Hazard Ranking System Issue Analysis:
Geohydrology
MITRE
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Hazard Ranking System Issue Analysis:
Geohydrology
Robert E. Gerstein
November 1986
MTR-86W62
SPONSOR:
U.S. Environmental Protection Agency
CONTRACT NO.:
EPA-68-01-7054
The MITRE Corporation
Metrek Division
7525 Colshire Drive
McLean, Virginia 22102-3481
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Department Approval: '
MITRE Project Approval:
^Lb\^
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ABSTRACT
Three principal hydrogeologic issues have been raised in public
comments on the National Contingency Plan and the National
Priorities List. All three issues relate to the identification, in
the EPA Hazard Ranking System (HRS), of the target population
potentially affected by a release of hazardous substances to ground
water. The three issues are that:
Ground water flow directions need to be considered in the
HRS.
The three-mile radius currently used to define the target
area in the HRS overestimates (or underestimates) the
distance contaminants can migrate.
The interconnection (or separation) of aquifers needs to be
defined in the HRS.
This study examines each of these issues and the requirements
involved in addressing the issues in the HRS. The requirements
examined include data needs, data acquisition costs, and data
uncertainties. Detailed hydrogeologic investigations would be
needed at every site to provide the necessary data. The cost and
time required to obtain the necessary data would be high relative to
that of current site inspections. Due to uncertainties inherent in
the data, the use of the data would introduce different types of
uncertainties than those currently present in the HRS.
In the course of this study, three additional issues were also
identified and examined. These are the influence of topography on
ground water flow, the behavior of ground water in karst terranes,
and the estimation of the future use of ground water supplies.
These issues focus on ground water conditions which are extremely
site specific and not consistently predictable on a national basis.
It does not appear feasible to develop quantitative, generic rating
factors to account for these issues in the HRS.
Suggested Keywords: Superfund, Hazard ranking, Hazardous waste,
Ground water, Hydrogeology.
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TABLE OF CONTENTS
Page
LIST OF ILLUSTRATIONS ix
1.0 INTRODUCTION 1
1.1 Background 1
1.2 Geohydrologic Issues Identified During the Rulemaking 3
Process
1.3 Organization of This Report 5
2.0 DEFINITION OF HYDROLOGIC TERMS 7
2.1 Underground Water 7
2.2 Aquifers and Confining Beds 7
2.3 Heads and Gradients 9
2.4 Hydraulic Conductivity 11
2.5 Transmissivity and Storativity 15
3.0 GEOHYDROLOGIC DATA ELEMENTS IN THE HAZARD RANKING SYSTEM 17
3.1 Overview of the HRS Requirements Pertaining to the 17
Ground Water Migration Route
3.2 Depth to the Aquifer of Concern 18
3.3 Permeability 21
3.4 Ground Water Use 23
3.5 Distance to the Nearest Well 24
3.6 Population Potentially Threatened 25
3.7 Summary 26
4.0 DETERMINING THE DIRECTION OF GROUND WATER FLOW 27
4.1 Overview of the Issue 27
4.2 General Considerations 27
4.3 Measuring Hydraulic Potential 28
4.3.1 Data Requirements 28
4.3.2 Data Density 29
4.3.3 Data Constraints 31
4.4 Measuring Hydraulic Conductivity 34
4.4.1 Piezometer Tests 34
4.4.2 Pumping Tests 38
4.4.3 Data Constraints 40
4.5 Findings 41
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TABLE OF CONTENTS (Continued)
Page
5.0 AQUIFER INTERCONNECTION 45
5.1 Overview of the Issue 45
5.2 Data Requirements 45
5.3 Findings 47
6.0 THE THREE-MILE RADIUS 49
6.1 Overview of the Issue 49
6.2 Data Related to Migration Distance 49
6.2.1 Survey of Contaminant Plume Geometries 49
6.2.2 Additional Illustrations of Contaminant Plume Size 50
6.3 Flow Velocity and Migration Distance 53
6.3.1 Darcy's Law 53
6.3.2 Data Requirements 54
6.3.3 Findings 55
6.4 Conclusions 56
7.0 OTHER RELATED ISSUES 59
7.1 Topography 59
7.2 Karst Terranes 61
7.3 Future Use of Ground Water Supplies 64
8.0 SUMMARY AND CONCLUSIONS 69
8.1 Direction and Velocity of Ground Water Migration 69
8.2 Aquifer Interconnection 70
8.3 The Three-Mile Radius 71
8.4 Related Issues: Topography, Karst Terranes, and the 72
Future Use of Ground Water Supplies
9.0 BIBLIOGRAPHY 75
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TABLE OF CONTENTS (Concluded)
APPENDIX A - GLOSSARY OF SELECTED TERMS 77
APPENDIX B - DETERMINING THE DIRECTION OF GROUND WATER FLOW 83
APPENDIX C - CORRELATION BETWEEN CONTAMINANT PLUME LENGTH 87
AND HYDRAULIC GRADIENT
APPENDIX D - ESTIMATED COST AND TIME REQUIREMENTS FOR 93
GEOTECHNICAL FIELD WORK
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LIST OF ILLUSTRATIONS
Figure Number Page
2-1 Aquifer Types and Associated Terms 10
2-2 Components Used to Determine Total Head 12
and Hydraulic Gradient
4-1 Possible Flow Path of Contaminants More 35
Dense versus Less Dense than Water
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1.0 INTRODUCTION
1.1 Background
The Comprehensive Environmental Response, Compensation, and
Liability Act of 1980 (CEROA) (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 concernt" 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 HRS. 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 Geohydrologic Issues Identified During the Rulemaking Process
The Hazard Ranking System and the National Priorities List have
been promulgated using the Federal notice and comment rulemaking
procedures. These procedures include a public review and comment
period during which comments and concerns were expressed by the
public regarding the HRS and how it affects the evaluation of sites
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for NPL listing. Those commenters who focused on geohydrologlc
issues identified three principal areas of concern.
One of these issues is that the ground water gradient is not
considered in the HRS. The commenters implied that gradient data
could be used to determine the direction of ground water flow, and
thus the direction of contaminant migration, so as to more
accurately define potential targets. Another implied use Is to
determine the speed of contaminant migration.
Secondly, several commenters cited the three-mile radius used
in the HRS as another major concern. The HRS uses a three-mile
radius to define the target area potentially affected by a site.
Commenters suggested that the three-mile radius overestimates the
distance contaminants can migrate and that, similar to the first
issue, the circular target area defined by the three-mile radius
ignores the direction of ground water flow.
A third hydrologlc issue is that the separation of hydrologic
units is undefined in the HRS. Commenters have suggested that
criteria need to be established which could be used to determine the
degree of interconnection between stratigraphlc units so that the
likelihood of vertical migration of contaminants, from a waste site
into one or more aquifers, can be evaluated. This issue involves
both defining the aquifer of concern and, in turn, identifying the
target population.
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1-3 Organization of This Report
In order to provide a basic ground water vocabulary, Chapter 2
contains definitions and explanations of some of the fundamental
principles and concepts that are important in the study of
geohydrology. A detailed glossary is provided in Appendix A.
Chapter 3 describes the geohydrologic data elements currently
in the HRS. This establishes a frame of reference for describing
the geohydrologic parameters which are being evaluated in this
report for possible inclusion in the Hazard Ranking System.
Chapter 4 addresses the issue of ground water flow direction.
Data collection and analysis schemes are described and their
inherent uncertainties are highlighted.
Chapter 5 discusses the aquifer interconnection issue. The
concepts and criteria for establishing interconnection are also
discussed.
Chapter 6 presents an evaluation of the three-mile radius
currently used in the HRS to delineate the potential target area.
In addition, case histories are used to provide examples of typical
ground water contamination plumes.
Chapter 7 describes three additional issues that have been
identified in the process of conducting this study. These issues
are: the influence of topography on ground water flow, the
specialized regime represented by karst terrane, and the future use
of ground water supplies.
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A summary of the findings and recommendations of this study is
presented in Chapter 8. Hie bibliography is presented in Chapter 9.
Appendix A contains the glossary. Appendix B provides an
illustration of how field data can be used to determine the direction
of ground water flow. Appendix C presents an analysis of the
correlation between the length of ground water plumes and their
respective hydraulic gradients. Appendix D provides estimates of
the cost and time requirements associated with hydrogeologic
investigations of hazardous waste sites.
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2.0 DEFINITION OF HYDROLOGIC TERMS
In order to address the issues which are the subject of this
report, a basic hydrologic vocabulary needs to be introduced. The
definitions presented here are summaries of more detailed discussions
in Heath (1983) and the Ground Water Manual published by the U.S.
Department of the Interior (1981). A detailed glossary is provided in
Appendix A.
2.1 Underground Water
Underground water is the term applied to all water beneath the land
surface. It occurs in two different zones, the unsaturated zone and
the saturated zone. The unsaturated zone contains both water and air
and it occurs at and immediately below the land surface. Beneath the
unsaturated zone is the saturated zone in which all interconnected
openings are full of water. The water table is the level in the
saturated zone at which the upward pressure exerted by the water is
equal to the downward pressure exerted by the atmosphere (i.e., the
level at which the hydraulic pressure is equal to atmospheric pressure).
Usually it can be thought of as the point below which all geologic
materials are saturated. Water in the saturated zone is the only
underground water that is available to supply wells and springs. It is
the only water to which the term ground water is correctly applied.
2.2 Aquifers and Confining Beds
All saturated geologic materials can be classified either as
aquifers or as confining beds. An aquifer is a geologic unit that
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will yield water in a usable quantity to a well or spring. A
confining bed is a geologic unit having low hydraulic conductivity,
relative to adjacent aquifers, that restricts the movement of ground
water either into or out of the adjacent aquifers.
Aquifers are classified as being either confined or
unconfined. An unconfined aquifer is one that does not have a
confining layer overlying it. Its upper boundary is the water table
and for this reason it is often referred to as a free or water table
aquifer or as being under water table conditions. Water at the
upper surface of an unconfined aquifer is in direct contact with the
atmosphere through the open pore spaces of the unsaturated zone. At
the water table the upward pressure exerted by the water in the
aquifer is in balance with atmospheric pressure. Typically, the
movement of the ground water in an unconfined aquifer is in direct
response to gravity.
A confined or artesian aquifer has an overlying confining layer
of lower hydraulic conductivity than the aquifer and has only an
indirect or distant connection with the atmosphere. In an artesian
aquifer, the water is under pressure such that when the aquifer is
penetrated by a tightly cased well or piezometer, the water will
rise above the bottom of the confining bed to an elevation at which
it is in balance with the atmospheric pressure and which reflects
the pressure in the aquifer at the point of penetration. If the
water level in an artesian well stands above the land surface, the
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well is a flowing artesian well. The imaginary surface which
conforms to the elevations to which water will rise in wells
penetrating artesian aquifers is known as the potentiometrie surface.
Special local conditions may exist within a hydrologlc regime
which give rise to a perched aquifer and perched water table. Above
the regional water table in some areas, hydrologic units with
relatively low hydraulic conductivities (e.g., silt, clay,
unfractured rock) may be surrounded by higher conductivity
material. Downward percolating water may be interrupted by the low
conductivity material forming a saturated zone of limited areal
extent. This saturated zone is a perched aquifer and its upper
surface is a perched water table. Unsaturated material lies between
the bottom of the saturated zone and the top of the regional water
table. A perched water table may be permanent or seasonal depending
upon the climatic conditions or overlying land use.
The types of aquifers and the terms associated with the ground
water regime are illustrated in Figure 2-1.
2.3 Heads and Gradients
Ground water moves from regions of higher potential energy to
regions of lower potential energy. In fluid mechanics the term head
describes the potential energy stored, in a fluid. Total head is
defined as the sum of the potential energy in a fluid which can be
attributed to the elevation, pressure, and velocity of the fluid.
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Well in
Unconfirmed
Aquifer
V
Non-flowing Well
in Confined Aquifer
Potentiometric Surface
Flowing Well
in Confined Aquifer
Confined Aquifer
Bedrock
Confining Layer
Source: Adapted from US. Department ot the Interior, 1981.
FIGURE 2-1
AQUIFER TYPES AND ASSOCIATED TERMS
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In ground water studies velocity head can be ignored because ground
water moves relatively slowly (Heath, 1983). The total head at a
well, then, is the sum of two components, elevation head and pressure
head. The total head can be determined by subtracting the depth to
water in a non-flowing well from the altitude of the measuring point.
Alternatively, the equation for total head (h ) is:
ht = Z + hp
where Z, the elevation head, is the distance from an arbitrarily
chosen datum plane (e.g., sea level) to the point where the pressure
head, h . is determined. This point usually corresponds to the
bottom of the well bore or the depth of the well screen. Figure 2-2
illustrates the components used to determine total head and hydraulic
gradient in an unconfined aquifer.
