PREPARED FOR:
 OFFICE OF
 UNDERGROUND
 STORAGE TANKS
US. ENVIRONMENTAL
PROTECTION AGENCY
CONTRACT NO: 68-01-6939
               INTERIM REPORT
               FATE AND TRANSPORT
               OF SUBSTANCES LEAKING
               FROM UNDERGROUND
               STORAGE TANKS
               VOLUME I-
               TECHNICAL REPORT

               1986
               COM
               CAMP DRESSER & McKEE INC.

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                           INTERIM REPORT
                  FATE AND TRANSPORT OF SUBSTANCES
               LEAKING FROM UNDERGROUND STORAGE TANKS
                     VOLUME 1 TECHNICAL REPORT
                 U.S. EPA CONTRACT NO:  68-01-6939

                DOCUMENT CONTROL NO: 998-TS6-RT-CDZN-1
                           Prepared For:

                Office of Underground Storage Tanks
                U.S. Environmental  Protection Agency
                         401 M Street, S.W.
                      Washington, D.C.  20460
                            Prepared By:

                      Camp Dresser & McKee Inc.
                          One Center Plaza
                         Boston, MA  02108

                          January 31, 1986
This document has been prepared for the U.S.  Environmental
Protection Agency under Contract No. 68-01-6939.   The material
contained herein is not to be disclosed to,  discussed with, or
made available to any person or persons for  any reason without
prior expressed approval  of a responsible official  of the U.S.
Environmental Protection Agency.

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                             EXECUTIVE SUMMARY
BACKGROUND AND OBJECTIVES

As part of the development of a comprehensive regulatory program for
underground storage tanks (USTs) by the U.S. Environmental  Protection
Agency (Office of Underground Storage Tanks), this work assignment  aims  to
investigate the fate and transport of regulated substances  leaking  from
USTs.

The specific objectives of the work assignment are:

    •  To summarize the current understanding of the physical,  chemical,
       biological, and toxicological  processes relative to  leaks of
       regulated substances stored in underground tanks.
    •  To evaluate existing methodologies  and data pertinent  to fate and
       transport processes with a potential  for applicability to the UST
       program.
    •  To establish priorities for further investigation, data  collection,
       and/or methodological development.

This report presents the results of the first phase  of  this project,
summarizing the current understanding of the mechanisms or  processes that
govern fate and transport of substances leaking from underground tanks.
This information is provided both as background to the  ongoing  program
development and implementation efforts, and  as a means  of directing the
further efforts of this work assignment to those critical areas where
additional study would be most useful  for  current and future  UST program
tasks.

In order to focus the technical  findings of  this first  phase  of the work
assignment, a list of relevant program activities was developed as  a tie to
on-going UST program deliberations.  Where possible,  the technical  findings
have been tied to these activities particulary in Section 7.0 of this
report.  A briefing for UST program staff  is planned  during the review
period of this report, both to present and discuss the  report's technical
findings, and to discuss their applicability to current  policy/regulatory
issues.
                                   -i-

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The report is organized as follows:   Section  1.0 gives  an  overview of  the
project and its objectives.  We present in Section 2.0  an  overview of  the
substances to be regulated under the UST program, specifically the
petroleum and hazardous substances covered by the RCRA  subtitle I.  While
the fate and transport of a chemical in the subsurface  are interwoven,  they
are for ease of presentation, discussed as separate mechanisms in  Sections
3.0 and 4.0.  The transport of a substance can be described by the
mechanisms that govern its movement  in the environment.  The fate  of a
substance includes all of the physical, chemical, and biological changes it
undergoes in the environment.  The physical  and chemical nature of the
substances stored in underground tanks, and particularly the physical,
chemical, and toxicological properties relevant to fate  and transport
mechanisms are then presented in Section 5.0.   The environmental factors
and/or settings which can be used to describe  leaking UST  scenarios  are
discussed in Section 6.0.  Finally,  in Section 7.0, we  summarize the
technical findings of this report in the context of their  applicability to
the development of the UST regulatory program.

REGULATED SUBSTANCES (SECTION 2.0)

Regulated substances under this program include petroleum  and  hazardous
substances.

Among the petroleum substances, those of interest include  the  following
petroleum products:

    •  Power fuels - aviation gasoline, motor  gasolines, diesel fuel oils,
       jet fuels, and gas turbine fuel  oils.
    •  Heating oils - fuel  oils (Nos. 1, 2,  4, 5 and 6).
    •  Solvents - Stoddard solvent,  petroleum  spirits,  petroleum extender
       oils, and aromatic solvents.
    •  Lubricants - automotive and industrial  lubricants.
These substances were selected for detailed study based  on  three criteria:
meets statutory definition of a liquid, is commonly stored  in  underground
tanks, and is widely used in the United States.
                                  -n.-

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Of the 698 hazardous substances regulated by CERCLA and by extension
regulated under the LIST program, 477 have been determined to be likely to
be stored in underground tanks based on available inventories of existing
USTs and on knowledge of substance properties that make underground storage
desirable or necessary.  A list of these chemicals, with their
corresponding CAS registry numbers, is included as Appendix A.  More
research is needed to determine, in time, the likelihood that other
chemicals among the 221 remaining CERCLA-regulated substances are stored  in
underground tanks.

At any rate, by far the greatest number of tanks  storing regulated
substances in the United States contain petroleum products -- specifically,
power fuels and heating oils.  This measure alone suggests that relatively
more attention be focused on the transport and fate of petroleum products
that may leak from USTs, than for other regulated substances.

TRANSPORT MECHANISMS (SECTION 3.0)

Liquid Phase Transport:

The transport (migration) of substances leaking from USTs  depends  on:  the
quantity released, the physical properties of the contaminant,  and  the
structure of the subsurface soils and  rock through  which  the  contaminant  is
moving.

Even though USTs leaks can be characterized as insidious  spills,  they  may
be undetected for long periods of time, and thus, can release substantial
volumes of a substance into the subsurface.  One  reason  the quantity of
released substance is important, is that it often determines  the  method,
and degree of groundwater contamination,  i.e., by direct  contact with  the
groundwater or through solution in percolating rainwater.

The physical properties of the leaked  substances  that are  important  to
transport are:  solubility, specific gravity, viscosity,  surface  tension,
evaporation rate, and vapor density.  The influence  of  these  properties on
transport mechanisms is discussed in detail  in Section  3.0.

                              -iii-

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Section 3.0 also discusses the transport mechanisms through various types
of subsurface media.  Movement of contaminants is discussed for three
characteristic zones of the subsurface:   the unsaturated zone lying below
the ground surface, the capillary zone that is a transition between the
unsaturated and saturated zones, and the saturated zone lying below the
water table.

Transport of a contaminant through the unsaturated zone is  characterized by
vertical flow, driven by gravity, and lateral  spreading associated  with
capillary forces and soil  heterogeneities.

In general, the contaminant plume will take a pear-shaped form as  it moves
through the unsaturated zone.   The plume will  be wider (the pear "fatter")
in a less permeable soil  than  in a more permeable soil.  The shape  will be
irregular in a stratified soil  with varying permeabilities.

The capillary zone has a significant impact on the movement only of
non-aqueous substances that are less dense  than water.   In  such cases,  the
primary direction of movement  is lateral, with flow extending farthest  in
the direction of the slope of  the water table.

The transport of contaminants  in the saturated zone can be  characterized
for three classes of substances as follows: miscible or dissolved
substances, immiscible substances with specific gravity of  less than 1.0,
and immiscible substance with  specific gravity of more  than 1.0.  Dissolved
substances will  enter the groundwater and will move in  the  general
direction of groundwater flow  according  to  the mass transport  laws  of
advection and dispersion.   Immiscible substances that are "lighter" than
water are typically only found  in the shallow part of the saturated zone.
Immiscible substances that are  denser than  water will  move  down through the
saturated zone.
                                  -iv-

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Vapor Phase Transport:

A liquid contaminant leaking from an UST will enter the vapor phase
according to its vapor pressure (the higher the vapor pressure of the
substance, the more likely it is to evaporate).  Once in the vapor phase,
the contaminant will move by advection -- being "blown along" by subsurface
air currents - and by diffusion.  The movement of vapors is predominantly
in the horizontal direction.

Much less research attention has in the past been given to vapor phase
transport than to liquid transport; several  research needs are identified.

FATE MECHANISMS (SECTION 4.0)

Solubility:

Contaminants can go into solution when: rainwater percolates through  the
unsaturated and capillary zones bringing dissolved contaminant with it,
possibly including those that were in a vapor phase, nonaqueous  phase
liquids migrate in the saturated zone  or dissolve from the bottom of an
"oil pancake"  in the capillary zone.

Irrespective of the density of the chemical, its  rate of solution  in
groundwater will  depend on its solubility in water,  area of contact,  and
groundwater flow rate.  In an inverse sense, these parameters determine how
long the nonaqueous phase liquid remains  in  the subsurface.  As  each  of
these factors  increases, the rate of  solution increases.

Vaporization:

Vapors can be  released by contaminants  in soil  or  water.  Vaporization
occurs from open bodies of water,  such  as lakes,  from subsurface soils, and
from dissolved substances in groundwater, in that  order of importance.
                                  -v-

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The tendency of a chemical  of limited solubility to evaporate is described
by Henry's Law constant, which expresses the driving force for transfer of
solute from aqueous to gaseous phase as the quotient of that chemical's
vapor pressure and solubility.  The higher the Henry's Law coefficient  for
a given substance, the greater its- tendency to evaporate.   The constant is
temperature-dependent; evaporation increases with temperature.

Adsorption:

Adsorption of organic chemicals is most significant on organic matter or
clay minerals.  When adsorption is due to organic matter in the aquifer,
the tendency for a chemical  to adsorb is expressed in terms of the  soil
adsorption coefficient, KQC.

Adsorption plays two significant roles in the fate of contaminants:   it can
retard the forward progress of a contaminant plume, and it can act  as a
residual  source of contamination, extending the period of  contamination.

Biotic Processes:

Some contaminants can be degraded by biological  activity;   both liquid  and
gaseous compounds can be biodegraded.  Degradation is most prevalent  in  the
unsaturated zone where conditions are more favorable for microbial
utilization of a contaminant.

There are three modes of biodegradation: aerobic degradation -- the
predominant mode  taking place in the unsaturated zone of  the subsurface
environment and in the saturated zone when dissolved oxygen is available;
anaerobic degradation -- is more important in the degradation of hazardous
substances as compared to petroleum hydrocarbons; and cometabolism  --
contaminants that tend to be resistant to biodegradation can be attacked
more rapidly when found in  the subsurface with other contaminants.
                                 -VI-

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The breakdown of a contaminant in the natural  environment will  depend on
the nature of the chemical, on the types of microorganisms present, and on
environmental factors.  Degradation of contaminant constituents is
microorganism-specific and site-specific.

Abiotic Chemical Transformation Processes:

Two types of chemical processes can occur in the subsurface:
Oxidation/Reduction and Hydrolysis.

The tendency of a substance to donate or accept electrons in  an abiotic
reaction may be given by two parameters: electrode potential  and redox
potential.  Only a relatively narrow portion of the range of  electrode and
redox potentials are available in soils.  Therefore, oxidation/reduction
can only be important when potent oxidizing or reducing agents  are present.
Direct oxidation or reduction by such agents is unlikely to be  a
significant fate of substances leaking from USTs,  except for  the most
reactive compounds.

The importance of hydrolysis from an environmental  fate point of view is
that the resulting product is usually more  susceptible to further attack by
processes of biodegradation, and  the additon of the hydroxyl  group that
makes the chemical more water-soluble.  The kinetics of hydrolysis are
influenced by environmental factors.  The rate of  disappearance of a
chemical is directly proportional to the concentration of the compound.
The hydrolysis half-life is independent  of  the concentration.
PROPERTIES OF REGULATED SUBSTANCES (SECTION  5.0)

The properties of regulated substances  in  USTs  have  been  identified  and
three databases were developed:   1.  properties  of petroleum  products  stored
in underground tanks; 2. properties  of  hydrocarbons  known  to be
constituents of gasoline and fuel  oil;  and 3. the properties of
CERCLA-regulated hazardous substances.
                               -vi i-

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Petroleum products, exept in rare instances,  have a specific gravity less
than 1.0 and will  float on groundwater.  Kinematic viscosity was  related to
transport in the unsaturated zone.  The lower the kinematic viscosity,  the
faster a product will  migrate through soil  (when gravity is the predominant
driving force).  High  vapor pressure products, such as  gasoline and  jet
fuels, have greater tendency to release vapor phase contaminants  than do
substances with low vapor pressures.

Relying on three parameters cited above,  petroleum products were  "grouped"
with respect to movement through the subsurface.  Group I,  or high vapor
pressure - low kinematic viscosity products,  included automotive  and
aviation gasoline, solvents, and No. 0-GT gas turbine fuel  oil.

The hazardous substances were grouped by using a "cause and effect"
approach.  Two-parameter "sortings of the database were used to assess
potential impact.   Three impact scenarios were related  to specific
parameters collected in the database, as  follows:   groundwater
contamination: solubility toxicity data,  human exposure to  toxic  vapors:
vapor pressure-toxicity data, and explosion/fire:  ignitibility-vapor
pressure data.

After assigning ranges which denote high, medium or low values of these
parameters, and after  removing highly reactive chemicals from the analysis,
the sorts were performed resulting in lists of substances considered to
have "similar potential" for UST-related  impact.

The results indicate that 32 chemicals with toxicity data (final  or
proposed RQ) fall  in the high toxicity/high solubility  group and  are
considered to have very high groundwater  impact  potential;  only 5 chemicals
may be considered  to have very high potential  for human health impact due
to inhalation of vapors (high vapor pressure/high  toxicity); and  9
substances are found in the highest ignitibility/high vapor pressure group.

A comparison was made  of hazardous substances  in the database with the
constituents of gasoline, fuel  oil, and petroleum solvents.   By comparing
the hazardous chemical lists resulting from the  sorts to the petroleum
product constituents some valuable insight  is  achieved.

                                -viii-

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The hazardous components of gasoline and fuel  oil  fall  primarily in  the
"mid-range" groups for potential  to impact drinking water aquifers,  the
potential to expose people to toxic vapor; and the potential  for
fire/explosion.  This mid-range finding reinforces what is felt
intuitively: certain hazardous substances have potential  for  greater
adverse .impact than the constituents of gasoline and other petroleum
products.  This does not mean, however, that petroleum  products  do not
present environmental problems when released in the subsurface.   The
hazardous components of gasoline  include benzene,  toluene, and  xylene which
are fairly soluble, toxic, and produce explosive/toxic  vapors.

ENVIRONMENTAL SETTINGS (SECTION 6.0)

In Section 6.0, the environmental  factors which influence the fate and
transport of substances leaking from USTs were identified. Two  types of
environmental  factors were presented:   natural  and non-natural.   The
natural environmental factors (e.g., climate and hydrogeology) determine
how vulnerable a site is to contamination, while the non-natural  factors
(e.g., man-made conditions, population density) exacerbate a  site's
vulnerability to contamination.  As such, non-natural factors can increase
the hazards resulting from leaking contaminants from USTs.

The ultimate significance of these factors with respect to leaking USTs
varies from site to site, and also depends on  the properties  and  quantity
of the leaked contaminant.  For some of the natural  environmental  factors,
such as climatic variables or the use  of groundwater for drinking water
supplies, it may be possible to assess the significant  variations  on a
nationwide geographic basis.  For other environmental factors, particularly
the non-natural factors, it is possible to assess  the influence  of these
factors only on a site-specific basis.  For example, in determining  the
effect of vapor contaminants, a knowledge of the specific
micro-stratigraphy in the area of a leak and the presence of  receptor
structures is very important.  It is also important  to  recognize  the
effects that an environmental factor has on fate and transport mechanisms
can be in similar or conflicting  directions.
                               -IX-

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 Since within  a  regulatory frameworks the development of generic
 environmental settings  is useful  to evaluate  the  potential  consequences of
 leaking  UST incidents,  three  existing methodologies were examined to
 determine  their applicability.   The methodologies  are used  in the
 following: RCRA Risk  and  Cost Analysis Model  (W-E-T), Liner Location Risk
 and  Cost Analysis  Model,  and  DRASTIC and its  revised version UST
 DRASTIC/IMPACT.

 DRASTIC  is a  simple and easy-to-use approach which when properly applied
 may  be a useful  planning  or screening tool.   The  NWWA assessed the
 feasibility of  revising DRASTIC  to petroleum  UST  related problems.  The
 need to  address site-specific factors was also  recognized;  these are
 included in IMPACT.   The  revised  DRASTIC and  IMPACT make up the proposed
 UST  DRASTIC/IMPACT system.  It appears that this  system could be a useful
 tool  for comparative  evaluation  of environmental  settings for hazards posed
 by leaking UST  incident for two  reasons:  1.  the  methodology includes many
 of the important environmental factors determined  here to be relevant to
 leaking  UST occurrences,  and  2.  the scale at which DRASTIC  develops an
.environmental setting is  appropriate for UST  related problems.  Because the
 UST  DRASTIC/IMPACT system appears to be the most  applicable of the
 available  methods  to  evaluate the groundwater pollution potential from a
 leaking  UST site,  some  notes  on  the use of the  system are presented in
 Section  6.0.
 APPLICABILITY TO UST PROGRAM  (SECTION 7.0)

 Section  7.0  offers  preliminary suggestions as to how scientific and
 technical  information — as presented in this report -- can be used in the
 development  and implementation of the UST regulatory program.  Although
 this  report  presents findings.of only the first phase of this work
 assignments, there  are  several ways in which the findings may have
 immediate  and important use.  Some of these are:
                               -x-

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•  Section 2.0 through 6.0 of this  report  assemble,  in  one  document,
   up-to-date information on the relevant  scientific topics.   This
   information can prove timely as  the program begins to  build its
   regulatory framework.  Also, the information can  be  "re-packaged" to
   be distributed to state and local  government agencies, or  industrial
   organizations, that are faced with real-world LIST incidents.

•  The three databases developed under this  work assignment thus  far
   (Volume II) can be of value in an  immediate way,  as  a  readily
   available reference for properties of UST-regulated  substances.
   Beyond the general value of such a reference, the data provided  in
   these computerized information systems  can  help to prioritize
   program efforts.

•  From a global  regulatory perspective (i.e.,  without  focusing on
   individual incidents for which specific chemical  and setting
   information is required), the technical  information  offered in this
   report generally indicates the following:

   -  From the point of view of environmental  settings, the use (or
      potential  use) of the underlying aquifer, the  presence  or absence
      of man-made conduits, and the proximity  to population,  are
      important  features relevant to  UST incidents.  However,  specific
      hydrogeologic details cannot  be easily incorporated in  a
      regulatory program without reference to  specific  and very
      localized  situations.

   -  As far as  the contents of UST-regulated  tanks  are concerned,  it
      may be possible to state certain findings more defensibly in  the
      "negative"  as opposed to the  "positive."   For  example,  one can
      state with  a high degree of certainty that some petroleum
      products are unlikely to cause  problems  because they are largely
      immobile if released in the subsurface environment.
      Unfortunately, issues such as acceptable  levels of toxicity and
      exposure make positive statements about  "very  hazardous"
      chemicals  a more subjective matter.   Also, information  on the
      distribution of tanks containing various  substances may be
      important  to decide whether and where  to  draw  "hazard"  standards.

•  On a site-specific basis, with information  regarding tank  contents
   and localized  hydrogeologic features in  hand, findings of  this
   report can be  used to prioritize actions  in  the areas of compliance
   monitoring, corrective action requirements,  and enforcement.  For
   example, state level  personnel  charged  with  implementing UST
   programs may do the following:

   -  Having received notification  of a leak and some degree  of
      information regarding the site  and the contents of the  tank,
      ranking systems such  as UST DRASTIC/IMPACT can  be useful in
      prioritizing or allocating available  resources.

   -  Use the databases developed herein to  assess the  degree  of hazard
      associated  with an UST incident.  In  particular,  if such
      databases were to be merged with "knowledge-based" systems for
                             -xi-

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   guiding UST incident response activity,  state and  local  level
   personnel  may be better able to make proper use of resources  and
   take actions scaled to the level  of hazard  posed in a  particular
   situation.

Finally, the  technical  and scientific findings offered in this
report indicated generally where and how research activities  may be
useful to the UST program -- for example, to study the movement  of
contaminants  in the unsaturated zone, fate  mechanisms in  general,
and vapor-phase movement.

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                              ACKNOWLEDGEMENTS
This report was prepared by Camp Dresser & McKee Inc. under Contract No.
68-01-6939 with the U.S. Environmental Protection Agency.  It was prepared
under the direction of Ms. Pamela Harris, the USEPA project monitor from
the Office of Underground Storage Tanks.

The project was managed by Dr. Guillermo J. Vicens.  Major portions of the
work were undertaken and reported by Bernadette H.  Kolb, Dr. Myron S.
Rosenberg, Vicki L. Gerwert, Sarah R. Fisher, Walter R. Niessen, Anthony M.
Lo Re, and Dr. Jonathan A. French.  Technical reviews of the report were
performed by: Donald G. Muldoon, Dr. Larry Partridge, Dr. Roger L. Olsen,
Prof. Lynn W. Gelhar (M.I.T.), Peter J.  Riordan (independent consultant),
Dr. Brendan M. Harley, Robert A. Weimar, Patricia Billig, and Thomas F.
Cheyer.  The technical editing of selected sections was performed by John
Bates.

COM acknowledges the assistance of Pamela Harris of USEPA in providing
overall guidance and coordination with other OUST program staff and others
throughout the Agency.
                                   -xm-

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                             TABLE OF CONTENTS
                                  VOLUME I

Section                                                                Page


EXECUTIVE SUMMARY 	 i

ACKNOWLEDGEMENTS 	 xi i1

TABLE OF CONTENTS 	 xiv

LIST OF FIGURES 	 xix

LIST OF TABLES 	 xxiii

1.0  INTRODUCTION 	 1-1

     1.1  Project Background 	 1-1

     1.2  Leaking UST Problems	 1-1

          1.2.1  Groundwater Contamination Incident 	 1-2
          1.2.2  Vapor Incidents 	 1-5

     .1.3  Objectives and Scope of Work 	 1-9

     1.4  Relation to Other Studies 	 1-11

     1.5  Relevant UST .Program Issues	 1-13

     1.6  Organization of Report 	 1-14

2.0  UST PROGRAM REGULATED SUBSTANCES. 	 2-1

     2.1  Overview 	 2-1

     2.2  Regulated Petroleum Substances in USTs 	 2-1

          2.2.1  Overview 	 2-1
          2.2.2  Petroleum Products 	2-2
          2.2.3  Petroleum Additives	 2-4

     2.3  Regulated Hazardous Substances in USTs 	 2-13

          2.3.1  Overview 	 2-13
          2.3.2  Data Sources 	 2-14
          2.3.3  Data Resources  Inc. and Quantum Analytics
                 Analysis of CA List 	 2-16
          2.3.4  Fire Protection Guide on Hazardous Materials 	 2-16
          2.3.5  Other Potential Sources of Data 	 2-20
                              -XIV-

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     2.4  Remaining CERCLA Hazardous Substances 	 2-24

          2.4.1  Likelihood of Storage in Underground Tanks 	 2-24
          2.4.2  Additional Data Gathering and Analysis 	 2-24

     2.5  Summary	 2-25

3.0  TRANSPORT MECHANISMS 	,	 3-1

     3.1  Overview	 3-1

          3.1.1  Factors Affecting Liquid Transport 	 3-1
          3.1.2  Transport Diagrams 	 3-12

     3.2  Darcy's Law 	 3-15

          3.2.1  Groundwater in the Saturated Zone 	 3-15
          3.2.2  Dissolved Contaminants in the Saturated Zone 	 3-18
          3.2.3  Immiscible Fluid Flow 	 3-22

     3.3  Transport of Liquids in the Unsaturated Zone 	 3-23

          3.3.1  Overview 	 3-23
          3.3.2  The Physics of Unsaturated Zone
                 Contaminant Transport 	 3-25
          3.3.3  Use of the Physics As It Applies To Leaking USTs  .... 3-44
          3.3.4  Estimating Relationships 	 3-54

     3.4  Transport of Liquids in the Capillary Zone 	 3-57

          3.4.1  Overview 	 3-57
          3.4.2  Transport of Floating Substances 	 3-58
          3.4.3  Estimating Relationships 	 3-66

     3.5  Transport of Liquids in the Saturated Zone 	;...   3-67

          3.5.1  Overview 	   3-67
          3.5.2  The Physics of Saturated Zone Transport 	 3-69
          3.5.3  Applications 	 3-73

     3.6  Vapor Transport in the Unsaturated Zone 	 3-80

          3.6.1  Overview 	 3-80
          3.6.2  Fundamentals of Transport 	 3-81
          3.6.3  Summary of Vapor Phase Transport 	 3-90

     3.7  Summary of Transport Mechanisms 	 3-90

4.0  FATE MECHANISMS	 4-1

     4.1  Overview 	 4-1
                                -xv-

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     4.2  Solubility	 4-2

          4.2.1  Overview 	 4-2
          4.2.2  Environmental  Factors That Influence Solubility 	 4-4
          4.2.3  Current Understanding 	 4-6

     4.3  Vaporization 	 4-8

          4.3.1  Overview 	 4-8
          4.3.2  Environmental  Factors Affecting Vaporization 	 4-11

     4.4  Adsorption 	 4-12

          4.4.1  Overview 	 4-12
          4.4.2  Environmental  Factors That Influence Adsorption 	 4-18
          4.4.3  Estimating Procedures 	 4-19

     4.5  Biotic Processes 	 4-22

          4.5.1  Overview 	 4-22
          4.5.2  Factors That Influence Degradation 	 4-27
          4.5.3  Types of Degradation 	 4-33
          4.5.4  Impacts of Contaminants 	 4-36

     4.6  Abiotic Chemical Transformation Processes 	 4-47

          4.6.1  Overview 	 4-47
          4.6.2  Hydrolysis 	 4-48
          4.6.3  Oxidation/Reduction 	 4-57

     4.7  Summary of Fate Mechanisms 	 4-59

5.0  PROPERTIES OF REGULATED SUBSTANCES IN USTs 	 5-1

     5.1  Overview 	 5-2

     5.2  Properties of Petroleum Products 	 5-2

          5.2.1  Overview 	 5-2
          5.2.2  Bulk Properties Database 	 5-2
          5.2.3  Grouping by Bulk Properties 	 5-8
          5.2.4  Constituents of Petroleum Products 	 5-14

     5.3  Hazardous Substances  in USTs 	 5-22

          5.3.1  Overview 	 5-22
          5.3.2  Development of The Database 	 5-22
          5.3.3  Uses of The Data Base 	 5-28
          5.3.4  Groups of Hazardous Substances 	 5-37
                                   -xvi-

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     5.4  Comparison of Hazardous Substances and
          Petroleum Products	 5-62

          5.4.1  Gasoline	 5-64
     5.4.2  Fuel  Oil 	      5-65
          5.4.3  Additives and Trace Components 	 5-67
          5.4.4  Petroleum Solvents 	5-68
          5.4.5  Key Findings 	 5-71

     5.5  Summary	 5-72

6.0  ENVIRONMENTAL SETTINGS	 6-1

     6.1  Overview 	 6-1

     6.2  Environmental Factors Affecting Liquid
          Fate and Transport	 6-2

          6.2.1  Natural  Environmental  Factors 	 6-2
          6.2.2  Non-Natural Environmental  Factors 	 6-7

     6.3  Environmental Factors Affecting Vapor Transport
          and Transport 	 6-9

          6.3.1  Natural  Environmental  Factors 	 6-10
          6.3.2  Non-Natural Environmental  Factors 	 6-10

     6.4  Environmental Setting Methodologies 	 6-18

          6.4.1  RCRA Risk and Cost Analysis Model (W-E-T) 	 6-18
          6.4.2  Liner Location Risk and Cost Analysis Model  	 6-21
          6.4.3  DRASTIC	 6-26

     6.5  Summary	 6-35

7.0  APPLICABILITY OF TECHNICAL FINDINGS TO THE LIST PROGRAM 	 7-1

     7.1  Introduction 	 7-1

     7.2  Problem Definition and Assessment 	 7-5

          7.2.1  Properties of Regulated Substances 	 7-5
          7.2.2  Manifestation of Problems 	 7-6
          7.2.3  Release Settings 	 7-7

     7.3  Regulation/Guidance Development 	 7-7

          7.3.1  Technical  Standards 	 7-8
          7.3.2  Corrective Action 	 7-10
          7.3.3  Notification and Reporting 	 7-12

     7.4  Compliance and Enforcement 	 7-14
                                -xvn-

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     7.5  Information Transfer 	 7-15

          7 = 5.1  Research and Development ..,,,,..	 7-15
          7.5.2  Training and Technical Assistance 	 7-17
          7.5.3  Demonstration Projects 	 7-18
          7.5.4  Outreach Programs 	 7-19

     7.6  Summary	  7-20

8.0  REFERENCES 	 8-1
                                   VOLUME 2
Appendix A - Properties of CERCLA-Regulated Hazardous Substances
Stored in Underground Tanks 	 A-l

Appendix B - Properties of Petroleum Products Stored in
Underground Tanks 	 B-l

Appendix C - Properties of Hydrocarbons Known to be Constituents of
Gasoline and Fuel Oil 	 C-l

Appendix D - Resulting Groups From Parametric Sort as of Hazardous
Substance Database 	 0-1
                           -xviij-

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                               LIST OF FIGURES


Figure                                                                 Page

1-1   Location Plan for Site Treatment Containment Wells and South
      Hollow Well  Field - Provincetown, MA 	 1-3

1-2   Vapor Incident - North Babylon, NY 	 1-6

1-3   Vapor Incident - Livingston, MT 	 1-8

1-4   Task Flow Chart 	 1-10

2-1   Comparison of the Hazardous Substances on the CERCLA List with
      the Hazardous Substances of the California List 	 2-17

3-1   Schematic Contaminant Plumes Showing Methods by Which
      Groundwater can Become Contaminated 	 3-3

3-2   Schematic Contaminant Plumes Showing the Effects of Specific
      Gravity on Immiscible Fluid Transport 	 3-6

3-3   Schematic of the Subsurface Environment 	 3-9

3-4   Schematic Plume Showing the Influence of Stratification of the
      Unsaturated Zone on Contaminant Distribution 	 3-13

3-5   Migration Patterns in Porous Subsurface Media 	 3-14

3-6   Schematic Representation of Contaminant Migration from an UST
      Through Fractured Porous Limestone 	 3-16

3-7   Macroscopic and Microscopic Concepts of Groundwater Flow 	 3-19

3-8   Determination of Wetting and Nonwetting Fluids  	 3-26

3-9   Fluid Saturation States (Regimes) of a Two-Phase System for a
      Hydrophilic Porous Medium 	 3-28

3-10  Pressures on a Capillary Interface 	 3-30

3-11  Typical Retention Curves in Soil  During Drainage 	 3-32

3-12  Effect of Moisture Content Hysteresis for a Coarse Material  .... 3-34

3-13  Relative Permeability of Water in Unsaturated Sand 	 3-35

3-14  Variations of Relative Permeability of Wetting  (K  ) a'nd
      Nonwetting (Krmw) Fluids With Saturation	 3-37

3-15  Distribution of Fluid Flow for the Three Saturation Regimes .... 3-38

3-16  Effect of Hysteresis on Relative Permeability 	 3-40
                                -XIX-

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3-17  Schematic Distribtion of Air,  Oil, and Water in a Porous
      Med1 a 	  3-42

3-18  Relative Permeability as a Function of Saturation for
      Ai r-Oi1-Water System 	  3-43

3-19  Schematic Diagrams of Different  Contaminant Plume Shapes in

      Soils of Varying Permeability in the Unsaturated Zone 	-3-47

3-20  Comparison of Primary Log Retention of Initially Dry and
      Saturated Ottawa Sand by Column  Drainage 	  3-53

3-21  Schematic of the Capillary Zone  with a Typical  Water
      Saturation Curve 	  3-59

3-22  Variation in Water Content in  the Capillary Zone with
      Elevation 	  3-60

3-23  Contaminating Effect on Soil  Caused by Fluctuating Water Table..  3-65

3-24  Types of Dispersion 	  3-70

3-25  Schematic Diagram Showing the  Contribution  of Molecular
      Diffusion and Mechanical Mixing  to the Spread of a
      Concentration Front 	  3-74

3-26  Effect of Layers and Lenses on Flow Paths in a Shallow
      Steady-State Groundwater Flow  System 	  3-75

3-27  Effect of Density on Migration of Contaminant Solution in
      Uniform Flow Field 	  3-78

3-28  Growth, Steady-State Maturity, and Decay of a Vapor
      Concentration Field, For a Point-Source Leak in a Deep
      Homogeneous Mediurn 	  3-84

3-29  Steady-State Concentration Field for a Point-Source Leak Just
      Above an Impermeable Lower Boundary (Ground Surface Permeable).   3-87

3-30  As  in Figure 3-29, but  now with  Ground Surface Impermeable
      (e.g. Frozen or Paved)	  3-88

3-31  Steady-State Conditions as in  Figure 3-28C, but with Vertical
      Diffusion Coefficient 1/10 that  of the Horizontal  Diffustion
      Coeff 1 ci ent 	  3-89

4-1   Langmuir and Freudlich  Isotherms 	  4-15

4-2   Basic Activities and Requirements for a Cell  to Utilize  A
      Contaminant 	  4-24

4-3   Contrasts in Electron and Carbon Flow 	  4-26
                                  -xx-

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4-4   pH Dependence of KT for Hydrolysis  by Acid-,  Water-,  and
      Base-Promoted Processes 	  4-51

4-5   Examples of the Range of Hydrolysis Half-Lives  for Various
      Types of Organic Compounds in Water at pH 7 and 25 C  	  4-55

5-1   Common Hydrocarbons in Petroleum Products 	  5-6

5-2   Distillation Curves of Common Petroleum Products 	  5-9

5-3   Petroleum Products  Grouped by Kinematic Viscosity 	  5-12

5-4   Solubility Data Distribution of Hydrocarbon Database  	  5-18

5-5   Percent Composition of Gasoline 	  5-19

5-6   Physical  State Distribution of UST  Hazardous  Substances  	  5-30

5-7   Percent Inorganic and Organic UST Hazardous Substances  	  5-31

5-8   Solubility in Common Solvents of Solid Chemicals 	  5-34

5-9   Density Data Distribution of Liquid Chemicals  	  5-35

5-10  Vapor Data Density Distribution of  Liquid Chemicals  	  5-36

5-11  Grouping Methodology 	  5-38

5-12  Toxicity Group Distribution 	  5-41

5-13  Solubility Data Distribution of Hazardous Substance Database  ...  5-43

5-14  High, Medium, and Low Solubility Group Distribution  	  5-44

5-15  Vapor Pressure Data Distribution of Hazardous  Substance
      Database 	  5-46

5-15  High, Medium, and Low Vapor Pressure Group Distribution  	  5-47

5-17  High, Medium, and Low Ignitabil ity  Group Distribution  	  5-49

5-18  Toxicity-Solubil ity Sort: High Solubility Results  	  5-55

5-19  Toxicity-Solubi 1 ity Sort: Medium Solubility Results  	  5-56

5-20  Toxicity-Solubility Sort: Low Solubility Results 	   5-57

5-21  Toxicity-Vapor Pressure Sort: High  Vapor Pressure  Results  	  5-58

5-22  Toxicity-Vapor Pressure Sort: Medium Vapor Pressure Results  ....  5-59

5-23  Toxicity-Vapor Pressure Sort: Low Vapor Pressure Results  	  5-60
                                   -xxi-

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5-24  Ignitability-Vapor Pressure Sort: High, Medium, and Low
      Vapor Pressure Results	  5-61

5-25  General  Make-Up of Petroleum Products 	  5-63

6-1   Contaminant Transport Through Homogeneous & Heterogeneous
      Soi 1 s 	  6-6

6-2   Climatic Effects on Subsurface Vapor Transport 	6-14

6-3   Three-Digit Zip Code Map  Florida 	  6-22

6-4   Liner-Nine Generic Groundwater Flow Fluids 	  6-24

6-5   Groundwater Regions of the United States 	  6-27

6-6   DRASTIC Hydrogeologic Setting 	  6-28

7-1   Chain of Events Linking UST Incidents with Potential  Impacts  ...  7-2
                                 -xxn-

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                               LIST OF TABLES


Table                                                                   Page

2-1   Major Products of Petroleum 	  2-3

2-2   U.S. 1983 Demand for Petroleum Products 	  2-5

2-3   U.S. Sales of Fuel  Oils, By Uses 	  2-6

2-4   Evaluation of Petroleum Products Stored in Underground Tanks ....  2-7

2-5   Additive Type by Product 	  2-10

2-6   High Volume Chemicals Found in California USTs	  2-18

2-7   Industry Classifications Not Included or Partially Included
      on California List 	  2-22

2-8   Standard Industrial  Classification Major Groups  Not Included or
      Partially Included on California List 	  2-23

3-1   Typical  Values of Hydraulic Conductivity and Permeability 	  3-21

3-2   Primary Residual Saturation of Mineral  Oil 	  3-51

3-3   Thickness of the Funicular Zone 	  3-64

4-1   Predominant Electron Acceptors Used in Electron-Transport Systems  4-25

4-2   Variables Potentially Affecting Biodegradation 	  4-32

4-3   Degradability of Hydrocarbons 	  4-39

4-4   Hazardous Substances Recognized as Readily Degraded 	  4-43

4-5   Resistance to Degradation of Pesticides 	  4-45

4-6   Persistance of Pesticides Likely to be Found in  USTs  	  4-46

4-7   Types of Organic Functional Groups That Are Generally Resistant
      to Hydrolysis 	  4-53

4-8   Types of Organic Functional Groups That Are Potentially
      Susceptible to Hydrolysis 	  4-54

4-9   Hazardous Substances for Which Hydrolysis is a Significant
      Fate Mechanism 	  4-56

5-1   Commercial Grade Classification of Petroleum Products 	  5-3

5-2   Kinematic Viscosity of Petroleum Products 	  5-10


                          -xxiii-

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5-3   Percent Aromatic Carbon Compounds 	 5-15
5-4   Percent Composition and Solubility of The Main
      Constituents of Gasoline 	 5-20
5-5   Properties Related to Environmental  Behavior 	 5-24
5-6   Chemical  Classes of Hazardous Substances 	 5-32
5-7   Definitions Solubility in Water 	 5-43
5-8   Ignitability Scales for Reportable Quantity Adjustments 	 5-48
5-9   Pyrophoric, Explosive and High Water Reactive
      Hazardous Substances 	 5-51
5-10  Chemicals Requiring Stabilization	 5-52
5-11  Gas Phase Toxicity, Ignitibility, Solubility Data
      Reportable Quantity Information 	 5-53
5-12  Hazardous Constituents  of Gasoline and No.  2 Fuel  Oil
      Parametric Comparison 	 5-66
5-13  Average Heavy Metal Composition of Fuel  Oil  	 5-69
5-14  Chemical  Groups of Petroleum Solvents and Other
      Industrial Solvents	 5-70
6-1   Standard  Environments for Human Health Risk Evaluation •	 6-20
6-2   Four Mobility Classes 	 6-25
6-3   Assigned  -Weights for DRASTIC Features 	 6-30
6-4   Ranges and Ratings for  Depth to Water 	 6-31
6-5   Assigned  Weights for UST DRASTIC Features 	 6-33
6-6   Ranges and Ratings for  IMPACT Factors 	 6-34
6-7   Environmental  Factors Affecting Liquid Fate and Transport  	 6-36
6-8   Environmental  Factors Affecting Vapor Fate  and Transport  	 6-37
7-1   Typical Program Activities 	 7-4
                              -XXIV-

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                          1.0 INTRODUCTION
1.1  PROJECT BACKGROUND

Substances leaking from underground storage tanks (USTs) can have adverse
impacts on public health and the environment.  These impacts may include
the contamination of subsurface soils, the loss of an aquifer as a potable
drinking water supply, and the toxic or explosive buildup of vapors in
basements and other underground structures.  To address this problem, the
1984 Resource Conservation and Recovery Act (RCRA) amendments call for the
development of a comprehensive regulatory program for underground tanks
storing petroleum and hazardous substances (subtitle I).  This program, the
responsibility of EPA's Office of Underground Storage Tanks (OUST) of the
Office of Solid Waste and Emergency Response (OSWER), is designed to remedy
-- and ultimately to prevent -- threats to public health and the
environment from leaking underground storage tanks (USTs).

