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RCRA FACILITY INVESTIGATION GUIDANCE
VOLUME II of III
SUBSURFACE INVESTIGATIONS
October 1986
. Waste Management Division
Office of Solid Waste
U.S. Environmental Protection Agency
S~Ui.. '.
U.S. Environmental Protection Agency
Library. Room 2404 PM-211-A
401 M Street, S.W.
Washington, DC 20460
-1.
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DISCLAIMER
This Draft Final Report was prepared for the Environmental Protection
Agency by the GCA Corporation, GCA Technology Division, Inc., Bedford,
Massachusetts 01730, in fulfillment of Contract No. 68-01-6871, Work
Assignment Nos. 45 and 51. The opinions, findings, and conclusions expressed
are those of the authors and not necessarily those of the Environmental
Protection Agency or the cooperating agencies. Mention of company or product
names is not to be considered as an endorsement by the Environmental
Protection Agency.
LI
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r
CONTENTS
Summary ..... ill
Figures
Tables. . .
VOLUME I
DEVELOPMENT OF AN RFI PLAN
1. Overview of RCRA Corrective Action Process for Continuing
Releases 1-1
3. General Strategy for Release Characterization 3-1
4. Quality Assurance/Quality Control Procedures. . . 4-1
5. Data Presentation 5-1
6. Health and Safety During RCRA Facility Investigations .... b-1
7. Waste and Unit Characterization 7*1
Appendices
A. Aerial Photography, Mapping, Surveying A-l
B. Partial List of Monitoring Constituents Applicable to
Specific Media B-1
ill
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CONTENTS (continued)
VOLUME II
CHARACTERIZATION OF SUBSURFACE RELEASES
8. Soil 8-1
Overview 8-1
Approach for Characterizing Releases to Soil 8-2
Characterization of Contaminant Source and the Environ-
mental Setting 8-6
Design of a Monitoring Program 8-28
Data Presentation 8-36
Field Methods 8-37
9. Ground Water 9-1
Overview '. . . . 9-1
Approach for Characterizing Releases to Ground Water . . 9-2
Characterization of the Contaminant Source and the
Environmental Setting. ........... 9-7
Design of a Monitoring Program 9-29
Data Presentation 9-47
Field Methods . . . . . . 9-54
10. Subsurface Gas ' 10-1
Overview ...... 10-1
Approach for Characterizing Subsurface Gas Releases to
Soil 10-2
Characterization of the Contaminant Source and the
Environmental Setting . . 10-2
Design of a Monitoring Program 10-15
Data Presentation. 10-22
Field Methods 10-22
Appendices
C. Geophysical Techniques C-l
D. Lists of Ground Water Monitoring Parameters D-l
E. Flow Net/Flow Line Document E-l
F. Subtitle D Subsurface Gas Migration Model ..... F-l
VOLUME III
CHARACTERIZATION OF AIR AND SURFACE WATER RELEASES
11. Air 11-1
12. Surface Water 12-1
iv
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FIGURES
Number Page
8-1 Important release mechanisms for various unit types 8-11
8-2 Hydrogeologic conditions affecting soil moisture transport. . . 8-18
8-3 Unified soil classification system (U.S.C.S.) 8-20
8-4 Example of a completed boring log 8-38
8-5 Typical ceramic cup pressure/vacuum lysimeter 8-45
9-1 Occurrence and movement of ground water and contaminants
through (1) porous media, (b) fractured or creviced media,
(c) fractured porous media • 9-12
9-2 Ground-water flow paths in some different hydrogeologic
settings 9-13
9-3 Monitoring well placement and screen lengths in a mature
karst terrain/fractured bedrock setting .... 9-17
9-4 Monitoring well location 9-30
9-5 Example of using soil gas analysis to define probable location
of ground-water plume containing volatile organics. ..... 9-36
9-6 Vertical Well Cluster Placement 9-44
9-7 General schematic of multiphase contamination in a sand
aquifer 9-45
10-1 Subsurface gas generation/migration in a landfill ........ 10-6
10-2 Subsurface gas generation/migration from tanks and units
closed as landfills 10-7
10-3 Schematic of a deep subsurface gas monitoring well. .• 10-20
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TABLES
Number
8-1
8-2
8-3
9-1
9-2
9-3
9-4
9-5
10-1
10-2
10-3
10-4
10-5
10-6
Recommended Strategy for Characterizing Releases to Soil. . . .
«
Recommended Strategy for Characterizing Releases to Ground
Factors Influencing the Intervals Between Individual Monitoring
Recommended Strategy for Characterizing Subsurface Gas
Release Characterization Tasks for Subsurface Gas Releases. . .
Summary of Selected Ons it e Organic Screening Methodologies. . .
Summary of Candidate Methodologies for Quantification of
Typical Commercially Available Screening Techniques for
i-
Page
8-3
8-4
8-7
9-3
9-4
9-15
9-40
9-55
10-3
10-4
10-23
10-24
10-26
10-28
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SECTION 8
SOIL
8.1 OVERVIEW
The objective of an investigation of a release to soil is to characterize
the nature, extent, and rate of migration of a release of hazardous waste or
constituents to that medium. This section provides:
• a recommended strategy for characterizing releases to soils, which
includes characterization of the source and the environmental
setting of the release, and conducting a monitoring program which
wilt^characterize the release itself;
• recommendations for data organization and presentation;
• * 'appropriate field methods which may be used in the investigation; and
• a checklist of information that may be needed for release
characterization.
The exact type and amount of information required for sufficient release
I ~ " • --'V "' *• "f" • V _ •' ' -'• ....'V--. : • . < . '-• .''.-.•- s-s
/characterization will be site-specific and should be determined through
interactions between the regulatory agency and the facility owner/operator
during the RFI process. This guidance does not define the specific data
required in all instances; however, it identifies possible information
necessary to perform release characterizations and methods for obtaining this
information. The RFI Checklist, presented at the end of this section,
provides a tool for planning and tracking information for release
characterization. This list is not meant to serve as a list of requirements
for all releases to soil. Some releases will involve the collection of only a
subset of the items listed.
8-1
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8.2 APPROACH FOR CHARACTERIZING RELEASES TO SOIL
Characterizing contaminant releases to soils may employ a phased
approach. Data collected during an initial phase can be evaluated to
determine the need for or scope of subsequent efforts. Table 8-1 presents a
recommended release characterization strategy. The intensity and duration of
the investigation will depend on the complexity of the environmental setting
and the nature and magnitude of the release.
A preliminary task in any soils investigation should be to review
existing site information that might help to define the nature and scope of
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the release. Information from the RFA sepofl°should indicate the likelihood
of a release to soils from the units at the facility, and may indicate site or
waste factors conducive to inter- or intra-media transport.
The owner/operator should plan the initial characterization effort with
background information on the site, including wastes and soil
characteristics. This initial phase should serve to determine the identity of
the contaminants of concern at the site and the approximate extent of the
release. Soil physical and chemical properties should be measured to
determine the potential mobility of the contaminants in the soil. Waste and
soil properties should be examined to formulate a conceptual or qualitative
model of contaminant persistance and transport in soil. Table 8-2 lists tasks
that are generally required to characterize a release to soils, and displays
the associated techniques and outputs from each of these tasks.
Depending on the results of the initial phase, the need for further
^ O^K^-lVw r-«:c<»v\w*j.fsdU4ifcx4 HK*. ol& " cx^jt*. o
characterization will be determined by the regulatory agency. Subsequent
phasesj—if nocoooar^fc^may involve expansion of the sampling network, changes
in the study area, quantification of contaminant leaching or volatilization,
or other objectives dictated by the earlier findings. The owner/operator may
propose to input initial data into a mathematical model to aid in the choice
of additional sampling locations or to estimate waste mobility in soil. The
results of all characterization efforts should be summarized and presented in
a format appropriate to the data.
8-2
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TABLE 8-1. RECOMMENDED STRATEGY FOR CHARACTERIZING RELEASES TO SOIL
INITIAL PHASE
I. Review existing data and identify missing information
- Site features and unit operating characteristics
* Waste characteristics
—/ So.il characteristics and other environmental factors
2. Initial. Sampling Strategy
- Determine soil stratigraphy (obtain limited number of soil cores)
- Select constituents to be monitored (may require exploratory
sampling)
- Formulate conceptual model of contaminant transport and _&_
transformation •- Cw ~ri«a. C«*>K. oF *n"">
- Plan initial sampling based on site/waste/soil characteristics:
I. Define study and background areas
2. Determine sampling methods, locations, depths and numbers
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3. Initial Sampling * 1 \ ~\
- Conduct initial soil sampling and field measurements
Analyze samples for selected parameters
- Evaluate analytical and geological data
- Summarize and present data in an appropriate format
SUBSEQUENT PHASES
1. Strategy for Subsequent Sampling
- Expand sampling area and/or density
- Add/delete constituents of concern
' —•*.., Estimate "worst case" migration
•4.,, - .' 'input initial data into mathematical model to aid in selection of
'future'sampling locations (optional)
- Determine intermedia transport of components
*
2. Subsequent Sampling
Conduct additional' sampling and field measurements
- Summarize and present data in an appropriate format
Determine completeness and adequacy of collected data
8-3
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TABLE 8-2. RELEASE CHARACTERIZATION TASKS FOR SOILS
Investigatory
Tasks
Investigatory
Techniques
Data Presentation
Formats/Outputs
1.
Waste/Unit
characterization
Characterization
of environmental
setting
Determine^ ^ .
surface
features and
topography
Characterize
soil strati-
graphy and
hydrology
Refer to Section 7
List of monitoring constituents
and their chemical /physical
properties
Aerial photography
or mapping
List of uni
tributing to releases to soils
Soil,,survey map
Topographic map
Photographs
Soil core examination Soil" boring logs
Measurement of soil
properties
Soil profile, transect, or
fence diagram
Particle size distribution
Table of unsaturated hydraulic
conductivities for each soil layer
Table of soil chemistry and
structure (eg. pH, porosity) for
each soil type 1
Meteorological
Conditions
On-site meteorological
monitoring
Temperature charts
3. Release Sampling and Analysis
Characterization
Tables of monthly and annual
precipitation, runoff, and
evapo-transpiration
Map of sampling points
Table of constituent
concentrations measured at each
sampling point
(continued)
8-4
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TABLE 8-2. (continued)
Investigatory
Tasks
Investigatory
Techniques
Data Presentation
Formats/Outputs
Soil Transport
Modeling
Area and profile maps of site,
showing distribution of
contaminants
Table of input values, boundary
conditions, output values, and
modeling assumptions
Haps of present or future extent
of contamination
8-5
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8.3 CHARACTERIZATION OF CONTAMINANT SOURCE AND THE ENVIRONMENTAL SETTING
8.3.L Waste/Unit Characterization
8.3.1.1 Waste Characteristics—
The physical and chemical properties of the waste constituents affect the
fate and transport of the hazardous constituents in soil, and the selection of
sampling and analytical methods. Sources of information and measurement
techniques for waste characteristics are discussed in detail in Section 7.
Chemicals released to soil may undergo transformation or degradation by
chemical or biological mechanisms, may be adsorbed on soil particles, or
volatilized into pore gases or the overlying air space. Table 8-3 summarizes
various physical, chemical, and biological transformation/transport processes
affecting waste constituents in soil.
The chemical properties of the contaminants of concern also influence the
choice of sampling method. Important considerations include the water
solubility and volatility of the contaminants, and the potential hazards to
equipment and operators during sampling. For example, water soluble compounds
that are mobile in soil water can be detected by pore water sampling in
addition to whole soil sampling. Volatile organic contaminants require
specialized sampling and sample storage measures to prevent losses prior to
analysis.
Reactive, corrosive, or explosive wastes may pose a potential hazard to
personnel during soil sampling. Landfills can produce methane gas that could
be ignited by sparks or heat from the drilling operation. Corrosive,
reactive, or explosive wastes could damage soil coring equipment or cause
fires or explosions. Appropriate precautions against these kinds of incidents
include having adequate safety plans in place, using explosimeters or organic
vapor detectors, and employing geophysical techniques (e.g., to locate buried
drums). All soil samples should be handled as though they contained dangerous
levels of toxic substances.
Identity and composition of contaminants—The owner/opera tor should
identify and provide approximate concentrations for any constituents of
concern found in the original waste and in leachate from any releasing unit.
8-6
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TABLE 8-3. TRANSFORMATION/TRANSPORT PROCESSES IN SOIL
Process
Key factor
Biodegradation
Photodegradat ion
Hydrolysis
Oxidat ion/reduc t ion
Volatilization
Adsorption
Dissolution
Waste degradability
Wastetoxicity
^Acclamation of microbial community
-Aerobic/anaerobic conditions
pH
Temperature
Solar irradiation
Exposed surface area
Functional group of chemical
soil pH
Chemical class of contaminant
Presence of oxidizing agents
Partial pressure
Henry's Law Constant
Soil diffusion
Temperature
• f' »,.* •';-., .^
Effective surface area of soil
Cation exchange capacity (CEC)
Fraction organic content (foc) of soil
Octanol/water partition coefficient
(kow) of waste
Solubility
soil pH
Complex formation •
8-7
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Identification of other (nonhazardous) waste constituents that may affect the
behavior of hazardous constituents in the subsurface environment is also
recommended. Such constituents may form a primary leachate with transport
behavior different from pure water, and may also mobilize hazardous
contaminants bound to the soil. - of oCcv^M -ypc^^etf ^ ^r\\ Q K^AI^M
Physical state of contaminants — The physical state (solid, liquid, or
gas) of the contaminants in the soil should be determined by inspection or
from site operating records. —
Viscpsity—The owner/operator should measure the viscosity of any bulk
liquid wastes in order to estimate potential mobility in soils.
pH—Bulk liquid pH affects contaminant transport in two ways: (1) it
alters the chemical form of acids and bases, metal salts and other metal
complexes, thereby altering their water solubility and soil sorption
properties, and (2) it may alter the soil chemical or physical makeup, leading
to changes in sorptive capacity or permeability.
Dissociation constant(pKa)--For compounds that are appreciably ionized
within the expected range of field pH values, the owner/operator $houl
provide the pKa of the compound. Ionized compounds have a positive or
negative charge and are often highly soluble in water. Therefore, they are
more mobile than in their neutral forms. Compounds that may ionize include
organic and inorganic acids and bases, phenols, metal salts, and other
inorganic complexes.
Density—The density of liquid waste should be measured and the density
of each major component of the waste should be determined. Compounds with
density greater than water, such as chlorinated solvents, may migrate to the
bottom of an aquifer, while compounds less dense than water, such as gasoline,
may float on the water table. •
8-8
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Water solubility—This chemical property influences the adsorption of
chemicals to the soil. Highly water-soluble compounds tend to be very mobile
in soil and ground water. Liquid wastes which are not soluble in water may
form a distinct phase in the soil, with flow behavior differing from that of
water.
Henry's Law constant (H_)--Thia parameter indicates the partitioning
ratio of a chemical between air and water phases at equilibrium.' The larger
the value of H , the greater the tendency of the chemical to volatilize from
the water surrounding soil particles into pore gases or into above ground
*
air. The H of a contaminant affects the intermedia transport from soil to
air..
Qctanol/Water partition coefficient (k )—The characteristic
distribution of a chemical between an aqueous phase and an organic phase
(octanol) is used to predict the adsorption of organic chemicals onto soils.
It is frequently expressed as the logarithm of the value (log k ). In
transport models, k is frequently converted to k , a parameter that
takes into account the organic content of the soil. The empirical expression
used to calculate k is: k = 0.63 k f , where f is the
oc oc ow oc oc
fraction by weight of organic carbon in the soil.
Biodegradability — Many organic chemicals are rapidly degraded by soil
microorganisms. The degradation rate depends on a number of factors,
including:
The molecular structive of the contaminants. Certain manmade
compounds, (e.g., PCBs and chlorinated pesticides) are essentially
non-degradable, while others (e.g., methanol) are rapidly consumed
by bacteria. The owner /operator should consult published lists of
compound degradability to determine the persistence of waste
components in soil.
The presence or absence of oxygen in the soil. Host chemicals are
degraded far more rapidly in aerobic (oxygenated) soil. Although
unsaturated soils are generally aerobic, anaerobic conditions may.
exist under landfills and other sites where leachate with a high \
biochemical oxygen demand (BOD) is present. M
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Microbial adaptation or acclimation. Biodegradation depends on the
presence in the soil of organisms capable of metabolizing the waste
'constituents. Site-specific information on microbial communities
will rarely be available; however, if an investigation fails to
v detect compounds known to have been released to soils, the
owner /operator may wish to verify experimentally that biodegradation
is occurring. ""'
c, 4W,
v
Rates of Hydrolysis, Photolysis, and Oxidation — Chemical transformation
of waste constituents can affect the identity, amounts, and transport behavior
of these chemicals. Photolysis is only important for chemicals on the land
surface. The owner/operator should consult published sources to determine
whether individual constituents are susceptible to degradation by these
processes, but should keep in mind that most literature values refer to
aqueous systems. Degradation will also be affected by soil characteristics
such as pH, water content, and soil type.
8.3.1.2 Unit and Release Characteristics
Source factors that affect the extent of the release include:
• unit characteristics,
• point-source or nonpoint-source release type,
• the depth of the release,
• the timing of the release, and
• the quantity of the release.
8.3.1.2.1 Unit Characteristics —
Information on design and operating characteristics of a unit can be
helpful in characterizing a release. Figure 8-1 presents important mechanisms
of contaminant release to soils for various unit types. This information can
be used to identify areas for initial soil monitoring.
8.3.1.2.2 Point Source or Non-Point Source Release—
The owner/operator should establish whether the release involved a
localized (point) source or a non-point source. Units that are likely sources
of localized releases to soil include container handling and storage areas,
8-10
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Unit Type
Surface Impoundment
Landfill
Release Mechanism
Releases from overtopping
Seepage
X'
Migration of spills and other releases outside
the unit's runoff collection and containment system
Migration of spills and other releases outside the
containment area from loading and unloading
operations
Seepage through dikes or unlined portions to
surrounding soils
Waste File Migration"of runoff outside the unit's runoff
collection and containment system
Migration of spills and other releases outside the
containment area from loading and unloading
operations.
Seepage through underlying soils
Land Treatment Unit Migration of runoff outside the containment area
• Passage of leachate into'the soil horizon
Container Storage Area Migration of runoff outside the containment area
Above-ground Tank
Incinerator
Class I and IV
Injection Well
Releases from overflow
Leaks through tank shell
Leakage from coupling/uncoupling operations
Leakage from cracked or corroded tanks
Routine spills or other releases from waste handling/
preparation activities
Leakage due to mechanical failure
Leakage from waste handling operations at the
well head
*Waste transfer stations and waste recycling operations generally have
mechanisms of release similar to tanks.
Figure 8-1. Important release mechanisms for various unit types
8-11
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tanks, waste piles, and bulk chemical transfer areas such as loading docks,
pipelines, and staging areas. Non-point sources include airborne
contamination originating from incinerators, surface impoundments, wastewater
treatment units (e.g., aeration basins), and waste piles. A less frequent
source of non-point releases is catastrophic fires or explosions. Land
treatment can also result in non-point releases beyond the treatment zone if
such- units are not properly designed and operated.
The primary characteristic of a localized release is a limited area of
high contamination surrounded by larger areas of clean soil. Therefore, the
release characterization should .determine the boundaries of the contaminated
area, and minimize the expenditure of resources in analyzing numerous clean
samples. Where appropriate, a survey of the area with an organic vapor
analyzer or other rapid screening technique will aid the owner/operator to
narrow the area under investigation. Stained soil may provide additional
evidence of contamination. It should be emphasized, however, that even if the
extent of contamination appears obvious, it is the responsibility of the
owner/operator to verify boundaries by analysis of samples from outside of the
contaminated area.
Non-point releases that occur via air deposition generally have a
characteristic distribution; concentrations often decrease logarithmically
away from the source, and generally have low variability within a small area.
The highest contamination concentrations tend to follow the prevailing wind
directions. Non-point releases occurring via other mechanisms (e.g., land
treatment) may be distributed more evenly over the affected area. In these
situations, a large area may need to be investigated in order to determine the
extent of contamination. However, the relative lack of "hot spots" may allow
the number of samples/unit of area to be smaller than for a point source
release.
8.3.1.2.3 Depth of Contamination—
The plan should consider the original depth of the release to soil, and
the depth to which contamination may have migrated since the release. Often,
releases occur initially at the soil surface, as a result of spillage or
leakage. Direct release to the subsurface can occur due to leaking
underground tanks, buried pipelines, waste piles, impoundments, and landfills.
8-12
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I
The depth to which a waste migrates after release depends on the volume
of waste and water infiltrating the soil, the age of the release, and the
chemical and physical properties of the waste and soil. In a homogenous soil,
contaminants tend to move directly downwards through the unsaturated zone.
Lateral movement will occur only through dispersion. Changes in soil
structure or composition with depth (stratification), zones of saturated soil,
fractures, and other discontinuities may cause contaminants to spread
horizontally for some distance before migrating downwards. Examination of
soil cores and accurate measurement of soil physical properties and moisture
content are therefore required to estimate the potential for contaminant
t ransport.
Mechanisms other than water transport can move contaminants downwards;
these include diffusion within the liquid and vapor phases of the"soil
including gravity flow of the liquid phase, turnover of the soil by burrowing
animals, freeze/thaw cycles and plowing, or other human disruption. All
factors that might affect the depth of contamination should be considered.
The owner/operator should use existing information to estimate the extent of
contamination, and should also propose sampling to confirm these estimates.
Monitoring of releases to soil will differ substantially depending on the
depth of the contamination. For both near surface and subsurface soils
*'•"•-• • • •
investigations, a phased approach may be used. Initial characterization will
often necessitate a judgemental approach in which sampling locations are
chosen based on existing information (e.g., topography, soil stratigraphy, and
— STC^C*^VC*A-
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8.3.1.2.4 Timing of the Release—
Many factors that affect the extent of contamination are time-dependent.
These factors include:
• Age of the release;
• Duration of the release; and
• Frequency of the release.
The length of time that has passed since a release occurred can affect
the extent of contamination, the chemical composition of the contaminants
present in soil, and the potential for inter-media transport. Recent releases
tend to be similar in composition to the parent waste material and may be more
concentrated within the original boundaries of the release. If a recent
release occurred at the land surface, volatilization to air or dissolution in
overland runoff may be important transport mechanisms. Older releases, on the
other hand, may have undergone extensive chemical or biological changes,
altering their original composition, and may have migrated a considerable
distance from their original location. If the contaminants are mobile in
soil, transport to ground water may be a concern, while soil-bound
contaminants may be affected by surface transport of soil particles by water
or air. These factors should be considered in the selection of monitoring
constituents and sampling locations.
The duration and frequency of the release-will affect the amounts of
waste released to the environment, and also the distribution of contaminants
in the soil. A release that consisted of a single episode, such as a spilled
drum or other short-term leaks, may move as a discrete "slug" of contamination
through the soil. On the other hand, multiple or continuous releases may
present a situation in which contaminants exist at different stages of
migration from the source and/or chemical and biological decomposition.
Estimations of contaminant fate and transport will generally require that the
owner/operator determine which of these cases apply at the site.
8-14
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8.3.1.2.5 Magnitude of the Release—
The quantity (mass) of waste released to soil and the rate of release
will affect the geographical extent and nature of the contamination. Every
oil type has a certain capacity to bind contaminants; this is called the
wj sorptive capacity of the soil. When the sorptive capacity is exceeded,
contaminants tend to leach through the soil to ground water. Therefore, a
"minor" release may be immobilized in shallow soils, while a "major" release
result in ground-water contamination. The physical processes of
volatilization and of dissolution in water are also affected by contaminant
A* concentrations; therefore, the extent of intermedia transport is affected by
the amounts of contaminants present in the soil. Information on the magnitude
of the release should be estimated from site operating records, unit design
features, and other sources.
8.3.2 Characterization of the Environmental Setting
The nature and extent of contamination is affected by environmental
processes such as dispersion and degradation, acting after the release
occurred. Factors which should be considered include soil physical and
chemical properties, subsurface geology and hydrology, and climatic or
meteorologic patterns. These factors are discussed below.
Three characteristics of the soil medium must be considered in order to
obtain representative samples for chemical or physical analysis:
• .the large spatial variability of soil properties and contaminant
distribution;
• spatial and temporal fluctuations in soil moisture content; and
• the presence of solid, liquid, and gaseous phases in the unsaturated
zone above the water table.
8.3.2.1 Spatial variability—Spatial variability, or inhomogeneity, can be
defined as horizontal or vertical differences in soil properties occurring
within the scale of the area under consideration. Vertical discontinuities
are found in most soil profiles as a result of climatic changes during soil
formation, alterations in topography or vegetative cover, etc. Soil layers
8-15
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show wide differences in their tendency to adsorb contaminants or to transmit
contaminants in a liquid form; therefore, a monitoring program that fails to
consider vertical stratification will likely result in an inaccurate
assessment of contaminant distribution. Variability in soil properties may •
also occur in the horizontal plane as a result of factors such as drainage,
slope, land use history, and plant cover.
Soil and site maps will aid in designing sampling procedures by
identifying drainage patterns, areas of high or low surface permeability, and
areas susceptible to wind erosion and contaminant volatilization. ~
Maps of unconsolidated deposits may be prepared from existing soil core
/
measurements, well drillers' logs, or from previous geological studies.
Alternately, the information must be obtained from new soil borings. Since
soil boring is an expensive and time-consuming procedure, it will usually be
to the advantage of the owner/operator to also obtain samples from these cores
for preliminary chemical analyses.
The number of cores required to characterize site soils depends on the
site's geological complexity and size, and on the perceived importance of
detecting small-scale discontinuities in surficial materials. Another
consideration is the potential risk of spreading the contamination as a result
of the sampling effort. For example, an improperly installed borehole casing
could lead to leakage of contaminated water through a formerly impermeable
clay layer. The risks of disturbing the subsurface must be considered equally
with the need for obtaining more data.
Chemical and physical measurements should be made for each distinct soil
layer, or boundary between layers, which might be affected by a release.
During drilling, the investigator should note on the drilling log depths of
soil horizons, soil types and textures, and the presence of cracks, channels,
and zones containing plant roots and animal burrows.
Soil variability should be accounted for by inclusion of replicate
measurements of soil chemical and physical properties. Determination of the
range and statistical variability of values for soil parameters will allow
more accurate prediction of the mobility of contaminants in the soil.
8-16
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8*3.2.2 Spatialand temporal fluctuations in soil moisture content—Movement
of hazardous waste or constituents through soil to ground water occurs
primarily by transport of dissolved chemicals in pore water. Soil moisture
affects the hydraulic conductivity of the soil and the transport of dissolved
wastes through the unsaturated zone. Therefore, it is very important to
characterize the storage and flow of water in the unsaturated zone. Moisture
in the unsaturated zone is in a dynamic state that is constantly acted.upon by
competing physical forces.
Water applied to the soil surface infiltrates downward under the
influence of gravity until the soil moisture content reaches equilibrium with
capillary forces. A zone of saturation may occur beneath the bottom of a unit
(e.g., an unlined lagoon) if the unit is providing a constant source of
moisture. In a low porosity soil, such a saturation front may migrate
downward through the unsaturated zone to the water table, and create a
ground-water or liquid "mound".
In a higher porosity soil, the saturation zone may only extend a small
distance below the unit, with liquid then draining through the soil toward the
water tables without complete saturation (see Figure 8-2).
In certain cases, soil moisture characterization can also be affected by
the presence of isolated zones of saturation and fluctuations in the water
table depth, as illustrated in Figure 8-2. These factors should be considered
in the investigation where there is evidence of migration below the soil
surface, by careful characterization of subsurface geology and measurement of
hydraulic conductivity in each layer of soil that could be affected by
subsurface contamination.
8.3.2.3 Solid, liquid, and gaseous phases in unsaturated zone—Soil in the
unsaturated zone contains solid, liquid, and gaseous phases. Depending upon
the physical and chemical properties of the waste, a hazardous constituent may
be found primarily bound to the soil, dissolved in the pore water, as a vapor
within the pores or interstitial spaces, or as a distinct liquid phase. The
investigation should therefore take into consideration the predominant form of
the contaminant in the soil. For example, some whole-soil sampling methods
lead to losses of volatile chemicals, while analysis of soil pore water will
not detect low solubility compounds such as PCBs that remain primarily
adsorbed to the solid phase. Release characterization procedures should
8-17
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8-18
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consider chemical and physical properties of both the soil and the waste to
assist in determining the nature and extent of contamination due to a release
to soil.
Soil Classification—The owner/operator should classify each soil layer
potentially affected by the release. The owner/operator should select one or
more of the classification systems cited below, based on the data provided by
the procedure and the objectives of the investigation.