If all other factors remain constant, the rate of ground water
movement depends on the hydraulic gradient. The hydraulic gradient
(dh/dl) is the change in total head (also known as the head loss)
per unit of distance (1) in a given direction. Referring to
Figure 2-2 it may be expressed as:
= htl ~ ht2
Hydraulic Gradient (dh/dl)
1
2.4 Hydraulic Conductivity
In the 1850's a systematic study of the movement of water
through a porous medium led Henry Darcy to conclude that the rate of
water flow through a filter bed of a given nature is proportional to
the difference in the height of the water between the two ends of
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FIGURE 2-2
COMPONENTS USED TO DETERMINE TOTAL HEAD AND HYDRAULIC GRADIENT
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the filter bed and inversely proportional to the length of the flow
path. He also determined that the quantity of flow is proportional
to a coefficient, K, which is dependent upon the nature of the
porous media. The flow was also found to be proportional to the
cross-sectional area of the test bed. These relationships have come
to be known as Darcy's law and are shown symbolically as:
Q . KA (ha ~ V or Q - KA(dbYdl)
1
where:
Q « quantity of flow or discharge expressed as a volume per
unit time (L3/T)
A " cross-sectional area (L^)
ha = total head at well a (L)
eft
ht - total head at well b (L)
1 distance between the two wells (L)
K ^ coefficient of proportionality (the hydraulic
conductivity) (L/T)
dh " hydraulic gradient (L/L)
dl
The above equations can be rearranged to solve for K, the
hydraulic conductivity:
K Q
A(dh/dl)
Dimensional analysis shows that hydraulic conductivity is expressed
in units of velocity (or distance divided by time):
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[Q] , when expressed in units of length (L) and time (T)
[A][dh/dl] equals
, which reduces to L/T.
[L2][L/L]
The factors involved in the definition of hydraulic
conductivity include the volume of water that will move in a unit
time under a unit hydraulic gradient through a unit area. Using a
unit gradient to express the value of hydraulic conductivity (rather
than an actual gradient at some particular point in the aquifer)
allows for easy comparison of hydraulic conductivity for different
types of rocks.
The hydraulic conductivity of rocks ranges through 12 orders of
magnitude (Freeze and Cherry, 1979; Heath, 1983). Not only is it
different for different types of rocks but the hydraulic
conductivity may vary from place to place in the same rock. When
hydraulic conductivity is variable within an aquifer, then the
aquifer is said to be heterogeneous . Alternatively, if it is
essentially the same in all areas, then the aquifer is termed
homogeneous.
Also, at any place within an aquifer, the hydraulic
conductivity may be different in different directions. Under these
conditions the aquifer is termed anisotropic. If the hydraulic
conductivity is essentially the same in all directions, the aquifer
is said to be isotropic.
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In USGS Water Supply Paper 2220, Heath (1983) makes the
following statement:
Although it is convenient in many mathematical analyses
of ground water flow to assume that aquifers are both
homogeneous and isotropic, such aquifers are rare, if they
exist at all. The condition most commonly encountered is
for hydraulic conductivity in most rocks and especially in
unconsolidated deposits and in flat-lying consolidated
sedimentary rocks to be larger in the horizontal direction
than it is in the vertical direction.
Hydraulic conductivity, then, is a very site specific variable in
the flow equation.
2.5 Transmissivity and Storativity
Heath (1983) defines the transmissivity of an aquifer as the
rate at which water is transmitted through a unit width of an
aquifer under a unit hydraulic gradient. It equals the hydraulic
conductivity multiplied by the aquifer thickness.
Storativity (the storage coefficient) is a dimensionless number
which represents the volume of water released from storage in a unit
prism of an aquifer when the head is lowered a unit distance.
According to Heath (1983), for confined aquifers the storage
coefficient ranges between 0.00001 to 0.001. For unconfined
aquifers the range is from 0.1 to about 0.3.
Both of these factors are important in the analysis of data
from pumping tests (see Section 4.4.2) and are used to determine the
hydraulic conductivity of the aquifer being tested.
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3.0 GEOHYDROLOGIC DATA ELEMENTS IN THE HAZARD RANKING SYSTEM
3.1 Overview of the HRS Requirements Pertaining to the Ground Water
Migration Route
A complete description of the HRS appears as Appendix A to the
National Contingency Plan (40 CFR 300) as published in the Federal
Register on July 16, 1982 (47 FR 31219). The HRS assigns three
scores to a site: migration, fire and explosion, and direct contact.
The ranking of sites as national priorities for further investigation
and possible remedial action is based primarily on the site migration
score. The scores for fire and explosion and direct contact are
used in identifying sites requiring emergency attention.
The site migration score reflects the potential for harm to
public health, welfare, or the environment by the migration of
hazardous substances away from the site. It is a composite of three
separate scores for three possible migration routes: ground water,
surface water, and air. A migration score for each applicable route
is first calculated by evaluating the site with respect to a number
of factors that characterize (a) the hazardous substances at the
site, (b) the containment of the hazardous substances, (c) the
potential for migration of the hazardous substances from the site by
that route, and (d) the presence and proximity of targets (i.e.,
human populations or sensitive ecological systems or environments).
The individual route scores are then combined to give an overall HRS
site migration score. The scoring system is structured so that HRS
site migration score can range from 0 to a maximum of 100. The
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higher the score the higher the relative threat ascribed to the site.
Under present policy, sites eligible for the National Priorities List
that have HRS scores greater than or equal to 28.50 are included on
the National Priorities List.
This study focuses on the ground water migration route and
particularly on those factors that pertain to ground water route
characteristics and targets. Three current HRS rating factors are
based on the geologic or hydrogeologic conditions at the site and
are used to evaluate the likelihood of contaminant migration via the
ground water route. These factors are as follows:
The depth to the aquifer of concern.
The permeability of the unsaturated zone or the intervening
geologic material.
The distance to the nearest well that draws water from the
aquifer of concern.
Other ground water related factors considered in the evaluation of
a site are the use of ground water (e.g., drinking, Industrial,
irrigation) and the population potentially threatened through regular
use of ground water. Each of these factors has unique informational
or data requirements as described in the following sections.
3.2 Depth to the Aquifer of Concern
When the ground water migration path is evaluated on the basis
of route characteristics, the depth to the aquifer of concern is one
of the principal scoring factors. The depth to the aquifer of
concern is defined as the distance between the lowest known point at
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which contaminants are present at the site and the top of the aquifer
which serves as a source of water for the target(s).
The key term is "aquifer of concern" which, for the purpose of
the HRS, is the aquifer which yields the highest HRS score for the
ground water pathway. At many sites, more than one aquifer is
present and potentially may be affected by releases from the site.
A common condition at a site is to observe a release of contaminants
to a surficial aquifer which serves as a source of supply for a
relatively small number of private wells. Often a deeper aquifer,
which serves as a source of supply for several community wells, is
also present in the site area and has the potential to be
contaminated by pollutants migrating through the intervening geologic
material. Therefore, data on both aquifers (in some cases it may
be three or more aquifers) and the intervening geologic material
needs to be evaluated separately in order to determine which of
these aquifers yields the highest HRS ground water route score for
the site. This is described more fully in Section 3.2, and is
illustrated in Figure 3 of Appendix A to the NCP (40 CFR 300;
47 FR 31226; July 16, 1982).
Identifying the aquifer of concern, and thus determining
the depth to the aquifer of concern, frequently requires more
hydrogeologic information than any other migration factor in the
ground water pathway. The data needed to clearly identify the
aquifer of concern at most hazardous waste sites are similar to the
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data needed for a complete hydrogeologic site characterization as
described by Freeze and Cherry, 1979; the Office of Technology
Assessment, 1984; and the U.S. Environmental Protection Agency,
1985a, among others.
Identification of the aquifer of concern begins with the
collection and examination of available reports, maps, and data.
Geologic maps and reports indicate the geologic formations in the
site area, together with their stratigraphic (i.e., the number and
types of rock units present) and structural relationships.
Topographic maps along with soils or surflclal geology maps provide
information on the genesis and distribution of unconsolidated
deposits and landforms. Hydrogeologic maps usually provide an
interpretation of the ground water flow characteristics, considering
the topographic, geologic, hydrogeologic, geochemical and water
resource data available for the area.
Air photo interpretation may also be used to prepare maps of
landforms, soils, land use, vegetation, and drainage in the vicinity
of a site. Each of these environmental features leads to Inferences
about the natural ground water flow systems and/or the presence of
potential aquifers.
The information described in the previous paragraphs relate for
the most part to surface geology. However, in order to identify the
ground water regime in the vicinity of a site (i.e., in order to
identify all potentially affected aquifers), it is also necessary to
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compile information on the subsurface geology. Ideally, on-site
boring logs would be used to depict the subsurface stratigraphic
relationships. The more usual case in the application of the HRS to
a site is to first use available records rather than boring logs.
Published geologic maps and available well log records can be used
to determine the local lithology (i.e., the physical characteristics
of the rock units), stratigraphy, and structure. Similar information
on a regional scale can help establish the context in which the local
geology is interpreted. Depending upon the availability of the data,
it may be possible to produce a wide variety of visual aids to
characterize a site. These could include stratigraphic cross
sections, geologic fence diagrams, isopach maps of overburden or
formation thickness, and lithofacies maps. Hydrogeologic information
might include water table contours and isopachs of saturated
thickness of unconfined aquifers.
The identification of the aquifer(s) of concern is of critical
importance to the HRS evaluation of the ground water migration route.
From the above discussion, it is apparent that initially a site
inspection needs to draw on a considerable amount of existing
information in order to characterize the surface and subsurface
geological conditions.
3.3 Permeability
In the HRS, permeability is another factor which is used when
the ground water migration path is being evaluated on the basis of
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route characteristics. As explained in Appendix A to the NCP
(40 CFR 300; 47 FR 31224, July 16, 1982), the permeability being
evaluated is either that of the unsaturated zone (when the aquifer
of concern is the uppermost aquifer) or that of the intervening
geological formations (when the aquifer of concern is a deeper,
confined aquifer). This factor is used as an indicator of the speed
at which a contaminant could migrate from a site. It is assigned a
value based on the least permeable continuous layer below the site.
A table entitled "Permeability of Geologic Materials" (Table 2
in Appendix A to the NCP) is used to select a rating value which
corresponds to the type of material beneath the site and its
approximate range of hydraulic conductivity. This table was derived
from Davis (1969) and Freeze and Cherry (1979). In this table the
terms "permeability" and "hydraulic conductivity" are used
interchangeably. The hydraulic conductivity depends upon the size
and arrangement of the fluid transmitting openings within the
geologic media (its specific or intrinsic permeability) and on the
dynamic characteristics of the ground water (i.e., kinematic
viscosity, density, and the strength of the gravitational field).
Usually, for HRS scoring purposes, qualitative data gathered
during a site inspection are used to define the stratigraphic
sequence at a site. Typically, the lithology is described based on
well logs from local or regional hydrologic studies. It is these
descriptions that form the basis for the evaluation of the hydraulic
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conductivity of the least permeable continuous layer which is used
to assign a value to the HRS permeability factor.
3.4 Ground Water Use
This rating factor indicates the use made of ground water
drawn, within three miles of the site boundary,* from the aquifer of
concern. A value of zero is assigned if the ground water is
evaluated as being unusable (e.g., a saline aquifer, or extremely
low yield). Intermediate factor values include both non-drinking
use (e.g., commercial, industrial, or irrigation) and drinking water
use with unthreatened alternate supplies readily available. A
maximum value is assigned to this factor when the aquifer of concern
is the only source of supply for drinking water.
In those cases where several separate aquifers exist beneath a
site, each aquifer must be evaluated separately on the basis of the
information that pertains to the particular aquifer. For example,
if two separate aquifers are present beneath a site, the HRS
precludes evaluating the ground water use based on one aquifer if
the other scoring factors are going to be based on a different
aquifer. This is explained more fully in Appendix A to the NCP
(40 CFR 300; 47 FR 31189, July 16, 1982).
Also, ground water use is evaluated in the HRS as it existed
prior to the occurrence of any remedial actions at the site and as
*The site boundary is the term used here to describe the edge of the
area of known contamination attributable to the site (Section 3.5
of Appendix A to the NCP, 40 CFR 300; 47 FR 31230, July 16, 1982).
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it existed before the effects of pollution attributable to the site
may have caused changes in usage patterns. For example, a domestic
well, abandoned because of contamination from the waste site, would
still receive the maximum ground water use score If it was a sole
source of supply prior to being contaminated and abandoned.
3.5 Distance to the Nearest Well
The distance to the nearest well factor is used In the HRS
as a measure of the likelihood of exposure of a population (see
Section 3.6) to contaminated ground water. The distance is measured
from the site boundary (i.e., the edge of the area of known
contamination attributable to the site) to the nearest well drawing
drinking or irrigation water from the aquifer of concern. There are
five distance intervals associated with this factor. The intervals
are ordered so that the factor value decreases as the distance
increases. The decreasing value of the factor serves as a surrogate
for attenuation of contaminants in ground water as it migrates to a
potential target. The highest value is assigned if the well is
within 2000 feet of the site boundary. The next highest value Is
assigned for a distance interval of 2001 feet to one mile. The
scoring values decline to zero as the distance increases, in one
mile increments, to over three miles.
In order to evaluate this factor it is necessary to identify
the nearest well to the site drawing drinking and/or irrigation
water from each aquifer. The depth of each well should be documented
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to show that it is actually completed in the aquifer to which it is
assigned. This factor is used in combination with another factor,
the population potentially threatened.