As part of the development of this program, this work assignment aims to
investigate the current understanding of the fate and transport of
regulated substances leaking from underground storage tanks.  The overall
goal is to provide assistance to EPA in establishing the scientific and
engineering basis for the UST program regulatory efforts.

1.2  LEAKING UST PROBLEMS

Leaks of petroleum or hazardous substances from underground storage tanks
pose a threat to the environment and to public health.  EPA has identified
spills or tank leaks of organic chemicals, especially hydrocarbons such as
gasoline, to be the most frequent contaminants of groundwater (USEPA,
1977).  Four recent case studies well illustrate the types of problems
caused by leaking USTs.
                                    1-1

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1.2.1  GROUNDWATER CONTAMINATION INCIDENTS
       Provincetown,  Massachusetts

       In  December 1977,  the  discovery  of a  leak  in  a  gasoline storage  tank
       at  a North Truro,  Massachusetts  automotive service station  forced
       the shutdown of Provincetown's  nearby municipal  well  field.   On
       excavating the tank,  it  was  estimated that as much as 2,000  or 3,000
       gallons  of high-test  unleaded  gasoline had been  discharged  into  the
       ground,  less than  600  feet  from  the nearest well  in the municipal
       well  field (Figure 1-1).

       The gasoline percolated  downward through the  underlying sands until
       it  reached the groundwater  table, about 45-50 feet below ground
       surface, where it  formed  a  pool  of pure product  that  overlaid and
       depressed the  water table.   Dissolved gasoline  concentrations of up
       to  20-80 parts per million  (ppm) formed a  plume  of contaminated
       water extending from  the  ends  of the  gasoline pool  in the direction
       of  groundwater flow toward  the well  field.

       To  solve the immediate water supply problem,  the Town installed  or
       gained temporary access  to  alternative sources  of supply and
       implemented a  strict water  conservation program.   Cleanup of the
       gasoline spill  proceeded  in  two  phases. The  first phase involved
       removing as much pure  gasoline  from the aquifer  as possible.  In
       North Truro, however,  the situation was complicated by the  fact  that
       the water table was betweeen 45  and 50 feet below ground surface,
       well  below suction lift  limits.   Consequently, trenching,
       excavation, and standard  skimming techniques  were ruled out.
       Because  of the limited extent  of the  gasoline lens, a two-well
       system was judged  sufficient for gasoline  collection.

       The second phase of the  cleanup  involved pumping  and  treating the
       groundwater affected by  the  spill  and recharging  the  treated
       groundwater to the aquifer.  The basic concept of this well  field
       cleanup  program is to  contain  and remove the  gasoline using  a
       36,000 gallon-per-day  (gpd)  groundwater recirculation cell.   One
       inner, shallow recirculation cell, consisting of  four recovery wells
       and a shallow  recharge bed,  contains  and removes  the  contaminated
       groundwater.   An outer,  deeper  recirculation  cell,  with one
       production well  and one  recharge chamber,  can be  operated at about
       108,000  gpd.   It provides additional  containment  in the event of
       contaminated groundwater  movement from the inner  cell.

       To  meet  stringent  discharge  criteria  set by the  State, the water is
       treated  by air stripping, filtration, and  granular activated carbon
       adsorption before  being  pumped to the recharge bed.  In operation
       since May 1985,  the treatment  facility will be operating for three
       to  five  years  before the'spill cleanup is  completed and the  system
       is  dismantled.
                                    1-2

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                                    OUTER CELL
                                   CONTROL WELL
                         APPROXIMATE
                          LIMITS OF
                       OUTER HYDRAULIC
                       CONTAINMENT CELL
                                                     SITE
                                                  TREATMENT
                                                    PLANT
INNNER HYDRAULIC
CONTAINMENT CELL
                                                 ^CONTAMINATED
                                                                      ,*oM
                                                                         &,<'
                                                                                    t'4
                                                                                 ^>
                                                                              ,,
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The sources and amounts for funding the various phases of the
project are:  the Town of Provincetown, $1.1 million; Massachusetts
Department of Environmental Quality Engineering (DEQE), $1.2
million; Massachusetts Executive Office of Community Development
(EOCD), $750,000; and the U.S. Department of Housing and Urban
Development, $250,000.  Although the well field is owned and
operated by Provincetown, it is located in neighboring Truro.  The
towns jointly applied for the state EOCD grant towards construction
of the site treatment system.

Hal pole, Massachusetts Oil Contamination

During September and October, 1983, a series of events led to the
discovery of possible oil contamination of two important wells in
the School Meadow Brook Well field in Walpole.

The well field consists of a 150-acre parcel of property in
southeastern Walpole.  The site is generally wetland, with areas
filled to construct the Town's public works facilities,  including
well  supplies and municipal garages.  The well field's five
production wells at the site have a combined pumping capacity of
approximately 2.0 million gallons per day, producing about 60
percent of the Town's supply.

Routine backhoe excavations by Walpole DPW personnel  to locate and
repair an existing drain in one area of the site indicated the
presence of subsurface oil contamination.  Test holes were excavated
to determine the extent of the problem; soil and water samples were
collected from the backhoe test pits and analyzed for oil  products.
Oil contamination was then confirmed in the backhoe test pits, the
test holes, and two nearby production wells.

Subsequent subsurface studies identified several  potential sources
of contamination in the area: the soil and fill present over much of
an old DPW garage area, soil in an area previously used for storage
of oil, and the groundwater in the area immediately surrounding the
gasoline pumps at the new DPW fuel storage depot.

The origin of the oil was likely several abandoned USTs, as well  as
previous on-site oil piping systems. The principal contaminants in
the garage areas are petroleum hydrocarbons dissolved in the
groundwater and adhering to the area soils.  Water samples show the
presence of several volatile organics indicative of gasoline
contamination (benzene, toluene, ethylbenzene, and xylenes).
Consequently, the groundwater around this fuel storage area may also
be somewhat contaminated due to spillage or leakage.   However, the
information collected indicated that the problem was  not widespread.
It is likely that naturally-occurring biodegradation  and adsorption
will  reduce much of the contamination to non-detectable levels.

The recommended plan for cleaning up the contaminated area involves
excavation of contaminated materials from the old DPW garage area
and disposal of the materials offsite.  This will  remove the major
source of contamination, and, coupled with implementation  of a site
management plan, should provide adequate protection of the drinking
water supply.
                             1-4

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       Removal  of contaminated soils and groundwater and off-site disposal
       was performed during May and June, 1985.  A total of 4,380 cubic
       yards of contaminated soil  was removed from the site and replaced
       with clean fill.  The total construction cost was $240,500.

1.2.2  VAPOR INCIDENTS
These case studies serve to demonstrate the potential  for vapor phase

impacts, as well  as groundwater and soil contamination, from UST leak

incidents.


    •  North Babylon, New York

       In May 1983, a 4,000 gallon underground storage tank at a service
       station in North Babylon, New York was discovered to be leaking
       after fumes were reported in a commercial  building next door
       (Peterec,  1985).  An estimated 100,000 gallons  of gasoline released
       underground reached the groundwater table  and migrated towards and
       under a subdivision of homes located across the street.  Clean-up
       began immediately to minimize groundwater  and soil contamination.
       An initial investigation in June 1983 discovered gasoline as a pure
      .product "floating" on the groundwater 4 feet below the basement slab
       of a home  in the subdivision, though indoor air test results did not
       detect the presence of any hydrocarbons.  By late November 1983,
       fumes were reported in numerous homes in the subdivision.  A new
       testing program then discovered a considerable  concentration of
       benzene in the basements and living areas  of several homes.  As
       shown in Figure 1-2, investigators have concluded that vapors from
       the floating gasoline pool migrated vertically  through several feet
       of coarse  sand and gravel into the basements of residences, driven
       in part by rising groundwater and the barrier to loss to the
       atmosphere of freezing conditions at the surface of the ground.
       Fume abatement systems were installed around these severely impacted
       residences; several vertical perforated PVC pipes were inserted into
       the soil and connected to a fan which withdrew  the vapors before
       they could enter the house.  Meanwhile, the basement areas were
       positively pressurized to reduce the vapor movement inward.

       Though these abatement systems were effective in lowering the vapor
       concentration in these residences, no standards for "safe levels"
       have been  established by state or federal  agencies.  In order to
       protect the health of the individuals in the affected area, the gas
       station owner was forced, through a court  agreement, to purchase
       twenty-one homes in the subdivision as well as  to continue clean-up
       and monitoring efforts.  Source control measures to remove the
       floating product and associated vapors were still underway as of
       October, 1985.  The remedial action program at  this site has
       provided more information and data than on any  other spill on Long
                                    1-5

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cr>
                SEASONAL FROST
 UNSATURATED ZONE
(COARSE SAND & GRAVEL)
                                                FREE FLOATING HYDROCARBONS
                                               • NOT DRAWN TO SCALE
     CAMP DRESSER & McKEE INC.
                                           FIGURE 1-2
                          VAPOR INCIDENT —NORTH BABYLON, NY

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   Island, an area highly dependent on groundwater.  This has  resulted
   in the North Babylon incident being possibly one of the best
   documented incidents in the country.

•  Livingston, Montana

   Beginning in December, 1978, intermittent  gasoline odors were
   detected in a restaurant and adjacent house (Reichmuth, 1984).   The
   odor intensity appeared to increase during the winter months and
   during periods of windy weather.  In  January,  1981, intense odors
   prompted notification of the local  fire marshal 1 who immediately
   ordered evacuation of both establishements.  Air monitoring found
   hydrocarbon concentrations up to 9,000 ppm.  This is just below the
   lower flammable limit of air-gasoline mixture  (14,000 ppm).

   A subsequent investigation of two nearby gasoline service stations
   found the service station furthest  away to have a leaking LIST.   The
   actual leak occurred in the pipe connecting the 6,000-gallon
   underground tank to the surface pump.  Poor inventory control
   practices and the uncertainty over  how long the distribution line
   had been actually leaking prevented an accurate determination of the
   total gasoline loss.  Estimates, however,  ranged from 2,500 to  5,000
   gallons.

   Further investigation revealed that because of the direction of the
   groundwater gradient, it was impossible for the groundwater to  carry
   gasoline between the leak and the restaurant.   Attention was then
   directed toward the subsurface soil environment which, as shown in
   Figure 1-3, consisted of 15-20 feet of silty clay material  above a
   gravel layer.  Vapor migration was  eventually  attributed to low
   water table levels, especially during the  winter months, which
   exposed the normally saturated gravel.  The high porosity of
   underlying gravel  allowed the vapors  to easily migrate laterally
   toward the restaurant and house located approximately 350 feet  away.
   During periods of high groundwater, the gravel  layer was totally
   saturated, thereby preventing the lateral  migration of vapors.   The
   soil profile beneath the restaurant,  in turn,  consisted of  7 feet of
   silty clay separating the gravel beneath the restaurant floor and
   the gravel layer.  Discontinuities  in the  silty clay layer  provided
   paths for the vapors to migrate relatively easily upward into the
   restaurant.  A residence 100 feet south of the leaking tank did not
   experience any vapor problems apparently because the gravel  layer in
   that area was normally saturated by water.  Two other conditions
   also contributed to the variable flux in vapors: frozen soil
   conditions and windy weather.  Frozen soil  conditions impeded
   vertical movement by restricting loss to the atmosphere,  thereby
   promoting greater lateral  movement.  Windy conditions were  closely
   tied to low barometric pressures.  This resulted in a "breathing"
   effect whereby gas was released from  the subsurface as pressure
   fluctuated.
                               1-7

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                   RESTAURANT
                                                                SOUTH STATION
                                                                                    RESIDENCE
                                                                              GASOLINE
                                                                              SATURATED
                                                                                ZONE
                                                                                WATER LEVEL APPROX. 4508.2
                              GASOLINE
                               VAPORS
HORIZONTAL
SCALE IN FEET
                   GRAVEL
  SOURCE: Adapted from Reichmuth (1984)
CAMP DRESSER & McKEE INC.
                            FIGURE 1-3
       VAPOR INCIDENT —LIVINGSTON, MT

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1.3  OBJECTIVES AND SCOPE OF WORK
The incidents described above clearly demonstrate the complexity of leaks
from underground storage tanks.  To assist in the development of a program
to address these incidents, the specific objectives of this work assignment
are:

    •  to summarize the current understanding of the physical, chemical,
       biological, and toxicological  processes relative to leaks of
       regulated substances from underground tanks,
    •  to evaluate existing methodologies and data pertinent to fate and
       transport processes with a potential  for applicability to the UST
       program, and
    •  to establish priorities for further investigation,  data collection,
       and/or methodological development.

To achieve these broadly-stated objectives,  a set of specific tasks was
developed, as presented schematically in Figure 1-4.  Our  work began with
the identification of the components  of the  UST program within OSWER for
which an understanding of fate and transport phenomena is  a prerequisite
(Task 1).  Concurrently, existing data was used to assess  the likely
character of current (and potential)  leaking UST incidents (Task 2).
Analogous to the problem definition efforts  of Task 2, Task 12 collected
data on actual leaking UST incidents  where vapor phase processes either
were demonstrably important or where  one might speculate that they could
have been.  Similarly, Task 13 focused on development of a list of
hazardous substances likely to be stored in  USTs primarily based on the
California UST notification program.

Task 3 (a review of relevant chemical, toxicological, and  physical
properties of substances of interest) and Task 4 (an examination of the
environmental settings applicable to  leaking UST incidents) start the key
technical/scientific aspects of the work.  Task 5 (an analysis of the
current state of understanding of the various fate and transport mechanisms
that appear to be most relevant to the substances and settings of interest
to the UST program) uses the information gathered in Tasks 3 and 4.
                                    1-9

-------
                                 IDENTIFY LUST PROGRAM
                               NEEDS W/R F&T PHENOMENA
                              • OSWER PROGRAM REQUIREMENTS
                              • FIELD LEVEL NEEDS
      i SUMMARIZE CHARACTER
        OF VAPOR INCIDENTS
         FROM LEAKING UST
         • INCIDENT DATA
         • DECISION NEEDS
SUMMARIZE CHARACTER
    OF LUST PROBLEMS
 •TANK POPULATIONS
 •QUANTITIES OF
  STORED MATERIALS
 •INCIDENTS
          SURVEY CHEMICAL
           AND PHYSICAL
            PROPERTIES
           • TOXICITY
           • SOLUBILITY
           •DENSITY
                            e
J   ANALYZE/REVIEW
PRESENT INFORMATION
  ON F&T PHENOMENA
    • FATE
    • TRANSPORT
                                 IDENTIFY AND EVALUATE
                              EXISTING F&T METHODOLOGIES
                                • OTHER EPA PROGRAMS
                                • INSTITUTIONAL /PRIVATE

                             IDENTIFY HAZARDOUS
                             SUBSTANCES STORED
                                 IN UST
                              CALIFORNIA LIST
                              LIKELY TO BE IN UST
                             ESTABLISH RELEVANT
                               ENVIRONMENTAL
                                  SETTINGS
                                •CLinflTOLOGV
                                •HVDROGEOLOGV
                                •DEHOGRflPHICS
                         7  RECOMMEND APPLICABLE METHODOLOGIES
                                  • FOR PROGRAM ISSUES
                                 1 FOR QUALITATIVE SCREENING
                                  IDENTIFY FUTURE NEEDS
                               •DATA COLLECTION
                               •METHODOLOGY DEVELOPMENT
                               • IMPLEMENTATION
                           NOTE:  F&T = FATE AND TRANSPORT
CAMP DRESSER & McKEE INC.
                  FIGURE  1-4
             TASK FLOW  CHART
                                        l-in

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This Interim Report is being submitted as part of Task 5.  It conveys the
reasoning (based on Tasks 1-5 and 12-13) that underlies the selection of

the specific fate and transport phenomena we believe to be important for
leaking UST decision-making.  For this selected set of fate and transport
phenomena, the report discusses the current state-of-the-art regarding the
technical/scientific understanding of (and data base supporting) each item.

The Interim Report is to be followed by Task 6, which involves a thorough
review of methodologies/techniques/procedures in professional use to
analyze those fate and transport phenomena judged to be important in Task
5.  With knowledge of current methods in hand, Task 7 would develop
recommendations of the methods most applicable to the needs of the UST
program.  At the completion of this task, a second interim report would be
prepared.  This report would outline the specific aspects of the UST
program's needs for which we recommend and suggest appropriate
methodologies with which to analyze fate and transport issues.  Task 8
would then outline the technical areas in need of future effort.  These
needs will reflect the recommendations developed in Task 7.

The objective of this report is to present the results of the first phase
of this project, specifically to summarize the current understanding the
mechanisms or processes which govern fate and transport of substances
leaking from underground tanks.  It is the intent of this report to provide
this information as background to the ongoing program development and
implementation efforts, and to seek comment to focus the further efforts of
this work assignment to those critical areas where additional study would
be most useful for current and future UST program tasks.

1.4  RELATION TO OTHER STUDIES

To summarize the current understanding of the mechanisms and processes that
govern fate and transport of substances leaking from underground storage
tanks, an extensive review of the literature was performed.  Also,
researchers, engineering consultants, and industry representatives were
contacted.  Through these contacts the project team gained an understanding

                                     1-11

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of the research being conducted and shared information with several OSWER
contractors who are or have been performing other pertinent work.


The literature review was facilitated by several  computerized literature

searches.  The data bases that were queried for information through DIALOG

information services were:


    •  Chemical Abstracts   1972-1985

    •  NTIS                 1964-1985

    •  Waternet             1971-1985

    •  Pollution Abstracts  1970-1985


The references obtained from these sources and through the contacts

described above that were relevant to this study  are included in Section
8.0.  Specific literature used in our study is cited throughout the report,
particularly in Sections 3.0 through 6.0 on the current state of the
science.


A sample of some of the contacts made and general topics discussed is worth
highlighting.  These are:


    •  Through OUST, information was gathered from a database being
       developed for the State Release Incident survey.

    •  Data on the hazardous substances likely to be found in USTs and the
       consituents of gasoline and other petroleum products was shared with
       ICF in conjunction with their on-going work with OSWER/OPPE.

    •  A meeting was held with several staff members of the EPA R.S. Kerr
       Research Laboratory in Ada, Oklahoma to discuss their current
       research projects related to this work assignment.

    t  Information on the analysis of the California LIST notificaton data
       base was obtained from Data Resources, Inc.

    •  Studies done by the American Petroleum Institute relating to fate
       and transport were obtained through Todd G. Schwendmann.

    •  A brief discussion was held with Linda Alien of the National Water
       Well Association on UST DRASTIC/IMPACT.
                                     1-12

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    •  A meeting was held with Jeff van Ee and other staff members from
       EPA/Environmental  Monitoring System Laboratory (Las Vegas) on the
       research they are  conduting on techniques for monitoring leaks
       outside USTs.

    •  Dr. David Kraemer  of Arizona State University was contacted
       concerning vapor transport mechanisms.

    •  A meeting with Steve Ragone of the USGS was held to discuss
       environmental factors that influence fate and transport of
       substances from leaking USTs and to discuss the USGS Toxic Waste
       Groundwater Contamination Program.

    •  Information on work being conducted by ICF, especially the chemical
       composition of petroleum products was obtained; and a meeting was
       held with James R. Janis and James Bunting to discuss the
       applicability of our technical findings to UST activities.

    •  A meeting was held with David Berg of EPA/ORD to discuss UST related
       research that is being conducted at the Las Vegas, Cincinnati,
       Edison, New Jersey and the Ada, Oklahoma facilities.


1.5  RELEVANT UST PROGRAM ISSUES


In order to focus the findings of this first phase of this work assignment,
a list was developed of relevant UST program issues.  Although the focus of

our efforts and nature of our findings are technical, the purpose of
developing this list of issues and reporting them herein is to provide a

tie to on-going UST program deliberations.  This list is probably not

al'1-inclusive, and in fact, a briefing for UST program staff is planned
during the review period  of this report to both present and discuss the

technical findings and to complete this list of relevant policy/regulatory

issues.


A sampling of the relevant UST program issues which are addressed in the
report is:


    •  Whether there is a technical basis for distinguishing between
       petroleum products and hazardous substances.  (Sections 5.2 and 5.4)

    •  Whether and how petroleum products and hazardous substances might be
       aggregated into groups for regulatory and standards-setting purposes
       (Sections 5.2.3 and 5.3.3).
                                     1-13

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    •  Whether representative (indicator)  substances or chemicals can be
       associated with certain chemical  groups (Sections 5.3.2 and 5.4).

    t  Whether leak incidents can be characterized and possibly prioritized
       by the mobility and/or toxicity of  tank contents.  (Sections 5.0
       and 7.0).

    •  Whether contamination of groundwater is a consequence of all UST
       leaks and must be addressed in all  corrective actions.  (Section 3.0
       and 7.0).

    •  Whether regulatory distinctions could be drawn based on
       environmental  settings (Sections  6.0 and 7.0).

    •  Whether and how distinctions in leak incidents can be attributed to
       the factors given in Section 9003 of subtitle I, e.g., location of
       tank, soil and climate conditions,  uses of tanks, hydrogeology,
       water table, size of tank, etc. (Section 3.0 and 6.0).

    •  Whether and how vapors resulting  from UST leaks pose a threat to
       human health and the environment.  (Section 3.6 and 4.3).

    •  Whether and how corrective actions  (in cases of vapor or liquid
       incidents) may be organized or prioritized.  (Section 7.0).

    •  Whether and how prioritizeation might be achieved by groups of
       products or substances matched with environmental settings.
       (Section 7.0).


We should note that although many of the references in the above bullets
are to Sections 5.0 and 7.0 of this report, it is the basic understanding
of fate and transport processes in Sections 3.0 and 4.0 that form the basis
for these findings.


1.6  ORGANIZATION OF REPORT
To evaluate the potential  seriousness  of a leaking  UST incident,  a  thorough
understanding of its fate  and transport in the subsurface environment  must
be developed.  The purpose of the next several  sections is to present  a
comprehensive picture of the current understanding  of the fate and
transport of substances (regulated by  the UST program) after they have
leaked from an underground tank.
                                    1-14

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First, we present in Section 2.0 an overview of the substances to be
regulated under the LIST program, specifically the petroleum and hazardous
substances covered by the RCRA subtitle I.  While the fate and transport of
a chemical in the subsurface are interwoven, they are, for ease of
presentation, discussed as separate mechanisms in Sections 3.0 and 4.0.
The transport of a substance can be described by the mechanisms that govern
its movement in the environment.  The fate of a substance includes all  of
the physical, chemical, and biological changes it undergoes in the
environment.

The objective of the transport sections is to answer the question:  What
governs the movement of a chemical  within each of the compartments?  The
fate of a chemical in the subsurface can be best understood in relation to
the following questions:  How will  a chemical partition among the major
compartments and their constituent  media, and where and to what extent  does
a chemical degrade and/or transform?  Five fate mechanisms were determined
to be most important for chemicals  leaking from USTs:  solubility,
vaporization, adsorption, biodegration, and abiotic chemical
transformations.

The physical and chemical nature of the substances stored in  underground
tanks, and particularly the physical, chemical, and toxicological
properties relevant to fate and transport mechanisms are then presented in
Section 5.0.  The environmental factors and/or settings which can be used
to describe leaking LIST scenarios are discussed in Section 6.0.

Finally, in Section 7.0, we summarize the technical findings  of this report
in the context of their applicability to the development of the UST
regulatory program.  All of the references used in the project are listed
in Section 8.0
                                     1-15

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                    2.0  UST PROGRAM REGULATED SUBSTANCES


2.1  OVERVIEW

As defined in RCRA subtitle I (the UST legislation),  the substances  to  be
regulated under the UST program include hazardous substances and  petroleum

products, as follows:


         "(A) any substance defined in section 101(14)  of the Comprehensive
         Environmental  Response, Compensation, and Liability Act  of  1980
         (but not including any substance regulated as  a hazardous waste
         under subtitle C), and

         (B) petroleum, including crude oil  or any fraction  thereof  which
         is liquid at standard conditions of temperature and pressure (60
         degrees Fahrenheit and 14.7 pounds  per square  inch  absolute
         pressure)."

The hazardous substances regulated under CERCLA include  698  substances,

which may be found in a variety of mixtures, tailored to a specific  end
use.  Petroleum is refined  into many products, some of which,  in  turn,  are
often used as ingredients for petroleum byproducts (e.g.,  petrochemicals,
plastics, or rubber).  The  purpose of this  section is to describe the
regulated substances stored in underground  tanks,  and more specifically,
those to be considered  in this study.


2.2  REGULATED PETROLEUM SUBSTANCES IN USTs


2.2.1  OVERVIEW


Petroleum substances can be considered in two major groups:


    •  Crude oil and its intermediate substances  — Crude  oil  is  petroleum
       in the state it  was  extracted from the earth.  Its  intermediate
       substances include all  other chemical  mixtures that may be present
       during the refining  process.

    t  Finished petroleum products — Petroleum products are primary stocks
       from the refining process that may have been blended,  compounded or
       otherwise modified to become commercial  products.   (Note that this
       definition does  not  include secondary products (byproducts) of
       petroleum, such  as products of the petrochemical  industry).


                                    2-1

-------
We have used three criteria to determine which petroleum substances  should
be studied in this report:
    •  whether the petroleum substance meets the statutory definition  of  a
       regulated petroleum substance (a liquid at 60 F and one atmosphere
       pressure),
    •  whether the petroleum substance is commonly stored in  underground
       tanks, and
    •  whether the substance is widely used in the United States.
The first two criteria isolate those petroleum substances  likely to  be
regulated under the LIST program,  while the third criterion helps to  focus
study on the petroleum substances of greatest concern under this program.

While crude oil. and most of its intermediate substances  meet  the legisla-
tive definition of a regulated petroleum-substance,  they are  not likely to
be stored in underground tanks (even when piping volumes are  considered).
The sheer volume of crude oil  processed at refineries makes it  impractical
to store crude oil or its intermediate products underground.  Therefore,
crude oil and its intermediate products need not be  considered  further  in
this report.

The finished petroleum products that meet the criteria stated above  for
regulated substances under the LIST program will be the focus  of this  study.

2.2.2  PETROLEUM PRODUCTS

A wide range of petroleum products can be refined from crude  oil  --
blended, compounded, or otherwise modified into commercial  products  ranging
from highly refined gasolines, to heavy fuel  oils, to asphalt.   Table 2-1
presents a summary of the names of petroleum products that can  be derived
from crude oil.  It should be  noted that these names  are not  necessarily
the refiner's names. Thus, different names may be used to  describe the  same
basic refiner's product.  For  example, mineral  spirits and Stoddard  solvent
                                    2-2

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                                 TABLE 2-1
                        MAJOR PRODUCTS OF PETROLEUM
Power Fuels

    Aviation Gasoline, Motor Gasoline, Diesel Fuel Oils, Naphtha-type Jet
    Fuel, Kerosene-type Jet'Fuel and Gas-Turbine Fuel  Oils.

Heating Oils

    Liquified Petroleum Gas and Fuel Oils (e.g., Nos.  1, 2, 4, 5, and 6).

Illuminating Oils

    Kerosene and Mineral Seal  Oil

Solvents

    Stoddard Solvents, Petroleum Spirits, Petroleum Extender Oils, and
    Aromatic Solvents

Lubricants

    Automotive Lubricants and  Industrial  Lubricants
r
Still  Gas

Petroleum Coke

Building Materials

    Asphalt Cements, Liquid Asphalts, Joint  Filler, Roofing Asphalt,
    Emulsified Asphalts and Road Oils

Insulating and Waterproofing Materials

    Transformer Oils, Cable Oils,  Waterproofing Asphalt  and Waxes

Other Products

    Cutting Oils, Heat Treating Oils, Heat-Transfer Oils,  Hydraulic Oils,
    Tree-Spray Oils, Insecticide-Base Oils,  Weed-Control  Oils, Medicinal
    White Oils, and Petroleum  Jelly
Sources:  Standen (1967) and U.S.  Department of Energy (1984)
                                    2-3

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are generally the same refined 300-400°F-boiling-range naphtha (Standen,
1967).  Also, distillate fuel  oils are blended to make a wide variety of
commonly-known fuel  products,  such as diesel  fuel  oil.  We recognize  that
by using these names some overlap in Table 2-1 occurs.

The major categories of petroleum products were assessed for  the  three
criteria defined in  2.2.1.  In order to determine the third criterion
(i.e., whether a product is widely used),  Table 2-2  gives the U.S.  1983
demand for petroleum products. (Note that  the categories of products  listed
in Table 2-2 do not  necessarily coincide with the products listed in  Table
2-1 because of differences between names in this table and refiner's
names).  The uses of distillate and residual  oils  are presented in  Table
2-3.

Table 2-4 presents results of  the three-criteria assessment.   The petroleum
products studied in  this report include:

    •  Power fuels - aviation  gasolines, motor gasolines,  diesel  fuel  oils,
       jet fuels, and gas turbine fuel  oils.
    •  Heating oils  - fuel oils (Nos. 1, 2, 4, 5,  and 6).
    •  Solvents - Stoddard solvent, petroleum spirits, petroleum  extender
       oils, and aromatic solvents.
    •  Lubricants -  automotive and industrial  lubricants.

The commercial grade classifications of the above  products,  including The
American Society of  Testing Materials (ASTM)  specifications,  are  presented
in Table 5-1, in Section 5.2.2.

2.2.3  PETROLEUM ADDITIVES

Additives are widely used in petroleum  products  -- including  many of  those
of importance in this report — to modify  the physical  and/or chemical
characteristics of the pure product, both  to  ensure  product quality
requirements and to  meet performance criteria.
                                    2-4

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                                 TABLE 2-2
                  U.S. 1983 DEMAND FOR PETROLEUM PRODUCTS
                             (Million Barrels)
                  Motor Gasoline                    2417
                  Distillate Fuel Oil                982
                  Residual Fuel Oil                  519
                  Jet Fuel (Kerosene Type)           306
                  Jet Fuel (Naphtha Type)             76
                  Lubricants                          53
                  Kerosene                            46
                  Special  Naphthas1                   30
                  Aviation Gasoline                    9
Source:  U.S. Department of Energy (1983)
 Special  naphthas are all finished products within the gasoline range that
 are used as paint thinners, cleaners or solvents, including all  commercial
 hexane and cleaning solvents.
                                  2-5

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                                 TABLE 2-3
                      U.S. SALE OF FUEL OILS, BY USES
                             (Thousand Barrels)
       Use
Vessels Bunkering

Electric Utility Company

Rail roads

Oil Company Use

Oil Company Fuel

Industrial  Use

Heating Oils

Military Use (3)

Diesel

Miscellaneous Uses

TOTALS
Residual  Oil

 136,290

 204,238

     140

  32,820

    (1)

  89,573

  46,743

   5,411

   N/A

   3,389

 518,604
Distillate Oil

   33,785

   11,744

   91,681

     (1)

   29,957

   69,332(2)

  311,289

   15,925

  399,013

   19,201

  981,927
Source:  American Petroleum Institute (1985)

N/A - not available

(1) - different headings were used in the source tables.   It is  uncertain
      if these two categories define the same use.

(2) - Includes oil company use.

(3) - Includes imports by military
                               2-6

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                                 TABLE 2-4

                      EVALUATION OF PETROLEUM PRODUCTS
                        STORED IN UNDERGROUND TANKS
PRODUCT*
I
Meets Statutory
Definition
Power Fuels 0
Heating Oi Is 0
Liquified Petroleum
Gas X
3
Illuminating Oils 0
Petroleum Solvents 0
4
Lubricants 0
Still Gas X
Petroleum Coke X
c
Building Materials X
Insulating and
Waterproofing
Materials S
Other Products7 S
0 = meets criteria X = does

CRITERIA
II
Tank
Storage
.
•
0
0
0
0
X
X
X
s
s
not meet criteria


III
Widel
Used
0
0
0
X
0
0
X
0
0
X
s
s


Included


y In This Study
0
0
X
X3
0
0
X
X
X
X
X7
= some products
criteria, see
on next page











meet
notes

Notes on next page.
                                 2-7

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                            TABLE  2-4  Continued


 See Table 2-1 for a  more  detailed list  of  these  product groups.

o
 Criteria are as  follows:  (1)  meets the  statutory definition of a petroleum
 substance, .i.e., a liquid at  60°F (15.6 C)  and one atmosphere pressure;
 (2) is known to  be stored in  underground storage tanks; and (3) is widely
 used.
3
 While  illuminating oils are liquids,  they  are not commonly stored in USTs
 when used for this purpose.  However, kerosene will be studied because it
 is essentially a No.  1 fuel oil and is  included  in that category.


 Note that underground storage of  lubricants  is probably only found with
 respect to bulk  sales which includes  only  a  fraction of their use.


 Building materials include both liquid  and  solids.  Most liquid building
 materials are highly viscous  and  are  not stored  in underground tanks.
 Some building materials may meet  all  three  criteria, such as road oils,
 which  are similar to  the  lighter  distillate  oils.
 'These materials  include  both  liquids  and  solids, having highly specialized
6
 uses.
 The other products,  as  defined  on Table 2-1, are liquids except for
 petroleum jelly.   The  frequency with which these liquids are stored in
 underground tanks  is unknown.   These products have highly specialized uses
 and may be found  in  a  variety of forms, however, due to the large variety
 of forms.  Cutting oils and  heat treating oils are used widely in
 industry; however, due  to  the large variety but limited volume of oil
 marketed, these were not studied.
                                2-8

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Additives are also used as supply extenders,  particularly in motor

gasoline, to economically increase the stock  without greatly sacrificing

product quality.  Additives usually constitute only a small  portion  of the

product volume, at concentrations ranging from a few parts per million to

as much as 10 percent.


