly as
water content decreases below saturation. In sandy soils,
though permeability is much higher at saturation than in loamy
soils, permeability decreases more rapidly as the matric
potential becomes more negative, eventually becoming lower
than in medium-textured soils.
Even under conditions of constant water content, K may change
due to swelling of clay particles or changes in the chemical
nature of soil water. Due to their negatively charged nature, soil
clay particles tend to repel each other and remain dispersed,
resulting in low permeability. The presence of sodium in a soil
often enhances dispersion and negatively affects permeability.
Positively charged cations in soil water neutralize the negative
charges and allow the soil particles to come close enough
together so that flocculation can occur, which increases soil
pore sizes and permeability. A soil low in salts but high in
sodium may have low permeability.
Other chemicals, especially organic compounds, may also
alter soil permeability (Sims etal. 1984). Increased permeability
may result from: (1) dissolution of clay minerals by organic
acids (Huang and Keller 1971); and (2) a change in the relative
volume of a clay as a result of interactions with an organic
solvent (Anderson et al. 1982). Potential for such changes
should be investigated in orderto predict and minimize leaching
of the organic compound, if it is of environmental concern, or to
determine if it would increase the leaching of associated waste
constituents of concern.
Soil permeability is primarily determined by soil texture, with
more coarse materials usually having higher conductivities.
However, the development of strong soil structure may increase
permeability in a finer-textured soil by increasing macropore
space.
The Soil Conservation Service has developed guidelines for
the classification of saturated soil permeability (Soil Survey
Staff, 1962). For remediation of waste-contaminated soils, the
most desirable soil would be a soil in which permeability was
large enough to maximize soil attenuation processes (e.g., to
maintain adequate aeration for aerobic microbial degradation)
but small enough to minimize leaching potential (assuming
that lower permeabilities protect against leaching).
Drainability
Drainability Is a term used to describe the relative rapidity and
extent of removal of water from a soil. Drainability is dependent
upon the permeability of the soil (i.e., K, the hydraulic conductivity)
and ground-water relationships that are controlled by soil
properties and the landscape position of the site (i.e., the
hydraulic gradient, dH/dL). A well-drained soil (e.g., a loamy
soil) is one in which water is removed readily but not rapidly; a
poorly drained soil (e.g., a poorly structured, fine-textured soil)
will remain saturated for extended periods of time, resulting in
the formation of reducing conditions and depletion of oxygen,
which may decrease biodegradation of organic waste
constituents; an excessively drained soil (e.g., a sandy soil) is
a soil in which water is removed so completely that droughty
conditions may occur. Drainage classes for soils developed by
the Soil Conservation Service are given in Table 4.
Another soil characteristic important in terms of potential
transport of waste constituents is the depth to an impermeable
layer, bedrock, or ground water, including seasonal variations.
Soil depth affects drainability of a soil and the effective depth for
waste constituent attenuation. Ryan and Loehr(1981) reported
that with depths of less than 1.5m, horizontal flow predominates,
and saturated hydraulic conductivity can be assumed to be
equal to the permeability of the saturated horizon with the
highest permeability. The hydraulic gradient is assumed to be
equal to the slope of the limiting layer and can be approximated
by the slope of the soil surface. When depth to an impermeable
layer, bedrock, or ground water is greater than 1.5 m, vertical
flow is predominant. The saturated hydraulic conductivity of the
soil can be assumed to be equal to the permeability of the most
limiting horizon, and the hydraulic gradient is assumed to be
one.
Table 4. Soil Conservation Service (SCS) Drainage
Classes (Soil Survey Staff 1962)
Drainage Class
Observable Symptom
Very poorly drained
Poorly drained
Somewhat
poorly drained
Moderately well drained
Well drained
Somewhat
excessively drained
Excessively drained
Water remains at or on the
surface most of the year
Water remains at or near the
surface much of the year
Soils are wet for significant
portions of the year
Soils are seasonally wet (e.g.,
high drained water table in
spring)
Water readily removed from the
soil either by subsurface flow or
percolation; optimum condition
for plant growth
Water is rapidly removed from
the soil; characteristic of many
uniform sands
Very rapid removal of water
with little or no retention
14
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Infiltration
Infiltration rate refers to the rate at which water enters the soil
from the surface. When the soil is saturated, the infiltration rate
is equal to the saturated hydraulic conductivity. However, when
the soil initially is relatively dry, the infiltration rate is higher as
water enters large pores and cracks. The infiltration rate is
reduced rapidly to a near steady-state value as large pores fill
and clay particles swell. Infiltration rates are affected by soil
texture and structure, ionic composition of the applied liquid,
condition of the soil surface, and the type of vegetation on the
soil surface.
Since infiltration governs the amount of water that will enter a
soil, engineering and agronomic practices that affect physical,
chemical, and biological soil characteristics may be utilized to
alter infiltration rates. Changes in bulk density, porosity, and
percent of water-stable aggregates will affect the capacity of a
soil for infiltration and the potential erodibility of a soil. Infiltration
rates can be increased by lower bulk densities and higher
porosity or by an increase in the number of macropores
connecting to the soil surface.