3.6 Population Potentially Threatened
This factor is used in the HRS to indicate the population
potentially threatened by regular use of ground water drawn from the
aquifer of concern. It includes residents, workers, and students.
Transient populations, such as customers or travelers passing
through an area, are excluded.
The potentially threatened population is that population using
water drawn from wells within three miles of the site boundary.
Detailed information about well locations, well construction, and
the water distribution system is needed in order to evaluate this
factor. Depending upon the nature of the water distribution system,
the population being counted may itself be in areas beyond the
three-mile limit, providing the water used by the population is
drawn from wells within the three-mile limit. Similarly, people
within three miles of the site boundary who do not use water from
the aquifer of concern are not counted. Six population ranges are
used to evaluate this factor with a population of greater than
10,000 individuals receiving a maximum value.
The distance to the nearest well and the population potentially
threatened are combined into a single factor which is used as a
measure of the risk to public health presented by the migration of
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contaminated ground water. The combined factor assesses the
likelihood of exposure and the number of people who might be exposed.
A complete description of the use of the combined factor appears in
Section 3.5 of Appendix A to the NCP (40 CFR 300; 47 PR 31230,
July 16, 1982).
3.7 Summary
The migration of contaminants via ground water is one of the
principal areas of concern in the HRS. In evaluating the ground
water migration route, several geohydrologic factors are
considered. These include identification of the aquifer of concern
and a determination of its use for drinking or non-drinking
purposes. The distance from the known extent of contamination to
the nearest point of withdrawal is also determined, as is the
population that would regularly use ground water derived from the
aquifer of concern within three miles of the site boundary.
Depending upon which aquifer is the aquifer of concern, the
hydraulic conductivity of the unsaturated zone or of intervening
geologic formations is also evaluated. The data used to evaluate
these factors are derived from existing information and the results
of field studies performed during site Inspections.
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4.0 DETERMINING THE DIRECTION OF GROUND WATER FLOW
4.1 Overview of the Issue
Determining the ground water flow regime in the vicinity of a
hazardous waste site has been cited as being important for the
identification of the target area potentially threatened by the
site. This chapter describes the methodology generally used for
determining the direction of ground water flow and identifies
important sources of uncertainty in the determination.
4.2 General Considerations
An isotropic aquifer provides the simplest example for
determining the direction of ground water flow. Under isotropic
conditions, flow is parallel to the direction of the greatest
decrease in the potential field (i.e., total head). The direction
of ground water movement and the hydraulic gradient can be determined
if the following data are available for three wells located in any
triangular arrangement in the area of study (Heath, 1983):
The relative geographic position of the wells.
The distance between the wells.
The total head at each well.
An illustrative example of how the data are used is provided in
Appendix B. As mentioned previously, however, isotropic aquifers
are rarely, if ever, encountered. Anisotropic aquifers are the more
common condition, and in these aquifers ground water flow is usually
not parallel to the hydraulic gradient.
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Fetter (1981) points out that in the majority of field
situations, the direction of ground water movement is a function of
two variables. The first is the potential field of the ground water
flow system and the second is the degree of anisotropy and the
orientation of the axes of permeability.
Therefore, directional hydraulic conductivity and hydraulic
potential (i.e., hydraulic head) are the principal parameters which
must be measured in the field in order to determine the direction of
flow in anisotropic aquifers.
4.3 Measuring Hydraulic Potential
4.3.1 Data Requirements
The direction of ground water flow is a function of both
horizontal and vertical components. Fetter (1981) describes the
procedure for resolving the directional components of ground water
flow. Similar to the example shown in Appendix B, the horizontal
components can be determined from three tightly cased boreholes.
These boreholes need to be located in the same aquifer in such a
manner that they form a right triangle, and they must all be
completed to the same elevation above a horizontal datum (usually
mean sea level). For the vertical component, a fourth borehole,
located adjacent to the borehole at the right angle of the triangle}
needs to be constructed and completed to a deeper part of the same
aquifer. The three components can then be resolved into a resultant
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vector. Fetter suggests that a typical horizontal spacing would be
100 feet for each leg of the triangle.
A single determination of ground water flow direction at a
single location and at a single point in time, then, requires some
prior knowledge of the hydrogeology of the site. This is necessary
for completing the three shallow piezometers at the same depth in
the same aquifer and is especially necessary for setting the vertical
piezometer within the same aquifer. Similarly, the locations of the
piezometers need to be accurately surveyed, including an indication
of the elevation of the measuring point (usually the top of the
casing). Water level measurements need to be taken simultaneously
(or as nearly as possible). Depending upon the nature of the
measuring and recording devices, water level measurement accuracies
of one hundredth of a foot are achievable (Everett, 1980, among
others).
4.3.2 Data Density
The direction of ground water flow can best be determined by
compiling a map of the potentiometric surface in an aquifer. Where
multiple aquifers are present, a potentiometric map will be needed
for each. Perhaps the single most important issue is the number of
data points needed to accurately characterize the potentiometric
surface at a site.
The scheme described by Fetter (1981) employs a 100-foot
horizontal spacing to derive one data point from a 5,000 square-foot
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area. At this level of detail, as many as nine data points could be
required per acre. (There are 640 acres per square mile.) An
alternative scheme has been suggested in the draft version of the
RCRA Ground Water Monitoring Technical Enforcement Guidance Document
(U.S. Environmental Protection Agency, 1985b). In this report an
initial 300-foot spacing is suggested; this spacing may be decreased
depending upon preliminary results and the need for greater detail.
According to this guidance, then, the number of boreholes needed at
a site could range between one and three per acre. R.W. deary
(personal communication, 1985) suggests that a 5 to 10 acre site
would require as many as 10 to 20 wells to characterize the flow
regime. This well density is comparable to the range specified by
the U.S. Environmental Protection Agency (1985b).
Appendix D provides estimates of the cost and time requirements
for installation of ground water wells and for collection and
analysis of data to determine the direction of ground water flow.
For a field program consisting of ten wells, representative total
cost for well installation and for data collection and analysis are
estimated to be $21,000 for 20-foot wells, $50,500 for 100-foot
wells, and $92,000 for 200-foot wells (see Table D-2 of Appendix D).*
Representative total time requirements for well installation and
*Based on the data presented in Table D-l of Appendix D, the
estimated range of total costs is $6,600 to $54,000 for 20-foot
wells, $20,000 to $329,000 for 100-foot wells, and $29,000 to
$635,000 for 200-foot wells.
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for data collection and analysis are estimated to be 4 weeks for
20-foot wells, 6 weeks for 100-foot wells, and 16 weeks for 200-foot
wells. It should be noted that wells installed for determining the
direction of flow could also be used for other purposes such as the
collection of ground water samples and the determination of
hydraulic conductivities (see Section 4.4). When this is done,
there would not be additional well installation costs for these
other activities.
4.3.3 Data Constraints
Fetter (1981), the Office of Technology Assessment (1984), and
the U.S. Environmental Protection Agency (1985a) cite several sources
of uncertainty associated with the determination of the direction of
ground water flow. One of these is the frequency with which ground
water level measurements are taken. The ground water flow regime is
subject to seasonal and temporal forces which may produce either
short-term or long-term variations in ground water levels and flow
patterns. The sources of these variations may include on-site or
off-site well pumping, intermittent natural processes such as tidal
processes or changes in river stage, and changes in land use
patterns. At any particular site, then, a one-time determination of
ground water levels is not likely to provide a reliable picture of
the ground water regime because any seasonal or temporal variations
would not be detected. For this reason, local conditions must
dictate the frequency of water level measurements.
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Another data constraint concerns measurement uncertainties
related to whether a site is in an area of recharge or discharge.
In such areas there can be erroneous interpretations of ground water
level measurements as described below.
A recharge area is defined by decreasing hydraulic head with
depth indicating downward flow. In an open (i.e., uncased) borehole
in a recharge area, ground water is free to move into the borehole
from any of the upper (higher hydraulic head) pores or joints above
the bottom of the borehole and then to move into regions of lower
hydraulic head within the borehole. In a cased borehole (e.g.,
piezometer), ground water is not free to move into the borehole from
the upper pores or joints and then to move to regions of lower
hydraulic head within the borehole; ground water can only move into
the borehole at the point at which the screen is set. Water levels
in an open borehole will therefore be higher than water levels in a
piezometer completed to a discrete, but the same, depth in a recharge
area.
Similarly, a discharge area is defined by an increasing
hydraulic head with depth. If piezometers set at different depths
within an aquifer indicate increasing heads with depth, then an
upward flow condition exists. In an open borehole in a discharge
area, ground water will move up the bore and out of the borehole
through pores or Joints having a lower hydraulic potential. Hie
artesian head will be relieved in the open borehole and the composite
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water level will be lower than in a cased borehole (e.g., piezometer)
completed at the same depth as the open borehole because the water
cannot move out of the cased borehole.
Ground water table or potentiometric surface maps based on
erroneous interpretations of ground water level measurements in
recharge or discharge areas can thus lead to misinterpretations of
the ground water flow regime in the area potentially affected by a
waste site.
Another source of measurement uncertainty has been described by
Saines (1981) as a back water effect in a discharge area. According
to this theory, sudden increases in water levels in piezometers in a
river valley after heavy rains and a rise in river stage have been
incorrectly ascribed to infiltration of rain water or flood water,
even though some of the piezometers may be over 330 feet deep. In
many cases the increase in the water levels in the piezometers is
due to the increased pressurization on the system caused by the
increased elevation of the discharge point. Again, this is a
temporal effect.
At some sites, the characteristics of the hazardous substances
themselves cause uncertainty regarding the direction of contaminant
migration. Hazardous substances (including leachate) which are more
dense or less dense than water may not follow the same flow path as
ground water. Their movement may instead be more controlled by
geology and gravity. For these contaminants, analysis of water level
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and piezometric data cannot adequately predict their transport
direction because the analysis assumes a single density fluid which
moves in the direction of ground water flow. Dense contaminants
may, however, move at an angle to the normal ground water flow
direction or vertically downward until they encounter a relatively
impermeable layer. At that point, they may move by gravity along
the contact between the aquifer and the impermeable material. Ihis
may be either in the general direction of ground water flow or
opposite to the ground water flow direction. Contaminants that are
less dense than water may "float" on top of ground water and move
randomly along the interface between the saturated and unsaturated
zone. Figure 4-1 illustrates a possible migration path for
contaminants more dense and less dense than water.
4.4 Measuring Hydraulic Conductivity
There are two types of field tests which can be used to measure
hydraulic conductivity: piezometer tests and pumping tests.
4.4.1 Piezometer Tests
Hydraulic conductivity values can be determined in the field by
means of tests carried out in a single piezometer. The water level
in the piezometer is suddenly changed by either introducing water (a
slug test) or removing water (a bail test) in the water column. The
recovery of the water level with time is then observed. The same
effect can be created by suddenly introducing or removing a solid
cylinder of known volume from the water column. According to the
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Source of Contaminant
(Greater Density Than Water)
Source of Contaminant
(Lesser Density Than Water)
Unsaturated
Zone
Waterjab|e_
Direction
of Ground Water Flow
. Flow of Dissolved
Contaminant
Saturated
Zone
Source: Adapted from Office of Technology Assessment, 1984.
FIGURE 4-1
POSSIBLE FLOW PATH OF CONTAMINANTS
MORE DENSE VERSUS LESS DENSE THAN WATER
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Practical Guide to Ground Water Monitoring (U.S. Environmental
Protection Agency, 1985a), another technique exists in which the
water level is depressed by pressurizing the well casing. The
pressure is rapidly released, and the recovery of the water level is
observed.
Slug/bail tests are suitable for relatively low conductivity
settings where the resultant changes in water levels take place
slowly and measurements accurate to one hundreth of a foot can be
made. For a slug test, water of a different quality than that in
the aquifer may be introduced into the system. This water must be
removed prior to any sampling of water quality in the well. For a
bail test, in a well where hazardous constituents are suspected to
be present, water removed from the well needs to be managed in an
environmentally protective manner. A single slug/ball test in a
30-foot thick confined aquifer, approximately 100 feet below the
surface, is estimated to cost between $300 and $1,600 (exclusive of
drilling costs) and to take between 3 and 18 hours (see Table D-3 of
Appendix D).
A significant limitation of slug and bail tests is that they
are heavily dependent on a high quality piezometer intake. Corroded
or clogged intakes may yield highly inaccurate measurements.
Alternatively, if the piezometer was developed by surging or
backwashing prior to testing, the values may reflect artificially
induced increased conductivities.
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The pressurization technique minimizes the disturbance of the
well, and it has the least potential for compromising the integrity
of water quality samples. It can also be used for conducting tests
on wells with very high hydraulic conductivities when pressure
transducers are used for the water level measurements. A single
pressurization test in a 30 foot aquifer, approximately 100 feet
below the surface, is estimated to cost $500 to $2,200 (exclusive
of drilling costs) and to take 4 to 18 hours (see Table D-3 of
Appendix D).
