Hazard Ranking System Issue Analysis:
Subsurface Geochemical Retardation
MITRE
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Hazard Ranking System Issue Analysis:
Subsurface Geochemical Retardation
Dr. Dash Sayala
Dr. Fred Price
November 1987
MTR-86W171
SPONSOR:
U.S. Environmental Protection Agency
CONTRACT NO.:
EPA-68-01-7054
The MITRE Corporation
Civil Systems Division
7525 Colshire Drive
McLean, Virginia 22102-3481
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Department Approval:
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V
< 7 '
MITRE Project Approval: .- x ^
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ABSTRACT
This report presents an analysis of the feasibility of
addressing geochemical retardation processes in the EPA Hazard
Ranking System. Seven processes are examined and one, sorption, is
identified as providing the most reasonable indicator of the
potential for subsurface media to retard the migration of hazardous
substances. It is proposed that the sorptive capacity of rocks and
sediments be estimated on the basis of their sum total clay and
organic carbon content and that this estimate be used to refine
evaluations of hazardous waste sites.
Several options are presented for EPA consideration with regard
to ranking sorptive capacity, measuring migration path lengths, and
characterizing relative retardation potential at sites. Suggestions
are made for incorporating this relative retardation potential into
the current MRS. The data requirements for these options are
discussed.
Suggested Keywords: Geochemical processes, Retardation potential,
Sorbents, Sorptive capacity, Subsurface migration path, Hazardous
waste/substance, Hazard Ranking System.
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ACKNOWLEDGEMENT
The authors wish to thank Dr. Lawrence M. Kushner whose
critical review and constructive suggestions have helped the
authors refine the report. Thanks are also due to Kris Barrett,
Nancy Cichowicz, and Robert Gerstein for their peer review and
suggestions.
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TABLE OF CONTENTS
Page
LIST OF FIGURES vii
LIST OF TABLES viii
1.0 INTRODUCTION 1
1.1 Background 1
1.2 Issue Description 3
1.3 Objectives 5
1.4 Approach 5
1.5 Organization of the Report 5
2.0 APPROPRIATENESS OF CONSIDERING GEOCHEMICAL PROCESSES 7
3.0 EVALUATION OF GEOCHEMICAL PROCESSES FOR THE HRS 11
3.1 Description of Geochemical Processes 13
3.1.1 Adsorption 13
3.1.2 Ion-Exchange 15
3.1.3 Hydrolysis 17
3.1.4 Dissolution and Precipitation 18
3.1.5 Redox Reactions 20
3.1.6 Acid-Base Reactions 22
3.1.7 Complexation 23
3.2 Selection of Geochemical Processes for Site Screening 25
4.0 ESTIMATION OF SORPTIVE CAPACITY OF ROCKS AND SEDIMENTS 29
4.1 Model for Sorptive Capacity Estimation 30
5.0 APPLICATION OF SORPTIVE CAPACITY TO RANKING OF HAZARDOUS 35
WASTE SITES
5.1 Ranking of Sorptive Capacities of Rocks and Sediments 37
by Sorbent Contents
5.1.1 Ordinal Scale 37
5.1.2 Proportional Scale 39
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TABLE OF CONTENTS (Concluded)
Page
5.2 Migration Paths 41
5.3 Characterization of Migration Path for Sorptive 44
Potential
5.4 Examination of Various Options for Ranking of Retardation 47
Potential
5.5 Data Needs and Sources 49
5.5.1 Data Needs 49
5.5.2 Data Sources 49
5.6 Suggestions for Incorporation of a Retardation Potential 51
Factor into the HRS Ground Water Route Score
6.0 LIMITATIONS 55
7.0 CONCLUSIONS 59
8.0 REFERENCES 61
9.0 SUGGESTED READINGS 71
APPENDIX A--SORPTIVE CAPACITY ESTIMATION DATA 73
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LIST OF FIGURES
Figure Number Page
1 Illustration of the Retardation/Attenuation 8
of a Hazardous Substance by Various Processes
in the Subsurface
2 Geochemical Processes; Example of Water-Rock- 10
Pollutant Interactions in the Subsurface
3 Schematic Cross Section Showing Three 42
Alternative Migration Paths for Evaluation
of Retardation Potential
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LIST OF TABLES
Table Number Page
1 Ranges of Clay and Organic Carbon Contents 31
of Six Common Groups of Rocks and Their
Equivalent Sediments
2 Range of Options for Ranking Sorptive 36
Capacities, Selecting Migration Path
and Characterizing Migration Path for
Retardation Potential
3 Relative Retardation Potentials (and Their 38
Factor Value) of Groups of Rocks and Their
Equivalent Sediments According to the Range
of Total Sorbent Contents—Ordinal Scale
4 Groups of Rocks and Equivalent Sediments 40
Ordered by Their Average Total Sorbents—
Proportional Scale
5 Example of Numerical Factor Values for the 46
Thickness-Weighted Average Sorbent Contents
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1.0 INTRODUCTION
1.1 Background
The Comprehensive Environmental Response, Compensation, and
Liability Act of 1980 (CERCLA) (PL 96-510) requires the President to
identify national priorities for remedial action among releases or
threatened releases of hazardous substances. These releases are to
be identified based on criteria promulgated in the National
Contingency Plan (NCP). On July 16, 1982, EPA promulgated the
Hazard Ranking System (HRS) as Appendix A to the NCP (40 CFR 300;
47 FR 31180). The HRS comprises the criteria required under CERCLA
and is used by EPA to estimate the relative potential hazard posed
by releases or threatened releases of hazardous substances.
The HRS is a means for applying uniform technical judgment
regarding the potential hazards presented by a release relative to
other releases. The HRS is used in identifying releases as national
priorities for further investigation and possible remedial action by
assigning numerical values (according to prescribed guidelines) to
factors that characterize the potential of any given release to
cause harm. The values are manipulated mathematically to yield a
single score that is designed to indicate the potential hazard posed
by each release relative to other releases. This score is one of
the criteria used by EPA in determining whether the release should
be placed on the National Priorities List (NPL).
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During the original NCP rulemaking process and the subsequent
application of the HRS to specific releases, a number of technical
issues have been raised regarding the HRS. These issues concern the
desire for modifications to the HRS to further improve its
capability to estimate the relative potential hazard of releases.
The issues include:
• Review of other existing ranking systems suitable for
ranking hazardous waste sites for the NPL.
• Feasibility of considering ground water flow direction and
distance, as well as defining "aquifer of concern," in
determining potentially affected targets.
• Development of a human food chain exposure evaluation
methodology.
• Development of a potential for air release factor category
in the HRS air pathway.
• Review of the adequacy of the target distance specified in
the air pathway.
• Feasibility of considering the accumulation of hazardous
substances in indoor environments.
• Feasibility of developing factors to account for
environmental attenuation of hazardous substances in ground
and surface water.
• Feasibility of developing a more discriminating toxicity
factor.
• Refinement of the definition of "significance" as it relates
to observed releases.
• Suitability of the current HRS default value for an unknown
waste quantity.
• Feasibility of determining and using hazardous substance
concentration data.
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• Feasibility of evaluating waste quantity on a hazardous
constituent basis.
• Review of the adequacy of the target distance specified in
the surface water pathway.
• Development of a sensitive environment evaluation
methodology.
• Feasibility of revising the containment factors to increase
discrimination among facilities.
• Review of the potential for future changes in laboratory
detection limits to affect the types of sites considered for
the NPL.
Each technical issue is the subject of one or more separate but
related reports. These reports, although providing background,
analysis, conclusions, and recommendations regarding the technical
issue, will not directly affect the MRS. Rather, these reports will
be used by an EPA working group that will assess and integrate the
results and prepare recommendations to EPA management regarding
future changes to the HRS. Any changes will then be proposed in
Federal notice and comment rulemaking as formal changes to the NCP.
The following section describes the specific issue that is the
subject of this report.
1.2 Issue Description
EPA has received public comments concerning the lack of
consideration in the HRS of geochemical processes that may retard
the migration and attenuate the concentration of hazardous
substances (organic and inorganic) released from hazardous waste
sites.
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Currently, the HRS ground water route uses selected retardation/
attenuation* parameters (or Indicators). These Include permeability
(as a measure of vertical retardation) under the route characteristics
category and persistence (as a measure of blodegradability) under the
waste characteristics category. These parameters address physical and
biological processes, respectively. By not considering geochemlcal
effects while addressing permeability, the HRS Implicitly assumes that
hazardous substances released from a site will migrate to, and In, an
aquifer of concern with the same velocity as the transporting solutions
and/or ground water. However, concern has been expressed by some
commenters that without consideration of geochemlcal processes, the HRS
Inadequately reflects the relative threat posed by sites. The public
comments received by EPA suggest that geochemlcal process related para-
meters should be Introduced Into the HRS to more realistically assess
the likelihood of released substances reaching target populations.
This report examines the geochemlcal processes that can affect the
fate of organic and Inorganic hazardous substances In the subsurface.
It also assesses the practicality of Introducing geochemlcal retarda-
tion parameters Into the HRS and suggests appropriate revisions to the
current HRS. These revisions should Increase Its ability to
^Retardation Is the Immobilization of a hazardous substance while
attenuation Is the reduction In the concentration of hazardous
substances. Since retardation and attenuation generally complement
each other, the term retardation will be used In this document to
Indicate retardation/attenuation.
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discriminate among sites on the basis of their relative potential
for hazardous substances to reach target populations.
1.3 Objectives
The objectives of this study are:
• To assess whether it is feasible and practical to
incorporate geochemical processes related to hazardous
substance retardation in the evaluation of the ground water
route.
• To develop methodologies to evaluate the relative potential
for the subsurface environment to retard the migration of
hazardous substances.
• To develop options for incorporating the appropriate
geochemical parameter(s) into a site ranking scheme.
1.4 Approach
In order to achieve these objectives, the following approach
was followed:
• Review published literature pertinent to geochemical processes.
• Review reports on remedial investigation and technology
assessments for remedial action.
• Identify potentially important geochemical process(es) for
retardation and assess their suitability for potential use in
a site screening scheme.
• Develop methods and options for incorporating geochemical
retardation into a site scoring scheme.
• Document the findings.
1.5 Organization of the Report
The main portion of this report summarizes information on
geochemical processes and related parameters and evaluates their
importance and utility for the purpose of ranking the relative
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potential hazard of hazardous waste sites. This material is
presented in six major sections.
Section 2 explains why it is appropriate to consider
geochemical processes for subsurface site evaluation. Section 3
briefly describes the important geochemical processes that control
the fate of hazardous substances in the subsurface. It also
identifies those geochemical process(es) and parameter(s) that are
most useful for evaluation of the retardation potential in the
subsurface environment between a site and the target populations.
Section 4 explains the basis and techniques of estimating the
sorptive capacity of rocks and sediments for ranking the retardation
potential. In Section 5, options for ranking the sorptive
capacities of rocks and sediments, for selecting subsurface
migration path lengths, and for characterizing the retardation
potential of a migration path are presented. Needs for and sources
of data for evaluating the retardation potential of a subsurface
geomedia are also presented in Section 5, along with suggestions for
incorporating a retardation potential factor into the HRS ground
water. Section 6 discusses the limitations of the methods presented
for evaluating the subsurface retardation potential. Section 7
presents the conclusions of this study.