Frozen soil often has a lower infiltration rate than unfrozen soil,
especially if the soil was frozen when moist. Since frost usually
penetrates deeper if soil is bare than if it is snow-covered,
practices that prevent snow from blowing away may lessen
frost penetration. However, the additional snow may increase
surface runoff.
Potential for Off-site Migration
An assessment of the flooding frequency of a contaminated
site should be made to determine potential for off-site migration
of waste constituents in flood waters. Only slight hazards exist
if the soil is usually not flooded any part of the year, moderate
hazards if occasional flooding occurs (10-50 percent chance of
flooding once every two years) and severe hazards if frequent
flooding occurs (greater than 50 percent chance of flooding
every two years) (Ryan and Loehr 1981).
Runoff is the portion of precipitation that appears in surface
waters. Surface runoff is water that travels over the ground
surface to reach a lake, stream, or river (overland flow). Surface
flow may carry constituents in solution, in suspension, or
adsorbed to suspended soil particles. Subsurface runoff and
ground water carry primarily soluble constituents not strongly
sorbed to soil particles.
Steenhuis and Walter (1979) described a method of categorizing
pollutants with regard to potential losses in soil and water
according to their relative concentrations in water and on soil
particles, as indicated by adsorption/desorption isotherms. An
adsorption partition coefficient, ks, for a given solution
concentration is calculated as the ratio of amount adsorbed to
that in solution:
concentration of substance adsorbed to particles (ppm, mg / kg)
s ~~
concentration of substance in solution (ppm, mg /1)
Group I pollutants are those with ks values approximately equal
to 1000. These include the strongly adsorbed and solid phase
pollutants. The loss of these pollutants in baseflowand interflow
is small. Their losses in overland flow may potentially be high
and are related to the amount of sediment in the water and the
amount of pollutant in the soil. Organic matter, which may be an
important sorbent of waste constituents, is more easily eroded
than most mineral particles and tends to remain in suspension
because of its low bulk density. Silt and clay are also more
erodible than sand and are usually higher in organic matter
content. Clays, because of the charge properties and surface
area, are also important sorbents of waste constituents.
Therefore, an "enriching process" occurs in overland runoff in
which concentrations of waste constituents in eroded sediments
may be much higher than in the original soil. Loss of Group I
pollutants can be decreased by erosion control practices that
minimize sediment detachment and transport.
Group II pollutants, with ksvalues of aboutS, include moderately
sorbed pollutants (e.g., many pesticides). Their loss in overland
flow is related to the amount of runoff water and not to the
amount of soil loss. Erosion control practices that prevent
sediment detachment and transport are not as effective as
practices that reduce the total amount of runoff volume. Transport
of sorbed substances by water passing through a soil is much
slower than transport by surface flow. An equilibrium exists
between substances dissolved in soil water and those sorbed
to the soil. The greatest contamination of subsurface flow water
is due to those waste constituents that are weakly adsorbed or
those slow to degrade.
Group III pollutants have ks of about 0 to 0.5 and non-sorbed or
soluble pollutants. Their primary pathway of loss is through
interflow and baseflow. Losses in surface runoff are small and
therefore are not greatly affected by practices to control runoff.
Reduced runoff may even increase subsurface flow and
increase losses of Group III pollutants to interflow and ground
water.
Moderately and weakly sorbed waste constituents usually
migrate fairly rapidly after soil contamination with initial
precipitation events. Strongly sorbed constituents , depending
on their stability and recalcitrance, may pose hazards for years
due to movement in overland flow.
Control of runoff can be accomplished by several means
(Steenhuis and Walter 1979). Decreasing runoff velocity will
reduce both surface runoff volume and sediment loss. More
water remains on the soil for a longer period of time, thus
permitting increased infiltration. Runoff velocity may be reduced
in several ways: (1) by forcing water to move laterally ratherthan
straight down slopes: (2) by reducing the slope of the land
through land-forming; or (3) by increasing the roughness of the
soil surface to dissipate the kinetic energy of the water.
An increase in surface storage will remove the stored water
from the total surface runoff volume, resulting in decreased
runoffvelocity and a reduced sediment carrying capacity. Surface
storage can be increased by various engineering and
agricultural practices (e.g., creation of ridges of soil orvegetation)
that allow water to pool. Moisture storage capacity in the soil
itself can be increased by addition of organic matter or by
draining or evaporating moisture already in the soil profile.
Runoff also can be controlled by reduction of the splash energy
of falling rain. Raindrop impact on bare soil may break soil
aggregates to component particles. These smaller particles
may be carried by water into larger pores, thus forming a thin
surface layerwith low hydraulic conductivity. Dissipating raindrop
energy by use of a plant canopy or mulch or by promotion of
aggregate stability with organic matter addition may reduce this
surface sealing effect.
Snowmelt occurring in the spring, often on frozen ground, can
potentially carry a higher contaminant load than rainfall runoff
15
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that has infiltrated the ground. Also, time frames in which
rainfall runoff and snow melt runoff occur are different. Rainfall
runoff occurs when the infiltration capacity of the soil is exceeded
by the rate of precipitation. Infiltration capacity may be exceeded
by the intensity of the rainfall event or its duration. However,
spring snowmelt occurs over a period of time, usually interrupted
periodically by subfreezing temperatures and continuing until
all of the snow melts.