The principal petroleum products  containing additives are power fuels, fuel

oils, and lubricants.  These products — often stored in  underground tanks

— are also the petroleum products in greatest demand (Table 2-2).  Table

2-5 identifies the additive types commonly found in these products

(kerosene and special naphthas are excluded from Table 2-5 because the use

of additives in these products could not be confirmed).   Although  the

additive types indicated are typically used in the  products  listed,  their

use is dependent on the manufacturer and on the product's end use  and thus

they may not be found in all cases.


The following paragraphs discuss  the additive types listed in Table  2-5

with respect to their uses and chemical  composition.

    t  Anti-foam — Anti-foam additives  are used principally in lubricating
       oils to reduce foaming.  These additives normally  consist of  sili-
       cones or organic copolymers.

    •  Anti-knock — Anti-knock additives are used  to increase the octane
       number of aviation and motor gasoline  in order to  achieve more ef-
       ficient combustion.  The most widely used anti-knock  agents have
       been lead derivatives such as tetraethyl lead and  tetramethyl  lead.
       Due to mandated reduction  in the  lead  content of gasoline,  agents
       containing lead have been  steadily replaced  by other  lead-free
       anti-knock additives such  as ethyl  alcohol  ("gasohol"), ether and
       aromatic compounds.

    •  Anti-oxidant — Anti-oxidants are used in fuels and lubricating oils
       to prevent the oxidation of unstable hydrocarbon components which
       could lead to the formation of gum deposits  or acidic bodies.   Com-
       mon anti-oxidant additives include amine and phenol  compounds  such
       as phenylene diamine and butylphenol.   Other anti-oxidants  include
       zinc, calcium, barium and  magnesium salts,  including  thiophosphates,
       salicylates, phenates, and sulphonates.

    •  Anti-corrosion -- Anti-rust agents inhibit  corrosion  by coating
       metal surfaces with a very thin protective  film that  prevents  water
       from contacting the surfaces.  This protection is  especially
       important in storage tanks, pipelines, tankers, and engine  fuel
       systems.  Common anti-rust additives include various  fatty  acid
       amines, sulfonates, alkyl  phosphates,  and amine phosphates.

                                      2-9

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                                      TABLE 2-5
                               ADDITIVE TYPE BY PRODUCT
                                     PRODUCT
ADDITIVE TYPE   	

Anti-foam

Anti-knock        •

Anti-oxidant      •

Anti-corrosion

Ant i-wea r/Ext reme
Pressure

Color Dyes        •

Deicers           •

Detergents/
Dispersants

Friction
Modifiers

Metal
Deactivators

Pour Point
Depressants

Static Dissipator

Supply/Octane
Improver

Tackiness Agents

Viscosity Index
Improver
          AVIATION
AVIATION  TURBINE
GASOLINE   FUEL
DISTILLATE
FUEL
OIL
LUBRICATING
    OILS
MOTOR
GASOLINE
RESIDUAL
  FUEL
  OIL
                                            2-10

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•  Anti-wear/extreme pressure — Anti-wear/extreme pressure additives
   are used to improve lubrication properties under high load and high
   temperature conditions.  Anti-wear/extreme pressure additives  in  use
   include fats, oils, and waxes and their derivatives, and compounds
   of sulfur, chlorine and phosphorus.  Metal dialkyldithiophosphates
   are common anti-wear/extreme pressure additives for lubricating
   oils.

•  Color dyes -- Organic dyes are added to gasolines and aviation fue.ls
   principally for the identification of various brands or grades of
   fuel.  They are also used in gasolines to increase sales appeal.
   The concentration of dye is relatively small.

•  Deicers -- Anti-icing agents are used to ensure that dissolved water
   in fuels, which will tend to come out of solution as the fuel  temp-
   erature drops, does not freeze in the fuel system.  The deicer may
   be either a freezing-point depressant or a surface-active agent.
   Freezing point depressants act in the same manner as does anti-
   freeze in an engine's cooling system and include a variety of  alco-
   hols and glycols.  The most common freezing-point depressants  are
   dipropylene glycol, hexylene glycol, ethanol , and iso-propanol.
   Surfactants prevent adherence of ice crystals to each other and on
   metal surfaces.  The surfactants used to provide this property
   comprise amides,  amines, and amine or ammonium salts of
   organophosphates.

•  Detergents/dispersants -- Detergents/dispersants are widely used  in
   gasolines and engine oils to prevent formation of deposits on  inter-
   ior engine surfaces and to remove deposits from dirty surfaces.
   This additive is  especially important in gasolines because of  the
   susceptibility of engine carburetors to fouling from deposits.
   Commonly used detergents in gasoline include  amides and
   alkylammonium dialkyl  phosphates.  Detergents used in engine oil  are
   most frequently barium and calcium sulfonates and phenates.

t  Friction modifiers — Friction modifiers are  used principally  in  lu-
   bricating oils to adjust the frictional  properties of the oil.  It
   is often desirable to reduce the frictional resistance  of oils  in
   order to facilitate more efficient operation.  Friction modifiers
   include organic acids, amines or natural  fat, oils and  waxes and
   their derivatives.

•  Metal deactivators -- Metal  deactivators  are  used in  fuel  and  lubri-
   cating oils and act (as chelating agents)  by  binding trace amounts
   of dissolved copper that may be present  in the fuel.   Free copper is
   undesirable because it is a powerful  oxidation catalyst.   If copper
   is present even the most effective antioxidant  might  not  be able  to
   prevent oxidation and the resulting gum deposits  without  the help of
   a metal deactivator.  Metal  deactivators  are  usually diamines,
   thriazoles, or thiazoles.

•  Pour point depressants -- Pour point  depressants  are  used  mainly  in
   lubricating and fuel oils to improve their fluidity at  low tempera-
   tures.  The pour  point is an indication  of the  lowest  temperature at
                                  2-11

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       which a fuel or lubricating oil  can be stored and still  be capable
       of flowing under gravity forces.  Common pour point depressants  are
       aromatic paraffins and alky!  methacrylate polymers.

    •  Static dissipator — Static dissipators, or electrical  conductivity
       additives, are used principally in aviation turbine fuels  to
       increase the conductivity of the fuel  and thereby reducing the
       possibility of an electrostatic discharge causing an explosion
       during high-speed refueling.   ASTM specifications for aviation
       turbine fuel additives restrict the use of a static dissipator to
       two proprietary additives, Shell ASA-3 and Stadis 450.

    •  Supply/octane improver — Supply/octane quality additives  have been
       used recently to meet gasoline demand  and quality requirements as
       economically as possible.  These additives are blended  with finished
       motor gasoline (in quantities upwards  of 10%) to increase  the supply
       at an economical  price.   Common additives include ethanol, t-butyl
       alcohol, methyl-t-butyl  ether, and methanol.  In addition  to serving
       as supply improvers, these additives have also proven beneficial to
       increasing the octane number, thus doubling as an effective anti-
       knock agent.

    •  Tackiness agent — Tackiness  agents are sometimes used  in
       lubricating oils to make the  oil adhere more firmly to  surfaces.
       These agents consist of  high  molecular weight polymers  such as
       polybutenes.

    •  Viscosity index improver — Viscosity  index improvers are  used in
       lubricants to modify the viscosity/temperaure characteristics of the
       lubricant.  Viscosities  of liquids are sensitive to temperature  and
       decrease with increasing temperature.   These additives  serve to  re-
       duce the rate at which viscosity decrease as temperature increases.
       Viscosity index improvers include oil-soluble olefin polymers, alkyl
       styrene polymers, and methacrylate fatty alcohol/lower  alcohol ester
       copolymers.

It is virtually impossible to create a detailed, specific,  and complete

picture of additives in petroleum products.  Each type of  additive

described above has within it many associated chemicals,  and the  lucrative

market for effective chemical additives generates new formulations daily.

Moreover, as most additives are patented,  their chemical  formulations and

uses in specific products are considered proprietary.   There are  both

single and multi-purpose additives,  and different batches  of the  same

petroleum product may contain either.  Because of these difficulties, it  is
usually possible to specifically identify only the  most  common chemical

additives.
                                    2-12

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Many petroleum additives are hazardous substances.   Therefore,  their
presence in a petroleum product is important when addressing the
environmental significance of leaks from underground storage tanks,  the
majority of which contain petroleum products.  Most of the additives are
only present in concentrations of several  parts per million and may not
pose a severe contamination threat by themselves.

Identifiable petroleum additives which are also on  the list of  hazardous
substances stored in underground tanks include:

                        Tetraethyl lead
                        Ethylene dibromide
                        Ethyl ene di chloride
                        Dimethyl amine
                        Methanol

As stated previously, other hazardous substances  regulated by CERCLA may
also be used as petroleum product additives.

2.3  REGULATED HAZARDOUS SUBSTANCES IN USTs

2.3.1 OVERVIEW

The hazardous substances regulated under CERCLA were published  by EPA as a
final rule in the Federal  Register (USEPA, 1985a).   This  list includes  693
hazardous substances (specific chemicals and waste  streams).

The list of substances that are designated as hazardous substances under
CERCLA was taken from five other statutory sources.   These sources are:

    •  Clean Water Act Section 311(b)(4);
    •  Clean Water Act Section 307(a);
    0  Clean Water Act Section 112;
    t  Resource Conservation and Recovery Act Section 3001;  and
    •  Comprehensive Environmental Response, Compensation, and  Liability
       Act Section 102.
                                    2-13

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The first four statutory sources listed above were used  under the  LIST
program to designate the hazardous substances on the CERCLA list.   Section
102 of CERCLA was used to designate the hazardous wastes under RCRA
hazardous substances under CERCLA.  Therefore, all specific chemicals
listed on the CERCLA list are regulated under the LIST program.  The waste
streams are included in the list of regulated hazardous  substances under
the LIST program until  the wastes are discarded or intended  to be discarded
as wastes.

Recent estimates of the universe of USTs to be regulated by EPA set the
hazardous substance portion of the tank population at less  than one-tenth,
with most regulatable  tanks containing  petroleum substances (e.g., Data
Resources, Inc. 1985).  Although the degree of risk from leaking USTs may
not be directly related to the number of tanks,  these estimates would
indicate that relatively more incidents may result from  tanks containing
petroleum substances than from tanks containing  hazardous substances.

2.3.2  DATA SOURCES

Although extensive information is currently available about hazardous sub-
stances, few sources of information specifically provide data about the
likelihood of storage  of these substances in underground tanks. There is
also no single characteristic of hazardous substances which immediately
points to the suitability of a substance for underground storage.   These
substances exist as liquids, gases, and solids at ambient temperatures and
pressures, and are produced and used in a variety of forms  and volumes.

The following describes the available data sources, and  the results of our
review of these in terms of a list of regulated  hazardous substances likely
to be found in USTs.
                                    2-14

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California List of Hazardous Substances

Description.  The most important source of data for this  task is the list
of hazardous substances in USTs which was developed by the State of
California.  This list contains substances that were reported by owners  of
USTs as part of a statewide registration program.   The California (CA)  list
of hazardous substances stored in underground tanks identifies 1778
substances (1248 chemicals and 530 generic names of mixtures, e.g., "acid
and caustic mix," "No. 3 fuel  oil").   The program  registered both product
and waste tanks.  Only those substances previously defined as hazardous  by
one of the following agencies  were included on the CA list of hazardous
substances in underground tanks:

        •  California Department of Health Services
        •  The Director's List of Hazardous Substances,  Director of the
           Department of Industrial Relations, State of  California
        •  Division of Water Quality, State Water  Resources Control  Board
        •  United States Environmental  Protection  Agency
        •  Registry of the Toxic Effects of Chemical  Substances, National
           Institute for Occupational Safety and Health,  U.S. Department of
           Health and Human Services

The substances on the California list are not all  stored  as pure product in
underground tanks.  The substances listed include  chemicals that comprise
mixtures and/or solutions, the "hazardous" chemical  components of the
mixtures, as well as pure chemicals.   The California data do not indicate
whether the chemicals are pure chemicals or chemical  components of  mix-
tures, or whether the chemicals/mixtures are wastes or products. Many of
the substances are solids or gases which will  only appear in USTs in aque-
ous solution or, more commonly, dissolved in organic solvents/carriers.

As noted by Data Resources, Inc. (1985), of the 14,000 records in the
California data base coded as  non-motor fuel tanks, approximately 5,500
have no entry in the field for type of  product stored.  This means  that
only about 60% of the data could be analyzed for tank contents.
                                    2-15

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Use in This Report.  The 698 substances on the CERCLA list were compared to
the 1,248 chemicals on the California list.  Cross-checking of both
chemical numbers and names was performed because chemical  numbers  other
than Chemical Abstracts Service Registry Numbers (CASRN)  were sometimes
used to identify chemicals on the California list.   The comparison allowed
the selection of the 462 hazardous substances from the California  list
which are also on the CERCLA list.  Figure 2-1 presents this comparison
graphically.  These 462 substances, listed in Appendix A,  are likely to be
found in USTs but may include chemicals both stored as products or as
wastes.  Those found in USTs only as wastes may not be subject to
regulation under the UST program.

2.3.3  DATA RESOURCES INC. AND QUANTUM ANALYTICS ANALYSIS  OF CA LIST

Data Resources, Inc. (DRI) and Quantum Analytics (QA) have analyzed the
California data and produced a final  draft report (Data Resources, Inc,
1985).  Using a variety of analyses, DRI identified the high volume CERCLA
chemicals found in USTs and ranked them by volume of product and percent of
regulated tank population.  Pertinent information is presented in  Table
2-6.

2.3.4  FIRE PROTECTION GUIDE ON HAZARDOUS MATERIALS

Description

The Fire Protection Guide on Hazardous Materials (1984), produced  by the
National Fire Protection Association, provides data on the flammabil ity of
hazardous substances as well as useful  information  about their physical
states, physical properties, and methods of shipping.  Two of the  four
documents that make up the Fire Protection Guide on Hazardous Materials
(1984) - NFPA 325 M (1984) and NFPA 49 (1984)  -  have been  used as  data
sources in this study.

NFPA 325 M (1984) summarizes available .data on the  fire hazard properties
of more than 1,500 substances.  Listed with each substance are,  if
applicable, its flash point, ignition temperature,  flammable limits,
                                    2-16

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        1248
     HAZARDOUS
      CHEMICALS
        ON
     CALIFORNIA
        LIST
                                        462
                                      HAZARDOUS
                                      SUBSTANCES
                                        ON
                                      CALIFORNIA
                                      AND CERCLA
                                        LISTS
CAMP DRESSER & McKEE INC.
                   FIGURE 2-1
COMPARISON OF THE HAZARDOUS SUBSTANCES
  ON THE CERCLA LIST WITH THE HAZARDOUS
     SUBSTANCES ON THE CALIFORNIA LIST

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                         TABLE 2-6
       HIGH VOLUME CHEMICALS FOUND IN CALIFORNIA USTs
     CAPACITY
                                               NO. OF TANKS
CHEMICAL
Sodium Hydroxide
Sul furic Acid
Toluene
Dimethyl Ketone (acetone)
Methyl Ethyl Ketone (MEK)
Chromium
Potassium Hydroxide
Nickel
Xylene, all monomers
Methyl Alcohol
PERCENT
9.3
6.4
5.4
4.6
4.4
4.3
3.6
2.8
2.6
2.5
RANK
1
2
3
4
5
6
7
8
9
10
PERCENT
6.1
4.4
5.9
6.5
5.4
2.6
0.7
1.8
3.8
3.4
RANK
2
5
3
1
4
11
33
17
6
7
Subtotal
45.9%
                                               40.6
Source:   Data Resources,  Inc.  (1985)
                            2-18

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specific gravity,  vapor density,  boiling point,  and  relative  solubility in
water.  In addition, there is a code listed for  extinguishing methods,  and
for three hazard identifications:  health, flammability and  reactivity.   Of
most immediate interest are the following five flammability codes:

Flammability Code          Description
    4                    Very flammable  gases  or very volatile flammable
                         liquids
    3                    Materials which can be  ignited under almost  all
                         normal  temperature conditions.
    2                    Materials which must  be moderately heated  before
                         ignition  will  occur.
    1                    Materials which must  be preheated  before ignition
                         can occur.
    0                    Materials that  will  not burn.

NFPA 49 (1984) is  a compilation of information on hazardous chemicals which
focuses on hazardous properties and fire fighting phases.   It includes,
along with the hazard codes for each chemical, a description  of the
chemical and data  regarding the fire and explosion hazards, life hazard,
personal protection, fire fighting phases, usual shipping  containers, and
storage.

Use in This Report

The remaining 236  hazardous substances  on the  CERCLA list  (i.e., those  not
already selected through comparison with the California list) were  compared
to the chemicals listed in NFPA 325 M and NFPA 49.  Liquids or gases  which
have a NFPA flammability code of  2, 3 or 4, and  are  on  the  CERCLA list  were
selected for further consideration.

The National Fire  Protection Association recommends  outside or detached
storage for these  substances.  One method of storage for flammable  or ex-
plosive substances is in underground tanks where the substances are
protected from high ambient temperatures and contact with  fire or other
flammable substances.  In addition, the  underground  tanks  are protected
                                    2-19

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from physical damage.  While the same substance may be stored above ground
in a tank, the fire codes require that flammable substances stored above
ground be kept at certain distances from other facilities.   It follows that
a flammable substance will most likely be stored below ground either to
maximize the use of space at the site or to protect the tank and stored
substance from fire, heat, and physical  damage.  Fifteen substances which
are regulated by CERCLA but are not on the California list  have been
selected on the basis of the criteria described above and are also included
in Appendix A.

Three additional  substances which also have a flammability code of 2,  3 or
4 and which are regulated by CERCLA have been determined to be unlikely to
be stored in underground tanks on the basis of information  found in the
Fire Protection Guide on Hazardous Materials.  One of these is paraldehyde
(CASRN 123637), which has a freezing point of 54°F.  It is  unlikely that a
substance with such a high freezing point would be stored below ground.
Another substance is crotonaldehyde (CASRN 4170303),  which  is stored and
shipped in small  containers.  Nickel carbonyl (CASRN 13463393) is shipped
in gas cylinders, and is also unlikely to be stored in an underground  tank.
These three chemicals were not included in Appendix A.

2.3.5  OTHER POTENTIAL SOURCES OF DATA

There remain 221 CERCLA substances which were not  included  in Appendix A .
following the screening steps described  above.  Some  of these substances
may be found in USTs, but there is not at this time sufficient evidence to
justify adding them to Appendix A.  Three of them  have been determined to
be very unlikely to be found in USTs.  Potential  sources of information for
further refinement of the list in Appendix A are discussed  below.

Industry SIC Code

Industries which use the specific chemicals of interest are potential
sources of data on hazardous substances.  A table  including Standard
Industrial Classification (SIC) codes for the hazardous substances  found in
USTs in California is reported by Data Resources,  Inc. and  Quantum
                                    2-20

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Analytics (Data Resources, Inc., 1985).   The DRI  list  was  compared to two
sources to try to confirm if all industry types are represented in the
California data.  The industries represented in California were compared to
a list with industry classification of the top 100 chemical  producers
(Chemical & Engineering News,  1985).  Five types  of industries  were
determined to be missing from, or only partially represented in,  the
California data (Table 2-7).  Agrichemicals, for  example,  was  listed  as  a
partially represented category because the California  data had  reported
only three tanks to contain agricultural  chemicals.

In addition, the SIC codes for the California list were compared  to the
Standard Industrial Classification Manual  (U.S. Office of  Management  and
Budget, 1972).  Twelve major groups of industries, listed  in Table 2-8,
were determined to be either missing from  the California data  or  partially
represented by it.  Two groups were considered to be partially represented:
Major Group 20 - Food and Kindred Products - because meats and  grains are
not represented in the California data;  and Major Group 30 - Rubber and
Miscellaneous Plastics Products - because  rubber  is not represented.

Additional  investigation of these industries and  their UST practices  may
reveal additional  regulated hazardous substances  in USTs that  are not
included in Appendix A.  This  work was not performed because most of  the
regulated .hazardous substances likely to  be found in USTs  are  included in
Appendix A, and thus, the possible addition of a  few substances to the list
(considering that  USTs containing hazardous substances represent  only a
small percentage of the total  number of  regulated tanks) was not  judged
significant to the objectives  of this study.

UST Notification Program

Another potential  source of data will  be the UST  notification program
currently being implemented by EPA, which  is due  to be completed  in 1986.
Under this  program, notification forms will  be distributed to owners  of
USTs that may contain regulated substances.  If in fact CERCLA-regulated
hazardous substances are stored in underground tanks on site, the owners
are required to complete the forms and return them to  the  responsible state
agency.
                                    2-21

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                                 TABLE 2-7

                   INDUSTRY CLASSIFICATIONS NOT INCLUDED
                  OR PARTIALLY INCLUDED ON CALIFORNIA LIST
              1.   Rubber Products - manufacturing
              2.   Glass Products - manufacturing
              3.   Alcoholic Beverages  - distilling
              4.   Non-metallic minerals
              5.   Agrichemicals
Source:  Chemical  & Engineering News  (1985),
                                    2-22

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   TABLE 2-8 STANDARD INDUSTRIAL CLASSIFICATION MAJOR GROUPS NOT INCLUDED
                  OR PARTIALLY INCLUDED+ ON CALIFORNIA LIST
      GROUP NUMBER
INDUSTRY
1.    Major group 08    Forestry
2.    Major group 10    Metal  Mining
3.    Major group 11    Anthracite Mining
4.'    Major group 12    Bituminous Coal  and Lignite Mining
5.    Major group 14    Mining and Quarrying of Non-metallic Minerals
6.    Major group 20    Food and Kindred Products
7.    Major group 21    Tobacco Manufacturing
8.    Major group 22    Textile Mill  Products
9.    Major group 23    Apparel  and Other Finished Products
10.   Major group 30+   Rubber and Miscellaneous Plastics  Products
11.   Major group 32    Stone, Clay Glass and Concrete  Products
12.   Major group 39    Miscellaneous Manufacturing Industries
                        (includes jewelry, flatware, musical  in-
                        struments, athletic goods, artists materials,
                        linoleum, fur &  pelt curing, etc.)
Source:  U.S. Office of Management  and  Budget  (1985)
                                    2-23

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The receipt and analysis of the data from the notification  program will  be
a  major step toward providing a solid data base on the universe  of tanks
containing hazardous substances.  The notification  program  will provide
extensive data on the owners of USTs containing hazardous substances,  and
useful, if limited, information on the hazardous contents of the  USTs  (if
there is a mixture of chemicals in a tank, owners are  required  to report
only the CERCLA hazardous substances of greatest quantity in the  mixture).

2.4  REMAINING CERCLA HAZARDOUS SUBSTANCES

2.4.1  LIKELIHOOD OF STORAGE IN UNDERGROUND TANKS

Of the 698 hazardous substances regulated by CERCLA,  221 are not  currently
included in Appendix A.   Of the 477 chemicals that  do  appear in Appendix A,
some may be present in waste mixtures that would not  be regulated under the
UST program.  Further research is  necessary to establish whether  these
chemicals are also found in USTs as products.

2.4.2  ADDITIONAL DATA GATHERING AND ANALYSIS

Additional data about the remaining 221 CERCLA hazardous substances  is
needed in order to determine which of them are likely  to be  found in USTs.

Two methods might be used to approach this task. One  is to  pursue
additional data directly from the  industries that were not well-covered by
the California inventory.  This might reveal  the storage of  certain
chemicals which are used by the industries inventoried.  Lists of the
industries not covered or partially covered in the  California inventory are
shown in Tables 2-7 and  2-8.

A second approach is to  collect information on the  manufacture, use, and
physical properties of each remaining substance.  Some substances may be
eliminated because, for  example, they are not produced in quantities large
enough to store in underground tanks.  Other substances  may  be solids that
                                    !-24

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are rarely dissolved prior to use.  It is likely that a few of the
remaining chemicals will  be added to the list of those found in USTs.
There is no need under this work assignment to attempt to extend the  list
presented in Appendix A.   Most CERCLA hazardous substances already appear
in the list, and hazardous substances as a class represent only a small
portion-of the total number of regulated USTs.  It  may be worthwhile  in
future efforts to identify substances that should be  deleted from the  list
because they exist in USTs as wastes only, and so are not subject to  UST
regulations.
2.5  SUMMARY

Based on three criteria (liquid,  stored  in  underground  tanks,  and wide
use), the petroleum substances of interest  for the  remaining work include
the following:
    •  Power fuels - aviation  gasolines,  motor  gasolines, diesel  fuel oils,
       jet fuels, and gas turbine oils.
    •  Heating oils - fuel  oils  (Nos.  1,  2,  4,  5,  and  6).
    •  Solvents - Stoddard solvent,  petroleum spirits,  petroleum  extender
       oils, and aromatic solvents.
    •  Lubricants - automotive and industrial  lubricants.
Of the petroleum products  listed  above,  power  fuels  and heating oils are
produced in greatest quantities,  and therefore,  are  probably found more
frequently in underground  tanks.   We will  focus  most  of our  efforts in this
area.

Of the 698 hazardous substances  regulated  by CERCLA,  477  have been
determined to be likely to be found  as  products  in USTs.  A  list of these
chemicals, with their corresponding  CASRN  numbers, is included in Appendix
A.
                                    2-25

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These substances have been selected based on two criteria.   Of the
substances listed in Appendix A,  462 are also found in the  California  list
of hazardous substances, which was developed from an inventory of existing
USTs.  Fifteen of the substances  are liquids or gases which are flammable
enough to suggest storage outdoors or in separate facilities,  and thus  seem
likely to be stored in underground tanks, although they were not included
in the California list.

The remaining 221 CERCLA substances are not  readily identifiable as  being
either likely or unlikely to be stored in underground tanks.  Some may  not
be produced in large enough quantities to warrant underground  storage.
Some of the solids will not be stored in underground tanks  in  the solid
phase, but might appear dissolved in another substance.   Additional
research must be done to confirm  or deny the likelihood  of  underground
storage of the remaining CERCLA chemicals.

A potential source of additional  data will be the nation-wide  notification
program for USTs.  The completed  notification forms are  due to be returned
to the states by the tank owners  on or before May 8, 1986.   This survey may
not reveal every chemical  stored  in underground tanks,  but  will provide
extensive data on the users of USTs.  Research using information from
industries and other facilities which use USTs could resolve any remaining
questions regarding the likelihood of CERCLA substances  being  stored in
underground tanks.

These efforts have several  implications.  First,  the list of 477 hazardous
substances given in Appendix A, rather than  the full  CERCLA list of  698
substances, will be the focus of  work in Sections 3.0 through  6.0,
particularly in the discussion of the properties  of the  substances.  Since
the other 221 CERCLA substances are unlikely to be stored in underground
tanks, they will not receive further attention.

Second, because many of the 477 substances are solids or  gases  that  appear
in aqueous solution, or more commonly, are dissoved in  organic
solvents/carriers, the fate and transport of these solutions and/or
solvents/carriers needs to be studied.
                                    2-26

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Finally, the relative emphasis given hazardous  substances  versus  petroleum
substances should take into account current estimates  that the latter
represent over 90 percent of the regulated UST  universe.   Many of the
studies in other sections of this report deal with  generic fate and
transport mechanisms, but those that deal  with  specific  substances  should
have relatively higher emphasis on petroleum substances.
                                    2-27

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                         3.0  TRANSPORT MECHANISMS
3.1  OVERVIEW

3.1.1  FACTORS AFFECTING TRANSPORT

Necessary to an evaluation of the potential  seriousness of leaking USTs is
a thorough understanding of how substances are transported through the
environment.  The transport of a substance can be described by the
mechanisms that govern its movement in the environment.

The objective of the text addressing transport phenomena, is to answer the
question:  What governs the movement of a chemical in liquid and vapor
phases within each of the compartments of the subsurface?  To answer this
question, Section 3.0 has been organized as follows:   first, a general
introduction to the factors affecting transport is presented at Section 3.0
below.  Readers who are not interested in the detailed technical  discussion
of transport may wish to move on to Section 4.0 after this overview; prior
to Section 3.2.

Section 3.2 introduces Darcy1s Law which is the foundation of quantitative
theory of the flow of fluids through porous media.

In the general introduction, three zones of the subsurface which  are
important to the transport of contaminants are defined:  The unsaturated
zone, the capillary zone and the saturated zone.   The transport of liquid
phase contaminants in each of these zones is presented in Sections 3.3
through 3.5, respectively.  Section 3.6 discusses vapor transport in the
unsaturated zone.

For each case involving leaks from USTs, the extent and impact of
contamination is unique.  The degree to which contamination migrates in the
subsurface depends on three major factors:
                                    3-1

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    0    the quantity released;
    •    the physical  properties of the leaked substance;  and
    •    the structure of the subsurface.

Some of the basic,  and decisive  parameters  of each of these factors  are
discussed below to  provide an initial  understanding of transport in  the
subsurface.

Quantity Released

Spills can be grouped into two major types  -  catastrophic  or "sudden"
spills and insidious spills.   A  catastrophic  spill involves the rapid
release of large volumes of a substance.  Because  leaks  from USTs  are  not
typically of this type (pipeline ruptures  and highway or railroad  accidents
are more common examples), catastrophic releases will  not  be considered
further.  Insidious spills, which are characteristic of leaking USTs,
involve slow, but often continuous, release of relatively  small  volumes  of
substances.  However, because a  tank often  leaks  over a long period  of
time, substantial  volumes of a substance can  be released into the
subsurface.

One reason the quantity of released substance is  important is that it  often
determines the method, and degree, of groundwater  contamination.   Two
methods of groundwater contamination are depicted  in Figure 3-1.   If the
volume of a release is large enough (i.e.,  exceeds the retentive capacities
of the soil media in the unsaturated zone), the leaked substance will  come
into direct contact with the groundwater (Figure 3-1 (A)).  Consequently,
contamination of the groundwater can be rapid and  extensive.  Even though  a
smaller release will most likely remain in  the unsaturated zone,
groundwater below the spill site can still  become  contaminated by  the
solution of contaminant into percolating  rainwater (Figure 3-1 (B)).  While
                                    3-2

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                                       RAINFALL
                                                 GROUND SURFACE
                                            FREE PRODUCT
                                         OR DISSOLVED PRODUCT
            WATER TABLE
                                                       DISSOLVED PRODUCT
                                   SATURATED ZONE
                 (A) CONTAMINANT IN DIRECT CONTACT WITH THE WATER TABLE
                                  RAINFALL
                                                 GROUNDSURFACE
                                         • FREE PRODUCT
                                                     UNSATURATED ZONE
                  WATER TABLE
                                         SATURATED ZONE


                (B) GROUNDWATER CONTAMINATION RESULTING FROM SOLUTION
                  OF CONTAMINANT IN PERCOLATING RECHARGE WATER
CAMP DRESSER & McKEE INC.
FIGURE 3-1 SCHEMATIC CONTAMINANT PLUMES SHOWING METHODS
          BY WHICH GROUNDWATER CAN BECOME CONTAMINATED
                                        3-3

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the contamination will  take a longer time to develop, the impact of
contamination can still be extensive.

Physical  Properties

The miscibility (or immiscibility) of a leaked substance with water or, in
other words, the solubility of the substance in water leads to one of two
very different transport processes.  For the purpose of the transport
section,  substances will only be considered to be miscible (readily
dissolved in water) or immiscible (two fluids that maintain a sharp
interface when in contact with one another).  Immiscible substances include
all of the petroleum products and non-aqueous phase liquid hazardous
substances.  Some of the hazardous substances stored in underground tanks
will be miscible.  Actually, the solubility of UST regulated substances in
USTs range from freely soluble to slightly soluble to pratically insoluble.
The different degrees of solubility, particularly with respect to hazardous
substances, are discussed in detail in Section 4.2 and Section 5.0.

Fluids miscible with water are fully displaced by water, and thus, travel
as a single phase fluid in the subsurface.  Immiscible fluids are
transported in the unsaturated zone under two- (or multi-) phase flow
conditions and are held back at residual saturation in the voids of the
soil matrix.  If the retentive capacity of the unsaturated zone is not
exceeded, substances held at residual saturation can only be further
transported by water according to their solubility (Schwille, No Date).

Note that Figure 3-l(A) also illustrates schematically transport of
miscible fluids or immiscible fluids released in sufficient quantity to
reach the water table.   On the other hand, the transport of the substance
depicted in Figure 3-l(B) is only relevant for immiscible fluids held at
residual  saturation in the unsaturated zone.

If the leaked substance is immiscible with water, the specific gravity
(S.G.) (or density relative to water) of the substance is the next
                                    3-4

-------
parameter which governs its migration pathway.  When the quantity of the
leaked substance is sufficiently large to reach the water table, as a
nonaqueous phase liquid (free product), the specific gravity of the
substance will determine whether the free product "floats" on the capillary
fringe (S.G. < 1) or "sinks" through the saturated zone to an impermeable
boundary (S.G. > 1).  Figure 3-2 presents schematics of these conditions.
Even small differences of a few tens of grams per kilogram groundwater can
be decisive (Schwille, No Date).

Other important physical properties of a leaked substance that govern its
movement in the subsurface are (Schwille, No Date):

    •    Viscosity - affects the rate of movement under saturated and
         unsaturated conditions of a substance moving as a separate phase.
    •    Surface tension - is responsible for capillary effects and
         determines the spreading of fluids on the capillary fringe at the
         end of the spreading phase.
    •    Evaporation rate - (dependent on the vapor pressure and latent
         heat of evaporation) determines the potential  for a substance to
         have an associated vapor phase.
    0    Vapor density - determines whether the vapor phase in the
         unsaturated zone rises or sinks to spread on the capillary fringe.

3.1.3 Structure of the Subsurface

The type and composition of the subsurface formation also affects the
transport of a leaked substance.  Subsurface materials  can be divided into
two basic media - porous media and fractured (or fissured) rock media.
Porous media (often called soil media) is comprised of  unconsolidated
particulate materials, for example gravel, sand or silt.  Fluids--air,
water or oi'l--are contained within the pore spaces of soils.  The size and
degree of interconnection of the pores affect the permeability of these
materials.  Rock is consolidated material which can transport fluids
through interconnected pores (primary porosity) or through fracture
(secondary porosity) or both.  Rock usually underlies a few inches to
                                    3-5

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                                 FREE PRODUCT
                                 DISSOLVED
                                  PRODUCT
                                                   UNSATURATED ZONE
                                                      SATURATED ZONE
                         IMPERMEABLE BOUNDARY
     (A) AN IMMISCIBLE SUBSTANCE WITH A SPECIFIC GRAVITY LESS THAN 1 WILL FLOAT ON
                                THE WATER TABLE
                                                 UNSATURATED ZONE
                                                     SATURATED ZONE
    (B) AN IMMISCIBLE SUBSTANCE WITH A SPECIFIC GRAVITY GREATER THAN 1 WILL SINK TO AN
                             IMPERMEABLE BOUNDARY
CAMP DRESSER & McKEE INC.
               FIGURE 3-2
SCHEMATIC CONTAMINANT PLUMES SHOWING
     THE EFFECTS OF  SPECIFIC GRAVITY
      ON IMMISCIBLE FLUID TRANSPORT

    3-6

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several  hundred feet of soil.  With respect to the transport of fluids,
there are three basic rock types - bedded sedimentary, carbonate, and

crystalline.
     •  Sedimentary rocks usually occur in distinct layers or beds.  The
        beds may be horizontal  or may slope or dip in some direction.
        Joints and fractures often follow the bedding planes. The movement
        of fluids may be affected by the orientation of these planes.
        These rocks may also have significant primary porosity.

     •  Carbonate rock, limestone in particular, is partially soluble in
        water.  These rocks can sometimes have extremely high permeability
        and porosity resulting from the development of large openings and
        fissues that range from less than an inch to many feet.

     •  Crystalline rocks, such as granite and gneiss, are usually not
        porous.  These rocks are often fractured and a certain amount of
        fluid may exist in and move through these openings.  Fractures near
        the surface may parallel the surface (sheet jointing); while the
        orientation of deeper fractures may vary as a result of techtonic
        stresses.
Although contaminant transport in fractured media is goverend by the same
processes as porous media, the factors listed above can result in quite
different migration pathways.  The standard technique used to predict flow
in fractured media is to treat the fractured medium as an equivalent porous

medium (Freeze and Cherry, 1979).  This technique is reasonable to describe
single-phase (miscible) fluid flow, but does not sufficiently describe

immiscible flow.  Birk and Vorreyer (1978)  listed several  prime reasons for
the dissimilarities between migration of oil (an immiscible fluid) in •
fractured aquifers as described by complex  mathematical solutions and seen
in the field.

