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  (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

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      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

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               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

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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


                                     9-18

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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
                                     9-19

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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.
                                     9-20

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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.
                                    9-21

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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
                                     9-22

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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
                                     9-23

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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.
                                     9-26

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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

                                     9-27

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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.
                                     9-28

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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.
                                     9-29

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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
                                     9-33

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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.
                                     9-34

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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
                                     9-37

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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.

                                     9-38

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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,
                                     9-39

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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
                                       9-40

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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.
                                     9-42

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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

                                      9-43

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                                            LEGSNO
                                         )   WELL ANO SCREEN
                                         •

                                        10'  SCREEN LENGTH


                                       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
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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.
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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
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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.
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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
  /
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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.
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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).

                                      9-61

-------
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).

                                     9-62

-------
      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.
                                     9-63

-------
     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.
                                     9-64

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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

-------
                          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
                                        9-66

-------
                                  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.
                                     9-67

-------
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).
                                     9-68

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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

-------
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

-------
 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
-------
 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
-------
 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
-------
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
-------
 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
-------
     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
-------
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
-------
     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
-------
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
-------
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
   PVC PIPE
                                                       SOIL  BACXRLL
S-r-21 BENTONITE
   PLUG
       BACKFILL

   2 PEA GRAVEL
   SOIL BACKFILL


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   PLUG

   SOIL BACKFILL
                                 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
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 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
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    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
-------
                                                             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
-------
                                                  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
-------
     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
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                                                               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
-------
                        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
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                 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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