• USDA Soil Classification system (USDA, 1975)—Primarily developed
for agricultural purposes, the USDA system also provides information
on typical soil profiles (e.g., 1-foot fine sandy loam over gravelly
sand, depth to bedrock 12 feet), ranges of permeabilities for each
layer, and approximate particle size ranges. These values are not
accurate enough for predictive purposes, however, and should not be
used to replace information collected on site. Existing information
on regional soil types is available, but suitable for initial
planning purposes only. U.S. Department of Agriculture (USDA)'
county soil surveys may be obtained for most areas.
• Unified Soil Classification Systerns-(USCS) (Lambe and Whitman,
1979)—Minimum procedure for qualitative field classification of
soils. This system should be used to identify layers on soil boring
logs. The USCS is based on field determination of the percentages
of gravel, sand and fines in the soil, and on the plasticity and
compressibility of fine-grained soils. Figure 8-3 displays the
decision matrix used in classifying soils by this system.
The above classification systems are adequate for descriptive purposes,
and for qualitative estimates of the fluid transport properties of soil
layers. Quantitative estimation of hydraulic*conductivity requires a
laboratory determination of the particle size distribution for each soil
layer, as described below.
Particle size distribution—A measurement of particle size distribution
should be made for each layer of soil potentially affected by the release.
The recommended method for measurement of particle size distribution is ASTM
D422 (ASTM, 1984).
' The particle size distribution has two uses in a soils investigation:
(I) estimation of the hydraulic conductivity of the soil by use of the Hazen
formula, and (2) assessment of soil sorptive capacity.
8-19
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8-20
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1. The hydraulic conductivity(K) may be estimated from the particle
size distribution using the Hazen formula:
K«Ad2
10
Where d^Q is equal to the effective grain size, which is that
grain-size diameter at which 10 percent by weight of the particles
are finer and 90 percent are coarser (Freeze and Cherry, 1979). The
coefficient A is equal to 1.0 when K is in units of cm/sec and d^Q
is in mm. Results should be verified with in-situ hydraulic
conductivity techniques.
2. Particle size affects sorptive capacity and, therefore, the
retardation of contaminants in the soil. Sandy soils generally have
low sorptive capacity while clays have a high affinity for heavy
metals and some organic contaminants. This is due in part to the
fact that small clay particles have a larger surface area in
relation to their volume than do large sand particles. Large
' surface areas lead to stronger interactions with waste molecules.
Clays also bind contaminants due to the chemical structure of the
clay matrix.
Porosity-—Soi1 porosity is the volume percentage of the total soil volume
not occupied by soild particles (i.e., the volume of the voids). In general,
the greater the porosity, the more readily fluids may flow through the soil.
. An.exception is clay soils, .in which fluids are held tightly by capillary
forces. Porosity is usually measured by oven-drying an undisturbed sample and
weighing it. It is then saturated with liquid and weighed again. Finally,
the saturated sample is immersed in the same liquid, and the weight of the
displaced liquid is noted. Porosity is the weight of liquid .required to
saturate the sample divided by the weight of liquid displaced, expressed as a
decimal fraction.
Hydraulic conductivity—An essential physical property affecting
contaminant mobility in soil is the hydraulic .conductivity. This property
indicates the ease with which water at the prevailing viscosity will flow
through the soil, and is dependent on the porosity of the soil, grain size,
degree of consolidation and cementation, and other soil-factors.
Measurement of hydraulic conductivity in the saturated soil below the
water table is fairly routine. Field and laboratory methods to determine
saturated conductivity are discussed in the section on ground-water
8-21
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investigations (Section 9). Measurement of unsaturated conductivity is
difficult because the value changes with changing soil moisture content.-
Therefore, conductivities for the entire range of field moisture content may
need to be determined for each type of soil at the site. It is also important
to note that, unless otherwise specified, unsaturated hydraulic conductivity
refers to flow in the vertical direction, while saturated conductivity refers
to flow in the horizontal direction.
A detailed discussion of field and laboratory methods for determining
saturated and unsaturated hydraulic conductivity is contained in Soil
Properties Classification and Hydraulic Conductivity Testing (U.S. EPA,
1984). In general, field tests are required when the soil is heterogenous,
while laboratory tests may suffice for a soil without stratigraphic changes.
Estimation of hydraulic conductivity from the particle size distribution may
be used as a rough estimate if precise values are not required.
Relative permeability—The hydraulic conductivity of a soil is usually
established using water as the infiltrating liquid. However, at sites where
there is the likelihood of a highly contaminated leachate or a separate liquid
waste phase, the owner/operator should also determine conductivity with that
liquid. The ratio of the permeability of a soil to a non-aqueous solution and
its permeability to water is known as relative permeability.
The importance of determining this value is due to the potential effects
of leachate on soil hydraulic properties. Changes in conductivity from
infiltration of leachate may result from differences in the viscosity or
surface tension of the waste, or the leachate may affect the soil structure so
as to alter permeability. Studies of waste migration through landfill liners
have demonstrated that certain wastes may cause shrinking or expansion of clay
molecular structures, dissolve clays and organic matter, clog soil pores with
fine particles, and cause other changes that affect permeability.
Soil sorptive capacity and Soil-Water Partition Coefficient (K )—The
mobility of contaminants in soil depends not only on properties related to the
physical structure of the soil, but also on the extent to which the soil
material will retain, or adsorb, the hazardous constituents. The extent to
which a constituent is adsorbed depends on chemical properties of the
8-22
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constituent and of the soil. Therefore, the sorptive capacity must be
determined with reference to particular constituent and soil pair. The
soil-water partition coefficient (K_) is generally used to quantify soil
sorption, K_ is the ratio of the adsorbed contaminant concentration to the
dissolved concentration, at equilibrium.
There are two useful approaches to determining K : (1) soil adsorption
laboratory tests, and (2) prediction from soil and constituent properties.
The Soil Adsorption Isotherm (AI) test is widely used to estimate the
extent of adsorption of a chemical (i.e., contaminant) in soil systems.
Adsorption is measured by equilibrating aqueous solutions containing
different, environmentally realistic, concentrations of the test chemical with
a known quantity of clean soil. After equilibrium is reached, the
distribution of the chemical between the soil and water(K_J is measured by a
suitable analytical method.
The AI test has several desirable features. Adsorption results are
highly reproducible. The test provides excellent quantitative data, readily
amenable to statistical analysis. In addition, it has relatively modest
requirements for chemicals, soils, laboratory space and equipment. The ease
of performing this test will depend on the physical/chemical properties of the
contaminant and the availability of suitable analytical techniques to measure
the chemical.
The AI test can be used to determine the soil adsorption potential of
slightly water soluble to infinitely water soluble chemicals. In general, a
chemical having a water solubility of less than 0.5 ppm is not tested with
this method because these chemicals are essentially immobile in soil.
The U.S. EPA Office of Pesticides and Toxic Substances (U.S. EPA 1982a,
1982b) has compiled information on the use of the AI test, including a
detailed discussion of apparatus, procedures, sources of error, statistical
requirements, calculation methods, and limitations of the test.
The second approach for determining K_ is to estimate the value from
soil and waste properties. Soil properties that should be considered under
this approach are: (I) particle size distribution; (2) cation exchange
capacity; and (3) soil organic carbon content. The waste properties that must
be determined will vary depending on the type of waste. Lyman et al. (1981)
8-23
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discuss several methods for estimating 1C. from chemical properties of the
constituent (i.e., K and water solubility) and the soil organic content.
Data collection needs for waste properties were discussed earlier in this
section.
Cation exchange capacity (CEC)—This parameter represents the extent to
which the clay and humic fractions of the soil will retain charged species
such as metal ions. The CEC is an important factor in evaluating transport of
lead, cadmium, and other toxic metals. Soils with a high CEC will retain
correspondingly high levels of these inorganics. The hazardous chemicals will
be prevented from leaching to ground water in the short-term, but in the
long-term these soils may be a reservoir for continuing releases. A method
for the determination of CEC is detailed in SW-846, Method 9081 (U.S. EPA,
1982c).
Organic carbon content—The amount of natural organic material in a soil
has a strong effect on retention of organic pollutants. The greater the
fraction by weight of organic carbon (f ), the greater the adsorption of
organics. Soil f ranges from under 2 percent for many subsurface soils to
over 20 percent for a peat soil. The owner/operator should use an estimate of
f based on literature value;
oc
information is not available.
f based on literature values for similar soils, if site specific
Soil pH—Soil pH affects the mobility of potentially ionized organic and
inorganic chemicals in the soil. Compounds in these groups include organic
and inorganic acids and bases, and metals.
Depth to Water Table—The thickness of the unsaturated zone may affect
the attenuation capacity of the soil and the time taken for contaminants to
migrate to ground water. Seasonal fluctuations should be identified if
significant, as well as changes due to pumping or other factors.
Pore Water Velocity—Pore water velocity impacts the time of travel of
contaminants from soil to ground water. The pore water velocity can be
calculated by the following equation:
8-24
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V * q/6
where: V ** pore water velocity, cm/day
q * volumetric flux/unit area, cm/day
6 » volumetric water content, dimensionless
Percolation (volumetric flux per unit area)—Movement of contaminants
from soil to ground water occurs primarily via dissolution and transport with
percolating soil water. It is important, therefore, to determine the volume
of water passing through the soil. The percolation rate, or volumetric flux,
must be determined in order to calculate pore water velocity through the
unsaturated zone. The rate of percolation can be estimated from the water
balance equation:
PER • P - ET -DR
where: PER = Percolation/Recharge to ground water, cm/yr
P « Precipitation and irrigation
ET = Evapotranspiration
DR = Direct Surface Runoff
Annual averages" for P," ET and DR*should be obtained from existing local
sources. Sources of information to estimate PER include:
' • State or Regional water agencies;
• Federal water agencies.(US Geological Survey, National Weather
Service, Forest Service); and
• Weather stations.
It is recommended that site-specific ET and DR data be used if possible, since
local conditions can vary significantly from regional estimates.
Volumetric Water .Content(6)—The volumetric water content is the percent
of total soil volume which is filled with water. It is equal to the amount of
water lost from the soil upon drying to constant weight at 105°C, expressed as
the volume of water/bulk volume of soil. This parameter affects the
8-25
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unsaturated hydraulic conductivity, and is required for calculation of pore
water velocity. At saturation, the volumetric water content is equal to the
porosity of the soil.
Additional Soil Conditions— Certain soil conditions will require special
consideration in data interpretation or in choice of sampling method.
i
• In certain dense, cohesive soils pore water moves primarily through
narrow solution channels or fracture zones, rather than permeating
the bulk of the soil. This condition can sometimes be recognized by
dark-colored deposits marking the fractures, or by the tendency of
soil cores to break apart at the discontinuity.
Alluvial "ffraveTsoils and fractured rocky soils generally have a low
primary porosity, but a high secondary porosity (i.e., most flow
occurs through large cracks and channels rather than through the
soil pores). As a result, the rate of flow is high, and the low
surface area of the fracture system provides a very low sorptive
capacity. Field variability is very high, because if the core does
not intersect the contaminated fracture(s), contamination will not
be detected. Pore water may travel for long distances along
horizontal fractures before reaching ground water.
Certain clay soils known as Vertisols, or expandable clays, will
fracture into large blocks when dry. These cracks can be a direct
route for ground water contamination. The owner/operator should
determine whether these soils are present at the site, by consulting
soil surveys. They occur in, but are not limited to, eastern
Mississippi and central and southern Texas. Other clay soils may
also develop cracking to a lesser degree. In these cases, it is
advisable to sample during both wet and dry seasons.
Sampling saturated soils may be accomplished with the same coring
and drilling techniques used for unsaturated soil core sampling.
Particular care must be taken to prevent contamination between soil
layers, and to preserve the borehole integrity. Generally, this
requires installation of a steel casing, which is lowered as the
borehole advances.
Frequently, the choice of sampling technique is dictated by
mechanical factors. Hard, rocky, or dense soils may prevent the use
of manual tube samplers or augers. Even power-driven corers
equipped with split-spoon samplers will fail to penetrate some
soils. Power augers will penetrate most unconsolidated materials,
but will not drill through rock, for which an air-driven rotary
drill is the preferred method. Loose sandy soils will often fail to
be retained in a tube sampler. Core sampling should generally be
carried out under the supervision of an experienced driller, as it
is easy to obtain poor results or to damage equipment.
8-26
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• Where unfavorable soil conditions interfere with proposed sampling
locations, it may be necessary to move the sampling point to a
nearby location. In the event that such conditions are encountered,
new locations should be chosen which are adequate to characterize
the release.
8.3.3 Sources of Existing Information
Considerable information may already be available to assist in
characterizing a release. Existing information should be reviewed to avoid
duplication of previous efforts and to aid in focusing the RFI. Any existing
information relating to releases from the unit, and to hydrogeological,
meteorological, and environmental factors that could influence the
persistence, transport or location of contaminants should be reviewed. This
information may aid in:
• Delineating the boundaries of the sampling area;
• Choosing sampling and analytical techniques; and
• Identifying information needs for later phases of the investigation.
Information may be obtained from readily available sources of geological and
meteorological data, waste characteristics, and facility operations records.
8.3.3.1 Geological and Climatological Data—
The Federal government and most state governments compile geological
data, soil surveys, land use records, and climatological information. These
sources should be consulted for local geology, soil types, historical
precipitation and water table records, and other useful data. Sources to be
consulted for soils data include the Soil Conservation Service (SCS),
Agricultural Stabilization and Conservation Service (ASCS), the U.S.
Geological Survey (USGS), state soils bureaus and agricultural extension
services, university soil science departments, and private consultants.
Additional sources of geologic information include geotechnical boring logs'
for foundation studies and well logs made during drilling of water supply
8-27
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wells. These logs should indicate the depth, thickness, and character of
geologic materials, and the depth to the water table. Climate and weather
information can be obtained from:
National Climatic Center
Department of Commerce
Federal Building
Ashville, North Carolina
Tel: (704)258-2850
28801
8.3.3.2 Facility Records and Site-Specific Investigations—
The owner/operator should review information from the RCRA Facility
Assessment (RFA) in planning the RFI. For example, if soil sampling was
performed in the RFA, analytical results should be reviewed. RFA sampling
data may help in defining the boundaries of a proposed sampling area. RFA
information may also note indications of releases such as visible surface
contamination, stressed vegetation, or obvious defects in unit design and/or
operation.
Facility records, the RCRA Part B permit application, and any previous
site reports should also be examined for any other information on unit
V
characteristics, wastes produced at the facility, and other factors relevant
to soil releases. Facility operating records should have data on wastes
treated, stored, or disposed of at the facility. Wastes regulated under the
RCRA manifest system are identified by a waste code which can aid in
identifying constituents of concern. Wastes originating within the facility
may have been analyzed for process control. Unit releases (e.g., losses from
leaking tanks) can sometimes be estimated from storage records.
8.4 DESIGN OF A MONITORING PROGRAM
8.4.1 Objectives of the Monitoring Program
Monitoring procedures which specify locations, numbers, depths, and
collection techniques for soil samples should be prepared by the
owner/operator prior to each sampling effort. These procedures should provide
8-28
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the justification for the proposed samples, in terms of their expected
contribution to the investigation. Examples of soil monitoring objectives
include:
N
• Search for soil contamination in a drainage channel where a spill is
known to have occurred.
* Establish a random or systematic grid sampling network to determine
average contamination levels in several zones of a-large area
affected by airborne deposition.
• Fill in data gaps concerning the transport of waste constituents
within a permeable soil layer.
j
In preparing soil monitoring procedures, the owner/operator should take
into consideration those factors discussed in Section 8.3.1 through 8.3.3 that
apply to their facility.
8.4.1.1 Initial Characterization Effort
The objectives of an initial soil characterization effort are generally
to begin characterizing releases identified or suspected in the RFA, and to
make associated soil physical and chemical measurements required for a more
comprehensive release characterization. In developing the approach the
owner/operator should determine the following:
• Parameters to be monitored:
• Sampling methods;
• Approximate study and background areas; '
• Sample locations (judgemental or systematic approach); and
• Number of samples to be collected.
8.4.1.2 Subsequent Characterization Phases
Depending on the outcome of the initial characterization effort, the
owner/operator may be required to. obtain additional data to characterize the
8-29
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release. The findings of the initial phase will dictate the objectives of any
later phases, but later phases will often involve:
• Expanding the number of sampling locations to a wider area or
depth, or increasing sampling density where data are sparse.
• Institution of a grid sampling approach to assess releases
identified by judgemental sampling (see Section 3).
• Addition of specific monitoring parameters.
• Use of indicator parameters when appropriate.
• Sampling in areas of interest identified by modeling or
sampling to confirm predictions of the model (i.e., extent of
transport).
There is no set or recommended number of phases to complete a RFI. The
owner/operator should determine by consultation with agency officials, whether
the collected data are sufficient to meet the objectives of the investigation.
8.4.2 Monitoring Constituents
8.4.2.1 Constituents for monitoring—The owner/operator should propose
hazardous constituents for monitoring based on the composition of wastes known
to be present or released to soils at the site. Appendix B may be used to
select monitoring constituents. Additional measurements may include
nonhazardous constituents which could indicate the presence of a hazardous
constituent (indicator parameters), or which could mobilize or otherwise
affect the fate and transport of hazardous constituents. Chemical and
physical properties of the soil that can be measured from soil samples should
also be included in the list of parameters.
Justification of constutuent selection may be provided through detailed
facility records or preliminary analytical results. In lieu of this
justification, the owner/operator may be required to perform a broader
analytical program (i.e., Appendix VIII of 40 CFR Part 261).
After selecting monitoring constituents, the owner/operator should
contact a laboratory with experience in soils analysis to obtain guidance on
compound-specific requirements for sampling and sample preservation. The
8-30
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laboratory should have knowledge of EPA protocols and analytical procedures.
Guidance on quality control (QC) requirements for sampling is given in
Section 4.
8.4.2.2 Sampling methods~Both soil and pore water sampling may be utilized
during initial monitoring. Chemical analysis of soil core samples may be used
to identify all constituents of concern present in the soil, whether they are
adsorbed to the solid matrix or primarily dissolved in the soil water.
Lysimeters can be installed in the boreholes created during core sampling, to
monitor mobile constituents that may migrate to ground water. Description of
sampling techniques for soils and pore water is provided in Section 8.6.
Appropriate sample collection and preservation techniques should be
specified. When a soil sample is removed from its surroundings, chemical and
physical changes begin immediately. These include moisture loss, oxidation,
gas exchange, loss of volatile components, increased or decreased biological
activity, and potential contamination of the sample. Therefore, the
owner/operator should propose measures to store and preserve samples to
minimize their degradation. Sampling techniques should not adversely affect
analytical procedures and results. For example, use of drilling muds or
fluids to lubricate soil augers can introduce organic or inorganic
contaminants that may make quantification of the contaminants of concern
impossible. The practice of coating metal parts with oils or greases to
prevent rust will have a similar effect.
Highly volatile compounds can sometimes be detected with a portable
photoionization detector (e.g., HNu or Fhotovac) or an organic vapor analyzer
(OVA) from the soil surface. Organic vapors can be detected and measured in
shallow boreholes and in ground-water monitoring wells. Vapor sampling is
especially appropriate for initial characterization, since it is a rapid,
semiquantitative survey technique. The benefits of this approach are:
• The investigator can learn quickly whether a sample is contaminated,
allowing attention to be redirected to other areas if necessary; and
• Samples that would undergo chemical changes with storage can be
analyzed immediately.
8-31
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However, there are several drawbacks Co onsite analysis:
* The number of samples that can be processed onsite is usually low;
and
• Maintenance of acceptable laboratory quality control practices in
the field is difficult.
8.4.3 Monitoring Locations
8.4.3.1 Determine study^n(| background areas—Determination of the area of
interest will depend on the site layout, topography, the distribution of
surface soils, soil stratigraphy, and information on the nature and source of
the suspected release. The size and type of unit may affect the size of the
area under consideration. For example, a landfill may only require monitoring
the surrounding soil while an inactive land treatment facility might require
sampling over the entire unit.
•—-.
High variability in the chemical composition of soils makes determination
of background levels for the constituents of concern essential. This is
particularly important for quantification of metals, since many toxic metals
can occur naturally at parts per million concentrations in soil.
Background areas (not affected by any facility releases) should be
selected based on their similarity to the study area in terms of soil type,
drainage, and other physical factors. Background soil samples should be taken
from areas that are not near a suspected source of contamination, and are
taken from the same stratigraphic layer as the study samples, if possible.
Selection and sampling of appropriate background areas is important, because
confirmation of a release in a heavily polluted area may depend on a
comparison of study and background concentrations.
The owner/operator could increase efficiency in the initial
characterization effort by using a surveying technique or a surrogate
measurement to establish the extent of the study area such as the HNu or OVA
to locate surface contamination by volatiles as described above. Subsurface
soil contamination can sometimes be identified by geophysical techniques such
as electromagnetic and resistivity techniques (see Appendix C). Surrogate
measurements are tests of chemical or physical waste parameters that are
8-32
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faster and generally less costly than a complete analysis for the constituents
of concern. For example, Total Organic Halogen (TOX) analysis may be used to
detect total chlorinated solvents in place of a GC/MS analysis.
8.4.3.2 Determine Location and Number of Samples—The owner/operator should
propose the area to be investigated and the number of samples to be collected
and analyzed. Samples should be take" fi-f«n «-HO vicinity of all units
identified as potential sources of soil contamination. The total number of
samples required for the initial investigation will depend on the extent of
prior information, the extent and severity of the release, and the objectives
of the characterization. However, the following general guidance will aid the
owner/operator to sample efficiently.
• Sampling efficiency is increased by use of proportional sampling,
which involves dividing the site into zones, based on proximity to
the release source or other factors. The number of samples taken in
each zone is proportional to the zone's area.
• Use of composite samples will allow detection of contamination over
a wide area with a smaller number of analyses. Compositing involves
> pooling and homogenization of multiple soil samples from a limited
area of the site. The' composite is then analyzed to give an average
*"" " value for soil contamination in that area. The following
limitations on compositing should be observed:
1. The possibility of detecting low levels of contamination
decreases as the number of samples forming the composite
increases. For example, if a single sample containing 100 ppb
of PCBs were composited with nine other samples with 0 ppb, the
composite would contain only 10 ppb. The high value would
therefore be less likely to be detected. If the owner/operator
proposes to use composite samples, he or she should include an
example calculation to show that target levels for -the
investigation can be detected with the proposed compositing
approach.
2. Compositing is most useful when large numbers of soil samples
can be easily collected (i.e., for shallow contamination). In
order to obtain the maximum information from deep soil coring,
it is recommended that compositing not be used for core samples.
3. Compositing should not be used for soils contaminated with
volatile organics, as the constituents of interest may be lost
during homogenization and storage of the samples.
8-33
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If the owner/operator chooses to employ a. statistical approach in
the investigation, the number of samples collected must be adequate
to meet the level of confidence proposed for the statistical
decision. For example, an investigator wishes to show that average
levels of heavy metals in soil downwind from a surface impoundment
are not significantly different from background levels in the
region. To obtain a valid test of the difference of the two means,
the number of samples must be chosen based on the desired level of
confidence (e.g., 95 percent) and the variability of the measured
parameter. Further guidance on use of statistical methods in soil
investigations may be obtained from the following documents.
1. Soil Sampling Quality Assurance User's Guide: by D. S. fiarth
and B. J. Mason, EPA 600/4-84-043, 1984.
2. Preparation of a Soil Sampling Protocol: Techniques and
Strategies, by B. J. Mason, EPA 600/4-83-020, 1983.
Characterization of contaminant distribution with depth requires
that samples be taken from every distinct layer of soil which might
be affected by the release, and from boundaries between soil
layers. If the soil profile contains thick layers of homogenous
soil, samples should be taken at regular intervals (e.g. every
5 feet). In addition, samples should be taken where cores intersect
fractures, animal burrows, or other features which could affect
waste transport. The owner/operator should also propose measurement
of soil physical and hydraulic properties in each distinct soil
layer. The aim in taking these measurements as part of the initial
characterization effort is to identify properties that vary with.
depth, so as to allow a stratified sampling approach to be used in
any future sampling phases. Also, these properties must be measured
to refine conceptual models of contaminant transport, or for input
into mathematical models of soil transport.
Prediction of contaminant fate and transport can range from a
"conceptual" model of waste behavior in the soil to complex computer programs
requiring extensive input of soil and water budget data. Predictive modeling
finds two common uses in soil investigations: input of site-specific data to
locate appropriate sampling locations, and estimation of the future rate and
extent of contaminant transport to predict future migration.
Estimation of waste transport in the unsaturated zone is difficult due to
the high spatial variability in soil physical and hydrologic properties.
Modeling should not be used to replace actual measured values (e.g., to
establish the limits of waste leaching or diffusion in soil). However, if
8-34
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used with caution, models can act as useful tools to direct the sampling
program by directing sampling towards site areas (e.g., a permeable soil
layer) identified as soil water flowpaths.
ComputerModels—A number of computer models of soil contaminant
transport have been developed. These vary widely in their complexity.
Because these models, particularly those including fate processes, require
extensive input of site-specific soils and waste data, mathematical modeling
should only be attempted if the site complexity warrants the time and resource
expenditure.
To select a model the owner/operator should consider the following
factors:
Applicability—The model should be suitable for the objectives of
the study. For example, if the parameter to be modeled is the
time-of-travel of an individual hazardous constituent from the
surface to a distant water supply well, the model should reflect
both saturated and unsatiirated zone transport,'and soil adsorption
behavior. However, if the site is located above a shallow water
table, and the constituent of interest is known to be very mobile in
ground water, a saturated flow model may suffice.
Limitations—Models vary in their ability to handle complex site
geometry, soil stratification, and other heterogeneities. The model
should have sufficient capabilities to approximate site conditions.
Data Requirements—The amount of site-specific data required to
model soil fate and transport increases with increasing model
complexity. Data requirements for analysis of contaminant .transport
in the unsaturated zone might include:
volumetric water content;
degree of water saturation;
- particle density;
bulk density;
- pressure potential;
relative permeability;
- field capacity; and
- water diffusivity.
In addition, the model may require specification of sampling grid
coordinates, spatial and temporal distribution of soil moisture,
waste chemical and physical properties, etc.
8-35
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• Effort and Resources Needed—The computation power and computing
time required to run a model generally increase with model
complexity. Another important consideration is the length of time
required to learn to use the model. The model should be available
in a format compatible with computer resources, and should include
adequate documentation and/or technical support by the supplier of
the program.
Numerous models have been developed to calculate flow and transport under
saturated and unsaturated soil conditions. Ground water (saturated flow)
models are discussed in Section 9. A U.S. Nuclear Regulatory Commission
Report (Oster, 1982) reviewed the applicability of 55 unsaturated flow and
transport models. Computer models selected for use in the RFI should I) be
well documented; 2) have been peer reviewed; and 3) have undergone extensive
field testing. All documentation (i.e., documentation on the model's theory,
structure, use, and testing) should be available for examination by the
regulatory agency. Access to the model, along with relevant data sets, should
also be available upon request. The EPA may require that a sensitivity
analysis be performed, and that the results of the analysis be submitted
together with the model results.
8.5 DATA PRESENTATION
Data obtained in a soil investigation may include:
•
*
Soil classifications;
Soil boring logs;
Measurements of soil physical or hydrologic characteristics;
Onsite survey results: HNu/Photovac, geophysical techniques; and
Chemical analyses results for constituents of concern or indicator
compounds
Soil and site map(s)—In addition to the required Part B site map, the
owner/operator should prepare map(s) displaying the location of surface soil
types, areas of paved or compacted soil, fill or otherwise disturbed soil, and
other features that could affect contaminant distribution.
8-36
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The owner/operator should develop maps of unconsolidated geologic
material at the site. These maps should identify the thickness, depth, and
textures of soils, and the presence of saturated regions and other
hydrogeological features. Soils should be identified according to accepted
USDA methods for description of soils (USDA, 1975). Figure 8-4 displays a
typical soil boring log.
Three graphical methods commonly used to display soil boring data are:
(1} cross-sections; (2) fence diagrams; and (3) isopach maps. Cross-sections
are derived from borings taken along a straight line through the site.
Plotting the stratigraphy of surficial deposits against horizontal distance
between sampling points gives a vertical profile, or transect. Fence diagrams
depict the same information between points that are not in a straight line.
An isopach map resembles a conventional topographic map, however, the contour
intervals represent units of thickness of a particular soil layer rather than
elevations. For example, a map of clay isopachs may be used to show the
thickness in feet of an impermeable layer.below a waste lagoon. Generally, to
verify lateral continuity, more than one- transect through a site will be
required. When it is important to indicate the areal extent of a layer, for
example where a clay lens is suspected to cause lateral transport in the
unsaturated zone, both vertical and horizontal presentations may be
necessary. Graphical methods are discussed in detail in Section 5, Data
Presentation.