Freeze and Cherry (1979) describe two methods for analyzing and
interpreting the water level versus time data that arise from slug
or bail tests. The simplest interpretation is that of Hvorslev which
is applicable for tests in a point piezometer (i.e., a piezometer
open only at one point). For a bail test Hvorslev reasoned that the
rate of inflow at the piezometer tip at any time is proportional to
the hydraulic conductivity of the geologic media and the unrecovered
head difference. A detailed description of the method of analysis
can be found in Freeze and Cherry (1979). According to these
authors, Hvorslev also developed formulas for anisotropic conditions
and for special conditions affecting the physical placement of the
piezometer openings.
The second test interpretation procedure described by Freeze
and Cherry (1979) applies to piezometers that are open over the
entire thickness of a confined aquifer. It assumes that the aquifer
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is horizontal, confined between impermeable formations, infinite in
horizontal extent, of constant thickness, and that hydrogeologically
it is homogeneous and isotropic. A graphical curve matching procedure
is used to determine the aquifer parameters of transmissivity and
storativity. Ihe hydraulic conductivity can then be determined by
dividing the transmissivity by the thickness of the aquifer.
4.4.2 Pumping Tests
A pumping test is made by pumping a well for a period of time
and observing the change in hydraulic head in the aquifer. These
tests are specifically well suited to determining the transmissivity
and storativity in confined and unconfined aquifers. Pumping tests
provide in-situ measurements that are averaged over a large aquifer
volume.
The mathematical principles behind pumping tests have been
described by Freeze and Cherry (1979), Bouwer (1978), Fetter (1980),
and Heath (1983), among others. For a given pumping rate, if the
transmissivity and storativity of an aquifer are known, it is possible
to calculate the time rate of drawdown (i.e., change in potential
level versus time) at any point in the aquifer. This response
depends solely on the values of transmissivity and storativity.
Therefore, measured values of drawdown versus time, at some
observational point in an aquifer, can be used to work backwards
through the equations to determine the values of transmissivity and
storativity. The hydraulic conductivity at the observation point can
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then be calculated from the transmissivity by dividing the
transmissivity by the aquifer thickness.
Freeze and Cherry (1979) point out that a single pumping test
can provide information on both the hydraulic conductivity and
storage properties of an aquifer. Also, important leakage properties
of an aquifer system can be investigated if observations are made in
both the aquifers and the potential confining layers.
The same authors, however, indicate that there are two major
disadvantages to conducting pumping tests. The principal technical
concern is that the interpretation of pumping test results is not
unique. Freeze and Cherry (1979, p. 376) illustrate that there is a
marked similarity in the time-drawdown response that can arise from
leaky, unconfined, and bounded systems. The authors state that "The
fact that a theoretical curve can be matched by pumping test data in
no way proves that the aquifer fits the assumptions on which the
curve is based."
Similarly, Driscoll (1986, pp. 559-579) presents several
examples of pumping tests set in different types of environments.
Driscoll draws alternative conclusions for each of these examples to
illustrate that pumping test data can be interpreted in more than
one way.
The second disadvantage which is cited is that pumping tests
are expensive. As shown in Appendix D (Table D-3), a single pumping
test for a 30-foot thick confined aquifer, approximately 100 feet
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below the surface, Is estimated to cost between $10,000 and $15,000
if drilling is not necessary and to take 10 to 15 days. If drilling
is necessary, the costs of pumping test is estimated to be about
$40,000. The costs associated with site security, drilling spoils
and pumped water disposal, and increased contractor costs for working
in a potentially hazardous environment are not included in the cost
estimate. The cost to test a two aquifer system is estimated to
range between $13,000 and $29,000 if drilling is not necessary and
to take 20 to 25 days (see Table D-3 of Appendix D). If drilling is
necessary, the total cost of the test is estimated to range between
$68,000 and $110,000.
Freeze and Cherry suggest that pumping tests should be used only
in cases where it is anticipated that the aquifer will be developed
for water supply purposes. They further state that it is usually
inappropriate to use pumping tests for geotechnical applications,
contamination studies, or regional flow analyses.
A third disadvantage is that the construction and placement of
pumping and observation/monitoring wells must be done with extreme
care. Driscoll (1986) notes that even under non-pumping conditions
poorly constructed or abandoned wells may serve as conduits for
contaminant migration.
4.4.3 Data Constraints
The limitations of slug and bail tests and the advantages
and disadvantages of pumping tests have already been described.
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Furthermore, as part of this study, a representative sample of the
data submitted to EPA in support of RCRA Part B permit applications
was reviewed. The data showed that even within one stratigraphic
unit the range of reported values of hydraulic conductivity could be
several orders of magnitude. Other sources of uncertainty in the
data reviewed included average values outside the range of reported
measured values and standard deviations as great as the range of
values presented in the field data. It would appear, then, that
field determinations of hydraulic conductivity are at present costly,
unreliable, and subject to reporting and/or interpretive errors.
4.5 Findings
As it is presently designed, the Hazard Ranking System relies
on a large amount of existing data to characterize the hydrogeologic
regime at hazardous waste sites. The additional data that are needed
to address the issues that have been raised concerning direction of
ground water flow are primarily field measurements of potential head
and hydraulic conductivity.
In order to evaluate the ground water regime in the area
potentially affected by the site, the data requirements imply a
significantly increased field effort, in terms of cost and time
requirements, compared to the current site inspection program. The
increased field effort is needed both to evaluate the ground water
regime at any one point in time and to account for time dependent
changes in the ground water regime. Typically, a one year cycle of
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water level measurements may be necessary to adequately characterize
flow conditions at a site.
Sections 4.3 and 4.4 discuss the increased cost and time
requirements associated with measurements of potential head and
hydraulic conductivity. As indicated, representative costs for a
ten well program for measuring the potential head are estimated to
range from $21,000 (for 20-foot wells) to $92,000 (for 200-foot
wells). Representative time requirements for well installation and
data collection and analysis are estimated to range from 4 weeks
(for 20-foot wells) to 16 weeks (for 200-foot wells). Hydraulic
conductivity testing is estimated to cost an additional $300 to
$2,200 for a single slug/bail test or pressurizatlon test (assuming
no additional wells need to be installed) and to take an additional
3 to 18 hours. If a pumping test is done instead to evaluate
hydraulic conductivity, the additional cost for a single pumping
test is $10,000 to $15,000 for a single aquifer and $13,000 to
$29,000 for a two aquifer system (assuming no additional wells need
to be installed). A pumping test is estimated to require 1.5 to
2 weeks in the former case and 3 to 3.5 weeks in the latter case.
Furthermore, the increased data collection effort does not
necessarily always translate into more accurate knowledge about
conditions at the site. Each of the additional data elements
needed to assess the direction of ground water flow has attendant
uncertainties as described in detail in Sections 4.3 and 4.4.
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Consequently, the use of these data elements in evaluating the
direction of ground water flow at a site would introduce different
types of uncertainties into the HRS process than those that are
currently present as a result of the direction of flow not being
considered. The primary concern would be in underestimating the
target area potentially threatened by the site. The extend to which
this could be a significant problem would depend upon site specific
conditions, the amount of data collected both temporally and
spatially, and the uncertainties inherent in the measurement methods.
In addition to the data reliability issues, these uncertainties also
create an added quality control/quality assurance burden because
judgments need to be made based on interpretations of the data.
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5.0 AQUIFER INTERCONNECTION
5.1 Overview of the Issue
Commenters have noted that the separation (or, conversely, the
connection) of hydrologic units is not defined in the HRS. EPA
personnel, contractors, and commenters have suggested that a
definition is needed so that a judgment can be made as to when two or
more water bearing units function as a single hydrologic unit. This
issue focuses on the vertical migration of contaminated ground water.
As discussed in Chapter 3, both the definition of the aquifer of
concern and the identification of the target population are affected
by the number of hydrologic units that are present in the vicinity of
a disposal site and the degree of connection between multiple water
bearing units.
5.2 Data Requirements
A primary piece of information needed to evaluate the connection
(or lack of connection) between two or more water bearing units is
the areal extent of confining layers beneath a disposal site. As
described in Chapter 3, evaluation of the ground water migration
pathway generally requires that a large body of existing information
be collected and analyzed in order to define the hydrogeologic
environment at the site under investigation. The collected
hydrogeologic information should help define both the lateral and
vertical extent of potential confining layers.
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A second key data element needed to determine the degree of
aquifer connection is the hydraulic conductivity of adjacent
strata. Fetter (1981) illustrates how the direction of ground water
flow is affected by the degree of anisotropy in the aquifer. Ratios
of horizontal to vertical hydraulic conductivity on the order of 100
or less require that the lateral, transverse, and vertical components
of flow be resolved into a resultant vector. As he points out in his
1980 textbook (Fetter, 1980), differences of more than two orders of
magnitude between horizontal and vertical hydraulic conductivity are
not uncommon. This finding was reiterated by Heath (1983). Under
these conditions the direction of flow would be in the direction of
greatest hydraulic conductivity.
This criterion is used at the U.S. Environmental Protection
Agency's Robert S. Kerr Environmental Laboratory when the placement
of injection wells is being evaluated (Thornhill, personal
communication, 1985). A real difference of two orders of magnitude
or more in the hydraulic conductivities of two successive strata is
considered sufficient to retard vertical migration. As mentioned in
Section 4.4, however, there are several uncertainties associated
with the direct measurement of hydraulic conductivity. These
include unreliable pumping test design and implementation, variable
interpretations of test results, and reporting errors. Field
determinations of hydraulic conductivity may be of questionable
value for making comparisons between different strata for reasons
discussed in Section 4.4.
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5.3 Findings
On the subject of aquifer interconnection, guidance can be
developed that can be incorporated into a revised HRS. The guidance
could establish that the hydrogeology of the site, and/or
contrasting values of hydraulic conductivity, should be used for
determining the degree of interconnection between aquifers. In so
doing, a decision would need to be made regarding the requirements
to determine hydraulic conductivity. Field determinations of this
parameter are both time consuming and costly (see Section 4.4), and
uncertainties are introduced at all levels of data aquisition,
analysis and interpretation. If the HRS continues to serve as a
screening tool intended to use only limited data, then qualitative
assessments of the hydraulic conductivity, based on descriptions of
the geologic units, are the recommended requirement.
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6.0 THE THREE-MILE RADIUS
6.1 Overview of the Issue
During public review and comment on both the HRS and the
National Priorities List, comments were received regarding the
three-mile radius used in the HRS to evaluate ground water targets.
Some commenters indicated that three miles is an overestimate of the
potential migration distance of contaminants from hazardous waste
sites while other commenters stated that it is not far enough.
In order to address this issue, data related to contaminant
plume size were collected from the open literature and other
available sources. Also, a general range of ground water velocities
was used to estimate annual migration distances. The following
sections describe the available data.
6.2 Data Related to Migration Distance
6.2.1 Survey of Contaminant Plume Geometries
As part of a 1985 study for the U.S. EPA Office of Ground Water
Protection, the consulting firm of Geraghty and Miller, Inc. prepared
a survey of 50 contaminant plumes and their geometries (i.e., length,
width, depth). The raw data is reproduced in Appendix C. Analysis
of the plume survey data has led to the following conclusions:
As of the time of observation, the average plume length is
greater than 2,800 feet and the maximum reported length is
18,000 feet.
There is little correlation in the reported data between
plume length and gradient (see Appendix C).
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Over ten percent of the plumes appear to be circular or
semi-circular (i.e., length equals width).
Data gaps limit further meaningful study of these plumes;
details of their age, content, and the nature of the source
(continuous or intermittent) are not provided.
The data does not show which plumes have stablized, nor does it
indicate which plumes discharge to surface water. The survey,
however, illustrates that plume geometries are highly variable and
site-specific.
6.2.2 Additional Illustrations of Contaminant Plume Size
In their 1974 paper "Leachate Plumes in a Highly Permeable
Aquifer," Kimmel and Braids present an Investigation of a landfill
located on sand and gravel on Long Island, New York. They delineated
a leachate plume which is more than 10,000 feet long and has reached
a depth of more than 160 feet.
In their chapter on ground water contamination, Freeze and Cherry
(1979) cite the example of a landfill located on a moderately
permeable, glaciodeltaic sand aquifer. A large plume of leachate
contaminated water has penetrated deep into the aquifer (approximately
65 feet) and has moved laterally one to two thousand feet in the
direction of ground water flow. They point out that contamination in
this landfill developed over a period of 35 years and that leachate
will continue to be produced as water infiltrates through the
landfill. The zone of contamination is expected to expand.
In a 1984 report prepared for the U.S. Environmental Protection
Agency, Geraghty and Miller, Inc. evaluated a number of surface
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impoundments and their effects on ground water quality. Several case
histories are cited below to illustrate certain unique characteristics
of ground water contamination.
At a site in Oregon, timber and wood products wastes were
disposed in a shallow alluvial sand and gravel deposit which supplied
nearby domestic wells. In August 1972, "only a few weeks after
disposal operation began," a plume of contaminated ground water had
migrated 1,000 feet downgradient and had contaminated 11 domestic
wells. The affected area covered about 4 acres. By January 1973,
the plume had advanced to a point about 1,500 feet from the disposal
pit and the affected area had increased to about 15 acres.