Hydrologic Budget
The preparation of a hydrologic budget or water balance for a
contaminated site will aid in prediction of leaching potential of
waste constituents and types and extent of attenuation
processes (biodegradation, sorption, etc.) that will act upon the
constituents. Inputs to the hydrologic budget include
precipitation, run-on, and any additional water that may be
required to accomplish the remediation technology. Factors
that may limit the amount of additional water that may be added
must also be considered, such as the total amount of
precipitation relative to evaporation, seasonal distribution of
precipitation, and changes in amount of precipitation from year
to year. Losses from the site include evaporation, transpiration,
percolation to ground water, and surface runoff.
Precipitation data may be based on measured rainfall events
or on frequency analyses (e.g., 10-year/24-hour storm,
25-year/1-hour storm, etc.). Information should include the total
rainfall a site receives as well as the intensity, duration, and
frequency of single precipitation events.
Evaporation is the transfer of liquid water into the atmosphere.
Factors affecting the rate of evaporation are the nature of the
evaporating surface and vapor pressure differences as affected
by temperature, wind, atmospheric pressure, ionic composition
of water, and available energy (Schwab et al. 1981). In saturated
soils, evaporation is expected to be the same as from open free
water surfaces. However, in unsaturated soils with water content
below field capacity, evaporation is low, as soil water movement
is slow when the soil is relatively dry.
Transpiration is the process by which water vapor passes into
the atmosphere through the tissues of living plants. Loss of soil
moisture by transpiration is often a substantial portion of the
total water available during a growing season. Transpiration is
dependent on the moisture available, the kind and density of
plant growth, climatic factors, and soil fertility and structure.
The measurement of evaporation and transpiration is usually
combined and referred to as evapotranspiration.
Evapotranspiration can either be measured directly or predicted
using various models (Schwab et al. 1981). Methods to measure
the movement of water out of the soil zone include tensiometric
measurements and water flux meters (Hillel 1971).
Soil aeration controls the levels of oxygen and carbon dioxide,
which are gases involved in both respiration and photosynthesis
(Brady and Weil 1999). In respiration, oxygen is utilized in the
oxidation of organic compounds, with the production of carbon
dioxide and water. In photosynthesis, the reaction is reversed,
and carbon dioxide and water are combined to form simple
organic compounds (i.e., sugars), and oxygen is released. For
respiration to occur in a soil, oxygen must be replenished and
carbon dioxide, which may reach toxic levels, removed. For
most plants, the supply of oxygen in a soil should be kept above
10 percent.
Poor soil aeration occurs when the moisture content of a soil is
too high so as to exclude gases in the pores, or when the
exchange of gases with the atmosphere is too slow to maintain
appropriate levels of soil gases (Brady and Weil 1999). Soil
moisture may reach too high levels in areas with naturally poor
drainage, in soils after excess amounts of water have been
added through rainfall or irrigation, or in soils that have been
compacted by plowing or machinery. Soil moisture content can
often be controlled by artificial drainage with canals and tile
drains.
Mass flow and diffusion control exchange of gases between
the soil and the atmosphere. Mass flow, which is movement of
air due to differences in pressure between the atmosphere and
soil air, is enhanced by variations in soil water content (i.e.,
water added to a soil forces air out of the soil). When water is
removed from soil by evapotranspiration, air is drawn into the
soil.
Diffusion is primarily responsible for gaseous interchange in a
soil. In diffusion, each soil gas moves in a direction determined
by its own partial pressure. Therefore, gas movement can
occur even when there is no overall pressure gradient for the
total mixture of gases in a soil, for there is a concentration
gradient for each individual gas, expressed as a partial pressure
gradient. A higher concentration of oxygen in the atmosphere
will result in movement of oxygen into a soil, even though the
total soil air pressure and atmospheric pressure are equal.
Carbon dioxide and water vapor generally move out of a soil to
the atmosphere, since their concentrations are usually higher
in soil than in the atmosphere.
The soil properties affecting the aeration of well-drained soils
are those that determine the volume of the soil macropores
(Brady and Weil 1999). Soil macropores affect the amount of
total air space as well as the potential for gaseous exchange
and biodegradation. These soil properties include: (1) soil
texture; (2) bulk density; (3) aggregate stability; and (4) soil
organic matter content.
POTENTIAL FOR
Soil properties that affect off-site migration of waste constituents
via air transmission are also important in site characterization.
The degree of migration in air is an interaction between soil/site
characteristics and waste constituent characteristics.
Soil properties that determine the extent and rate of volatility of
waste constituents are those related to soil permeability and
soil moisture. The total porosity of the soil, the distribution of
macro- and micropores, and the tortuosity of soil pores should
be characterized. The range of air-filled porosities exhibited by
soils under moisture regimes encountered at a specific
contaminated site should also be investigated, for wetter soils
are less permeable to gases than dry soils. At lower soil water
contents, there is also an increase in sorption of constituents.
The dispersion characteristics of an area are an important
component of air migration potential. Greater dispersion
associated with open lands is more favorable than areas with
channel type dispersion, such as in valley and depressional
areas. Determination of prevailing wind directions and wind
velocities will give an indication of the extent and direction of
migration.