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2.0 APPROPRIATENESS OF CONSIDERING GEOCHEMICAL PROCESSES
Reliable and meaningful assessment of the relative hazard posed
by hazardous waste sites to target populations via subsurface*
migration requires estimation of the potential of subsurface
geologic media (geomedia) to retard hazardous substances released
from a site.
The overall subsurface retardation potential is the result of a
variety of physical, biological and chemical processes. Generally,
retardation is affected by a group of processes at a given site
which depend on the specific subsurface characteristics of the area
and the types of hazardous substances that are present. The
relative contribution of the physical, biological and geochemical
processes to the retardation further depends on site-specific
circumstances. Conceptually, it is the combination of all active
processes in the subsurface that can reduce the amount and
concentration of hazardous substances as they migrate from a site to
A target (see Figure 1). This report addresses the major
geochemical processes, which include adsorption, ion-exchange,
hydrolysis and complexation.
Geochemical processes include physical and chemical inter-
actions between geologic media and their environment. Biological
interactions are not included under geochemical processes.
*The subsurface as used in this report includes both saturated and
unssturated zones.
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00
Top Soil Zone
Potable
Waterwell
Sorption +
Filtration +
Biodegradatbn +
Dilution + Hydrolysis +
Precipitation
Dilution + Degradation + Sorption + Hydrolysis + Complexation +
Filtration + Diffusion + Precipitation
Target
(C2)t2
Legend
Q0
- Rock/Sediment Types
- Hazardous Substance Amount at the Release Point (QQ > QI }
- Hazardous Substance Amount at the Aquifer Entry Point
- Initial Concentration of a Hazardous Substance (C0>C1 >C2)
- Concentration of a Hazardous Substance at the Aquifer Entry Point
- Concentration of a Hazardous Substance at a Target
- Initial Time of Release
- Time After Migration Through Unsaturated Zone
- Time After Migration, in Saturated Zone, to a Target
SAXSSSF"*
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Geochemical processes such as water-rock, water-pollutant, and
pollutant-rock interactions are ubiquitous when hazardous substances
are released from a site and migrate in the subsurface (see
Figure 2). These interactions can play a significant role in
determining the fate of hazardous substances migrating in the
subsurface. The literature pertinent to the migration of hazardous
substances is replete with data indicating that geochemical
processes can significantly retard the migration and attenuate the
concentration of both organic and inorganic substances (for
examples, see Nowak, 1980a and 1980b; Roberts et al., 1982).
The literature cited throughout this report provides evidence
that geochemical processes can result in significant retardation
effects. Therefore, it is appropriate to consider these effects for
incorporation into a site ranking scheme.
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Calcite
Precipitates
REDUCING
OXIDIZING
^
reas
N
y
V
Pyrrte
pH Increases
H2S
Bacterial Reduction
of Sulfate
?
H2,CO2.CH4,NH3
Ethyl Alcohol
Acetic and Succinic Acid
>
U0?+ U020H+
^ ^
* Eh Decreases
Fermentation of
Organic Materials
Sorption
by
Clays -i- Organic Matter + Fe Oxides/Hydroxides
Pyrite Oxidized by
Ferric Sulfate
Ferrous Sulfate and Sulfuric Acid
V
*Eh Increases
'i
02
Bacterial oxidation to
Fernc Sulfate
Uranium and Vanadium
Oxidized and Mobilized
u 6+
"
j
I
T
Hydrolysis to
Sulfuric Acid and Ferric Hydroxide
Uranium Reduced
Stable
U4+
pH Decreases
Vanadium Reduced
Stable V3+
* Eh is the measure of the ability of chemical environment to gain or loose electrons.
* pH is the measure of the acidity or alkalinity of a chemical environment/liquid substance.
FIGURE 2
GEOCHEMICAL PROCESSES; EXAMPLE OF WATER-ROCK-POLLUTANT INTERACTIONS
IN THE SUBSURFACE
(This Figure Shows The Fate Of Uranium And Vanadium When Migrating
In Oxidizing Ground Water And Subsequently Encountering Reducing
Conditions Along The Flow Path. This Model Is Used To Explain The
Subsurface Migration/Deposition Of Uranium By Ground Water.)
(Modified After Rackley, 1972.)
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3.0 EVALUATION OF GEOCHEMICAL PROCESSES FOR THE HRS
Geochemical processes which can affect the fate of hazardous
substances in the subsurface include adsorption, ion-exchange,
hydrolysis, dissolution and precipitation, reduction-oxidation
(redox), acid-base reactions and complexation. Any combination of
these processes can operate in a given subsurface environment
depending on the geochemical and hydrologic conditions, and can
affect, to various degrees, the migration of hazardous substances in
the unsaturated and saturated zones.
Prior to a discussion of these processes, it should be noted
that there are different types of hazardous substance releases.
These types of release may have an influence on geochemical
processes and thus affect their retardation along the migration path.
Types of hazardous substance releases include, both single- and
multiple-point releases. Such releases have been observed during
EPA remedial investigation studies of sites with contaminated ground
water. These releases may be either continuous or discontinuous
depending on conditions such as influx of water for leachate
generation, leachability of the disposed waste, and adequacy of
waste containment. In comparison to the volume of both geologic
materials and ground water within the migration medium, the volume
of leachate generated from the waste is usually small. If the
volume of leachate generated and released from a site is small in
comparison to the volume of the migration path, then there is a high
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probability that small volumes of released hazardous substances can
be retarded in the subsurface. If the total amount of hazardous
substances in the leachate exceeds the retardation or retaining
capacity of a medium, then migration through the medium becomes much
more likely.
Hazardous substance migration can be divided into three major
types. The three types of migration in ground water are:
• Miscible migration: migration of substances dissolved in
water.
• Immiscible migration: migration of substances not dissolved
in water.
• Facilitated migration: migration of substances aided by
water suspended materials.
The first two depend on the solubilities of the hazardous substances
along the subsurface migration path; the third refers to situations
in which migration is aided by the presence of water suspended solids
such as colloids, clays and organic matter, that carry the hazardous
substances. The rate at which migration occurs in a heterogeneous
subsurface system is governed by geochemical interactions along the
migration path, including pollutant-water-rock interactions.
Chemical reactions leading to transformation or degradation are
faster and more effective with the miscible substances than
immiscible ones (Stumm and Morgan, 1981). Generally, those
substances that migrate as immiscible forms or with the aid of
suspensates (particulates) are more prone to retardation by sorption,
without chemical degradation or transformation, than are their
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miscible counterparts (Tinsley, 1979; Stumm and Morgan, 1981; and
Morrill et al., 1982).
3.1 Description of Geochemical Processes
A brief description of the major geochemical processes is given
in the following sections. The geochemical processes described in
the following sections occur in the unsaturated and saturate zones of
the subsurface migration path.
3.1.1 Adsorption
Adsorption is a retardation mechanism arising from the physical-
chemical interaction of hazardous substances with the surfaces of
charged mineral constituents or organic matter in the subsurface
medium. This interaction reduces the mobility of the substances
relative to the ground water flow. Adsorption of the substances onto
solid surfaces takes place as a result of electrostatic forces, by
weak Van Der Waals interaction, or by covalent bonding. It occurs on
all surfaces (including water suspended particles) where the necessary
bonding conditions are present. Laboratory experiments conducted by
Kinniburgh and Jackson (1981), Rao and Davidson (1980), and others
indicate that adsorption of inorganic and organic substances takes
place rapidly, within a matter of minutes. Generally, adsorption
refers to an equilibrium distribution of a substance between a solid
phase and a solution phase (mobile phase). For a fixed quantity of
migrating chemical species, the dissolved concentration of the
species decreases as the degree of adsorption increases.
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Although adsorption is an important process for the retardation
of hazardous substances in the subsurface, under conditions of either
high or low pH, desorption can temporarily release the absorbed
substances. However, desorption is slower than adsorption (Helling
and Dragun, 1981) and, generally, the extreme pH conditions that
promote desorption are not common in ground waters. Among several
absorption-desorption studies, the studies by DiToro and Horzempa
(1982) and by others mentioned in Hague (1980), Marking and Kimberle
(1979), and Mayer and Hamelink (1977) conclude that persistent
organic substances (e.g., DDT, PCB) tend to strongly sorb to
absorbents present in a migration media. They further conclude that
a significant portion of the absorbed concentration is extremely
difficult to desorb.
Adsorption in the subsurface is promoted by the presence of
sorbents, such as clays, organic matter (organic carbon), Fe-Mn-Al
oxides/hydroxides, zeolites, and sulfides. In addition, adsorption
tends to increase as the surface area of the solid matrix increases
(i.e., as grain size decreases) and as the ionic strength or
electrolytic concentration of the ground water increases (Mortensen,
1959; Hurle and Freed, 1972; Chiou et al., 1979; Lyman et al., 1982;
and Stuart, 1983). Based on the works of Schwarzenbach and Westall
(1981), Means et al. (1980); and Karickhoff et al. (1979), the
adsorption of organic substances by sediments increases as the
organic carbon (organic matter) content and the surface area (i.e.,
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silt/clay-size fraction) of the sediments increase. Similarly,
studies of sedimentary uranium deposits indicate that uranium and
associated inorganic elements, such as vanadium, selenium, lead, and
zinc are concentrated through adsorption onto clays and organic
matter found within fine-grained sedimentary rocks (Sayala, 1983;
Wayland and Sayala, 1983; Leventhal and Santos, 1981; and
Schmidt-Collerus, 1979).
Two important observations can be made about the adsorption
process. Namely, that (1) adsorption occurs under a wide range of
physical and chemical conditions, and (2) adsorption can affect a
variety of both organic and inorganic substances. These observations
are discussed by several researchers, including Rao et al. (1986),
Siegrist et al. (1986), Panter et al. (1985), and Karickhoff et al.
(1979).
3.1.2 Ion-Exchange
Ion-exchange refers to the exchange of bound ions in the solid
matrix of a geomedium with mobile ions in the aqueous phase. The
bonding involved, being ionic, is strong. Ion-exchange differs from
adsorption in that the latter is strictly a surface phenomenon,
whereas the former can involve penetration of ions or molecules into
the crystal structure of the solid to replace existing ions. The
single term, sorption, is commonly used to refer to the combination
of ion-exchange and adsorption processes.
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The naturally-occurring materials that exhibit ion-exchange
capacity are generally the same as those noted for their absorptive
capacities, i.e., clays, organic matter, Fe-Mn-Al oxides/hydroxides,
zeolites, and sulfides. The ion-exchange capacity of these geologic
materials is commonly expressed in terms of their cation-exchange
capacity, or CEC. Generally; the CEC of organic material in a
geomedium is about 200 milliequivalents per 100 grams; whereas, those
of clays range from 8 to 150 meq/100 gm depending on the type of
clays (Brady, 1974). The CEC of hydrous oxides is generally lower
than 8 meq/100 gm. As a result, a medium with large amounts of clays
and organic matter will commonly have high ion-exchange capacities
and can be effective in retarding the subsurface migration of both
organic and inorganic substances.
Cation exchangers are also known to exchange their cations for
polar organic compounds. For example, McAttee (1959 and 1963) found
that organic compounds containing amines and carbonyl groups are
exchanged for the cations in clays. Studies by Fripiat et al.