In the Las Vegas-Henderson area of Nevada, the case history data
provides a comprehensive picture of a long-term, multi-source
contamination problem. Much of the contamination in the ground water
is derived by seepage from industrial waste impoundments and, to some
extent, from municipal waste impoundments. The waste generating
facilities include a major industrial complex housing several
companies engaged chiefly in metal refining and the manufacture of
chemical products. The other waste generating facilities in the area
include four sewage treatment plants. The companies at the industrial
complex began operation in 1946 after the U.S. Army ceased using the
area for disposal of wastes from the manufacture of magnesium
products. In 1971, a plume of nitrate-rich ground water in the near-
surface aquifer extended downgradient to a distance of 3 to 4 miles.
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In yet another example, between 1953 and 1959 a pulp and paper
mill at Brokaw, Wisconsin, discharged sulfite waste liquors into a
six-acre percolation pond on an island in the Wisconsin River.
Migration in the aquifer was reported by the company to be 3,300 feet
downgradient in 11 years (equivalent to 0.8 feet per day). The
contaminant plume, which was composed of spent liquor of high
specific gravity, migrated beneath the Wisconsin River to contaminate
supply wells southeast of the percolation ponds. Barrier wells were
constructed in the mid to late 1960's to prevent further migration
of the plume and to remove the spent liquors from the aquifer.
A case history from Long Island, New York, describes the growth
of a contaminant plume since the early 1940's. In 1942, plating
wastes from an aircraft manufacturing plant in South Farmlngdale
were observed in the water table aquifer. By 1948, a domestic well
1,500 feet from the disposal basin was found to be contaminated (a
migration rate of 0.7 feet per day). In 1962 the plume was about
4,200 feet long, with an average width of 750 feet. In this latter
phase the longitudinal migration rate had diminished somewhat to
0.53 feet per day.
Migration rates for contaminant plumes range between 0.5 and
1.0 foot per day for these latter four case histories reported by
Geraghty and Miller. Further, the case histories show a range of
plume lengths from a few hundred feet to beyond three miles. Also,
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based on these limited data, it appears that the age of the disposal
facility may be related to the length of the plume.
6.3 Flow Velocity and Migration Distance
Over the course of several NPL notice and comment rulemaking
cycles, several commenters have stated that hydraulic gradient data
could be used to determine ground water migration velocities which,
in turn, could be used to calculate expected migration distances at
specific sites. In this section the governing equations are defined
and the data needs are identified.
6.3.1 Darcy's Law
As described in Section 2.4, Henry Darcy developed, from
empirical data, an expression for the factors controlling ground
water movement. He found that the total flow is proportional to the
cross-sectional area perpendicular to the direction of flow,
multiplied by the hydraulic gradient in the aquifer, and by the
hydraulic conductivity of the aquifer. Algebraically Darcy's law is
written as:
Q = KA (dh/dl)
where:
Q = total flow
K = hydraulic conductivity
A = cross-sectional area
dh/dl = hydraulic gradient
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Similarly, as shown in Heath (1983), the velocity equation of
classical hydraulics states that the total flow (Q) is a product of
cross-sectional area and velocity (V). Written algebraically:
Q - AV
The two equations can be combined to yield an expression known as
the Darcy (or discharge) velocity in an aquifer:
V - K (dh/dl)
Fetter (1980) notes that the discharge velocity "is an apparent
velocity, representing the velocity at which water would move through
an aquifer if the aquifer were an open conduit." To determine the
actual velocity of ground water through a porous medium, the
hydraulic conductivity is divided by a dimenslonless porosity term
(n) to account for the actual open space available for flow. The
equation for velocity becomes:
V - K/n (dh/dl)
Multiplying the velocity by an appropriate time factor yields the
migration distance:
D - Vt
6.3.2 Data Requirements
As shown above, the velocity equation is dependent on three
field parameters: the hydraulic conductivity, the hydraulic
gradient, and the porosity. The data requirements, constraints, and
costs for both hydraulic gradient and hydraulic conductivity have
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been described in Sections 4.3 and 4.4, respectively. The porosity
parameter presents a special problem in determining flow velocity.
Traditionally, porosity is defined as the ratio of the volume
of openings in a rock to the total volume of the rock. In evaluating
the flow of ground water through porous media, it is necessary to
account for surface adhesion which will fill some of the available
pore space. The net result, then, is that the effective porosity
for flow (n ) is somewhat less than the total porosity. Rigorous
solutions to the flow equation would require that the effective
porosity be used to calculate ground water flow velocities. As
stated in the Practical Guide to Ground Water Sampling (U.S.
Environmental Protection Agency, 1985a), methods are still being
developed by which effective porosity can be measured. In order to
solve the Darcian flow equation, it is necessary to use surrogate
values for the effective porosity of the geologic material being
investigated.
6.3.3 Findings
The parameters needed to evaluate ground water migration
velocity and distance include hydraulic conductivity, hydraulic
gradient, and effective porosity. The uncertainities associated
with field measurement of these parameters have been previously
discussed and include non-unique interpretations of analytical
results, reporting errors, extensive data collection requirements
(some with associated long time requirements) and the inability (as
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yet) to measure effective porosity in the field. The costs
associated with the field determination of some of these parameters
are described in detail in Appendix D and have been summarized in
Chapter 4.
The costs and uncertainties which surround the parameters
needed to evaluate migration velocity and distance suggest that, if
the HRS continues to serve as a screening tool, it is not feasible
to require a distance/velocity analysis at each site. Also, the
hydrologic literature (e.g., Princeton Associates, 1985) generally
cites a velocity range of 5 feet per day to 5 feet per year as being
typical of ground water flow. These values imply that on an annual
basis, ground water can migrate as little as 5 feet or as much as
1,800 feet. Consequently, for hazardous wastes sites, which may
have a history of disposal that spans several decades, it is not
surprising that some migration distances are found to approach or go
beyond three miles.
6.4 Conclusions
Plume geometries and case histories have been used to study
the three-mile radius issue. Complete data sets describing the
hydrogeologic conditions at each site reviewed are not available.
Therefore, analytical studies have not been performed to verify the
plume geometries and to establish age and migration rate
relationships.
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The empirical data from the plume survey and the case histories,
and the generally accepted rate of ground water migration, indicate
that, in general, ground water contamination plumes are less than
four miles in length. Of the fifty-seven plumes examined, 93 percent
(53 of 57) are less than three miles in length, 86 percent (49 of 57)
are less than two miles in length, and 82 percent (47 of 57) are
less than one mile in length.
In selecting a distance to define a zone of influence around a
hazardous waste site, the Agency needs to consider to what extent an
overestimate (or underestimate) of the potentially exposed targets
is undesirable. For example, using the data on fifty-seven plumes,
a decision to use a four-mile radius will rarely underestimate the
potential targets, but will overestimate (compared to, for example,
a three-mile radius) in up to 93 percent of the cases examined.
Likewise, a three-mile radius overestimates in up to 86 percent of
the cases and underestimates in about 7 percent of the cases.
Similarly, a two-mile radius overestimates the potentially exposed
targets in up to 82 percent of the cases and underestimates in
14 percent of the cases. Additionally, the available data indicate
that the current HRS practice for measuring the three-mile radius
(i.e., starting the measurement from the furthest point of known
contamination attributable to a site rather than from the source of
the contamination) is likely to add to overestimates of the potential
targets at many sites. In practical terms, an overestimate of the
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potentially exposed targets may result in a site being included on
the NPL whereas an underestimate may result in a site not being
listed. Thus, the decision as to what distance is appropriate for
inclusion in the HRS needs to take into consideration the objectives
of the Agency in preparing the NPL. It is recommended that whatever
distance is selected, the current HRS practice for dealing with
aquifer discontinuities be retained (i.e., not counting populations
within the target distance limit that are beyond an aquifer
discontinuity).
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7.0 OTHER RELATED ISSUES
This study originated, in large part, in response to issues and
concerns raised by the public during review and comment cycles of
NPL and NCP rulemaking. In the course of conducting this study,
additional issues have been identified which are worthy of some
consideration. These include the influence of topography on ground
water flow regimes, the movement of ground water in karst terranes,
and the future use of ground water supplies.
7.1 Topography
The authors already cited in this report (e.g., Freeze and
Cherry, Fetter, and Heath) generally agree that in unconfined
aquifers the water table is a subdued expression of the surface
topography. In confined aquifers stratigraphic, structural, and
hydraulic controls are the dominant influence. Beyond the general
statement about topography, no relationship is established that
defines the effect of topography on the direction of ground water
flow. The overriding principle is that ground water will flow from
an area of high potential head to an area of low potential.
The explanation for this uncertainty about the influence of
topography on the direction of ground water flow is related to the
primary purpose of the study of hydrogeology. For the most part
that purpose has been to explore and evaluate ground water resources
with an aim towards developing sources of water supplies. As an
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analogy, in the petroleum or gas industries, traditional exploration
schemes begin with research and reconnaissance. This is followed by
exploratory drilling which in turn leads to test drilling. If test
drilling proves successful and there appears to be sufficient yield
from the oil or gas reservoir, development proceeds. Development
continues by step-out drilling (new wells located no more than one
mile from successful development wells) until yields decrease to
uneconomic levels.
So it is with ground water resource development. General
knowledge of an area may be assembled from reconnaissance and
preliminary research studies. However, the specific conditions
which define the hydrogeologic regime in a particular area cannot be
described until exploratory wells are drilled and field data are
collected and analyzed.
Because the primary purpose of the study of hydrogeology has
been to develop water supplies, there has been a heavy emphasis on
the use of field data. This reliance on empirical information has
obviated the need for lengthy explanations or theoretical
discussions of the relationship between topography and the direction
of ground water flow. For this reason, no scales or mechanisms have
been developed specifically to define the relationship between
topography and ground water flow. As a result, then, it appears
that there is presently no quantitative way to incorporate
topography into the MRS.
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7.2 Karst Terranes
Karst terranes have also been included as part of this study.
The following discussion defines karst terranes and explains why
generic HRS rating factors cannot be developed for facilities located
in these areas.
Karst terranes (named for the Karst Region of Yugoslavia) are
characterized by a landscape which exhibits irregularities in surface
form caused by rock dissolution. This landscape usually occurs in
limestone (a rock composed of calcium carbonate) although it may also
form in dolomite (a rock in which some of the calcium in the limestone
has been replaced by magnesium), or in areas of gypsum (calcium
sulfate) or rock salt (sodium chloride). Dissolution may occur along
joints, bedding planes or other openings. If the limestone beds are
horizontal or dip at a low angle, the land surface develops sinkholes
and solution valleys. In addition, large networks of interconnected
caves may form. Freeze and Cherry (1979) state that "in major karst
regions thousands of kilometers of caves exist . . . ." According to
Quinlan and Ewers (1986), about 20 percent of the United States is
underlain by carbonate rock. At least half of this rock is "maturely
karsted" (i.e., it contains a well developed, integrated conduit
system). Examples which indicate the wide geographic distribution of
these features include Luray Caverns in Virginia, Howe Caverns in New
York, Mammoth Caves in Kentucky, Carlsbad Caverns in New Mexico, Wind
Cave in South Dakota, Wyandotte Cave in Indiana, and Manama Caverns
in Florida.
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In the context of evaluating hazardous waste sites located in
karst terranes, Quinlan and Ewers (1986) point out that pollutant and
ground water flow in most karst aquifers is not described by the
dispersive Darcian flow characteristics of granular aquifers.
Restricting their comments to "maturely karsted carbonate rocks," they
state that most pollutant and ground water flow is analogous to flow
in surface stream networks. Flow is turbulent; it occurs in conduits;
it commonly has velocities of 30 to 1,500 feet per hour; and it
terminates in springs.
From the site inspection perspective, however, karst terranes
present special problems. Quinlan and Ewers state that "the
probability of a single well Intercepting a conduit near a typical
site, where the approximate flow direction is known, is about 1 in
2,600." These authors suggest a strategy for establishing a
monitoring network at hazardous waste sites located in karst terranes.
This strategy can be adapted to serve site inspection purposes and
includes the following steps:
Locate all springs, streams in sinkhole bottoms, and major
streams in caves.
Establish connections between the site, springs, and
underground streams using dye-tracing techniques.
Sample the connected points.
Determine the background values by sampling one geochemically
similar spring which was shown by dye-tracing not to be
connected to the site.
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As Indicated In the steps outlined above, dye-tracing techniques
play an important role in the investigation of a ground water basin
in karst terrane. Quinlan and Ewers suggest that dye-tracing would
have to be carried out under both base flow and flood conditions.
This, they explain, is because during flood flow, cave streams may
occupy alternate high-level routes which lead to springs other than
those delineated during base flow conditions. In some situations it
may also be desirable to use dye-tracing to determine the boundaries
of the ground water basin.
Quinlan and Ewers also address the issues of off-site
investigations and sampling frequency. Because the network of
conduits in a karst terrane may be very large, they suggest that the
area that must be evaluated for any particular site might extend to
a radius of five miles or more. And, because the flow velocity can
range between 30 and 1,500 feet per hour, they point out that
quarterly sampling may completely miss discharge events. At the
other extreme, continuous monitoring may be wholly impractical.
Quinlan and Ewers candidly admit that the issue of determining the
proper sampling frequency in karst terrane is still an open question.