The upper layers of soil contain large numbers and a diversity
of microorganisms. Biodegradation of organic waste
16
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constituents is accomplished by enzymes produced by the
microorganisms. Since many enzymes are not released by
microbial cells, substances to be degraded must contact or be
transported into the cells. Enzymes are generally specific in the
substances they affect; therefore, many types may be required
in order to complete degradation of organic constituents. The
production of enzymes is genetically controlled, thus mutations
and adaptations of the native soil microorganisms can improve
the ability of the populations to degrade organic substances.
Microbial ecologists have identified ranges of critical soil
environmental conditions that affect the activity of soil
microorganisms (Table 5). Many of these conditions are
controllable and can be managed to enhance biodegradation
of organic constituents.
Water is necessary for microbial life, and the soil water matric
potential against which microorganisms must extract water
from the soil regulates their activity. (The soil matric potential is
the energy required to extract water from soil pores to overcome
adhesive and cohesive forces.) Soil water also serves as the
transport medium through which many nutrients and organic
constituents diffuse to the microbial cell, and through which
metabolic waste products are removed, and influences soil
aeration status. The nature and amount of soluble materials,
soil water osmotic pressure, and pH of the soil solution are
additional important aspects of the influence of soil water on
microbial activity (Paul and Clark 1989).
Fungi appear to be more tolerant of low soil water potential than
are bacteria (Gray 1978, Harris 1981). Microbial degradation of
organic materials in drier soils is probably primarily due to
fungi. When a soil becomes dry, many microorganisms form
spores, cysts, or other resistant forms, while others are killed
by desiccation.
Clay content of soil and presence of organic matter may affect
oxygen content in soil. Clayey soils tend to retain a higher
moisture content, which restricts oxygen diffusion (though
development of soil structure may increase oxygen diffusion
Table 5. Critical Environmental Factors for Microbial
Activity (Sims et al. 1984)
Environmental
Factor
Optimum Levels
Available
soil water
Oxygen
Redox potential
PH
Nutrients
Temperature
25% - 85% of water holding
capacity; -0.01 MPa
Aerobic metabolism: Greater than
0.2 mg/l dissolved oxygen,
minimum air-filled pore space of
10%; Anaerobic metabolism: O2
concentrations less than 1 %
Aerobes and facultative
anaerobes: greater than
50 millivolts; Anaerobes: less than
50 millivolts
5.5 - 8.5
Sufficient nitrogen, phosphorus,
and other nutrients so not limiting
to microbial growth (Suggested
C:N:P ratio of 120:10:1)
15° -45°C (Mesophiles)
potential), while the presence of organic matter may increase
microbial activity and deplete available oxygen.
Loss of oxygen as a metabolic electron acceptor induces
change in the activity and composition of the soil microbial
population. Facultative anaerobic organisms, which can use
oxygen when it is present or can switch to alternative electron
acceptors such as nitrate (i.e., denitrifying bacteria) or sulfate
(i.e., sulfate reducing bacteria) in the absence of oxygen, as
well as obligate anaerobic organisms become the dominant
populations. Generally, microbial metabolism shifts from
oxidative to fermentative, becoming less efficient in terms of
biosynthetic energy production.
Another soil parameter that describes the effect of the soil
environment on metabolic processes is the redox potential of
the soil (Paul and Clark 1989). Biological energy is obtained
from the oxidation of reduced materials. Electrons are removed
from organic and inorganic substrates to capture the energy
that is available during the oxidative process. Electrons from
reduced compounds are moved along respiratory or electron
transport chains composed of a series of compounds. In an
aerobic process, molecular oxygen (O2) acts as the terminal
electron acceptor. In some cases where O2 is not available,
nitrate (NO3~), iron (Fe3+), manganese (Mn4+), and sulfate (SO42~
) can act as electron acceptors if the organisms have appropriate
enzyme systems. A measurement of the oxidation-reduction
potential (redox potential) of a soil provides a measurement of
the electron density of the system. As a system becomes
reduced, O2 is depleted, and other substances are used as
terminal electron acceptors. There is a corresponding increase
in electron density, resulting in a progressively increased
negative potential. Redox potential is measured as Eh,
expressed in millivolts, or as Pe, which is equal to -log [e~],
where [e~] is the concentration of negatively charged electrons.
Oxygen levels in soil systems can be maintained by: (1)
prevention of saturation with water; (2) presence of sandy and
loamy soil materials (excessive clay contents are undesirable);
(3) moderate tilling; (4) avoidance of compaction of soil; and (5)
limited addition of supplementary carbonaceous materials.
Soil pH also affects the activity of soil microorganisms. Fungi
are generally more tolerant of acidic soil conditions (below pH
5) than are bacteria. The solubility of phosphorus, an important
nutrient in biological systems, is maximized at a pH value of 6.5.
A specific contaminated soil system may require management
of soil pH to achieve levels that maximize microbial activity.
Control of pH to enhance microbial activity may also aid in
immobilization of hazardous metals in soil systems. A pH level
greater than 6 is recommended to minimize metal transport.
Microbial metabolism and growth are dependent upon adequate
supplies of essential macro- and micronutrients. Required
nutrients must be present and available to microorganisms in:
(1) a usable form; (2) appropriate concentrations; and proper
ratios (Dragun 1988). If the wastes present at a site are high in
carbonaceous materials and low in nitrogen (N) and
phosphorus (P), the soils may become depleted of available N
and P required for biodegradation of organic waste constituents.