(1962), Stuart (1983), and JRB Associates (1984) indicate similar
instances of the exchange of organic compounds for the cations of
clays. This exchange and its effect on retardation of hazardous
substances can be influenced by the nature and amount of exchangeable
agents and the ionic strength of ground water.
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3.1.3 Hydrolysis
The reaction of a chemical compound or a metal ion with water is
called hydrolysis. The process may be expressed as:
R1OOOR2 + H20 RiCOOH + R2OH
Cu+2 + H20 CuOHf + H+
Decomposition of inorganic compounds by hydrolysis is an important
process in ground water, sometimes causing relatively insoluble
compounds to form. Similarly, decomposition of some organic
compounds by hydrolysis can transform some toxic compounds to less
toxic constituents or less toxic compounds into more toxic forms.
Metal ions, such as copper, lead, zinc, and cadmium, can be
transformed in ground water into their hydroxides (or hydrous
oxides) and then be retarded by sorption. Generally, the hydrolysis
of metal ions is dependent on their hydropotentials; that is, the
ratio of their charge to their ionic radius (Siegel, 1974).
It is difficult to generalize about the relative rate of
hydrolysis of organic compounds in ground water since that rate is
dependent on pH and the structural nature of the compound (Mabey and
Mill, 1978; Cherry et al., 1984; Kaplin and Likhovidova, 1984; and
Tinsley, 1979). The relationship between hydrolysis rate and pH
depends on specific acid-base catalytic process constants. At a
fixed pH, the half-life (T-,/?) of an organic compound due to
hydrolysis is independent of its concentration (Tinsley, 1979); i.e.,
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Tl/2 = °-693/Kh
where K, = Hydrolysis constant
The half-lives of organic compounds in the subsurface might be
a useful measure of their persistence with regard to hydrolysis.
However, estimation of the hydrolysis potential of a migration
medium is not possible without spacial and temporal data such as the
Eh,* pH and hydrochemistry (i.e., water composition) of the ground
water and the nature of hazardous substances and their solubilities
in the water. Site specific ground water data are difficult to
obtain and may be variable and unreliable. The spacial and temporal
data need in situ (i.e., subsurface) water sample collection and
analyses that are costly and time consuming. Further, field
techniques used for Eh/pH measurements are not well excepted.
3.1.4 Dissolution and Precipitation
When ground water interacts with a soil matrix containing
hazardous substances, dissolution (solubilization) and/or
precipitation of the hazardous substances may occur. A detailed
discussion of the processes of dissolution and precipitation is
given by Garrels and Christ (1965), Krauskopf (1967), and Stumm and
Morgan (1981). The kinetics of these processes are controlled by a
variety of migration media characteristics including pH, Eh,
hydrochemistry, velocity of the ground water, and the geochemistry
*Eh is the measure of the ability of an environment to supply
electrons to an oxidizing agent, or to take up electrons from a
reducing agent (i.e., electromotive potential).
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of the solid matrix along flow paths. The kinetics of dissolution
and precipitation, in turn, influence the retardation of hazardous
substances.
The characteristics of the hazardous substances, including the
solubilities of many inorganic and organic substances, can also
influence their retardation. Generally, the solubilities of these
substances are reduced in high ionic-strength ground water. In
supersaturated solutions, the hazardous substances may precipitate.
Dissolution and precipitation reactions are usually slower than
reactions involving dissolved species only, although it is difficult
to generalize about their rates. The dissolution-related reactions
modify the chemical characteristics (e.g., change the valence or
charge) of hazardous substances and may aid in sorption-related
retardation. However, reactions leading to dissolution and
precipitation are slower than sorption and, therefore, may have less
impact on retardation, particularly in relatively fast-moving ground
water.
The Eh, pH and ionic strength of ground water, the geochemistry
of sediments and rocks, and the nature and solubilities of hazardous
substances are important factors in determining the probable
occurrence of dissolution and precipitation. However, it is
difficult to estimate the dissolution and precipitation potential
along a migration pathway because spatial and temporal data on the
Eh, pH and ionic strength of the ground water along the path are
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needed and such data are not easily and accurately measured or
reliably predicted (Plummer et al., 1983; Rubin, 1983; Lindberg and
Runnells, 1984; and Holm et al., 1986).
3.1.5 Redox Reactions
Redox reactions are those in which the valence or oxidation
states of ions or neutral species in ground water are altered,
indicating a gain (reduction) or loss (oxidation) of electrons.
Redox reactions can result in decomposition, dissolution or
precipitation of a hazardous substance. The Eh of the ground water
system is a measure of the tendency for redox reactions to occur.
Eh and pH control the solubility of certain hazardous substances
and, therefore, influence their migration or retardation. A good
review of redox reactions is given by Stumm and Morgan (1981).
Degradation and precipitation as a result of redox reactions
are more likely to occur with inorganic than organic substances
(Utah State University and Arthur D. Little, Inc., 1984).
Generally, under oxidizing acidic conditions, copper, lead, nickel,
and uranium, for example, are mobile; whereas, under reducing
alkaline conditions, the mobility of these inorganics is hampered
and they may precipitate. In contrast arsenic, chromium, mercury,
molybdenum, and selenium are mobile under oxidizing alkaline
conditions and immobile under reducing acidic conditions.
Among the organic compounds, phenols, aldehydes, aromatic
amines, and certain organic sulfur compounds are more prone to
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oxidation-related degradation than are the alcohols, nitro- and
alkyl-substituted aromatics, unsaturated alkyl groups and aliphatic
ketones, acids, esters, and amines (JRB Associates, 1982).
Halogenated hydrocarbons, saturated aliphatic compounds, benzene and
chlorinated insecticides are less affected by oxidation reactions.
PCBs, chlorinated compounds, kepone, atrazine, and others are prone
to reduction-related degradation (Sweeney, 1981).
In ground water, the species that promote oxidation are
fluorine, hydroxide, oxygen, hydrogen peroxide, perhydroxyl,
hypochlorous acid, and chlorine (Rice, 1981). Those that promote
reduction are sulfides, organic matter, ammonia, methane, and
reducing bacteria (Sayala, 1983; Kobayashi and Rittman, 1982; Stumm
and Morgan, 1981). Generally, the absence of dissolved oxygen and
presence of dissolved organic carbon (DOC) confer reducing or
anaerobic conditions on ground water. Approximately 4 milligrams
per liter of DOC is apparently sufficient to produce, locally, a
reducing environment in ground water (Langmuir, 1972).
Although redox reactions may promote the retardation of
hazardous substances in the subsurface, the extent to which these
reactions are likely to occur and result in retardation cannot be
easily or reliably estimated (Plummer et al., 1983; Rubin, 1983; and
Lindberg and Runnells, 1984). As noted previously, spatial and
temporal rock/sediment and water chemical data (e.g., Eh and pH) are
21
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needed to accurately assess the potential of a medium to retard
hazardous substances through redox reactions.
3.1.6 Acid-Base Reactions
Acid-base reactions comprise an important category of geochem-
ical processes, as some wastes deposited at hazardous waste sites
may be acidic or basic. When very acidic or basic wastes are
released into a geomedium, and subsequently into an aquifer,
reactions such as dissolution, hydrolysis, and precipitation can
occur or there can be changes in the permeability of the geologic
media. Hence, hazardous substances may be chemically altered and
mobilized or their movement may be retarded. Along the subsurface
migration path the pH of waters containing hazardous substances can
become gradually neutral due to natural buffers such as carbonates,
sulfates, silicates, or ammonia, or the mixing of ground waters of
differing compositions. A pH near 7 (neutral) generally favors
precipitation of hazardous substances and, therefore, retardation.
The fate of acidic hazardous substances in terms of buffering is
explained by Haji-Djafari et al. (1979) and Highland et al. (1981).
Nigrini (1971) demonstrated that subsurface waters with low pH can
carry metals in solution. Horsnail and Elliot (1971), from their
study of molybdenum and copper in acidic ground waters in Canada,
concluded that acidic conditions facilitate the mobility of copper,
but retard the mobility of molybdenum. In remedial technology
assessment studies by the Utah State University and Arthur D.
22
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Little, Inc. (1984), pH control Is suggested to remedy the inorganic
substance problem of a local ground water system.
Although acid-base reactions can aid in the retardation of
primarily inorganic substances, prediction of their occurrence is
complex, and estimation of the retardation capacity of a geologic
medium that may result from acid-base reactions is unreliable (Stumm
and Morgan, 1981, and Plummer et al., 1983).
Hydrochemical data which was systematically collected along the
subsurface migration path would be required to estimate the effect
of acid-base reactions on the hazardous substance migration and,
even then, the estimate would be uncertain. The detailed hydro-
chemical information needed to assess the affect of acid-base
reactions on retardation is not currently available from a CERCLA
site inspection. Furthermore, a reliable method for predicting
acid-base conditions in the subsurfaces is not known to exist.
3.1.7 Complexation
In aqueous solutions, metal ions can combine with anionic or
neutral species to form ionic or neutral complexes. This process is
called complexation. The major complexing anions or inorganic
ligands in ground water are carbonate, chloride, sulfate, phosphate,
hydroxide, and, in some cases, fluoride, nitrate and ammonia. The
major neutral species or organic ligands are organic acids (such as
the amino acids and fatty acids), polysaccharides, and aliphatic and
aromatic compounds. Whether a hazardous substance released to a
23
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subsurface system exists in a complex molecular or ionic form depends
on site-specific water-rock and water-pollutant interactions. The
various forms or chemical species in ground water exhibit differing
sorption, precipitation and mobility characteristics. A compre-
hensive treatise on aqueous complexes and complexation is given by
Ringbom (1963), Stumm (1967), Morel and Morgan (1972), Langmuir
(1979), Stumm and Morgan (1981), and Singer (1982).
Complexation, in some cases, may solubilize and hence mobilize
hazardous substances; in other cases, it may result in insoluble
compounds (and colloids) and, therefore, promote hazardous substance
retardation. The metals are particularly prone to formation of a
variety of complex species in aqueous media (Stumm and Morgan,
1981). Long-chain organic complexes (polymers) have low solubility
in ground water; hence their retardation is provided mainly by
filtration and sorption. Generally, the nature and fate of the
complexes are determined by the chemical nature of the ground water
and the geochemistry of the migration medium. In cases where
organic and inorganic substances in leachates are mixed, the
tendency is for formation of insoluble organometal polymers whose
mobility is retarded.
Through the use of chemical equilibrium models, it is possible
to predict the complex form in which a given inorganic metal ion is
likely to be found. These models include the REDEQL model of Morel
and Morgan (1972); the SOB1NEQ model of Kharaka and Barnes (1973)•
24
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the MINEQL model of Westall et al. (1976); the WATSPEC model of
Wigley (1977); the WATEQ2 model of Ball and Jenne (1979); and the
GEOCHEM model of Mattigod and Sposlto (1979). These models require
spatial and temporal hydrochemical data, including equilibrium-based
redox potential data to accurately predict metal complexation and
precipitation. Also, their use is limited to the prediction of
complexation and precipitation of inorganic substances. No suitable
models have been identified as currently available for organic
substances.