In practice, then, a site monitoring strategy described by
Quinlan and Ewers (and modified for site inspection) would be very
site-specific and time consuming. Although they do not specifically
address cost considerations, they imply that because of the time
involved (generally an annual cycle) and the number of samples
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required, an adequate site inspection is quite costly. Because the
hydrogeologic settings of karst terrenes are highly variable and
because sites can only be characterized by site-specific studies, it
does not appear feasible to develop a meaningful, generic HRS
scoring factor applicable to sites located in these areas.
With regard to selecting a distance to use in estimating the
potential targets that may be affected by the migration of
contaminants in karst terranes, it is necessary to consider the
extent to which inherent overestimates or underestimates of potential
targets is undesirable. For example, Qulnlan and Ewers state that a
radius of investigation may extend to five miles or more. A decision
to extend the HRS ground water radius needs to consider not only the
fact that contaminants may migrate greater distances in karst
terrane, but also that this migration is more localized. Therefore,
an expansion of the radius would tend to greatly overestimate the
potential targets, except in the situation where the bulk of the
potential targets are centrally located (e.g., a municipal well)
and are drawing from a solution channel containing water with
contaminants from the site. Also, potential targets may not be
overestimated in those areas where solution channels are generally
well connected (e.g., portions of Florida).
7.3 Future Use of Ground Water Supplies
Another related issue is the feasibility of assessing the
future use of potentially affected aquifers. As discussed in
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Driscoll (1986), Fetter (1980), and Freeze and Cherry (1979), among
others, both geologic and socio-economic factors need to be
considered when discussing the projected use of ground water. Fetter
(1980) has synthesized from several sources a definition for the
"safe yield" of an aquifer that incorporates both the geologic and
socio-economic concerns which need to be considered in evaluating an
aquifer: the "safe yield is the amount of naturally occurring ground
water that can be withdrawn from an aquifer on a sustained basis,
economically and legally, without impairing the native ground water
quality or creating an undesirable effect such as environmental
damage."
The principal geologic factor which needs to be determined is
the volume of water in the aquifer. This can be calculated by
multiplying the thickness and areal extent of the aquifer by its
specific yield. The specific yield is the ratio of the volume of
water that drains from a rock (under the influence of gravity) to the
total volume of the rock. This ratio is expressed as a percentage
and ranges from an average value of 2 percent for clay to 27 percent
for coarse sand (Fetter, 1980).
The data requirements and constraints associated with the
determination of thickness and areal extent of an aquifer have
already been described in Sections 3.2 and 5.2. Specific yield may
be determined by both laboratory and field methods.
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In the laboratory, a sediment column is flooded from the bottom
driving out all the air. This creates a known volume of fully
saturated sediment. Water is then drained from the column in such a
manner so as to avoid evaporation losses. The drainage period
required for the column to reach equilibrium can be as long as
several months. The ratio of the volume of water drained to the
volume of the soil column is the specific yield. To express the
specific yield as a percentage the ratio is multiplied by 100.
The specific yield of sediments and rocks can also be determined
in the field from pumping tests. Water wells are pumped and the
rate at which water level falls in nearby wells is measured. A more
detailed discussion of pumping tests and their data requirements and
constraints appears in Section 4.4.2.
To estimate the volume of water available from an aquifer would
thus require a significant commitment of time and resources. A
field program of hydraulic conductivity testing would be required.
The costs associated with hydraulic conductivity testing and, if
necessary, drilling are described in Sections 4.3.2 and 4.4.2.
Also, as previously discussed, there are significant uncertainties
associated with the determination of the key parameters needed to
estimate the volume of water available from an aquifer.
Furthermore, from the socio-economic perspective, several
additional issues have been discussed in the literature already
cited. Aside from geologic concerns, Fetter's definition of safe
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yield refers to legal, as well as economic, constraints. One major
concern is the variation in State water laws. Each State has its
own body of water law which encompasses both ownership and
appropriation of water. Although generally concerned with surface
water, many States consider ground water to be the source of some
part of their surface water resources. Rules governing ownership of
water rights and appropriation of the resource are very complex and
State specific. These differences make it very difficult to
consistently estimate national trends or even local trends in the
future use of ground water resources.
Similarly, other socio-economic factors which affect projections
of ground water use cannot be consistently estimated on a national
basis. Two such socio-economic factors are regional economic growth
(or contraction) and population increases (or decreases).
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8.0 SUMMARY AND CONCLUSIONS
This study has examined the feasibility of developing additional
scoring factors for the HRS related to the geohydrology of hazardous
wastes sites. Of particular interest was the direction and velocity
of ground water movement. Associated with this issue was the use of
a three-mile radius circle to delineate the area of concern around a
hazardous waste site. Another major issue that was investigated for
this study was the criteria for determining the interconnection of
aquifers. During the course of this research, some related issues
were identified and investigated. The principal findings of these
collected studies are summarized in the following sections.
8.1 Direction and Velocity of Ground Water Migration
In fractured or cavernous aquifers (typified by aquifers in
karst, as discussed in Section 7.2), the direction and velocity of
ground water flow is largely unpredictable. In granular aquifers,
however, laminar flow predominates. The direction of ground water
flow, the migration distance, and the flow velocity are all related
through Darcy's Law and the basic flow equation of hydraulics. These
are empirical relationships and, because of this, they rely on
actual field data. The required data elements include hydraulic
conductivity, effective porosity, and the hydraulic gradient as
determined from measuring both the total hydraulic head in
observation/monitoring wells and the distance between the wells.
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Regarding the field data, there are considerable uncertainties
attached to determining values of hydraulic conductivity in the field.
Laboratory determinations of hydraulic conductivity yield results that
are not equivalent to field values. Similarly, the effective porosity
for flow has been recognized as being significantly different (i.e.,
10 to 20 percent less) from the bulk porosity of an aquifer. This is
an active research area and, for the moment, surrogate values are being
used to solve flow equations. Although relatively straight-forward, the
determination of hydraulic gradient in the area around a hazardous waste
site requires a large amount of data. Well locations, elevations, and
head measurements must all be accurately determined. Temporal factors
such as base flow and flood flow must also be considered.
As described in Sections 4.3 and 4.4, the level of effort and the
associated costs needed to collect the field data are very high
compared to those of current site inspections. The use of these data,
furthermore, would not necessarily always translate into more accurate
knowledge about conditions at the site. Use of the data would introduce
different types of uncertainties into the HRS process than those that
are currently present as a result of the direction of flow not being
considered. The primary concern would be in underestimating the target
area potentially threatened by the site.
8.2 Aquifer Interconnection
To evaluate the connection (or lack of connection) between two or
more water bearing units, information is needed on the areal extent of
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confining layers beneath a site. In addition to the hydrogeologic
characterization of the site, the primary data element on which to
base a decision would be the hydraulic conductivity of adjacent rock
units. Other site specific conditions, such as poorly cased or
abandoned wells and channels left by decayed root material, could
also have a local influence on aquifer interconnection. As described
in Section 4.4, field determinations of hydraulic conductivity are
generally unreliable and costly. Developing a rating factor which
would require these field data is not recommended, assuming the HRS
remains a screening tool intended to use only limited data.
8.3 The Three-Mile Radius
A three-mile radius, measured from the furthest point of known
contamination attributable to a site, is currently used in the HRS
to evaluate ground water targets. Data from case histories and a
plume survey have been used to examine the likely areal extent of
ground water plumes. This data, along with data on generally
accepted ground water migration velocities, indicate that plumes
are typically less than four miles in length. The data further
indicate that the use of target distances less than four miles will
overestimate potential targets at some sites and underestimate
potential targets at other sites. The greater the target distance
used, the greater the potential for overestimating target
populations. On the other hand, the lesser the target distance used,
the greater the potential for underestimating target populations.
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Therefore, the objectives of the Agency, with regard to the NPL,
need to be considered in reaching a decision as to the appropriate
distance to be used in identifying a zone of influence around a
hazardous waste site.
8.4 Related Issues; Topography, Karst Terranes, and the Future Use
of Ground Water Supplies
Three related issues that have been identified in the course of
this study have also been examined. The influence of topography on
ground water movement is not rigorously described in the ground water
literature. Because of this, no quantitative way of Incorporating
topographic influences into the HRS can be developed.
In karst terranes (e.g., cavernous limestone), ground water flow
is wholly unpredictable and site-specific. Consequently, it Is not
feasible to develop a meaningful, generic, rating factor applicable
to such areas.
Future use of ground water supplies depends upon both geologic
and socio-economic factors. The principal geologic factor Is the
volume of water in the aquifer. As previously discussed, estimation
of the volume of water available is costly and time consuming
compared to the time and cost of current site inspections. There
are also significant uncertainties associated with the determination
of the key parameters needed to make the estimate. Furthermore,
water supplies, water laws and other socio-economic factors which
affect the future use of ground water supplies exhibit a high degree
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of local variability. Consequently, it is not possible to
consistently predict changes related to these factors on a national
basis.
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9.0 BIBLIOGRAPHY
American Geological Institute, Dictionary of Geological Terms,
Anchor Press, Garden City, 1976.
Bouwer, H., Groundwater Hydrology, McGraw-Hill, New York, 1978.
Cleary, R.W., Personal Communication, Princeton Associates, July
1985.
Davis, S.N., "Porosity and Permeability of Natural Materials" in
Flow Through Porous Media, R.J.M. DeWeist (ed.), Academic Press,
pp. 54-89, New York, 1969.
Driscoll, F.G., Groundwater and Wells, 2nd ed., Johnson Division,
St. Paul, 1986.
Ecology and Environment, memorandum to the U.S. Environmental
Protection Agency, TDD No. HQ-8608-06, September 3, 1986.
Everett, L.G., Groundwater Monitoring, General Electric Co.,
Schenectady, NY, 1980.
Fetter, C.W., Applied Hydrogeology, Charles E. Merrill Co., Columbus,
OH, 1980.
Fetter, C.W., "Determining the Direction of Ground Water Flow", in
Ground Water Monitoring Review, Vol. 1, No. 3, 1981.
Fetter, C.W., "Potential Sources of Contamination in Ground Water
Monitoring", in Ground Water Monitoring Review, Vol. 3, No. 2, 1983.
Freeze, R.A. and J.A. Cherry, Ground Water, Prentice-Hall, Inc.,
Englewood Cliffs, 1979.
Geraghty and Miller, Inc., letter report to the U.S. Environmental
Protection Agency Office of Ground Water Protection, November 1985,
Heath, R.C., Basic Ground Water Hydrology, USGS Water Supply Paper
2220, Reston, VA, 1983.
Keely, J.F., "Optimizing Pumping Strategies for Contaminant Studies
and Remedial Actions", in Ground Water Monitoring Review, Vol. 4,
No. 3, 1984.
Kimmel, G. and O.C. Braids, "Leachate Plumes in a Highly Permeable
Aquifer", Ground Water, Vol. 12, pp. 388-393, 1974.
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NUS Corporation, letter report to the U.S. Environmental Protection
Agency, C582-9-6-18, September 3, 1966.
Office of Technology Assessment, Protecting the Nation's Groundwater
from Contamination. OTA-0-233, Washington, DC, 1984.
Princeton Associates, Groundwater Pollution and Hydrology, Course
Notes, July 1985.
Quinlan, R. and A. Ewers, "Ground Water Monitoring in Karst
Terranes", Guest editorial in Ground Water Monitoring Review,
Vol. 6, No. 1, 1986.
Saines, M., "Errors in Interpretation of Ground Water Level Data",
in Ground Water Monitoring Review, Vol. 1, No. 1, 1981.
Scalf, M.R. et al., Manual of Ground Water Quality Sampling
Procedures, National Water Well Association, Worthington, OH, 1981.
Thornhill, J.T., Personal Communication, U.S. Environmental
Protection Agency, Robert S. Kerr Environmental Laboratory, Ada, OK,
November 1985.
U.S. Code of Federal Regulations40 CFR Part 300, The National Oil
and Hazardous Substances Contingency Plan; Appendix A; Uncontrolled
Hazardous Waste Site Ranking System - A User's Manual, 47 FR 31219,
July 16, 1982.
U.S. Department of the InteriorWater and Power Resources Service,
Ground Water Manual, Washington, DC, 1981.
U.S. Environmental Protection Agency, National Surface Impoundment
Assessment Report, January 1984.
U.S. Environmental Protection Agency, Practical Guide for Ground
Water Sampling. EPA/600/2-85/104, 1985a.
U.S. Environmental Protection Agency, RCRA Ground Water Monitoring
Technical Enforcement Guidance Document - Draft, 1985b.
Walton, W.C., Groundwater Resource Evaluation, McGraw-Hill Book Co.,
New York, 1970.
Ward, C.H. et al. (ed.), Ground Water Quality. John Wiley & Sons,
Inc., New York, 1985.
Zuras, A.D. et al., "The National Priorities List Process", in
conference proceedings, Management of Uncontrolled Hazardous Waste
Sites, HMCRI, Silver Spring, MD, November 1985.