     •  Oil  migration is not uniform as in  unfissured porous media, but is
        along preferred fissures.

     •  Joint planes can retain or deflect  oil  movement.

     0  Water table flucuations can open up new fissures causing migration
        in different directions.

     •  Oil  contents of polluted groundwater vary more in  fissured media
        than in unfissured porous media.
                                    3-7

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     •  Because of insignificant porosity and smaller capillary effects
        terms such as 'the thickness of the oil layer is...1 are not
        helpful in fissured rocks since such terms suggest a more or less
        uniform distribution of oil underground.
For the reasons listed above, this report will  not address transport of
immiscible substances in fractured rock media.

When considering hydrology (water content) and  the placement of USTs,
several zones of the subsurface can be defined  for porous media:  the
weathered soil zone;  the excavation zone;  the vadose zone;  the capillary
zone;  and the saturated zone.  Figure 3-3 presents a schematic of these
subsurface zones.  With the exception of the weathered soil zone,
substances leaking from underground tanks will  encounter these zones in
succession.  Each of these zones is discussed in general below, and
transport mechanisms in the latter three zones  is discussed in detail in
Sections 3.3 through 3.5.

Weathered Soil Zone.  The weathered soil zone is the upper region of the
unsaturated zone and extends from the ground surface to the level impacted
by major vegetative roots.  This zone will not  exist where rock outcrops
are present.  Because USTs are generally located at least several feet
below the ground surface, substances leaking from underground tanks will
not typically encounter this zone.  If leaked substances are transported
through the weathered soil  zone, downward migration may be relatively rapid
and strongly localized compared to the underlying media because of presence
of root channels, animal  burrows or dessication cracks (Convery, 1979).
Leaked substances may also be highly adsorbed in this zone, because of the
organic-rich nature of the soil.

Excavation Zone.  The excavation pit of USTs are filled either with
disturbed soil removed from the pit or artificial  fill.  In the first case,
the backfilled material  may be more permeable particularly in the vertical,
than that removed because it has been disturbed.  These excavation areas
offer a migration route of minimum vertical  resistance and fluids may move
                                    3-8

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                                                       GROUND SURFACE .
         UNSATURATED
            ZONE
                          TANK
                                                          WEATHERED SOIL ZONE
        EXCAVATION ZONE
                       VADOSE ZONE
                                  CAPILLARY ZONE
                    VWATER TABLE
                                                     SATURATED ZONE
                                                     (GROUNDWATER)
                            IMPERMEABLE BOUNDARY
CAMP DRESSER & McKEE INC.
                    FIGURE 3-3
SCHEMATIC OF THE SUBSURFACE ENVIRONMENT
                                    3-9

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downward more rapidly in them than in natural  soils.  Artificial fill,
typically sand and gravel, is often used when tanks are installed in
clay-rich or rock media.  In this case, the artificial fill is typically
much more permeable than the natural  media.  When a leak occurs, the
substance pools in the excavation pit, which then behaves as an areal
source of contamination.  Otherwise.,  transport of a leaked substance in the
excavation pit is governed by the same mechanisms as vadose zone movement.

Vadose Zone.  The vadose zone is the  lower region of the unsaturated zone
and extends from the weathered soil zone to the capillary zone.  The terms
vadose zone and unsaturated zone are  used interchangably, except where
their distinctions are of significance.  Under uncontaminated conditions,
the interstices (pore spaces or fractures) in the vadose zone, are filled
with air and water.  Leaked substances move downward through the
unsaturated zone under the influence  of gravity.  Some lateral movement
occurs due to capillary forces.  Lateral movement in the unsaturated zone
also results from heterogeneities in  the media and variations in water
saturation of the unsaturated zone (as discussed in Section 3.3).

If the leaked substance is an immiscible fluid, a certain amount of the
substance is immobilized (by capillary or interfacial  forces) in the
interstices of the soil.  The immobilized substance is termed to be held at
residual  saturation which is dependent on .the soil  structure and moisture
content, and the leaked substance.  Neglecting gradients that affect the
flow of the immiscible substances, if the quantity of  released substance
does not exceed the residual saturation of the media,  the substances will
not reach the capillary zone (Schwille, No Date).

As the leaked substance migrates downward through the  unsaturated zone,
volatile substances will vaporize and form .a gas envelope around the
infiltration core.  Vapors are also generated from immiscible substances
floating on the capillary fringe and  to a lesser extent from dissolved
contamination.  Vapor migration in the unsaturated zone is due to diffusion
and advection.  Vapor transport is presented in Section 3.6.
                                    3-10

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Capillary Zone.  The capillary zone is the transition zone between the
unsaturated and saturated zones.  The moisture content of this zone ranges
                                                          •     •
from the residual  saturation of water at its upper surface to complete
saturation at the water table.  Water in the interstices of this zone are
at subatmospheric pressures.  The capillary zone will be thicker in fine-
grained media than in coarse-grained media.

Immiscible substances that reach the capillary zone will either float on
the capillary fringe or sink to an impermeable boundary depending on the
specific gravity of the substance.  Substances with a specific gravity less
than water may form a mound on the capillary fringe.  This occurs when the
infiltration rate of the leaked substance exceeds the lateral spreading on
the capillary fringe (Figure 3-2(A)).  Under a high infiltration rate, the
substance can be depressed below the capillary zone into the saturated
zone.  Downward migration of substance with a specific gravity greater than
water is slowed down when the substance reaches the water table because
groundwater must be displaced.  These substances, however, sink through the
saturated zone to form a mound on an impermeable boundary (provided the
specific retention of the media/substance is exceeded).  Spreading of the
mound then takes place under the pressure head of the leaked substance and
follows the slope of the impermeable boundary (Figure 3-2(B)) (Schwille, No
Date).

Saturated Zone.  In the saturated zone below the water table, the
interstices are completely saturated with water (termed groundwater) which
exists in excess of atmospheric pressure.  Groundwater flows under the
prevailing hydraulic gradients.  Both free product and dissolved substances
can migrate in the saturated zone.  Dissolved substances include miscible
substances and immiscible substances which dissolve at saturation
concentrations.  Dissolved substances can move with the groundwater flow
and can travel long distances.  The mechanisms that govern transport of a
contaminant in the saturated zone are advection - the movement of
contaminant with the mean groundwater flow - and dispersion - a mixing
                                    3-11

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process that occurs on molecular (due to diffusion), microscopic (due to
variations in velocity within individual pores), and macroscopic (due to
soil heterogenieties) levels.  The transport of a dense (S.G. > i) free
product in the saturated zone will be downward due to gravity.  This
downward movement stops when the product is exhausted or reaches an
impermeable boundary.  A free product may also move laterally in the
saturated zone under the influence of horizontal hydraulic gradients.  A
blob of immiscible fluid of lower viscosity than water will move more
rapidly than water.   If the product reaches an impermeable boundary, it
forms a pool that flows along the slope of the boundary.  As free product
moves through the saturated zone it can  leave a trail  of contaminated
soil.  Movement of the free product in the saturated zone has many
similarities to this process in the unsaturated zone.  The contaminant is
free to dissolve to its saturation concentration.

3.1.2  TRANSPORT DIAGRAMS

Following are several schematic diagrams showing the influence of many of
the variables discussed above on the migration patterns of substances
leaking from USTs.  Figure 3-4 depicts the influence of stratification of
the unsaturated zone on the distribution of a contaminant.  The excavation
zone around a tank is assumed to be more permeable than the surrounding
natural soils.  This figure shows that the different in permeabilities
between the "disturbed" excavation zone and the undisturbed soil is often
large enough that the leaked substance pools (builds up hydraulic head) in
the excavation zone before moving into the undistrubed  soil.

Figure 3-5 presents three schematics of contaminant with different physical
properties moving through porous media.

    (A)  Miscible substance - moves through the subsurface as a single-
         phase fluid.
    (B)  Immiscible substance (S.G. < 1) - moves through the unsaturated
         zone as a second phase.  Note that the infiltration rate is high
                                    3-12

-------
                                                       GROUND SURFACE
                                 TANK
       /UNSATURATED £.,•-•:•,  -
          ZONE   f: .;;•-. ,-;• -. ..'.
                                                     _  EXCAVATION ZONE
                                                     -; (HIGH PERMEABILITY)
                                                INTERMEDIATE PERMEABILITY i
 SOURCE: Adapted from Convery (1979)
CAMP DRESSER & McKEE iNC.
                  FIGURE 3-4
SCHEMATIC PLUME SHOWING THE INFLUENCE OF
    STRATIFICATION OF THE UNSATURATED

    ZONE ON CONTAMINANT DISTRIBUTION

           3-13

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              (A)
            miscible
               (B)
            Immiscible
             SG<1
               (C)
            Immiscible
              SG<1
                                                               Ground surface
                                                            . Unsaturated zone
                                                                    Saturated  •.':.'..
                                                                      zone   \'"i'
                                                                 Ground surface
                                              Second phase
           Unsaturated zone

                 Capillary zone
                                              dissolved phase -"""."""saturated   •:..
                                                                      zone
   Second              .       i'::lv>\"
   Phase                     ••.'••':'.':•':
            Unsaturated zone   /••'•'.•••'•'•
                 Capillary zone  '•:'.'.;•:':/.

          dissolved    '       : ;'•'/: ••'/ -
          products  saturated   v/g*
          -_._„zone:'••'.'•'•'''''':.
                                                   Impermeable
SOURCE:B,C taken from Schwille (No Date)
K - hydraulic
   conductivity
   CAMP DRESSER & McKEE INC.
                 FIGURE  3-5
 MIGRATION PATTERNS IN  POROUS
           SUBSURFACE MEDIA

3-14

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         enough to cause the free product to be forced below the water
         table under the infiltrating core.
    (C)  Immiscible substance (S.G. > 1) - moves through the unsaturated,
         capillary and saturated zones as a second-phase fluid to form a
         mound on the impermeable boundary.  While lateral  spreading of the
         mound occurs in all directions, the predominant direction is with
         the slope of the impermeable boundary.  This figure assumes a
         static groundwater condition and also shows the influence of
         stratification of the saturated zone on the dissolved plume (k is
         hydraulic conductivity or permeability of the strata).
Figure 3-6 presents the contaminant distribution of a miscible substance in
a porous fractured aquifer.  This figure shows how the contaminant
primarily migrates along the fractures and also diffuses into the pores of
the rock.  As time passes, the zone of contamination will  diffuse farther
into the porous matrix (Freeze and Cherry, 1979).

A comprehensive and more detailed description of the transport processes in
each of the major subsurface zones follows in Sections 3.3 through 3.5.
Section 3.2 discusses Dairy's Law—the basic equation governing groundwater
flow.  For the reader not interested in the detailed technical discussion
of transport mechanisms which follows, you may want to move on to the
discussion of fate mechanisms in Section 4.0.

3.2  DARCY'S LAW

3.2.1  GROUNDWATER FLOW IN THE SATURATED ZONE

Darcy's Law is the foundation of quantitative theory of the flow of fluids
through porous media.  Darcy, who conducted experiments in sand filters,
concluded that the rate of flow of water, Q, through the filter bed was
proportional to the area, A, of the sand and to the difference Ah, between.
the fluid heads at the inlet and outlet faces of the bed,  and inversely
proportional to the distance, L, between inlet and outlet, or expressed
analytically that:
                                    3-15

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                                       GROUND SURFACE
                     DIFFUSION
                     POROUS ROCK       	.
                                  	     SECONDARY
                                        . OPENINGS ALONG
                              TX'.C^r7:^ BEDDING PLANES
                             '""•"" '""' """"     AND FRACTURES
SOURCE: Adapted From Freeze and Cherry (1979)
CAMP DRESSER & McKEE INC.
                 FIGURE 3-6
SCHEMATIC REPRESENTATION OF CONTAMINANT
          MIGRATION FROM AN UST
 THROUGH FRACTURED POROUS LIMESTONE
                                  3-16

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               Q = cA Ah
                     L

where c is a combined constant of proportionality characteristic of the
sand.  There are many variations of this equation which takes into account
the properties of the media, the fluid and the type of driving force.
These equations and factors, which affect their values in the subsurface,
are the subject of this section.

The flow of groundwater in any direction in the saturated zone is described
by:
               Q--KA*
where:         h = the hydraulic head (L)

               — " the hydraulic gradient (dimensionless)
               dl

               K = the hydraulic conductivity (L/T)
                                              p
               A = the cross-sectional  area (L )

               Q = the flow rate (L3/T)

The negative sign is the result of defining dh in the same direction as  dl

This equation is often written as:
                q  ==  -  K-
                q    A      K dl

where q (L/T) is defined as  specific discharge or Darcy velocity
                                    3-17

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Specific discharge is a macroscopic concept; i.e., it refers to the average
velocity of water across a cross-sectional area of porous medium and must
be clearly differentiated from the microscopic velocities associated with
the actual paths of individual particles of water as they wind their way
through the grains of the medium (Freeze and Cherry, 1979).  These
differences are illustrated in Figure 3-7.

While specific discharge has units of velocity, it does not define the
average travel time of water between two points.  Because the grains of the
media occupy much of the volume in the saturated zone, water flows only in
the interconnected pore spaces of a medium.  The fraction of the total
volume of soil media available for the flow of water is called the
effective porosity, n .  The effective porosity is generally smaller than
the total porosity (the volume of voids/total  volume) of a medium because
some of the voids are not interconnected.  To calculate the average
velocity of water, the following expression can be used:
                   e   ne
where V  is known as the mean effective or seepage velocity (L/T)

3.2.2  DISSOLVED CONTAMINANTS IN THE SATURATED ZONE
In general, the advective movement of dissolved (in groundwater)
contaminants in the saturated zone can be described by Darcy's Law (as
described above).  Dispersion -- a mechanical  mixing process -- is also
important to describing to migration of dissolved  contaminants.  Dispersion
is discussed in Section 3.5.2.  The simpler version of Darcy's Law
presented above holds only if the properties (density and viscosity)  of the
groundwater with dissolved contaminant are not significantly different than
water.  The hydraulic conductivity (K) includes both the properties  of the
fluid and the media, or:
                                     3-18

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                  Q
OJ
I
      SOURCE: Freeze and Cherry (1979)
     CAMP DRESSER & McKEE INC.
                 FIGURE 3-7
MACROSCOPIC AND MICROSCOPIC CONCEPTS OF
            GROUNDWATER FLOW

-------
                         K = kpg//a
                                                    ^
where k = the intrinsic permeability of the media (L~)
      P = the density of the fluid (M/L3)
                                         2
      g = the gravitational constant (L/T )
      M = the dynamic viscosity of the fluid (M/LT)

The relationship between intrinsic permeability and hydraulic conductivity
for various media is shown in Table 3-1.  These terms are often shortened
to permeability (intrinsic permeability) and conductivity (hydraulic
conductivity) and these shortened terms are generally used herein.  Darcy's
Law can thus be written as:
The discussion to this point has assumed that the flow of groundwater can
be calculated in any direction.  In field situations, the permeability of
the medium varies.  These variations are primarily caused by
heterogeneities and anisotrophy.  In an aquifer 'that is heterogeneous, the
permeability is dependent on the position within a geologic formation.
Freeze and Cherry (1979) define three types of heterogeneities:  layered
discontinuous, and trending.  Examples of these are given below.

    •  Layered -  A stratified soil common in consolidated or
       unconsolidated sedimentary deposits.
    •  Discontinuous - Interlayered deposits of clay in sand, the presence
       of faults, the overburden-bedrock contract.
    •  Trending - Sedimentation process that creates deltas or glacial
       outwash plains.
When the permeability of a medium is dependent on the direction of
measurement, the medium is anisotrophic.  The most common form of
a.nisotrophy is transverse isotrophy which means that the permeability in
the horizontal direction is different than the vertical.  Typical
                                     3-20

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                      .  TABLE  3-1
TYPICAL VALUES OF HYDRAULIC CONDUCTIVITY AND PERMEABILITY
— log,,, • K' (em seel —
Permeability
Aquifer
Soils
Roeks
-Ing,,, -Kern2)
2 -
1

) 1
1
Pervious

Clean
gravel
t
M
( 4
1
S (
Semipervious
Good
Clean sand or
xand and gravel


.
'

<
6
1
5 -1

7 S 4 10 II
1
lincvervious
Poor
Very line sand,
loess, loam, soK
Peat
sill.
net/
Stratified clay
Oil roeks
).
3

) 1
Sandstone
0 1
(
1
None

tJnwcalhcred c ay
Good
,. Breccia,
limestone,
. . . granite
dolomite
: 1> 14 15 16
1
-: -.1 -4 -5
  Source:   Bear (1979)
                         3-21

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horizontal  to vertical  anisotrphy ratios are from 10:1 to 3:1 (Freeze and
Cherry, 1979).
3.2.3  IMMISCIBLE FLUID FLOW

The flow of two or more immiscible fluids (e.g.,  air and water or air,  oil
and water) is more complex because of the need to consider the interactions
between the fluids and between the fluids and the soil  matrix.  These
concepts are discussed in detail  in Section 3.3.1 but can be summarized as
follows.  The flow of immiscible  fluids depends on capillary pressures
which are the result of surface tensions between the fluids, and the
saturation levels of each of the  fluids which determines the relative
permeability of the fluid in the,medium.  Darcy's Law in this case is
expressed for each fluid as:
where:  kr = the relative permeability of the fluid (it varies from 0  to 1
             and is determined from relative permeability graphs (Section
             3.3.2).

        ^£ = the pressure gradient
        dx

        d£ = the change in elevation over the distance travelled
        dx
When the pressure gradient is 0,  and flow is vertically downward,  dz  =  -1,
                                                                  dx
and the movement of these fluids is expressed by:
                    q =	r-
                           M
                                    3-22

-------
where  v is the kinematic viscosity of the fluid.  In Section 3.3.2, this
version of Darcy's Law is used to express the relative (to water) rate of
movement of immiscible substance in the unsaturated zone.

3.3  TRANSPORT OF LIQUIDS IN THE UNSATURATED ZONE

3.3.1  OVERVIEW

In the unsaturated zone, the transport of a substance leaking from an LIST
can occur as a solute in the water phase, a constituent in the immiscible
phase, or vapor in the air phase.  Only transport mechanisms for the first
two conditions (liquids) are presented in this section;  vapor transport  is
presented in Section 3.6.

Transport of fluids in the unsaturated zone is a more complex phenomena
than in the saturated zone, which under uncontaminated conditions is
described by single-phase flow mechanics.  This complexity results because
fluids in the unsaturated zone are immiscible, i.e.,  the fluids displace
each other without mixing, and there is a distinct fluid-fluid interface
within each pore (Freeze and Cherry, 1979).  Thus, transport of liquids in
the unsaturated zone is described by multi-phase flow phenomena.  Subsets
of multi-phase flow can be defined depending on the number of immiscible
fluids involved.

Under uncontaminated conditions in the unsaturated zone, the voids are
partly filled with water at less than atmospheric pressure,  the remainder
of the pore space being taken up by air.  Two-phase flow phenomena govern
the movement of these fluids, or more generally, any  two immiscible fluids.
The introduction of a contaminant miscible with water (a distinct interface
does not exist between the water and the contaminant) will  also be governed
by two-phase flow, neglecting possible density differences.   When another
fluid that is immiscible with water and air (e.g., oil  or pure solvent) is
introduced into a soil medium, three-phase flow phenomena describe their
                                     3-23-

-------
movement.  The terms immiscible substance or oil  will  be used for this
fluid.

Besides the parameters of the fluids and the soils, the physics  of
multi-phase flow involves taking into account relative permeablities and
capillary pressures, which depend on the saturations of the various fluids
and their movements (Fried et. al.,  1979).  The physics of unsaturated zone
movement is presented in Section 3.2.  Section 3.3 discusses the use of the
physics as it relates to leaking substances from USTs.

The unsaturated zone (also called the zone of aeration) is located above
the water table and above the capillary zone (Freeze and Cherry, 1979).
When it rains, water infiltrates the weathered soil zone and then
percolates through the vadose zone.   Simply described, water percolates
through the unsaturated zone, traveling primarily through the large voids
in the soil matrix, and is retained  in the smaller voids by capillary
forces.  In general, as the pore size increases,  the percolation rate
increases.  Depending on the pore size and distribution, percolation rates
range from inches per hour to inches per year.  The moisture content of
these zones is less than the porosity of the soil medium and is  described
in more detail below.

The moisture content in the weathered soil zone is relatively dynamic and
is affected by conditions at the ground surface (seasonal  and diurnal
fluctuations of precipitation, irrigation, air temperature and humidity),
and/or by the presence of a shallow  water table (Bear, 1979).  The vadose
zone (also called the intermediate zone) is generally  envisioned as the
region of the unsaturated zone where the moisture content is not changed
rapidly by changing conditions at either the ground surface or the water
table (Stallman, 1964).  The thickness of this zone is dependent on the
depth of the water table below the ground surface.  The moisture content of
this zone, after an extended period  of gravity drainage without  the
introduction of additional water at  the surface,  is at the irreducible
saturation of water (also called field capacity).  This water is held by
                                    3-24

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hygroscopic and capillary forces  (Bear, 1979).  It is recognized that
under certain environmental  conditions (e.g., desert regions where it has
not rained for a long period of time or possibly under a building or a
parking lot) the moisture content of the soil in this zone may be very low.

3.3.2 THE PHYSICS OF UNSATURATED ZONE CONTAMINANT TRANSPORT

Transport in the unsaturated zone is very difficult to predict
quantitatively under field conditions.  The flow of one immiscible fluid is
impeded by the presence of the other fluids.  This section presents a
description of two- and three-phase flow conditions.

Two-phase Flow

This section describes the movement of two fluid phases that are immiscible
with each other in the unsaturated zone.  Although the concepts presented
here were largely developed  by petroleum engineers for oil  and water, they
can be extended to describe  the transport of any two immiscible fluids in
the unsaturated zone.  Two-phase flow also partly describes the transport
mechanisms govering movement of an immiscible substance below the water
table, but because other factors also affect transport, this condition is
considered in Section 3.5.

Two-phase flow is not completely quantitatively understood.  Four factors
have been selected to provide a qualitative description of two-phase flow
mechanisms:  wettability, capillary pressure, moisture retention, and
relative permeability.

Wettability.  In the case of air-water movement, air is the nonwetting
fluid and water is the wetting fluid.  Movement of two immiscible fluids in
the unsaturated zone is described in terms of this distinction.  The
determination of which immiscible fluid wets a solid depends on the contact
angle between the solid surface and the liquid-gas or liquid-liquid
interface, measured through  the denser fluid as shown in Figure 3-8.  When
                                    3-25

-------
                          NONWETTING FLUID
                           (LIQUID OR GAS)
                                                             WETTING FLUID
                                                                (LIQUID)
                                                     CONTACT
                                                      ANGLE
                                       SOLID
                               EXAMPLES:

                            NONWETTING FLUID  WETTING FLUID
                                  AIR
                                  AIR
                                  OIL
                              PURE SOLVENT
         WATER
           OIL
         WATER
         WATER
SOURCE: Adapted From Bear (1979)
 CAMP DRESSER & McKEE INC.
              FIGURE 3-8
DETERMINATION OF WETTING AND
        NONWETTING FLUIDS


3-26

-------
the contact angle is less than 90 degrees, the denser fluid wets the solid;
when the contact angle is greater than 90 degrees, the denser fluid is the
nonwetting fluid. The contact angle is a function of the interfacial
tension between the fluids.  A more detailed discussion of wettability can
be found in Bear (1979).

Four stages (or regimes) of the saturation of water relative to air (or any
two immiscible fluids) can be distinguished and are depicted in Figure 3-9:
pendular, insular, funicular and saturated.  The following description was
taken from Bear (1979) and Convery (1979).

At very low saturations, water exists as pendular rings around grain
contacts within the porous medium (Figure 3-9 (A)).  These rings of fluid
are completely isolated from one another, and do not form a continuous
water phase, except for a very thin film of water of nearly molecular
thickness (that does not behave as ordinary liquid water) on solid
surfaces.  Hydraulic pressures cannot be transmitted through water in the
pendular regime, since it is not continuous.

If the saturation of the wetting phase increases, the pendular rings expand
until a continuous water phase is formed.  Flow of water is then possible.
There is a coincident decrease in the saturation of air.  The saturation
regime is called funicular (Figure 3-9 (B)).  The phase distribution and
flow behavior of fluids in the funicular regime are complex, and are
strongly a function of the saturation history of the porous medium.

With increasing water saturation, air eventually becomes discontinuous and
is commonly isolated in the larger pores of the medium.  Air is in a
condition of insular saturation (Figure 3-9 (C)).  The air globules will
become mobile only if a pressure discontinuity exists across them within
the water to force them through capillary restrictions.  Otherwise, the
droplets are immobile and remain trapped within the pores.  The insular
drops will impede flow of water to some extent.
                                   3-27

-------

                                                             I
                 (A) PENDULAR
                        (C) INSULAR
                 (B) FUNICULAR
                     (D) SATURATED
                     WATER
                         AIR
SOURCE: Adapted From Convery (1979)
CAMP DRESSER & McKEE INC.
                FIGURE 3-9
 FLUID SATURATION STATES (REGIMES) OF
A TWO-PHASE SYSTEM  FOR A HYDROPHILIC
             POROUS MEDIUM
                                  3-28

-------
Under saturated conditions (Figure 3-9 (D)), one fluid, in this case water,
completely occupies the interstices of the media.

Capillary Pressure.  When two immiscible fluids are in contact in a soil
medium, there is a distinct fluid-fluid interface with each pore.  Because
of the interfacial tension which exists between these fluids, a
discontinuity in pressure exists across the interface separating them.  The
magnitude of the pressure difference (called capillary pressure) depends  on
the curvature of the fluid-fluid interface inside the pore space.  In a
general two-phase system, the capillary pressure (P ) is the difference
between the pressure on the nonwetting side of the interface and that on
the wetting side (Bear, 1979).

                    c ~ P nonwetting ~ P wetting

The capillary pressure is thus the tendency of a partially saturated porous
medium to suck in the wetting fluid or to repel the non-wetting fluid.  The
capillary pressure has a negative value and this is often called suction  or
tension (Bear, 1979).

These principles are illustrated below, for the case of a gasoline-water
system.  The example was taken from Williams and Wilder (1971).

When gasoline is at an insular state of saturation, it is discontinuous (in
globules or bubbles) and is primarily located in the larger pore spaces
where it is surrounded by water.  Figure 3-10 shows the pressures at such
an interface between two soil particles.  For equilibrium to exist:

                  Pgas - Pw = Pc =  2ocos9
                                      rc
                                    3-29

-------
                    GASOLINE
                                     ;:•;•.: '•'•• • .'.';•'..•.'•;'•:'.'.•'••..•'. WATER ."•
                                      '               ''
                                                  NECK OR
                                                PORE THROAT
SOURCE: Williams and Wilder (1971)
 CAMP DRESSER & McKEE INC.
                FIGURE 3-10
PRESSURES ON  A CAPILLARY INTERFACE
                                    3-30

-------
where:  P    = Pressure on the gasoline side of the interface
        P  =   Pressure on the waterside of the interface
         W

        P  =   Capillary Pressure
         c

         a =   Surface energy (a function of the fluids)

         9=   Angle of contact between the denser fluid and the soil.

        rc =   Radius of the capillary

Until the pressure drop across the interface is greater than the capillary
pressure (P_,_ - P.. > P_) , the globule of gasoline will  not move through
           ^uS    W    L
the pore throat.  If gasoline is present at this state of saturation in a
porous medium on a macroscopic scale, it is immobile.  At the macroscopic
scale, the average capillary pressure is equal  to the difference between
the average pressure in the water and that in the gasoline (Bear, 1979).

Moisture Retention.  Moisture retention curves indicate how water is
retained in a soil  medium by capillary forces against gravity.  The
pore-size distirbution and pore shapes of a porous medium affect the shape
of the retention curve (Bear, 1979).  Figure 3-11 shows typical  moisture
retention curves for drainage of (A) a well graded and poorly graded soil

(schematically); and (B) of various soils (experimentally).  The important
aspects of these curves are:

    •  Critical  capillary head - Point A in Figure 3-11 (A) is the point at
       which the larger pores of a saturated medium will  start to drain.
    •  Irreducible water content - Point B in Figure 3-11 (A) is the water
       content level at which a certain quantity of water remains at the
       pendular saturation level even at very high capillary pressures
       (Bear, 1979).
                                    3-31

-------
0
<
UJ
i
UJ
DC
D
V)
(/)
U)
oc
a
£'
<
cu
<
u

!
\
\
\ /-Well-graded soil
V^
\
\
\
\ I

V ^v
\, || N.
^^^^^^ N.
i\
/ Vi
Poorly graded soil \U
0 B,, B, .












*n




WATER MOISTURE CONTENT
(A) SCHEMATIC CURVES




3
/'/•
2

n c-


md < 500 u
W a X« 	 v 	 Ramona sand
V2 \ \ 	 H 	 Placentia clay loam
|\\ o • 	 A 	 Hanford/ sandy. loam
v-^ \ \ 	 a 	 Yolo clay loam
TV +\ °v • 	 • 	 Chino silty clay loam
i\\\ \
K\ \ \
n \ \ov \
\\ \ \X \
Vj^x \ \ \ X-
^^H, \1 \ \
% ^\ ° \
r * V \
I I I I I I I
0 0 20 30 40 50 60 70
i i
80 90 1 00
WATER SATURATION (%)
(B) CURVES OBTAINED DURING SATURATION
SOURCE: Bear (1979)

FIGURE 3-11
CAMP DRESSER & MCKEE INC. TYPICAL RETENTION CURVES IN SOIL
DURING DRAINAGE
3-32

-------
    •  Irreducible water saturation - Analogous to irreducible water
       content.Figure 3-11 (B) shows that values of irreducible water
       saturation are approached at lower capillary pressures for more
       permeable than less permeable soils.
Different moisture retention curves would be obtained for the wetting
(imbibition) than for drainage of a porous medium.  This phenomena is
called hysteresis.  As shown in Figure 3-12 a higher moisture content is
obtained when a soil is being drained rather than during imbibition (Bear,
1979).  Hysteresis is more complicated than depicted in Figure 3-12 because
the drying or wetting process can start at any point on the boundary
curves.  This leads to drying and wetting scanning curves which are drawn
between the boundary curves.  The interested reader is referred to Chapter
6-1 in Bear (1979).

Relative Permeability.  The concept of relative permeability is perhaps
most easily understood as follows.  Because two immiscible fluids occupy
the voids of a medium, only a portion of the voids is available for flow of
each fluid.  In effect, the permeability of the formation to one of these
fluids is dependent on the. saturation level  of the other fluid.

Early researchers assumed that two immiscible fluids flow simultaneously
through a medium;   each fluid established its own  set of tortuous paths
through the medium, which were assumed to be relatively stable.   This
would imply that the relationship between fluid saturation and relative
permeability would be linear.

However, as discussed above, the effects of capillary pressure must also be
considered.  The relationship between permeability and saturation is then
better described as a cubic parabola.   The relationship of the relative
permeability of water with saturation as established by experiments by
Wyckoff and Botset (1936) is shown in Figure 3-13.   This figure indicates
a rather rapid reduction in the permeability as moisture content decreases
from saturation.  Essentially,  the large pores drain first so that the flow
takes place through the smaller pores.   At point A on this figure,  water is
at irreducible saturation or is held at a pendular state (Bear, 1979).
                                    3-33

-------
              UJ




              UJ

              CC
              UJ
              oc
              0.


              I
                                   IMBIBITION (OR WETTING)





                                   . BOUNDARY DRYING CURVE
                           BOUNDARY

                           WETTING CURVE
                               DRAINAGE (OR DRYING)
                                                                 STARTING WITH A


                                                                 SATURATED SAMPLE
                                 MOISTURE CONTENT
                                            ENTRAPPED AIR

                                            (MAY BE REMOVED WITH TIME)
SOURCE: Adapted From Bear (1979)
 CAMP DRESSER & McKEE INC.
               FIGURE 3-12

    EFFECT OF MOISTURE CONTENT


HYSTERESIS FOR A COARSE MATERIAL
                                    3-34

-------
                                   MOISTURE CONTENT
                  1.0
             DC
             UJ


             1
             m
             <
             UJ
             5
             oc
             UJ
             Q.

             UJ



             1

             UJ
             oc
0.75
0.50
0.25
oc



1

HI
_i
CD

O
Z>
Q
UJ
DC
CE
                               25
                                        50
                               75
                                100
                                    SATURATION (%)
SOURCE: Bear (1979)
 CAMP DRESSER & McKEE INC.
                             FIGURE 3-13

                RELATIVE PERMEABILITY OF WATER

                       IN UNSATURATED SAND
                                 3-35

-------
Figure 3-14 depicts schematically the relationship between relative
permeability and saturation level for both a wetting fluid and nonwetting
fluid.  The interesting features of this figure are (van Dam, 1967 and
Convery, 1979):

     •  The permeability of a given porous medium to one fluid in the
        presence of another fluid is reduced with respect to single-phase
        permeability.
     •  The reduction in permeability is dependent on the wetting of the
        porous medium by one of the two fluids.
     •  A minimum saturation for each fluid is required before the medium
        is permeable to that fluid, i.e., the fluid can flow.  This
        saturation is called the residual saturation of the fluid (or
        primary retention value).  For water, the residual saturation is
        often termed the irreducible saturation.  Residual saturation is
        discussed in detail in Section 3.3.3.
     •  Throughout most of the saturation range, the sum of the relative
        permeabilities is less than unity.

With respect to Figure 3-14, Stallman (1964) notes that these relationships
may not hold for the water-water vapor and air system or for other
relatively volatile fluids.  Under these conditions, the interaction
between the liquid and its vapor state may preclude their separation and
treatment as fluids in the pore space.  For a further discussion of vapor
movement, see Section 3.6.

Figure 3-14 also shows the degree of water saturation in terms of the fluid
saturation regimes defined previouisly.  Using water (the wetting fluid)
and oil (the nonwetting fluid), Williams and Wilder (1971) described
schematically the movement of two immiscible fluids in a soil matrix.
These stages are shown in Figure 3-15.

In Figure 3-15 (A), water is at irreducible saturation or at the pendular
state.  It occupies the small  interstices in the soil  matrix and is
                                     3-36

-------
                    100% -
                   1.0
                    0.5
                                                1.0
                                                 0.5

                                               100%
SOURCE: Van Dam. 1967
 CAMP DRESSER & McKEE INC.
                FIGURE 3-14
VARIATIONS OF RELATIVE PERMEABILITY OF
      WETTING (Krw) AND NON-WETTING
      (Krnw) FLUIDS  WITH SATURATION
                                 3-37

-------
                    A) PENDULAR REGIME
                        OIL FLOW ':
                            —HYDRAULIC GRADIENT
                    B) FUNICULAR REGIME
                          OIL FLOW
                                          WATER FLOW
                               •HYDRAULIC GRADIENT
                    C) INSULAR REGIME    WATER FLOW
                              -HYDRAULIC GRADIENT
    \^'0\   OIL PHASE
          WATER PHASE
SOURCE: Williams and Wilder (1971)
 CAMP DRESSER & McKEE INC.
             FIGURE 3-15
DISTRIBUTION OF FLUID FLOW FOR
THE THREE SATURATION REGIMES
                                  3-38

-------
immobile.  Oil  occupies most of the available void space, particularly the
larger pores and flows at its relative permeability.

Water held at the funicular state is at a higher saturation level  than at
the pendular state.  It now is present as a continuous fluid.  As  shown in
Figure 3-15 (B), both oil and water flow in the medium under these
conditions.

When oil  is in  the insular state, water occupies most of the void  space.
The oil phase is discontinuous and does not contact the solid, which is
wetted with water.  Oil under these conditions generally does not  flow.
Figure 3-15 (C) shows the water is the flowing fluid.

The relative permeability to a fluid at a given saturation depends on
whether that saturation is obtained by approaching from a higher or lower
value (i.e., the medium is wetting or drying).  This phenomena is  called
hysteresis.  Therefore, there is no unique value of relative permeability
for a given saturation.  Figure 3-16 depicts the effects of hysteresis on
relative permeability.

Three-phase Flow

When a third immiscible is added to the two-phase flow system described
above, then three-phase flow phenomena describe the migration process.
Although three-phase flow is more complicated, due to complex boundary
conditions and  the hysteresis phenomena, it follows the same principles of
two-phase flow.  It should be noted that the transport mechanisms  discussed
here are applicable to any immiscible contaminant substance, e.g.,
gasoline, fuel  oil, and chlorinated solvents; however, crude oils  and  other
petroleum products that have to be heated to flow behave differently.   When
these substances cool to a lower temperature, than at a which all
components are  dissolved in each other, they do not behave as Newtonian
fluids (Schwille, No Date).
                                     3-39

-------
                     100%
SOURCE: van Dam (1967)
 CAMP DRESSER & McKEE INC.
            FIGURE 3-16
EFFECT OF HYSTERESIS ON RELATIVE
          PERMEABILITY


   3-40

-------
In a three-phase flow system, water is the wetting fluid relative to air or
oil, and oil is the wetting fluid relative to air.  This is shown more
clearly below.