Generally, complexation occurs slowly, and may not result in
retardation when ground water velocity is high. Furthermore, it is
difficult to estimate a generalized complexation-aided-retardation
capacity of a migration medium, since the process promotes the
retardation of some hazardous substances and increases the mobility
of others given similar subsurface conditions. Finally, because the
models used to predict complexation are equilibrium-based models, it
is not clear how applicable they would be to nonequilibrium
conditions frequently found in nature.
3.2 Selection of Geochemical Processes for Site Screening
The preceding sections described seven geochemical processes
that play a major role in the natural retardation of released
hazardous substances. The first step in evaluating the role of
these geochemical processes would be to obtain site-specific
information about the Eh, pH and ionic strength of ground water, the
25
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geochemistry of sediments and rocks, and the nature and solubilities
of hazardous substances. These data are rarely available during the
current evaluation of CERCLA sites. Furthermore, as discussed
above, comprehensive evaluation of the partial or combined effect of
these processes is beyond present modeling capability, even if it
were feasible to obtain all data necessary to characterize them
adequately at a site and along the migration path.
If geochemical retardation effects are to be considered in the
evaluation of the relative hazard of sites, then a method must be
employed which makes use of the limited and generalized information
expected to be available at from CERCLA site inspections. One
method, which is described in Section 4.1, is to concentrate on the
sorption process (adsorption plus ion-exchange) in evaluating the
relative potential of subsurface media to retard released hazardous
substances.
The reasons for selecting sorption are:
• Its generality, as compared to other competing retardation
processes, regarding the types of hazardous substances
(organic and inorganic) and the wide range of geochemical
conditions under which it will occur.
• The possibility that the sorptive capacity of various rocks
and sediments can be represented, if only qualitatively, by
easily obtainable geological information. (This topic is
discussed more fully in Section 4.)
• Sorption promotes retardation but unlike other geochemical
processes it rarely promotes increased mobility.
In contrast, each of the other types of geochemical processes
tends to be limited to specific types of hazardous substances and
26
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chemical conditions under which they are operative. Additionally,
processes such as complexation, redox and acid-base reactions,
dissolution, and precipitation promote not only retardation but also
migration. Thus, any credible prediction of the effect of other
types of geochemical processes at a site requires more detailed
information than is generally available or obtained from CERCLA site
inspections. Further, there are no reliable techniques to obtain
and model hydrochemical data (such as Eh, pH and ionic strength)
along a subsurface migration path in order to predict retardation of
hazardous substances.
Additional support for the selection of sorption is provided by
the following:
1. The research literature consistently alludes to the
importance of sorption in the retardation of both organic
and inorganic substances. For example, Siegrist et al.
(1986) stress the importance of sorption for remedial
action in their study of sorption systems for treating and
disposing of municipal wastewaters. Ward et al. (1985)
conclude that sorption of trace organic compounds by
organic matter in a geomedium is a primary geochemical
mechanism for determining the fate of an organic
substance. A modeling study of a creosote site in Conroe,
Texas by Bedient et al. (1984) indicates good agreement
between absorption-related retardation of organic compounds
and the amount of organic matter in the migration media,
the explanation lying in the sorptive capacity of the
organic matter. From their sorption study of subsurface
samples, Banerjee et al. (1985) conclude that total organic
carbon content of geologic samples is a reasonably good
predictor of sorption of organic compounds. From their
study of tidal marsh soils at a municipal landfill site in
New York, Panter et al. (1985) conclude that organic matter
and clays are effective in sorption-related retardation of
halogenated organic substances. Based on evaluation of the
potential for ground water contamination from geothermal
fluid releases, Summers et al. (1980) indicate the
27
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Importance of sorption in the retardation of various
metals. From their open literature review (more than 70
published articles), Steelman and Ecker (1984) conclude
that sorption is a primary mechanism for retardation of
organic compounds. In research related to nuclear waste
repositories, sorption is regarded as one of the viable
mechanisms for radionuclide retardation in natural geomedia
and in engineered backfill barriers (Nowak, 1980, 1980a,
I980b; Pigford et al., 1980; Serne and Relyea, 1981; and
Moody, 1982). A good summary on sorption of organic and
inorganic substances is given by JRB Associates (1984) and
Rai et al. (1984) respectively.
2. Studies of uranium deposits in sedimentary rocks of
different geologic ages indicate that sorption by organic
matter and clays is one of the most important processes
leading to concentration of uranium and associated metals
in such deposits (Sayala and Ward, 1983; Wayland and
Sayala, 1983; Schmidt-Coilerus, 1979; and Leventhal and
Santos, 1981). These results strongly suggest that clays
and organic matter can retard inorganics for a long time
unless the sorptive system is drastically disturbed.
3. Although other geochemical processes that have been
discussed are not suitable for direct use in a site ranking
scheme, they generally enhance sorption by controlling the
type of ionic compounds (ion-specification), providing ions
(through dissolution) for sorption sites, and creating net
surface charges for sorption (see Rai et al., 1984).
28
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4.0 ESTIMATION OF SORPTIVE CAPACITY OF ROCKS AND SEDIMENTS
Quantification of the potential of subsurface media to retard
hazardous substances by sorption requires extensive (in both time
and space) data on a number of physical and chemical properties of
hazardous substances, rocks, sediments, and ground water along the
migration path. For example, knowledge of the mineral composition,
grain size, particle surface area, and thicknesses of the different
geologic layers that are encountered by the migrating hazardous
substances is required. Knowledge of geochemical parameters such as
the ionic strength and pH of the ground water is also necessary.
However, even if the data were available, models to quantify the
sorptive capacity are beyond the present state-of-the-art.
If sorption effects are to be used as a basis for predicting
the retardation of hazardous substances in a site-screening
evaluation scheme, then a simple model is necessary. This model
should provide a basis for qualitatively estimating the sorptive
capacities of rocks and sediments and ranking the overall sorptive
or retardation potential of the subsurface migration path. The
theoretical basis for the model should be sound, and the level of
detail should be consistent with that of other factors in the site
evaluation scheme. The model and ranking schemes described in the
following sections meet all the above mentioned criteria.
29
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4.1 Model for Sorptive Capacity Estimation
The model developed here assigns relative sorptive capacities
to various types of rocks and unconsolidated sediments based on
their total clay plus organic carbon content. Organic carbon is
used as a measure of organic matter or carbonaceous materials.
There are two main reasons why total clay plus organic carbon
content have been selected as an indication of the sorptive
capacities of rocks and sediments. They are:
(a) The inherent sorptive capacity of clays and organic
matter found in rocks and sediments is much greater
for a wide variety of organic and inorganic
substances than are those of the other sorbents (see
Brady, 1974; Fuller, 1980; Rai et al., 1984; Panter
et al., 1985; and Siegrist et al., 1986). Increasing
amounts of these two sorbents in a geomedium, in
general, increases the medium's ability to sorb
hazardous substances (see Griffin and Shimp, 1978;
Karickhoff et al., 1979; Karickhoff, 1984; Means
et al., 1980; Hassett et al., 1980; Schwarzenbach and
Westall, 1981; Stuart, 1983; and JRB Associates,
1984).
(b) Clays and organic carbon commonly occur together in
rocks and sediments, and the range of their
concentrations in different types of rocks and
sediments is well known.
Generally, fine-grained sedimentary rocks contain more clays
and organic carbon than their coarse-grained counterparts. Igneous
rocks do not contain any organic carbon, while some metamorphic
rocks contain small amounts. However, both types of rocks usually
contain small amounts of clays, generally as great as five percent
(see Table 1 and Williams et al., 1954 and Huang, 1962). Available
data on the organic carbon and clay contents of consolidated rocks
30
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TABLE 1
RANGES OF CLAY AND ORGANIC CARBON CONTENTS OF SIX COMMON GROUPS
OF ROCKS AND THEIR EQUIVALENT SEDIMENTS1
Organic
Rock Type Sediments Carbon (%) Clays (%)
Coal Seams Peat, muck, & 58-80 3-14
organic-rich
sediments
Claystones, Mudstones, Clayey, muddy 0.4-10 37-81
Shales & Siltstones^ & silty sediments
Sandstones
Carbonates^
Metamorphic Rocks
Igneous Rocks
Sandy sediments
Limey sediments
Talus,4
clean sand,
clean gravel
0.01-0.5
0.2-0.51
0.0-0.17
Or\
• U
8-32
1-10
1-10
1_ c
J
1Source data in Tables A-l, A-2, and A-3 in Appendix A.
^Tar sands with 4 to 10 percent organic carbon and oil shales with
about 5 to 10 percent organic carbon can be included in this
category.
^Argillaceous carbonates mixed with claystones and mudstones
generally have higher clay contents than those indicated.
4Talus is composed of rock fragments of variable sizes and shapes
and usually contains very little clay (usually less than 6 percent)
and organic carbon (usually less than 0.18 percent) (Richard, 1986).
Therefore, talus is considered equivalent to metamorphic rocks in
terms of its total clay and organic carbon content.
31
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and unconsolidated sediments are presented in Appendix A (Tables
A-l, A-2, and A-3). The clay and organic carbon contents of
unconsolidated sediments are generally the same as those noted for
their consolidated counterparts (i.e., rocks).
The ranges of clay and organic carbon contents for six common
groups of rocks and their equivalent sediments are drawn from the
data presented in Appendix A. These ranges are given in Table 1.
From the table, one can infer that coal seams and their equivalent
sediments have the highest relative capacity for sorption while
igneous and metamorphic rocks, have the lowest capacity for
sorption. The increase of sorptive capacity with increasing clay
and organic carbon contents is shown for a variety of inorganic and
organic substances in Appendix A (Table A-4 and Figures A-l, A-2,
A-3, and A-4).
The values given in Table A-4 of Appendix A are a measure of
adsorption by total clays plus organic carbon and do not represent
total sorption. However, these values shed some light on the
combined sorptive capacity of clay and organic carbon contents. The
inorganics, with few exceptions, generally show that sorption
increases with increasing contents of clays and organic carbon.
However, there is no single quantitative relationship between
sorbent content and amount of hazardous substances sorbed for all
inorganic substances. Some substances are sorbed more than others
by about the same amounts of sorbents (see Table A-4 and Figures A-l
32
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through A-4 in Appendix A). Similar behavior is also shown for the
examples of organic substances. These data are by no means
extensive or totally conclusive. However, they do suggest that
increasing contents of clays plus organic carbon increases the
sorptive capacity of a geomedium, and therefore, its ability to
retard various hazardous substances by sorption.
33
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5.0 APPLICATION OF SORPTIVE CAPACITY TO RANKING OF HAZARDOUS WASTE
SITES
In the previous section, data were shown that suggest a
generalized proportionality between the clay plus organic carbon
contents of rocks and sediments and their sorptive capacities for a
variety of hazardous substances (see Appendix A). This section deals
with questions such as how to rank the relative sorptive capacities
of various rocks and sediments, how to estimate and rank the
subsurface sorptive or retardation potential, and finally, how to
incorporate the retardation potential factor value into a site
ranking scheme.