76
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APPENDIX A
GLOSSARY OF SELECTED TERMS
(Adapted from Fetter, 1980; Heath, 1983; and
the American Geological Institute, 1976)
Anisotropy
Aquifer
Aquifer, Confined
Aquifer, Unconfined
Bedrock
Confining Bed
Datum Plane
Density
The condition under which one or more of the
hydraulic properties of an aquifer vary
according to the direction of flow.
Rock or sediment in a formation, group of
formations, or part of a formation which is
saturated and sufficiently permeable to
transmit usable quantities of water to wells
and springs.
An aquifer that is overlain by a confining bed.
The confining bed has a significantly lower
hydraulic conductivity than the aquifer.
An aquifer in which there are no confining beds
between the zone of saturation and the surface.
There will be a water table in an unconfined
aquifer. Water-table aquifer is a synonym.
A general term for the consolidated (solid) rock
that underlies soils or other unconsolidated
surficial material.
A body of material that is stratigraphically
adjacent to one or more aquifers and that has a
low hydraulic conductivity relative to the
adjacent aquifers. It may lie above or below
the aquifer.
An arbitrary surface (or plane) used in the
measurement of ground water heads. The datum
most commonly used is the National Geodetic
Vertical Datum of 1929, which closely
approximates sea level.
The mass or quantity of a substance per unit
volume. Units are kilograms per cubic meter or
grams per cubic centimeter.
77
-------
Discharge Area
Discharge Velocity
Dynamic Viscosity
Effective Porosity
Equipotential Line
Equipotential
Surface
Fence Diagrams
Ground Water
Ground Water,
Confined
Ground Water Flow
An area in which there are upward components of
hydraulic head in the aquifer. Ground water is
flowing toward the surface in a discharge area
and may escape as a spring, seep, or baseflow,
or by evaporation and transpiration.
An apparent velocity, calculated from Darcy's
law, which represents the flow rate at which
water would move through an aquifer if the
aquifer were an open conduit. Also called
specific discharge.
See Viscosity.
The amount of interconnected pore space through
which fluids can pass, expressed as a percent
of bulk volume. Part of the total porosity
will be occupied by static fluid being held to
the mineral surface by surface tension, so
effective porosity will be less than total
porosity.
A line in a two-dimensional ground water flow
field such that the total hydraulic head is the
same for all points along the line.
A surface in a three-dimensional ground water
flow field such that the total hydraulic head
is the same everywhere on the surface.
Three or more geologic cross-sections showing
the relationship of wells or outcrop sections
to formations.
The water contained in interconnected pores
located below the water table in an unconfined
aquifer or located in a confined aquifer.
The water contained in a confined aquifer.
Pore-water pressure is greater than atmospheric
at the top of the confined aquifer.
The movement of water through openings in
sediment and rock which occurs in the zone of
saturation.
78
-------
Ground Water,
Perched
Ground Water,
Unconfined
Head, Total
Heterogeneous
Hydraulic
Conductivity
Hydraulic Gradient
Isopach
Isotropy
Kinematic Viscosity
Lithofacies Map
Lithology
The water in an isolated, saturated zone located
in the zone of aeration. It is the result of
the presence of a layer of material of low
hydraulic conductivity, called a perching bed.
Perched ground water will have a perched water
table.
The water in an aquifer where there is a water
table.
The sum of the potential energy that can be
attributed to the elevation, pressure, and
velocity of the ground water at a given point
in an aquifer.
Pertaining to a substance having different
characteristics in different locations. A
synonym is non-uniform.
A coefficient of proportionality describing the
rate at which water can move through a permeable
medium. The density and kinematic viscosity of
the water must be considered in determining
hydraulic conductivity. Also referred to as
permeability.
The change in total head with a change in
distance in a given direction. The direction
is that which yields a maximum rate of decrease
in head.
A line on a map drawn through points of equal
thickness of a designated unit.
The condition in which hydraulic properties of
the aquifer are equal in all directions.
The ratio of dynamic viscosity to mass density.
It is obtained by dividing dynamic viscosity by
the fluid density. Units of kinematic viscosity
are square meters per second.
A map showing the areal variation in overall
physical characteristics of a stratigraphic
unit.
The physical character of a rock (e.g., grain
size, color, mineral constituents).
79
-------
Observation Well
Permeability
Piezometer
Pore Space
Porosity
Potentiometric
Surface
Pumping Test
Recharge Area
Rock
A nonpumping well used to observe the elevation
of the water table or the potentiometric
surface. An observation well is generally of
larger diameter than a piezometer and typically
is screened or slotted throughout the thickness
of the aquifer.
See Hydraulic Conductivity.
A nonpumping well, generally of small diameter,
which is used to measure the elevation of the
water table or potentiometric surface. A
piezometer generally has a short well screen
through which water can enter.
The volume between mineral grains in a porous
medium.
The ratio of the volume of void spaces in a
rock or sediment to the total volume of the
rock or sediment.
A surface that represents the level to which
water will rise in tightly cased wells. If the
head varies significantly with depth in the
aquifer, then there may be more than one
potentiometric surface. The water table is a
particular potentiometric surface for an
unconfined aquifer.
A test made by pumping a well for a period of
time and observing the change in hydraulic head
in the aquifer. A pumping test may be used to
determine the capacity of the well and the
hydraulic characteristics of the aquifer. Also
called aquifer test.
An area in which there are downward components
of hydraulic head in the aquifer. Infiltration
moves downward into the deeper parts of an
aquifer in a recharge area.
Any naturally formed, consolidated or
unconsolidated, material (but not soil)
consisting of two or more minerals.
80
-------
Safe Yield
Saturated Zone
Seepage Velocity
Slug Test
Soil
Specific Retention
Specific Yield
Storage, Specific
Storativity
The amount of naturally occurring ground water
which can be economically and legally withdrawn
from an aquifer on a sustained basis without
impairing the native ground water quality or
creating an undesirable effect, such as
environmental damage. It cannot exceed the
increase in recharge or leakage from adjacent
strata plus the reduction in discharge, which
is due to the decline in head caused by pumping.
The zone in which the voids in the rock or soil
are filled with water at a pressure greater than
atmospheric. The water table is the top of the
saturated zone in an unconfined aquifer.
The actual rate of movement of fluid particles
through porous media.
An aquifer test made by either pouring a small
instantaneous charge of water into a well or by
withdrawing a slug of water from the well. A
synonym for this test, when a slug of water is
removed from the well, is a bail-down test.
The layer of material at the land surface that
supports plant growth.
The ratio of the volume of water the rock or
sediment will retain against the pull of gravity
to the total volume of the rock or sediment.
The ratio of the volume of water a rock or soil
will yield by gravity drainage to the volume of
the rock or soil. Gravity drainage may take
many months to occur.
The amount of water released from or taken into
storage per unit volume of a porous medium per
unit change in head.
The volume of water an aquifer releases from or
takes into storage per unit surface area of the
aquifer per unit change in head. It is equal
to the product of specific storage and aquifer
thickness. In an unconfined aquifer, the
storativity is equivalent to the specific yield.
Also called storage coefficient.
81
-------
Stratification
Stratigraphic Unit
Transmissivity
Unsaturated Zone
The layered structure of sedimentary rocks.
A unit consisting of stratified, mainly
sedimentary rocks, grouped for description,
mapping, or correlation.
The rate at which water of a prevailing density
and viscosity is transmitted through a unit
width of an aquifer or confining bed under a
unit hydraulic gradient. It is a function of
properties of the liquid, the porous media, and
the thickness of the porous media.
The zone between the land surface and the water
table. It includes the root zone, intermediate
zone, and capillary fringe. The pore spaces
contain water at less than atmospheric pressure,
as well as air and other gases. Saturated
bodies, such as perched ground water, may exist
in the unsaturated zone.
Viscosity
Water Table
Well, Full
Penetrating
Well, Partially
Penetrating
Zone of Aeration
The property of a fluid describing its
resistance to flow. Units of viscosity are
newton-seconds per meter squared or pascal-
seconds. Viscosity is also known as dynamic
viscosity.
The surface in an unconfined aquifer or
confining bed at which the pore water pressure
is atmospheric. It can be measured by
installing shallow wells extending a few feet
into the zone of saturation and then measuring
the water level in those wells.
A well drilled to the bottom of an aquifer,
constructed in such a way that it withdraws
water from the entire thickness of the aquifer.
A well constructed in such a way that it draws
water directly from a fractional part of the
total thickness of the aquifer. The fractional
part may be located at the top or the bottom or
at any point in between in the aquifer.
See Unsaturated Zone.
82
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APPENDIX B
DETERMINING THE DIRECTION OF GROUND WATER FLOW
This appendix illustrates how field data from three piezometers
can be analyzed to indicate the direction of ground water flow.
This problem is similar to a three-point problem in structural
geology wherein it is sometimes necessary to determine the attitude
of various strata at depth, given only stratigraphic logs.
In order to determine the direction of ground water flow from
piezometer data, a uniform aquifer is assumed. Beyond this
assumption, there are three basic data requirements. The relative
geographic position of the wells must be known along with the
distance between the wells. Also the total hydraulic head at each
well is required data. For this illustrative example, the following
information is given:
The total head at well 1 is 26.26 feet.
Well 2 is 165 feet southwest of well 1. The total head at
well 2 is 26.20 feet.
Well 3 is south-southeast of Well 1 and southeast of
well 2. The intervening distances are:
1 to 3 - 215 feet
2 to 3 - 150 feet
The total head at well 3 is 26.07 feet.
Figure B-l is a schematic representation of the raw data.
The following steps, and the accompanying Figure B-2, outline
the solution to this example problem:
83
-------
Well 1
(Head, 26.26 ft.)
Well 2
(Head, 26.20 ft.)
25
50
100 feet
Well 3
(Head, 26.07 ft.)
FIGURE B-1
RAW DATA FOR EXAMPLE PROBLEM
-------
26.26 ft.
(A) Well 2
W.L = 26.20 ft.
(B) (26.26-26.20) _ (26.26-26.07)
(E) 26.2-26.07
133
215
0.13 ft.
133ft
Direction of
Ground Water
Movement
26.07 ft.
0.001
FIGURE B-2
SOLUTION TO EXAMPLE PROBLEM
85
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A. Identify the well with the intermediate value of total head
(i.e., well 2).
B. Locate, along the line connecting the wells with the highest
and lowest heads, the point at which the head is the same
as the intermediate value. This is done by interpolation.
C. Draw a straight line from the intermediate well to the point
identified in the previous step. This segment represents a
water level contour of equal total head.
D. A line perpendicular to the water level contour and through
either the well with highest or lowest head parallels the
direction of flow.
E. The difference between the head of the well intersected by
the line and that of the contour divided by the distance
between them is the hydraulic gradient.
86
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APPENDIX C
CORRELATION BETWEEN CONTAMINANT PLUME LENGTH
AND HYDRAULIC GRADIENT
Analysis of the correlation between the length of the contaminant
plumes and their respective hydraulic gradients was performed on the
data from the contaminant plume survey. The plume survey data are
shown in Table C-l. As shown in Table C-2, gradient information was
not available for many of the contaminant plumes. Table C-2 lists
the plumes for which both data points (i.e., the plume length and the
hydraulic gradient) are available. The results of the correlation
analysis are shown at the bottom of the table. The analysis
demonstrates that for the plumes identified in the survey there is
little correlation between length and gradient (i.e., the coefficient
2
of determination, r , is only about 18 percent).
87
-------
TABLE C-l
SURVEY OF CONTAMINANT PLUME GEOMETRIES
Case
1
3
4
6
7
8
9
10
11
12
14
18
19
20
21
22
25
26
27
Length
(feet)
18,000
1,000
5,000
700
3,000
1,300
2,200
8,000
4,300
7,000
1,000
2,200
1,600
4,400
1,000
1,800
500
3,600
2,000
Width
(feet)
4,000
600
5,000
100
1,600
1,300
1,300
6,000
1,000
4,500
800
256
715
2,500
1,000
700
300
1,800
700
Depth Transmissivity
(feet) (gpd/ft)*
30
75
80 2,000-400,000
60-70
60
20
55-140
100
50
15
30 40,000
50 20,000
20
40
30
80 40,000
20
Gradient
(feet /mile)
14
6.6
29
10.5
1-19.6
63
21
42
370
VHK
88
-------
TABLE C-l (Continued)
Case
28
29
30
31
32
33
34
37
38
39
40
41
43
46
47
48
50
51
52
53
Length
(feet)
1,000
800
1,600
10,600
5,000
9,800
2,000
450
400
1,000
3,000
3,000
1,000
700
4,000
2,500
1,400
450
400
Width
(feet)
400
500
1,200
2,000
1,000
3,600
1,500
120
150
250
1,000
600
1,000
700
2,000
700
900
200
240
Depth
(feet)
15
60
70
170
135
180
16
60
50
50
80
150
20
25
40
95
25
Transmissivity
(gpd/ft)*
24,000
140,000
340,000
110,000
3,000
40,000
160,000
200,000
2,000
Gradient
(feet /mile)
422
5.3
264
316.8
316.8
15.84
31.7
13.2
158
89
-------
TABLE C-l (Concluded)
Case
54
55
56
57
58
60
62
65
66
67
68
Length
(feet)
1,100
5,000
1,600
1,000
600
350
2,000
400
1,500
6,000
700
Width
(feet)
800
2,300
800
700
200
100
450
200
400
2,500
400
Depth
(feet)
70
165
120
26
110
40
100
50
30
50
90
Transml ss 1 vity
(gpd/ft)*
46,000
71,000
66, 000
270,000
Gradient
(feet /mile)
158
*gpd/ft = Gallons per day per foot.