Graphical display of contaminant distributions in soil may include:
• Area/site maps with concentrations indicated by values, symbols or
isoconcentration lines;
• Three-dimensional contour plots of concentrations, such as are
produced by computer graphics; and
• Vertical concentration contours (isopleths) plotted along a transect
or fence diagram.
8.6 FIELD METHODS
Soil sampling methods can be divided into three categories:
8-37
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8-39
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• Surface sampling techniques;
• Manual coring and augering; and
* Methods requiring deep drilling or coring.
Sampling in the upper 15 cm of soil can be accomplished with a variety of
simple tools, including shovels, spatulas and soil punches'. Contaminants that
have moved downwards in the soil profile require tools such as tube samplers
and augers, Manually operated tools are useful to 1 to 2 meters depth,
depending on the soil type. Below this, hydraulically or mechanically driven
equipment is needed.
Methods that sample soil fluids will be presented in later text.
8.6.1 S urface Sampling Technique a
Surface soils consist of not only the soil itself, but also "non-soil"
materials such as rocks, vegetation, plant roots, and man-made items. The
investigator must define how these materials will be treated; whether they
will be discarded or analyzed separately.
8.6.1.1 Soil Punch—
A soil punch is a thin-walled steel tube that is 15 to 20 cm long and
1/2-in. to 2-in. in width. The tube is driven into the ground with a wooden
mallet and twisted to free the sample. The punch is pulled out and the soil
J pushed or shaken from the tube. This technique is fast and inexpensive. It
is not useful in rocky areas, or in unconsolidated soils that will not stay in
the punch.
8.6.1.2 Ring Samplers—
This device consists of a 15 to 30 cm steel ring which is driven into the
ground, then the soil and ring are removed for analysis. This technique is
useful when results are to be expressed on a unit area basis, since the soil
ring contains a known area of soil. Ring samplers will not work in loose,
sandy soils or stiff clays.
8-40
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8.6.1.3 Grab Samplers—
Collection of grab samples by shovel, spatula, or scoop is not
recommended if area or volume determinations are required. The
reproducibility of sample size is poor and subject to bias by the individual
collecting the sample. The principal advantage of grab sampling is the ease
of collection.
8.6.1.4 Soil Probes—
Manual soil probes are designed to acquire samples from the upper two
meters of the soil profile. The soil probe is a stainless-steel or brass tube
that is sharpened and beveled on one end, and fitted with a T-handle. Soil
probes are common agricultural tools, and can be obtained in several
diameters. The probe is pushed into the soil, 20 to 30 cm at 'a time. After
each increment, the tube is pulled out and the soil extruded. Whether the
soil is "disturbed" or "undisturbed" depends on whether the soil can be
removed in one piece; the.samples are considered disturbed for .the purposes of
engineering or physical measurements. Loose soils will be difficult to remove
with this tool, and the borehole will tend to collapse when the tube is .
withdrawn to obtain samples.
8.6.1.5 Hand Augers— .
Augers have a spiral cutting blade that transports soil cuttings
upwards. Hand-operated augers may be used to a depth of approximately
30 feet. Single flight augers are pulled from the ground periodically and
soil samples taken from the threads of the auger. Continuous flight augers
transport the loosened soil to the top of the borehole, where it can be
collected. Augers provide highly disturbed samples. No information can be
obtained on soil structure, bulk density, or permeability. Cross-
contamination between soil layers is likely, and depth information is not
reliable. Therefore, reliance on augering as a sole technique is not
recommended. Augering can be used, however, in conjunction with tube sampling
to obtain undisturbed samples.
8-41
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8.6.2 Deep Sampling Methods
The subject of deep drilling is discussed more extensively in the section
on ground-water sampling (see Section 9), since deep cores will generally be
taken in conjunction with drilling for monitoring well emplacement. There are
some techniques that are of particular importance to soil sampling, and
therefore, a brief discussion is included here. Procedures for sampling with
split-spoon and thin-wall tube corers are presented in Section 7.
8.6.2.1 Hollow-Stem Augers—
Hollow-stem augers have a continuous flight-cutting blade around a hollow
metal cylinder. A stem with a plug is ordinarily kept inside the auger barrel
to prevent soil from entering. When core samples are desired, the stem is
withdrawn and a tube sampler may be inserted to the bottom of the borehole.
This sampling method is also known as continuous soil sampling. Hollow-stem
augers have the additional advantage that they do not require drilling fluids.
8.6.2.2 Solid-Stem Augers—
Solid-stem augers, as the name implies, are augers that do not have an
inner barrel. As with the manual variety, single-flight augers must be-
withdrawn each time a sample is desired, or samples may be taken from the
cuttings brought to the surface by augers of the continuous flight type.
Augers may be used in conjunction with tube samplers by withdrawing the auger
and obtaining a sample from the bottom of the borehole. This approach is only
useful with soils that 'do not cave in or crumble.
8.6.2.3 Power Corers—
Soil coring devices that may be used with hydraulically or mechanically-
driven drilling rigs include thin-walled Shelby tubes and split-spoon
samplers.. These are two of the most common samplers; many other types are
available.
8.6.2.4 Thin-Walled Tube Samplers—
The Shelby tube is a metal cylinder with the end sharpened and beveled
for cutting into the soil. Common sizes used for field investigations are
1-in. to 3-in. in diameter. The tube is pushed down into the soil with a
8-42
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smooth even motion; the pressure required to penetrate to a certain depth is a
standard engineering parameter indicating soil density. Thin-walled tubes
produce the highest quality undisturbed cores for engineering and hydraulics
testing but are only useful in cohesive soils. However, because the soil must
be extruded from the tube, it is sometimes difficult to remove the core in one
piece. Therefore, soil stratigraphy characterization may be accomplished more
readily with split-spoon samplers.
8.6.2.5 Split-Spoon Samplers—
A split-spoon consists of a metal cylinder, split in half, and screwed
into a solid outer tube that is connected to the drilling rig. The tube is
forced into the soil by dropping a heavy weight (hammer) onto the tube. After
the tube is pulled from the soil, the inner cylinder is removed and opened,
exposing the soil core. Core samples may be used to determine stratigraphy,
for chemical and grain-size analysis, or for pore water extraction.
Split-spoons are the preferred method for sampling unconsolidated soils, and
may also be used to penetrate some types of rock.
8.6.2.6 Trenching—
Trenches and test pits are useful where detailed examination of soil
stratigraphy and hydrogeology is required. Trenching is limited to the top
two meters of soil. Shallow trenches may be dug manually, but in most
instances a backhoe will be faster and easier.
An example of the appropriate use of trenching in a site investigation
might involve extensive subsurface contamination with fuel oil. Samples
gathered from split-spoons and test wells have indicated that extensive
contamination exists throughout the soil profile, leading to the conclusion
that the oil layer is very thick. Trenching should be used to allow a more
careful examination of the extent of contamination. In this example, it is
discovered from examinations of the trench walls that -the oil is actually
i
present only in a thin layer on top of the water table. Apparently, oil was
forced upwards within the soil core due to pressure on the soil from the
sampling device. In this instance, the use of trenching is appropriate,
because precise knowledge of the extent of contamination allows for more
effective selection of corrective measures.
8-43
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8.6.3 Pore Water Sampling
Under conditions where a contaminant is suspected to migrate readily
through the soil with infiltrating water, it is sometimes appropriate to
monitor water quality in the unsaturated zone. By sampling the soil pore
water before it reaches the water table, this approach can give early warning
of threats to aquifers.
Compounds for which pore water sampling is useful are those that are
moderately to highly water soluble, and are not appreciably retained on soil
particles. Examples include poorly adsorbed inorganics such as cyanide or
sulfate, halogenated solvents such as ICE, and organic acids. Due to the
mobility of these compounds, pore water sampling will be most useful when the
release is recent.
The most common pore water collection technique uses a suction device
called a vacuum lysimeter, which consists of a porous ceramic cup, connected
by tubing to a collection flask and vacuum pump (Figure 8-5). The lysinister
cup is permanently installed in a borehole of the appropriate depth, and the
hole is backfilled with sand. Suction from the pump works against soil
suction to pull water out of the silica flour. The method will not work in
dry soils.
The primary advantage of this method is that the installation is
"permanent", allowing multiple samples from one spot, to detect changes in
contamination levels with tune. Limitations include:
Measurements cannot be related accurately to soil concentrations,
because the sample is obtained from an unknown volume of soil;
Lysimeters are occasionally subject to plugging, and are difficult
to install in fractured or rocky soils;
Some organic and inorganic constituents may be adsorbed by the
ceramic cup (Teflon porous section lysimeters can overcome this
problem); and
Volatile organics will be lost unless a special organics trap is
installed in the system.
8-44
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PRESSURE-VACUUM
ACCESS TUBE
BENTONITE
NEOPRENE PLUG
PLASTIC BODY
BENTONITE
POROUS
CERAMIC
TIP
ACCESS LINES
(I/A"POLYETHYLENE
TUBING)
AUGERED HOLE
• V DIAMETER
DISCHARGE TUBE
BACKFILLED
"NATIVE SOIL"
POWDERED QUARTZ
BENTONITE
Figure 8-5. Typical ceramic cup pressure/vacuum Lysimeter.
8-45
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EXHIBIT 8-1. RFI CHECKLIST - SOILS
Site Name/Location
Type of unit _
1. Does waste characterization include the following information? (Y/N)
Constituents of concern
Indicator parameters
Concentrations of constituents
Physical state of bulk waste
Viscosity
pH
pka
Density
Water Solubility
Henry's Law Constant
kow
Biodegradability
Rates of hydrolysis, photolysis and oxidation
2. Have the following information requirements to
characterize a release to soil been determined (Y/N)
Site Soil Characteristics
Surface soil distribution map ._«_____
Surficial geology
Soil classification ________
Particle size distribution
Porosity
Hydraulic conductivity (saturated and unsaturated) __.._
Relative permeability _______
Soil sorptive capacity _
Cation exchange capacity ________
Soil organic content
Soil pH
Unsaturated Transport Characterization
Depth to water table
Pore water velocity
Percolation
Volumetric water content
(continued)
8-46
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EXHIBIT 8-1 (continued)
3. Has the following information on the release to soil
been determined? (Y/N)
Area of contamination
Distribution of contaminants within study area
Depth of contamination ______
Chemistry of contaminants _______
Vertical rate of transport :: L,-_J..__J_-__
Lateral rate of transport in each stratum ______
Persistance of contaminants in soil _______
Potential for release from surface soils to air
Potential for release from surface soils to surface
water ________
Potential for release to ground water .
Potential receptors
8-47
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REFERENCES
ASTM, D422-63. Particle Size Analysis for Soils. Annual Book of ASTM
Standards, Vol. 4.08. 1984. p. 116-122.
ASTM, D2488-69. Standard Recommended Practice for Description of Soils.
Annual Book of ASTM Standards, Vol. 4.08. 1984. p. 399-405.
Barth, D. S., and B. J. Mason. Soil Sampling Quality Assurance User's Guide.
EPA 600/4-84-043. U.S. EPA. 1984.
Callahan, M. A., et al. Water-Related Environmental Fate of 129 Priority
Pollutants. Vol. I and 2, EPA 440/4-79-029a, U.S. EPA, Washington, DC.
1979.
Ford, P. J., et al. Characterization of Hazardous Waste Site - A Methods
Manual, Vol. II, Available Sampling Methods. EPA 600/4-84-076. 1984.
Lambe, T.W. and R.V. Whitman. 1979. Soil Mechanics, SI Version. John
Wiley and Sons, Inc., New York, New York. 553 pp.
Lyman, W. J. Reehl, W. F. and D. U. Rosenblatt. Handbook of Chemical
Property Estimation Methods. McGraw Hill. 1981.
Mason, B. J. Preparation of a Soil Sampling Protocol: Techniques and
Strategies. EPA/4-83-020. 1983.
Oster, C. A. Review of Ground Water Flow and Transport Models in the
Unsaturated Zone. NUREG/CR-291. PNL-4427, Battelle, Pacific Northwest
Laboratory, Richland, WA. 1982.
U.S.D.A. (U.S. Department of Agriculture). 1975. Soil Taxonomy: A Basic
System of Soil Classification for Making and Interpreting Soil Surveys.
Soil Survey Staff, Soil Conservation Service. Washington, DC. 754 pp.
U.S. EPA Office of Pesticides and Toxic Substances. Sediment and Soil
Adsorption Isotherm. Test Guideline No. CG-1710. In: Chemical Fate
Test Guidelines, EPA 560/6-82-003. 1982a.
U.S. EPA. Office of Pesticides and Toxic Substances. Sediment and Soil
Adsorption Isotherm. Support Document No. CS-1710. In: Chemical Fate
Test Guidelines, EPA 560/6-82-003. 1982b.
8-48
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U.S. EPA. Test Methods for Evaluating Solid Waste. SW-846, 2nd Ed. 1982c.
U.S. EPA. Soil Properties, Classification and Hydraulic Conductivity
Testing. SW-925. 1984.
-------
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SECTION 9
GROUND WATER
9.1 OVERVIEW
The objective of an investigation of a release to ground water is to
characterize the nature, extent, and rate of migration of a release of
hazardous constituents to that medium. This section provides:
• a recommended strategy for characterizing releases to ground water,
which includes characterization of the source and the environmental
setting of the release, and conducting a monitoring program which
will characterize the release itself;
• recommendations for data organization and presentation;
• appropriate field methods which may be used in the investigation; and
• a checklist of information that'may be needed for release
characterization.
The exact type and amount of information required for sufficient release
characterization will be site-specific and should be determined through
interations between the regulatory agency and the facility owner/operator
during the RFI process. This guidance does not define the specific data
required in all instances; however, it identifies possible information
necessary to perform release characterizations and methods for obtaining this
information. The RFI Checklist, presented at the end of this section,
provides a tool for planning and tracking information for release
characterization. This list is not meant to serve as a list of requirements
for all releases to ground water. Some releases will involve the collection
of only a subset of the items listed.
9-1
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9.2 APPROACH FOR CHARACTERIZING RELEASES TO GROUND WATER
The owner/operator should develop a monitoring program to quantify
contaminant releases from SWMUs* to ground water. The initial monitoring
phase should include a limited number of monitoring wells, located and
screened in such a way that they are capable of providing background water
quality and of intercepting any release. The regulatory agency will evaluate
the adequacy of an existing monitoring system, if proposed for use in the
initial monitoring phase. The owner/operator may be required to install new
wells if the existing well system is found to be inadequate.
Initial ground water sampling and analysis may be conducted for a limited
set of monitoring constituents. This set may include a subset of the..
hazardous constituents of concern, and may also include indicator parameters
(e.g., TOX). Sampling frequency and duration should also be proposed in the
RFI Plan.
Investigation of a given release may be terminated based on results from
an initial monitoring phase if these results show that a suspected release was
not, in fact, an actual release. If, however, contamination is found, the
release must be adequately characterized through a subsequent monitoring
phase(s).
Subsequent characterization involves determining the detailed chemical
composition and the area! and vertical (i.e., three dimensional) extent of the
contaminant release, as well as its rate of migration. This should be
accomplished through direct sampling and analysis and, when appropriate, can
be supplemented by indirect means such as geophysical methods and modeling
techniques.
Table 9-1 outlines a recommended strategy for characterizing a ground
water release. Table 9-2 lists the specific tasks and data outputs for the
release characterization process. The steps delineated in these tables should
generally be performed in sequential order, although some may be accomplished
concurrently. For example: the site's hydrogeology may be investigated at
^Guidance in this section applies to any units not already regulated under
the 40 CFR Parts 265 and 264, Subpart F monitoring requirements. The RFI
guidance does not replace existing requirements for regulated units.
9-2
-------
TABLE 9-1. RECOMMENDED STRATEGY FOR CHARACTERIZING
RELEASES TO GROUND WATER
INITIAL PHASE
1. Site hydrogeology should be adequately characterized, including:
- obtaining/reviewing existing information
•• - performing additional subsurface characterization, if needed
- characterizing the ground-water flow system
2. Site waste/unit should be characterized, considering:
- adequacy of existing information
- collection of new information, as needed
- waste physical and chemical properties
3. An adequate initial monitoring system should be installed, involving:
- appropriate areal well location (background and downgradient)
- collection of additional hydrogeologic data if necessary
proper well screen interval selection
4. A monitoring program must be established, including:
- selection of monitoring constituents . • •
- determination of sampling frequency
- - determination of duration of monitoring
- choice of data evaluation procedures to be used
5. Conduct ground-water sampling and evaluate analytical results
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the same time as waste and unit characterization; soil borings installed
during hydrogeologic characterization may be converted into monitoring wells;
and additional wells may be installed to more accurately characterize a plume
while a sampling and analysis program is in effect at existing wells.
The tasks to be conducted for each release will be determined on a
site-specific basis. It should be noted that some of the characterization
tasks may have been previously accomplished in conjunction with the
40 CFR Parts 264 and 265, Subpart F (ground-water monitoring) regulations.
9.3 CHARACTERIZATION OF THE CONTAMINANT SOURCE AND THE ENVIRONMENTAL
' SETTING
9.3.1 Waste and Unit Characterization
9.3.1.1 Waste Characteristics—
Knowledge of the waste constituents (historical and current) and their
characteristics at the units of concern is essential in selecting monitoring
constituents and well locations. Waste (source) information should include
identifying volumes and concentrations of hazardous waste or constituents
present, and their physical and chemical characteristics.
Identification of hazardous constituents may be a relatively simple
matter of reviewing records of unit operations, or may require field efforts,
such as direct sampling and analysis of the waste in the unit. Hazardous
constituents may be grouped by similar chemical and physical properties to aid
in developing a more focused monitoring program.
Knowledge of physical and chemical properties of hazardous' constituents
can help to determine their mobility, and their ability to degrade or persist
in the environment. Mobility of chemicals in ground water is related to their
solubility, volatility, adsorbability, partitioning, and density.
Section 7 provides additional guidance on waste characterization. The
following discussion describes several waste-related factors and properties
which, if understood, can aid in developing ground-water monitoring procedures:
• The mobility of a waste is highly influenced by its physical form.
Solid and gaseous wastes are less likely to come in contact with
ground water than liquid wastes, except in situations where the
ground-water surface directly intersects the waste, or where
infiltrating liquids are leaching solids on route to the saturated
zone.
9-7
-------
The concentration of any constituent at the waste source may provide
an indication of the concentration at which it may appear in the
ground water.
The chemical class (i.e., organic, inorganic, acid, base, etc.)
provides an indication of how the waste might be detected in the
ground water, and how the various components might react with the
subsurface geologic materials, the ground water, and each other.
The pH of a waste can provide an indication of the pH at which it
would be expected to appear in the ground water. A low pH waste
could also be expected to cause dissolution of some subsurface
geologic materials (e.g., limestone), causing channelization and
differential ground-water flow.
The acid dissociation constant of a liquid (pKa) is a value which
indicates its equilibrium potential in water, and is equal to the pH
at which the hydrogen ion is in equilibrium with its associated
base. If direct pH measurements are not feasible, the concentration
of a waste in combination with its pKa can be used to estimate the
likely pH which will oc'cur at equilibrium (in ground water), at a
given temperature. Acid dissociation values can be found in most
standard chemistry handbooks, and values for varying temperatures
can be calculated using the Van't Hoff equation (Snoeyink and
Jenkins, 1980).
Viscosity is a measure of a liquid's resistance to flow at a given
temperature. The more viscous a fluid is, the more resistant it is
to flow. Highly viscous wastes may travel more slowly than the
ground water, while low-viscosity wastes may travel more quickly
than the ground water.
Water solubility describes the mass of a compound that dissolves in
or is miscible with water at a given temperature and pressure.
Water solubility is important in assessing the fate and transport of
the contaminants in ground water because it indicates the chemical's
affinity for the aqueous medium. High water solubility permits
greater amounts of the hazardous constituent to enter the aqueous
phase. Therefore, this parameter can be used to establish the
potential for a constituent to enter and remain in the hydro logic
cycle.
The density of a substance (solid or liquid) is its weight per unit
volume. The density of a waste will determine whether it sinks or
floats when it encounters ground water, and will assist in locating
well screen depths when attempting to monitor for specific hazardous
constituents released to ground water.
The log of the octanol/water partition coefficient (Kow) is a
measure of the relative affinity of a constituent for the neutral
organic and inorganic phases represented by n-octanol and water,
9-8
-------
respectively. It is calculated from a ratio (P) of the equilibrium
concentrations (C) of the constituent in each phase:
p
octanol . „ ,
. log p
water
The KQW has been correlated to a number of factors for determining
contaminant fate and transport. These include adsorption onto soil
organic matter, bioac cumulation, and biological uptake. It also
bears a relationship to aqueous solubility. •
i,
The Henry's Law Constant of a constituent is the relative
equilibrium ratio of a compound in air and water at a constant
temperature. It can be estimated from the equilibrium- vapor
pressure divided by the solubility in water and has the units of
atm-nr/mole. The Henry's Law Constant expresses the equilibrium
distribution of the constituent between air and water and indicates
the relative ease with, which the constituent may be removed from
aqueous solution.
Other influences of the waste constituents should also be
considered. Constituents may react with soils, thereby altering the
physical properties of the soil, most notably hydraulic
conductivity* Chemical interactions among waste constituents should
also be considered. Such interactions may affect mobility,
reactivity, solubility., or toxicity of the constituents. The
potential for wastes or reaction products to interact with unit
construction materials (e.g., synthetic liners) should also be
considered".*
The analytical reference books listed in Section 7 may be used to obtain
information on the parameters discussed above. Other waste information may be
found in facility records, permits, or permit applications. It should be
noted that mixtures of chemicals may exhibit characteristics different than
those of any single chemical. In cases where mixtures occur, waste sampling
may be necessary.
9.3.1.2 Unit Information—
Indirect releases to ground water may occur as a result of contaminant
releases to soil and/or surface water that percolate to ground water. These
releases may result from point sources, spills, leaks, or from overland
run-off, and can vary considerably in areal extent. Direct releases to ground
water may occur when waste materials are in direct contact with ground water
(e.g., when a landfill rests below the water table).
9-9
-------
Certain unsound unit design and operating practices can allow waste to
migrate from a unit and possibly mix with natural runoff. Examples include
surface impoundments with insufficient freeboard allowing for periodic
overtopping; leaking tanks or containers; or land based units above shallow,
low permeability materials which, if not properly designed and operated, can
fill up with water and spill over, creating a "bathtub effect". In addition,
precipitation falling on exposed wastes can dissolve and thereby mobilize
hazardous constituents. For example, at uncapped active or inactive waste
piles and landfills precipitation and leachate are likely to mix at the toe of
the active face or the low point of the trench floor.
Unit dimensions (e.g., depth and surface area) and configuration
(e.g., rectangular, parallel trenches), as well as volume (e.g., capacity)
should also be described, since these factors will have a bearing on the
development of a suitable monitoring network.
9.3.2 Characterization of the Environmental Setting
Hydrogeologic conditions at the site to be monitored must be evaluated
for the potential impacts the setting may have on the development of a
monitoring program and the quality of the resulting data. Several
hydrogeologic parameters should be evaluated, including:
• Types and distribution of geologic materials;
• Occurrence and movement of ground water through these materials;
• Location of the facility with respect to the regional ground-water
flow system;
• Relative permeability of the materials; and
• Potential interactions between contaminants and the geochemical and
biological constituents of the formation(s) of interest.
These conditions are interrelated and will, therefore, be discussed
collectively below.
9-10
-------
There are three basic types of geologic materials through which ground
water normally flows. These are: (1) porous media; (2) fractured media; and
(3) fractured porous media. In porous media (e.g., sand and gravels, silt,
loess, clay, till, and sandstone), ground water and contaminants move through
the pore spaces between individual grains. In fractured media (e.g.,
dolomites, some shales, granites, and crystalline rocks), ground water and
contaminants move through cracks or solution crevices in otherwise relatively
impermeable rock. In fractured porous media (e.g., fractured tills, fractured
sandstone, and some fractured shales), ground water and contaminants move '
through both the intergranular pore spaces as well as cracks or crevices in
the rock or soil. The occurrence and movement of ground water through pores
and cracks or solution crevices depends on .the relative effective porosity and
degree of channeling occurring in cracks or crevices. The fractured porous
media include fractured tills, fractured sandstone, and some fractured
shales. Figure 9-1 illustrates the occurrence and movement of ground water
and contaminants in the three types of geologic materials presented above.
The distribution of these three basic types of geologic materials is
seldom homogeneous or uniform. In most settings, two or more types of
materials will be present. Even for one type of material at a given site,
large differences in hydrologic characteristics may be encountered. The
heterogeneity of the materials can play a significant role in the rate of
contaminant transport, as well as in developing appropriate monitoring
procedures for a site.
, Once the geologic setting is understood, the site hydrology should be
evaluated. The location of the site within the regional ground-water flow
system should be determined to evaluate the potential for contaminant
migration on the regional scale. Potentiometric surface data (water level
information) for each applicable geologic formation at properly selected
vertical and horizontal locations is needed to determine the horizontal and
vertical ground-water flow paths (gradients) at the site. Figures 9-2(a)
and 9-2(b) illustrate two g'eohydrologic settings commonly encountered in
eastern regions of the United States, where .ground water recharge exceeds
evapotranspirational rates. Figure 9-2(c) illustrates a common geohydrologic
setting for the-arid western regions of the United States.
9-11
-------
•<-)
§1
u e
-------
(a) LOCAL AND REGIONAL GROUND WATER
FLOW SYSTEMS IN HUMID ENVIRONMENTS
(b) TEMPORARY REVERSAL OF GROUND-WATER FLOW DUE TO
FLOODING OF A RIVER OR STREAM
Ttrnporary
rtvtrs*l of
groundwutr flew
Flood tugt
(c) TYPICAL GROUND-WATER FLOW PATHS IN ARID ENVIRONMENTS
| I Infiltrating watt r from precipitation
I I _-X"*' mtv or may not ittsh thi waicr tibl*
Figure 9-2. Ground-water flow paths in some different hydrogeologic
settings.
9-13
-------
In addition to determining the djlrections of ground-water flow, it is
essential to determine the approximate rates of ground-water movement to
properly design a monitoring program. Hydraulic conductivity, hydraulic
gradient, and effective porosity data are required to estimate the average
linear velocity of ground water and, therefore, assist in the determination of
the rate of contaminant migration. Hydraulic conductivity data should be
determined using single well (slug) test data. Hydraulic conductivities
determined in the laboratory have been shown to vary by orders of magnitude
from values determined by field methods and are, therefore, not recommended
for use in the RFI. Several hydraulic conductivity measurements should be
made on materials penetrated by individual wells to provide data on the
relative heterogeneity of the materials in question.
To determine the actual or linear flow velocities, the effective porosity
of the materials should be determined. Porosity can have an important
controlling influence on hydraulic conductivity (a measure of the conductive
property of a geologic material). Materials with high porosity values
generally also have high hydraulic conductivities. An exception is clayey
geologic materials which, although possessing high porosities, have low
hydraulic conductivity values (resulting in low flow rates) due to their
molecular structure. All of the pore spaces within geologic materials are not
available for water or solute flow. Dead-end pores and the portion of the
total porosity occupied by water held to soil particles by surface tension
forces do not participate in the transport of ground water or solutes.
Methods for measuring effective porosity are being developed currently. In
the absence of measured values, the values provided in Table 9-3 should be
used.
Knowledge of the rates of ground-water flow is essential to determine if
the locations of the monitoring wells are within reasonable flow distances of
the contaminant sources. Flow rate data can also be used to calculate
reasonable sampling frequencies. This is particularly important when
attempting to monitor the potential migration of a spill or intermittent
contaminant source.
Geochemical and biological properties of the aquifer matrix should be
evaluated in terms of their potential interference with the goals of the
monitoring program. For example, chemical reactions or biological
9-14
-------
TABLE 9-3. DEFAULT VALUES FOR EFFECTIVE POROSITY
Soil Textural Classes
Effective
Porosity of
Saturation
Unified Soil Classification System
GC, GP, GM, GS
SW, SP, SM, SC
ML, MH
CL, OL, CH, OH, PT
0.20
(20%)
.10
(10%)
0.01
USDA, Soil Textural Classes
Clays, silty clays,
sandy clays
Silts, silt loams,
silty clay loams
All others
Rock- Units (all)
Porous media (nonfractured
rocks such as sandstone and
some carbonates)
Fractured rocks (most carbonates,
shales, granites,, etc.)
0.01
0.10
(10%)
0.20
(20%)
0.15
(15%)
0.0001
(0.01%)
aAssumes de minimus secondary porosity. If fractures or soil structure are
present then effective porosity should be 0.001 (0.1%).
9-15
-------
transformations of the monitoring constituents of concern may introduce
artifacts into the results. Physical and hydrologic conditions will determine
whether or not information on chemical or biological interactions can be
collected. If the potential for these reactions or transformations exists,
consideration should be given to monitoring for likely intermediate or
transformation products.