A variety of options are available for estimating the sorptive
capacities of rocks and sediments and ranking the sorption-related
retardation potential of a subsurface migration path. These options
result from combinations of choices regarding: (1) the means for
ranking the sorptive capacities of different types of rocks and
sediments; (2) the specific subsurface migration path considered; and
(3) the means for characterizing the sorptive or retardation
potential of the selected subsurface migration path. Table 2
illustrates the range of options available. In the following
sections, each option is discussed. Then, feasible combinations are
discussed along with their advantages and disadvantages. The types
of data required by each option and possible sources of the data are
also discussed. Finally, suggestions regarding incorporation of the
35
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TABLE 2
RANGE OF OPTIONS FOR RANKING SORPTIVE CAPACITIES, SELECTING MIGRATION
PATH AND CHARACTERIZING MIGRATION PATH FOR RETARDATION POTENTIAL*
Ranking of Sorptive
Capacities of Rocks and
Sediments by Sorbent Contents
Migration Paths
Characterization of
Migration Path
for Retardation Potential
Ordinal scale: using the
ranges of total clay plus
organic carbon contents
Proportional scale: using
the averages of total clay
plus organic carbon contents
u>
The total migration
path: both total vertical
migration path and
horizontal migration
path, including aquifer,
from point of deposition
to point of exposure
The total vertical
migration path, including
the aquifer of concern
The vertical migration
path above the aquifer
of concern
1. Characterization by
considering the most
sorptive layer; i.e.,
the layer with greatest
sorbent content along
the migration path
of concern
2. Characterization by
considering the thickest
layer along the migration
path
3. Characterization by
thickness-weighted
averages of the sorptive
capacities of geomedium
layers along the
migration path
4. Characterization by
arithmetic mean of
sorptive capacities of
geomedium layers along
the migration path
*0rder of presentation represents sequence, not order of preference,
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resulting retardation potential factors into a site ranking scheme
are made.
5.1 Ranking of Sorptive Capacities of Rocks and Sediments by Sorbent
Contents
Two scales have been developed for ranking sorptive capacities
using the data on clay plus organic carbon contents (sorbent content)
of rocks and sediments. These are:
• An ordinal scale that uses the ranges of sorbent content.
• A proportional scale that uses the averages of sorbent content.
5.1.1 Ordinal Scale
The ordinal scale simply ranks the sorptive capacities of
different types of rocks and sediments according to the ranges of
their clay and organic carbon content. For this ranking scheme, it
is estimated that rocks and sediments with high sorbent content have
a correspondingly higher retardation potential than those with
medium and low ranges (see Table 3). Thus, ordinal values are
assigned to the different groups of rocks and sediments, and these
values are intended to suggest the rank order of the retardation
potential of those rocks and sediments. For the purposes of
retardation potential factor scoring, rocks and their equivalent
sediments are divided into three broad groups as follows:
1. The first group is composed of coal seams, claystones,
mudstones, shales and siltstones, and their equivalent
unconsolidated sediments such as peat, muck and organic-
rich soils, and clayey, muddy, and silty soils. Based on
Table 1, these rocks and sediments contain the greatest
percent of sorbents (37 to 94 percent) when compared to the
two other groups.
37
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TABLE 3
RELATIVE RETARDATION POTENTIALS (AND THEIR FACTOR VALUE) OF
GROUPS OF ROCKS AND THEIR EQUIVALENT SEDIMENTS ACCORDING
TO THE RANGE OF TOTAL SORBENT CONTENTS1—ORDINAL SCALE
Rock Type
Sediments
Relative
Retardation
Potential
Assigned
Factor
Value3
Coal seams, claystones
mudstones, shales and
siltstones^
Peat, muck and
organic-rich
sediments, clayey,
muddy, and silty
sediments
High
Low
Sandstones
Carbonate, Metamorphic
and Igneous Rocks
Sandy sediments
Limey sediments
gravels Talus
clean sands,
clean gravel
Medium Medium
Low High
-'-Ranges are given in the text and in Table 1.
^Tar sands and oil shales are included.
^Relative values for the purpose of illustration.
38
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2. The second group is composed of sandstones and sandy
soils. These generally contain from about 8 to 32 percent
clays and organic carbon.
3. The third group is composed of carbonate, metamorphic and
igneous rocks, and limey soils and talus. The clay and
organic carbon content of these rocks and sediments is
generally below 10 percent.
Since Group 1 rocks and sediments have the highest values for
percent clays and organic carbon, it is expected that Group 1 rocks
and sediments have a higher potential for retardation of hazardous
substances than those of Groups 2 and 3. Group 3 rocks and
sediments have a lower potential for retardation.
5.1.2 Proportional Scale
The proportional scale, shown in Table 4, is based on the
mid-point of the average clay plus organic carbon content of various
rocks and sediments. For the purpose of rating the relative
retardation potential, the commonly occurring rocks and their
equivalent sediments are divided into five groups according to their
average total sorbent contents. The relative magnitude of these
averages are intended to reflect semi-quantitatively the relative
sorptive capacities. Thus, for example, coal seams and their
equivalent sediments, containing an average clay and organic carbon
content of 77 percent, are presumed to have a sorptive capacity
about 26 times greater than that of metamorphic and igneous rocks
which have an average sorbent content of 3 percent.
While a proportional scale confers advantages not possible with
an ordinal scale, the precise quantitative relationship implied in
39
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TABLE 4
GROUPS OF ROCKS AND EQUIVALENT SEDIMENTS ORDERED BY THEIR
AVERAGE TOTAL SORBENTS—PROPORTIONAL SCALE
Rock Type
Sediments
Average Total Sorbentsl
(Percent Clays Plus
Percent Organic Carbon)
Coal seams
Peat, muck, and
organic-rich
sediments
•^-Mid-point value of the ranges given in Table 1.
^Tar sands and oil shales are included.
77
Claystones, muds tones,
shales and siltstones^
Sandstones
Carbonates
Metamorphic and
Igneous Rocks
Clayey, muddy and
silty sediments
Sandy sediments
Limey sediments,
gravels
Talus, clean sands,
clean gravels
64
20
6
3
40
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Table 4 between the average clay plus organic carbon content and
sorptive capacity (and, therefore, retardation potential) of rock and
sediment groups has no experimental or empirical foundation. This
relationship, as mentioned in Section 4, is based on the premise that
sorptive capacity increases with an increase in clay plus organic
carbon content.
5.2 Migration Paths
In order to evaluate the potential for retardation of hazardous
substances, the length of the subsurface migration path associated
with a site must be defined. Three possible migration paths, which
are shown in Figure 3, are evaluated. These are:
Path A; This path considers the entire migration distance from
point of deposition to point of exposure; i.e., the total vertical and
horizontal migration paths, including the depth of an aquifer of
concern. This path extends from the lowest point of waste disposal to
the nearest target well. Since hazardous substances can migrate
vertically to an aquifer of concern and horizontally within an aquifer
to the nearest target, it is prudent to evaluate the entire migration
path for retardation potential.
Path B; This path considers the total vertical distance from the
lowest point of waste disposal to the bottom of an aquifer of
concern. This path does not consider the retardation potential of the
horizontal migration path in an aquifer.
41
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Top Soil
Path-B •
Aquifer
WjJterJ^abJe.
Target Well
Rock/Sediment Type 1
t
Path-C
Path-A
FIGURE 3
SCHEMATIC CROSS SECTION SHOWING THREE ALTERNATIVE MIGRATION PATHS FOR
EVALUATION OF RETARDATION POTENTIAL
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Path C; In this case, only the vertical distance to the upper
surface of an aquifer of concern is considered. This option does
not consider the retardation potential of a migration path (either
vertically or horizontally) in an aquifer. This path definition
conforms to the one used for evaluating "permeability of the
unsaturated zone," in the current HRS ground water route
characteristics category.
Although any of the three migration path lengths could be
selected to evaluate the subsurface retardation potential of the
subsurface migration path, the HRS as currently designed excludes
consideration of hazardous substance migration in an aquifer of
concern (either vertical or horizontal). By considering only the
vertical migration path to an aquifer of concern, the present HRS
attempts to assess the probability that released hazardous
substances will reach that aquifer. In order to assess the
probability that hazardous substances will reach a given target, a
site ranking scheme should also consider the migration path in the
aquifer, as this can comprise a major portion of the entire
subsurface path. Thus, the migration path length to include in a
site ranking scheme for the evaluation of subsurface retardation
potential depends upon the intended objectives of the ranking scheme
and the type of data available for characterising the sorptive
capacity along the migration path.
43
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5.3 Characterization of Migration Path for Sorptive Potential
Once the length of a migration path is defined, a means of
characterizing that path for its sorptive potential must be
established in order to complete the evaluation of the subsurface
retardation potential. For this purpose, four methods of migration
path characterization are proposed. These are:
• Method 1; Characterization by considering only the most
sorptive geomedium layer (the layer with the greatest sorbent
content) along the migration path.
• Method 2; Characterization by considering only the thickest
geomedium layer along the migration path.
• Method 3; Characterization by a thickness-weighted average
sorptive capacity (sorbent content) of geomedium layers along
the migration path.
• Method 4; Characterization by the arithmetic mean of the
sorptive capacities (sorbent contents) of geomedium layers
along the migration path.
Method 1
Under this method the most sorptive layer along the migration
path is selected to characterize the retardation potential of the
subsurface. This layer should have a specified minimum thickness in
order to be considered in site evaluation. A minimum thickness of
6 feet is suggested for naturally occurring rock/sediment layers
because it is twice the thickness of artificially created
barrier-clay layers (liners) generally considered for remediation of
a hazardous waste migration problem (Otte, 1986).
44
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Method 2
This method would characterize the retardation potential of the
migration path by considering the thickest layer along the path. No
minimum thickness is imposed by this method. As in the case of
Method 1, the sorptive potential of the path would be characterized
by only a single layer.
Method 3
This method uses the thickness-weighted average sorptive
capacity of geomedium layers along the migration path to evaluate the
subsurface retardation potential. The following is a step-by-step
methodology for calculating the thickness-weighted average sorptive
capacity along the subsurface migration path:
• Collect the data on type (i) and thickness (t) of the "n"
number of rock and sediment layers along the migration path.
• Calculate the total length (L) of the migration path.
• Identify, from Table 4, the average total sorbent (percent
clay + percent organic carbon) contents (TSC) of rock and
sediment layers along the migration path.
• Calculate the thickness-weighted average sorbent content
(TWASC) of the media along the migration path using the formula:
£ t.
TWASC = 2- (TSC x :pi)
i=l L
• Assign a factor value to the site according to the assigned
retardation potential factor values given in Table 5.
Method 4
This method characterizes the retardation potential of the
migration path by the arithmetic mean of the average total sorbent
45
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TABLE 5
EXAMPLE OF NUMERICAL FACTOR VALUES FOR THE
THICKNESS-WEIGHTED AVERAGE SORBENT CONTENTS
Thickness-Weighted
Average Sorbent
Content (%)
60
60 to 40
39 to 20
20
Relative
Retardation
Potential
Highest
Medium High
Medium Low
Lowest
Assigned
Factor
Value1
Lowest
Medium Low
Medium High
Highest
^Values presented for purposes of illustration only.
46
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content of rock and sediment layers along the path. The arithmetic
mean of the sorbent content (AMSC) is calculated using the formula:
1 n
AMSC = - £ TSCi
n i=l
where n, i and TSC. are the same as those defined under Method 3
above. The site could be assigned retardation potential factor
values in the same manner as in Method 3.