Source: Geraghty and Miller, 1985.
90
-------
TABLE C-2
DATA USED FOR CORRELATION ANALYSIS BETWEEN PLUME LENGTH
AND HYDRAULIC GRADIENT (from Table C-l)
Case
1
7
8
9
18
19
20
25
29
33
34
37
38
41
43
48
52
57
Results of
Length (feet)
18, 000
3,000
1,300
2,200
2,200
1,600
4,400
500
800
9,800
2,000
450
400
3,000
1,000
4,000
450
1,000
the Correlation Analysis:
Length
Mean Value 3116.67
Standard Deviation 4338.2
Gradient (feet /mile)
14
6.6
29
10.5
63
21
42
370
422
5.3
264
316.8
316.8
15.84
31.7
13.2
158
158
Gradient
125.43
145. 72
Correlation Coefficient (r) = -0.429
91
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APPENDIX D
ESTIMATED COST AND TIME REQUIREMENTS
FOR GEOTECHNICAL FIELD WORK
This appendix provides estimates of the current cost and time
requirements associated with a hydrogeologic investigation of a
hazardous substance release site. In particular the estimates are
for the costs associated with addressing the principal issues cited
in this report (i.e., the direction of ground water flow, aquifer
interconnection, and the three-mile radius). The data presented in
this appendix are 1986 estimates developed by the EPA Field
Investigation Teams (FIT) as follows: NUS Corporation (estimates
for FIT-Zone I, EPA Regions I-IV) and Ecology and Environment
(estimates for FIT-Zone II, EPA Regions V-X).
Tables D-l and D-2 present cost and time estimates for well
drilling and for the collection and analysis of water level data.
Table D-l shows the estimated range of cost and time requirements
for the two FIT Zones. The table also indicates representative cost
and time requirements; the representative values are based upon the
data provided for the individual EPA Regions within the two FIT
Zones. Table D-2 provides representative cost and time estimates
for a field program consisting of the installation of ten wells and
the collection and analysis of water level data from those wells.
93
-------
TABLE D-l
COST AND TIME ESTIMATES FOR WELL DRILLING AND
COLLECTION AND ANALYSIS OF WATER LEVEL DATAa
Cost Factors
(dollars)
Mobilization0
Drilling cost (per
single 4" well)
20 Foot Depth
100 Foot Depth
200 Foot Depth
Data Collection and
Analysis (assumes
10 well drilling
program)^
20 Foot Depth
100 Foot Depth
200 Foot Depth
FIT
Zone Ib
1,500-20,000
850-3,000
1,500-30,000
2,500-60,000
3,500-4,000
3,500-9,000
2,500-15,000
Time Requirement for
A Ten Well Drilling FIT
Program (weeks)
Drilling
20 Foot Depth
100 Foot Depth
200 Foot Depth
Data Collection and
20 Foot Depth
100 Foot Depth
200 Foot Depth
Total Representative
20 Foot Depth
100 Foot Depth
200 Foot Depth
Zone Ib
2-5
2-12
4-16
Analysis^
2-3
7
11
Timee»f
-
-
-
FIT
Zone IIb
1,000-6,000
400-1,300
2,000-4,000
4,000-7,000
1,600-2,400
2,400-3,200
3,200-4,000
Representative
Cost
6,000
1,200
4,000
8,000
3,000
4,500
6,000
FIT Representative
Zone IIb
2-3
6-10
4-16
1
1
1-2
-
-
-
Time
3
8
12
2
4
6
4
10
16
94
-------
TABLE D-l (Concluded)
aCost estimates do not include costs associated with site security
or disposal of drill spoils, or increased costs associated with
working in a potentially hazardous environment.
"The indicated range represents the range of estimates provided
for each EPA Region within the FIT Zone.
cMobilization costs include contract administration, rig
outfitting, and initial transportation to site.
dIncludes water level measurements, well description, and water
level contour mapping.
^Assumes some data analysis occurs concurrently with the latter
phase of drilling.
^Additional time would be required for mobilization. Mobilization
time includes time for such activities as contract bidding, contract
negotiation, rig outfitting, and transportation to the site.
Source: U.S. Environmental Protection Agency Field Investigation
Teams (FIT), Zones I and II, 1986, unpublished data.
95
-------
TABLE D-2
REPRESENTATIVE COST AND TIME ESTIMATES FOR
A FIELD PROGRAM CONSISTING OF TEN WELLS3
Cost
(dollars)13
Mobilization0
Drilling
Data Collection
and Analysis^
Total Cost
Time (weeks)
Drilling
Data Collection
and Analysis"
Total Time6
20-Foot
Wells
6,000
12,000
3,000
21,000
20-Foot
Wells
3
2
4
100-Foot
Wells
6,000
40,000
4,500
50, 500
100 -Foot
Wells
8
4
6
100-Foot
Wells
6,000
80, 000
6,000
92,000
100-Foot
Wells
12
6
16
Representative estimates are based upon Table D-l.
bCost estimates do not include costs associated with site security
or disposal of drill spoils, or increased costs associated with
working in a potentially hazardous environment.
cMobillzation costs include contract administration, rig outfitting,
and initial transportation to site.
dIncludes water level measurements, well descriptions, and water
level contour mapping.
eAssumes some data analysis occurs concurrently with the latter phase
of drilling.
Source: U.S. Environmental Protection Agency Field Investigation Teams
(FIT), Zones I and II, 1986, unpublished data.
96
-------
Table D-3 presents cost and time estimates for determining
hydraulic conductivities. The data are for slug/bail tests,
pressurization tests, and pumping tests. The assumptions behind the
estimates are indicated on the table. If well installation is
necessary, the installation costs would be those shown in Tables D-l
and D-2. However, wells previously installed for water level
measurements could also be used for determining hydraulic
conductivities, providing the wells are properly installed. If such
existing wells were used, there would be no additional cost for well
installation.
Geophysical surveying techniques may also be used to supplement
conductivity testing and well drilling. These surveys are used
either to characterize the stratigraphy beneath a release site or to
locate the actual areas of waste deposition (e.g., buried drums may
be located by a magnetometer survey). Because each site and each
site inspection may be different, it is not possible to generalize
the exact nature of the additional field work that could be conducted
at any given site. For information purposes, however, representative
costs and time requirements of various geophysical field surveys are
presented in Table D-4.
97
-------
TABLE D-3
COST AND TIME ESTIMATES FOR DETERMINING HYDRAULIC CONDUCTIVITY
Time
Cost Factors3 Requirements3
(dollars) (days)
FIT FIT FIT
Type of Field Test Zone I Zone II Zone IIb
Slug/Bail Test 270-1,600d 300-1,500d 3-18d»e
(single well)c
Pressurization Test 500-l,100d 530-2,200d 4-l8d»e
(single well)c
Pumping Test If 10,000d»8 9,800-15,000d»8 10-15d
(single aquifer)0 38,000-43,000h
Pumping Test II1. 12,700d>8 19,600-29,200d 20-25d
(two aquifers)J 68,000-110,000h
aThe indicated range represents the range of estimates provided for
each EPA Region within the FIT Zone, except where otherwise noted.
^Estimates not available for FIT Zone I.
cAssumes 90 feet of overburden above a 30-foot thick aquifer.
dEstimate does not include drilling costs and drilling time.
eTime in hours, not days.
*Test is intended to determine hydraulic conductivity.
^Estimate for Zone I, Region I only.
hEstimate for Zone I, Region II only; includes drilling costs.
^Test is intended to determine hydraulic conductivities and degree of
aquifer interconnection.
JAssumes two aquifers: one is 30 feet thick and begins 90 feet below
the ground surface; the other is 50 feet thick and begins 200 feet below
the ground surface.
Source: U.S. Environmental Protection Agency Field Investigation Teams
(FIT), Zones I and II, 1986, unpublished data.
98
-------
TABLE D-4
COST AND TIME ESTIMATES FOR GEOPHYSICAL SURVEYS3
ELECTRICAL EARTH RESISTIVITY
FIT Zone I
Cost Factorsb:
Cost Per 100 Linear Feet $ 42
Survey Line Spacing 300 feet
Survey Line Length approx. 1,500 feet
Total Survey Length approx. 7,500 feet
Survey Cost (does not include labor costs) $ 3,150
Field Team Labor Cost $24,000
Data Analysis $ 1,120
Total Estimated Cost $28,270
Time Requirement:
Daily Coverage 200 feet
Time for 7,500 Foot Survey approx. 6 weeks
Time for Data Analysis 1 week
Total Time 7 weeks
FIT Zone II
Cost Factorsb:
Cost Per 100 Linear Feet $ 250
Survey Line Spacing 100 feet
Survey Line Length 1,500 feet
Total Survey Length 22,500 feet
Survey Cost 456,250
Data Analysis $2,600-$5,200
Total Estimated Cost approx. $60,000
Time Requirement:
Daily Coverage 1,000-2,000 feet
Time for 22,500 Foot Survey 3 weeks
(use 1,500 feet/day) (15 days)
Time for Data Analysis 2-3 weeks
Total Estimated Time 5-6 weeks
99
-------
TABLE D-4 (Continued)
ELECTROMAGNETIC TERRAIN CONDUCTIVITY
FIT Zone I
Subcontract Cost in 1983 for a 15,000 $27,000
Foot Survey15
FIT Zone II
Cost Factors15:
Cost Per 100 Linear Feet $10-$50
Survey Line Spacing 25 feet
Survey Line Length 90,000 feet
Survey Cost (based on $30/100 feet) $27,000
Data Analysis 1 1,800
Total Estimated Cost $28,800
Time Requirement:
Daily Coverage 2,000-5,000 feet
Time for 90,000 Foot Survey approx. 4 weeks
(based on 4,000 feet/day)
Time for Data Analysis 1 week
Total Estimated Time 5 weeks
SEISMIC SURVEY
FIT Zone I
Subcontract Cost (1983-1985) for a $20,700
9,000 Foot Survey15
FIT Zone II
Cost Factors15:
Cost Per 100 Linear Feet $200-$350
Survey Line Length (assumed) 9,000 feet
Survey Cost $18,000-$31,500
Data Analysis $ 1,800
Total Estimated Cost Jl9,800-$33,300
Time Requirement:
Daily Coverage 200-500 feet
Time for 9,000 Foot Survey approx. 6 weeks
(based on 350 feet/day)
Time for Data Analysis 1 week
Total Estimated Time 7 weeks
100
-------
TABLE D-4 (Continued)
PROTON MAGNETOMETER SURVEY
FIT Zone I
Cost Factorsb:
Cost Per 100 Linear Feet $ 60
Survey Line Spacing 10 feet
Survey Line Length (assume 25 percent approx. 60,000 feet
of the site area needs this detailed
survey)
Survey Cost $36,000
Data Analysis $ 4,200
Total Estimated Cost $40,200
Time Requirement:
Daily Coverage 1,000-2,000 feet
Time for 60,000 Foot Survey approx. 8 weeks
(assume 1,500 feet/day)
Time for Data Analysis 3 weeks
Total Estimated Time 11 weeks
FIT Zone II
Cost Factors15:
Cost Per 100 Linear Feet $20-fc30
Survey Line Spacing 10 feet
Survey Line Length (assume 25 percent approx. 60,000 feet
of the site area needs this detailed
survey)
Survey Cost (based on $25/100 feet) $15,000
Data Analysis $ 1,800
Total Estimated Cost $16,800
Time Requirement:
Daily Coverage 2,000-3,000 feet
Time for 60,000 Foot Survey approx. 5 weeks
(assume 2,500 feet/day)
Time for Data Analysis 1 week
Total Estimated Time 6 weeks
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TABLE D-4 (Concluded)
GROUND PENETRATING RADAR SURVEY
FIT Zone I
Cost Factorsb:
Subcontract Cost for 1983 for a 5,000
Foot Survey
Data Analysis
Total Estimated Cost
Time Requirement:
Daily Coverage
Time for 5,000 Foot Survey
Time for Data Analysis
Total Estimated Time
FIT Zone II
Cost Factorsb:
Cost Per 100 Linear Feet
Survey Line Length (assume 5,000 foot
survey)
Survey Cost (assume $400/100 feet)
Data Analysis
Total Estimated Cost
Time Requirement:
Daily Coverage
Time for 5,000 Foot Survey
(assume 400 feet/day)
Time for Data Analysis
Total Estimated Time
$14,000
$ 560
$14,560
5,000 feet
1 day
2 days
3 days
$200-$600
5,000 feet
$20,000
$ 2,800
$22,800
200-600 feet
approx. 3 weeks
approz. 2 weeks
5 weeks
aFor all these estimates, a 50-acre survey is assumed, unless
otherwise noted.
bSurvey costs include labor cost unless otherwise noted.
Source: U.S. Environmental Protection Agency Field Investigation
Teams (FIT), Zones I and II, 1986, unpublished data.
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