The monitoring system design is influenced in many ways by a site's
hydrogeologic setting. Determination of the items noted in the subsurface
stratigraphy and flow systems discussions will aid in logical monitoring
network configurations and sampling activities. For example:
• Background and downgradient wells should be screened in the same
stratigraphic horizon(s) to obtain comparable ground-water quality
data. Hydraulic conductivities should be determined to evaluate
preferential flowpaths which will require monitoring and to
establish sampling frequencies.
• The distances between and number of wells (well density) should be a
function of the spatial heterogeneity of a site's hydrogeology, as
is sampling frequency. For example, formations of unconsolidated
deposits with numerous interbedded lenses of varying hydraulic
conductivity or consolidated rock with numerous fracture traces will
generally require a greater number of sampling locations to ensure
that contaminant pathways are intercepted.
•• The slope of the potentiometric surface and the slope of a retarding
geologic formation strongly influence the migration rates of light
and dense immiscible compounds.
• The hydrogeology will strongly influence the applicability of
various geophysical methods, and should be used to establish
boundary conditions for any modeling to be performed for the site.
• Analyses for contaminants of concern in the ground water monitoring
program can be influenced by the general water quality present.
Naturally-occurring cations and anions can affect contaminant
reactivity, solubility, and mobility.
• Sites with complex geology will generally require more hydrogeologic
information to provide a.reasonable assurance that well placements
will intercept contaminant migration pathways. For example,
Figure 9-3 illustrates a cross-sectional and plan view of a waste
landfill located in a mature Karst environment. This setting is
characteristic of carbonate environments encountered in various
parts of the country, but especially in the southeastern states. An
assessment of the geology of the site through the use of borings,
geophysical surveys, aerial photography, tracer studies, and other
geological investigatory techniques, identified a mature Karst
9-16
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geologic formation characterized by well defined sinkholes, solution
channels and extensive vertical and horizontal fracturing in an
interbedded limestone/dolomite. Using potentiometric data,
ground-water flow was found to be in an easterly direction.
Solution channels are formed by the flow of water through the
fractures. The chemical reaction between the carbonate rock and the
ground-water flow in the fractures produces solution channels.
Through time these solution channels are enlarged to the point where
the weight of the overlaying rock is too great to support;
consequently causing a "roof" collapse and the formation of a
sinkhole. The location of these solution channels dictate the
placement of monitoring wells. Note in the plan view the placement
of well No. 2 is offset 50 feet from the perimeter of the landfill.
The horizontal placement of well No. 2, although not immediately
adjacent to the landfill, is necessary in order'to monitor all
potential contaminant pathways. The discrete nature of these
solution channels dictate that each potential pathway be monitored.
The height of the solution channels ranges from three to six feet
directly beneath the sinkhole to one foot under the landfill. The
vertical extent of the cavities allow for full screening of the
horizontal solution channels. (Note the change in orientation of
solution channels due to the presence of the shell hash layer).
Chapter I of the RCRA Ground Water Monitoring Technical Enforcement
Guidance Document (TEGD) (U.S. EPA, 1986) provides guidance in
characterization of site hydrogeology. Although the TEGD was written far
enforcement officials to establish compliance with 40 CFR Part 265, Subpart F
(interim status ground-water monitoring) regulations, various sections of the
document will be useful to the facility owner/operator in developing
monitoring plans for RCRA Facility Investigations.
In order to characterize a release to ground water, data should be
collected to assess the following:
* subsurface stratigraphy,
• ground-water flow systems, and
• ground-water quality.
9.3.2.1 Subsurface Stratigraphy—
In order to adequately characterize the hydrologic setting of a site, an
analysis of site geology must first be completed. Geologic site characteri-
zation consists of both a characterization of subsurface stratigraphy, which
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includes unconsolidated material analysis, bedrock features such as lithology
and structure, as well as depositional information,, which indicates the
sequence of events which resulted in the present subsurface configuration.
Some of the information needed to characterize a site's subsurface are
discussed below, including:
• Grain size distribution and gradation;
• Hydraulic conductivity;
• Soil porosity;
• Discontinuities in soil strata; and
. • Degree and orientation of subsurface stratification and bedding.
Refer to Section 8 of this document for further detail regarding soils.
Grain size distribution and gradation—A measurement of the percentage of
sand, silt, and clay should be made for each distinct layer of the soil.
Particle size can affect contaminant transport through its impact on
adsorption and hydraulic conductivity. Sandy soils generally have low
sorptive capacity while clays tend to have a high affinity for heavy metals
and some organic contaminants. This is due in part to the fact that small
clay particles have a greater surface area in relation to their volume than do
large sand particles. Greater surface areas lead to stronger interactions
with contaminant molecules. Clays also bind contaminants due to the chemical
structure of the clay. Methods for determination of sand/silt/clay fractions
are available from ASTM, Standard Method No. D422-63 (ASTM, 1984).
Hydraulic conductivity—This property represents the ease with which
fluids can flow through a formation, and is dependent on porosity, and grain
size, as well as on the viscosity of the fluid. Hydraulic conductivity can be
determined by the use of slug tests, as discussed in subsection 9.6.
Porosity—Soil porosity is the volume percentage of the total volume of
the soil not occupied by solid particles (i.e., the volume of the voids). In
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general, the greater the porosity, the more readily fluids may flow through
the soil, with the exception of clays (high porosity), in which fluids are
held tightly by capillary forces.
Discontinuities in soil strata—Folds are layers of rock or soil that
have been naturally bent over geologic time. The size of a fold may vary from
several inches wide to several miles wide. In any case, folding usually
results in a complex structural configuration of layers (Billings, 1972).
Faults are ruptures in rock or soil formations along which the opposite
walls of the formation have moved past each other. Like folds, faults vary in
size. The result of faulting is the disruption of the continuity of
structural layers.
Folds and faults may act as either barriers to or pathways for
ground-water (and contaminant) flow. Consequently, complex hydrogeologic
conditions may be exhibited. The existence of folds or faults can usually be
determined by examining geologic maps or surveys. Aerial photographs can also
be used to identify the existence of these features. Where more detailed
information is needed, field methods (e.g., borings or geophysical methods)
may need to be employed.
Joints are relatively smooth fractures found in bedrock. Joints may be
as long as several hundred feet (Billing, 1972). Most joints are tight
fractures, but because of weathering, joints may be enlarged to open
fissures. Joints result in a secondary porosity in the bedrock which may be
the major pathway of ground-water flow though the formation (Sowers, 1981).
Interconnected conduits between grains may form during rock formation
(Sowers, 1981). The permeability of a bedrock mass is often defined by the
degree of jointing. Ground water may travel preferentially along joints,
which usually govern the rate of flow through the bedrock. The degree and
orientation of joints and interconnected voids is needed to determine if there
will be any vertical or horizontal leakage through the formation. In some
cases, bedrock acts as an aquitard, limiting the ground-water flow in an
aquifer. In other cases, the bedrock may be much more productive than
overlying alluvial aquifers.
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Geologic naps available from the USGS may be useful in obtaining
information on the degree and orientation of jointing or interconnected void
formation. Rock corings may also be used to identify these characteristics.
Degree and orientation ofsubsurface stratification and bedding—The
owner/operator should develop maps of the subsurface structure for the areas
of concern. These maps should identify the thickness and depth of formations,
soil types and textures, the locations of saturated regions and other
hydrogeological features. For example, the existence of an extensive,
continuous, relatively horizontal, shallo strata of low permeability can
provide a clue to contaminant routing. In such cases, the contaminants may
migrate at shallow depths,-which are above the regional aquifer. Such
contamination could discharge into nearby, low-lying structures (e.g., seepage
into residential basements).
9.3.2.1.1 Subsurface Characterization—A variety of direct and indirect
methods are available to characterize a site geologically. Direct methods
utilize-soil borings and rock core samples for subsequent lab analysis which
provide the necessary data to evaluate sediment grain size, texture
uniformity, mineralogy, soil moisture content, bedrock lithology, porosity,
and structure. Combined, these data provide the basis for delineating the
geologic nature of the site and, in turn, provide the data necessary to
evaluate the hydrologic setting.
Indirect methods of geologic investigation, such as geophysical
techniques and aerial photography can be used to supplement data gathered by
direct field methods, through extrapolation and correlation of data on surface
and subsurface geologic features. Borehole geophysical techniques can be used
*
to extrapolate direct data from soil borings and bedrock cores. Surface
geophysical methods can provide indirect information on depth, thickness,
lateral extent, and variation of subsurface features that can be used to
extrapolate information gained from direct methods. Applicable surface
geophysical methods include seismic refraction, electrical resistivity,
electromagnetics, magnetics, and ground penetrating radar.
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Subsection 9.6 provides a detailed description of direct and indirect
methods used in geologic site characterization. Geophysical methods are .
described in Appendix C, and aerial photography applications are described in
Appendix A.
9.3.2.2 Flow Systems—
In addition to characterizing the subsurface geology, the owner/operator
/
should adequately describe the ground-water flow system. To adequately
describe the ground-water flow paths, owner/operators should:
• Establish the direction of ground-water flow (including horizontal
and vertical components of flow);
• Establish the seasonal, temporal, and artificially induced
(e.g., offsite production well pumping, agricultural use) variations
in ground-water flow; and
• Determine the hydraulic conductivities of the hydrogeologic units
underlying the site.
Hydrologic and hydraulic properties and other relevant information needed
to fully evaluate the ground-water flow system are listed and discussed below:
•• Hydraulic conductivity;
• Hydraulic gradient;
• Direction and rate of flow;
• • Aquifer type/identification of aquifer boundaries;
• Specific yield (effective porosity)/storativity;
• Depth to ground water; '
i
• Identify uppermost aquifer;
• Identify recharge and discharge areas; and
• Use of aquifer.
Hydraulic conductivity—In addition to defining the direction of ground
water flow in the vertical and horizontal directions, the owner/operator
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should identify the distribution of hydraulic conductivity (K) within each
formation. Variations in the hydraulic conductivity of subsurface materials
can'affect flow rates and alter directions of ground-water flow paths. Areas
of high hydraulic conductivity represent areas of greater ground-water flow
and, if contaminants are present, zones of potential migration. Therefore,
information on hydraulic conductivities is needed to make reasonable decisions
regarding well placements. Hydraulic conductivity measurement is described in
Section 9.6.
Hydraulic gradient—The hydraulic gradient is defined as the change in
static head per unit distance in a given direction. The hydraulic gradient
defines the direction of flow and may be expressed on maps of water level
measurements taken around the site. Ground-water velocity is directly related
to hydraulic gradient. " " •*-
• Direction and rate of flow—-A thorough understanding of how ground water
flows beneath the facility will aid the owner/operator in locating wells to
provide suitable background and downgradient samples. Of particular
importance is the direction of ground-water flow and the impact that external
factors^(intermittent well pumping, temporal variations in recharge patterns,
etc.) may have on ground-water flow patterns.' In order to account for these
factors, monitoring procedures should include precise water level measurements
in piezometers or observation wells. These measurements should be made in a
sufficient number of wells and at a frequency sufficient to adequately gauge
both seasonal average flow directions and to show any seasonal or temporal
fluctuations in flow directions. Horizontal and vertical components of
ground-water flow should be assessed. Methods for determining vertical and
horizontal components of flow are described in subsection 9.5.4.
Aquifer type/identification of aquifer boundaries—Aquifer boundaries
define the flow limits and the degree of confinement of an aquifer. There are
two major types of aquifers: unconfined and confined. An unconfined aquifer
has a free water surface at which the fluid pressure is the same as
atmospheric. A confined aquifer is enclosed by retarding geologic formations
and is, therefore, under pressure greater than atmospheric. A confining'unit
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consists of consolidated or unconsolidated earth materials that are
substantially less permeable than aquifers. Confining units are called
aquitards or aquicludes. Aquifer boundaries can be identified by consulting
geologic maps and state geologic surveys. Observation wells and piezometers
can be used to determine the degree of confinement of an aquifer by gathering
water level data.
Specific yield/storativity-~Specific yield and storativity are both terms
used to characterize the amount of water an aquifer is capable of yielding.
In an unconfined system, the specific yield is the ratio of the drainable
volume to the.bulk volume of the aquifer medium (some liquid will be retained
in pore spaces). The storativity of a confined aquifer is the volume of water
released from a column of unit area and height per unit decline of pressure
head. Specific yield or storativity values may be necessary to perform
complex ground-water modeling.
Depth to ground water~The depth to ground water is the vertical, distance
from the land's surface to the top of the saturated zone. A release from a
unit not in contact with the water table will .first percolate through the
unsaturated zone and may, depending upon the nature of the geologic-material,
disperse horizontally. Thus, a release of this nature may encounter a deep
water table with some lateral extent. Depth to ground water can influence the
selection of sampling methods as well as geophysical methods.
A shallow water table can also facilitate releases to other environments
via volatilization of some compounds into the unsaturated zone, seepage into
basements of buildings in contact with the saturated zone, or the transport of
contaminants into wetlands where the water table reaches the level of the
ground surface. Sufficient mapping of the water table with relation to these
features should provide an indication of where these interractions may exist.
Identification of uppermost aquifer—As described in Chapter one of the
Technical Enforcement Guidance Document (TEGD) (U.S. EPA, 1986), the uppermost
aquifer extends from the water table to the first confining layer and includes
any overlying isolated zones of saturation. The identification of the
confining layer is an essential facet of the definition of the uppermost
9-24
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aquifer. The uppermost aquifer includes all interconnected saturated zones
overlying the confining layer. There should be no interconnection, based upon
pumping tests, between the uppermost aquifer and lower aquifers, although
pumping tests may not conclusively show lack of interconnection.
The investigator should consider the definition of the uppermost aquifer
to encompass all saturated zones that serve as pathways for contaminant
migration, including isolated zones of saturation. Saturated formations, even
those consisting of relatively impermeable materials, should be considered as
part of the uppermost aquifer. Chapter One of the TEGD describes criteria
useful in identifying which portions of the uppermost aquifer should be
monitored.
The investigator should assess any hydraulic interconnections between the
uppermost and deeper aquifers when attempting to demonstrate that a confining
unit is able to prevent the passage of contaminants to the deeper aquifers.
Examples illustrating the determination of hydraulic interconnection in
various geologic settings are given in Chapter One of the TEGD.
Identification of recharge and discharge areas"Ground-water recharge can
be defined as the entry into the saturated zone of water made available at the
.water table surface, together with the associated flow away from the water
table within the saturated zone. Ground-water discharge can be defined as the
removal of water from the saturated zone across the water table surface,
together with the associated flow toward the water table within the saturated
zone (Freeze and Cherry, 1979). Ground-water recharge and discharge areas
also represent areas of potential cross-media contamination.
Recharge can be derived from the infiltration of precipitation,
inter-aquifer leakage, inflow from streams or lakes, or inadvertently by
leakage from lagoons, sewer lines, landfills, etc. Discharge occurs where
ground water flows to springs, streams, swamps, or lakes, or is removed by
evapotranspiration or pumping wells, etc. Information on the source and
location of aquifer recharge and discharge areas may be obtained from state
water resource publications, geologic surveys, or existing site information.
Comparison of aquifer water levels with nearby surface water levels may also
provide an indication of the source and location of aquifer recharge and '
discharge areas.
,9-25
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Flow nets can be used to determine areas of aquifer recharge and
discharge. Subsection 9.5.4 describes the use of flow nets to determine
ground-water flow patterns.
Use of aquifer—-The proximity and extent of local ground-water use
(e.g., pumping) may dramatically influence the rate and direction of
ground-water flow at a RCRA facility site, possibly causing seasonal or
eposodic variations. These factors should be considered when designing and
implementing a ground-water monitoring system. Information on local aquifer
use may be available from the USGS, and state and local water authorities.
Aquitard type and location—Aquitard type refers to the type of geologic
formation that serves to bound ground-water flow for a given aquifer. Such
boundaries may be rock or may be an unconsolidated unit such as clay, shale,
or glacial till. The identification of such formations and their hydraulic
characteristics is essential in determining ground-water flow paths. Aquitard
locations can be determined by consulting geologic maps and boring log
information. .Although aquitards are substantially less permeable than
aquifers, they are not totally impermeable and can allow significant
quantities of water to pass through them over time. The location of. an
aquitard should be used in determining monitoring well depths.
9.3.2.3 Ground.Water Quality—
All of the data requirements discussed above are necessary to support the
goal of obtaining .representative background and downgradient ground-water
samples which will be capable of characterizing the ground-water quality
beneath the facility, and determining the extent and rate of migration of a
release.
9.3.3 Sources of Existing Information
A complete review of relevant existing information on the facility is an
essential part of the release characterization. This review can provide
valuable knowledge and a basis for developing monitoring procedures.
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Information that may be available and useful for the investigation includea
both site-specific studies and regional surveys available from local, state,
\
and Federal agencies.
Information from the RCRA Facility Assessment (RFA) should be thoroughly
reviewed in developing monitoring procedures, and should serve as a primary
information source. It may. also provide references to other sources of
information. For example, the facility's Part B Permit Application may
contain.other relevant information not explicitly contained in the RFA files.
These reports and all of the facility's RCRA compliance/permit files will
provide an understanding of the current level of knowledge about the facility,
and will assist in identifying data gaps to be filled during the investigation.
• Public information is available from local, state, and Federal
governments concerning the topics discussed below.
9.3.3.1 Geology--
Knowledge of local bedrock types and depths is important to the
investigation of a site. Sources of geologic information include
United States Geological Survey (USGS) reports, maps, and files; State
geological survey records; and local well drilling logs.
9.3.3.2 Climate-
Climate is also an important factor affecting the potential for
contaminant migration from a release source. Mean values for precipitation,
evaporation, evapotranspiration, and estimated percolation will help determine
the potential for onsite and offsite contaminant transport. The investigator
should consult monthly or seasonal precipitation and evaporation (or
temperature) records. Climate and weather information can be obtained from:
National Climatic Center
Department of Commerce
Federal Building
Ashville, North Carolina 28801
Tel: (704)258-2850
9.3.3.3 Ground-Water Hydrology—
The investigator will need to acquire information on the ground-water
hydrology of a site and its surrounding environment. Ground-water use in the
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area of the site should be thoroughly investigated to find the depths of local
wells, and their pumping rates. Sources of such information include the USGS,
state geological surveys, local well drillers, and State and local water
resources boards* A list of all state and local cooperating offices is
available from the USGS, Water Resources Division in Res ton, Virginia, 22092.
This list has also been distributed to EPA Regional Offices. Water quality
data, including surface waters, is available through the USGS via their
automated NAWDEX system. For further information, telephone (703)860-6031.
9.3.3.4 Available Aerial Photographs—
Aerial reconnaissance can be an effective and economical tool for
gathering information on waste management facilities. For this application,
aerial reconnaissance includes aerial photography and thermal infrared
scanning. See Appendix A for a detailed discussion of the usefulness of
aerial photography in release characterization and availability of aerial
photographs.
9.3.3.5 Other-
Other sources of information-for subsurface and release characterization
include:
• U.S. EPA files (e.g., CERCLA-related reports);
• U.S'. Geological Survey; '
• U.S. DOA Soil Conservation Service;
• U.S. DOA Agricultural Stabilization and Conservation Service;
• U.S. DOI - Bureau of Reclamation;
• State Environmental Protection or Public Health Agencies;
• State Geological Survey;
• Local Planning Boards;
• County or City Health Departments;
• Local Library;
• Local Well Drillers; and
• Regional Geologic and Hydro logic Publications.
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9.4 DESIGN OF A MONITORING PROGRAM
9.4.1 Objectives of the Monitoring Program
The objective of initial monitoring is to begin characterizing known or
likely contaminant releases to ground water. To help accomplish this
objective, the investigator should evaluate any existing monitoring wells to
determine if they are capable of providing samples representative of
background and downgradient ground-water quality for the unit(s) of concern.
Figure 9-4 illustrates three cases where existing well systems are evaluated
with regard to their adequacy for use in a ground-water investigation. If the
monitoring network is found to be inadequate for all or some of the units of
concern, additional monitoring wells must be installed. Further
characterization, utilizing both direct and indirect investigative methods, of
the site's hydrogeology should be completed to identify appropriate locations
for the new monitoring-wells.
If initial monitoring confirms a contaminant release, the owner/operator
should extend the monitoring program to determine the vertical and horizontal
concentrations of all hazardous constituents in the release. The rate of
contaminant migration should also be determined. A variety of investigatory
techniques are available for such monitoring programs.
Monitoring procedures must include direct methods of obtaining
ground-water quality information -(e.g., sampling and analysis of ground water
from monitoring wells). Indirect methods of investigation may be used when
appropriate to aid in determining locations for monitoring wells (i.e.,
through geologic and/or geochemical interpretation of indirect data). For
many cases, the use of both direct and indirect methods may be the most
efficient approach.
Elements to be addressed in the ground-water monitoring program include:
• Constituents to be monitored;
• Frequency and duration at which samples will be taken;
• Sampling and analysis techniques to be used; and
• Monitoring locations.
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[Note: Permit application regulations in 40 CFR §270.14(c)(2)
require applicants to identify the uppermost aquifer and hydraulically
interconnected aquifers beneath the facility property if the facility has
any "regulated" units. The application must indicate ground-water flow
directions and provide the basis for the aquifer identification (i.e., a
report written by a qualified hydrogeologist on the hydrogeologic
characteristics of the facility property supported by at least the well
drilling logs and available professional literature). However, some RCKA
Part B permit applications did not require hydrogeologic
characterizations (e.g., storage only facilities) prior to the HSWA
Amendments of 1984. Now, such characterizations may be required
according to RCRA Section 3004(u) when SWMU releases to ground water are
likely or known. The RCRA Ground Water Monitoring Technical Enforcement
Guidance Document (TEGD) (U.S. EPA. 1986). and the Permit Applicant's
Guidance Manual for Hazardous Waste Land Treatment, Storage, and Disposal
Facilities (U.S. EPA, 1984) should be consulted for further information
on regulatory requirements.]
9.4.2 Monitoring Constituents
Initial monitoring should be focused on rapid, effective release
characterization at the downgradient limit of the waste management area.
Monitoring constituents must include waste-specific subsets of hazardous
constituents from 40 CFR Part 261, Appendix VIII. Indicators (e.g., TOX,
specific conductance) of these hazardous constituents may also be proposed.
Such" indicators alone are not sufficient to characterize a release of
hazardous constituents, since the natural background variability of indicator
constituents can be quite high. Furthermore, indicator concentrations do not
precisely represent hazardous constituent concentrations.
In developing a list of monitoring constituents, the following items
should be considered:
The nature of the wastes managed at the facility should be reviewed,
to determine which constituents (and any chemical reaction products
if the processes are understood) are relatively mobile and
persistent;
The effects of the unsaturated zone (if present) beneath the
facility on the mobility, stability and persistence of the waste
constituents;
The detection limits of the constituents; and
The concentrations and related variability of the proposed
constituents in background ground water.
9-31
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The list of constituents should be representative of constituents at least as
mobile as the most mobile hazardous constituent reasonably expected to be
derived from the unit's waste.
In the absence of detailed waste characterization information, the owner
or operator may employ the monitoring constituent lists in Appendix D. The
use of these lists is contingent upon the level of detail provided by the
waste characterization. The regulatory agency may modify these lists as
necessary on a facility-specific basis.*
List A is a core set of constituents that can be used at any unit needing
further investigation. Lists Bl, 82, etc., include constituents useful at
sites known to contain certain specific waste types. For example, Bl is a
list of constituents useful at units known to contain metal-finishing wastes
and associated water treatment wastes. List C is a set of parameters useful
at units with unknown or variable wastes. List A should be used along with
either List B (or relevant parts) or List C, as appropriate.
If hazardous constituents are found during initial monitoring or are
known to be present,.the owner or operator will be required to perform a more
detailed analysis to determine the presence of any other hazardous
constituents in ground water. For such cases, Appendix D may be used to
select monitoring constituents. If the unit of concern cannot be monitored
separately from regulated units, then monitoring for all constituents in 40
CFR Part 261, Appendix VIII will be necessary.
A useful reference on the mobility and fate of some hazardous
constituents is Water-Related Fate of 129 Priority Pollutants. Further
guidance on the selection of monitoring constituents is given in the Permit
Writer's Guidance Manual for Subpart F, Ground Water Protection.
The owner/operator should consider monitoring for additional inorganic
constituents that characterize the general quality of water at the site
(e.g., chloride, iron, manganese, sodium, sulfate, calcium, magnesium,
potassium, nitrate, phosphate, silicate, ammonium, alkalinity or pU).
Baseline data on such constituents can be used for subsequent monitoring
phases, if necessary. ,
*Note: The Appendix D lists are currently incomplete, and would need further
refinement if adopted by the regulatory agency.
9-32
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For example, information on the major anions and cations that make up the bulk
of dissolved solids in water can be used to determine reactivity and
solubility of hazardous constituents and therefore predict their mobility
under actual site conditions.
Subsequent ground-water monitoring phases will usually require monitoring
for a waste-specific subset of the Appendix VIII constituents, and in some
cases for all hazardous constituents in 40 CFR Part 261, Appendix VIII.
9.4.3 Monitoring Schedule
9.4.3.1 Monitoring Frequency—
Monitoring frequency should be based on various factors, including:
* *
• Ground-water flow rate and flow patterns;
• Trends in the monitoring data;
• Climato logical characteristics (e.g., precipitation patterns);
Generally, the greater the rate of ground-water flow, the greater the
monitoring frequency needed. For example, monitoring frequency in an
intergranular porosity flow aquifer of low permeability materials would likely
be less than for a fracture or solution porosity flow aquifer with
unpredictable and high flow rates. In the case of a fracture or solution
porosity flow aquifer, it is possible that contaminants could migrate past the.
facility boundary in a matter of days, weeks, or months, requiring frequent
monitoring.
The adequacy of existing hydrogeologic and background monitoring data
will strongly influence the monitoring schedule. For example, a facility
which has adequately completed a hydrogeologic characterization and has .
performed adequate background monitoring under interim status requirements
will likely have a good data base against which to compare initial monitoring
results. At the other end of the spectrum are facility.es lacking
hydrogeologic data and monitoring systems. Owners or operators of these
facilities will need to design and install an adequate monitoring well system
and establish background values of the selected monitoring constituents. It
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is recommended that these facilities undergo an accelerated monitoring
program, as described below.
The determination of whether contamination has occurred will be based on
a comparison of background and downgradient monitoring values obtained for a
list of waste-specific hazardous constituents and, where appropriate,
indicator constituents. The monitoring constituents should be selected based
on the waste characterization and the hydrogeologic information and should
include constituents that would be expected to be at the leading edge of any
plume of contamination. The comparison should be based on the mean of pooled
data obtained over a relatively short period of time. The results of this
comparison will be evaluated by the regulatory agency to determine if
subsequent monitoring is necessary. .,
If there is a high temporal correlation between background and
downgradient concentrations of the monitoring constituents of concern,
background values may be established by sampling background wells each time
ground water is sampled. With this approach, background concentrations are
not established by averaging values obtained over time; rather, background
values are established at each sampling event.
9.4.3.2 Duration of Monitoring—
The duration of the initial monitoring phase will vary with
facility-specific conditions (e.g., hydrogeology, wastes present). Initial
monitoring does not necessitate the collection of a full year of background
data. The initial characterization may be accomplished through accelerated
monitoring over a period of weeks or months. The regulatory agency will
evaluate initial monitoring results to determine how long monitoring should
continue and to determine the need for adjustments in the monitoring schedule
and the list of monitoring constituents. If the regulatory agency determines
that a release to ground water has not occurred, the investigation process for
that release can be terminated. If contamination is found during initial
monitoring, the facility will be -required to perform further monitoring to
fully characterize the release.
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9.4.4 Monitoring Locations ....
If there is no existing monitoring system or if the system is inadequate
to effectively characterize ground-water contamination, the owner/operator
should design and install a well system capable of intercepting the suspected
contaminant plume(s). The system should also be used for obtaining relevant
hydrogeologic data. The monitoring well network configuration should be based
on the site's hydrogeology, the layout of the facility and the units of
concern, and should reflect a consideration of any information available on
the nature and source of the release. It is important to recognize that the
potential pathways of contaminant migration are three dimensional.
Consequently, the design of a monitoring network which intercepts these
potential pathways requires a three dimensional approach.
In many cases, the initial monitoring system will need to be expanded for
subsequent phases. Additional downgradient wells will often be needed to
determine the extent of the contaminant plume. A greater number of background
wells may also be needed to account for spatial variability in ground-water
quality. •
Prior to the installation of additional downgradient monitoring wells, an
estimation of plume geometry should be made from a review of current and past
site characterization information. Additional hydrogeologic investigations
may also be appropriate. For example, piezometer readings surrounding the
well(s) showing a release should be taken to determine the current hydraulic
gradient(s). These values should be compared to the potentiometric surface
map developed for the site hydrogeologic characterization to better describe
the direction(s) of plume migration. Seasonal (natural or induced) or
regional fluctuations should be considered during this comparison.. A
re-evaluation of the facility's subsurface geologic information should be
performed to identify preferential pathways of contaminant migration. In some
situations, it may be appropriate to develop ground-water flow nets to show
vertical and horizontal components of flow. Guidance on construction of flow
nets is provided in Appendix E.