5.4 Examination of Various Options for Ranking of Retardation
Potential
In this section, possible options for ranking the subsurface
retardation potential of a migration path are examined. The options
are based on applicable combinations of choices with respect to
scales for ranking sorbent capacities, length of migration path, and
means for characterizing migration path retardation potential.
Options Using the Ordinal Scale
If the ordinal scale is used for ranking sorptive capacities,
then ranking of the retardation potential of a migration path is
necessarily limited to Method 1 or 2, that is, site evaluation by
the most sorptive or the thickest geomedium layer along a migration
path (see Section 5.3). It is not possible to use Method 3 or 4
because ordinal rankings cannot be averaged meaningfully. The use
of Method 1 or 2 simply suggests that the layer selected has higher
retardation potential than the other layers. Consideration of the
layer with the greatest sorbent content infers that retardation of
migrating hazardous substances will occur, most effectively, by
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sorption in that layer. This assumption is similar to that
currently made in the HRS for evaluation of the permeability of the
unsaturated zone. Alternatively, consideration of the thickest
sorptive layer assumes that most hazardous substances are sorbed and
retarded during the course of longer migration, i.e., with longer
residence time, in that layer.
Options Using the Proportional Scale
If the proportional scale is used, then ranking of the
retardation potential of a migration path can use any of the methods
described in Section 5.3 although its greatest potential would be
realized in Methods 3 and 4. Evaluation of the retardation
potential of a migration path based on the thickness-weighted
average is expected to give the best estimate of the subsurface
sorptive potential because this method reflects the total sorptive
capacities of rock and sediment layers along the entire migration
path. Undoubtedly, this evaluation would be more meaningful if the
quantitative relationship between sorbent contents and the capacity
to sorb various hazardous substances were well understood. However,
the proportional scale provides a basis for obtaining a "relative
average" retardation potential of media along a migration path when
the thickness of different rock and sediment layers along that path
are known.
The retardation potential of a migration path based on the
arithmetic mean of total sorbent contents is a qualitative estimate
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of the overall subsurface sorption-related retardation potential.
By not considering the thicknesses of geologic layers, this
methodology provides only a partial sorptive capacity of the
subsurface. However, estimation of the retardation potential based
on the arithmetic mean should be used only when data on thicknesses
of various geomedium layers along the migration path are not
available.
5.5 Data Needs and Sources
The data required for evaluating the sorptive capacity of media
along a migration path and the sources of these data, along with
their logistics, are discussed in this section.
5.5.1 Data Needs
The data needed for evaluating the sorptive capacity of a
migration path are as follows:
• Length of the migration path.
• Type of rock or sediment in each layer along the path.
• Thicknesses of rock and/or sediment layers, as measured
along the path.
5.5.2 Data Sources
The required data can be obtained from at least four possible
sources which, if available, can be obtained at little or no
additional cost to the site inspection program. One source is
regional and local stratigraphic reports and publications prepared
by Federal and State agencies, and by universities. These data are
usually as good as established regional or local stratigraphic
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layers. Often, the well-established stratigraphic layers and marker
beds of a regional stratigraphic sequence are used, in combination
with limited local data, to establish a local stratigraphic
sequence. The collection of such stratigraphic data involves no
major additional costs to the site inspection program.
A second source of local data is well logs (descriptive and
geophysical) from wells located in the vicinity of a site. The
source of these wells would include private or municipal water
wells, and shallow exploration/production wells from industry or
government. Frequently, these well logs will include a lithologic
description of rock/sediment layers along with their apparent
thicknesses.
A third source of information is data gathered by the
Department of Transportation or State Highway Departments in
preparation for bridge building, overpass construction or general
roadway work. This may include road cuts, boring logs and cross
sections. The availability and quality of this information is
variable, and probably confined to shallow depths.
A fourth data source which can be used for evaluating sorptive
capacity of a media is monitoring wells and soil borings taken at
the site as a part of the site inspection or monitoring program.
This site-specific information is valuable but will vary in quality
and quantity, depending on the site.
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5.6 Suggestions for Incorporation of a Retardation Potential Factor
into the HRS Ground Water Route Score
A retardation potential factor, such as that described in
Section 5.3 and 5.4, can be incorporated either into the route
characteristics or waste characteristics category of the current HRS
ground water route. The route characteristics category is a
reasonable location for a retardation potential factor because it
could improve the capability of that category to assess the
probability that hazardous substances will migrate to an aquifer.
Alternatively, incorporation of a retardation potential factor into
the waste characteristics category is also reasonable because it may
improve the assessment of the nature of a hazardous substance
released within an aquifer of concern, prior to reaching a target.
The retardation potential factor could be incorporated into the
HRS route characteristics category in either of two ways:
• As an additive factor with the current permeability factor;
i.e.,
Total Route Characteristics Score = [depth to aquifer of
concern factor value] + [net precipitation factor value] +
[retardation potential factor value] + [permeability factor
value] + [physical state factor value].
• As a multiplicative factor with the permeability factor value,
i.e.,
Total Route Characteristics Score = [depth to aquifer of
concern factor value] 4- [net precipitation factor value] +
[(permeability factor value) x (retardation factor value/
maximum retardation factor value)] + [physical state factor
value].
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Incorporating the retardation potential factor value as an
additive factor may require adjusting the maximum score for route
characteristics, whereas incorporating it as a multiplicative factor
(ranging between zero and one) with the current permeability factor
will not. In the current HRS, permeability of the unsaturated zone
is used as an indicator of the rate at which hazardous substances
migrate from a site to an aquifer of concern. This assumes that
hazardous substances migrate at the same rate as transporting
solutions and/or ground water. However, in reality this may not be
the case. Therefore, it may be more realistic to adjust the
permeability factor value by multiplying it by a retardation factor
value (ratio).
Under the waste characteristic factor category, the retardation
potential could be incorporated as an additive factor:
Total Waste Characteristics Score = [toxicity/persistence
factor value] + [hazardous waste quantity factor value] +
[retardation potential factor value].
Currently; toxicity/persistence and hazardous waste quantity
are used in the waste characteristics factor category to evaluate
the nature of the substances that might be released to an aquifer.
Along with biodegradation which is now used to evaluate persistence,
retardation potential can be used as an additional factor to
evaluate the total waste characteristics category. This is a viable
approach since it is the retardation potential of the migration
path, in combination with other factors, that affects the
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concentration of released hazardous substances prior to reaching a
target.
Note that if the route characteristics factors are meant to
evaluate the vertical migration path from the point of deposition to
the aquifer, then the retardation potential factor should reflect
only the sorptive capacity of that part of the migration path. In
such case, the retardation potential of that portion of the
migration path which occurs in the aquifer could be incorporated
separately into the waste characteristics factor category as an
additive factor as shown above.
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6.0 LIMITATIONS
This section discusses the possible limitations of and concerns
with the approach described in Section 5 for incorporating a
geochemical retardation factor in the HRS. This approach involves
an estimation of the potential of subsurface media to retard various
hazardous substances. A simple, but practical method has been
described to estimate the sorptive capacities of rocks and sediments
in terms of the sum of their clay and organic carbon content. Four
limitations are discussed, along with a series of general concerns.
The first limitation of this approach is that while the clay
and organic carbon content are the major contributors to the
sorptive capacity of a rock or sediment, they may not represent the
total sorptive capacity. This is due to the possible presence of
other sorbent components, such as Fe-Mn-Al oxides/hydroxides and
sulfides. Unfortunately, unlike the clay and organic carbon
content, the average quantity of these sorbents in a variety of
rocks and sediments is unknown. However, since clays and organic
carbon, unlike other sorbents, can occur together and sorb a wide
variety of hazardous substances, they are usd for estimating the
total sorptive capacities of rocks and sediments.
Second, the organic carbon contents of each rock/sediment type
in Tables A-l and A-2 exhibit a range of values. This may be due to
different sample collection techniques and natural variability of
organic matter in rocks and sediments. Samples collected from
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outcrops are subject to oxidation which will alter the organic
matter content while core samples are subject to thermal alteration
which can affect the organic carbon content. The organic carbon
content of rocks and sediments could vary spacially depending on the
environments in which they are deposited.
Variations in clay contents of each rock/sediment type is
evident from Tables A-3 and A-4. Variations in clay contents can
also be due to sample collection techniques and natural variability
of clay contents in rocks and sediments. It is possible that both
organic carbon and clay contents of each rock/sediment type could
vary spacially at a site and among sites. However, by the use of
the average total sorbents (carbon and clay contents, see Table 4),
the effect of sorbent variability on the estimation of subsurface
retardation potential is minimized. The average total sorbent
values in Table 4 account for the variability mentioned above since
these values are derived from the diverse sample data summarized in
Table 1.
Third, the effect of geologic structure (such as faults,
fractures, joints, and fissures) on sorptive capacity is not
considered. Such effects depend on the type of rock and the nature
of the structures present. However, the presence of geologic
structure in rocks does not necessarily mean open, pipe-like
conduits, except in karst terrane or in some igneous and metamorphic
rocks where joining is prevalent. Generally, the structural gaps or
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spaces in most sedimentary rocks and in some igneous and metamorphic
rocks are filled with a fine to clay-size matrix, derived from the
crushed wall—rock. In some instances, these spaces are sealed by
reprecipitation and/or recrystallization of filled rock-matrix
(Lahee, 1961; Koch and Vogel, 1980; Birnbaum and Radlick, 1982; and
Babcock, 1982). Therefore, in most sedimentary environments, and in
some igneous and metamorphic environments, structure is not expected
to circumvent the sorptive capacity intrinsic to the rock or sediment
itself. Clearly, in those settings where geologic structure controls
the migration paths (such as in some karst terrane), sorption may not
be an effective hazardous substance retardation mechanism. In the
context of site screening, however, sorptive capacity adequately
addresses the potential of most subsurface media to retard pollutants.
Fourth, data are not available to substantiate a quantitative
relationship between the sum of clay and organic carbon content and
sorptive capacity of a rock or sediment. However, the preponderance
of the data available in the literature indicates that an increase in
the sum of clay and organic carbon content results in an increase in
the sorption of hazardous substances (see Table A-5; Figures A-l to
A-4). In the absence of carefully developed experimental data on
sorption of a variety of hazardous substances by different rocks and
sediments, the relationship between the sorbent content and sorption
appears to be reasonable in establishing a relative sense of sorptive
capacity.
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In addition to limitations to the approach previously described,
there are some general concerns with this approach. Adding a geochem-
ical retardation potential factor to the HRS will generally reduce the
score of the sites being evaluated relative to the current HRS.
Without consideration of retardation that may occur in the subsurface,
the current HRS implies that hazardous substances migrate at the same
velocity as ground water and that there is no change in their concen-
tration after they are released. This is a worst-case approach with
respect to hazardous substance. Thus, incorporation of retardation
potential provides a more realistic assessment of the likelihood that
released hazardous substances will reach a target. It is clear,
however, that the methodologies offered here for evaluating the
subsurface sorption-related retardation potential of a migration path
involve simplification of the actual situation in subsurface
migration. Undoubtedly, the use of a single generalized parameter
(sorption) to represent retardation potential results in limitations
with respect to the accuracy of the value assigned to the retardation
potential. However, the degree of simplification does not detract
from the purpose of the methodology which is to evaluate the
subsurface geochemical retardation potential. In general, a more
complicated evaluation scheme might require a more sophisticated
treatment of subsurface geochemistry, but additional or more detailed
site-specific data would not by itself result in an improved
evaluation. Rather, advances in the state-of-the-art of modeling
complex geochemical processes are also needed.