         Wetting Fluid                  Nonwetting Fluid

           Water                            Air
           Water                            Oil
           Oi 1                              Air

As with the concept of relative permeability in a two-phase flow system,
the pores of a soil medium are occupied by air, oil, and water;  Figure
3-17 is a schematic of three immiscible fluids  (air, water, and oil) in a
soil pore.  Each of these fluids sees a relative permeability of the medium
depending on the saturation level of each fluid.  The relationships between
relative permeability and saturation level are  shown in the triangle
diagram (Figure 3-18).

The most important aspects of this diagram are  illustrated  in Figure 3-18
(B).  They show:

    •  There is a very small region where all three fluids  can flow
       simultaneously, and thus, that are even  larger areas where at least
       one of the fluids is immobile.
    •  The zones where each fluid is immobile or at its residual
       saturation.
As mentioned at the beginning of this section,  transport in the unsaturated
zone is very difficult to characterize quantitatively under field
conditions.  Numerical techniques are available to solve the system of
differential equations which govern unsaturated zone movement but
computations may not be practical because of the fine discretization
required.  Other important complicating factors are: (1) the lack of data
for the soil hydraulic parameters in the system of equations; and (2)
possible irregularities of boundaries and or heterogenity of medium.  Even
                                    3-41

-------
SOURCE: van Dam (1967)
 CAMP DRESSER & McKEE INC.
             FIGURE 3-17
SCHEMATIC DISTRIBUTION OF AIR, OIL,
   AND WATER IN A POROUS MEDIA
                                 3-42

-------
               (A)
                        100%
                           	 Air

                           — Oil
                           	Water
                (B)
                      WATER
                       100%
        AIR   Immobile Saturation
        100%
                    OIL
                    100%
 SOURCE: van Dam (1967)
CAMP DRESSER & McKEE INC.
                 FIGURE 3-18
RELATIVE PERMEABILITY AS A FUNCTION OF
  SATURATION FOR AIR-OIL-WATER SYSTEM
                                    3-43

-------
so many useful concepts and relationships using the parameters described in
this section have been developed to aid in the prediction of transport
mechanisms.  These concepts and relationships are presented in the next two
sections.

3.3.3 USE OF THE PHYSICS AS IT APPLIES TO LEAKING USTS

When a substance leaks from an UST, the most important transport question
to be answered is:  what is the maximum migration of a contaminant?  In
relation to the unsaturated zone, this question can be divided into two
parts:  (1) how much of the unsaturated zone media will become contaminated
and at what rate; and (2) will the contaminant reach the water table?
Because answers to these questions are not known in a quantitative sense,
following is a qualitative discussion of the important parameters.  Section
3.3.4 presents some estimation techniques.

The mobility of a substance in the unsaturated zone depends on:

    •  viscosity, which affects the velocity of the percolation process;
    •  quantity released and infiltration process, which may have an effect
       on the percolation process;
    •  permeability of the medium, which affects the spill  geometry; and
    •  for immiscible substances, residual saturation levels,  which is the
       retentive capacity of the soil medium.
Viscosity

The viscosity affects the velocity of the flow process through the Darcy
equation which shows that velocity is inversely proportional to viscosity
(see Section 3.2).  Therefore, heavy oils do not readily penetrate the
soil;  whereas, lighter products, gasoline in particular, move-through the
soil more easily than water does (Clean Environment Commission, No Date).
Section 5.1 applies these concepts in grouping petroleum products by
kinematic viscosity.
                                    3-44

-------
In dry soils (e.g., deserts or under buildings), essentially  two-phase  flow
phenomena (air and oil) govern mobility.  The  relative  permeability  of  the
immiscible substance can be used in Darcy's Law, if  saturated flow is
assumed.  This means that enough oil has to be  released to  displace  the air
to its residual saturation level.  In this case, velocities of percolation
are inversely proportional to kinematic viscosity, all  other  parameters
being equal  (Schwille, No Date).

     Velocity of contaminant -  -JCJIgffltjcjflscosUy of water	(velocity of Water)
                            Kinematic viscosity of contaminant

TP. is relationship assumes that gravity forces are dominant,  >.e.,  the
influence of pressure gradients on a flow is negligible.  For example,  a
                                             o
light heating oil (kinematic viscosity = 4 mm /second)  will move  4 times
slower than water, and trichloroethylene (kinematic  viscosity =
     o
0.4mm /second) will move 2.5 times faster than water.   Schwille (No  Date)
notes that modeling studies confirm these results.

The viscosity, and thus pecolation velocity, can also change  over  time  due
to "weathering" of a leaked substance.  With respect to the  lighter
petroleum products (those with a larger volatile content), the loss  of
volatile compounds to evaporation or solubilization  will  increase  the
viscosity and slow migration.  This phenomena is probably most important in
environments where the depth to the water table is large.

Quantity Released

Movement in the unsaturated zone occurs because of hydraulic  gradients
which are the sum of gravitational forces and pressure-head gradients
(Freeze and Cherry, 1979).  If the quantity released is  large enough to
achieve saturated conditions and the infiltration rate  exceeds that  which
can be moved by gravity, the pressure-head gradient  from  the  pooled
                                     3-45

-------
substance will increase the the hydraulic head, and thus, the percolation
rate.

Spill Geometry

Transport in the unsaturated zone is primarily downward, due to
gravitational forces, but lateral spreading will also occur.  This section
address how the extent of contamination (spill geometry or shape of the
contaminant plume) varies with different subsurface permeability and
structure.

The extent to which the unsaturated zone will  be contaminated is affected
by different permeabilities of uniform media,  stratification or
heterogeneities in this zone and variations in water content.  Schematic
drawings of different plume shapes assuming the capillary zone is not
reached are shown in Figure 3-19.

In a uniform (homogeneous) media, the contaminant plume is pear-
shaped;   percolation is primarly vertical  and capillary forces act to
spread the plume laterally with increasing  depth.  Figures 3-19 (A) and (B)
show the differences in relative shape for  homogeneous soils that are
highly permable and less permeable, respectively.  In less permeable media,
capillary forces are more significant and the  plume is broader at its base
(CONCAWE, 1979).  Thus, a larger zone of contamination is expected in
tighter soils.

As shown in Figure 3-19 (C), the shape of a contaminant plume in a
stratified soil (assume these are sand layers) has an undulating
appearance.  As the contaminant moves from  a layer of high permeability to
lower permeability, it has to overcome more resistance to flow.  The result
is that the contaminant spreads out laterally  and mounds at the interface
between the layer until sufficient head is  reached to move it into the next
strata.  Conversely, with a change of strata from low to higher
permeability, the plume shape is laterally  contracted because the
                                   3-46

-------
         (A)
      (B)
(C)
          HIGHLY PERMEABLE
          HOMOGENEOUS SOIL
        LESS PERMEABLE
       HOMOGENEOUS SOIL
  .. . •  STRATIFIED SOIL WITH
     VARYING PERMEABILITY
 SOURCE: A, B, C FROM CONCAWE 1979
CAMP DRESSER & McKEE INC.
                  FIGURE 3-19
SCHEMATIC DIAGRAMS OF DIFFERENT CONTAMINANT
 PLUME SHAPES IN SOILS OF VARYING PERMEABILITY
	  IN THE UNSATURATED ZONE	
                3-47

-------
resistance to flow is lower.  Because of the significant lateral  spreading
in a stratified soil, there is a considerable reduction in the depth of
penetration of an equal  spill  volume in a stratified soil  than in a
homogeneous soil.

Heterogeneities of much  lower permeability than the surrounding soil (e.g.,
a clay lens in sands) can add a significant lateral spreading component to
the shape of a contaminant plume (Figure 3-19 (D)).  In this example, the
resistance to vertical  flow through the heterogeneity is large enough to
cause the contaminant to migrate laterally across the surface of  the
heterogeneity before continuing its downward migration.  If the zone of
much lower permeability  is continuous (a clay layer), the  vertical  movement
of a contaminant can be  arrested if it reaches residual saturation  as shown
in Figure 3-19 (E) or it travels to the surface at an outcrop (Figure 3-19
(F)).

As discussed in the previous section, the flow of the immiscible  substance
is only moderately reduced, compared to full saturation, when the soil
moisture content is less than  irreducible saturation.  This is because the
water is held in the less pervious areas of the pore space (Schwille, No
Date).  As water content increases, the relative permeability of  the
immiscible substance is  more highly reduced (see the curves for the
nonwetting fluid and oil in Figures 3-14 and 3-18 respectively).   If the
percolating contaminant  encounters an area of perched water (water  at
saturation in the unsaturated  zone), the movement of the contaminant will
be slowed as it tries to displace some of the water (plume shape  similar to
stratified soils) or it  will flow over the surface of the  perched water as
if it were like a clay  lens.  Schwille (No Date) notes that since the water
content of a media in the field is rarely known, the relative decline in
permeability can only be estimated to an order of magnitude.
                                    3-48

-------
Residual Saturation of Immiscible Substances

For immiscible substances, the residual saturation of the leaked substance
is a measure of the retentive capacity of a soil medium for the substance.
Residual saturation is defined as the minimum content a fluid must attain
in order to move in a porous medium, or alternatively, the threshold
content below which it is immobile (CONCAWE, 1979).  Note that residual
saturation levels are always much less than complete (100 percent)
saturation and other terms used to describle this phenomena - retention
capacity or specifc retention -  although they are less frequently found in
the literature, may better describe the phenomena.

While it is a relatively easy laboratory procedure to determine the
residual saturation of an immiscible substance, estimates extrapolated to
field conditions are less accurate unless a good understanding of the
variation of water content is developed.  In the absence of water, the
residual saturation of oil will be greater than when water is present.  As
water content increases, the residual saturation of oil becomes constant.
Some of the values of residual saturation reported in the literature are
listed below:

    •  Schwille (No Date) reported residual  saturation for oil in the
       unsaturated zone is typically 3-5 1/m  for high permeability media
       and 30-50 1/m  for low permeability media.
    •  He also noted that values for low molecular weight chlorinated
       hydrocarbons (solvents) had been determined to be on the same order
       of magnitude as oil.
    •  Dietz (1971) assumes  that immobile oil  saturation ranged from 10
       percent of the pore volume for light oils (gasoline), to 15 percent
       for medium oils (diesel, light fuel oil) to 20 percent for heavy  oils
       (lube oil, heavy fuel oil).

Convery (1979) makes a distinction between primary specific retention and
ultimate specific retention.  This distinction recognizes that the terms
"residual" and "immobile" are relative.  Primary specific retention
determines the spread of the immiscible substance and is the volume of a
                                   3-49

-------
nonwetting fluid retained after one passage of the fluid through the medium
followed by gravity drainage.  As water (a wetting fluid) repeatedly passes
through the medium, some of the nonwetting fluid will  be displaced.  This is
the ultimate specific retention and represents the long-term retention of
the medium.  Therefore, care must be used in selecting a value of residual
saturation because at the primary residual saturation the immiscible
substance may have some mobility left and the substance can be removed from
the pores of a medium by draining, solubilizing or vaporizing.

The residual  saturation of a leaked substance is primarily a function of
pore size (grain size and packing), pore size distribution (sorting), pore
shape (grain size) of the soil  medium and its water content.  The volume of
a liquid retained by a medium is an inverse function of grain and pore size
of the medium (Pfannkuch, 1983).  Because permeability of a medium varies
directly with grain size and sorting, an inverse relationship is also
expected between retention and  permeability and as indicated in observations
by Convery (1979).  Thus, in general, a tighter soil will retain more of a
leaked substance than a more permeable soil.

Column drainage experiments reported by Pfannkuch (1983) were conducted to
measure primary specific retention of mineral oil  under the following
conditions:

    •  Natural  soils representing geologic materials and conditions typical
       of the glaciated regions- of the north central and northeastern United
       States;
    •  Dry Ottawa sand; and
    t  Ottawa sand with water at its irreducible saturation.

The results for the specific geologic strata experiments are shown in Table
3-2.  In general, a broad trend of increasing primary oil retention with
decreasing grain size was indicated.  However, below silt-sized particles,
there was a dramatic drop in primary retention.  This  is believed to be
caused by mineralogical change.  Sand and silt consist primarily of quartz
                                    3-50

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                                  TABLE 3-2

                 PRIMARY RESIDUAL SATURATION OF MINERAL OIL
    Geologic Material

    Alluvium, Outwash  sand weathered
    Old Gray Till

    Sandy Till  (Red Till)

    Old Gray Till

    Loess
 Primary Residual Saturation
  	(% Pore Volume)

15.0 - 23.4%
25.0 - 27.3%

20 - 43%

41.2 - 51.5%
Source:   Phannkuch (1983)
                                   3-51

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and feldspar and have a low negative surface charge.  Clay minerals, which
have a smaller mean grain size, are characterized by high negative surface
charges.  Thus it was concluded that the mineral oil (non-polar and no
significant charge) might be replused by the clay minerals, reducing
retention.

The experiments with natural soils also attempted to define the role of
sorting on residual saturations.  This is difficult to do because the
tighter soils (highest retention) also tend to be the most poorly sorted.
Thus, the conclusion was simply:  higher primary retention occurs in finer,
more poorly sorted soils.

In the Ottawa sand experiments, similar relationships were seen for the dry
and "wet" sands between grain size and residual saturation of mineral oil,
although the primary retention of oil in the wet soil (with water at
irreducible saturation) was lower than the dry soil.  As shown in Figure
3-20, these experiments also indicate that the primary residual saturation
of mineral oil will be lower for an initially water saturated system
(characteristic of the saturated zone) than initially oil saturated systems
(characteristic of the unsaturated zone).

Immiscible substances which travel as a second-phase in the saturated zone
also have to exceed residual saturation levels before they can flow.  As
.shown in the experiments discussed above, the volume of nonwetting substance
retained in a porous medium will be smaller when the medium'is saturated
with water (characteristic of the the saturated zone) than when it is
partially saturated with water (characteristic of the unsaturated zone).
Figure 3-14, the two-phase flow relative permeability graph,  also
illustrates this principle.  The .residual saturation for water (the wetting
fluid at the pendular state) is less than the residual  saturation of oil
(the nonwetting fluid at the insular state).
                                    3-52

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  UJ
  N
  cc
  o
        2.50 I
        2.00 -
        1.50 -
        1.00 -
        .50 -
                           .9      1.0      1.1

                                 LOG RETENTION

                     • INITIALLY SATURATED

                     O INITIALLY DRY
                    1.2
 i
1.3
                                    1.4
SOURCE: Pfannkuch (1983)
CAMP DRESSER & McKEE INC.
                 FIGURE 3-20
COMPARISON OF PRIMARY LOG RETENTION
OF INITIALLY DRY AND SATURATED OTTAWA
        SAND BY COLUMN DRAINAGE
                                     3-53

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3.3.4 ESTIMATING RELATIONSHIPS

Two well-known estimation relationships have been developed which help
answer the questions stated at the beginning of Section 3.3.3 and repeated
below:
    •  How much of the unsaturated zone media will become contaminated and
       at what rate?
    •  Will the contaminant reach the water table?
Both relationships were developed by or for the petroleum industry for use
with crude oil and petroleum products.  Because residual saturation is the
key factor in both relationships, and Schwille (No Date) noted similar
values for residual  saturation of chlorinated hydrocarbons as petroleum,
these equations will  be assumed to apply to these immiscible substances as
well.   It should be stated, at the outset that no situations are known
where these equations have been applied in field situations and thus their
validity remains to  be demonstrated.

The first relationship was reported in Davis et. al . (1972), and is called
the saturation equation.  It is an estimation technique to determine the
volume of soil required to immobilize a spill.  This equation will  give a
conservative estimate of the required volume of soil because it does not
take into account the heterogeneous nature of field  conditions.

        soil required to attain      =  0.2V
        immmobile saturation            PSn
        (cubic yards)

where:  V  = the volume of oil spilled (barrels)
        P  = the porosity of the soil (percent)-
        Sp = residual saturation
             use 0.10 for gasoline, 0.15 diesel  and  light fuel  oil  and 0.20
             for lube or heavy fuel oil.
                                    3-54

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With regard to the rate at which the contaminant reaches the water table,
Shepherd (No Date) notes that one common misunderstanding is that it can
take months or years for a contaminant to reach the water table.  While
this can be true under certain environmental conditions, he states that
storage and petroleum handling facilities are typically located over
permeable sands or glacial sediments where the depth to the water table is
less than 25 feet.  Under these conditions and with a sufficient quantity
of release, he states that a leaked substance, such as gasoline, can reach
the water table in a few hours or, at most, a few days.

The second relationship reported by CONCAWE (1979) estimates the maximum
depth of penetration of infiltrating oil.  It is simply a more detailed
version of the previous equation which has been rearranged to determine the
depth of penetration.  This relationship assumes that oil infiltrated at
the ground surface and requires an estimate of the infiltration area.
Because the leaks of concern herein are from USTs, the applicability of
this relationship to this problem is suspect.  While it might be possible
to estimate the cross-sectional area of an infiltrating leak, this would
require accounting for the effects of the excavation zone around the tank.

                                 D = 1000V
                                     A R k
where:  D = maximum depth of penetration,  m
        V = volume of infiltration oil, m
                                   2
        A = area of infiltration, m
        R = retention capacity of soil  (1/m )  (see table below)
        k = approximate correction factor for  various oil  viscosities
            k = 0.5 for low viscosity products,  e.g., gasoline
            k = 1.0 for kerosene, gas oil  and  other products  with
                      similar viscosities.
            k = 2.0 for more viscous oils, such  as light fuel  oil.
                                   3-55

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Typical values of R, the oil retention capacity of porous soil with natural
moisture content are given below.


          Soil	.R (1/m3)

    Stone, coarse gravel                       5
    Gravel, coarse sand                        8
    Coarse sand, medium sand                  15
    Medium sand, fine sand                    25
    Fine sand, silt                           40

If the soil consists of layers with different retention capacities, an
intermediate value of R should be selected.  Note, however, that the
heterogeneities in the soil, which can rarely be properly represented, will
result in a decrease in downward penetration in the field than indicated by
this calculation.  This equation is probably most useful  for estimating
whether or not a spill of known volume will reach the water table.


Because both these relationships are simple, their underlying assumptions
and interpretation of their possibly results need to be highlighted:


    •  Both relationships assume a homogeneous, or nearly so, soil media.

    0  The relationships do not account for the lateral spreading of an
       immiscible substance due to capillary forces.

    9  If the soil media is stratified, a worst case for both conditions
       can be calculated using the most porous  media.

    •  The equations do not recognize variations in water content, nor the
       hysteresis phenomena.

    •  These equations do not consider the flow process which creates  the
       contaminated zone;  it is assumed that the oil is  distributed
       uniformly at residual saturations.
                                    3-56

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3.4  TRANSPORT OF LIQUIDS IN THE CAPILLARY ZONE

3.4.1'  OVERVIEW

The capillary zone is the transition zone between the unsaturated and
saturated zones.  As in the unsaturated zone, transport in the capillary
zone is governed by three-phase (air, water, and oil) flow phenomena
determined by capillary and gravitational forces and by the relative
permeability of these fluids.  Unlike the unsaturated zone where flow is
primarily in a vertical direction, flow in the capillary zone is mainly
horizontal (CONCAWE, 1979).

In terms of the substances leaking from USTs, transport of immiscible
substances with a specific gravity less than water ("floaters") is of the
most significance in this zone and these are discussed in Section 3.4.2.
Transport of miscible substances and immiscible substances with a specific
gravity greater than water will simply slow down as they encounter the
increasing degrees of moisture saturation in this zone.  An estimation
relationship used for this zone is presented in Section 3.4.3.

In the capillary zone, moisture content increases with depth and pressure
heads are less than atmospheric.  Looking at the moisture content
distribution from the opposite direction (moving up from the water table),
water held by capillary forces occurs in all, then smaller, and finally the
smallest, connected pore spaces.

To describe the transport of floating immiscible substances (e.g.,
petroleum products) in the capillary zone, the capillary zone is often
divided into two regions:  the capillary fringe and the funicular zone.
The region where all the pores are filled with water held by capillary
forces is called the capillary fringe, and the region where the water
content transitions from complete saturation to irreducible saturation is
called the funicular zone.  Recall that for two-phase flow conditions with
                                     3-57

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water at funicular saturation (Figure 3-14), both immiscible fluids will
flow.  Figure 3-21 presents a schematic of these zones with a typical
moisture content curve.

The thickness of the capillary zone depends on the size of the voids and on
the degree of homogeneity of the soil, mainly the pore size distribution
(Bear, 1979).  The table presented below lists two different estimates of
capillary zone thickness for various soil types.
                                Capillary Rise   (in cm)
                            CONCAWE,(1979)     Bekchurin (1958)
                                               from Bear (1979)
      Coarse grain sands       12-15             2-5
      Medium grain sands       40 - 50            12 - 35
      Fine grain sands         60 - 110           35 - 70
      Silts and clays         175 - 250
      Silt                                        70 - 150
      Clay                                       200 - 400
      Chalk                   120 - 900
While these estimates vary for a particular soil  type, they clearly show
that capillary rise increases with finer pore sizes. The effect of pore
size distribution on capillary rise is depicted schematically in Figure
3-22.  In Figure 3-22 (A), the thickness of the capillary rise is shown to
be relatively constant for a homogeneous soil;   this is an idealized
condition not likely to be encountered in the field.  Large variations in
capillary rise are shown in Figure 3-22 (B) for non-homogeneous soils.

3.4.2  TRANSPORT OF FLOATING SUBSTANCES

If an immiscible substance with a specfic gravity less than water (the term
oil is used here) is released in sufficient quantity to exceed the residual
saturation of the unsaturated zone soil medium, it will  reach the capillary
zone.  Transport in the capillary zone is typically describing in terms of
                                    3-50

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     CAPILLARY ZONE
SOURCE: Adapted from Dietz (1967)
                                               UNSATURATED ZONE
                                                	  	 	   Capillary Rise Above Phreatic Level
                                                          In Slimmest Continuous Pores

                                               FUNICULAR ZONE
                            Capillary Rise Above Phreatic Level
                            In Widest Pores
                                                                 CAPILLARY FRINGE
                                                                   Phreatic Level, As Found In
                                                                   Observation Wells
                                                              SATURATED ZONE
                                                         100%
                                                WATER SATURATION
  CAMP DRESSER & McKEE INC.
                  FIGURE 3-21
SCHEMATIC OF THE CAPILLARY ZONE WITH
   A TYPICAL WATER SATURATION CURVE
                                         3-59

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                       (A)
                       (B)
                         capillaries of '
                         like diameter
         0   50  100

          Water content (% )
                       capillaries of
                       unlike diameter
                                                              V1

       0   50   100

        Water content (%)
 SOURCE: Schwille (1967)
CAMP DRESSER & McKEE INC.
              FIGURE 3-22
VARIATION IN WATER CONTENT IN THE
  CAPILLARY ZONE WITH ELEVATION

      3-60

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mounding on the capillary fringe and lateral  spreading in the funicular
zone.  These phenomena are not well understood.  A general description of
these phenomena is given below.

When free oil  initially reaches the capillary zone, its vertical  movement
is stopped at the top of the capillary fringe.  As more oil reaches this
region a layer of increasing thickness, a mound, begins to form on the
capillary fringe under the influence of the infiltrating oil.  Van Dam
(1967) has reported that a minimum thickness  of oil must be established
before lateral migration can occur.  The weight of the oil will result in
the depression of the capillary fringe and possibly the water table.
Buoyant forces act to restore the water to its initial level  and if the
minimum thickness is exceeded, the lateral spreading begins.   In general,
lateral spreading occurs in the funicular zone and the oil is described in
terms of its characteristic shape a pancake or lens.  Lateral spreading
will occur in all directions, but the predominant movement will be with the
slope of the water table.  Lateral migration  ceases when oil  is at its
residual saturation.

Shape of the Oil  Pancake

The infiltration rate, local  water table configuration and permeability
distribution will determine the shape of free oil  in the capillary zone
before lateral spreading begins.  Van Dam (1967) states that "...oil will
spread preferentialy within the top of the capillary fringe in the
direction of the groundwater gradient or towards parts of the porous medium
where the highest permeabilities occur.  The  latter effects may prevail in
the case of strong capillary pressures, which phenomenon may  limit the
extent of oil  migration along a sloping groundwater table."

The degree of mounding on the water table is  affected by the  infiltration
rate.  In general, significant mounding occurs only when the  infiltration
rate exceeds the rate of lateral spreading.  Shepherd (No Date) states that
                                    3-61

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only an extremely rapid product-loss rate will  depress the water table
significantly under a spill site.

While lateral  spreading occurs in all  directions, the predominant direction
is with the slope (gradient) of the water table.  If the water table
gradients are low, a more circular pancake will  form than the narrow
elongated pancakes which are characteristic of steeper gradients.

The permeability of the soil affects the rate and thickness of lateral
spreading of free oil.  In low permeability soils, resistance to flow is
high and a thicker free oil pancake will form.   High permeability soils
will allow the formation of thin and faster moving pancakes.

Two qualitative descriptions of leaks  showing the impact of these factors
are cited below (Shepherd, No Date):

     •  A slow product leak in a very  permeable  soil  with a steep hydraulic
        gradient could result in a long narrow plume with a
        capillary-induced saturated (oil and water)  soil  zone of less than
        two feet.
     •  A rapid product loss with a relatively  flat hydraulic gradient
        could result in a oval plume around the  leak site with a saturated
        soil zone several  feet thick,  before lateral  movement takes place.

Rate of Movement

As decribed above, movement of free oil  in the capillary zone is governed
by multi-phase flow principles described in Section 3.2.2.  The rate of
lateral spreading varies considerably  over time  (CONCAWE, 1979).  Even with
greatly increased gradients resulting  from the  height of the mound under a
leaking UST, the rate of lateral spreading is slow (Shephard, No Date).

Within the capillary zone, the rate of movement  will  be governed by the
degree of water saturation.  At the bottom of this zone,  water saturation
is high, and oil may displace some of  the water  but this oil will encounter
                                     3-62

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more resistance to flow.  Nevertheless, experimental and field evidence
indicates that considerable migration occurs within the capillary fringe at
or very near the water table (Freeze and Cherry, 1979).  Shepard (No Date)
notes that the maximum rate of movement is at about two-thirds of the
height of the capillary zone.  This happens because near the top of the
funicular zone, the water saturation is low and the relative permeability
of oil will  be higher.

The effects of viscosity described in section 2.3.3 will also impact the
movement of free product in the capillary zone.  Gasoline which is less
viscous than water will move faster than the water in this zone.

Thickness of the Pancake Away From the Percolating Core

Dietz (1971) states that oil will  spread over water in a layer than is
roughly equivalent to  the thickness of the funicular zone.  Table 3-3 lists
average funicular zone thickness.

The ultimate spread of the free oil is also affected by flucuation in the
water table.  The flucuations can be caused by seasonal precipitation,
tidal forces or the turning on and off of a pumping well.  Unless the
flucuations are rapid, the free oil will stay on the top of the capillary
fringe and migrate with the moving water table.  Under rapidly flucuating
conditions (e.g., an increase of several feet because of the flooding of an
irrigation ditch), oil may be temporarily trapped at higher than residual
saturation under the water table.   Bouyant forces will then act to move the
oil to the top of the water table.

Fluctuating water tables expose soil  that was previously uncontaminated to
oil, as shown in Figure 3-23.  The oil will be retained in these areas at
residual  saturation levels.  Thus, the effect of flucuating water tables is
to disperse the oil over a larger region.  Because some of the oil  is
retained in previously uncontaminated area, there is an apparent reduction
in the volume of the free oil.  In fractured rock media, flucations in the
                                    3-63

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                                 TABLE 3-3
                      THICKNESS OF THE FUNICULAR ZONE
    Sand
Average
 Grain
Diameter
  (mm)
  Thickness
   of the
Funicular Zone
     (cm)
    Extremely coarse-very coarse

    Very coarse-moderately coarse

    Moderately coarse-moderately fine

    Moderately fine-very fine
  2 - 0.5

 0.5 - 0.2

 0.2 - 0.05

0.05 - 0.015
   1.8 -

   9.0 -

  22.4 -

  28.1 -
9.0

22.4

28.1

45.0
Source:   Dietz (1971)
                                    3-64

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                      Original
                      water table
                  Oil at residual
                  saturation
          Free oil
                 New water table
 SOURCE: Davis et. al. (1972)
CAMP DRESSER & McKEE INC.
              FIGURE 3-23
CONTAMINATING EFFECT ON SOIL CAUSED
     BY FLUCTUATING WATER TABLE


         3-65

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water may increase the apparent volume of free oil  due to "washing out" of
oil from fractures previously unconnected to the water table (CONCAWE,
1979).

3.4.3 ESTIMATING RELATIONSHIPS

CONCAWE (1979) reported an estimation relationship to determine the area!
spread of free oil in the capillary zone.  As with the estimation
relationships reported in Section 3.3, this relationship assumes that oil
infiltrated at the ground surface and requires an estimate of the
infiltration area.  Because the leaks of concern herein are from USTs, the
direct applicability of this relationship to our problem is difficult.
While it might be possible to estimate the cross-sectional  area of an
infiltrating leak, this would require accounting for the effects of the
excavation zone around the tank.

                  S = 1000 V-AxRxdxk
                               F
                                                       2
where:  S = Maximum spread of oil on the water table, m
        V = Volume of infiltrationg oil, m3
                                              2
        A = Area of infiltration at surface, m
        R = Retention capacity of soil above the water table (see table
            below), 1/m
        d =.Depth to the water table, m
        F = Oil contained immediately above the capillary fringe (see table
                       2
            below), 1/m
        k = Correction factor for various oil  viscosities

            k = 0.5 for low viscosity petroleum products, e.g.  gasoline
            k = 1.0 for kerosine, gasoil and products with  similar
                viscostiy
            k = 2 for more viscous oils such as light fuel  oils

Values for R and F are given below for various porous soils.
                                   3-66

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son on

Stones, coarse gravel
Gravel , coarse sand
Coarse sand, medium sand
Medium sand, fine sand
Fine sand, silt
R
Retention Capacity
1/nT
5
8
15
25
40
F 2
1/nr

5
8
12
20
40
Further, the application of this relationship to field conditions has not
been documented.  Until  such documentation is gathered, use of this
relationship should be used with great discretion.   The Davis et. al.
(1972) states that the ultimate extent of the oil  pancake can be
calculated, but doing so under field conditions involves so many
assumptions and estimates that the result can easily be inaccurate by more
than 100 percent.

3.5    TRANSPORT OF LIQUIDS IN THE SATURATED ZONE

3.5.1  OVERVIEW
The saturated zone is located below the water table and under
uncontaminated conditions the interstices are filled with water (called
groundwater).  The water table is defined as the surface where hydraulic
pressures are at atomospheric pressure.  Groundwater exists under pressures
in excess of atmospheric pressure.  Groundwater flow is quantitatively
described by Darcy's Law (see Section 3.2).

Both dissolved and immiscible contaminants migrate in the saturated zone.
There are several ways contaminants from leaking USTs can reach the
saturated zone.  They include:
                                   3-67

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    •  Dissolved Substances
         -  A miscible contaminant leaked from an UST migrates through the
            subsurface in the groundwater and/or dissolves in subsurface
            water before reaching the water table.

         -  An immiscible substance can be exhausted at residual  saturation
            in the unsaturated zone and infiltrating water passing by this
            substance can dissolve some of the components and carry them to
            the saturated zone.

         -  At the contact between a pancake of immiscible contaminant and
            the water table,  solution of contaminant can take place.

         -  An immmiscible substance can enter the saturated zone as  a
            second phase and  available for solution in groundwater.

         -  Percolating rainwter can dissolve the vapors of a contaminant
            in the unsaturated zone and transport the redissolved
            contaminant to the water table.


    •  Immiscible Substances
       The location below the water table of an immiscible substance as a

       second phase is a function of the specific gravity of. the substance.


       -  If the specific gravity is less than water,  the immiscible
          substance is usually found in the upper parts of the saturated
          zone, just below the water table and is at residual  saturation.
          There are two general ways for substances to migrate below the
          water table: (1) as a result of-the mounding process during
          percolation; or (2) flucatuating water tables.

       -  Substances with a specific gravity greater than water will  sink
          through the saturated zone until they are immobilized at residual
          saturation or reach an impermeable boundary.


The rate and direction of movement of a contaminant in the saturated zone

are functions of the groundwater flow region, the local  geology and the

chemicals and physical properties of the contaminant.   Section 3.5.2

presents the qualitative and quantitative aspects of advection and
dispersion for a homogeneous media.  In Section 3.5.3, applications of

these principles to the more complex situations likely to be found in the
field will be presented.'
                                    3-68

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3.5.2  THE PHYSICS OF SATURATED ZONE  TRANSPORT

Dissolved substances are carried along in the groundwater as a  single  phase
system.  Transport of dissolved substances is governed  by advection
(Darcy's Law) and dispersion.   Transport of immiscible  substances in  the
saturated zone are governed by Darcy's Law and the  two-phase flow
principles discussed in Section 3.3.   They do not flow  according to  the
laws of hydrodynamic dispersion (Guswa and Lyman, 1982).

The transport of dissolved substances in the saturated  zone is  governed by
advection and dispersion.  Advection  is the movement of a contaminant  plume
with the mean groundwater flow which  is described by Darcy's Law (Section
3.2).  Dispersion describes how a contaminant spreads out and is diluted  as
it to occupies more of the saturated  zone than can  be explained by
advection only.  Dispersion occurs on three levels;  these are shown  in
Figure 3-24.

    •  Molecular diffusion - the movement of contaminant  with concentration
       gradients;
    •  Microscopic dispersion  (or mechanical  mixing)  -  the variations  of
       direction and magnitude of velocity within a  single pore space  and
       between pore spaces of  different sizes;
    •  macroscopic (or field-scale) dispersion -  due to variations in  the
       permeability (heterogeneities).
Together, the first two levels of dispersion are  often  called hydrodynamic
dispersion.  While all  three types of dispersion  can occur in the saturated
zone, the degree to which any  one type governs dispersion depends on  the
characteristics of the zone.  Molecular diffusion may be  the principle
dispersion phenomenon  at low  groundwater velocities, such as those  found
in clay or shale (Guswa and Lyman, 1982).  At high groundwater  velocities
in relatively homogenous permeable media (e.g., a homogeneous sand in  a lab
column), pore scale mechanical  mixing may be prinicpally  responsible  for
dispersion.  However, practically all natural geologic  materials are
heterogeneous, and consequently, macroscopic dispersion will  be the
principle phenomenon in most field situations.
                                    3-69

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                                                                 DIRECTOR OF AVERAGE
                                                                       FLOW
                                                        VELOCITY
                                                      DISTRIBUTION
  MICROSCOPIC DISPERSION
       MOLECULAR DIFFUSION
 SOURCE: Bear (1979)
      DISPERSION AROUND A CLAY
       LENS IN A SANDY AQUIFER
          DISPERSION CAUSED BY A
       SAND LENS IN A SILTY AQUIFER
                            MACROSCOPIC DISPERSION
 SOURCE: Johnson & Dendrou (1984)
CAMP DRESSER & McKEE INC.
        FIGURE 3-24
TYPES OF DISPERSION
                                      3-70

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Molecular Diffusion

Diffusion is the process whereby ionic or molecular constituents move under
the influence of their kinetic activity in the direction of their
concentration gradient.  It can occur in conjunction with mechanical
dispersion or it can occur in the absence of any bulk hydraulic movement of
the solution.  Diffusion ceases only when concentration gradients are
nonexistent.

Pick's first law states that the mass of a diffusing substance passing
through a given cross-section per unit time is proportional  to the
concentration gradient and is expressed as:

                                 F = - D dC_
                                         dX
where:

              F is the mass flux per unit area per unit time,
              D is the diffusion coefficient,
              C is the solute concentration,
              dC_ is the concentration gradient.
              dX

In porous media, the diffusion coefficient, D, is replaced by  an
empirically derived "apparent diffusion coefficient, D*' which is
represented by the relation:

                                 D* = AD

where A is an empirical coefficient, less than unity, that takes into
account the effect of the solid phase of the porous medium on  the
diffusion.  In laboratory studies of diffusion of non adsorbed ions  in
porous geologic materials, values of A between 0.5 and 0.01  are commonly
observed.  (Freeze and Cherry, 1979).
                                    3-71

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The Advection and Dispersion Equation

The advection dispersion equation is the fundamental  differential  equation
for contaminant transport.   Neglecting the loss or gain of solute  mass due
to reactions, the equation  in one-dimension for a non-reactive substance
can be written as:

          change in concentration   =    dispersion  +  advection
          over time
                              _    dC
                      at     ax    37
where:
       C is the solute concentration
       D is the dispersion coefficient
       V is the average linear pore velocity
       x is the distance
       t is the time

As discussed earlier, the coefficient of hydrodynamic dispersion can  be
expressed in terms of two components, mechanical  dispersion,  and molecular
diffusion:
                                 D = aL V + D*
where:
       D is the dispersion coefficient
       a,  is the longitudinal  dispersivity of the medium
       V is the average linear pore velocity
       D* is the apparent diffusion coefficient
                                    3-72

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Many laboratory experiments have found the longitudinal  dispersivity
to be on the order of millimeters or centimeters.