Fertilization may be required at some contaminated sites as a
management technique to enhance microbial degradation. A
recommended ratio of carbon to nitrogen to phosphorus (C:N:P)
to promote biodegradation of organic substances is
approximately 120:10:1, the approximate ratio found in bacterial
biomass (Alexander 1977, Kowalenko 1978).
17
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Biodegradation of organic waste constituents declines with
lowering of soil temperature, due to reduced microbial growth
and metabolic activity. Biodegradation has been shown to
essentially cease at a temperature of 0° C. Soils exhibit a
variation in the temperature of surface layers, both diurnally and
seasonally. Diurnal changes of temperature decrease with
depth of the soil. Due to the high specific heat of water, wet soils
are less subject to large diurnal changes than dry soils (Paul
and Clark 1989). Factors that affect soil temperature include:
(1) soil aspect (direction of slope); (2) steepness of slope; (3)
degree of shading; (4) soil color; and (5) surface cover.
Soil mineralogy and organic matter are also important factors
in soil microbial ecology. The high cation exchange capacity of
certain clays (e.g., montmorillonite) and organic matter
contributes to the buffering capacity of soils, reducing the
impact of protons (i.e., hydrogen ions) released as a product of
microbial metabolism. Sorption of organic and inorganic waste
constituents by clays and organic matter affects the availability
of substrates and nutrients to microorganisms. Sorption of
waste constituents on soil materials strongly depends on soil
solution pH. Critical pH values are defined by the pKa of the
sorbate and the magnitude of charge on the soil material.
Soils contaminated by waste materials may contain elevated
concentrations of salts. In addition, remedial technologies that
utilize fertilizers or chemical agents may add salinity to the soil.
Increased salinity increases the osmotic potential of soil
solution, which may restrict the activity of soil microorganisms
due to osmotic stress.
The environmental factors presented in Table 5, as well as soil
and waste characteristics, interact to affect microbial activity at
a specific contaminated site. Computer modeling techniques
are useful to attempt to describe interactions and their effects
on treatment of organic constituents in a specific situation.
FIELD FACTORS THAT AFFECT
SOIL/WASTE CHARACTERIZATION AND
MANAGEMENT
Climatic Information
Meterological and climatological data are also required to
assess the attenuation of waste constituents in soil systems
as well as to assess public health and environmental impacts
from the migration of waste constituents to receiving ground
and surface waters and through air transmission.
Temperature of both the air and soil affects the rate of biological
and chemical attenuation processes in the soil, volatilization of
compounds, and the hydrologic budget. In general, soil
temperature is difficult to control in a field situation, but may be
affected by the use of natural mulches or artificial materials and
soil water control.
Most soil microorganisms are mesophiles; i.e., they exhibit
maximum growth and activity in the 20° to 35° C temperature
range. Soils also contain microorganisms that grow best at
temperatures below 20° C (psychrophiles) and others that
exhibit maximum growth rates between 50° to 60° C
(thermophiles). In general, organic matter decomposition
increases with increasing temperature. The influence of
temperature on microorganism activity may be used to estimate
required times for utilization of biological remediation techniques
at contaminated sites.
The availability of soil temperature data is much less than the
availability of airtemperature data. K.W. Brown and Associates,
(1980) discussed a method developed by Fluker (1958) for
predicting annual soil temperature cycles. They also presented
isotherm maps of soil temperature data at the 4-inch depth for
the spring of 1979 in the United States.
Location Topography
Potential for contaminant migration due to soil permeability,
depth to ground water, erodibility, and flooding potential can
sometimes be predicted by knowledge of the landform type on
which a contaminated site is located. The landform type may
also indicate possible site modifications required to minimize
transportofwasteconstituents (Phungetal. 1978). Forexample,
on upland crests and valley side landforms, surface water is
limited to incident precipitation and controllable off-site runoff.
These landforms may require diversion of surface waters to
reduce the amount of water entering and infiltrating the site.
Upland crests or valley sides may also pose a hazard to ground
water since they are often in ground-water recharge areas.
Upland flat areas with fine-grained soils of low permeability
pose less risk of high ground water and erosion and have
greater attenuation potential than terrace landforms. Terrace
landforms are often underlain by highly permeable coarse-
grained soils, sometimes at shallow depths. Contamination
from these sites may occur at nearby surface expressions of
underlying ground water. The possibility of ground water
intersecting a terrace site increases as the site position
approaches either the valley wall or the level of the modern
floodplain.
Warner (1976) describes four site conditions where pollution
potential is especially high. They are as follows:
(1) Sloping sites with relatively impermeable bedrock (e.g.,
shale, dense limestone, crystalline igneous rock) 0.6 m or
less from the surface have a high potential for erosion,
seepage, and overland flow of contaminated runoff.
(2) Sites located in karsttopography, with clayey soils overlying
limestone or dolomite with fracture and solution porosity
and permeability have a high potential for contamination of
ground water, for although infiltration into soil is slow,
liquids can rapidly enter bedrock where soil is absent,
creating sinkholes and paths for direct flow into ground-
water systems.