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7.0 CONCLUSIONS
Assessment of the relative hazard posed by hazardous waste
sites via subsurface migration can be improved by evaluating the
potential of subsurface media to retard and/or attenuate released
hazardous substances. The most practical way to incorporate
geochemical retardation into a site evaluation scheme is by
estimating the sorptive capacity of a migration path. The clay and
organic carbon contents of rocks and sediments is the most useful
indicator of the sorption-related retardation potential of geologic
media along a subsurface migration path.
Among the methodologies developed to evaluate retardation
potential, the method utilizing the proportional ranking scale and
the thickness-weighted average sorptive capacity is conceptually
attractive. Given that stratigraphic information is routinely
collected for each site, little additional information, such as
thicknesses according to rock/sediment type is required to evaluate
the geochemical retardation potential of subsurface geologic media.
It appears to be both appropriate and feasible to include a
geochemical retardation potential factor in the HRS. Incorporation
of the retardation potential will conceptually further improve the
capability of the HRS to discriminate among hazardous waste sites.
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Rai, D., J. M. Zachara, A. P. Schwab, R. L. Schmidt, D. C. Girvin,
and J. E. Rogers, 1984. "Chemical Attenuation Rates, Coefficients,
and Constants in Leachate Migration, Volume 1: A Critical Review,"
Report EA-3356, prepared by Battelle Pacific Northwest Laboratories,
Electric Power Research Institute, Palo Alto, CA.
Rao, P. S. C. and J. M. Davidson, 1980. "Estimation of Pesticide
Retention and Transformation Parameters Required in Non-Point Source
Pollution Models," in Environmental Impact of Non-Point Source
Pollution, M. R. Overcash and J. M. Davidson, eds., Ann Arbor Science
Publishing Inc., Ann Arbor, MI, pp. 23-67.
Rao, P. S. C., P. NKedi-Kizza, and J. M. Davidson, 1986. "Abiotic
Processes Affecting the Transport of Organic Pollutants In Soil," in
Land Treatment—A Hazardous Waste Management Alternative, R. C. Loehr
and J. F. Malina, Jr., eds., Water Res. Symposium, No. 13, pp. 63-72.
Rice, R. G., 1981. "Ozone for the Treatment of Hazardous Materials,"
Water-1980, Symposium Series, American Institute of Chemical
Engineers, 209(77), pp. 79-107.
Richard, A., 1986. Personal communication, U.S. Soil Conservation
Service.
Ringbom, A. , 1963. Complexation in Analytical Chemistry,
Interscience Publishers, New York.
Roberts, P. V., M. Reinhard, and A. J. Valocchi, 1982. "Movement of
Organic Contaminants in Groundwater: Implications for Water Supply,"
Journal of the American Water Works Association, 74:408-413.
Ronov, A. B., 1958. "Organic Carbon in Sedimentary Rocks (In
Relation to Presence of Petroleum)," Geochemistry, Vol. 5, p. 510.
Rubin, J., 1983. "Transport of reacting solutes in porous media:
Relation between mathematical nature of problems formulation and
chemical nature of reactions," in Water Resources Research, Vol. 19,
No. 5, pp. 1231-1252.
67
-------
Sayala, D. , 1983. "Geochemical Studies," in Multidlsclpllnary
Studies of Uranium Deposits in the Red Desert, Wyoming, U.S.
Department of Energy Open-File Report, GJBX-1(83), pp. 79-104.
Sayala, D. and D. L. Ward, 1983. "Multidisciplinary Studies of a
Uranium Deposit in the San Juan Basin, NM," U.S. Department of Energy
Open-File Report, GJBX-2(83), p. 227.
Schmidt-Collerus, J. J., 1979. "Investigation of the Relationship
Between Organic Matter and Uranium Deposits," U.S. Department of
Energy Open-File Report, GJBX-130(79), p. 281.
Schwarzenbach, R. P. and J. Westall, 1981. "Transport of Nonpolar
Organic Compounds from Surface Water to Ground Water—Laboratory
Sorption Studies," Environ. Sci. Technol., 15(11), pp. 1360-1367.
Serne, R. J. and J. F. Relyea, 1981. "The Status of Radionuclide
Sorption—Desorption Studies," performed by the WRIT Program,
PNL-SA-9787, Battelle Pacific Laboratory, Richland, WA.
Siegel, F. R., 1974. Applied Geochemistry, John Wiley and Sons
Publishers, New York, NY, p. 353.
Siegrist, R. L., D. L. Anderson, and D. L. Hargett, 1986. "Large
Soil Absorption Systems for Wastewaters from Multiple-Home
Development," Water Engineering Research Laboratory, Cincinnati, OH,
U.S. Environmental Protection Agency, EPA/600/S2-86/023, p. 2.
Singer, P. C., 1982. "Trace Metals and Metal-Organic Interactions in
Natural Waters," Ann Arbor Science Publishers Inc., Ann Arbor, MI,
p. 350.
Steelman, B. L. and R. M. Ecker, 1984. "Organic Contamination of
Groundwater—An Open Literature Review," PNL-SA-12709, Battelle
Pacific Laboratory, Richland, WA.
Stuart, J. D., 1983. "Organics Transported Through Selected Geologic
Media: Assessment of Organics Transported Away from Industrial Waste
Disposal Sites," Institute of Water Resources, The University of
Connecticut, PB83-224246, p. 20.
Stumm, W., 1967. "Metal Ions in Aqueous Solutions," in Principles
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Wiley Publishers, New York, NY.
Stumm, W. and J. J. Morgan, 1981. Aquatic Chemistry, Wiley
Publishers, New York, p. 583.
68
-------
Summers, K., S. Gherini, and C. Chen, 1980. "Methodology to Evaluate
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Protection Agency, (EPA-600/7-80-117), U.S. Environmental Protection
Agency, Industrial Environmental Research Laboratory, Cincinnati, OH.
Sweeney, K., 1981. "Reductive Treatment of Industrial Wastewaters:
I—Process and Description and II—Process Application," in Water
1980, (G. F. Bennett, ed.), American Institute of Chemical Engineers
Symposium, Series 209 (77), pp. 67-78.
Teodorovich, G. I., 1958. "Study of Sedimentary Rocks,"
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John Wiley and Sons Publishers, New York, NY.
Tissot, B. P. and D. H. Welte, 1984. Petroleum Formation and
Occurrence, Springer-Verlag Publishers, New York, NY, p. 648.
Tomlinson, G., D. Gray, and M. Neuworth, 1984. "Effect of Coal Rank
on Direct Coal Liquefaction Processes: Solvent Refined Coal (SRC)-II
Process Experiment," Sandia National Laboratories Report SAND-1984.
Utah State University and Arthur D. Little, Inc., 1984. "Review of
In-Place Treatment Techniques for Contaminated Surface Soils," U.S.
Environmental Protection Agency Report EPA-540/2-84-003a, Vol. 1,
p. 163.
Vassoevich, N. B., I. V. Visotskiy, A. N. Guseva, and V. B. Olenin,
1967. "Hydrocarbons in the Sedimentary Mantle of the Earth,"
Proceedings of the 7th World Petrology Congress, 2, pp. 37-45.
Ward, C. R., 1977. "Mineral Matter in the Springfield-Harrisburg
(No. 5) Coal Member in tne Illinois Basin," Illinois State Geological
Survey, Circular 498, p. 35.
Ward, C. H., M. B. Thomson, P. B. Bedient, and M. D. Lee, 1985.
"Transport and Fate Processes in the Subsurface: Special Report,"
National Center for Groundwater Research, Rice University,
Houston, TX, May 21, p. 33.
Wayland, T. and D. Sayala, 1983. "Multidisciplinary Studies of
Uranium Deposits in the Red Desert, Wyoming," U.S. Department of
Energy Open-File Report, GJBX-K83), p. 395.
69
-------
Westall, J. C., J. L. Zachary, and F. M. M. Morel, 1976. "MINEQL:
Computer Program for the Calculation of Chemical Equilibrium
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Engineering, MIT, Cambridge, MA, p. 91.
Wigley, T. M. L., 1977. "WATSPEC: A Computer Program for
Determining the Equilibrium Specification of Aqueous Solutions,"
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W.H. Freeman and Company, San Francisco, CA, p. 403.
70
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9.0 SUGGESTED READINGS
Anderson, M. A., and A. J. Rubin, 1981. Adsorption of Inorganics at
Solid-Liquid Interfaces, Ann Arbor Science Publishers, Ann Arbor, MI,
p. 357.
Lindsay, W. L. , 1979. Chemical Equilibria in Soils, Wiley-
Interscience Publication, New York, NY, p. 423.
Manahan, S. E., 1977. Environmental Chemistry, Willard Grant Press,
Boston, MA, p. 527.
Morrill, L. A., B. C. Mahilum, and S. H. Mohiuddin, 1982. "Organic
Compounds in Soils," in Sorption, Degradation and Persistence, Ann
Arbor Science Publishers, Ann Arbor, MI, p. 279.
Rai, D., J. M. Zachara, A. P. Schwab, R. L. Schmidt, D. C. Girvin,
and J. E. Rogers, 1984. "Chemical Attenuation Rates, Coefficients,
and Constants in Leachate Migration," Electric Power Research
Institute (Palo Alto, CA) Report EA-3356, Vol. 1.
Siegel, F. R., 1974. Applied Geochemistry, Wiley-Interscience
Publication, New York, NY, p. 345.
Stumm, W. and J. J. Morgan, 1981. Aquatic Chemistry,
Wiley-Interscience Publication, New York, NY, p. 767.
Tinsley, I. J., 1979. Chemical Concepts in Pollutant Behavior,
Wiley-Interscience Publication, New York, NY, p. 257.
Utah State University and Arthur D. Little, Inc., 1984. "Review of
In-Place Treatment Techniques for Contaminated Surface Soils," U.S.
Environmental Protection Agency Report EPA-540/2-84-003a, p. 163.
Ward, C. H., W. Giger, and P. L. McCarty, 1986. Ground Water
Quality, Wiley-Interscience Publication, New York, NY, p. 533.
71
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APPENDIX A
SORPTIVE CAPACITY ESTIMATION DATA
This appendix contains a collection of tables and figures
presenting data useful in estimating the sorptive capacity of various
consolidated rocks and unconsolidated sediments defined in terms of
organic carbon and clay content.