At facilities where it is known or likely that volatile organics have
been released to the uppermost aquifer, organic vapor analysis of soil gas
from shallow holes may provide an initial indication of the areal extent of
the plume (Figure 9-5). An organic vapor analyzer (OVA) may be used to
9-35
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measure the volatile organic constituents in shallow hand-augered holes.
Alternatively, a sample of soil gas may be extracted from .a shallow hole and
analyzed in the field using a portable gas chromatograph. These techniques
are limited to situations where volatile organics are present. Further,.the
presence of intervening, saturated, low permeability sediments strongly
interferes with the ability to extract a gas sample. Although it is not
necessarily a limitation, optimal gas chromatography results are obtained when
the analyte is matched with the highest resolution technique, (e.g., electron
capture/halogenated species). The effectiveness of this approach should be
evaluated by initial OVA sampling in the vicinity of wells known to be
contaminated.
Other direct methods that may be used to define the extent of a plume
include sampling of seeps and springs. Seeps and springs occur where the
local ground-water surface intersects the land surface resulting in
ground-water discharge into a stream, rivulet, or other surface water body.
Seeps and springs may be observed near marshes, at road cuts, or near
streams. As discharges from seeps and springs reflect the height of the
potentiometrie surface, they are likely to be most abundant during a wet
season.
•'• To-minimize the installation of new wells, the use of applicable
geophysical and modeling methods may be proposed to describe geologic
conditions and contaminant plume geometry/characteristics. ,
Such methods can aid in the placement of new monitoring wells.
A variety of indirect geophysical methods are currently available to aid
in characterizing geologic conditions and ground water-contamination.
Geophysical methods do not provide detailed, constituent-specific data;
however, they can be useful in estimating the general areal extent of a
plume. This may reduce speculation involved in determining new well
locations. Details on the use of geophysical methods are presented in
Appendix C.
Mathematical and/or computer modeling results may be used in conjunction
with the results of geophysical investigations to assist in well placement
decisions. The owner/operator should not, however, depend solely on such
models to determine the placement of new monitoring wells. Since models may
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not accurately account for the high spatial and temporal variability of
conditions encountered in the field, modeling results should be limited to
estimating contaminant concentrations at various locations and times.
In order, to estimate the potential extent of a release in the direction
of ground-water flow, Darcy's Law should be applied to determine ground-water
velocity (see Subsection 9.5.2). This velocity should then be multiplied by
the age of the unit of concern (i.e., assuming the unit began releasing
immediately) to estimate the potential distance of contaminant migration.
This distance.should be used as a "yardstick" in determining well locations.
More complex modeling (e.g., solute transport), may be proposed by the
owner/operator to assist in locating additional monitoring wells. However,
modeling results cannot be used in lieu of field monitoring data when
characterizing releases.
The International Ground Water Modeling Center (IGWMC), supported largely
by the U.S. Environmental Protection Agency, operates a clearing-house for
ground-water modeling software, organizes and conducts short courses and
seminars, and carries out a research program supporting the Center's
technology transfer and educational activities. Two major functions of IGWMC
are the dissemination of information regarding ground-water models and the
distribution of modeling software. The Center maintains computerized-data
bases, including updated computer codes and test files, and descriptions of a
large number of ground-water models. By means of a search and retrieval .
procedure, this information is easily accessible and readily available. The
IGWMC can be contacted at the following address:
International Ground Water Modeling Center
Holcomb Research Institute
Butler University
Indianapolis, Indiana 46208
Telephone: (317)283-9458
The Center will send, upon request and free of charge, a listing of available
publications, and a copy of its Newsletter.
In selecting and applying models, it is important to remember that a
model is an artificial representation of a physical system used to
characterize a site. A model cannot replace field data, nor can it be more
accurate than the available site data.
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Use of computer models requires special expertise. Time and experience
are needed to select the appropriate code and subsequent calibration. If
these resources are not available, modeling should not be attempted. Models
are used in conjunction with scientific and engineering judgment; they are an
aid to, not a surrogate for, a skilled analyst.
If a model is proposed in the monitoring procedures, the owner/operator
should describe all assumptions used in applying the model to the site in
question. A sensitivity analysis of the model should be run to determine
which input parameters have the most influence on model results, and the
model's results should be verified by field sampling.
9.4.4.1 Background and Downgradient Wells- ' * ;
Background monitoring wells should provide baseline ground-water quality
data in the uppermost aquifer. Background wells (preferably upgradient)
should be located so as to obtain samples that are not affected by releases
from the unit(s) of concern. These wells should be screened at the same
stratigraphic horizon(s) as the downgradient wells to ensure comparability of
data. Background wells should be of sufficient number to account for any '
heterogeneity in background ground-water quality.
Downgradient wells must be located, screened, and of sufficient number to
provide a high level of certainty that any releases of hazardous constituents
from the units of concern to the uppermost aquifer can be discovered as soon
as possible. Determination of the appropriate number of wells to be included
in an initial monitoring system should be based on unit size and the
complexity of the hydrogeologic setting (e.g., degree of fracturing and
variation in hydraulic conductivity). At a minimum, downgradient monitoring
wells should be located at the limit of the waste management area of the units
of concern.
9.4.4.2 Well Spacing—
The horizontal spacing between wells should be a design consideration.
Site specific factors as listed in Table 9-4 should be considered when
determining the horizontal distances between initial monitoring system wells.
These factors cover a variety of physical and operational aspects relating to
the facility including hydrogeologic setting, dispersivity, seepage velocity,
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TABLE 9-4. FACTORS INFLUENCING THE INTERVALS BETWEEN INDIVIDUAL MONITORING
WELLS WITHIN A POTENTIAL MIGRATION PATHWAY
Wells Intervals May Be
Closer If the Site:
Wells Intervals May Be
Wider If the Site:
• Manages or has managed liquid waste
• Is very small (i.e., the downgradient
perimeter of the site is less than
150 feet)
• Has waste incompatible with liner
materials
• Has fill material near the waste
management units (where preferential
flow might occur)
• Has buried pipes, utility trenches,
etc., where a point-source leak
might occur
• Has complicated geology
•- closely spaced fractures
- faults
- tight folds
- solution channels
- discontinuous structures
• Has heterogenous conditions
- variable hydraulic conductivity
- variable lithology
• Is located in or near a recharge
zone
• Has a high (steep) or variable
hydraulic gradient
• Low dispersivity
• High seep velocity
Has simple geology
— no fractures • • '
- no faults
- no folds
- no solution channels:
- continuous structures
Has homogeneous conditions
- uniform hydraulic conductivity
- uniform lithology
• Has a low (flat) and constant
hydraulic gradient
• High dispersivity
• Low seep velocity
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facility design, and waste characteristics. In the less common homogeneous
geologic setting where simple flow patterns are identified, a more regular
well spacing pattern may be appropriate. Further guidance on the
consideration of site specific conditions to evaluate well spacing is
described in Chapter Two of TEGD (U.S. EPA, 1986).
Subsequent phase monitoring systems should be capable of identifying the
full extent of the contaminant plume and establishing the concentration of
individual constituents throughout the plume. Well installation and
monitoring should concentrate on defining those areas that have been affected
by the release. A well cluster network should be installed in and around the
plume to define the horizontal and vertical extent of contamination. Networks
of monitoring wells will vary from site to site, depending upon
hydrogeological complexity and contaminant characteristics. Surface
geophysical techniques and modeling may also be used, where appropriate, to
help facilitate plume definition. The well density or amount of sampling
undertaken to completely identify the furthest extent of migration should be
determined by the variability in subsurface geology, present at the site.
Formations such as unconsolidated deposits with numerous interbedded lenses of
varying permeability or consolidated rock with numerous fracture traces will
generally require more extensive monitoring to ensure that contamination is
appropriately characterized.
• Monitoring should also be performed to characterize the interior
portion(s) of any plume found at the site. This is"important because
constituents can migrate at differing rates and may have been released at
different times. Monitoring only at the periphery of the plume may not
identify all the constituents in the release, and the concentration of
monitoring constituents measured at the periphery of the plume may be
significantly less than in the interior portion(s). Patterns in
concentrations of individual constituents can be established throughout the
plume by sampling along several lines that perpendicularly transect the
plume. The number of transects and spacing between sampling points should be
based on the size of the plume and variability in geology observed at the
site. Sampling locations should also be selected so as to identify those
areas of maximum contamination within the plume. In addition to the expected
hazardous constituents, the plume may contain degradation and reaction
products, which may also be hazardous.
-------
Results of geophysical methods may be correlated with data from the
monitoring well network. The monitoring program should be flexible so that
adjustments can be made to reflect plume migration and changes in direction.
The spacing between initial downgradient monitoring wells should have
been designed to ensure the measurement of releases near the unit(s) of
concern. However, it is possible that the initial spacing3 between wells will
only provide for measurements in the peripheral portion of a plume. This
might result in water quality measurements that do not reflect the maximum
concentration of contaminants in the plume. Therefore, additional
downgradient wells may be needed adjacent to the units of concern during
subsequent monitoring phases.
A similar effect may be observed, even with a closely spaced initial
downgradient monitoring network, if a narrow, localized plume migrates past
the limit of the waste management area. Such a plume may originate from a
small leak in a liner and/or from a leak located close to the downgradient .
limit of the waste management area, thereby limiting the amount of dispersion
occurring in the plume prior to its passing the monitoring wells.
Consequently, if relatively wide spacing exists between wells or there is
reason to expect a narrow, localized plume, the installation of additional
monitoring wells may be necessary in the immediate vicinity of those wells in
which a release has been measured. Such an expansion of the monitoring
network is recommended when a release has been measured in only one or two .
monitoring wells, indicating a localized plume.
9.4.4.3 Depth and Screened Intervals--
The depth and screened intervals for initial phase monitoring wells
should be based on: (1) geologic factors influencing the potential
contaminant pathways of migration to the uppermost aquifer;
(2) physical/chemical characteristics of the contaminant controlling its
likely movement and distribution in the aquifer; and (3) hydrologic factors
likely to have an impact on contaminant movement. The consideration of these
factors in evaluating the design of monitoring systems is described in
Section 2.1 of the TEGD (U.S. EPA, 1986), including examples of placement in
some common geologic environments. Subsection 9.6 provides guidance on
borings and monitoring well construction.
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In order to establish vertical concentration gradients of hazardous
constituents in the plume during subsequent monitoring phases, well clusters
or multi-depth monitoring wells should be installed. The first well in a
cluster (or initial sampling interval in a multi-depth well) should be
•screened at the horizon in which contamination was initially discovered.
Additional wells in a cluster should be screened, where appropriate, above and
below the initial well's sampling interval until the margins of the plume are
established.
Basically, several wells should be placed at the fringes of the plume to
define its vertical margins, and several wells should be placed within the
plume to identify contaminant constituents and concentrations.
Care must be taken in placing contiguously screened wells close together
since one well's drawdown may influence the next and thus change the horizon
from which its samples are drawn. Alternating lower and higher screens should
reduce this effect (see Figure 9-6).
The specifications of sampling depths should clearly identify the
interval over which each sample will be taken. It is important that these
sampling intervals be sufficiently discrete to allow vertical profiling of
constituent concentrations in ground water at each sampling location.
Sampling will only provide measurements of the average contaminant
concentration over the interval from which that sample is taken. Samples
taken from wells screened over a large vertical interval may be subject to
dilution effects from uncontaminated ground water lying outside the plume
limits. The proposed screened interval should reflect the expected vertical
concentration gradients within the plume.
At those facilities where immiscible contaminants have been released and
have migrated as a separate phase (see Figure 9-7), specific techniques will
be necessary to evaluate their migration. Chapter 5 of the TEGD (U.S. EPA,
1986) contains a discussion of ground-water monitoring techniques that can be
used to sample multi-phased contamination. The formation of separate phases
of immiscible contaminants in the subsurface is largely controlled by the rate
of infiltration of the immiscible contaminant and the solubility of that
contaminant in ground water. Immiscible contaminants generally have limited
solubility in water. Thus, some amount of the immiscible contaminant released
from a unit(s) will dissolve in the ground water and thus migrate in
solution. However, if the amount of immiscible contaminant reaching ground
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IM%M> WATER TABLE
Figure 9-6. Vertical Well Cluster Placement.
9-44
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water exceeds the ability of ground water to dissolve it, the ground water in
the upper portion of the water table aquifer will become saturated and the
contaminant will form a separate immiscible phase. t
At this point, the behavior and migration of an immiscible contaminant
will be strongly influenced by its density relative to ground water. If the
immiscible is less dense than ground water, it will tend to form a separate
immiscible layer and migrate on top of the ground water. If the density of
the immiscible contaminant is similar to that of ground water, it will tend to
mix and flow as a separate phase with the ground water, creating a condition
of multiphase flow.
If the density of the immiscible is greater than ground water, it will
tend to sink in the aquifer (see Figure 9-7). As the immiscible sinks and
reaches unaffected ground water in a deeper portion of the aquifer, more of
the immiscible contaminant will tend to enter into solution in ground water
and begin to migrate as a dissolved constituent. However, if enough of the
dense immiscible contaminants are present, some portion of these contaminants
will continue to sink as a separate immiscible phase until a .geologic
formation of reduced permeability is reached. At this point, these dense
contaminants will tend to form a layer that migrates along the geologic
formation (boundary).
Immiscible phase contaminants may migrate at rates different than-that of
ground water. In addition, immiscible contaminants may not flow in the same
direction as ground water. However, it is important to re-emphasize that some
faction of these contaminants may dissolve in ground water and migrate away
from the facility as dissolved constituents.
Light immiscible contaminants tend to migrate downgradient as a floating
layer above the saturated zone (see Figure 9-7). The hydraulic gradient is a
major factor in the movement of this light immiscible layer. Other important
factors involved in the migration rate of a light immiscible phase include the
intrinsic permeability of the medium, and the density and viscosity of the
contaminants. Oftentimes, an ellipsoidal plume will develop over the
saturated zone as depicted in Figure 9-7. While it may be possible to analyze
the behavior of a light immiscible layer using analytical or numerical models,
the most practical approach for determining the rate and direction of
migration of such a layer is to observe its behavior over time with
appropriately located monitoring wells.
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The migration of a layer of dense immiscibles resting on a retarding
geologic formation may be strongly influenced by gravity. Depending on the
slope of the retarding formation, the immiscible layer may move with or in a
different direction from the flow of the ground water. Consequently, the
evaluation of the rate and direction of migration of a dense immiscible layer
should include a determination of the configuration of the* retarding formation
on which the immiscible layer is migrating. The direction of migration and
estimates of migration rates of dense immiscibles can then be obtained by
including the gravitational forces induced by the slope of the retarding
formation in the gradients used to calculate contaminant flow rates. If a
dense immiscible layer(s) is expected or known, the monitoring plan should
include procedures to verify its direction and rate of flow.
9.5 DATA EVALUATION AND PRESENTATION
The objective of monitoring is to estimate the rate and extent of
migration and the concentration of constituents in the plume. Data are,
therefore, collected from a set of monitoring wells that will allow
characterization of the dimensions and concentrations of constituents in the
plume,--as weir as the'rate of flow. ' •
9.5.1 Data Presentation
Subsequent monitoring phases will often include the measurement of many
more constituents in a more extensive well network than initial monitoring.
This will often necessitate the collection of large amounts of data.
Sections 6.8 and 6.9 of the TEGD (U.S. EPA, 1986) provide useful guidance on
organizing, evaluating, and presenting monitoring data. Section 4.7 of TEGD
addresses evaluation of the quality of ground water data. '
Section 5 of this document describes data presentation methods with •
examples. In addition to sorted data tables, the methods described for
isopleth maps of concentrations (Figure 5-3); geologic cross-sections
(Figure 5-6); cross-sectional concentration contours (Figure 5-7,); and fence
diagrams (Figure 5-8) should be useful for presenting ground-water
investigation findings. At a minimum, a topographic map meeting the
requirements of a Part B permit application and which shows the monitoring
well locations will be required.
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9.5.2 Rate of Migration
. Procedures to be used for determining the rate of hazardous constituent
migration in ground water should be proposed. Migration rates can be
determined by the concentration of monitoring constituents over a period of
time in wells aligned in the direction of flow. If these wells are located
both at the edge of the plume and in the interior of the plume, subsequent
analysis of 'the monitoring data can then provide an estimate of the rate of
migration both of the contaminant front as a whole and of individual
constituents within the plume. This approach does not necessarily provide a
reliable determination of the migration rates that will occur as the
contaminant plume moves further away from the facility due to potential
changes in geohydrologic conditions. More importantly, this approach requires
the collection of a time series data of sufficient duration and frequency to
gauge the movement of contaminants. Such a delay is normally inappropriate
during initial characterization of ground-water contamination since a
relatively quick determination of at least an estimate of migration rates is
needed to deduce the impact of ground-water contamination and to formulate an
appropriate reaction.
Rapid estimates of migration rates should be made from aquifer properties
obtained during the hydrogeologic investigation. The average linear velocity
(v) of the ground water should be calculated using the following form of
Darcy's equation:
where (K) is hydraulic conductivity! (i) is hydraulic gradient, and (n ) is
effective porosity. This assumes that contaminants flow at same rate as
ground water. This equation can be used to roughly estimate the rate of
migration, both of the contaminant front as a whole, and of individual
dissolved constituents within the plume.
Rough estimates of migration rates beyond the facility property boundary
can be made based on aquifer properties obtained during the site hydrogeologic
characterization, and knowledge of the physical and chemical properties of
9-48
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contaminants known to be present. By recognizing the various factors which
can affect the transport of monitoring constituents, the owner/operator can
determine approximate migration rates. Continued monitoring of the plume over
time should be conducted to verify the rate(s) of migration. Information on
rate(s) of migration should be used in determining any additional monitoring
well locations.
More refined estimates of contaminant migration rates should consider
potential differential transport rates among various monitoring constituents.
Differential transport rates are caused by several factors, including:
• Dispersion due to diffusion and mechanical mixing;
• Retardation due to adsorption and electrostatic interactions; and
• Transformation due to physical, chemical, and/or biological
processes.
Dispersion results in the overall dilution of the contaminant; however,
chromatographic separation of the contaminant constituents and differential
dispersion effects can result in a contaminant arriving at a particular
location before the arrival time computed solely on the average linear
velocity ,of ground-water flow. Alternately, retardation processes can delay
the arrival of contaminants beyond that calculated using average ground-water
flow rate(s). Local geologic variations will also affect constituent
migration rates. Relating constituent migration rates to ground-water flow
rates is a reasonable and relatively quick way to estimate ground-water flow
rates. Where possible, contaminant-specific migration rates should also be
determined.
9.5.3 Statistical Evaluations
If the owner/operator disputes the regulatory agency's interpretation of
initial monitoring results, he/she may propose a monitoring program, including
statistical data evaluation procedures, to refute the agency's
determinations. Statistical procedures must include a level of significance
that will strike an appropriate balance between the probability of false
positives and false negatives.
9-49
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The owner/operator should describe the proposed procedures, state all
underlying assumptions, and show that the procedures are consistent with
available data on each monitoring constituent. In some cases, different data
evaluation procedures may be appropriate for different monitoring
constituents. In these cases, the owner/operator should clearly show which
procedure(s) will be applied to each constituent.
Subsequent monitoring phases should stipulate and document procedures for
the evaluation of monitoring data. These procedures vary in a site-specific
manner, but must all result in determinations of the rate of migration,
extent, and hazardous constituent composition of the contaminant plume. In
some cases, where the release is obvious and/or chemically simple, it may be
possible to characterize it readily from a descriptive presentation of
concentrations found in monitoring wells and geophysical characterization. In
other cases, where contamination is less obvious or the release is chemically
complex, the owner/operator may propose a statistical inference approach.
Owner/operators should plan initially to take a descriptive approach to data
analysis in order to broadly delineate the extent of contamination.
9.5.4 Use of Flow Nets
A flow net provides a graphical technique for obtaining solutions' to
steady state ground-water flow. A properly constructed flow net can be used
to determine the distribution of heads, discharges, areas of high (or low)
velocities, and the general flow pattern (McWhorter and Sunada, 1977).
The Ground Water Flow Net/Flow Line Technical Resource Document (TRD)
(U.S. EPA, 1985), provided as Appendix E, provides detailed discussion and
guidance in the construction of flow nets. Although the focus of this
document is on the construction of vertical flow nets, the same data
requirements and theoretical assumptions apply to horizontal flow nets. The
fundamental difference between vertical and horizontal flow nets is in their
application. A flow net in the horizontal plane may be used to identify
suitable locations for monitoring wells whereas a flow net in the vertical
plane would aid in determining the screened interval of a well.
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The following excerpts from the Flow Net Document (U.S. EPA, 1985)
explain data needs for flow net construction. Several assumptions must be
made to construct a flow net:
• Ground-water flow is steady state, which means flow is constant with
time;
• The aquifer is completely saturated;
• No consolidation or expansion of the soil or water occurs;
• The same amount of recharge occurs across the system; and
• Flow is laminar and Darcy's Law is valid.
v
Knowledge of the hydro logic parameters of the ground-water system is
required to properly construct a flow net'. These parameters include:
• Head distribution, both horizontally and vertically;
• Hydraulic conductivity of the saturated zone;
: • Saturated zone thickness; and
"• Boundary conditions.
The distribution of head can be determined using time equivalent water
level measurements obtained from piezometers and/or wells. Plotting the water
level elevations on a base map and contouring these data will provide a
potentiometric surface. Contour lines representing equal head are called
lines of equipotential. Changes in hydraulic head, both horizontally and
vertically within an aquifer, must be known for proper flow-net construction.
These changes can be delineated with piezometers or monitoring wells installed
at varying depths and spatially distributed. The data must be time equivalent
since water levels change over time. Ground—water flow directions can be
determined by drawing lines perpendicular to the equipotehtial lines. Ground
water flows from areas of high hydraulic head to areas of low hydraulic head.
The hydraulic conductivity of a material depends on the properties of the
fluid and the media. Clayey materials generally have low hydraulic
conductivities, whereas sands and gravels have high conductivities (U.S. EPA
9-51
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TRD, 1985). Where flow crosses a boundary between different homogeneous media
the ground-water flowlines refract and flow velocity changes due to an abrupt
change in hydraulic conductivity. The higher permeability formation serves as
a conduit to ground-water flow. This is visually apparent in a properly
constructed flow net, since flow tubes are narrower in layers with higher
conductivity because less area is necessary to conduct the same volume of
ground water. In media of lower conductivity, flow tubes will be wider in
order to. conduct the same volume of flow (Cedergren, 1977). Construction of
flow nets for layered geologic settings (heterogeneous, isotropic systems) are
discussed in Section 2 of the flow net document (U.S. EPA, 1985).
The boundary conditions of an aquifer must also be known to properly
construct a flow net. The boundary conditions will be used as the boundaries
of the flow net. The three types of boundaries are: 1) impermeable
boundaries; 2) constant head boundaries; and 3) water table boundaries (Freeze
and Cherry, 1979). Ground water will not flow across an impermeable boundary;
it flows parallel to these boundaries. A boundary where the hydraulic head is
constant is termed a constant head boundary. Ground-water flow at a constant
head boundary is perpendicular to the boundary. Examples of constant head
boundaries are lakes, streams, and ponds. The water table boundary is the
upper boundary of an unconfined aquifer, and is a line of known and variable
head. Flow can be at any angle in-relation to the water table due to-recharge
and the regional ground water gradient. The boundary conditions of an aquifer
can be determined after a review of the geohydrologic data for a site (U.S.
EPA, 1985).
Although a complete understanding of the mathematics of ground-water flow
is not necessary for proper flow-net construction by graphical methods, a .
general understanding of the theory of ground-water flow is required. For a
brief discussion of ground-water flow theory as applied to flow nets, refer to
Section 1 of the flow net document (U.S. EPA, 1985). Detailed guidance on
graphical construction of flow nets is given in Section 2 of that document.
Mathematical techniques can be used to construct flow nets although graphical
techniques are the simplest and most commonly used. It is worth noting that
flow nets are dimensionless. .
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Construction of flow nets is not appropriate or valid in certain
instances. As discussed in the flow net document, these situations occur when
there is a lack of three-dimensional hydrologic data for a ground-water
system, and when ground-water flow in a system does not conform to the
principles expressed by and assumptions made in Darcy's Law.
Scaling problems occur when the aquifer and/or geologic layers associated
with a particular ground-water system are thin in relation to the length of
the flow net. If a flow net is constructed for this situation, the flow net
will be made up of squares that are too small to work with unless -the scale is
exaggerated.
For sites where the assumption of steady-state flow is not valid, the
construction of flow nets is very difficult. The flow net must be redrawn
each time the flow field changes to simulate the transient conditions.
Lack of three-dimensional hydrologic data or hydrologically equivalent
data for a ground-water flow system makes proper flow-net construction
impossible. Hydrologic testing at various depths within an aquifer and
determination of the vertical hydraulic conductivity of an aquifer are
essential to provide the necessary data. If these data are not available it
will be necessary to obtain them before a flow net can be constructed. -
There are three types of ground-water systems in which-the principles *
expressed'by Darcy's Law do not apply. The first is' a system in which the
flow is through materials with low hydraulic conductivities under extremely
low gradients (Freeze and Cherry, 1979). The second is a system in which a
large amount of flow passes through materials with very high hydraulic
conductivities; and the third is a system in which the porous media assumption
is not valid. Darcy's Law expresses linear relationships and requires that
flow be laminar (flow in which stream lines remain distinct from one
another). 'In a system with high hydraulic conductivity, flow is often
turbulent when it has a high velocity. Turbulent flow is characteristic of
karstic limestone and dolomite, cavernous volcanics, and fractured rock
systems. Construction of flow nets for areas of turbulent flow would not be
valid. The use of Darcy's Law also requires the assumption of porous media
flow. This assumption may not be valid for many fractured bedrock and karst
environments where fracture flow is dominant or large solution features are
present.
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9.6 FIELD METHODS
9.6.1 Geophysical Techniques
During the past decade, extensive development of remote sensing
geophysical equipment, portable field instrumentation, field methods,
analytical techniques and related computer processing have resulted in an
improvement in the capability to characterize hydrogeology and contaminant
releases. Some of these geophysical methods offer a means of detecting
contaminant plumes and flow directions in both the saturated and unsaturated
zones. Others offer a way to obtain detailed information about subsurface
soil and rock characteristics. This capability to rapidly analyze subsurface
conditions without disturbing the site offers the benefit of low cost and
risk, and provides better overall understanding of complex site conditions.
Appendix C discusses various geophysical techniques, including
electromagnetic, seismic refraction, resistivity, ground penetrating radar,
magnetic, and several borehole methods applicable to RCRA Facility
Investigations. Each technique is described in terms of how it can be used as
a supplementary investigative tool to assist in site lithologic, structural,
and hydrogeologic setting characterization, as well as to assist in the.
location of wells and the delineation of buried waste and contaminant plumes
in both initial and subsequent monitoring phases.
9.6.2 Soil Boring and Monitoring Well Installation
9.6,2.1 Soil Borings—
Soil borings shoul be sufficient to characterize the subsurface geology
below the site. Section 1.2 of TEGD (U.S. EPA, 1986} describes the
requirements of borings performed for interim status:
• Installation of initial boreholes at a density based on criteria
described in Table 9-5 and sufficient to provide initial information
upon which to determine the scope of a more detailed evaluation of
geology and potential pathways of contaminant migration.
,9-54
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TABLE 9-5. FACTORS INFLUENCING DENSITY OF INITIAL BOREHOLES
Factors That Hay Substantiate
Reduced Density of Boreholes:
Factors That May Substantiate
Increased Density of Boreholes:
Simple geology (i.e., horizontal,
thick, homogeneous geologic strata
that are continuous across site
that are unfractured and are
substantiated by regional geologic
information).
Use of geophysical data to correlate
well log data.
• Fracture zones encountered
during drilling.
• Suspected pinchout zones (i.e.,
discontinuous units across the
site).
• Geologic formations that are
tilted or folded.
• Suspected zones of high per-
meability that would not be
defined by drilling at 300-foot
intervals.
• Laterally transitional geologic
units with irregular permea-
bility (e.g., sedimentary facies
changes).
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Initial boreholes should be drilled into the first confining layer
beneath the uppermost aquifer. The portion of the borehole
extending into the confining layer should be plugged properly after
a sample is taken.