The effect of the dispersion coefficient is to cause some of the
contaminant to a move faster than the average linear velocity of the
groundwater and some of the contaminant to move slower than the average
linear velocity.  This results in a smearing out and dilution of an
originally sharp boundary between contaminated and uncontaminated water.
Figure 3-25 illustrates the combined effect of molecular diffusion and
mechanical mixing to the spread of a concentration front.  The dispersion
process causes spreading of the contaminant species in directions
transverse to the flow path, as well as parallel  to it.

3.5.3  APPLICATIONS

The examples that will  be presented here are for dissolved and immiscible
substances in the subsurface.  Concepts to be presented include:  how
variations in geology affect contaminant plumes,  the effect of density on a
dissolved contaminant plume and transport of nonaqueous phase liquid
substances in the saturated zone.

Effects of Heterogeneities on a Dissolved Contaminant Plume

Macroscopic dispersion is caused by variations in permeability in the
saturated zone.  These variations can occur at a small scale (a clay lens
in a sand aquifer) or at a large scale (the stratification of the
subsurface).  Changes in permeability result in changes in the direction
and rate of groundwater flow.  Field experiments have shown that
macroscopic longitudinal dispersivities can be many orders of magnitude
larger than those from lab tests;  values in the range of 1 to 100 meters
are common (Anderson, 1979).  Field data also seem to indicate that
macroscopic dispersivities.are dependent on the scale of the contamination
problem.
                                     3-75

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                                                   V POSITION OF INPUT
                                                    WATER AT TIME T
    RELATIVE
 CONCENTRATION  0.5
     (C/Co)
TRACER FRONT IF
 DIFFUSION ONLY
                              DISTANCE X
 DISPERSED
TRACER FRONT
SOURCE: Freeze and Cherry (1979)
  CAMP DRESSER & McKEE INC.
                   FIGURE 3-25
 SCHEMATIC DIAGRAM SHOWING THE CONTRIBUTION
    OF MOLECULAR DIFFUSION AND MECHANICAL
MIXING TO THE SPREAD OF A CONCENTRATION FRONT
                                    3-74

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Freeze and Cherry (1979)  present a series of contaminant plumes  in
cross-section to show the effect of simple layered heterogeneities  on
transport patterns.   These cross-sections are shown in Figure 3-26.

Figure 3-26 (A)  shows the groundwater flow domain.  At steady-state
groundwater flow occurs through a cross-section which is assumed to  be
isotrophic with  respect to hydraulic conductivity.  The contaminant  used  in
this figure is assumed to be conservative and the effect of dispersion  on
the plume is neglected.  In Figure 3-26 (B) the contaminant is shown
flowing through  a homogeneous medium.  Figure 3-26 (C) shows the effects  of
a thin continuous high horizontal hydraulic conductivity layer.   The
contaminant flows almost totally within this layer and the  travel  time  is
one-fifth of that in the homogeneous medium.  The effects of discontinuous
layers of low conductivity materials in shown in Figure 3-26 (D).   The
contaminant plume moves over the first lens, under the second one and  then
through the second one to reach the discharge point.    Figure 3-26  (E)  is a
variation on case C where the high conductivity layer is discontinuous.
The contaminant  plume moves within the high conductivity layer until this
layer ends in the middle of the cross-section.  In the central  part  of  the
cross-section the contaminant plumes spreads out occupying  almost the
complete depth of the aquifer.

This set of figures was presented to show in a simple fashion the  effect  of
heterogenities.   In the field, large conductivity changes across sharp
discontinuities  are common.  Thus, it can be seen that determining where  a
contaminant plume will move in the saturated zone can be complicated.

Density Effects  on a Dissolved Contaminant Plume

The advection-dispersion equation presented in Section 3.5.2 assumes that
there is no significant density differences between the groundwater
containing dissolved contaminant and the ambient groundwater.  Because  the
equations that include density differences are complex, only a qualitative
example is presented here.  Basically, as the density of the contaminated
                                    3-75

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                   Contaminant source
                                                     Stream-
                                    Steody water tnbIe

                                     No flow             Divide
                            (A) BOUNDARY CONDITIONS
                             (B) HOMOGENEOUS CASE
                                                  K2/K,=100
                       (C) SINGLE HIGHER-CONDUCTIVITY LAYER
                                                  K1/K2. =
                        (D) TWO LOWER-CONDUCTIVITY LENSES
                                                  K2/K,=100

                        (E) TWO HIGHER-CONDUCTIVITY LENSES
SOURCE: Freeze and Cherry (1979)
CAMP DRESSER & McKEE INC.
                FIGURE 3-26
   EFFECT OF LAYERS AND LENSES ON
FLOW PATHS IN A SHALLOW STEADY-STATE
      GROUNDWATER FLOW SYSTEM

     3-76

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groundwater increases relative to the ambient groundwater,  the more a
contaminant plume is likely to sink in an aquifer.   Figure  3-27 shows the
relative locations for three plumes of increasing density in  an initially
uniform flow field.   Figure 3-27 (A) shows that a contaminant plume at
about the same density of ambient groundwater spreads in  the  shallow part
of the saturated zone.  In contrast, Figure 3-27 (C)  shows  how large
density differences  result in a steeply downward sinking  plume (Freeze and
Cherry, 1979).

The density of contaminated groundwater is a function of  the  molecular
weight and the concentration of the dissolved contaminant.   The density of
ambient groundwater  also can vary,  although most likely to  a  lesser
degree within a local area, because of the presence of dissolved minerals.
The largest density  differences would be expected for high  molecular weight
contaminants at high concentrations.  This is somewhat of an  anomaly,
however, because as  discussed in Section 5.0,  the solubility  (saturation
concentration) within a given chemical group (e.g., paraffins or aromatics)
generally increases  with decreasing molecular weight. Therefore,
significant density  differences are most likely to  occur  when (1)  low
molecular weight contaminants are found in very high  concentrations;  or (2)
high molecular weight contaminants  are found near their saturation
concentrations.,

Transport of Immiscible Substances

It is more difficult to predict the migration path  of an  immiscible
substance- in the saturated zone than dissolved substances.  This is because
flow of an immiscible substance is  controlled by its  own  flow potential,
which depends on pressure, gravity  and surface forces, and  is not
necessarily similar  to the groundwater flow potential  (Faust,  1985).   The
migration path can be characterized in terms of density (with respect to
water) and viscosity.  Less viscous fluids will  move  more rapidly  than more
viscous fluids.  If  the leaked substance is less viscous  than water,  it
will  move faster than the prevailing hydraulic gradient.  The effects of
                                   3-77

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U)


CO
                    CONTAMINANT
                       SOURCE
                         \
                      (A)
                                  I
                               (C)
                                                                                                WATER TABLE
(B) (C)
                                                         LEGEND
                                                 Slightly More Dense Than Groundwater
                                                 Larger Density Contrasts
      SOURCE: Freeze and Cherry (1979)
       CAMP DRESSER & McKEE INC.
                        FIGURE 3-27
  EFFECT OF DENSITY ON MIGRATION OF CONTAMINANT
            SOLUTION IN UNIFORM FLOW FIELD

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density are such that a more dense immiscible substance can pose a greater
danger in terms of migration potential than less dense substances, i.e.,
less dense substances are typically only found in the shallow part of an
aquifer, while more dense substance can penetrate deep into the saturated
zone.  Migration of a substance less dense than water was described in
Section 3.4; the discussion below focuses on substances more dense than
water.

The movement of a dense immiscible substance in the saturated zone is
affected by two-phase phenomena and the groundwater gradients.  In simple
terms, the density of the fluid leads to vertical downward movement and
horizontal groundwater gradients will tend to move the substance laterally.
The actual migration route (analogous to a trajectory) will depend on the
magnitude of each of these factors.  Other factors, such as anisotrophy of
the porous medium and the presence of heterogeneities, will also influence
the migration route.  As the immiscible substance moves through the
saturated zone, some of it will be retained at residual saturation.  As
discussed in section 3.3.3, the quantity of substance held at residual
saturation will be smaller for the same medium in the saturated zone than
unsaturated zone.  If the quantity of release exceeds the retention
capacity of both the unsaturated and saturated zones, the substance will
move downward until it encounters an impermeable boundary.  The substance
will pool on this boundary and the primary flow direction will be along the
slope of the boundary and the gradient in the groundwater will have small
effects.

Numerical techniques for predicting the movement of immiscible fluids were
developed for study of petroleum reservoirs.  Only recently has these
methods been applied to the study of immiscible groundwater contaminants
(e.g., Abriola and Pinder, 1985a, b; Faust, 1985).  Only one case study
(Hyde Park Landfill in Niagara Falls, N.Y.) has been reported where these
techniques were applied to a field site (Osborne and Sykes, 1986).
However, the scale of that problem (80,000 tons of waste were dumped)  does
not lend itself to analogies with leaks from USTs.  Even so, the prediction
                                    3-79

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of the migration pathway at this site and all  sites involving immiscible
fluid transport is limited by the lack of generic and site specific data
which include:  laboratory measurements of relative permeabilities and
capillary pressures; field determinations of saturations of immiscible
substances; and estimates of the quantity released (Faust, 1985).

3.6  VAPOR TRANSPORT IN THE UNSATURATED ZONE

3.6.1  OVERVIEW

The preceding sub-sections have been focused on liquid phase  transport.
The movement of vapor phase contaminants -- a significant issue regarding
leaking USTs -- is addressed in this section.   However,  far less is known
and understood about the physics and chemistry of subsurface movement of
vapors than is known about liquids.   As a results, the nature and  treatment
of information provided here is different than in previous sections.   Here,
in a sense, basic (fundamental) science is molded to the need to understand
vapor phase movement in the subsurface environment.  Specifically, the text
does the following:

    •  introduces and describes the  basic physical relationships that
       underlie current theories regarding vapor phase movement; and
    •  adapts the basic physics (albeit crudely)  to illustrate
       phenomena/concepts of importance to the UST Program -- e.g., travel
       times, migration pathways, end-point concentrations.
In preparing this section, the most directly-relevant references were found
to stem from research addressing the movement of landfill-generated vapors.
Unfortunately, however, even this topic has not been studied  extensively.
To be certain that relevant information was not overlooked, a thorough
literature search was conducted and attempts were made to personally
contact individuals engaged in relevant work.   All of this leads to the
conclusion that scientific inquiry into the area of subsurface vapor
movement is scant.  Thus, treatment of the topic from an UST  program
perspective, relies largely on fundamental physical laws.
                                   3-80

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For vapors to move in the subsurface environment, the soils must be
sufficiently dry to permit interconnection of air passages among the soil
pores.  For simplicity,  consider this to be the case and further consider
the subsurface to be made up of uniform, fine sand.   Two fundamental
parameters will  govern the movement of vapors in this setting:  vapor
concentration and flux.   The leaked substance will  have its greatest vapor
concentration at the leak site, where the free liquid product is
evaporating at the liquid-vapor interface.  The rate of vaporization is
dependent on the vapor pressure of the substance, the pore pressure in the
soil, moisture content in the soil, and the temperature.  The flux or vapor
flow is away from areas  of high concentration to regions of ever less
concentration, ultimately the atmosphere.

3.6.2  FUNDAMENTALS OF TRANSPORT

The principal modes of vapor transport in soils are  diffusion and
advection.  It is important to consider the molecular diffusion coefficient
and the permeability of a particular soil  to any particular consituent. It
is also important to consider the adsorption of vapor onto'soil particles
and its subsequent release to the vapor state.  Vapor flux patterns are
determined by the presence of impermeable boundaries (e.g.,  frozen or paved
land surfaces),  permeable boundaries such as the ground surface where the
concentration of vapor is kept essentially at zero by wind action, and
liquid-vapor boundaries  where the vapor concentration is a function of the
vapor pressure of the substance.

Diffusion is the mass transport that occurs via the  random motion of vapor
molecules.  Because it is probable that more molecules will  move from a
region of high concentration to one of low concentration than in the
opposite direction, one can predict a net flux of matter away from the
region of high concentration towards that of low concentration.

With pure diffusion, pore pressure is assumed to be  uniform throughout the
soil medium; it is only  the presence of a concentration gradient, and the
                                    3-81

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tendency of random molecular motion to erase a gradient, that results in

transport.


In particularly fine-grained soils such as tight clays, where the
characteristic pore space is of a size less than that of the mean free path
length of a vapor molecule the diffusion process may better be called
effusion.  Transport is still driven by the concentration gradient, but at

a pace severely retarded by collisions of vapor molecules with soil
particles (Kraemer, 1986).


Advection is the mass transport that occurs when the vapor field is

subjected to a total pressure gradient resulting in a down-gradient flux of
the subject vapor and all  other pore gases present.  The gradient may be
imposed by any of several  causes:

    •  Barometric Pumping  - Pore air pressure deep in the unsaturated zone
       will, on the whole, reflect the mean atmospheric pressure at the
       ground surface.   A rise or fall in atmospheric pressure with respect
       to the pore pressure will result in a flux into or out of the soil.
       This mechanism is most important where the depth to the contaminant
       source is small  compared to the depth of the unsatured zone.   It can
       increase the rate at which free product in the soil  volatilizes.

    •  Imposed Pressure Gradient - This may result from many causes, among
       them:

       -  A heated building basement.  In cold weather, the density of the
          column of air in a building will be less than that found outside
          and in the ground, so that the.basement air pressure is lower
          than that surrounding the basement.  Gas and vapor will seep into
          the basement from the soil  pores, given a pathway, e.g., a crack
          in the basement wall or floor or with unfinished floors or
          crawl spaces.

       -  Generation of gas or vapor at a rate such that removal  of gas or
          vapor by diffusive flux is not rapid enough to prevent a buildup
          of total pore pressure in the soil  near the generation or
          evaporation site.  For example, methane and carbon dioxide are
          generated in municipal landfills.  In warm weather, these gases
          readily escape by diffusion upward, and there is negligible
          buildup of pore pressure; but when the ground surface is frozen,
          the gases can only escape by lateral underground travel over
          relatively long distances.   In such instances, the increase in
          pore pressure may be as much as 30 kilo-Pascals, or about one ft
                                    3-82

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          head of water,  (Mohren et.  al.,  1980;  McBean,  1982;  Merz  and
          Stone,  1969).

       -  Atmospheric horizontal  pressure  gradients,  which  result in  wind
          above the ground surface,  will  generally result in a small
          horizontal  advection of subsurface  pore gases.  Turbulent air
          pressure fluctuations may  also  induce  flow  in  the soil.

    t  Density Difference -  If a vapor has a  density  sufficiently different
       from that of other gases (such as  air)  in the  soil  pores, and  it  is
       present in sufficiently great concentration, there will  be a
       gravity-driven density current of  the  vapor.   In  particular, a
       relatively heavy  vapor will tend to "pour" down  to the  bottom  of  the
       unsaturated zone  and  pool  as  a lens on top of  the water-saturated
       zone (or on top of a  lens of  yet-denser vapor).   Included in this
       category are thermal  gradients in  the  top few  meters of the  soil ,
       caused by diurnal, short-term and  seasonal air temperature changes.
       For example, in winter the presence of cold, denser  air at the soil
       surface and warm  less dense air below  the surface may lead to
       convection currents.

The Concept of a Concentration Field:  Diffusive Transport  Only


The growth of a vapor envelope about a leak in an isotropic unsaturated

soil medium is shown in  Figure 3-28.   Early in the leak  history (Figure

3-28 (A)), the vapor is  close about  the spilled  product.  The  concentric

circles shown represent  concentric spheres in the actual  three-dimensional

case, and are surfaces of constant concentration of vapor.   On the  free

product surface at the leak  site, the concentration is  at its  maximum

value.  Surfaces where the concentration  are  various  fractions of M/(DH)

are shown, where M is the mass vaporization rate, D is  the  vertical

molecular diffusion constant, and H  is the depth of the  leak below  the

ground surface.


Somewhat later in the leak history,  the vapor envelope  has  grown (Figure

3-28 (B)).  If the leak  persists at  a constant rate for  a sufficiently long

time and other environmental  factors do not change, the  vapor  envelope,  or
more precisely the vapor concentration field,  will  approach a  steady  state

and not appreciably grow or  change further (Figure 3-28  (O).   This

steady-state distribution of vapor concentration is of  fundamental

importance in leaking UST vapor impact assessment,  in that  it  tells us the
                                    3-83

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          A: VAPOR ENVELOPE INITIALLY

     GROWING AROUND POINT-SOURCE LEAK.
                                             y
                                             H
                                             H
                                                          LEGEND
                                                               CDH
                                                                M
                                     <.03

                                                               1 <
                                     CDH
                                      M
                                                      C = Vapor concentration
                                                      D = Molecular diffusivity
                                                      H = Depth of leak
                                                      M = Leak rate or volatilization rate
   B: APPROACHING STEADY-STATE CONDITIONS
CAMP DRESSER & McKEE INC.
                   FIGURE 3-28A
GROWTH, STEADY-STATE MATURITY, AND DECAY OF A VAPOR
CONCENTRATION FIELD, FOR A POINT-SOURCE LEAK IN A DEEP
HOMOGENEOUS MEDIUM.
                                           3-84

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           C: STEADY-STATE CONDITIONS
                                            T
                                                        LEGEND
                                                             CDH
                                                                 < .03
                                                                 M
                                                       C = Vapor concentration
                                                       D = Molecular diffusivity
                                                       H = Depth of leak
                                                       M = Leak rate or volatilization rate
           D: RELAXING TO BACKGROUND

CONCENTRATION FOLLOWING CURTAILMENT OF LEAK.
CAMP DRESSER & McKEE !NC.
                  FIGURE 3-28B
GROWTH, STEADY-STATE MATURITY, AND DECAY OF A VAPOR
CONCENTRATION FIELD, FOR A POINT-SOURCE LEAK IN A DEEP
HOMOGENEOUS MEDIUM.
                                         3-85

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relative concentration of vapor ultimately to be expected at any location

below ground.


Suppose that a continuous leak, that has established a steady-state vapor

envelope about it, is terminated.   The vapor concentration field, or

envelope, surrounding the leak, will  gradually dissipate, and

concentrations will relax towards background levels (Figure 3-28 (D)).   The

time required to reach essentially steady-state conditions following first
leakage, and the time to relax from a steady-state condition to a vapor

free state following stop of a leak,  are both of interest.
    I  Boundary Conditions -  The vapor concentration fields depicted in
       Figure 3-28 were for the simple case of a "point-source"  leak
       beneath the surface of a deep, homogeneous unsaturated soil  layer
       with a permeable surface.   The concentration field will  be
       significantly different if there are impermeable boundaries  across
       which the vapor cannot flow.   For example, Figure 3-29 shows the
       steady-state concentration field for a point-source leak just above
       a horizontal lower boundary such as the surface of the saturated
       zone.  The ground surface  is  still  permeable, as in Figure 3-28.
       However, Figure 3-30 shows the case of a point-source leak over an
       impermeable lower boundary and under an impermeable ground surface
       of considerable extent, such  as a paved parking lot.

    •  Anisotropic Soil Conditions - Very often in undisturbed  sedimentary
       soils, where coarse and fine  grained soils are found  in  alternate
       layers, diffusion and advection occur much more readily  in the
       horizontal direction along the layers than in the vertical
       directions which requires  transversing fine layers as well as coarse
       layers.  This favoring of  horizontal  flow over vertical  means that
       the surface of constant concentration will appear "stretched"
       horizontally.  For example, the spheres of Figure 3-28 will  be
       replaced by the ellipsoids of Figure 3-31.

Pipeline Trenches, and Other Channels of Opportunity


The advection and diffusion of gas and vapor through tight,  undisturbed

soils may proceed relatively slowly, compared to the ease with  which gas or

vapor can flow in a coarse-grained homogeneous soil  -- such  as  the  crushed

rock typically used for bedding and  backfill of buried pipeline.  In fact,

pipeline trenches are notorious as efficient conduits for transporting
                                   3-86

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CO

CD
/'/ /'' /// /// m it' /// // ///  /// 111 inn; in Will /n ][/  i//  I//  i// !'/ i/f

             IMPERMEABLE LOWER BOUNDARY
    CAMP DRESSER & McKEE INC.
                                                                             LEGEND
                                                                                 CDH
                                                                                 M
                                                < .03
                                                                                    CDH
                                                                                    CDH
                                                                                   CDH
                                                                                    M
                                                                         C = Vapor concentration
                                                                         D = Molecular diffusivity
                                                                         H = Depth of leak
                                                                         M = Leak rate or volatilization rate
                      FIGURE 3-29
STEADY-STATE CONCENTRATION FIELD FOR A POINT-SOURCE
            LEAK JUST ABOVE AN IMPERMEABLE
  LOWER BOUNDARY (GROUND SURFACE PERMEABLE.)

-------
                                                     IMPERMEABLE
                                                   GROUND SURFACE
CO
i
co
co
                      IMPERMEABLE LOWER BOUNDARY
                                                                                 LEGEND
                                                                                     CDH
                                                                                         < .03
                                                                                          M
                                                                              C = Vapor concentration
                                                                              D = Molecular diffusivity
                                                                              H = Depth of leak
                                                                              M = Leak rate or volatilization rate
   CAMP DRESSER & McKEE INC.
                    FIGURE 3-30
AS IN FIGURE 3-29, BUT NOW WITH GROUND SURFACE
        IMPERMEABLE (E.G. FROZEN OR PAVED).

-------
                                                            H
                                                                          LEGEND
                                                                              CDH
                                                                               M
                                              < .03
                                                                                 CDH
                                                                              1  <
                                              CDH
                                               M
                                                                       C = Vapor concentration
                                                                       D = Molecular diffusivity
                                                                       H = Depth of leak
                                                                       M = Leak rate or volatilization rate
CAMP DRESSER & McKEE INC.
                     FIGURE 3-31
     STEADY-STATE CONDITIONS AS IN FIGURE 3-28C,
      BUT WITH VERTICAL DIFFUSION COEFFICIENT
1/10 THAT OF THE HORIZONTAL DIFFUSION COEFFICENT

-------
gases and vapors from the part of the trench nearest the gas source to
buildings hundreds of feet distant.  Therefore, any analysis of the vapor-
concentration field surrounding a spill  should include a careful, complete
mapping of all buried pipelines in the study area.  Furthermore,  the design
of any pipeline which conceivably could lead harmful  or explosive vapors or
gases to a valuable or inhabited structure should include adequate venting
of its trench for winter (frozen ground)  conditions,  and possibly trench
interruption by bringing the pipeline above ground for a short distance.

Adsorption and Release

In general, some of the vapor molecules of concern will  be adsorbed onto
the surface of the soil particles.  The rate at which they are adsorbed is
dependent on the vapor concentration in the soil  pores and on the fraction
of the potential attachment sites on the soil  particles which are occupied
by absorbed vapor molecules.  With the passing of a vapor plume and
reduction in vapor concentration, the vapor molecules are once more
released from the particle.  The process can be considered in terms of
reversable storage - release process.  It has the effect of retarding the
establishment of a steady-state concentration field,  and of retarding its
relaxation to background conditions after plume passage.

3.6.3  Summary of Vapor Phase Transport

The material  provided above has been organized and presented to provide an
understanding of the basic physics which underlies vapor transport in an
UST setting.   The most salient of the findings are summarized below:

    •  Evaporation is the rate at which a contaminant will enter  the vapor
       phase from the liquid phase is governed primarily by the vapor
       pressure of the chemical of interest.
    •  Advection is a flux (movement of mass)  process wherein a contaminant
       in vapor phase is entrained in a subsoil  current of. air, and "blown
       along" with the air, should the air in the soil  pore spaces be
       itself moving.  The air vapor may be driven by fluctuations in
       barometric pressure, by pressure gradients across building
                                   3-SO

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       foundations,  by the density difference between air and vapor denser
       or less dense than air, and by evaporation or chemical generation of
       vapor at a rate much greater than can be removed by diffusion alone.

    •  Diffusion is a flux that in general  is superimposed on the advection
       process.  Diffusion is predominantly a gradient-driven process, the
       rate of which is governed by the molecular diffusion coefficient and
       the structure of the porous medium.

    •  Adsorption/Release address the processes whereby, as vapor phase
       contaminants move through the soil matrix, there is a tendency for
       molecules of the gas to adhere to soil particles (adsorption) and,
       when concentrations are reduced subsequent to the passage of a
       "plume", the contaminant may release itself from the soil particles
       again.  An indicator of the tendency for adsorption and release is
       the octanol  water partition coefficient (K  ) of the contaminant.
                                                 ow

    •  Varying Permeability Due to Structures and Trenches block or inhibit
       vapor flux,  but the backfill surrounding such structures is often
       more permeable to vapor flow than the undisturbed natural soil.  In
       particular,  vapors are known to tend to move readily along buried
       pipelines through the surrounding backfill.

    •  Molecular Diffusion Coefficient and  Permeability of the Undisturbed
       Soil are often much less in the vertical  direction than in the
       horizontal direction.  Thus diffusion and advection fluxes of vapor
       both tend to be significantly greater in the horizontal than in the
       vertical direction.

    •  Studies on the subject of vapor transport in unsaturated soils has a
       much shorter history and a far less  plentiful body of technical
       literature than the subject of groundwater flow.  Yet with the
       profileration of hazardous volatile  chemicals manufactured,
       transported,  stored, used, and discarded by our industrial  society,
       and with the increasing age of the thousands of automotive service
       stations across the country and the  world with poorly installed,
       maintained or monitored storage tanks, continued research on a broad
       front appears justified.
3.7  SUMMARY OF TRANSPORT MECHANISMS


The transport (migration)  of contaminants through the subsurface depends on
the quantity released from a leaking LIST, on the physical  properties of the

leaked substance,  and on the structure of the subsurface soils and rock
through which the contaminant is moving.
                                    3-91

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Even though UST's leaks can be characterized as insidious spills, they may
be undetected for long periods of time, and thus, can release substantial
volumes of a substance into the subsurface.  One reason the quantity of
released substance is important is that it often determines the method, and
degree, of groundwater contamination, i.e., by direct contact with the
water table or through solution in percolating rainwater.

The physical properties of the leaked substance that are important to
transport are:
   . •  Solubility - determines if single or multi-phase flow phenomena
       will govern transport.
    •  Specific gravity - determines whether a nonaqueous phase liquid
       will "float" on the water table or "sink" through the saturated
       zone to an impermeable boundary.
    •  Viscosity - affects the rate of movement under saturated and
       unsaturated conditions of a substance moving as a separate phase.
    0  Surface tension - is responsible for capillary effects and
       determines the spreading of fluids on the capillary fringe at the
       end of the spreading phase.
    •  Evaporation rate - (dependent on the vapor pressure and latent
       heat of evaporation) determines the potential  for a substance to
       have an associated vapor phase.
    •  Vapor density - determines whether the vapor phase in the un-
       saturated zone rises or sinks to spread on the capillary fringe.

This section  discusses the transport mechanisms through various types of
subsurface media.  Movement of contaminants is discussed for three
characteristic zones of the subsurface:  the unsaturated zone lying below
the ground surface, the capillary zone that is a transition between the
unsaturated and saturated zones, and the saturated zone lying below the
water table.
Unsaturated Zone:
Transport of a contaminant through the unsaturated (or vadose)  zone is
                                     3-92

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characterized by vertical  flow, driven by gravity, and lateral  spreading
associated with capillary forces and soil heterogeneities.

The migration of contaminant through this zone is most commonly a three-
phase flow process, in which fluids which are immiscible in each other,
air, water, and the contaminant — say, gasoline — are the three phases.
Under certain conditions, as when a contaminant is dissolved in water or
when subsurface soils are dry, the migration is a two-phase flow process.
The flow of two or more immiscible fluids depends on capillary pressures,
which are the result of surface tensions between the fluids, and the
saturation levels of each of the fluids which determines the relative
permeability of the fluids in a medium.

The velocity of the immiscible fluid is inversely proportional  to
viscosity.  Therefore, heavy oils do not readily penetrate the soil;
whereas lighter petroleum products, gasoline in particular, move through
the soil more easily than water does.  In dry soils, (gravity dominated
flow with no pressure gradient) the velocities of downward flow (or
percolation) will be inversely proportional  to the kinematic viscosities;
                                                         p
for example, light heating oil (kinematic viscosity = 4mm /second)  will
move four times more slowly than water.

The downward flow of the contaminant will be arrested when:

    •  The contaminant reaches the water table,
    •  The contaminant encounters an impermeable layer, or
    •  An immiscible contaminant (i.e., one that will  not mix with
       water) reaches residual saturation levels.

Until one of the above occurs, the shape that the contaminant plume takes
in its migration through the subsurface depends on the permeability of the
soil medium, the viscosity of the contaminant, the capillary effects, and
on the degree of heterogeneities.  In general, less permeable soils cause
more lateral spreading than do more permeable soils.  The presence  of
                                     3-93

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heterogeneities, or a stratified soil, also increases the lateral spreading
and reduces the contaminant's depth of penetration in the subsurface.

In general, the contaminant plume will take a pear-shaped form as it moves
through the unsaturated zone.  The plume will be wider (the pear "fatter")
in a less permeable soil than in a more permeable soil.  The shape will be
irregular in a stratified soil with varying permeabilities.

for immiscible substances, the residual saturation of the contaminant is an
-important factor in estimating its spread and is a measure of the retentive
capacity of the soil with respect to a particular fluid.  In  general,
there is an inverse relationship between permeability and retention:  less
permeable soils retain more of an immiscible substance than do more
permeable soils.   In dry soils, the residual saturation of a substance will
be higher than in wet soils (which means the soil will tend to retain the
substance until a higher volume is reached).  As the water content of the
soil increases, the residual saturation of the substance tapers off to a
constant level.

A leaked substance such as gasoline can travel  through 25 feet of permeable
alluvial or glacial sediments in a few hours or, at most, a few days,
depending on specific conditions at the site.

Capillary Zone:

The capillary zone has a significant impact on the movement only of non-
aqueous phase liquids that are less dense than water.

In such cases, the primary direction of movement is lateral, with flow
extending farthest in the direction of the slope of the water table.
When free oil initially reaches the capillary zone, its vertical  movement
is stopped at the top of the capillary fringe.  As more oil reaches this
region, a layer of increasing thickness, a mound, begins to form on the
capillary fringe under the influence of the infiltrating oil.  The weight
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of the oil will  result in the depression of the capillary fringe and
possibly the water table.  Buoyant forces act to restore the water to its
initial level and if a critical  minimum thickness (which varies with soil
and contaminant properties) is exceeded, and lateral spreading begins.
Lateral spreading will, occur in all directions, but the predominant
movement will be with the slope of the water table.  Lateral migration
ceases when the oil  is at its residual saturation.  The flow results in a
characteristic shape called an "oil pancake."

The shape of the oil  pancake depends on permeability of the soil, the
percolation rate, and the local  water table configuration.  In general, the
more permeable the soil, the more the contaminant will  spread and the less
thick the recharge mound will be.  The steeper the hydraulic gradient, the
narrower the plume will  be, elongated in the direction of groundwater flow.

As the water table fluctuates, over time, the nonaqueous phase liquid
contaminant can be spread out somewhat in the vertical  plane.  In porous
media, this can lead to an apparent decrease in the volume of free product,
as previously uncontaminated soil will now retain some of the contaminant
at residual saturation.   In fractured rock media, on the other hand, there
may be an apparent increase in the volume of free product as contaminant
trapped in dead-end fractures may be "washed out" previously unconnected by
water table.

Saturated Zone:

The transport of contaminants in the saturated zone can be characterized
for three classes of substances as follows:

    •  miscible or dissolved substances,

    •  immiscible substances with specific gravity of less than 1.0, and

    •  immiscible substances with specific gravity of more than 1.0.
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Dissolved substances will  enter the groundwater and will  move in the
general direction of groundwater flow according to the mass transport laws
of advection and dispersion.  Advection is the movement of a contaminant
plume with the mean groundwater flow.  Dispersion describes how a con-
taminant spreads out and is diluted as it occupies more of the saturated
zone than can be explained only by advection.  Dispersion occurs on three
levels - molecular diffusion, microscopic dispersion and macroscopic
dispersion.  In most field situations, macroscopic dispersion, which is due
to variations in permeability (heterogenities), will  be the principal
phenomenon.

Immiscible substances that are "lighter" than water are typically only
found in the shallow part of the saturated zone.  The substance typically
takes the form of an emulsion and the rate of transport will  depend on the
local groundwater gradients and the viscosity of the substance.

Immiscible substances that are denser than water will  move downward through
the saturated zone.  A dense immiscible substance can pose a greater danger
in terms of migration potential  than less dense substances because more
dense substances can penetrate deep into the saturated increasing the
potential for its solution and the subsequent migration of dissolve
components with the prevailing groundwater flow pattern.   Although some
residual saturation (and therefore retention in the soil)  occurs as the
substance moves downward,  it is less than in the unsaturated zone.  If the
quantity of release exceeds the retention capacity of the unsaturated and
saturated zones, the nonaqueous phase liquid reaches an impermeable
boundary, it forms a mound and spreads with the slope of that boundary,
pooling in depressions in the boundary.

Vapor Phase Transport:

A liquid contaminant leaking from an UST will enter the vapor phase
according to its specific vapor pressure (the higher the vapor pressure of
the substance, the more likely it is to evaporate).
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Once in the vapor phase, the contaminant will  move by advection -- being
"blown along" by subsurface air currents (which themselves are the result
of several forces)  — and by diffusion.  The movement of vapors is
predominantly in the horizontal direction.

Molecules of gas may adhere to soil  particles by adsorption and then be
released after passage of the plume.  Movement of the gas can be blocked by
monolithic buried structures, but vapors will  move readily through the
backfill  surrounding such structures (such as along the route of a buried
pipeline).

Much less attention has been given to vapor phase transport than to liquid
transport; several  research needs are identified.
                                    3-97

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                            4.0  FATE MECHANISMS
4.1 OVERVIEW

In the previous section on the transport of regulated substances through
the subsurface, the substances were assumed to be non-reactive (or
conservative);  i.e., no change in the mass or phase of the substance
occured as it migrated through different zones or media of the subsurface.
This section discusses the fate of regulated substances which includes the
physical, chemical, and biological changes a substance may undergo in the
subsurface environment.

These fate mechanisms can be divided into two types of processes --
physical  processes which include solubility, vaporization, and adsorption,
and kinetic processes which include biotic processes and abiotic chemical
transformations.  Physical processes are natural  processes which transfer
the substances across media/phase interfaces.  Kinetic processes are
processes which decrease the concentration of a chemical by degrading it
into other products.

Five fate mechanisms were determined to be important for substances leaking
from USTs:
       Solubility - the partitioning of a chemical  between  nonaqueous  and
       dissolved phases (chemical  and soil-water).
       Vapori zation - the partitioning of a chemical  between  its  liquid  and
       vapor phases (chemical  and  soil-air).
       Adsorption - the partitioning of a chemical  between  the  soil  and
       soil water.
       Biotic Processes - the  degradation of a chemical  by  a
       microorganisms.
       Abiotic Chemical Transformations - the degradation of  a  chemical
       through chemical reactions.  Two specific reactions  are  examined:
       hydrolysis and oxidation/reduction.
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Whether or not a substance is subject to a fate mechanism depends on a
number of chemical-specfic and environment-specific factors.   In most
situations, not all  of the fate processes may occur at  the same time.  One
fate mechanism may dominate or several  fate mechanisms  may "share
responsibility" for the removal of a chemical  from a given media and/or
phase.  This is especially true for a mixture of chemicals released  in the
subsurface.  The presence of two or more chemicals can  inhibit or stimulate
the fate mechanisms to which any one substance is subject.  These
interactions are highly complex and poorly understood.

In the simplest case, however, for any one fate mechanism a chromatographic
process takes place.  This means that there is an order in which the
chemicals in a mixture may be, for instance, solubilized or biodegraded
which leads to a change in the relative concentration of the  components of
mixtures with distance from the leak location.  The changes in the relative
concentrations of the components of mixtures is called  the weathering of
the mixture.

The next five sections describe the fate mechanisms which were determined
to be important for the regulated substances in USTs.  In these sections,
the underlying theory, effects of environmental  factors and,  to the  extent
possible, the degree to which petroleum products and their constituents and
hazardous substances are transfered or  degraded in the  subsurface
environment are presented.