(3) Sites with little topographic relief where the ground water is
at or nearthe surface (e.g., old lake beds, floodplains) have
a high pollution potential.
(4) Sites with fractured bedrock and a shallow soil depth (e.g.,
in granitic areas) have high ground-water pollution potential.
Geological Factors
In addition to soil characteristics, geological information in the
form of subsurface geological characteristics and
hydrogeological factors are important in determining potential
for off-site migration from contaminated soils and
implementation of remedial techniques.
The geological framework of the site consists of the rocks or
sediments in the formations beneath the site. Information is
necessary on the composition, stratification, and thickness of
the geological layers. For example, sedimentary layers (e.g.,
limestones, sandstones, and shales) tend to channel flows
along bedding planes. Thus, flow directions may be determined
by dips in the strata. In humid climates, solution channels may
form in limestones, which may allow rapid transport of pollutants
18
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over long distances with little attenuation. Fracture zones that
occur in igneous and metamorphic bedrock (e.g., granites,
diorite, marble, quartzite,slate, gneiss, and schist) may also
permit rapid transport of polluted ground water (Blackman et al.
1980). The most favorable areas would be those covered by
thick deposits of unconsolidated low permeability materials
overlying shales or undisturbed fine-grained sedimentary
bedrock formations that have no major structural variations or
fractures affecting formation stability (Corbin 1980). Large
thicknesses of unconsolidated materials allow opportunities
for natural attenuation while providing a protective barrierto any
usable aquifer system.
Hydrogeological factors relating to ground water are also
required to assess potential for pollution from contaminated
soils. For ground water in unconsolidated formations, less
hazard exists if there is no connection with surficial or buried
drift aquifers, especially if the contaminated site is overlain with
low permeable materials to bedrock. For ground water in
bedrock formations, more favorable conditions to minimize
pollution potential from contaminated sites exist if the site is
away from recharge areas to major freshwater aquifers or there
is no direct connection to a usable bedrock aquifer (Corbin
1980). Confined ground water, which is isolated from the
surface by a relatively impermeable bed consisting of clay,
shale, or dense limestone, is not easily contaminated, nor is it
affected much by local sources of recharge (Warner 1976).
Knowledge of the nature of ground-water flow patterns is also
critical. Localized, rather than regional, ground-water flow
patterns, preferably with known discharge points and a large
distance to the water table are the most favorable conditions.
Information required to assess hazards might include:
(1) Elevations of water table and potentiometric surface
(ground-water gradients);
(2) Fluctuations in ground-water levels due to natural inputs
and outputs of water;
(3) Drawdowns of ground-water levels from wells (cones of
depression caused by pumping can alter ground-water
levels from that which would naturally exist);
(4) Effects on ground-water flow patterns from changes in
surface water flows or levels; and
(5) Hydraulic characteristics of the aquifer, including
transmissivity, specific yield, and specific retention.
Trafficabiiity of for Characterization
Activities Implementation of Remedial
Techniques
Site trafficability may be a determining factor in the performance
of site characterization activities and the selection and
implementation of remedial technologies at a contaminated
site. Trafficability refers to the capability of a soil to permit
movement of a vehicle over a land surface (Reeve and Fausey
1974). The trafficability of a soil under different climatological
and soil moisture conditions should be assessed. There may
be restrictions on the type of equipment that can be used and
times when the equipment can be used. For example, the
presence of boulders, steep slopes, or excessively wet
conditions may cause difficulties in the use of equipment.
The primary concern at waste sites is for successfully performing
given operations on the land without damaging the soil. Such
damage might include: (1) decreased permeability to air and
water; (2) altered thermal relations; and (3) resistance to root
penetration. Generally, trafficability means being able to perform
required operations in such a way as to create a desired soil
condition or to get an operation completed expediently.
Operations that require manipulation (i.e., tillage) require a
different interpretation of trafficability than do operations in
which soil is used as a surface on which to operate (i.e., non-
tillage).
The U.S. Army Corps of Engineers Waterways Experiment
Station (1956) identified four soil characteristics that relate to
trafficability of soils: (1) bearing capacity; (2) traction capacity;
(3) slipperiness; and (4) stickiness. Any one or a combination
of these may cause vehicle immobilization.
The trafficability of a soil is considered adequate for a vehicle if
the soil has sufficient bearing capacity to support the vehicle
and sufficient traction to develop the forward thrust necessary
to overcome the rolling resistance. Bearing and traction
capacities are related to soil strength or shear resistance. Soil
strength can be determined by laboratory tests (e.g., direct
shear, triaxial shear, and unconfined compression) or by a field
test using a cone penetrometer.
Slipperiness is the condition of deficient traction capacity in a
thin surface layer of soil that is otherwise trafficable. When soils
adhere and build up on the running gears of a vehicle, increasing
rolling resistance and making steering difficult, the condition is
called sticky. Soil stickiness and slipperiness usually occur on
soils high in clay. When the soil surface is cooler than the
underlying soil, moisture migrates from the lower layers to the
surface. If the evaporative demand is not large, the moisture
accumulates at the surface and causes decreased traction.
This condition is not a problem that can be alleviated by drainage
but by water management. Decreased traction is especially a
problem in seasons when the radiant energy input is low.