Installation of additional boreholes in numbers and locations
sufficient to characterize the geology beneath the site. The number
and locations of additional boreholes should be based on data from
initial borings and indirect investigation.
Collection of samples of every significant stratigraphic contact and
formation, especially the confining layer should be taken.
Continuous cores should be taken initially to ascertain the presence
and distribution of small and large scale permeable layers. Once
stratigraphic control is established, samples taken at regular,
e.g., five foot intervals could be substituted for continuous cores.
Boreholes in which permanent wells are not constructed should be
sealed with materials at least an order of magnitude less permeable
than the surrounding soil/sediment/rock in order to reduce the
number of potential contaminant pathways.
Samples should be logged in the field by a qualified professional in
geology.
Sufficient laboratory analysis should be performed to provide
information concerning petrologic variation, sorting (for
unconsolidated sedimentary units), cementation (for consolidated
sedimentary units), moisture content, and hydraulic conductivity of
each significant geologic unit or soil zone above the confining
layer/unit.
Sufficient laboratory analysis should be performed to describe the
mineralogy (X-ray diffraction), degree of compaction, moisture
content, and other pertinent characteristics of any clays or other
fine-grained sediments held to be the confining unit/layer. Coupled
with the examination of clay mineralogy and structural
characteristics should be a preliminary analysis of the reactivity
of the confining layer in the presence of the wastes present.
ASTM methods should be used for soil classification, specifically:
ASTM Method D422-63 for the particle size analysis of soils, which
describes the quantitative determination of the distribution of
particle sizes in soils; and
ASTM Methods D2488-69, for the identification and description of
soils based on visual examination and simple manual tests.
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An adequate number of geologic cross-sections should be presented by the
owner/operator. These cross-sections should adequately depict major geologic
or structural trends and reflect geologic/structural features in relation to
ground-water flow. Additionally, an owner/operator should provide a surface
topographic map and aerial photograph of the site.. Details regarding the
specific requirements for the presentation of geologic data -are presented in
Section 1.2.3 of the TEGD (U.S. EPA, 1986). Figure 5-8 depicts a fence
diagram.
9.6.2.2 Monitoring Well Installation—
The owner/operator is advised to consult Chapter Three of the TEGD
(U.S. EPA, 1986} for guidance on monitoring well installation. This chapter
provides information on the following topics:
• Drilling Methods for Installing Wells—Section 3.1 discusses a
variety of well drilling methods and corresponding applicability to
the installation of RCRA monitoring wells. The selection of the
actual drilling method that an owner/operator should use at a
particular site is a function of site-specific geologic conditions.
Of utmost importance is that the drilling method the owner/operator
uses will minimize the disturbance of subsurface materials and will
not cause contamination of the ground water. ~ " ~
• Monitoring Well Construction Materials--Section 3.2 discusses the
selection of construction materials for RCRA monitoring wells which
are durable enough to resist.chemical and physical degradation, and
do not interfere with the quality of ground-water samples. Specific
well components that are of concern include well casings, well
screens, filter packs, and annular seals.
• Design of Well Intakes—Section 3.3 discusses 'the design and
construction of the intake of monitoring wells so as to: (1) allow
sufficient ground-water flow to the well for sampling; (2) minimize
the passage of formation materials (turbidity) into the well; and
(3) ensure sufficient structural integrity to prevent the collapse
of the intake structure.
• Development of Wells—Section 3.4 discusses the requirements for
proper development of the monitoring wells to ensure turbid-free
ground water samples.
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Documentation of Well Construction Activity--Section 3.5 lists the
information required for the design and construction of wells as
follows:
date/time of construction;
drilling method and drilling fluid used;
well location 0+ 0.5 ft);
bore hole diameter and well casing diameter;
well depth (+ 0.1 ft);
drilling and lithologic logs;
casing materials;
screen materials and design;
- casing and screen joint type;
screen slot size/length;
filter pack material/size;
- filter pack volume calculations;
filter pack placement method;
- sealant materials (percent bentonite); . .
sealant volume (ibs/gallon of cement);
sealant placement method;
- surface seal design/construction;
- well development procedure;
type of protective well Cap;
- ground surface elevation (^ 0.01 ft);
top of casing elevation (+~0.01 ft); and
- detailed drawing of well (include dimensions).
Specialized Well Design—Section 3.6 discusses two cases which
require special monitoring well design: (I) where dedicated pumps
are used to draw ground-water samples; and (2) where lightr. and/or
dense phase immiscible layers are present.
Evaluation of Existing Wells—Section 3.7 discusses how to evaluate
the ability of existing wells to produce representative ground-water
samples.
Particular attention should be paid to the discussion regarding well
casing materials (3.2.1). It is imperative that well materials are
nonreactive to contaminants that may be present in the ground water. In cases
where the facility has existing monitoring wells which could potentially be
used in the RFI, the owner/operator must evaluate whether these wells are
capable of producing representative ground water samples. -A demonstration
involving the installation of new well(s) near existing wells and the analysis
and comparison of samples for the same parameters from both wells may be
necessary if the existing wells' integrity is in question.
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9,6,3 Aquifer Characterization
9.6.3.1 Hydraulic Conductivity Tests—
In addition to defining the direction of ground-water flow in the
vertical and horizontal direction, the owner/operator must identify areas of
high and low hydraulic conductivity (K) within each formation. Variations in
the hydraulic conductivity of subsurface materials can create irregularities
in ground-water flow paths. Areas of high hydraulic conductivity represent
areas of greater ground-water flow and, if contaminants are present, zones of
potential migration. Therefore, information on hydraulic conductivities is
required before owner/operators can make reasoned decisions regarding well
placements. It may be beneficial to use analogy or laboratory methods to
corroborate results of field tests; however, only field methods provide direct
information that is-adequate to define the hydraulic conductivity.
Hydraulic -conductivity can be determined in the field using single well
tests, more commonly referred to as slug tests, which are performed by
suddenly adding or removing a slug (known volume) of water from a well or
piezometer and observing the recovery of the water surface to its original
level. Similar results can be achieved by pressurizing the well casing,
depressing the water level, and suddenly releasing the pressure to simulate
removal of water from the well.
When reviewing information obtained- from slug tests, several criteria
^should be considered. First, slug tests are run on one well and, as such, the
information obtained from single well tests is limited in scope to the
geologic area directly adjacent to the well. Second, the vertical extent of
screening will control the part of the geologic formation that is being tested
during the slug test. That part of the column above or below the screened
interval that has not been tested during the slug test will not have been .
adequately tested for hydraulic conductivity. Third, the. methods used to
collect the information obtained from slug tests should be adequate to measure
accurately parameters such as changing static water (prior to initiation,
during, and following completion of slug test), the amount of water added to,
or removed from the well, and the elapsed time of recovery. This is
especially important in highly permeable formations where pressure transducers
and high speed recording equipment should be used. Lastly, interpretation of
the slug test data should be consistent with the existing geologic information
/
9-59
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(boring log data). It is, therefore, important that the program of slug
testing ensures that enough tests were run to provide.representative measures
of hydraulic conductivity, and to document lateral and vertical variation of
hydraulic conductivity in the hydrogeologic subsurface below the site.
It is important that hydraulic conductivity measurements define hydraulic
conductivity both in a vertical and horizontal manner across a site. In
assessing hydraulic conductivity measurements, results from the boring program
*
used to characterize the site geology should be considered. Zones of high
permeability or fractures identified from drilling logs should be considered
in the determination of hydraulic conductivity. Additionally, information
from coring logs can be used to refine the data generated by slug tests (TEGD,
Section 1.3.3).
Techniques for determining hydraulic conductivity are specified in Method
9100, Saturated Hydraulic Conductivity, Saturated Leachate Conductivity, and
Intrinsic Permeability; from SW-846, Test Methods for Evaluating Solid Waste.
2nd edition. Method 9100 includes techniques for:
• Laboratory
- sample collection;
- constant head methods; and
- falling head methods.
• Field
well construction;
- well development;
- single well tests (slug tests); and
- references for multiple well (pump) tests.
Cedergren, 1977 also provides an excellent discussion on aquifer tests,
including laboratory methods (constant head and falling head), multiple well
(pump) tests (steady-sta_te and nonsteady-state), and single well (borehole)
tests (open-end, packer, and others).
9.6.3.2 Water Level Measurements—
Water level measurements are necessary for determining depth to the water
table and mapping ground-water contours to determine hydraulic gradients and
flow rates.
9-60
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Depths to water are normally measured with respect to the top of casing
as in well depth determinations. Several methods are available including the
electric sounder and the chalked steel tape.
The electric sounder, although not the most accurate, is recommended for
initial site work because of the minimal potential for equipment contamination
and simplicity of use. Sounders usually consist of a conductivity cell at the
end of a graduated wire, and a battery powered buzzer. When the cell contacts
the water the increased conductivity completes the circuit and allows current
to flow to the alarm buzzer. The depth to water can then be read from the
graduations on the wire or the wire can be measured directly. This device may
not be suitable for use if a potentially flammable or explosive layer is
present in the well, unless it is an intrinsically safe device.
The chalked steel tape is a more accurate device for measuring static
water levels. The lower 0.5 to 1.0 meters of a steel measuring tape is coated
on either side with either carpenter's chalk or any of the various indicating
pastes. A weight is attached to the lower end to keep the tape taut and it is
lowered into the center of the well (condensate on the casing wall may
prematurely wet the tape). A hollow "plopping" sound occurs when the weight
reaches water, then the tape is lowered very slowly for at least another
15 cm, preferably to an even increment on the measuring tape. Next, the tape
is carefully withdrawn from the well; water depth is determined by subtracting
the wetted length of tape from the total length of tape in the well. In small
diameter wells, the volume of the weight may cause the water to rise by
displacement. In general, the use of indicating paste or chalk should be
discouraged although they may not present a significant problem if water
samples are not collected. As with all depth measurement devices, the wetted
section of the tape and the weight must be thoroughly cleaned before reuse to
avoid cross contamination.
The following sections of TEGD (U'.S. EPA, 1986) should be consulted for
water level measurement requirements, and information on data interpretation:
• Ground-water level measurement (1.3.1.1);
,• Interpretation of ground water level measurements (1.3.1.2);
• Establishing vertical components of ground water flow (1.3.1.3); and
• Interpretation of flow direction (1.3.1.4).
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9.6.3.3 Dye Tracing—
Dye tracing is a field method used to measure the velocity of ground
water for highly permeable strata (such as karst terrain and highly fractured
rock media). When, at a common point, the velocity of flowing water and the
hydraulic gradient are known, the permeability can be estimated. The
hydraulic gradient (i) of an existing water table can be estimated from wells
in the area. If not, observation wells must be installed (Cedergren, 1977).
The procedure used in dye tracing involves the insertion of a dye such as
fluorescein sodium into a test hole and observation of the time it takes to
emerge in a nearby test pit or on a bank from which seepage is emerging. The
seepage velocity, v , is determined by dividing the distance traveled, L, by
S
the time of travel, t. The effective porosity, n , is determined from test
data for the in-place soil; if no tests are available, it is estimated. The
hydraulic conductivity is calculated from the equation:
v n
k--^
It should be noted that the time required for tracers to move even short
distances can be very long unless the formations contain extremely permeable
strata (Cedergren, 1977).
9.6.4 Ground Water Sample Collection Techniques
The procedure for collecting a ground water sample involves the following
steps presented in Chapter 4 of TEGD (U.S. EPA, 1986):
• Measurement of static water level elevation (4.2.1);
• Monitoring of immiscible layers (4.2.2);
• Well evacuation (4.2.3);
• Sample withdrawal (4.2.4);
• In situ or field analyses (4.2.5);
• Sample preservation and handling (4.3); and
• Chain-of-custody procedures (4.4).
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Collection of static water level elevations on a continuing basis is
important to determine if horizontal and vertical flow gradients have changed
since initial site characterization, which could necessitate modification of
the ground-water monitoring system. . .
Steps should be taken to monitor for the presence and/or extent of light
and/or dense phase immiscible organic layers before the well is evacuated for
conventional sampling if wastes of this type are present at the facility.
The water standing in the well prior to sampling may not be
representative of in situ ground-water quality. Therefore, the owner/operator
should remove the standing water in the well so that water which is
representative of the formation can replace the standing water.
The technique used to withdraw a ground-water sample from a well should
be selected based on a consideration of the parameters which will be analyzed
in the sample. To ensure the ground-water sample is representative of the
formation, it is important to avoid physically altering or chemically
contaminating the sample during the withdrawal process. In order to minimize
the possibility of sample contamination, the owner/operator should:
1. Use only polytetrafluoroethylene (PFTE) or stainless steel sampling
devices; and
2. Use dedicated samplers for each well. (If a dedicated sampler is
not available for each well, the sampler should be thoroughly
cleaned between sampling events, and blanks should be taken and
analyzed to ensure that cross contamination has not occurred.)
Section 4.2.4 of TEGD (U.S. EPA, 1986) includes specific factors to take
into consideration regarding sample withdrawal.
Some'parameters are physically or chemically unstable and must be tested
either in the borehole using a probe (in situ) or immediately after collection
using a field test kit. Examples of several unstable .parameters include pli,
redox potential, chlorine, dissolved oxygen, and temperature. Although
specific conductivity (analogous to electrical resistance) is relatively
stable, it is recommended that this characteristic be determined in the
field. Most conductivity instruments require temperature compensation;
therefore, temperatures of the samples should be measured at the time
conductivity is determined.
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Many of the constituents and parameters that are included in ground-water
monitoring programs are not stable and, therefore, sample preservation is
required. TEGD refers to methods from Test Methods for Evaluating Solid Waste
- Physical Chemical Methods (SW-846, Section 1.4.6.2.3) for sample preservation
procedures, and SW-846, Section 1.2.2 for sample container requirements.
Improper sample handling may lead to distortion of contaminant consti-
tuents in the samples. Samples must be transferred into their containers in
such a way as to minimize distortion. Handling methods are analyte
dependent. Special handling considerations for various analyte types are
discussed in Section 4.3.3 of TEGD (U.S. EPA, 1986).
An adequate chain-of-custody program will allow for the tracing of
possession and handling of individual samples from the time of field
collection through laboratory analysis. An owner/operator's chain-of-custody
program should include:
• Sample labels which prevent mis identification of samples;
• Sample seals to preserve the integrity of the sample from the time
it is collected until it is opened in the laboratory;
• Field logbook to record information about each sample collection
during the ground-water monitoring program;
• Chain-of-custody record to establish the documentation necessary to
trace sample possession from the time of collection to analysis;
• Sample analysis request sheets which serve as official communication
to the laboratory of the particular analysis(es) required for each
sample and provide further evidence that the chain-of-custody is
complete; and
• Laboratory logbook which is maintained at the laboratory and records
all pertinent information about the sample.
Chapter Four of TEGD (U.S. EPA, 1986) should be consulted for sample
collection techniques as well as for analytical procedures, field and
laboratory QA/QC requirements, and suggestions for reporting of ground water
data. The owner/operator may also find the following publication useful for
sampling information:
Practical Guide for Ground Water Sampling, EPA/600/2-85/104,
September 1985.
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EXHIBIT 9-1. RFI CHECKLIST - GROUND WATER
Site Name/Location
Type of Unit
I. Does waste characterization include the following information (Y/N)
• Constituents of concern __
• Concentrations of constituents
• Physical form of waste
• Chemical properties of waste (organic, inorganic,
acid, base)
• pH
• pka
• Viscosity
• Water solubility
* Density
* kow
• Henry's Law Constant
2. Have the following information requirements to characterize
a release to ground water been gathered (Y/N);
Site Soil Characteristics
• Grain size distribution and gradation
• Hydraulic Conductivity
• Porosity
• Discontinuities in soil strata (e.g., faults)
• Degree and orientation of subsurface stratification
and bedding
(continued)
9-65
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EXHIBIT 9-1 (continued)
Ground Water Flow System Characterization
• Use of aquifer
• Depth to water table
• Direction of flow
• Rate of flow
• Hydraulic conductivity
0 Storativity/specific yield (effective porosity)
• Aquifer type (confined or unconfined)
• Aquifer characteristics (e.g., homogeneous,
isotropic, leaky)
• Hydraulic gradient
• Identification of recharge and discharge areas
• Identification of aquifer boundaries (i.e.,
areal extent)
• Aquitard characteristics (depth, permeability
degree of jointing, continuity)
Ground Water Quality Characteristics
• Presence of minerals and organics
• RCRA Parts 264 and 265 - Detection monitoring
requirements
• Appendix VIII constituents
3. Has the following information on the release to ground water
been ascertained (Y/N):
Extent
Location
Shape
Hydraulic gradient across plume
Depth to plume
Chemistry and concentration
Velocity
Potential receptors
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REFERENCES
ASTM. 1984. Annual Book of ASTH Standards, Volume 4.08: Natural Building
Stones; Soil and Rock, American Society for Testing and Materials,
Philadelphia, PA.
Balch, A. H., and W. W. Lee. 1984. Vertical Seismic Profiling Technique,
Applications and Case Histories: International Human Resource
Development Corp.
Billings. 1972. Structural Geology. 3rd Edition. Prentice-Hall, Inc.,
Englewood Cliffs, N.J.
Brady. 1974. The Nature and Properties of Soils. 8th Edition. MacMillan
Publishing Co., Inc., New York, N.Y.
Callahan, et al., 1979. Water-Related Environmental Fate of 129 Priority
Pollutants. EPA-440/4-79-029.
Cedergren. 1977. Seepage, Drainage, and Flow Nets, 2nd Edition, John Wiley &
" Sons, New York, N.Y.
Freeze and Cherry. 1979. Ground water. Prentice-Hall, Inc., Englewood
Cliffs, N.J.
Linsley, R.K., M.A. Kohler, and J. Paulhus. -1982. Hydrology for Engineers,
Third Edition. McGraw-Hill, Inc., New York, NY
McWhorter and Sunada. 1977. Ground Water Hydrology and Hydraulics, Water
Resources Publications, Litteton, Colorado.
Snoeyink and Jenkins. 1980. Water Chemistry, John Wiley & Sons,
New York, N.Y.
Sowers, G. F. 1981. Rock Permeability or Hydraulic Conductivity - An
Overview in Permeability and Ground Water Transport, T. F. Zimmic and
C. 0. Riggs/Eds., ASTM Special Technical Publication 746.
Technos, Inc. 1982. Geophysical Techniques for Sensing Buried Wastes and
Waste Migration, for U.S. EPA, Environmental Monitoring Systems
Laboratory.
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U.S. Department of Agriculture. 1975. Soil Taxonomy: A Basic System of
Soil Classification for Making and Interpreting Soil Surveys. Soil
Survey Staff, Soil Conservation Service, Washington, D.C.
U.S. Department of the Army. 1979. Geophysical Explorations, Corps, of
Engineers, Engineering Manual 1110-1-1802, 31 May 1979.
U.S. EPA. 1985. Characterization of Hazardous Waste Sites - A Methods
Manual, Volume I - Site Investigations, EPA-600/4-84/075, by GCA
Corporation for Office of Research and Development.
U.S. EPA. 1984. Characterization of Hazardous Waste Sites - A Methods
Manual: Volume II: Available Sampling Methods, 2nd Edition,
EPA-600/4-84-076, by GCA Corporation for Office of Research and
Development.
U.S. SPA. 1985. Ground Water Flow Net/Flow Line Technical Resource
Document (TRD), Final Report (Revised).
U.S. EPA. 1985. Guidance on Remedial Investigations Under CERCLA, Hazardous
Waste Engineering Research Laboratory, Office of Research and Development,
U.S. EPA. 1982. Handbook for Remedial Action at Waste Disposal Sites,
EPA-625/6-82-006.
U.S. EPA. 1984. Permit Applicant's Guidance Manual for Hazardous Waste -
Land Treatment, Storage, and Disposal Facilities.
U.S. EPA. 1983. Permit Writer's Guidance Manual for Subpart F, Ground-Water
Protection.
U.S. EPA. 1985. Permit Writers' Guidance Manual for the Location of
Hazardous Waste Land Storage and Disposal Facilities - Phase II: Method
for Evaluating the Vulnerability of Ground Water, Draft, Office of Solid
Waste.
U.S. EPA. 1985. Practical Guide for Ground Water Sampling, EPA-600/2-85/104.
U.S. EPA. 1985. RCRA Ground Water Monitoring Compliance Order Guidance
(Final).
U.S. EPA. 1986. RCRA Ground Water Technical Enforcement Guidance Document
(Draft).
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SECTION 10
SUBSURFACE GAS
10.1 OVERVIEW
The objective of an investigation of a subsurface gas release is to
characterize the nature, extent, and rate of migration of a release'of gaseous
hazardous waste or constituents to the soil. This section provides:
• a recommended strategy for characterizing subsurface gas releases,
which includes characterization of the source and the environmental
setting of the release, and conducting a monitoring program which
will characterize the release itself;
• recommendations for data organization and presentation;
• appropriate field methods which may be used in the investigation; and
* a checklist of information that may be needed for release
characterization.
The exact type and amount of information required for sufficient release
characterization will be site-specific and should be determined through
interactions between the regulatory agency and the facility owner/operator
during the RFI process. This guidance does not define the specific data
required in all instances; however, it identifies possible information
necessary to perform release characterizations and methods for obtaining this
information. The RFI Checklist, presented at the end of this section,
provides a tool for planning and tracking information for release
characterization. This list is not meant to serve as a list of requirements
for all subsurface gas releases to soil. Some releases will involve the
collection of only a subset of the items listed.
10-1
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10.2 APPROACH FOR CHARACTERIZING SUBSURFACE GAS RELEASES TO SOIL
Characterization of subsurface gas releases can be accomplished through a
phased monitoring approach. An initial phase can confirm or refute suspected
releases and/or begin initial characterization of a release. If a release has
already been confirmed, then the initial monitoring phase may be by-passed and
the owner/operator should conduct a subsequent monitoring phase(s) to
adequately characterize the release.
A recommended strategy for characterizing subsurface gas releases is
shown in Table 10-1. The strategy allows for flexibility based on site or
unit specific conditions, and some of the steps indicated may be omitted if
deemed unnecessary.
Characterization of a subsurface gas release can involve a number of
tasks to be completed throughout the course of the investigation. These tasks
are listed in Table 10-2 with associated techniques and data outputs.
10.3 CHARACTERIZATION OF THE CONTAMINANT SOURCE AND THE ENVIRONMENTAL SETTING
Subsurface gas generation depends primarily on waste type and the type of
unit. The type of waste managed by the unit will determine the conditions:
under .which the gas can be generated, and the type of unit and characteristics
of the surrounding environment (e.g., soil type) establishes potential
migration pathways. Units which may be of particular concern for subsurface
gas releases include:
t
• Below grade landfills with unsaturated soils adjacent to some parts
of the unit;
• Units closed as landfills (e.g., surface impoundments); and
• Underground tanks.
These types of units may have waste deposited or stored at such depths as
to allow for subsurface gas generation by volatilization or decomposition of
organic wastes and subsequent migration (see Figures 10-1 and 10-2).
10-2
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TABLE 10-1. RECOMMENDED STRATEGY FOR CHARACTERIZING SUBSURFACE GAS RELEASES
Gather Information for Use in Designing a Monitoring Program
- Waste characteristics.
- Gas generation mechanisms..
* Unit design and operational factors.
Environmental factors.
Design a Monitoring Program to Characterize the Nature, Extent, and Magnitude
of the Releases of Concern
Conduct Initial Monitoring Phase
Monitor ambient air and shallow boreholes around the site using
portable survey instruments to detect methane.
Use subsurface gas migration model to estimate plume dimensions.
Use results of above two steps to determine sampling locations and
depths; conduct limited well installation program. Monitor well gas
and shallow soil boreholes for methane. Monitoring at well sites
and shallow soil boreholes should be repeated several times.
- Monitor surrounding buildings and engineered conduits for methane.
Conduct Subsequent Monitoring Phases (as necessary) to Adequately Characterize
the Release
- Perform expanded monitoring of area for methane. Monitor for other
specific volatile organic compounds, if suspected to be present
(e.g., vinyl chloride, benzene, methylene chloride, and other
potentially harmful constituents). May sample shallow soil borings
but probably will require installation of additional gas monitoring
wells.
- Further monitoring of specific contaminants in surrounding
buildings, if warranted.
10-3
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The nature and extent of contamination are affected by environmental
processes such as dispersion, diffusion, and degradation, acting after the
release occurred. Factors which should be considered include soil physical
and chemical properties, subsurface geology and hydrology, and climatic or
meteorologic patterns.
The principal components of "landfill gas" are generally methane and
carbon dioxide, produced by the anaerobic decomposition of organic materials
in wastes. Methane is of particular concern due to its explosive/flammable
properties, although other hazardous gases of concern could be present. The
presence of these other gases in a unit is dependent upon the types of wastes
managed, the volatilities of the waste constituents, temperature, and possible
chemical interactions amongst wastes.
10.3.1 Waste and Unit Characterization
10.3.1.1 Waste Characteristics—
Subsurface gas generation occurs by biological, chemical, and physical
decomposition of disposed or stored wastes. Waste characteristics can affect
the rate of decomposition. The owner/operator should review unit-specific
lists (waste receipts, waste composition surveys, and any other records of
wastes managed) to determine waste type, quantities, location, dates of
disposal, waste moisture content, organic content, etc.
Three decomposition processes are known to occur in the production of
subsurface gases: biological decomposition, chemical decomposition, and
physical decomposition.
10.3.1.1.1 Biological Decomposition—The extent of biological decomposition
and subsequent gas generation from a given waste is related to the type of
unit. Biological decomposition is significant in most landfills and units
closed as landfills containing organic wastes due to anaerobic microbial
degradation. Generally, the amount of gas generated in a landfill is directly
related to the amount of organic matter present.
Organic wastes such as food, sewage sludges, and garden wastes decompose
rapidly, resulting in gas generation shortly after burial with high initial
yields. Much slower decomposing organic wastes include paper, cardboard,
10-8
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wood, leather, some textiles and several other organic components. Inorganic
and inert materials such as plastics, man-made textiles, glass, ceramics,
metals, ash, and rock do not contribute to biological gas production. At •
units closed as landfills, waste types that undergo biological decomposition
might include bulk organic wastes, food processing sludges, treatment plant
sludges, and composting waste.
Waste characteristics can increase or decrease the rate of biological
decomposition. Factors that enhance anaerobic decomposition include: high
moisture content, adequate buffer capacity and neutral pH, sufficient
nutrients (nitrogen and phosphorus), and moderate temperatures.
Characteristics that generally decrease biological decomposition include: the
presence of acidic or basic pH, sulfur, soluble metals and other microbial
toxicants. The owner/operator should review the waste characteristic
information to document if biological decomposition and subsequent gas
generation is occurring.
Under'anaerobic conditions, organic wastes are primarily converted, by
microbial action, into'carbon dioxide and methane. ' Trace amounts of hydrogen,
ammonia, aromatic hydrocarbons, halogenated organics, and hydrogen sulfide may
also be present. With regard to subsurface migration, the gases of concern
are methane (because of its explosive properties) and other volatile organics
that may be present in amounts hazardous to human health or the environment.
10.3.1.1:2 Chemical Decomposition—Gas production by chemical reaction'can
result from the disposal or storage of incompatible wastes. Reactive or
ignitable wastes can produce explosive or heat-producing reactions, resulting
in rapid production of gases, increased pressures, and increased
temperatures. In a landfill, under acidic conditions, a strong oxidizing
agent can react with organic wastes to produce carbon dioxide and ammonia
which can migrate from the unit, possibly providing a transport mechanism for
other gaseous components.
Under typical conditions, gas production from chemical reactions is not
expected to occur at landfills or units closed as landfills. However,
volatile liquids stored in underground tanks may have a significant potential
to create a release by chemical reaction. Good waste management practices,
-------
particularly the proper design and operation (e.g., pressure-relief valves and
leak detection systems) of underground tanks can minimize the potential for
gas release.
10.3.1.1.3 Physical Decomposition—Physical decomposition phenomena include
volatilization and combustion. Volatilization can result in subsurface gas
generation in underground tanks if there is a leak or puncture. The greater a
compound's vapor pressure, the greater will be its potential to volatilize.
Maintenance of underground tanks (e.g., pressure-relief valves and leak
detection systems) can minimize volatilization.
Combustion processes (e.g., underground fires) sometimes occur at active
landfills and can result in subsurface gas release. In a landfill, combustion
can convert wastes to byproducts such as carbon dioxide, carbon monoxide, and
trace toxic components. The owner/operator should review records of
subsurface fires and also review waste records to determine if combustion has
occurred and when. At landfills, combustion processes can accelerate chemical
reaction rates and biological decomposition, creating greater potential for
subsurface gas generation and subsequent release.