4.2  SOLUBILITY

4.2.1  OVERVIEW

Solubility can be defined as the mass of a substance that will  dissolve in
a unit volume of solute under specified chemical  or physical  conditions
(Freeze and Cherry,  1979).  The solute  of interest is the water in the  soil
matrix (soil water)  and the term solubility used throughout this  section
means solubility in  water.  Solubility can be  interpreted as  the
partitioning of a contaminant between its nonaqueous and dissolved phases,
in particular between the chemical  and  the soil  water.   While some of the
                                    4-2

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substances in USTs may be partly dissolved in  water,  many more  are  stored
in the nonaqueous liquid phase,  especially the petroleum products.

The water solubility of a substance  is  one of  its  most  important
environmental parameters.  Not only does solubility determine the extent to
which a contaminant will dissolve,  it  also impacts other fate mechanisms.
For example, a highly soluble substance often  has  a relatively  low
adsorption coefficent and also tend  to  be more readily  biodegradable  by
microorganisms.  The other fate  mechanisms —  volatilization and chemical
transformations (hydrolysis and  oxidation-reduction)  — are also affected
by the degree of solubility (Lyman,  1982b).

Substances dissolve in water because of a favorable energy change
associated with the formation of bonds  between the water molecules  and  the
molecules of the substance in comparison with  the  cohesive bonds between
the molecules of the substance itself.   The  process may involve strong
ionic interactions (as with the  solution of  metallic  salts) or  less
energetic hydrogen bonds (as between organic alcohols and  water).

When considering the "solution"  of one  liquid  in another,  the term
"immiscible" is used to describe a  situation of limited solubility  such
that two distinct phases will  exist. However, that immiscible  liquids
always exhibit some degree of mutual solubility.   Indeed,  the immiscible
combination of gasoline and water involves sufficient solubility of toxic
gasoline fractions to contaminate a  water supply.   Aternatively, two
liquids mixing/dissolving completely in one  another are termed  "miscible in
all proportions."  Such a circumstance  characterizes, for  example, mixtures
of methyl or ethyl alcohol  and water.

The saturation concentration, the maximum amount of a chemical  that will
dissolve in water at a specified temperature,  is an important property
value in UST leak studies.  This is  the maximum concentration that will be
reached at the interface between the leaking substance  and the  soil  water;
the maximum concentration which, via Henry's Law,  sets  the peak vapor
pressure of volative substance in the subsurface;  the maximum concentration
defining molecular diffusion of  "floating" substances into the  depth of the
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saturated zone; and the maximum concentration  of  a  substance  that  will  be
found in the groundwater at a spill  site.   Thus,  the saturation
concentration of a substance can be  used to scale the environmental
significance of a leak.

A word of caution should be interjected about  placing undue  reliance  on  the
relative solubility terms in judging environmental  significance.   The
solubility of most substances studied herein are  described  in  analytical
chemistry texts are being "very si ightly soluble" or "insoluble."   In fact,
no substance is completely insoluble, it is just  that in  the  practice of
analytical  chemistry,  these low solubilities are  not important.   In the
study of groundwater contamination,  the slightly  soluble  compounds  (the
organic species are often called hydrophobic compounds) may be the
compounds of greatest  interest.   They can  travel  long distances in  aquifer
systems and can be toxic to a level  of a few parts  per billion (ppb).
Further comment on this topic is provided  in Section 5.0.

4.2.2  ENVIRONMENTAL FACTORS THAT INFLUENCE SOLUBILITY

In the laboratory, the solubility of a chemical is  determined  by placing an
excess of chemical in  very pure water and  allowing  it to  equilibrate  at a
constant temperature.   The chemical  composition of  water  in a  soil medium,
however, is different  than very pure water because  of the solution of
minerals.  Its composition along with other factors  determine  the
solubility of a substance in soil  water.   The  factors that have been
determined  to influence the solubility of  a substance are:  temperature,
pH, dissolved organic  matter, dissolved salts, the  purity of the chemical,
redox potential, and the relative concentrations  of other substances  in
solution.  The interactions of these factors,  make  it difficult to predict
the solubility of a substance in soil water (A.D. Little, 1981).  From this
list of factors, the effects of temperature, pH,  dissolved salts and
dissolved organic matter are discussed below (Lyman  et. al., 1982b).
                                    4-4

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Temperature

The effect of temperature on the solubility of substances  is  usually
profound.  For most chemicals an increase in temperature results  in  an
increase in solubility.  A few chemicals, such as  p-dichlorobenzene,  show a
decrease in solubility with increasing temperature over certain temperature
ranges.

fill

The pH of soil water also affects the solubility of chemicals.  Organic
acids may be expected to increase in solubility with increased pH.   Organic
bases, on the other hand, behave in  the opposite way.   The solubility of
netural organic chemicals (e.g., alkanes and chlorinated hydrocarbons is
also reported to be affected by pH,  although the direction of the effect  is
not indicated.

Dissolved Salts

The presence of dissolved inorganic  salts generally reduces the solubility
of both organic and (other) inorganic substances.   This "salting out"
effect can be significant.  Also, relatively soluble substances (e.g.,
sodium stearate) can be converted to relatively insoluble  substances  (e.g.,
calcium or magnesium stearate) by cation exchange  with  groundwater-borne
ions.  Dissolved salts may also affect" the stability of emulsions and other
colloidal mixtures.

Dissolved Organic Matter

Several investigations have been reported to show  an increase in solubility
in the presence of dissolved organic matter.  Among the compounds that
exhibited this effect were:  n-alkanes,  DDT and  phthalate  esters.  The
solubilities of aromatic hydrocarbons were reported to  be  unaffected by the
presence of dissolved organic matter.
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4.2.3  CURRENT UNDERSTANDING

Nonaqueous phase liquids in the subsurface represent  a source of soluble
contaminaints for groundwater.  Chemicals dissolve at the interface  between
the chemical  and the soil  water.  The location of  the nonaqueous phase
liquid in the subsurface determines the way contaminants  can  be
solubilized.   In the unsaturated and capillary zones, chemicals are
dissolved by percolating rainwater.  Vapors can also  be solubilized  in
percolating rainwater.   The interface between  the  capillary and saturated
zones (the water table) is the location of much of the solution of floating
nonaqueous phase liquids.   In the saturated zone,  chemicals are dissolved
in the groundwater.  The density of a nonaqueous phase liquid determines
where the liquid will  be distributed in the subsurface.  In general,
nonaqueous phase liquids with a density less than  water will  be found  in
the unsaturated zone,  capillary zone and possibly  in  the  shallow part  of
the saturated zone.  In additon to these zones, more  dense  nonaqueous
liquids can be found throughout the saturated  zone.   Regardless of the
density of the fluid,  the  rate of solution of  a chemical  in soil  water will
primarly depend on  its  water solubility, the chemical/soil  water contact
area, and the flow  rate of the water (Guswa and Lyman, 1982).

The relationship between these variables and the extent of  groundwater
contaminant ion can, in  general, be simply stated:   the higher  the
solubility of the nonaqueous phase liquid, the chemical/soil water contact
area and the  flow rate, the higher the potential for  a greater extent  of
groundwater contaminantion.  In an inverse sense,  these parameters
determine how long  the  nonaqueous phase liquid remains in the  subsurface.
Another distinction that can be made is between continuous  and
discontinuous sources  of groundwater contamination.   The  nonaqueous phase
liquid in direct contact with groundwater represents  a continuous source of
contaminants.  The  solution of contaminants in percolating  rainwater is  a
discontinuous source;   available only when water is mobile  in  the
unsaturated zone (Convery, 1979).
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In general, the lighter the petroleum product (e.g., gasoline versus fuel
oil), the greater the content of soluble components (Davis et. al.,  1972).
If these products reach the water table, the concentration of the dissolved
components depends on the surface area of the "oil"-water contacts  and on
the flow rate of water.  Experiments by Fried et. al.  (1979) showed  that at
flow rates higher than those typically encountered in  sand and gravel
aquifers, an equilibrium concentration was achieved (for gas, oil,  toluene
and iso-octane) in about 10 centimeters.  At low flow  rates, solubilization
of components of petroleum products may be limited by  diffusion.  The
surface area of oil-water contacts is difficult to estimate because  the
nonaqueous phase liquid is unlikely to be a continuous fluid in the
subsurface.  In the unsaturated and saturated zones, immiscible fluids
leave a trail of nonaqueous product in the soil  matrix (residual
saturation).  Schwille (1967) has also reported the presence of oil
emulsions in the shallow part of the aquifer, which further increase the
contact area for solubilization of contaminants to take place.

Shepherd (No Date) reported concentrations of dissolved hydrocarbons that
can be expected in various places in the subsurface:  when agitated  in a
well  bore, dissolved concentrations may reach 200 ppm;  in an undisturbed
aquifer, concentrations for light hydrocarbons are approximately 50  ppm;
leaching of product-saturated soil  from the capillary  zone, both  above and
below a rising water table, provides soluble components at concentrations
of 1 to 10 ppm.  These ranges reflect both saturation  effects and the
formation of emulsions and other physical processes as well as differences
in dilution rates.

The concentrations reported above are for bulk petroleum products.   The
concentrations of individual  constituents of a petroleum product  will  vary
with their water solubility (the chromatographic separation process
mentioned previously).  Certain hydrocarbon components of petroleum
products and additives have a relatively high solubility, and thus,  higher
concentrations of these chemicals may be found than those reported for the
bulk petroleum product.  Section 5.0 discusses these factors and  the
solubilites of the hazardous  substances in detail.
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4.3  VAPORIZATION

4.3.1  OVERVIEW

Vaporization is the change in state from a  condensed  phase  (solid  or
liquid) to the gaseous state.  In this study of leaking  UST problems, the
evaporation of liquids is of primary interest.   Vaporization is  accompanied
by heat absorption (the heat of vaporization);  the  quantity of heat
required is unique to the specific chemical  involved.  For  substances
introduced into the subsurface environment  by leakage  from  USTs, the rate
of vaporization is controlled by the rate of transfer  of heat to the
evaporating substance and by the rate of reduction  in  the concentration of
the substance at the liquid-gas interface.   The evaporating substance is
removed from the liquid surface by diffusion, by adsorption onto soil
particles, and/or by solution in water in the soil.   In  the subsurface,the
significance of vaporization as a fate of a chemical  is  limited mainly to
the unsaturated, capillary and shallow part of  the  saturated zone  (Guswa
and Lyman, 1982).

For pure materials, the concentration of the substance in the gas  at the
liquid-gas interface is directly related to the vapor  pressure; essentially
a direct function of temperature only.   For solutions, the  concentration of
a substance in a gas phase [C.] in equilibrium  with the  solution is related
to the concentration in the liquid phase [X.].   The partition coefficient
which expresses the proportionality between the two is H ,  the Henry's Law
                                                        A
Constant.  Henry's Law expresses the relationship:

                               CC,] - Hx [X,]

where H  is tabulated (or capable of being  estimated)  for many organic
compounds in water solution.  It can be seen, for example,  that at the
liquid concentration corresponding to the limit of solubility (saturation
of water with the substance), the gas phase concentration corresponding to
the vapor pressure of the pure substance (p.) must  be  in  equil ibrium both
                                           I                          ' "' " '
                                    4-8

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with the pure material and with the material  in solution at  the same
temperature.  Noting that at an absolute temperature T and a total  pressure
of one atmosphere, the concentration of an ideal  gas is given by:

                               [C.] = P^RT

where R is the Universal  Gas Constant and P.  is the partial  pressure of the
vapor.

                           Hx sat.  =   [P.]
                                      CX^RT  sat.

It should be noted that the Henry's Law "Constant"  is not a  true constant
but varies with the concentration of the substance.  Indeed, the
determination of H  at saturation conditions  will  almost certainly  produce
                  A
a value quite different than that measured in highly dilute  solutions.
Also, the Henry's Law Constant is affected by the  presence of other
substances in the water such as inorganic ions and  other organic compounds.
However, in dilute solutions, the effect of the concentration of other
contaminants on the partial  pressure over the liquid is generally small.
Since for most chemicals, the vapor pressure  is a  much stronger function of
temperature than is the solubility, the Henry's Law constant will increase
with temperature in roughly the same proportion as  increasing vapor
pressure (Guswa and Lyman,-1982).

For many petroleum products,the materials stored  in USTs will  be complex
hydrocarbon mixtures.  For these mixtures, the concentration of the  pure
compound over the liquid  will not equal  the vapor  pressure of the pure
compound but will be defined by a complicated function of the specific
proportions and materials involved.  Moreover, since the proportions  of
each component in the gas phase are not usually matched to the proportions
in the liquid phase, it can  be seen that, as  evaporation proceeds, the
relative concentration of each component in both the liquid  and gas  phases
will be constantly changing.
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As an aid to understanding the  various  physical  and  chemical  factors

influencing the generation, fate and transport  of  contaminants  via the

gaseous state,  the following simplified scenario,  perhaps  typical of  a  UST

leak of gasoline,  is presented.


    1.  The UST (assume it is above  the water table)  is  perforated and  the
        contents flow out of the tank.   The  soil retains some of  the
        leaking substance but the contaminated  zone  grows  and sinks toward
        the water  table.

    2.  At the  surface of the contaminated soil  volume,  the gasoline
        vaporizes.  Locally, the soil  is cooled  in proportion to  the  latent
        heat of evaporation.  Heat flows to  the  cooled region by
        conduction.   The  hydrocarbon mixture at  the  liquid-gas  surface  is
        enriched by the more volatile,  lower molecular weight fraction  of
        the gasoline, such as the pentanes and  hexanes added  to improve
        winter  starting.

    3.  If the  soils are  sandy (porous), the migration of  hydrocarbon
        vapors  is  relatively rapid.   Since the  adsorptive  capacity of sand
        for hydrocarbons  is quite limited, there is  little adsorption.  In
        comparison to situations with  soils  of  similar porosity but higher
        adsorption capacity, the evaporation rate  is  lower since  continued
        evaporation  is paced by the  reduction of the  vapor concentration at
        the liquid interface by diffusion alone  and  is not augmented  by the
        "loss"  mechanism  of adsorption.

    4.  If it rains, the  vapor  will  (partially) dissolve in the rainwater
        percolating  down  through the subsurface.   As  the contaminated
        recharge water moves further downward (ahead  of  the advancing
        gasoline plume) it may encounter air free  of  hydrocarbon  vapors
        whereupon  dissolved hydrocarbons begin to  diffuse  out of  the water
        into the "clean air."

    5.  Eventually,  the gasoline plume  reaches  and spreads out  on the
        groundwater  surface and begins  to move  in  the direction of water
        flow.  Thus, the  surface area  for vaporization grows.   Further, the
        more soluble components of the  gasoline  hydrocarbons  (e.g.,
        benzene) dissolve in the groundwater.  Downstream, these  materials
        also volatile in  accord with a  Henry's  Law kind  of relationship.
        As the  vapors from this and  all  other hydrocarbon-gas interfaces
        move adsorption,  differing in  degree for each different
        hydrocarbon, may  retard certain  species more  than  others:  a
        chromatographic-type of separation which fractionates the vapor
        stream.

    6.  At some point, the vapor flux  reaches a  boundary.  The  boundary
        could be the surface of the  ground with  subsequent direct loss to
        the atmosphere.  The boundary could  also be an impermeable surface
        such as a  rock ledge or a tightly packed clay stratum.  Then, the
        gas phase  concentration would  build  and the movement  of contaminant
        in other directions would be accelerated.


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    7.  A third kind of boundary would be a cellar wall  or unfinished
        cellar floor.  Diffusion of the gasoline vapors  will  be  slowed  by
        the relatively high resistance of concrete but,  given the  increase
        in concentration which then occurs, vapor infiltration by  diffusion
        may be increased through cracks or sewer penetrations.  An
        unfinished floor would offer little resistance to  vapor  migration.
        As the vapor concentration  builds in the cellar, the  impact  can
        fall  into two categories.  At the lower concentration end, one  must
        be concerned about inhalation toxicity (e.g.,  for  carcinogenic
        benzene vapor).  At higher  concentrations, the potential grows  that
        the lower explosive limit may be reached such  that ignition  by  an
        electrical spark, operation of a direct-fired  water heater or
        furnace could initiate a catastrophic explosion.

4.3.?  ENVIRONMENTAL FACTORS AFFECTING VAPORIZATION
Environmental  factors which  affect  vaporization  of  a  chemical  from the
subsurface include:  soil  water content,  airflow  rate  over  the  ground
surface, humidity,  temperature and  bulk  properties  of the  soil, e.g.,
organic matter content,  porosity, density,  clay  content, and surface area.
(Thomas, 1982).  To  some  extent many of  these  factors e.g., porosity,
density, and soil  water  content affect the  movement of a vapor from the
spill  site in  that  they  determine the degree of  interconnections  in the
soil.   These factors are  discussed  in relation to vapor transport in
Section 3.6.  Other  factors  primarily determine  the degree of adsorption of
a chemical; these  include organic matter content, clay content, surface
area and indirectly humidity.   The  influence of  these factors on  adsoption
is described in Section  4.4.   The interrelationship of adsorption to
vaporization is discussed below.

The effects of adsorption on  the vaporization  of pesticides is discussed in
Thomas (1982).  He assumes that these concepts are  relevant to other
organic chemicals.   This  discussion  can  be  summarized as follows:
Adsorption competes  with  vaporization and solubility  as fate mechanisms
affecting the  distribution of  chemicals  in  the subsurface.  For weakly
adsorped chemicals,  an increase in  the moisture  content of the unsaturated
zone can increase  the vaporization  of the chemical  because water will
displace the adsorped chemical  from the  soil,  and the chemical .is then free
to dissolve in the soil water  or vaporize in the soil  air.
                                    4-11

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Vaporization can also be important  for persistent  chemicals.   DDT and
polynuclear aromatics, for example, tend to be strongly adsorbed  in  the
soil and are resistant to degradation there.   As their transport  downward
is negligible, their vaporization -- however  slow  — may be the only
pathway for their gradual removal  from the soil.

4.4  ADSORPTION

4.4.1  OVERVIEW

Adsorption, in the context of this  report, is the  mechanism that  relates
the affinity between a chemical  and the soil.  The chemical can be in  any
phase:  a nonaqueous phase liquid,  a solute in a water phase,  or  a vapor
phase.  Adsorption can occur on  almost all soil types, although for  many
soils, adsorption is negligible.   In general, adsorption  is most
significant on organic matter or clay inorganic soil  minerals.

Adsorption of vapors and non-aqueous phase liquids  has not  been as widely
studied as of solutes.  The discussion of adsorption in this section is
limited to solutes.   When viewed  in the context of  solutes, adsorption
describes the distribution of a  solute between a liquid and soil.
Adsorption in this section is also  limited to organic chemicals.
Adsorption of inorganic chemicals  is complex;  an  introduction to  this
process can be found in Freeze and  Cherry (1979).    As related to  organic
chemicals, most of the research,  and thus the understanding, of adsorption
has been done of low concentrations of organic compounds.

Adsorption has two effects on the  movement of contaminants  in  the
subsurface:  1. it retards the forward progress of  a  contaminant;  and  2.
the adsorped contaminant can act  as a residual  source, effectively
extending the contaminantion period.

Adsorption is generally considered  to be either an  affinity for the  solid
or a lack of affinity for the liquid (Weber,  1972).   In the latter case,
adsorption is called hydrophobic  bonding and  occurs  because of the
lyophobic (solvent-disliking) character of the solute relative to  a
                                    4-12

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particular solvent.  The solutes, thus, usually have a low solubility in
that liquid.  If the solute has a high affinity for soils, the three
primary types of surface phenomena that may be involved are (Hounslow,  No
Date, and Tinsley, 1979.):

    o  Physical  adsorption - results from van der Waals forces,  which is
       caused by electrostatic interaction between atoms and molecules
       arising from the fluctuations in their elecron distributions.
       Adsorbed molecules are not fixed to a specific site but are free to
       move within the interface.
    o  Chemisorption - involves the direct formation of a chemical bond
       between the adsorped molecule and the soil.
    o  Exchange adsorption -  occurs when ions of one substance  concentrate
       at the surface of the soil as a result of electrostatic attraction
       to charge sites at the soil surface.

Tinsley (1979) notes that classification of different adsorptive forces is
somewhat arbitrary and that adsorption of an organic chemical  to a soil
surface, may involve several  types of bonding.

The technique used to determine adsorption of a specific chemical  on a
specific soil is to equilibrate a known mass of uncontaminanted  soil  sample
with a known volume of solution of specific  concentration and  measure the
resultant equilibrium concentration.  The quantity adsorped can  be
determined by difference.  This experimental  information is usually
expressed as an  adsorption isotherm, where the quantity of chemical
adsorbed per unit mass of soil  is expressed  as a function of the
equilibrium concentration of the chemcial  (Tinsley,  1979).  When the data
is plotted, the  shape of the data may give some insight to the nature and
possibly the mechanisms that describe the interactions.  Hounslow (No Date)
gives a thorough description  of this.  The graphical  relationship  between
these parameters and their equivalent mathematical  relationships are known
as isotherms.  This information,  while certainly applicable to the chemical
and soil of interest, may be of limited value towards  explaining adsorptive
behavior of other chemicals because the underlying mechanisms  causing the
adsorption are not well understood.
                                    4-13

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Two relationships have been developed that tend to be more generally
applicable, the langmuir and Freundlich isotherms.  These two widely used
relationships are shown in Figure 4-1.

Langmuir Isotherm

Langmuir isotherms were developed to describe the adsorption  of gases by
solids.  The Langmuir isotherm is generally linear at low concentrations
and shows limiting adsorption and high concentrations.   It is generally
expressed as:

                           S = (SmbC) / (1 + bC)

where S is the number of moles of solute adsorbed per gram of adsorbent;
S  is the number of moles of solute  adsorbed per  gram of  adsorbent  in
forming a complete single layer of adsorbed solute molecules  on the surface
of the adsorbent; C is the concentration of solute in the liquid  phase; and
b is a constant.

In developing this relationship,  the following assumptions were made:

  -  •  The energy of adsorption is a constant and is  independent  of the
       extent of surface coverage.
    ••  The adsorption is on localized sites and there is  no interaction
       between the adsorbed molecules.
    •  The maximum adsorption possible is  that of a  complete  monolayer.
Tinsley (1979) states that because the heterogeneous  nature of  a  soil
surface invalidates the first assumption,  the equation  is  generally not
useful in the study of the adsorption of compounds from solution;
particularly onto soils.  It is more widely used  in  the adsorption  of gases
onto solids.
                                    4-14

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I
I—'
01
               CO
                     LANGMUIR
                               C
                                                      CO
              FREUDLICH
                                                                            n=1
                             S=
                               Smbc
                               1+bc
                        S=KCn
    SOURCE: Hounslow (No Date)
      CAMP DRESSER & McKEE INC.
             FIGURE 4-1

LANGMUIR AND FREUDLICH ISOTHERMS

-------
Freundlich Isotherm

The Freundlich isotherm is an empirical  relationship primarily used  to
describe nonlinear relationships.   It has wide used in  the study of
adsorption of solutes and can be expressed as  follows:

                                  S = KdCn

where K., the distribution coefficient,  and n  are  constants  and S and C
have the same definition as in the Langmuir isotherm.  Some  physical
significance has been attributed to the  constants:   n is  a measure of the
degree of nonlinearity; and K. is  indicative of the strength of adsorption
(Tinsley, 1979).  A linear form of this  equation is used  in  experimental
analysis:

                        log S' = log Kd + (l/n)log  C

If n = 1, the isotherm is linear and the change is  the  amount  adsorbed, S,
per change in concentration of the solute, C,  is equal  to  a  distribution
coefficient.

The distribution coefficient is widely used in studies  of  groundwater
contamination and is a valid representation of the  partitioning between
liquids and solids only if the reactions that  cause the partitioning are
fast and reversible and only if the isotherm is linear  (Freeze and Cherry,
1979).

Retardation

As stated above, one of the effects of adsorption" is  to retard the apparent
movement of a contaminant plume relative to the movement of  a  contaminant
not subject to adsorption (a conservative contaminant).  Under certain
conditions, the relative velocity of the adsorbed  contaminant  can'be
estimated.  These conditions are described below.
                                    4-16

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When the partitioning of a contaminant  can be adequately described  by the

distribution coefficient,  the retardation of the contaminant  relative to
the bulk mass of the water can be  described by the retardation  equation:
where v is the average Linear velocity of the groundwater;  v  is the

velocity of the c/co = 0.5 point on the concentration  profile of the
retarded constituent; p.  is the bulk density of the soil  and 9 is  the
moisture content of the soil.  The term  1+ Kd(^b/9)is referred to  as  the
retardation factor.  The reciprocal of the retardation factor is the
relative velocity of the contaminant as compared to the bulk flow of
groundwater.  Some comments on the use of this equation as  it applies  to
substances leaking from USTs are necessary.
    •  Applicat ion - The retardation equation when expressed  as  above
       applies generally to unsaturated  and saturated  flow.   For strictly
       saturated flow,  the effective porosity of the porous media can  be
       substituted for  the moisture content.

    •  Chemicals - Most of the availble  data  is  for pesticides,  and  to a
       lesser degree, aromatic and  polycyclic aromatic  ("energy-
       related") compounds for adsorption  onto organic  matter (Lyman,
       1982a).  The available data  is  presented  in Section 3.4.3.

    •  Equilibrium - Adsorption reactions  for contaminants in groundwater
       are normally viewed as being relatively rapid to the flow velocity.
       Tinsley (1979) states that there  is some  evidence that suggests  that
       equilibrium is slowly reached and cites some studies where
       equilibrium has  not been reached  after 23 days.

    •  Reversible - The desorption  process is not  as well studied as
       adsorption.  Tinsley (1979)  states  tht in some cases where
       desorption has been studied, the  rate  of  desorption is substantially
       slower than adsorption.  Also,  some of the  adsorbed substance may  be
       diffucult to remove, such that  100  percent  recovery of the
       contaminant cannot be reached.

    •  Linearity - When the Freundlich isotherm  is linear, n=l.   When  the
       relationship between the mass adsorbed and  the concentration  of  the
       solution is nonlinear, no method  is available for estimating  the
       exponential fraction, and some  value for  1/n must be assumed.   Lyman
       (1982a) presents a table showing  the errors that  can be expected
       with  use of the  linear form  of  the  Freundlich isotherm when the
       relationship is  nonlinear.   The magnitude of the  error depends  on
       the degree of nonlinearity and  how  far one  must  extrapolate from the
       range  at which K, was measured.
                                    4-17

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4.4.2  ENVIRONMENTAL FACTORS THAT INFLUENCE ADSORPTION


The primary environmental  factors that influence whether or not  adsorption

occurs, and to what degree,  are the properties  of the  soil, temperature,

and pH.  The most important  property of the soil  is its  surface  area  which

determines the availability of adsorption  sites.
Soi 1 s
As stated above,  adsorption  primarly occurs  on  two  types  of  soil--clays  and

organics.  Some of the properties  of these soils  relevant to adsorption  are

listed below (Guswa and Lyman,  1982  and  Tinsley,  1979):


    •  Clay mineral s - are layered structures of  silicates and  metal
       hydroxides.  This layering  has a  planar  geometry and  thus  a  very
       large surface area with  a very high residual  negative charge.  Clay
       m inerals  can be distributed  throughout  a  soil, but when found in
       cohesive lens or layers  have  a very low  permeability.

    •  Organics - are somewhat  hydrophobic and  organophilic, which  means
       they have  an affinity for organic substances.  Organic matter also
       provides a very large surface area and has a  high  cation exchange
       capacity (i.e., it is negatively  charged).   Adsorption on  organics
       is typically reported in relation to  its organic carbon  content.
       Lyman (1982a) reports that  while  the  ratio of  organic carbon to
       organic matter varies somewhat from soil to  soil,  a value  of 1.724
       is often assumed. The organic carbon  content  of a  soil generally
       decreases  with depth.  In the weathered  soil  zone  and peat,  the
       organic carbon content is high (up to about  40 percent).  Below this
       zone, organic carbon  content  is much  lower and are typically less
       than 0.1 percent below one  to two meters depth.  However,  in some
       locations  significant organic carbon  content  is found at significant
       depths in  the subsurface.

Because leaks from USTs usually occur several feet  below  the ground

surface, the role of organic matter  in adsorption may be  diminished.  Guswa

and Lyman (1982)  state.that  at  these depths  the relative  importance of

adsorption on clay minerals, or the  organic  fraction, may be very uncertain

and can only be resolved by  laboratory experiments with the  chemicals and

soils in question.
                                    4-18

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Temperature

Adsorption is an exothermic process.  The heat released during bonding
(called the heat of adsorption) can usually give some index of the strength
of the bond.  Small heats of adsorption (less than 10 kcal/mole)  usually
denote physical  adsorption, which explains the ability of physcially
adsorpted molecules to move freely within the interface.   Chemisorption has
a large heat of adsorption (from 30 - 50 kcal/mole) and results in a
stronger bond.   Because the heat of adsorption is exothermic, the effect
of temperature on adsorption is such that an increase in  temperature  will
usually result in a decrease in adsorption (Tinsley,  1979).  Some examples
of the effect of temperature change on K^ are cited in Lyman (1982a).

pH

For netural molecules, the effect of pH on adsorption is  very small.  A
more pronounced effect is seen for weak acids and bases which can assume a
charged form depending on the pH of the soil.  The charged  species is
always more water soluble than the neutral species, and thus, will  be
adsorbed to a smaller degree (Tinsley, 1979).  The charge on surface
adsorption sites of the solid also changes with pH.

4.4.3  ESTIMATING PROCEDURES

Most of the early work in estimating adsorption in natural  systems was  done
for pesticides and in soil near the ground surface (the weathered soil
zone).  For example, Karickhoff. (1979) reports that work  done by  Lambert
and his co-workers demonstrated that for a given soil  type, the sorption of
neutral organic pesticides was well  correlated with the organic matter
content of the soil.  The soils in the weathered soil  zone  are organic-rich
and can have organic matter content  up to 40 percent.   The  soils  of
interest in relation to leaks from USTs lie below the weathered soil zone.
As stated above, the organic carbon  content generally decreases with depth
and may reach negligible amounts (< 0.1 percent) below one  to two meters
depth.  The significance of adsorption for substances  leaking from USTs,
therefore, depends on the amount of organic matter in the subsurface soil
of interest and on adsorption to clay minerals.
                                    4-19

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Adsorption on clay or organic matter is most  important  for natural  organic
chemicals, e.g., hydrocarbon fuels or chlorinated solvents (Guswa  and
Lyman, 1982).  The water solubility of the chemical  and its  concentration
are other considerations that affect adsorption.

Adsorption studies have primarily been done on  hydrophobic (compounds
having a water solubility of less than a few parts per  million)  organic
compounds because the fate of these chemicals in  natural  systems is  highly
dependent on their sorptive behavior (Karickhoff, 1979).   In general,  the
amount of contaminant adsorbed by soils is a  function of  the concentration
of the solute in solution.  Karickhoff (1985) states that it has been
determined that for hydrophobic compounds  that  if the equilibrium  aqueous
phase pollutant concentration is below 10"  moles/liter or less  than one
half of the solute water solubility (whichever  is lower)  sorption  isotherms
to natural sediments are generally linear.  Linear isotherms result  when
the adsorption coefficient is independent  of  concentration.

McCarty et. al. (1981) stated that while the  interactions of hydrophobic
solutes with organic matter have been shown to  be reasonably predictable
over a wide range, interactions with inorganic  surfaces do not generally
follow a simple pattern.  Also, the role of the inorganic matrix is  likely
to dominate not only the solute transport  of  polar and  ionized species, but
also of npnionized, hydrophobic solutes if the  organic  content is  below a
critical level.  The theory and some experimental  data  to determine  this
critical level of organic content are presented in this paper.   They
conclude that if aquifer materials have a  very low organic content,  it is
possible that sand and clay may dominate the  retardation  effect.   The  data
are not presented here because the authors state  that "some  of the
assumptions made in arriving at these conclusions  are tenuous."  Some  of
the other factors that they suggest that need to  be  investigated are
effects of multiple dissolved organic solutes on  one another, effects  of
metal-organic complexes, interactions of ionized  organic  molecules and the
specific role of various aquifer minerals, including clay and silica.
                                    4-20

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When adsorption is due to organic matter, the tendency for a chemical  to
adsorp is expressed in terms of K  , the soil adsorption coefficient.   K
                                 (J C.                                     DC*
is defined as the ratio of the amount of chemical  adsorbed per unit weight
of organic carbon in the soil to the concentration of the chemical  in
solution at equilibrium (Lyman, 1982a):

                    K    =  g adsorbed/g organic carbon
                                 g/ml solution
Expressed in these units, K   may range from 1 to 10 million.  As with  the
distribution coefficient, K,, this relationship assumes that adsorption is
fast and reversible and that the isotherm is linear.  The relationship  of
K   to K. is as follows:

                                Koc • Kd/foc

where f   is the fraction of organic carbon in the soil.  Lyman (1982a)
reports that numerous studies have shown that values of K   obtained in
this manner (for a specific chemical) are relatively constant and
reasonably independent of the soil used.  If an experiment for a specific
soil/chemical  combination is not done to determine K. several techniques
for estimating K   are available.

The estimation techniques are empirically derived equations which relate
K   to some other property of the chemical, most commonly the octanol-water
partitioning coefficient (K  ),  the water solubility (S) and others  have
                           ow
been used.  They are usually expressed in log-log form as follows:

                       log KQC = a log (S or KQW) + b

where a and b are constants.  Lyman (1982a) and Hounslow (No Date)
summarize several of these estimation relationships.
                                    4-21

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4.5  BIOTIC PROCESSES

4.5.1  OVERVIEW

In the soil environment two biotic processes occur:  biodegradation and
biotransformation.  These processes are oxidation/reduction reactions which
are mediated by microorganisms.  Biodegradation is the decomposition of a
contaminant by microorganisms to ultimately produce energy, carbon dioxide,
and water.  Biotransformation can be, or is for the purposes of this
report, assumed to be partial biodegradation.  Contaminant compounds are
not degraded to carbon dioxide via the biotransformation process,  but are
degraded to simpler compounds, termed degradative products.  These
degradative products, however, may be more or less soluble and/or  toxic
than the original  compounds.  The microorganisms primarily responsible for
biodegradation in  the soil  include bacteria, fungi,  and yeasts.  In this
section, when a specific biotic degradation process  is not referred to, the
term degradation is used.

Biotic processes in the subsurface are complex.  Biotic processes  may occur
in three zones of  the subsurface: the unsaturated, capillary,  and  saturated
zones.  In these zones, microbes may degrade substances that are present in
various states.  These states include substances which are:

    •  adsorbed to soil particles,
    •  dissolved in water,  or
    •  held at residual saturation in the soil  matrix.

Even though a contaminant may be present in all three states,  degradation
may not occur in all  states.  Degradation by microbes in each  state is
dependent on environmental, microbial, and contaminant factors. Each of
these factors must be appropriate for biotic processes to occur.  It should
be noted that degradation of vapors does occur; vapors that are
resolubilized in the pore water may be readily biodegraded (Atlas,  1984).
                                    4-2?

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Basic to biotic processes in the soil  is the microbial  utilization of a
contaminant as a source of carbon and energy.  Carbon,  the major 'elemental
requirement of compounds for cell construction, is  released in either
organic form or as carbon dioxide (Brock and Brock, 1978).  Energy,  which
is used to drive biosynthetic reactions, is obtained from the degradative
oxidation/reduction reactions.  Biosynthetic reactions  require both  energy
and a source of carbon in order for cell growth to  occur.  Basic steps
involved in cell utilization of contaminants are shown  in Figure 4-2.

Microorganisms can be divided into two groups based on  their carbon  and
energy source:  (1.) autotrophs and (2.) heterotrophs.   Microorganisms  may
degrade a contaminant in either an aerobic or anaerobic environment  and may
be classified as facultative, possessing the ability to degrade both
aerobically and anaerobically, or obligate, possessing  the ability to
degrade a contaminant in only one type of environment,  aerobic or anaerobic
(Brock, 1979).