Damage to the soil by vehicular traffic usually results from
compressing and puddling the soil. To avoid such damage, the
soil should be manipulated or traversed only when the soil is
below some critical moisture level, which is dependent on the
type of soil. Wet soils are easily compacted by both tillage and
nontillage operations. Clay soils are especially a problem,
since they hold a large amount of water that must be removed
by internal drainage or evaporation before tillage is possible.
Soil compaction can be reduced in several ways. Reducing the
load intensity on a soil or reducing the number of trips over a soil
can be accomplished by changing machinery configurations or
tractor tire designs. Subsurface and surface drainage systems
can also be used to reduce soil moisture content.
Other site conditions that may affect trafficability include slopes
and the presence of coarse fragments, such as boulders.
Reeve and Fausey (1974) presented a review of methods
regarding determination of soil trafficability using predictive
equations and empirical rating systems.
If the remedial technology requires certain site conditions,
modifications must be made, if possible, to achieve that
condition. An assessment must be made to determine if
required modifications are feasible at that particular site. For
example, the initial steps in biodegradation of a chlorinated
organic compound may require anaerobic conditions followed
by aerobic conditions. Thus, the site/soil infiltration, permeability,
and drainability characteristics all determine whether anaerobic/
aerobic conditions can be achieved.
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CLASSIFICATION
In the United States, there are two systems under which soils
are likely to be classified: (1) the Unified Soil Classification
System (USCS); and (2) the U.S. Department of Agriculture
(USDA) System (Fuller 1978).
The USCS was developed to describe engineering properties
of soils (Fuller 1978). Classification of soil types into 15
categories are based on particle (grain) sizes and responses
to physical manipulation at various water contents. An
abbreviated description of the system (not including information
on manipulation (i.e., liquid limit and plasticity index)) is given
in Table 6.
The USDA System, developed for agricultural and other land
management uses, is based on both chemical and physical
properties of the soil. The first system (1938-1960) was based
on soil genesis, i.e., how soils formed or were thought to have
formed, while the present system is based on quantitatively
measurable properties of soils as they exist in the field. The
present system is constantly being refined but is in general use
by U.S. soil scientists. The highest level of the present USDA
System, the category referred to as soil order, is based on soil-
forming processes as indicated by the presence or absence of
major diagnostic horizons (Brady, 1990).
Fuller (1978) developed a comparison of the USDA System
and USCS. The part of the USDA System that can be compared
Table 6. IVIajor Divisions, Soil Type Symbols, and Type Descriptions for the Unified Soil Classification System (USCS) (Fuller 1978)
Major Divisions
Symbol
Description
Coarse-
grained soils
More than half
of material is
larger than
No. 200 sieve size
Fine-grained
More than half of
material is
smaller than
No. 200 sieve size
Gravels
More than half of
coarse fraction is
larger than No. 4
sieve size
Sands
More than half of
coarse fraction is
smaller than
No.4 sieve size
Silts and Clays
Liquid limit is
less than 50
Clean gravel
(little or no fines)
Gravels w/fines
(appreciable fines)
Clean sands
(little or no fines)
Sands w/fines
(appreciable fines)
Silts and Clays
Liquid limit is
greater than 50
silty soils, elastic silts.
G W Well graded gravels, gravel-sand
mixtures, little or no fines.
G P Poorly graded gravels or gravel-
sand mixtures, little or no fines.
GM Silty gravels, gravel-sand-silt mixtures.
G C Clayey gravels, gravel-sand-clay mixtures.
S W Well graded sands, gravelly sands, little or
no fines.
SP Poorly graded sands or gravelly sands,
little or no fines.
SM Silty sands, sand-silt mixtures.
S C Clayey sands, sand-clay mixtures.
M L Inorganic silts and very fine sands, silty or
clayey fine sands or clay silts
with slight plasticity.
C L Inorganic clays of low to medium
plasticity,
gravelly clays, sandy clays, silty clays, lean clays.
O L Organic silts and organic silty clays of low
plasticity.
MH Inorganic silts, micaceous or
diatomaceous fine sandy or
CH Inorganic clays of high plasticity fat clays.
Highly Organic Soils
O H Organic clays of medium to high plasticity, organic
silts.
P Peat and other highly organic soils.
Notes: ML includes rock Hour. The No. 4 sieve opening is 4.76 mm (0.187 in); the No. 200 sieve opening is 0.074 mm (0.0029 in).
20
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most directly in the USCS is soil texture and associated modifiers
(e.g., gravelly, mucky). The size rangesforthe USDAand USCS
particle designations were shown in Table 1. The two systems
are not directly comparable, since the soil texture designation
in the USDA System is based only on amounts of sand-, silt-,
and clay-sized particles (Figure 5), while in the USCS, soil type
is determined both on amounts of certain sizes of soil particles
as well as on the response of the soil to physical manipulation
at varying water contents. Correlations of the USDA soil textures
and USCS soil types are presented in Table 7.
Texture is a major criterion in the USCS but only a minor
criterion in the USDA System. A soil of a given texture can be
classified into only a limited number of the 15 USCS soil types.
However, in the USDA System, soils of the same texture may be
found in many of the 11 orders and 47 suborders of the system
because of differences in the chemical properties orthe climatic
areas in which they are located.