10.3.1.2 Unit Design and Operation--
Unit design (waste depth, fill configuration, and cover materials) also
affects gas generation. Generally, the amount of gas generated increases with
landfill volume and often with landfill depth. Deeper landfills have a
proportionally larger anaerobic zone, greater insulation and compaction, and
are more likely to confine gas production. Deeper landfills, such as trench
fills or canyon fills, can trap gases along confining sidewalls and bottom
bedrock or ground water. Daily, interim, and final cover soils can confine
gases within the landfill. This is particularly true for low permeability
.cover soils (e.g., clays) which impede vertical gas migration. Conversely,
mound or shallow landfills have large surface areas through which gases can
more easily vent.
Unit operations, such as methods and procedures used to segregate and
isolate inert wastes, to prevent moisture infiltration, to compact and
increase the density of the waste, and to minimize or prevent mixing of waste
10-10
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types can affect resultant releases of subsurface gases. Daily covering of
the unit may inhibit decomposition and thus gas generation and subsequent
migration.
Certain units have a high potential for allowing the movement of
subsurface gas. These units are those that receive and/or store large volumes
of decomposable wastes, volatile organic liquids, 'or highly reactive
materials. Subsurface gas migration may occur especially when major portions
of a land-based unit is below grade. Gas generated by these units can migrate
vertically and laterally from the unit, following the path of least resistance.
Some units are operated above grade or in relatively shallow soils
(e.g., surface impoundments, land treatment units). The potential for
subsurface gas migration from such units is usually low. Gases generated by
such units will generally be vented to the atmosphere unless prevented by a
natural barrier (e.g., frozen ground) or an engineered barrier (e.g., soil
cover).
Generally, the units that pose the greatest potential for subsurface gas
migration include: landfills, sites closed as landfills, and underground
storage tanks.
10.3.1.2.1 Landfills—Gas generated in landfills "can vent vertically to the
atmosphere and/or migrate horizontally through permeable soil, as shown in
Figure 10-1. Closure of the landfill or periodic covering of cells or lifts
with impermeable caps will impede the vertical movement of the gases, forcing
them to migrate laterally from the unit. Gas migration laterally, through the
subsurface (e.g., through underground utility line channels or sand lenses)
may accumulate in structures on or off the facility property.
10.3.1.2.2 Units Closed as Landfills—Gas generation and subsequent migration
are likely to occur at units closed as landfills containing wastes previously
discussed. Although surface impoundments and waste piles may be closed as
landfills they tend to generally produce less gas than landfills because they
generally contain smaller quantities of decomposable and volatile wastes and
are at shallow depths. Closure of such units covered with an impermeable
cover will increase the potential for lateral gas movement' and accumulation in
onsite and offsite structures (see Figure 10-2).
10-11
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10.3.1.2.3 Underground Tanks—Subsurface gas release and subsequent migration
may occur only if an underground tank is leaking. Underground tanks frequently
contain volatile liquids that could enter the unsaturated zone should a leak
occur (see Figure 10-2).
10.3.2 Characterization of the Environmental Setting
Subsurface conditions at the site to be monitored must be evaluated to
determine likely gas migration routes. Due to the inherent mobility of gases,
special attention must be paid to zones of high permeability created by
man-made, biological, and physical weathering action. These zones include
backfill around pipes, animal burrows, as well as solution channels, sand
and/or gravel lenses, desiccation cracks, and jointing in bedrock.
Natural and engineered barriers can also affect gas migration, generally
by inhibiting migration pathways as discussed below. Natural barriers to gas
migration include surface water, ground water, and geologic formations.
Engineered barriers include walls, onsite structures, underground structures,
caps, liners, and other design features. On the other hand, preferred
pathways for subsurface gas migration may result from previous underground
construction (e.g., underground utility lines) that can facilitate gas flow.
10.3.2.1 Natural and Engineered Barriers—
10.3.2.1.1 Natural Barriers'—Surface water, ground water, and saturated soils
can slow down or control the direction of subsurface gas migration. Gases
encountering these barriers will follow the pathway of least resistance,
usually through unsaturated porous soil.
Geologic barriers can also impede or control the route of subsurface gas
migration. Soil type is an important factor in gas migration. For example,
gravels and sands allow gas to migrate readily, particularly sand/gravel
lenses, while clayey gravels and sandy and organic clays tend to impede gas
movement. Underground utility trenches, backfilled with granular materials;
filled-in mine shafts; and tunnels or natural caverns can serve to channel
subsurface flow. Climatic conditions such as precipitation or
10-12
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freezing can reduce the porosity of surface soils, thereby impeding upward gas
movement. The location of natural barriers should be used in determining
monitoring locations.
Information regarding characterization of soils is provided in Section 8
(Soils).
10.3.2.1.2 Engineered Barriers—Landfills and units closed as landfills may
use caps and liners to prevent moisture infiltration and leachate percolation
to ground water. Caps can contribute to horizontal gas movement when upward
migration to the surface is restricted (as shown in Figure 1U-1). Liners tend
to impede lateral migration into the surrounding unsaturated soils. The
owner/operator should evaluate cap/liner systems (type, age, location, etc.)
to determine if gas migration could occur. Similar to liners, slurry walls
are used to border landfill units and* can retard lateral gas movement. With
respect to underground tanks, caps and liners are.not typically used. These
tanks are often placed into soils with sand or gravel backfill during
installation, followed by paving on the surface. Thus, any escaping gases
from a leaking underground tank may migrate laterally along the path of least
resistance adjacent to the units. The owner/operator should evaluate tank
construction, age, integrity, and location.
10.3.3 Sources of Existing Information
Much of 'the existing information to be reviewed at the initial stages of
the RFI should be provided by the RFA. The RFA addresses:
». unit characteristics, • "
• waste characteristics,
• gas migration pathways,
» • evidence of releases, and
• potential receptors.
Based on a review of RFA information the investigator should be able to
identify additional data needs and begin to develop the monitoring plan.
10-13
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Existing data should indicate which.units will generate methane or other
gases of concern (e.g., vinyl chloride, benzene, methylene chloride) to cause
a subsurface gas release. Such information can be found in construction or
design documents, permit and inspection reports, records of waste disposal,
unit design and operation records, and documentation of accidents, spills, and
releases.
Units of concern should be identified on the facility's topographic map.
The location and areal extent of these units can be determined from historical
records, aerial photographs, or field surveys. The depths and dimensions of
underground structures, locations of surrounding buildings, and waste-related
information should be identified. Waste management records may provide
information on.waste types, quantities managed, location of waste units, and
dates of waste disposal. Haste receipts, waste composition surveys, and
records of waste types (e.g., municipal refuse, bulk liquids, sludges,
contaminated soils, industrial process wastes or inert materials) should be
reviewed. For underground tanks, liquid waste compositions, quantities, and
physical properties should be determined.
Review of unit design and operation records may provide background
information on units of concern. These records may include: engineering
design plans, inspection records, operation logs, damage or nuisance
litigation, and routine monitoring data. Also, for landfills and units closed
as landfills, data may include the presence and thickness of a liner, ground-
water elevations, waste moisture contents, type and amount of .daily cover,
records of subsurface fires, and in-place leachate and/or gas collection
systems. Historical information on underground tank integrity may be
contained in construction and monitoring records. Records of accidents,
spills, and releases may provide information on problems, corrective measures,
and controls initiated.
The owner/operator should review records of subsurface conditions to
determine potential migration pathways. Aerial photographs or field
observations should identify surface water locations. Infrared aerial
photography or geological surveys from the USGS can be used as preliminary
aids to identify subsurface geologic features and ground-water location. In
addition to obtaining and reviewing existing information, a field
investigation may be necessary to confirm the location of natural barriers.
The local soil conservation service will often have information describing
10-14
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soil characteristics (soil type, permeability, particle size, etc.) or a site
specific investigation may need to be conducted. The U.S. Weather Bureau may
have weather statistics (temperature, rainfall, snowfall) for the facility
area. Historical records of the site (prior use, construction, etc.) should
also be reviewed to identify any factors affecting gas migration routes. The
monitoring program should also address any engineered structures affecting the
migration pathway.
«
In addition to the above, the owner/operator should examine the units and
surrounding area for signs of settlement, erosion, cracking of covers,
stressed or dead vegetation, contamination of surface waters, odors, elevated
temperatures in any existing monitoring wells, and for venting of smoke or
gases. The condition of any existing gas monitoring systems and containment
or collection systems should also be examined, as well as any structural
defects in tanks or liners. Any overflow/alarm shut off systems, subsurface
leak detection systems, secondary containment structures (e.g., concrete pads,
dikes or curbs) or other safety systems for early detection of potential gas
releases should be checked.
By reviewing all existing information, the owner/operator should be able
to identify any additional information needed to design a monitoring program.
10.4 DESIGN OF A MONITORING PROGRAM
10.4.1 Objectivesof the Monitoring Program
Characterization of subsurface gas releases can be accomplished through a
phased monitoring approach. The objective of initial monitoring is to begin
characterizing known or likely subsurface gas'releases. If initial monitoring
confirms a contaminant release, the owner/operator should expand the
monitoring program to determine the vertical and horizontal extent of the
release, as well as the concentrations of all constituents of concern in the
release.
Monitoring techniques used in subsequent monitoring phases should be
similar to those used in the initial phase. The full extent of the release
can be determined through additional shallow borehole and gas monitoring well
locations. The goal of this further testing will be to identify the boundary
of gas migration including the leading edge of the migration.
10-15
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A great deal of the effort conducted under subsequent phase(s) will
involve investigating anomalous areas where subsurface conditions are
non-uniform and thus the gas migration characteristics differ from surrounding
areas. Consequently, non-random sampling techniques will be utilized to
isolate these areas. The location of additional gas wells and shallow
boreholes at the sites of subsurface anomalies will provide answers to
questions regarding the migration pattern around these anomalous areas. Also,
because gas well installation was conducted only to'a limited extent under the
initial monitoring phase, additional wells may need to be installed along the
perimeter of the unit.
1 The monitoring program should address the following:
• The selection of constituents of concern for monitoring;
• Sampling frequency and duration; and
• Monitoring system design.
10.4.2 Monitoring Constituents
Methane should be used as the primary indicator a of subsurface gas
migration during the initial and any subsequent monitoring phases.. Other
hazardous gases that are suspected to be present should be monitored, if
warranted, in subsequent phases.
Constituents that have been commonly detected in subsurface gas are
listed in Appendix B. Constituents to be included for monitoring depend on
the type of wastes received, site-specific data on- construction, operations,
monitoring, spills, and inspections performed at the site. For example, if an
underground storage tank contains acetaldehyde, it should be.considered during
subsurface gas monitoring activities. The investigator should monitor
existing and new shallow boreholes and gas wells for the constituents of
concern. It is suggested that the investigator use-Appendix B in selecting
constituents for subsequent phases of monitoring. These constituents need not
be monitored at every sampling location. A subset of the sampling locations
can be tested for these constituents provided that sufficient gas
characterization is performed for all zones of varying gas composition and
varying horizontal and vertical sampling locations.
10-16
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10.4.3 Monitoring Schedule
A schedule for sampling should be established and described in the RFI
plan; This schedule should describe:
• Sampling frequency;
• Duration of the sampling effort; and
• Conditions under .which sampling should occur. •
During the- initial monitoring phases, bar punch probe monitoring for
methane should be conducted at least twice over the course of one week.
Monitoring the wells for methane should be conducted at least once a week for
one. month. Surrounding buildings should be monitored for methane at least
once a week for one month.
During any subsequent monitoring phases, more -extensive sampling will be
needed to adequately characterize the nature and extent of the release.
Monitoring of wells and buildings -for methane and, if warranted, other gaseous
constituents of concern should be conducted every other day for a two week
period to account for daily fluctuations.in gas concentrations.
Conditions for sampling should also be defined. Sampling .should
generally not be performed if conditions conducive to decreasing gas
concentrations are present (e.g., frozen ground, high atmospheric pressures,
extremely low temperatures).
In these cases, sampling should be delayed until such conditions pass.
10.4.4 Monitoring Locations
10.4.4.1 Shallow Borehole Monitoring—
It is recommended that the areas identified by the RFA and any subsequent
onsite surveys be investigated for volatile organic gas concentrations during
the initial monitoring phase. Shallow borehole monitoring using a bar punch
probe method or equivalent is recommended. Details of this method are
discussed in Appendix C. The bar punch is simply a steel or metal bar which
is hand-driven or hammered to depths of 3 feet. Once this hole is made it is
covered with a stopper or seal to confine the headspace in the hole. The hole
10-17
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should be allowed to equilibrate for up to an hour prior to sampling to
provide sufficient time for subsurface gas to replace the air in the hole.
The ease of installation of bar punch holes and the ability to obtain
real-time direct measurements from field survey instruments combine to make
this task a relatively simple operation. It should be recognized that shallow
borehole monitoring is a rapid screening method and therefore has its
limitations. A major limitation is that negative findings cannot assure the
absence of a release at a greater depth.
The number of locations to monitor will vary from site to site. However,
due to the ease of this operation, it is recommended that many locations be
surveyed during the initial monitoring phase. Selection of locations along
the perimeter of the unit of concern at intervals of 100 feet is an initial
approach. Individual site conditions and anomalies should be considered to
determine whether the number of sampling locations should be increased or
decreased. A large site with homogeneous subsurface conditions could require
a lesser number of sampling locations by increasing the distance between
sampling points. A site with many subsurface anomalies, such as engineered
barriers or varying soil strata, would require a greater number of sampling
locations. In general, sampling locations should be established where
conditions are conducive to gas migration, such as in sands, gravels and
permeable soils, and near engineered conduits (e.g., underground utility
lines). The appropriate precautions should be taken when sampling near
engineered conducts so as not to damage such property and to assure the safety
of the investigative team and others.
The distance from the unit at which to sample can best be determined
through consideration of site-specific characteristics (e.g., soil conditions)
and can be aided by the use of the gas concentration contour map generated by
the predictive model described in Appendix F. The shallow borehole survey
should be fairly extensive, ranging from sampling locations very near to the
unit to locations at the property boundary and beyond.
10.4.A.2 Gas Monitoring Wells—
Gas monitoring wells should be installed to obtain data on subsurface gas
concentrations at depths greater than the three feet accessible with a bar
punch probe. Wells should be installed to a depth equal to that of the unit.
10-18
-------
Multiple probe depths may be installed at a single location as illustrated in
Figure 10-3 . Where buried material is. fairly shallow (e.g., 10 feet), single
depth gas monitoring probes may be sufficient. When buried material exceeds
this depth below ground, multiple depth probes should be installed in the well.
The location and depth of gas monitoring wells should be based on any
natural highly permeable zones, such as dry sand or gravel, alignment with
offsite structures, proximity of the waste deposit, areas where there is dead
or unhealthy vegetation (that may be due to gas migration), and any engineered
channels which would promote the migration of a subsurface gas release
(e.g., utility lines). This information should be gathered during a review of
subsurface conditions, as discussed previously.
Gas monitoring well installation usually requires the use of a drilling
rig or power auger.. Once a borehole has been drilled to the desired depth,
the gas monitoring probes can be installed as illustrated' in Figure 10-3.
Equilibration times of at least 24 hours should be allowed prior to collection
of subsurface gas samples for analysis. Individual site characteristics or
anomalies which can create significantly different subsurface conditions will
require an increased number of wells to sufficiently determine the presence of
gas migration. For example, if the predominant soil strata along one side of
a unit changes from sandy clay to gravel, a well should be installed in both
of these areas. Also, if the amount of gas producing waste'buried at the site
varies greatly from one area to another, gas monitoring wells should be
installed near each area.
10.4.4.3 Monitoring On Site Buildings—
Monitoring should also be conducted in onsite structures in the areas of
concern, since methane gas migrating through the soil can accumulate in
confined areas. Explosimeters for methane and FID (Flame Ionization Detector)
and FID (Photo-Ionization Detector) survey meters for other vapor phase
organics are the recommended monitoring techniques.
Sampling should be conducted at times when the dilution of the indoor air
is minimized and the concentration of any migrated soil gas is highest. Best
sampling conditions would be to sample after the building has been closed for
the weekend or overnight arid when the soil surface outside the building and
over the unit of concern has been wet or frozen for several days. These
10-19
-------
,|/2"DIA.SCR40
PVC PIPE
i/a" OIA.
PERFORATIONS
FIBERGLASS
SCREENING TO "
BE WRAPPED
AROUND S TAPED
TO TUBE.
MONITORING PROSE DETAIL
1/2" OIA. SCH. 40
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S-r-21 BENTONITE
PLUG
BACKFILL
2 PEA GRAVEL
SOIL BACKFILL
'-2' BENTONiTE
PLUG
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MONITORING
PROBE TYR
2' PEA GRAVEL
SOIL BACKFILL
1 - 2' BENTONITE
PLUG
SOIL BACKFILL
2' PEA GRAVEL
Figure 10-3. Schematic of a deep subsurface gas monitoring well.
10-20
-------
conditions will maximize the potential for lateral migration of gas into
buildings rather than vertically into the ambient air. Recommended sampling
locations within the building include basements, crawl spaces, attics, and
around subsurface utility lines such as sewer or electrical connections.
Access conduits such as manholes or meter boxes are good sampling locations
for water, sewer, or gas main connections.
The threat of explosion from accumulation within a building makes this
monitoring activity important as well. The monitoring of gas levels within
buildings is a simple process involving a walk through inspection of areas
with portable field instruments. Such measurements should begin during the
initial monitoring phase. The importance of identifying potential releases to
onsite buildings warrants a complete inspection of all suspect areas.
If significant concentrations are measured in onsite structures during
initial monitoring, subsequent monitoring may need to be expanded to include
buildings which are outside the site boundaries, with the permission of the
offsite building owner. As with onsite structures, indoor sampling locations
and times should be selected based on conditions favoring maximum gas
concentration in the structures.
10.4..4..4.._.Use.of Predictive Models— . .....
In addition to monitoring potential gas releases using portable survey
instruments, the owner/operator should consider the use of predictive models
to estimate the configuration and concentration of gas releases. A subsurface
gas predictive model has been developed by EPA to estimate methane gas
migration from sanitary landfills. This model is based on site conditions,
waste-related data, and other environmental factors.
As part of the initial monitoring phase, the EPA "Subtitle D model" (see
Appendix F) or another acceptable predictive model should be used to estimate
the extent of subsurface gas migration. Results from this model can be used
in determining appropriate monitoring locations. The methane gas migration
model presented in Appendix F yields a methane concentration isopleth map of a
release. It should be recognized that predictive models may not be sensitive
to relevant site conditions. Therefore, model results should be used
cautiously to supplement actual field data.
10-21
-------
10.5 DATA PRESENTATION
Subsurface gas data collected during the RFI should be presented in
formats that clearly define the composition and extent of the release. The
use of tables and graphs is highly recommended. Section 5 provides a detailed
discussion of data presentation methods.
In general, release data should be initially presented in tabular form.
To facilitate interpretation, graphs of concentrations of individual
constituents plotted against distance from the unit should be used to identify
migration pathways and areas of elevated concentrations. Concentration
isopleth maps can also be drawn to identify the direction, depths, and
distances of gas migration, and concentrations of constituents of concern.
Specific examples of these and other data presentation methods are provided in
Section 5.
10.6 FIELD METHODS
Field methods for subsurface gas investigations involve sample collection
and analysis. Sample collection methods are discussed to summarize the
monitoring techniques described above. Since subsurface gas monitoring is
similar to air monitoring, the available methods for the collection and
analysis of subsurface gas samples are presented here only in tabular format
with further discussion in the Air Section of this document. Tables 10-3
through 10-5 summarize various methodologies available to collect and analyze
air samples. These methodologies range from real-time analyzers to the-
collection of organic vapors on sorbents. with subsequent laboratory analysis.
There are three basic monitoring techniques available for sampling
subsurface gas during initial or subsequent monitoring: above ground air
monitoring, shallow borehole monitoring, and gas well monitoring. These are
discussed in Appendix F, and summarized below.
It is important that all monitoring procedures be fully documented.
Information should include: locations and depth(s) of sampling points,
methods used (including sketches and photographs), survey instruments used,
date and time, atmospheric/soil temperature, analytical methods, and .
laboratory used, if any.
10-22
-------
TABLE 10-3. SUMMARY OF SELECTED ONSITE ORGANIC SCREENING METHODOLOGIES
Instrument
or detector
Measurable
parameters
Low
range of
detection
Comments
Century Series 100
or AID Model 550
(survey mode)
Volatile organic Low ppm
species
Uses Flame lonization
Detector (FID)
HNU Model PI-101
Volatile organic
species
Low ppm Photo-ionization (PI)
detector - provides
especially good
sensitivity to low
.molecular weight
aromatic compounds
(i.e., benzene, toluene)
*Does not detect methane
Century Systems
OVA-128 (GC mode)
Volatile organic
species
Low ppm Uses GC column for pos-
sible specific compound
identification and
detection
Photo Vac 10A10
(GC mode)
Volatile organic
species
Below ppm Uses PI detector. Es-
pecially sensitive to
aromatic species. May
be.used for compound
identification if
interferences are not
. present
10-23
-------
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-------
10.6.1 Above Ground Monitoring
This technique consists of the collection of samples of the subsurface
gas after it has migrated out of the soil and into buildings or along under-
ground utility lines. Basically there is no difference in the apparatus from
that described for ambient air monitoring. The locations at which sampling is
conducted, however, are selected to focus on areas where gases might accumu-
late. Sampling methods can utilize any of a number of types and brands of
portable direct-reading survey instruments (see Table 10-5). However, since
methane gas is frequently the major constituent of the soil gas, those which
are most sensitive to methane, such as explosimeters and FID and FID organic
vapor analyzers, are the preferred instruments. More selective air sampling
methods must be used, however, when compound-specific analyses are required.
10.6.2 Shallow Borehole Monitoring
Shallow borehole monitoring involves subsurface gas monitoring to depths
of up to 6 feet below the ground surface. Bar punches or metal rods which can
be hand-driven or hammered into the ground are used to make boreholes from
which gas samples are removed. Table 10-6 provides the basic procedure for
shallow borehole testing. Sample collection should follow the same methods
employed during above ground monitoring.
10.6.3 Gas Well Monitoring
: Monitoring gas wells will involve either the lowering of a sampling probe
(made of a nonsparking material) through a sealed cap on the top of the well
to designated depths or the use of fixed-depth monitoring probes (see
Figure 10-3). The probe outlet is usually connected to the desired gas
monitoring instrument. A more detailed discussion of gas well monitoring is
presented in Appendix F.
10-27
-------
TABLE 10-6. SUBSURFACE SAMPLING TECHNIQUES
SHALLOW (Up to 6 ft deep)
• Select sampling locations based on soil data and existing monitoring
data.
• Penetrate soil to desired depth. A steel rod 1/2 to 3/4 in. dia.
and a heavy hammer are sufficient. A bar punch is better for
numerous holes. It is a small, hand operated pile driver with a
sliding weight on the top. Hand augers may also be used.
• Insert plastic tubing to bottom of hole. Tubing may be weighted or
attached to a small diameter stick to assure that it gets to the
bottom of the hole. Tubing should be perforated along bottom few
inches to assure gas flow.
• Close top of hole around tubing.
• Before sampling record well head pressure.
* Attach meter or sampling pump and evacuate hole of air-diluted gases
before recording gas concentrations or taking samples.
• When using a portable meter, begin with the most sensitive range
(0 - 100 percent LEL for methane). If meter is pegged, change to
the next least sensitive range to determine actual gas concentration.
• Tubing shall be marked, sealed, and protected if sampling will be
done later.
* If at all possible, monitoring should be repeated a day or two after
probe installation to verify readings.
DEEP (More Than 6 ft deep)
• Same general procedures as above.
• Use portable power augers or truck-mounted augers.
• For permanent monitoring points, use rigid PVC tubing and the
general construction techniques shown in Figure 10-3.
CAUTION
When using hand powered equipment, stop if any unusually high
resistance is met - it could be a gas pipe or an electrical caole.
Before using powered equipment, confirm that there are no
underground utilities in the locations selected.
10-28
-------
EXHIBIT 10-1. RFI CHECKLIST - SUBSURFACE GAS
SITE NAME/LOCATION
TYPE OF UNIT
1. Hase the following information about the unit been gathered (Y/N):
• Age of unit .
• Construction integrity
• Presence of liner (natural or synthetic)
• Location relative to ground-water table or
bedrock or other confining barriers
• Unit operation data
• Presence of cover or other surface covering
to impede vertical gas migration
• Presence of gas collection system
• Presence of on/offsite buildings
•. -.Depth and dimensions of unit
• - .—Inspection records - -
* Operation logs
» Past fire, explosion, odor complaint reports
• Existing gas/ground-water monitoring data
• Presence of natural or engineered barriers near unit
• Evidence of vegetative stress
2. ' Does waste characterization include the following information (Y/N):
• Physical form of waste
• Chemical composition and concentrations
(continued)
10-29
-------
EXHIBIT 10-1 (continued)
• Quantities managed and dates of receipt
• Volatility of organic compounds
• Location of wastes in unit
• Viscosity of constituents of concern
• Solubility of constituents of concern
• Waste material moisture content and temperature
Have the characteristics of the soil surrounding the unit been
obtained (Y/N):
• Soil type and depth
• Texture (granular or cohesive)
• Grain size distribution and gradation
• Permeability
• Porosity
» Composition (e.g., organic content)
* Discontinuities in soil strata (e.g., sand lenses)
Have gas monitoring data been collected (Y/N):
• Extent and configuration of gas plume
• Measured methane and other gaseous constituent
concentration levels in subsurface soil and
buildings/structures
Sampling locations and schedule
• Number of wells and shallow soil borings
• Depth of monitoring wells
• Distance of wells from unit
• Frequency of sampling
10-30
-------
REFERENCES
U.S. EPA, RCRA Facility Assessment Guidance, Draft, March 1986. Permits and
State Programs Division, Office of Solid Waste, U.S. EPA, Washington, DC.
U.S. EPA, Guidance Manual for the Classification of Solid Waste Disposal
Facilities, Office of Solid Waste, Washington, D.C. January 1981.
10-31
-------
-------
APPENDIX C
GEOPHYSICAL TECHNIQUES
The methods presented in this Appendix have been drawn primarily from two
sources. The first, Geophysical Techniques for Sensing Buried Wastes and
Waste Migration (Technos, Inc., 1982) was written specifically for application
at hazardous wastes sites, and for an audience with limited technical
background. All of the surface geophysical methods discussed below can be
found in this document.
The second, Geophysical Explorations (Army Corps of Engineers, 1979) is a
more generic application-oriented manual which contains the borehole methods
described.in this section, unless other references are cited.
Care should be taken in the use of geophysical methods involving the
I
introduction or generation of an electrical current in the presence of
contaminants which may exhibit explosive tendencies, particularly borehole
methods.
C.I ELECTROMAGNETIC SURVEYS
The electromagnetic (EM)* method provides a means of measuring the
electrical conductivity of subsurface soil, rock, and ground water.
Electrical conductivity is a function of the type of soil and rock, its
*The term "electromagnetic" has been used in contemporary literature as a
descriptive term for other geophysical methods, including ground penetrating
radar and metal detectors which are based on electromagnetic principles.
However, this document will use electromagnetic (EM) to specifically imply
the measurement of subsurface conductivities by low frequency electromagnetic
induction. This is in keeping with the traditional use of the term in the
geophysical industry from which the EM methods originated.
C-l
-------
porosity, its permeability, and the fluids which fill the pore space. In most
cases the conductivity (specific conductance) of the pore fluids will dominate
the measurement. Accordingly, the EM method is applicable both to assessment
of natural geohydrologic conditions and to mapping of many types of
contaminant plumes. Additionally, trench boundaries, buried wastes and drums,
as well as metallic utility lines can be located with EM techniques.
Natural variations in subsurface conductivity may be caused by changes in
soil moisture content, ground water specific conductance, depth of soil cover
over rock, and thickness of soil and rock layers. Changes in basic soil or
rock types, and structural features such as fractures or voids may also
produce changes in conductivity. Localized deposits of natural organics,
clay, sand, gravel, or salt-rich zones will also affect subsurface
conductivity.
Many contaminants will produce an increase in free ion concentration when
introduced into the soil or ground water systems.. This increase over
background-conductivity enables detection and mapping of contaminated soil and
ground water at hazardous waste sites (HWS), landfills and impoundments.
Large amounts of organic fluids such as diesel fuel can displace the normal
soil moisture, causing a decrease in conductivity which may also be mapped,
although this is not commonly done. The mapping of a plume will usually
define the local flow direction of contaminants. Contaminant migration rates
can be established by comparing measurements taken at different times.
The absolute values of conductivity for geologic materials (and
contaminants) are not.necessarily diagnostic in themselves, but the variations
in conductivity, laterally and with depth, are significant. It is these
variations which enable the investigator to rapidly find anomalous conditions
(see Figure C-l).