73
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TABLE A-l
ORGANIC CARBON CONTENT OF ROCKS AND SEDIMENTS
Rock/
Sediment Type
Clay Stones
(or Clays)
Shales
Siltstones
Carbonate Rocks
Sandstones
Shales
Siltstones
Carbonate Rocks
Shales + Siltstones
Calcareous Shales
Carbonate Rocks
Petroliferous
Shales
Carbonate Rocks
Sandstones
Non-Petroliferous
Shales
Carbonate Rocks
Sandstones
Number of
Samples Analyzed
About 1,200
(200 formations
in 60 basins -US A)
Over 1,000
(USSR)
418 (Europe)
97 (Europe)
118 (Europe)
No Data (USSR)
1,105 (Composite
Average
Percent
Organic
Carbon
3.4
1.7
1.43
0.23
0.04
.90
.45
0.20
2.16
1.90
0.67
1.40
0.51
0.42
Samples* -
0.40
0.16
0.18
Source
Hunt (1963 and 1961)
Hunt (1963 and 1961)
Hunt (1963 and 1961)
Hunt (1963 and 1961)
Hunt (1963 and 1961)
Vassoevich et al. (1967)
Vassoevich et al. (1967)
Vassoevich et al. (1967)
(In Tissot & Welte, 1984)
(In Tissot & Welte, 1984)
(In Tissot & Welte, 1984)
Ronov (1958)
Ronov (1958)
Ronov (1958)
USSR)
Ronov (1958)
Ronov (1958)
Ronov (1958)
*A composite sample is made by combining the samples from different parts
of the same rock.
-------
TABLE A-2
ORGANIC CARBON CONTENTS OF ROCKS AND SEDIMENTS (RANGES)
Rock/
Sediment Type
Petroleum Reservoir Rocks
Limestones
Sandstones
Non-Reservoir Rocks
Limestones
Dolomites
Gray Shales
Black Shales
Calcareous Shales
Coals
Western U.S. Coals
Illinois Basin Coals
Appalachian Coals
Recent Sediments
Red beds
Peats
Metamorphic Rocks
Slates; Argillites
Sandstones
Siltstones + Shale +
Clays tones + Muds tones*
Sandstones
Shales + Siltstones +
Claystones*
Number of
Samples
Analyzed
(USA)
2
2
(USA)
3
3
5
3
5
29
110
22
Percent
Organic
Carbon
0.27; 0.51
0.52; 0.76
0.20-0.43
0.11-0.25
0.87-1.75
2.10-8.8
1.4-10.0
58-74
62-80
63-80
(USA)
4 About 0.1
No Data About 37.0
(USA)
2
840 Samples
(Red Desert
Basin, Wyoming)
5,578 Samples
(San Juan Basin
New Mexico)
0.17; 0.16
0.05-0.2
0.64-10.0
0.01-0.5
0.4-10.0
Source
Hunt (1979)
Hunt (1979)
Hunt (1979)
Hunt (1979)
Hunt (1979)
Hunt (1979)
Hunt (1979)
Radian Corp. (1980)
Radian Corp. (1980)
Radian Corp. (1980)
Hunt (1979)
Phillippi (1965)
Hunt (1979)
Sayala (1983)
Sayala (1983)
Sayala (1983)
Sayala (1983)
*Interbedded.
75
-------
TABLE A-3
AVERAGE CLAY CONTENTS OF ROCKS/SEDIMENTS
Rock/
Sediment Type
Number of
Samples Analyzed
Average
Percent
Clay
Content
Source
Arenites (Medium
to Coarse Grained
Sandstones)
Graywackes (Fine
to Medium Grained
Sandstones)
186 9 Wayland & Sayala (1983)
(Red Desert Basin, WY)
68 32 Wayland & Sayala (1983)
(Red Desert Basin, WY)
Siltstones
Mudstones
Claystones
Triassic Arenites
Sub Graywackes
(Red
(Red
(Red
8
Desert
40
Desert
32
Desert
2
35
Basin,
Basin,
Basin,
WY)
WY)
WY)
(USA)
(USA)
37
69
81
8
9
Wayland
Wayland
Wayland
&
&
&
(In Huang
(In Huang
Sayala
Sayala
Sayala
, 1962)
, 1962)
(1983)
(1983)
(1983)
(Most Common
Sandstones of
Different Ages)
Graywackes
(Sandstones of
Different Ages)
Graywackes
Argillaceous
Carbonates
Carbonates
14 (USA)
10
(Palo Duro Basin, TX)
(Gibson Dome, UT)
11
(Gibson Dome, UT)
25 (In Huang, 1962)
Fukui & Davault (1985)
24 Fukui (1982)
1 Fukui (1982), Fukui &
Allen (1982)
76
-------
TABLE A-4
RANGE OF CLAY CONTENT OF ROCKS
Rock Type
Number of
Samples Analyzed
Percent
Clay
Content
Source
Fossiliferous
Carbonates
Clayey Dolomites
(Argillaceous)
Dolomites
(Non-Argillaceous)
Clayey Limestones
(Argillaceous)
Slightly Clayey
Dolomitic Limestone
Slightly Clayey
Limestone
Limestones
(Non-Argillaceous)
Dolomitic Limestones
(Non-Argillaceous)
Coals
No Data (USA) 0.2-1.0 Barrett (1964)
No Data (USSR) 3-10.0 Teodorovich (1958)
No Data (USSR)
No Data (USSR)
No Data (USSR)
No Data (USSR)
No Data (USSR)
No Data (USSR)
43 (USA)
0-5.0 Teodorovich (1958)
10.0-30.0 Teodorovich (1958)
5.0-10.0 Teodorovich (1958)
5.0-10.0 Teodorovich (1958)
0-5.0 Teodorovich (1958)
0-5.0 Teodorovich (1958)
3-14 Ward (1977); Harvey
et al. (1983);
Tomlinson et al.
(1984)
77
-------
TABLE A-5
EXAMPLES OF INCREASE IN SORPTIVE CAPACITY WITH INCREASING
CLAY PLUS ORGANIC CARBON CONTENT FOR DIFFERENT
INORGANIC AND ORGANIC POLLUTANTS
Metal
Arsenic
Boron
Cadmium
Lead
Inorganics
Percent Sorbents
(Clay + Organic
Carbon)
47.1
25.3
17.3
11.0
65.4
59.2
58.0
28.4
22.4
26.0
5.6
53.1
31.0
29.1
22.9
22.0
18.0
6.3
32.5
25.35
21.0
10.7
5.8
(Source: Rai et
J-CEC
(meq/lOOgm)
___
__
31.0
58.0
35.2
18.5
18.0
16.2
7.8
14.4
21.0
5.1
11.1
10.3
11.5
4.0
18.2
12.2
10.4
6.96
3.8
al., 1984)
2 Estimated Value of
Retention Capacity or
Langmiur Adsorption
Maximum (umol/gm)
4.47
3.55
3.60
3.31
14.1
8.8
3.13
1.5
2.91
0.73
0.41
11.5
10.0
0.8
6.0
10.5
4.1
1.0
23.0
16.5
11.5
7.0
8.5
78
-------
TABLE A-5 (Continued)
Inorganics (Source: Rai et al., 1984a)
Metal
Percent Sorbents
(Clay + Organic
Carbon)
2 Estimated Value of
Retention Capacity or
•HlEC Langmiur Adsorption
(meq/lOOgm) Maximum (umol/gm)
Selenium
19.3
18.2
18.0
10.0
9.2
16.0
13.7
12.3
10.0
10.4
16.5
8.5
18.0
10.0
2.5
Zinc
50.0
32,
25.
22,
21.0
14.5
14.4
10.7
9.2
18.2
12.2
15.1
10.4
10.9
9.3
7.0
4.3
24.0
49.0
31.5
10.6
24.0
31.8
36.4
16.5
14.5
79
-------
TABLE A-5 (Continued)
Organic
Compound
Endrin
Diazinon
Trifluraline
Terbacil
Silvex
Pharate
2,4-D Amine
Organics (Source:
Percent Sorbents
(Clay + Organic
Carbon)
58
15
9
7
58
15
9
7
42
23
16
4
42
23
16
4
48
18
13
37
28
24
9
8
42
16
4
JRB Associates, 1984)
2 Estimated Freundlich
Adsorption Constant - K
(ug/gm)
11,115
660
222
90
325
5
20
7
2.73
1.6
0.5
0.2
2.5
0.4
0.4
0.1
162
42
34
75
14
9
5
2
4.62
0.65
0.76
80
-------
TABLE A-5 (Concluded)
Organic
Compound
lerbufos
Chlorpyrifos
Carbon Tetra-
chloride
Trichloro-
ethylene
Phenol
Organics (Source:
Percent Sorbents
(Clay + Organic
Carbon)
37
28
24
9
8
58
28
24
15
9
8
7
100
36
34
100
36
34
64
59
54
30
18
4
JRB Associates, 1984)
2 Estimated Freundlich
Adsorption Constant - K
(ug/gm)
50.8
20.5
7.9
10.6
3.1
7,095
389
147
102
37
24
13
1.75
1.18
0.62
70.0
1.6
3.9
19.4
22.7
10.5
11.0
2.5
0.2
J-CEC—Cation Exchange Capacity.
2Both the Langmiur Adsorption maximum and Freundlich Adsorption
constant are used as measures of adsorption. For a detailed
explanation of their use, see Rai et al. (1984) and JRB Associates
(1984).
81
-------
•£•
£
(fl
o
.y
c
0)
&
+
Sorbents (clay
c:
tt)
Q.
100
^
70
60
50
40
30
20
10
0
LEG
» B
r — — O~~ L
— 0 — c
— A— z
~ — d— s
B« -•- A
, — w
• F
/ ^
|l As Cd Pb
A« ^ ^ xA
TJ<^A/^ ^SeAxx" Zn
PV^X ,''' A A
P D j^
cf'
i i i i i
END
oron
ead -
admium
inc
elenium
rsenic
10 20 30 40 50
Retention Capacity or Langmiur Adsorption Maximum (nmol/gm)
Based on data from Rai et al. (1984).
FIGURE A-1
RELATIONSHIP BETWEEN SORBENT CONTENT AND RETENTION
CAPACITY/ADSORPTION MAXIMUM FOR SELECTED METALS
82
-------
100 -
s.
I
I
10 30 40
CECmeq/100gm
50
60
Based on data from Rai et al. (1984)
FIGURE A-2
RELATIONSHIP BETWEEN SORBENT CONTENT AND CATION
EXCHANGE CAPACITY FOR SELECTED METALS
83
-------
100
•p 70
.a 60
50
5"
5.
u
§
c 30
Q-
20
10
0
11.1151
_ Trifiuraline
I
Q 162
Pharate
11 Terbacil /
*J ^
f Silvex
1 1 1 1 1 1 1
Endrin ^,
-.*.--"
_.090 ----- '•122
1 1 1 > 1 > 1 >l
^
0 10 20 30 40 50 70 80 90 M60 170 230
Estimated Freundlich Adsorption Constant (ug/gm)
Based on data from JRB Associates (1984).
330 66011,120
FIGURE A-3
RELATIONSHIP BETWEEN SORBENT CONTENT AND ADSORPTION
CONSTANT FOR SELECTED ORGANIC COMPOUNDS
-------
100
§
o 60
i
?
1
$
C •30
8
Pe
20
10
A*
f\
"
f
5
e\f
I
I
/
I
' 'o
A ,
I/
I / ° '
?/ /
M/
TCE
7,095
— ^ — — O -- lerbufos
O147
^389
O 102
Cn|orpyrifos
i
i
i
+r
i
1
1
10 20 30 40 50 60 70 100 110 '140 150 380 390
Estimated Freundlich Adsorption Constant (ug/gm)
7,100
Based on data from JRB Associates (1984).
FIGURE A-4
RELATIONSHIP BETWEEN SORBENT CONTENT AND ADSORPTION
CONSTANT FOR SELECTED ORGANIC COMPOUNDS
85
------- |