The names of the classification units are combinations of
syllables, most of which are derived from Latin or Greek. Since
each part of a soil name conveys a concept of soil properties or
genesis, the name reflects the properties of the soil being
classified. Identification of the types of soils at a contaminated
site in terms of the categories of the USDA System provides
information that is relevantto identifying potential for attenuation
and migration of waste constituents.
A complete description of the USDA System is presented in Soil
Taxonomy (Soil Survey Staff 1975).
OF
Soil Information for a Contaminated Site
Many waste sites may have been subjected to earlier acute
emergency response cleanup actions or preliminary
investigations to determine the degree and extent of
contamination and the necessity for remedial action. Therefore,
Table 7. Corresponding USDA and USCS Soil Classifications
(Fuller 1978)
United States Department of
Agriculture (USDA)
Soil Textures
Corresponding Unified
Soil Classification
System (USCS)
Soil Types
1. Gravel, very gravelly loamy sand GP,GW, GM
2. Sand, coarse sand, fine sand SP.SW
3. Loamy gravel, very gravelly sandy loam, GM
very gravelly loam
4. Loamy sand, gravelly loamy sand, SM
very fine sand
5. Gravelly loam, gravelly sandy clay loam GM.GC
6. Sandy loam, fine sandy loam, loamy SM
very fine sand, gravelly sandy loam
7. Silt loam, very fine sandy clay loam ML
8. Loam, sandy clay loam ML.SC
9. Silty clay loam, clay loam CL
10. Sandy clay, gravelly clay loam, gravelly clay SC.GC
11. Very gravelly clay loam, very gravelly GC
sandy clay loam, very gravelly silty clay
loam, very gravelly silty clay and clay
12. Silty clay, clay CH
13. Muck and peat FT
there may be significant existing data on site and soil
characteristics.
Examples of sources of existing information include the
following:
(1) Government investigative reports;
(2) Engineering data from public and private agencies or firms
(e.g., university soil science and geology departments,
state water resource agencies, state geological surveys,
city water departments);
(3) Water well boring logs;
(4) Geotechnical and soil reports from nearby facilities; and
(5) Soil surveys.
Soil surveys are especially useful in providing information on
soil characteristics and properties for specific soils at a
contaminated site. A soil survey is a systematic examination,
description, classification, and mapping of soils in an area. A
soil survey report usually consists of two parts: (1)the soil map,
which shows where specific soils are found; and (2) a
description of the area shown on the map, which also provides
information on the suitability of the soils (quantified in terms of
slight, moderate, or severe limitations) for various agricultural,
engineering, wildlife, recreational, and environmental uses.
Each particular soil mapped is described with respect to:
(1) genesis and formation; (2) topographical location; (3) climatic
conditions; (4) hydraulic properties, such as infiltration,
permeability, runoff, erosion potential, and water holding
capacity; (5) organic matter content; (6) use and management;
(7) selected chemical properties, such as CEC and pH; and
(8) soil classification. Soil surveys in the U. S. are being
developed for county and regional areas through the
Cooperative Soil Survey, a joint effort of the U. S. Department of
Agriculture, the Agricultural Experiment Station, and other
agencies in each state. These surveys are easily accessible
through government agencies and federal depository libraries.
Following an initial site inspection of a contaminated site for
which a detailed soil survey exists, additional field work may
only involve spot verification of the survey using a hand-held soil
auger (Crites 1984). If the survey is more general, or if more
information is required on specific soil features, backhoe pits
should be used. Backhoe pits are recommended over soil
borings because they allow direct viewing of the soil profile,
including such conditions as fractured, near-surface rock,
hardpan or clay layers, and mottling or blue/gray color streaks
(an indication of high water table) and also allow accurate
sampling of the soil profile.
Existing data should be assessed to determine adequacy for
meeting further information requirements and to design
subsequent data collection activities. If additional data collection
is needed, soil scientists, geotechnical engineers, geologists,
and other persons trained in appropriate disciplines should be
consulted to generate the required information.
Methods forthe collection and analysis of soil samples should
be conducted according to recognized standard methods.
Documents with a compilation of soil analysis standard
methods include:
(1) Test Methods for Evaluating Solid Waste: Physical/
Chemical Methods. Third Edition, SW-846 (U. S.
Environmental Protection Agency 1986); and
(2) Methods of Soil Analysis, Parti: Physical and Mineralogical
Methods. Second Edition (Klute 1986), and Methods of Soil
21
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Analysis, Part 2: Chemical and Microbiological Methods.
Second Edition (Page et al. 1982).
Further Information on Fundamentals of Soil Science
Additional information on properties and uses of soils can be
obtained from introductory soil textbooks; e.g., Brady and Weil
(1999), Foth (1984), Hausenbuiller (1978), and Donahue et al.
(1983). Specific information concerning soils and hazardous
waste management can be obtained from references such as
Sims (1990), Sims et al. (1990), Sawhney and Brown (1989),
Dragun (1988), Loehr and Malina (1986), Sims et al. (1984),
Overcash and Pal (1979), and Guenzi (1974).
NOTICE
The U.S. Environmental Protection Agency through its Office of
Research and Development funded the research described
here. It has been subjected to the Agency's peer and
administrative review and 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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