At hazardous waste sites (HWS), applications of EM can provide:
• Assessment of natural geohydrologic conditions;
• Locating and mapping of burial trenches and pits containing drums
and/or bulk wastes;
•. Locating and mapping of plume boundaries;
C-2
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Coil
INDUCED
CURRENT
LOOPS
SECONDARY FIELDS
FROM CURRENT LOOPS
SENSED BY
RECEIVER COIL
Figure C-l. Block diagram showing EM principle of operations.
C-3
-------
• Determination of flow direction in both unsaturated and saturated
zones;
* Rate of plume movement by comparing measurements taken at different
times; and
• Locating and mapping of utility pipes and cables which may affect
other geophysical measurements, or whose trench may provide a
permeable pathway for contaminant flow.
Chapter V of Geophysical Techniques for Sensing Buried Wastes and Waste
Migration (herein after referred to as GTSBWWM) (Technos, Inc., 1982) should
be consulted for further detail regarding use, capabilities, and limitations
of electromagnetic surveys.
C.2 SEISMIC REFRACTION SURVEYS
t
Seismic refraction techniques are used to determine the thickness and
depth of geologic layers and the travel time or velocity of seismic waves
within the layers. Seismic refraction methods are often used to map depths to
specific horizons such as bedrock, clay layers, and water table. In addition
to mapping natural features, other secondary applications of the seismic
method include the location and definition of burial pits and trenches at HWS.
Seismic waves transmitted into the subsurface travel at different
velocities in various types o£ soil and rock, and are refracted (or bent) at
the interfaces between layers. This refraction affects their path of travel.
An array of geophones (transducers that respond to the motion of the ground)
on the surface measures the travel time of the seismic waves from the source
to the geophones at a number of spacings. The time required for the wave to
complete this path is measured, permitting a determination to be made of the
number of layers, the thicknesses of the layers and their depths, as well as
the seismic velocity of each layer. The wave velocity in each layer is
directly related to its material properties such as density and hardness.
Figure C-2 depicts the seismic refraction technique.
Seismic refraction can be used to define natural geohydrologic
conditions, including thickness and depth of soil and rock layers, their
composition and physical properties, and depth to bedrock or water table. It
C-4
-------
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can also be used for the detection and location of anomalous features, such as
pits and trenches, and for evaluation of the depth of burial sites or
landfills.
Specific details regarding the use of seismic refraction surveys, and the
capabilities and limitations of this method can be found in Chapter VII of
GTSBWWM.
C.3 RESISTIVITY SURVEY
The resistivity method is used to measure the electrical resistivity of
the geohydrologic section which includes the soil, rock, and ground water.
Accordingly, the method may be used to assess lateral changes and vertical
cross-sections of the natural geohydrologic settings. In addition, it can be
used to evaluate contaminant plumes and locate buried wastes at hazardous
waste sites. Figure C-3 is a graphical representation of the concept of a
resistivity survey.
Applications of the resistivity method at hazardous waste sites include:
• Locating and mapping contaminant plumes;
• Establishing direction and rate of flow of contaminant plumes;
• Defining burial sites by:
locating trenches,
defining trench boundaries, and
- determining the depths of trenches; and
• Defining natural geohydrologic conditions such as:
- depth to water table or to water-bearing horizons; and
depth to bedrock, thickness of soil, etc.
Chapter VI of GTSBWWM discusses methods, use, capabilities, and
limitations of the resistivity method.
C-6
-------
Current
Source
.Current Meter
Surface
Current Flow
Through Earth
Current
Voltage
Figure C-3. Diagram showing basic concept of resistivity
measurement.
C-7
-------
C.4 GROUND PENETRATING RADAR SURVEYS
Ground penetrating radar (GPR)* uses high frequency radio waves to
acquire subsurface information. From a small antenna which is moved slowly
across the surface of the ground, energy is radiated downward into the
subsurface, then reflected back to the receiving antenna, where variations in
the return signal are continuously recorded; this produces a continuous
cross-sectional "picture" or profile of shallow subsurface conditions. These
responses are caused by radar wave reflections from interfaces of materials
having different electrical properties. Such reflections are often associated
with natural geohydrologic conditions such as bedding, cementation, moisture
and clay content, voids, fractures, and intrusions, as well as man-made
objects. The radar method has been used at' numerous HWS to evaluate natural
soil and rock conditions, as well as to detect buried wastes. Figure C-4
depicts the ground penetrating radar method.
Radar responds to changes in soil and rock conditions. An interface
between two soil or rock layers having sufficiently different electrical
properties will show up in the radar profile. Buried pipes and other discrete
objects will also.be detected.
Radar has effectively mapped soil layers, depth of bedrock, buried stream
channels, rock fractures, and cavities in natural settings. Radar
applications to HWS assessments include:
• Evaluation of the natural soil and geologic conditions;
• Location and delineation of buried waste materials, including both
bulk and drummed wastes;
• Location and delineation of contaminant plume areas; and
• Location and mapping of buried utilities (both metallic and
nonmetallic).
*GPR has been called by various names: ground piercing radar, ground probing
radar, and subsurface impulse radar. It is also known as an electromagnetic
method (which in fact it is); however, since there are many other methods
which are also electromagnetic, the term GPR has come into common use today,
and will be used herein. •
C-8
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GRAPHIC RECORDER
ANTENNA
CONTROLLER
5-300 Meter
Cable
Radar
Waveform
O
O
SOIL
TAPE RECORDER
GROUND SURFACE
1 ROCK
Figure C-4. Block diagram of ground penetrating radar system.
Radar waves are reflected from soil/rock interface.
C-9
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In areas where sufficient ground penetration is achieved, the radar
method provides a powerful assessment tool. Of the geophysical methods
discussed in this document, radar offers the highest resolution. Ground
penetrating radar methods are further detailed in Chapter IV of GTSBWWM, as
are this method's capabilities and limitations.
C.5 MAGNETOMETER SURVEYS:
Magnetic measurements are commonly used to map regional geologic
structure and to explore for minerals. They are also used to locate pipes and
survey stakes or to map archeological sites. They are commonly used at HWS to
locate buried drums and trenches.
A magnetometer measures the intensity of the earth's magnetic field. The
presence of ferrous metals creates variations in the local strength of that
field, permitting their detection. A magnetometer*s response is proportional
to the mass of the ferrous target. Typically, a single drum can be detected
at distances up to 6 meters, while massive piles of drums can be detected at
distances up to 20 meters or more. Figure C-5 shows the use of a magnetometer
in detecting a buried drum.
At HWS, magnetometers may be used to:
• Locate buried 55 gallon drums;
• Define boundaries of trenches filled with ferrous containers;
• Locate ferrous underground utilities, such as iron pipes or tanks,
and the permeable pathways often associated with them; and
• Aid in selecting drilling locations that are clear of buried drums,
' underground utilities, and other obstructions.
The use, capabilities, and limitations of magnetometer surveys at
hazardous waste sites are provided in Chapter IX of GTSBWWM.
A. 6 BOREHOLE GEOPHYSICAL METHODS
There are several different types 'of borehole geophysical methods used in
the evaluation'of subsurface lithology, stratigraphy, and structure, teuch of
the data collected in boreholes is analyzed in conjunction with surface
C-10
-------
Amplifiers
and
Counter
Circuits
Chart and
Mag Tape
Recorders
Ground Surface
Figure C-5. Simplified block diagram of a magnetometer. A
magnetometer senses change in the earth's magnetic
field due to buried iron drum.
C-ll
-------
geophysical data to develop a more detailed description of subsurface
features. In this section, the major and most applicable types of borehole
geophysical methods for HWS investigations are identified and briefly
discussed. They include:
I. Electrical Surveys
a. Spontaneous Potential
b. Resistivity
II. Nuclear Surveys
<- a. Natural Gamma
b. Gamma Gamma
. c. Neutron
III. Seismic Surveys
a. Up and Down Hole
b. Crosshole Tests
c. Vertical Seismic Profiling
IV. Sonic Surveys
a. Sonic Borehole Imagery
b. Sonic Velocity
V. Auxiliary Surveys
a. Temperature
b. Caliper
c. Fluid Resistivity
All of the borehole methods presented in this section can be further
referenced in the Army Corps of Engineers Geophysical Explorations Manual
(Engineering Manual 1110-1-1802, May 31, 1979), with the exception of vertical
seismic profiling. This method is relatively new and further information can
be found in Balch and Lee, 1984.
C.6.1 Electrical Surveys
The two types of electrical subsurface surveys of geotechnical interest,
both of which involve continuous logging with depth of the electrical
characteristics of the borehole walls, are the spontaneous potential (SP) log
and the borehole resistivity log.
The SP log (also known as self potential) is a record of the variation
with depth of naturally occurring electrical potentials (voltages) between an
electrode at the depth in a fluid filled borehole and another at the surface.
C-12
-------
The known origins for spontaneous potentials arise from the relative
mobility and concentrations of the different elemental ions dissolved in tne
borehole fluid and the fluid in adjacent strata. The electrochemical
activities of the minerals in the strata also cause a component of the
measured spontaneous potentials (Figure C-6). The relative senses and
magnitudes of the several causes from which spontaneous potentials arise are
affected by the nature of the borehole fluid, by the mineralogical
characteristics of all the strata the borehole penetrates, and by the
dissolved solid concentration in the ground water in all potential layers.
The second type of electric survey is the electrical resistivity log.
The electrical resistivity of strata is one of the basic parameters that
correlates to lithology and hydrology. Direct access to individual layers of
the subsurface materials by means of the borehole is the primary advantage of
electrical resistivity logging over the more indirect use of apparent
electrical resistivity surveys from the surface.
Electrical current can be passed through in situ earth materials between
two electrodes. Electric fields created within the three dimensional (3D)
earth medium are related to the medium's structure and the nature of the
aqueous fluid in the medium. Figure C-7 demonstrates the conceptual field
configuration for borehole electrical resistivity survey.
Resistivity logging is a valuable tool in correlating beds from borehole
to borehole. In addition, they can be used together with knowledge of ground
water and rock matrix resistivities (obtained from samples) to calculate
porosities and/or water saturations. Also, if porosity is known and a
borehole temperature log is available, contaminant concentrations can be
inferred by electrical resistivity variations.
C.6^2 Nuclear Logging
<
Nuclear borehole logging can be used quite effectively in the area of HWS
investigation for borehole depths ranging from 10 to more than I,QUO feet. At
considerable depths, as for large buried structures, nuclear logging is a very
cost effective means of expanding a small number of data points obtained from
direct measurements on core samples to continuous records of clay content,
bulk density, water content, and/or porosity. The logs are among the simplest
C-13
-------
VOLT
METER
E, = BOREHOLE-FORMATION
FLUID POTENTIAL
E( = SHALE-SAND.POTENTIAL
E. * MUO FILTRATE-FORMATION
D FLUID POTENTIAL
-------
POWER
AND
RECORDER
INSULATED
SHEAVE
OOWNHOLE
ELECTRODE
REFERENCE
ELECTRODE
Figure C-7. Single-point resistance log (prepared by WES),
C-15
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to perform and interpret, but the calibrations required for meaningful
quantitative interpretations must be meticulously complete in attention to
detail and consideration of all factors affecting nuclear radiation in earth
materials. Under favorable conditions, nuclear measurements approach the
precision of direct density tests on rock cores. The gamma-gamma density log
and the neutron water content log require the use of isotopic sources of
nuclear radiation, Potential radiation hazards mandate thorough training of
personnel working around these sources. Strict compliance with U.S. NRG
Title 10, Fart 20, as well as local safety regulations, is required.
Additional information on natural gamma, gamma-gamma, and neutron gamma
methods is provided below.
The natural gamma radiation tool is a passive device measuring the amount
of gamma radiation naturally occurring in the strata being logged. The
40
primary sources of radiation are trace amounts of the potassium isotope K
40
and isotopes of uranium and thorium. K is most prevalent, by far,
existing as an average of 0.012 percent by weight of all potassium. Because
potassium is part of the crystal lattices of illites, micas, montmorillanites,
and other clay materials, the engineering gamma log is a mainly qualitative
indication of the clay content of the strata.
The natural gamma log is put to its simplest and most frequently used
applications in qualitative lithologic interpretation (specifically
identification of shale and clay layers) and bed correlations from hole to
hole. Since clay fractions frequently reduce the primary porosity and
permeability of sediments, inferences as to those parameters may sometimes be
possible from the natural gamma log. Environmentally based surveys may
utilize the log for tracing radioactive pollutants. If regulatory
restrictions allow the use of radioactive tracers, the natural gamma log can
be used to locate ground water flow paths. The natural gamma radiation level
is also a correction factor to the gamma-gamma density log.
In the gamma-gamma logging technique, a radioactive source and detector
are used to determine density variations in the borehole. An isotopic source
of gamma radiation can be placed on the gamma radiation tool and shielded so
that direct paths of that radiation from source to detector are blocked. The
source radiation then permeates the space and materials near itself. As the
gamma photons pass through the matter, they are affected t>y several factors
C-16
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among which is "Compton scattering." Part of each photon's energy is lost-to
orbital electrons in the scattering material. The amount of scattering is
proportional to the number of electrons present. Therefore, if the portion of
radiation able to escape through the logged earth materials without being
widely scattered and de-energized is measured, then that is an inverse active
measure of electron density. A schematic representation of the borehole
gamma-gamma tool is shown in Figure C-8.
The neutron water detector logging method is much liKe the gamma-gamma
technique in that it uses a radioactive source and detector. The difference
is that the neutron log measures water content rather than density of the
borehole material. A composite isotopic source of neutron radiation can be
placed on a probe together with a neutron detector. A neutron has about the
same mass and diameter as a hydrogen nucleus and is much lighter and smaller
than any other geochemically common nucleus. Upon collision with a hydrogen
nucleus the neutron loses about half its kinetic energy to the nucleus and so
is slowed down as well as scattered. Collision with one of the larger nuclei
scatters the neutron but does not slow it. After a number of collisions with
hydrogen nuclei, a neutron is slowed, or it is captured by a hydrogen atom and
produces a secondary neutron emission of thermal energy plus a secondary gamma
photon. Detectors can be "tuned" to. be sensitive to the epithermal (slowed)
neutron or to the thermal neutron or to the gamma radiation. One of these
detectors plus the neutron source is then a device capable of measuring the
amount of hydrogen in the vicinity of the tool. In the geologic environment,
hydrogen exists most commonly in water (H»0) and in hydrocarbons. If it can
be safely assumed that hydrocarbons are not present in appreciable amounts,
then the neutron-epithermal neutron, the neutron-thermal neutron, and the
neutron-gamma logs are measures of the amount of water present if the tool is
calibrated in terms of its response to saturated rocks of various porosities.
The neutron log can be used for hole to hole stratigraphic correlation.
Its designed purpose is to measure water quantities in the formation.
Therefore, the gamma-gamma density, the neutron water detector, the natural
gamma, and the caliper logs together form a "suite" of logs that, when taken
together, can produce continuous interpreted values of water content, bulk
density, dry density, void ratio, porosity, and percent of water saturation.
C-17
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RADIATION
SHIELDING
FORMATION
BACKS CATTERED
GAMMA PHOTONS
MEDIUM ENERGY
GAMMA PHOTONS
EMITTED FROM
ISOTOPIC SOURCE
COMPTON
COLLISIONS
WITH
^FORMATION
ELECTRONS
i
Figure O8. Schematic of the borehole gamma-gannna
density tool (prepared by WES).
C-18
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C.6.3 Seismic Surveys
The principles involved in subsurface seismic surveys are the same as
those discussed earlier under surface seismic surveys.
The travel times for F- and S-waves between source and detector are measured,
and wave velocities are determined on the basis of theoretical travel paths.
These calculated wave velocities can then be used to complement and supplement
other geophysical surveys conducted in the area of investigation.
..; Three common types of borehole seismic surveys are discussed in this
section. They include: Uphole and Downhole surveys, Crossfaole Tests, and
Vertical Seismic Surveys. The applications and limitations are discussed for
each of these methods.
In the uphole and downhole seismic survey, a seismic signal travels
between a point in a borehole and a point on the ground near the hole. In an
uphole survey the energy source is in-the borehole, and the detector on the
ground surface; in a downhole survey, their positions are reversed. The raw
data obtained are the travel times for this signal .and distances between the
seismic source and the geophones. A plot of travel time versus depth yields,
from the slope of the curve, the average wave propagation velocities at
various intervals in the borehole. Figure C-9 depicts a downhole seismic
survey technique.
Uphole and downhole surveys are usually performed to complement other
seismic tests and provide redundancy in a geophysical test program. However,
because these surveys force the seismic signals to traverse all of the strata
between the source and detector, they provide a means of-detecting features,
such as a low velocity layer underlying a higher velocity layer or a "blind"
or "hidden" zone (a layer with insufficient thickness and velocity contrast to
be detected by surface refraction).
Crosshole tests are conducted to determine the P- and S-wave velocity of
each earth material or layer within the depth of interest through the
measurement of the arrival time of a seismic signal that has traveled from a
source in one borehole to a detector in another. The crosshole test concept
is shown in Figure C-10.
C-19
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RECORDING'
SYSTEM ^
HAMMER WITH
MICROSWITCH
- .H-GEOPHONE .~-r-
GEOPHYSICAL
TEST HOLE —
..^%-
;±»
J-!
h-r
FILLED WITH FLUID
FOR HYDROPHONES OR
FOR GEOPHONES
GEOPHYSICAL
TEST HOLE—2
ORTHOGONAL SIDE-
WALL CLAMPED,
VELOCITY DETECTORS
4fl
,<£
VERTICAL GEOPHONE V
76.2 MILLIMETRES. t.D.
3.0 INCHES. I.D.
Figure C-9. Downhole survey techniques for P-wave data (after Vikspe).
C-20
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RECORDER
(TIMER)
Y
A,
I SOURCE
RECEIVE*
Figure C-10. Basic crosshole test concept (prepared by WES),
O21
-------
In addition to providing true P- and S-wave velocities as a function of
depth, their companion purpose is to detect seismic anomalies, such as a lower
velocity zone underlying a higher velocity zone or a layer with insufficient
thickness and velocity contrast to be detected by surface refraction seismic
tests.
The vertical seismic profiling (VSF) technique involves the recording of
seismic waves at regular and closely spaced geophones in the borehole. The
surface source can be stationary or it can be moved to evaluate seismic travel
times to borehole geophones, calculate velocities, and determine the nature of
subsurface feature in the vicinity of the borehole.
VSF surveys are different from downhole surveys in that they provide data
on not only direct path seismic signals, but reflected signals as well. By
moving the surface source to discrete distances and azimuths from the
borehole, VSP provides a means of characterizing the nature and configuration
of subsurface interfaces (bedding, ground water table, faults), and anomalous
velocity zones around the borehole.
The interpretation of processed VSP data is used in conjunction with
surface seismic surveys as well as other geophysical surveys in the evaluation
of subsurface lithology, stratigraphy, and structure. VSP survey
interpretations also provide a basis for correlation between boreholes'.
C.6.4 Sonic borehole surveys
In this section, two types of continuous borehole surveys involving high
frequency sound wave propagation are discussed. Sound waves are physically
identical to seismic P-waves. The term sound wave is usually employed when
the frequencies include the audible range and the propagating medium is air to
water. Ultrasonic waves are also physically the same, except that the
frequency range is above the audible range.
The Sonic borehole.imagery log provides a record of the surface
configuration of the cylindrical wall of the borehole. Pulses of high
frequency sound are used in a way similar to marine sonar to probe the wall of
the borehole and, through electronic and photographic means, to create a
visual image representing the surface configuration of the borehole wall. The
physical principle involved is wave reflection from a high impedance surface,
C-22
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the same principle used in reflection seismic surveying and acoustic
subbottom profiling. The sonic borehole imagery logging concept is depicted
in Figure Oil.
The sonic borehole imagery log can be used to detect discontinuities in
competent rock lining the borehole. Varying iithologies, such as shale,
sandstone, and limestone, can sometimes be distinguished on high quality
records by experienced personnel.
Another method of sonic borehole logging is referred to as the continuous
sonic velocity logging technique. The continuous sonic velocity logging
device is used to measure and record the transit time of seismic waves along
the borehole wall between two transducers as it is moved up or down the hole.
A diagram of the continuous sonic velocity logging device is provided in
Figure C-12.
This subsurface logging method provides data on fractures and abrupt
lithology changes along the borehole wall that can be effective in
characterizing the nature of surrounding material as well as borehole
correlation in lithology and structure.
C.6.5 Auxiliary Surveys
An auxiliary survey is the direct measurement of some parameter of the
borehole or its contained fluid to provide information that will either permit
the efficient evaluation of the lithology penetrated by the boring or aid in
the interpretation or reduction of the data from other borehole logging
operations. In most instances, auxiliary logs are made where the property
recorded is essential to the quantitative evaluation of other geophysical
logs. In some instances, however, the auxiliary results can be interpreted
and used directly to infer the existence of certain lithologic or hydrologic
conditions.
Discussed here are three different auxiliary logs; fluid temperature,
caliper, and fluid resistivity, that are especially applicable to the logging
methods discussed in this text. A description of each auxiliary log is
presented below.
C-23
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POWER SUPPLY AND
IMAGING DEVICE
o o o
COMPASS OR
DIRECTION
SENSOR
ULTRASONIC
ACOUSTIC
BEAM
ROTATING
PIEZOELECTRIC
TRANSDUCER
w
RECORD SHOWING
AZIMUTHAL DIRECTIONS
ABOVE "UNWRAPPED"
BOREHOLE IMAGE
WITH IMAGES OP
TWO DIPPING PLANES
Figure C-ll. Sonic imagery logger (prepared by WES),
C-24
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TNANSMITTEK __,
ACOUSTIC I$01*10*
MCIIVE*
PdHLIMC
Figure C-12. Diagram of 3D velocity tool (courtesy of Seismograph
Service Corporation, Birdwell Division).
C-25
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Temperature logs are the continuous records of Che temperature _
—^—-^^—^—— ' t /"""M
encountered at successive elevations in a borehole. The two basic types of ;v •
temperature logs are standard (gradient) and differential. Both types of logs
rely upon a downhole probe, containing one or more temperature sensors
(thermistors) and surface electronics to monitor and record the temperature
changes encountered in a borehole. The standard temperature log is the result
of a single thermistor continuously sensing the thermal gradient of the fluid
in the borehole as the sonde is raised or lowered in the hole. The
differential temperature log depicts the difference in temperature over a
fixed interval of depth in the borehole by employing two thermistors spaced
from one to several feet apart or through use of a single thermistor and an
electronic memory to compare the temperature at one depth with that of a
selected previous depth.
Temperature logs provide useful information in both cased and uncased
borings and are necessary for correct interpretation of other geophysical logs
(particularly resistivity logs). Temperature logs.can also.be used directly
to indicate the source and movement of water into a borehole, to identify
aquifers, to locate zones of potential recharge, to determine areas containing
wastes discharged into the ground, and to detect sources of thermal
pollution. The thermal conductivity and permeability of rock formations*can
be inferred from temperature logs as can be the location of grout behind
casing by the presence of anomalous zones of heat buildup due to the hydration
of the setting cement.
The caliper log is a record of 'the changes in borehole casing or cavity
size as determined by a. highly sensitive borehole measuring device. The
record may be presented in the form of a continuous vertical profile of the
borehole or casing wall, which is obtained with normal or standard caliper
logging systems, or as a horizontal cross section at selected depths, used for
measuring voids or large subsurface openings. There are two basic methods of
obtaining caliper logs. One technique utilizes mechanically activated
measuring arms or bow springs, and the other employs piezoelectric transducers
for sending and receiving a focused acoustic signal. The acoustic method
requires that the hole be filled with water or mud, but the mechanical method
operates equally well in water, mud, or air. Reliable mechanically derived
caliper logs can be obtained in small (2 in.) diameter exploratory borings as
well as large (36 in.) inspection or access calyx-type borings.
026
-------
Caliper or borehole diameter logs represent one of the most useful and
possibly the simplest of all techniques employed in borehole geophysics. They
provide a means for determining inhole conditions and should be obtained in
all borings in which other geophysical logs are contemplated. Borehole
diameter logs provide information on subsurface lithology and rock quality.
Borehole diameter varies with the hardness, fracture frequency, and
cementation of the various beds penetrated. Borehole diameter logs can be
used to accurately identify zones of enlargement (washouts) or constriction
(swelling), or to aid in the structural evaluation of an area by the accurate
location of fractures or solution openings, particularly in borings where core
loss has presented a problem. Caliper logs also are a. means of identifying
the more porous zones in a boring by locating the intervals in which excessive
mud filter cake has built up on the walls of the borehole. One of the major
uses of standard or borehole caliper logs is to provide information by which
other geophysically derived raw data logs can be corrected for borehole
diameter effects. This is particularly true for such nonfocused logs as those
obtained in radiation logging or the quantitative evaluation of flowmeter logs
or tracer and water quality work where inhole diameters must be considered.
Caliper logs also can be useful to evaluate inhole conditions for placement of
water well screens or for the selection of locations of packers for .
permeability testing.
The fluid resistivity log is a continuous graphical record of the
resistivity of the fluid within a borehole. Such records are made by
measuring the voltage drop between two closely spaced electrodes enclosed
within a downhole probe through which a representative sample of the borehole
fluid is channeled. Some systems, rather than recording in units of
resistivity, are designed to provide a log of fluid conductivity. As
conductivity is merely the reciprocal of resistivity, either system can be
used to collect the information on inhole fluid required for the correct
interpretation of other downhole logs. ,
The primary use of fluid resistivity or conductivity logs is to provide
information for the correct interpretation of other borehole logs. The
evaluation of nuclear and most electrical logs requires corrections for
salinity of the inhole fluids, particularly when quantitative parameters are
desired for determining porosity from formation resistivity logs.
C-27
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01
-------
APPENDIX D
LISTS OF GROUND-WATER MONITORING CONSTITUENTS
LISTS A, B, C
-------
-------
LIST A
Constituent
s odium .
calcium
magnes ium
sulfate
chloride
PH
total organic carbon
total organic halogen
total phenols
1,1,1-trichloroethene
lead
cadmium
Chemical Abstract System Number
7440-23-5
7400-70-2
7439-95-4
71-55-6
7439-97-6
7440-43-9
D-l
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LIST 81
INDUSTRY SPECIFIC PARAMETERS
METALFINISHING
Constituent
chromiurn
copper
cyanide
iron
z lac
triehloroethene
tetrachloroethylene
vinyl chloride
phenan threne
nickel
Chemical Abs trac t System Number
7440-47-3
7550-50-8
57-12-5
7439-89-6
7440-66-6
79-01-6
127-18-4
75-01-4
85-01-8
D-2
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LIST B2
IRON AND STEEL
Constituent
arsenic
chromium
cyan ide
tie
zinc
benzene
benzo(a)pyrene
tetrachloroethylene
Chemical Abstract System Number
7440-38-2
.7440-47-3
57-12-5
7440-T31-5
7440-66-6
71-43-2
50-32-8
127-18-4
D-3
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LIST B3
PESTICIDES
Constituent
Chemical Abstract Systen Number
arsenic
cyanide
copper
benzene
carbon tetrachloride
chlordane
chlorobenzene
chloroform
1,4-dichlorobenzene
2,4-dichlorophenol
hep tachlor
hexachlorocyclopentadiene
methyl chloride
methylene chloride
4-ni trophenol
phenol
tetrachloroethylene
tdluene
Manufactured pesticides*
7440«
57-
7550-
71-
'56-
57-
67-
108-
120-
76-
77«
74-
75-
100-
108-
127-
108-
38-2
•12-5
50-8
43-2
23-5
•74-9
66-3
•90-7
46-7
•83-2
•44-8
•47-4
87-3
•09-2
02-7
•95-2
•18-4
•88-3
*Any specific pesticides, residues, off-specification products,
or other similiar items known to have been disposed of at the
site, or, in the case of a dedicated facility, known to .have been
manufactured at the site.
D-4
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LIST C
\
Constituent
bis<2-ethylhexyl)phthalate
PCB-1016
PC8-1221
PCB-1232
PCB-1248
PCB-1254
PCB-1260
PCB-1242
arsenic
benzene
chlorobenzene
ethyl benzene
toluene
chromium
copper
cyanide
tetrachloroethylene
vinyl chloride
erichoroethylene
iron
manganese
nap"h thal'ene
nickel
phenanthrene
phenol
zinc
ChemicaIAbstract System Number
117-81-7
12674-11-2
11104-28-2
11141-16-5
12672-29-6
11097-69-1
11096-82-5
53469-21-9
7440-38-2
71-43-2
108-90-7
100-41-4
108-88-3
7440-47-3
7550-50-8
57-12-5
127-18-4
75-01-4
79-01-6
7439-89-6
7439-96-5
91-20-3
7440-02-0
85-01-8
108-95-2.
7440-6-6
D-5
U.S. Environmental Protection Agency
Library, Room 2404 FM-211-A
401 M Street, S.W.
Bashtoston. DC 20460
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