Requiring an electron-transport system,  the release of  energy from
contaminant degradation is primarily by respiration and may occur both
aerobically and anaerobically.  Aerobic  respiration utilizes oxygen  as  an
electron acceptor in the oxidative/reductive process.   Anaerobic
respiration utilizes an electron  acceptor other than oxygen.  Accepting
electrons from the energy source, a contaminant, an electron acceptor
becomes reduced.  Table 4-1 lists some of the common aerobic and anaerobic
electron acceptors with their respective reduced chemicals and/or
compounds.  The contrasts in electron  and carbon flow in  the oxidation  of
an organic energy source is shown in Figure 4-3. Oxidative and  reductive
processes are always paired, hence, without the presence  of an electron
acceptor microbial utilization of a contaminant will  not  occur.
                                    4-23

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          DEGRADATION:


          BIODEGRADABLE
           CONTAMINANT
                          MICROBES
    ENERGY RELEASED + CO2 + H2O +
      INTERMEDIATE END PRODUCTS
I
ro
BIOSYNTHESIS:


 INTERMEDIATE
END PRODUCTS
                             + ENERGY RELEASED
MICROBES
              NEW MICROBIAL CELLS
      CAMP DRESSER & McKEE INC.
                                            FIGURE 4-2
                            BASIC ACTIVITIES AND REQUIREMENTS FOR A CELL
                                      TO UTILIZE A CONTAMINANT

-------
                                 TABLE 4-1

                     PREDOMINANT ELECTRON ACCEPTORS USED

                      IN THE ELECTRON-TRANSPORT SYSTEM
     Aerobic
   Respiration
                        Acceptor
    Oxygen (02)
                              Reduced Product
     Water
   Anaerobic
  Respi ration
Nitrate (NO,")
                   Ni
 itrite (N02~)
                   Sulfate (S042~)

                   Carbon dioxide (CO,,)

                   Ferric (Fe  )

                   Elemental  Sulfur (S°)
                                      2_
                   Tetrathionate (S/,0C
Nitrate (N02~), nitrous
 oxide (NpOT, nitrogen (N2)

Nitrous oxide, nitrogen

Sulfide (H2S)

Methane (CH4)
           2+
Ferrous (Fe  )
                           Sulfide (H2<

                           Thiosulfate
                 2-,
Source:  Brock (1979)
                                    4-25

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                            AEROBIC RESPIRATION
             ORGANIC COMPOUND
                                        CARBON FLOW
      ELECTRON
        FLOW
                          ANAEROBIC RESPIRATION
             ORGANIC COMPOUND.
                                        CARBON FLOW
     ELECTRON
       FLOW
          NO3
                    so-
SOURCE: Brock (1979)
 CAMP DRESSER & McKEE INC.
              FIGURE 4-3
CONTRASTS IN ELECTRON AND CARBON FLOW
                                    4-26

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4.5.2  FACTORS THAT INFLUENCE DEGRADATION

Regardless of whether a chemical is readily degradable, the ultimate
breakdown of a chemical, in many cases, will  be determined by:

    •  environmental factors,

    •  the type of microorganisms, and

    •  the nature of the chemicals.

Environmental Factors

Environmental factors control metabolic activity in general  rather than
degradation specifically.  The importance of particular parameters varies
with each ecosystem.  In one environment a contaminant  can persist
indefinitely, whereas under a different set of environmental  conditions the
same contaminant can be completely degraded in a few hours or days.
Environmental variables which may affect biodegradation include:

    •  temperature,
    •  pH,.
    •  moisture,
    •  oxygen availability, and
    •  nutrients.

The following discussions are a condensation  of a detailed review by Scow
(1982).

Temperature.  The growth of microbes has been observed  in temperatures
ranging from -12 to 100°C.  Individual species are usually adapted to a
30-40 degree range within these extremes.  Depending on the  temperature in
which microorganisms have a competitive advantage over  other species,
microbes may be classified in one of three groups:  psychrophiles (< 25°C),
mesophiles (25°C - 40°C), and thermophiles (> 40°C). Temperatures outside

                                    4-27

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a microbes range are not necessarily lethal; many microbes have a dormant
state that permits survival until conditions are supportive for growth.  The
rates of biological  reactions increase with increasing temperature, within
the range tolerated by a microorganism.  This relationship may be
complicated, however, by microbial  populations that are adapted to
temperature extremes, for example,  psychrophil ic organisms may show
increased efficiency during the winter.

pH.  As a group, microorganisms have adapted to the entire pH range
normally encountered in soil  environments.   Optimal  growth for. fungi
primarily occurs under slightly acidic conditions, between pH 5 and 6.
Bacterial growth is favored by slightly alkaline conditions and is
inhibited when the pH decreases to  approximately 5.  Microbial  oxidation is
most rapid between pH 6 and 8.

Moisture and Oxygen.  In the unsaturated zone, air and moisture compete  for
soil pore space.  The amount of oxygen in the soil pore spaces  is dependent
on relative saturation levels of water and  air.  The amount of  available
oxygen affects the type and rate of biotic  degradation reactions.  Oxygen
levels are reduced by microbial  utilization of non-replaceable  oxygen
during aerobic metabolism and by encroachment of water into pore spaces
containing air.  Encroachment of water can  reduce oxygen diffusion rates by
as much as two thirds.  It is important to  note that the measure of soil
moisture that is most relevant to microorganisms is not moisture content
but "water pressure."  This is the  difference between the energy state of
the soil and the free energy of'water and represents the total
contributions of gravity, soil matrix, and  osmotic pressure.

Nutrients.  Microorganisms require  substances other than substrates for
induction of enzymes and degradation.  The  presence or absence  of such
substances influences the rate of biodegradation.  The major  elemental
requirements of a cell are carbon,  nitrogen, phosphorous, sulfur,
potassium, magnesium, calcium, iron, and sodium (Brock and Brock, 1978).
The three elements in the soil environment  that are most important in
determining growth are carbon, nitrogen, and phosphorous because of their
high concentration in a cell  (Mitchell, 1974).  If the elements are not
                                    4-28

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present in sufficient concentration (termed "limiting nutrients"),  the full
growth range of an organism, will  be inhibited.

Nature of Chemical
Correlations between physical  and chemical  properties  and the
degradability of organic compounds have been reported.  These properties
include:

    •  compound structure,
    •  compound solubility,  and
    •  chemical adsorption coefficients.

In general, highly branched and short chained compounds are less  degradable
than unbranched and long chained compounds.   For example, normal  alkanes
from n-octane to n-licosane are more susceptible to biodegradation  than
their lower molecular weight analogs, i.e.,  n-heptane  through methane.
Additional  compound structural  factors affecting the degradability  of a
compound are listed in Scow (1982).

Water-insoluble compounds are  believed to  persist longer than those that
are water soluble.  Possible reasons for  this behavior include:
    1.  inability of the compound to reach  the  reaction  site  in  the
        microbial  eel 1,
    2.  a reduced  rate  of reaction when  degradation  is  regulated by  the
        rate of solubilization,  and
    3.  the inaccessability of insoluble compounds because  of increased
        adsorption.
Chemicals that are readily adsorbed  may persist  in  the  environment.   In
addition, adsorption may be the primary factor preventing,  or significantly
delaying, the degradation in soil  of some  usually metabolizable  compounds.
Some compounds are physically trapped within  the lattice  structure of  clay
pores too small  for penetration by microorganisms.   Alternately,  by
combining with clay or other material, a compound may become  unable  to
penetrate cell membranes, i.e., undergo degradation.
                                    4-29

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The concentration of substrate is also influential  in- the  microbial
process.  If the concentration is too low,  degradation may be limited,
possibly due to the lack of sufficient stimulus  to  initiate enzymatic
response.  High concentrations of substrate may be  toxic  or inhibitory  to
metabolism, depleting the supply of oxygen.  Optimum concentration  of a
substrate is chemical and species-specific.

Type of Microorganisms

Microbial influences include the species:

    •  distribution and concentration,
    •  previous history,
    •  inter-and intra-specific interactions,  and
    •  ability to synthesize the enzyme systems  required  for  the  breakdown
       of organic compounds.

The distribution and concentration of microbes  in the  soil  varies.  Both
environmental  parameters or the presence of toxic substances  may  limit
microbial colonization.  Soil  microbial  populations  in  the  same vicinity
may differ in  numbers of microorganisms because  of  the  wide  temporal and
spatial distribution of organic matter available for microbial utilization.
The number of  microbes in the soil- is not as important  as  other factors
because microbes can rapidly increase their population  once they  have
acclimated to  a new substrate.  However, if short time  periods are of
concern, the size of the microbial  population may have  a significant
effect.  For example, complete degradation  of  glucose  can  vary from a few
hours to days  depending on  the number of organisms  present  (Scow, 1982).

The previous history of microbes in relation to  the  particular compound
undergoing degradation may influence  the reaction rate.  If a microbe has
not been recently exposed to the compound,  the microbe  may have to
acclimate to the compound.   Acclimation periods  vary from  a few hours to a
few days or even longer.  Four types  of acclimation  may take  place (Wilson
and Hensen, 1985):
                                    4-30

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    1.  Reproduction - Microbial  cells capable of degrading the contaminant
        are actively present and readily grow as degradation occurs.
    2.  Induction - Microbial  cells are not readily capable of degrading
        the contaminant and, hence, produce enzymes that aid in the
        transformation of contaminant to a degradable condition.  For
        example, enzymes may induce solubilization of a compound that  is
        not readi 1 y soluble.
    3.  Coloni zation - Microbes recently exposed to a similar contaminant
        can be transported to a new area of contamination.
    4.  Genetic - Microbial  species in a genera will  exchange plasma until
        the presence of a particular plasma in a species is conducive  to
        degradation.
Inter- and intra-specific interactions among  species  indirectl y affect the
rate of degradation.  Processes such as competition and predation can
inhibit degradation, whereas,  the degradation of a substrate may be
cooperative, with successive species degrading the initial  substrate in
sequential steps.  For example, extracellular enzymes of one organism  may
break down a compound such as  pol ysaccharide  sufficiently for uptake and
metabolism by another organism.

Microbial  species utilization  of a substrate  is enzyme-specific.  A
compound subjected to structural  alteration may require a  different enzyme
catalyst.   The absence of appropriate enzymes for microorganisms in a
genera will inhibit degradation.  For example, if a compound is degraded to
an alcohol, then to an acid, and then to inorganic matter  by
enzyme-specific species I, II, and III of the Norcardia genera,
respectively, the compound will remain an acid if the required  enzymes for
species III are not present.

Table 4-2  summarizes the variables potentially influencing  degradation.
                                    4-31

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                             TABLE 4-2

           VARIABLES POTENTIALLY AFFECTING BIODEGRADATION
ENVIRONMENTAL FACTORS

   •  Temperature
   •  pH
   •  Oxygen
   t  Moisure
   •  Nutrients

NATURE OF CHEMICAL

   0  Physical  and Chemical  Properties
   •  Concentration

NATURE OF MICROORGANISMS

   •  Spatial  Distribution and Concentration
   •  Previous  History
   •  Intra- and Inter-species Interactions
   •  Synthesis of Enzymes
                                4-32

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 4.5.3  TYPES OF DEGRADATION

 Degradation is the enzyme-catalyzed transformation of chemicals.  In all
 zones of  the subsurface  a contaminant may be degraded in three ways:

     •  aerobic degradation,

     •  anaerobic degradation, and

     •  cometabolically.
 The  degradation of a contaminant, however, may be hindered by sparing.
 Sparing  is  the process  in which the presence of a chemical will reduce or
 inhibit  the utilization of another chemical.  Such diauxic phenomena, the
 phenomenon  in which, given two carbon sources, an organism preferentially
 metabolizes one (completely) before commencing to metabolize the other,
 does not  alter the metabolic pathways of degradation, but rather determine
 whether  enzymes necessary for metabolic attack of a particular contaminant
 are  produced or active.  A lag phase commonly separates the two phases of
 growth.   For example, the presence of acetate, an intermediate product in
'hydrocarbon degradation, has been found to reduce utilization of hexadecane
 (Atlas,  1981).  The effects of such sparing processes have a marked
 influence on the  persistance of particular contaminants.

 Aerobic  and Anaerobic Degradation

 Aerobic  and anaerobic degradation occur in all zones of the soil.  Aerobic
 conditions  are more common in the unsaturated zone where conditions are
 more favorable for oxygen and nutrient exchange.  Aerobic degradation in
 the  saturated  zone is controlled by the amount of dissolved oxygen in the
 groundwater.  Anaerobic conditions may result from aerobic conditions in
 which oxygen levels are reduced by microbial depletion of non-replaceable
 oxygen during metabolism or by filling of soil pore spaces with
 oxygen-depleted water.  Certain organic compounds have been found to be
 degraded  only under anaerobic conditions, e.g., tetrachloroethylene.  The
                                    4-33

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energy yield from oxidation/reduction in anaerobic degradation is lower
than aerobic degradation (Mitchell, 1974).  This results in:

    •  much lower cell  yields,
    •  a much slower rate of decomposition, and
    •  incomplete degradation of the substrate to organic acids.

Bouwer and McCarty (1984) reported experimental results that indicated  that
overall  decomposition rates  under methanogenic (anaerobic)  conditions
appeared to be five to ten times slower than under aerobic  conditions.

Cometabolism

Cometabolism is an important transformation process  of  contaminants  by
microbes.  Cometabolism is the process in which microbes are able to
transform a secondary substrate because of the presence of  another
substrate, the primary substrate.  Unable to provide growth support  to  a
microbe, a secondary substrate is utilized due to the microbial growth
support  on a primary substrate.  Because of the transformation process  of
Cometabolism, less degradable contaminants may be attacked  more rapidly
when found in the subsurface with other contaminants that can  function  as  a
primary substrate (VanLooke  et. al., 1975).

Kinetics of Degradation

The rate at which a compound is degraded is dependent on the structure  of
the compound and the metabolic capacities of the microbes in the  ecosystem
receiving the compound.  The metabolic capacities of the microbes are
controlled by the environmental factors presented in Section 4.5.2.

The factors that affect which of the basic rate equations are  applicable in
deriving a biodegradation rate constant include:   whether a chemical is
degraded cometabol ical ly or  metabolical ly, strongly  adsorbed,  and/or
subject  to competing reactions (Scow, 1982).  For example,  one rate  law may
not adequately describe a chemical  over its total  degradative  curve  because
                                    4-34

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of changes in the chemical's concentration-dependency and  availability over
time.  In most cases, however, one rate order is assumed to be in effect
over the entire degradation curve.  The rate of growth of  a microbial
population, reflecting the rate of biodegradation, is expressed in three
equations representing:

    •  Degradation in a non-limiting environment,
    •  Degradation in a limiting environment, and
    •  Degradation by cometabolism.

These equations are discussed in detail  in Scow (1982) and Bouwer and
McCarty (1984).

VanLooke et.  al.  (1975) reports in biodegradation  trials,  betweeen 10  and
90 percent of the hydrocarbons are converted to microbial  biomass.  Field
studies by Raymond et al. (1976a) indicated the average reduction of oil
concentration due to biodegradation  ranged from 48.5 to 90 percent
depending upon the type of oil and soil.   However, due to  the large number
of different  hydrocarbons and the many different soil types,  no general
rules may be  proposed with regard to petroleum elimination in the
subsurface (VanLooke et.  al., 1975).  Any attempt  to integrate information
on rates is inhibited by the diversity in measurement techniques, soils,
environmental conditions, and the quality and quantity of  petroleum
products.

The extent of microbial degradation  of hydrocarbons in the saturated zone,
groundwater,  is not as well understood as in the unsaturated  zone.  Rates
of degradation are usually low because nitrogen and phosphorous
concentrations are commonly too low, and  oxygen is often limited  (Convery,
1979).  Under anaerobic conditions,  sometimes resulting from  microbial
utilization of limited oxygen in aerobic  conditions, rates may be only 10
percent of aerobic values.  Bouwer and McCarty (1984) reported experimental
results which indicated that overall decomposition rates under methanogenic
(anaerobic) conditions appeared to be five to ten  times slower than under
aerobic conditions.
                                    4-35

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4.5.4 IMPACT OF CONTAMINANTS

The microbial  degradation of contaminants has received considerable
attention over the past decade.  The factors that influence degradation,
the metabolic pathways of compounds, the rate of degradation,  and the
applicability of degradation as a method of corrective action  include some
of the areas studied to date.  The current understanding of microbial
degradation of petroleum products and hazardous substances in  the
subsurface is discussed below.

Petroleum Products

The majority of research on microbial  degradation of  hydrocarbons has been
conducted in laboratory studies under controlled conditions, and  the
difficulty in estimating degradation in the natural soil  environment  from
such laboratory results is stressed by several  researchers (Brookman  et.
al., 1985).  Factors such as oxygen, nutrient content, pH, soil
composition, temperature, mi.crobial populations, moisture content,  and
composition and concentration of substrate influence  the microbial
degradation in the soil environment.

ZoBell (1946) reviewed the action of microbes on hydrocarbons  and
recognized that many microorganisms have the ability  to utilize
hydrocarbons as sole sources of energy and carbon and that such
microorganisms are widely distributed in nature.  He  further recognized
that the microbial utilization  of hydrocarbons  is highly dependent  on the
nature of the chemical composition of the petroleum product and on
environmental  factors.

Twenty-one years after ZoBell's classic review, the attention  of  the
scientific community was dramatically focused on the  problems  of  oil
pollution due to the sinking of the supertanker Torrey Canyon  in  the
English Channel.  After this incident, several  studies were initiated on
the fate of petroleum in various ecosystems.
                                    4-36

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In 1968, the first detailed case study of petroleum product soil  and
groundwater contamination was conducted by McKee et. al.  (1972) at a
gasoline leakage site in Los Angeles County, California.   Experimental
results indicated that gasoline at residual saturation would not  be
mobilized by fluctuating water tables, several  bacterial  species  of the
Pseudomonas and Arthrobacter genera utilize gasoline as a source  of energy
for growth, and oxidation of gasoline occurred  rapidly in the zone of
aeration and more slowly in the saturated zone.

These initial  observations of hydrocarbon degradation, provide the base for
current research.  To date, researchers have identified:

    •  microorganisms responsible for degradation of hydrocarbons,
    •  the degradabil ity of hydrocarbon constituents,
    •  the processes responsible for hydrocarbon constituent degradation,
       and
    •  areas in the contaminated soil  where degradation  is more prevalent.

Microorganisms Responsible for Degradation.  The microorganisms primarily
responsible for the aerobic degradation of hydrocarbons  are well
documented.  Atlas (1984) reports bacteria and  fungi are  the microbes
mainly responsible for the degradation of hydrocarbons.   In decreasing
order, Pseudomonas, Arthobacter, Alcaligenes,  Corynebacterium,
Flavobacterium, Achromobacter, Microccus, Nocardia, Mycobacterium, and
Acinetobacter  are identified to be the most consistently isolated
hydrocarbon-degrading bacteria in the soil and  Trichoderma, Penicillium,
Aspergillus and Mortierella to be the hydrocarbon degrading fungi  isolated
repeatedly in the soil.  Four of the most consistently isolated bacteria,
Arthobacter, Pseudomonas, Alcaligenes, and Flavobacterium, are the most
dominant bacterial genera found  in soils indicating most  soils may possses
some of the bacteria responsible for hydrocarbon degradation.

The relative contribution of bacteria and fungi to hydrocarbon aerobic
degradation is not clear.  Bacteria tend to respond more  rapidly  to
hydrocarbon contamination of soil, whereas, fungi may be  inhibited
                                    4-37

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initially.  The activity of fungi, however, tends to persist after
bacterial activity has tapered off.  Atlas (1984) reports fungi  utilize
long chain alkanes, e.g., C~0, and bacteria prefer to metabolize straight
chain aliphatics but also attack branched and aromatic hydrocarbons.
Specifically, Raymond et. al.  (1976b) indicate Pseudomonas genera are
responsible for aromatic hydrocarbon degradation and Nocardia are probably
responsible for the major paraffinic hydrocarbon component degradation.

Several microorganisms have been isolated that anaerobically degrade
hydrocarbons.  The genera of Pseudomonas bacteria are facultative, i.e.,
capable of aerobically and anaerobically degrading hydrocarbons.  The
Moraxella genera of bacteria have been shown to metabolize aromatic
hydrocarbons anaerobically (Davis, 1967).

Degradability of Hydrocarbons.  Researchers have found normal  (straight
chain) paraffinic hydrocarbons are the most susceptible to degradation  and
aromatics are the least degradable.  Varying degradabil ity of hydrocarbons
reported by researchers are shown in Table 4-3.  Atlas (1981),  however,
reports the qualitative hydrocarbon content of the petroleum product
influences the degradability of individual  hydrocarbon components.
Examination of the susceptibility to microbial  degradation of hydrocarbons
in weathered heavy No. 6 fuel  oil (Bunker C) and light No. 2 fuel  oil
revealed far less degradation  of heavy No.  6 fuel  oil.  Major differences
in the susceptibility to degradation of each of the identical  compounds
within the context of the different hydrocarbon mixture,  i.e.,  No. 6 and
No. 2 fuel oil, were reported.

Processes Responsible for Hydrocarbons Degradation.   Knowledge  of the
metabolic pathways of hydrocarbon degradation provides insight  to
identifying the degradative processes utilized  by hydrocarbons.   The
anaerobic degradation of hydrocarbons has been  regarded  as negligible to
nonexistent.  This is based on the existance of petroleum in the
subsurface, in an anaerobic environment, for millions of years.   Wilson and
Rees (1985), however, have reported laboratory experiments indicate
anaerobic degradation of benzene, toluene,  ethylbenzene  and o-xylene.   The
disappearance of toluene was  rapid, whereas,  the other compounds required
considerable lag time before degradation occurred.

                                    4-38

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Class I
                                 TABLE 4-3

                       DEGRADABILITY OF HYDROCARBONS


                        DECREASING BIODEGRADABILITY
Class II
Class III
 Class IV
Reference
n-Paraffins
r tn T
V> Q U U "^ 1 O

Paraffinic
Med. M.W.-459


Branched
alkanes

Paraffinic
MW. 407
Aromatic
MW. 347
n-Paraffins
C5 to C8

Aromati c-
naphthenic
MW. 740

Cyclic HCs,
aliphatics,
aromatics
Naphthenic
MW. 1194


VanLooke
et.al. (1975)

Davis (1967)



HCs C1Q-C16
                                               Davis (1967)
n-alkanes,
n-alkylaro-
 matic,
 aromatic HCs
 (CIQ - c22)
n-alkanes,
 a Ikyl aromatic
 aromatic HCs
 C5 - Cg
 Branched

 C10 " C22
n-ankanes,
Hi ghly
Atlas (1984)
 alkyl aromatic,  condensed
 aromatic HCs    aromatic,
 (> C2o)         cycloparaffinic
                 w/ ^ 4 rings
Normal
 (straight
 chain)
 paraffinic
 HCs
Branched-
 chain
 paraffins
 and cyclo-
 paraffins
Aromatic HCs
               Davis et.al
               (1972)
Content:

High
 paraffinic
 HCs
Content:

High
 naphthenic
 HCs
Content:

High aromatic
 HCs
               Davis  (1967)
                                    4-39

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The importance of oxygen for hydrocarbon degradation is indicated by the
fact that the major degradative pathways for both saturated and aromatic
hydrocarbons involve oxygenases and molecular oxygen (Atlas,  1981).
Saturated (alkanes) and aromatic hydrocarbon degradation utilize molecular
oxygen for the subsequent formation of fatty acids and catechols,
repectively, with eventual oxidation and release of carbon dioxide (Brock,
1979).  Atlas (1981) reports fatty acids, some which are toxic to
microorganisms, have been found to accumulate during hydrocarbon
degradation, indicating incomplete oxidation of saturated hydrocarbons.
Particularly caprilic and laurinic fatty acids may be inhibitory towards
hydrocarbon tnetabol i zing microorganisms (VanLooke et. al., 1975).

Dissimilar to alkanes which are usually attacked at the end methyl  by
microbes, unsaturated hydrocarbons (alkenes) are attacked by microbes at
various locations producing a variety of end products.  Watkinson (1978)
reports alkene degradative end products include, but are not  limited  to:

    •     1,2 diols,
    •     w-unsaturated acids,
    •     w-unsaturated 1° and 2° alcohols,  and
    •     1,2 epoxide.

The degradation of some petroleum hydrocarbons by cometabolism has  been
reported.  Bouwer and McCarty (1984) report  experimental results indicate
ethyl benzene and napthalene are degraded by  cometabolism in the presence of
acetate.  VanLooke et. al. (1975) states researchers have found
alkyl benzenes with a methyl- or ethyl-side chain can only be  metabolized by
cometabolism, congruent to Bouwer's findings.  There have been several
reports of the cometabolic degradation of alicyclic compounds, which  are
particularly resistant to microbial attack (Dart, 1980).  The degradation
of cyclohexane after a cometabolic transformation by microbes who are
unable to metabolize cyclohexane independently has been reported (VanLooke
et. al., 1975).  Cooxidation involving gaseous hydrocarbons has been
reported by researchers.  Atlas (1984) reports, researchers have found
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ethane, butane, and propane cooxidized by Pseudomonas methanica in the
presence of methane.  In addition, the utilization of cyclohexane by
propane-grown cells has been reported by researchers.

Areas of Prevalent Degradation.  The areas in the contaminated soil
environment where degradation is most prevalent and rapid is dependent on
the environmental factors and nature of microorganisms discussed in  Section
4.5.2.  Soil microorganisms degrade petroleum contaminants rapidly under
optimum conditions, however, these conditions hardly ever exist in natural
soils.

McKee et. al. (1972) were the first to report the areas of varying degree
of degradation.  Bacterial  degradation of gasoline at residual saturation
in test columns was observed to be rapid in the zone of aeration above the
water table but much slower and less effective in the water-saturated zone.
Davis et. al. (1972) reported similar findings:  the rate of degradation
will be highest near the surface of the contaminated soil and lowest in the
zone where the oil  pancake  spreads over the water table.   The rate is low
in this area primarily for three reasons:  reduced aeration, reduced
nutrient supply, and markedly increased saturation of the soil  by oil.
Borden et. al. (No Date) developed a conceptually simple  model  to predict
the degradation of organic  contaminants under oxygen limiting conditions.
Model simulations indicate  that an anoxic (oxygen depleted)  zone will
develop between the advancing contaminant plume and the oxygenated
formation water.  The rate  of degradation will  be most rapid at the  sides
of the plume near the source where the anoxic zone is narrowest rather than
the downstream edge where the anoxic zone is widest.

Hazardous Substances

Like petroleum products, the degradation  of hazardous substances has
recently received considerable attention.  Encompassing a broad range of
chemical properties, the study of  the degradation of hazardous  substances
has focused in the area  of  determination  of those hazardous  substances  that
are degradable.  Some hazardous substances  are  not  naturally found in  the
subsurface, influencing  their persistance in the subsurface.
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As discussed in the previous subsections, degradation of a chemical  is
site-specific, i.e., dependent on environmental  factors, the nature  of
microorganisms.  For this reason it is difficult to "label" those
substances that are degradable.  In this section the following topics of
the degradability of hazardous substances will  be discussed:

    •  Hazardous substances recognized by researchers as readily degradable,
    •  Susceptibility of pesticides to degradation,
    •  Hazardous substances susceptable to biotransformation;  and
    •  The influence of concentration on the degradation of hazardous
       substances.

Hazardous Substances Readily Degraded.  Some hazardous  substances have  been
widely recognized as substances that are readily degraded in various
environments based on experimental  and field observations.  These hazardous
substances are listed in Table 4-4.  The 47 substances  comprising Table 4-4
were identified from the following  sources:

    •  Water-Related Environmental  Fate of 129  Priority Pollutants (Versar,
       1979), and
    •  Technical  Background Document to Support  Rulemaking Pursuant  to
       CERCLA Section 102 (Environmental Monitoring & Service,  1985)- RQ
       requirements of a hazardous  substance were increased if  a  chemical
       possessed a predominant fate process which influenced the
       elimination of the chemical  in the environment.
Pesticides.  Due to the recalcitrant nature of  pesticides, the  degradation
of pesticides has received considerable attention.   The recalcitrance of a
pesticide can be predicted on the basis of molecular structure.   An
approximation of comparable resistance to degradation as a function  of
structure is listed in Table 4-5.  The persistence  of common pesticides
likely to be found in USTs is shown in Table 4-6.  As indicated  in the
tables, the chlorinated hydrocarbons are the most resistant to  degradation.
                                    4-42

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                                 TABLE  4-4

                      HAZARDOUS SUBSTANCES RECOGNIZED
                            AS READILY  DEGRADED
FROM TECHNICAL BACKGROUND DOCUMENT FOR RQ ADJUSTMENTS1

    Acetaldehyde
    Acetic Acid
    Acetic Anhydride
    Acetone
    Acetonitrile
    Ammonia
    Amyl Acetate
    Aniline
    Ben zon it rile
    1 - Butanol
    Butyl acetate
    Cresol
    Cumene
    Dipropylamine
    Dimethyl amine
    Ethyl acetate
    Ethylenediamine
    Furfural
    Isobutyl Alcohol
    Methanol
    Methyl Ethyl  Ketone
    Methyl Isobuytl Ketone
    Phenol
    n - Propylamine
    Pyridine
    Quinoline
    Resorcinol
    Tri ethyl ami ne
    Vinyl Acetate

FROM 129 PRIORITY POLLUTANTS2

METALS AND INORGANICS

    Antimony
    Arsenic
    Cyanide
    Lead
    Mercury
    Selenium
                                    4-43

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                                 TABLE 4-4

                                (continued)




PESTICIDES, PCBs, AND RELATED COMPOUNDS

    Alorin
    Endosulfan and Endosulfan Sulfate
    2-Chloronaphtalene

MONOCYCLIC AROMATICS

    Phenol
    2,4 - Dichlorophenol
    Pentachlorophenol

PHTHALATE ESTERS AND POLYCYCLIC AROMATIC HYDROCARBONS

    Acenaphthene
    Napthalene
    Anthracene
    Phenanthrene
    Benzo [b] Fluoranthene
    Pyrene
Sources:   1.   Environmental  Monitoring & Services (1979)
          2.   Versar (1979)
                                    4.44

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                                 TABLE 4-5

                  RESISTANCE TO DEGRADATION OF PESTICIDES
      Decreasing
    Biodegradability
Aliphatic Acids

Organophosphates

Long chain Phenoxyaliphatic Acids

Short chain Phenoxyaliphatic Acids

Monosubstituted Phenoxyaliphatic Acids

Dissubstituted Phenoxyaliphatic Acids

Trisubstituted Phenoxyaliphatic Acids

Dinitrobenzene

Chlorinated Hydrocarbons (DDT)
Source:  Mitchell (1974)
                                    4-45

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                                 TABLE 4-6



                      PERSISTANCE OF PESTICIDES LIKELY



                            TO BE FOUND IN USTs
     Pesticide
Degradation Time in Soil  or Water
Chlordane - chlorinated hydrocarbon



DDT - chlorinated hydrocarbon



Dieldrin - chlorinated hydrocarbon



Parathion - organophosphorus



Malathion - organophosphorus
           11 years



            3 years



            3 years



            3 years



            1 week
Source:  Mitchell  (1974)
                                   4-46

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Biotransformation.  Researchers have identified chemicals that are
biotransformed.  Bouwer et. al. (1981) found that chloroform and several
other halogenated methanes were transformed readily in anaerobic water and
tri- and tetrachloroethylene disappeared at a slower rate.  Parsons (1983)
found carbon tetrachloride was transformed to chloroform in anaerobic
subsurface materials.  Similarily, tetrachloroethylene was transformed to
trichloroethylene, then to all three dichloroethylenes, and to vinyl
chloride.  Such a reaction is significant for the resulting substances may
be more toxic than the parent compound, such as the transformation of
tetrachloroethylene to vinyl chloride.

Influence of Concentration.  Wilson and McNabb (1983)  have studied the
effect of concentration on the prospect of degradation of a hazardous
substance.  The relationship between the concentration of a substrate and
its fate is complex.  At reasonably high concentrations (>100 ug/1)
utilization of a pollutant may provide an ecological  advantage resulting  in
an increase of microbial population that metabolize the organic pollutant.
However, at low concentrations (<10 ug/1) the use of the pollutant does not
provide enough ecological  advantage to lead to enrichment of microbes.  As
a result, compounds that usually are considered degradable may not be
transformed by subsurface microbes.  At high concentrations (>1,000-10,000
ug/1) metabolism of a pollutant  can entirely deplete the oxygen or other
metabolic requirements in groundwater resulting in partially degraded
compounds that may only be degraded further after dispersion or other
physical processes that mix contaminated water with oxygenated water.

4.6.  ABIOTIC CHEMICAL TRANSFORMATION PROCESSES

4.6.1  OVERVIEW

Chemical transformations decrease  contamination concentration  by degrading
the chemicals into other products.  These chemical  mechanisms  collectively
define the persistence of a contaminant in  the subsurface.   Significant
subsurface abiotic chemical transformation  mechanisms  include:   hydrolysis,
and oxidation and reduction.
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4.6.2  HYDROLYSIS

Overview

Hydrolysis is the reaction of a chemical with water, usually resulting in
the introduction of a hydroxyl function (H+, OH-) into a molecule and loss
of a leaving group (X).  The importance of hydrolysis from an environmental
fate point of view is the resulting product usually is more susceptible to
further attack through the process of bidegradation and the addition of a
hydroxyl group may make the chemical more water soluble (Neely,  1985).
Although hydrolysis of organic and inorganic compounds may occur, for the
purposes of this discussion, only organic compounds will  be addressed.
This section also does not address chemicals that are "water reactive,"
i.e., when in contact with water the chemical violently reacts (e.g.,
chlorosulfuric acid).

Hydrolysis belongs to the general  class of nucleophilic displacement
reactions where the nucleophile (N) displaces a leaving group (X):

                           R - X - N —> RN + X

When an organic compound undergoes hydrolysis, a "nucleophile" (N)  e.g.,
water or hydroxide ion, attacks an "electrophile" (R) e.g.,  carbon  atom',  or
phosphorous atom, and displaces a  "leaving group" (X) e.g.,  chloride or
phenoxide.  These displacement reactions are usually represented  by two
distinct Substitutions Nucleophilic kinetic patterns: S^,  (Unimolecular)
and SN£ (Bimolecular).

The "unimolecular" SNI process is  a two-step process characterized  by a
rate independent of the concentration and nature of the .nucleophile, (HJD+,
OH).  The S^,  .process is dependent on the concentration of the organic
compound (R-X), which undergoes a  relatively rapid  nucleophilic attack
following the formation of chemical  constituents products  from a  compound
and is favored by:
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    •  R-systems that form stable carbonium ions,  e.g.,  tributyl  and
    triphenyl methyl  systems;

    •  X-systems that are good leaving groups,  e.g.,  halide ions,
       p-toluene-sulfonate ions; and

    •  high dielectric-constant solvents  such  as water.

In an S^.p process the rate depends on the concentration  and nature  of  the
nucleophile.  The S^ process is characterized  as  a one-step bimolecular
process (Harris, 1982) and is favored by:

    •  R-systems with low carbonium stability,  e.g.,  methyl  and  other
       primary alkyl  systems;

    •  X-systems that are poor leaving groups,  e.g.,  NH2"  or CH^CH^O";  and

    •  Organic solvents such as acetone.
Hydrolysis of compounds in  water has  been  assumed  to  follow  a  first-order
decay in the concentration  of the organic  species  (Harris,  1982).   The  rate
of disappearance of RX, is  directly proportional to the  concentration of
the compound.  This first-order dependence is  important,  because  it  implies
that the hydrolysis half-life of RX is  independent of the RX concentration.
The half-life can be expressed as:

                              t1/2  =  0.693/KT

The total  hydrolysis rate constant, K-r, is generally  assumed to depend  on
the rate constants for neutral  and  specific  acid-  and base-catalyzed
hydrolysis, hydrogen ion concentration  and hydroxyl ion  concentration.  It
should be noted that Harris (1982)  states, "estimation of hydrolysis  rates
is only feasible when the hydrolysis  pathway is  reasonably simple and
straight-forward and the product(s) can be predicted."
                                    4-49

-------
Environmental Factors

Environmental factors control  the kinetics of hydrolysis.   The factors  that
have the potential for impacting the chemical transformation process  of
hydrolysis include:

    •  temperature,
    •  pH, and
    0  reaction medium - freshwater and salt water.

Based on the likelihood of leaking substances from USTs  interfacing with
salt water, the influence of this reaction medium will not  be discussed.

Temperature.  The rate of hydrolysis of chemicals increases with  increases
in temperature.  The quantitative relationship between the  rate constant
and temperature is often expressed by the Arrhenius  equation.  The
temperature dependence of the  hydrolysis rate constant is complex because
constants in the Arrhenius equation are themselves temperature dependent.
Despite these complexities,  Harris (1982) states  some useful  "rules of
thumb" for temperatures in the vicinity of 0-50°C:

    t  a 1° change in temperature causes a 10% change in k  (the rate
       constant);
    •  a 10° change in temperature causes a factor of 4.5 change  in k;  and
    •  a 25° change in temperature causes a factor of 10 change in k.

pH.  The pH dependence of the  hydrolysis rate is  summarized in Figure 4-4.
Representing a compound, the shape of the pH-rate profiles  for hydrolysis
depend on the magnitude of the natural  hydrolysis rate constant,  K  ,
compared to those of the specific acid/base-catalyzed processes.  The three
transition points, Iw, I.g,  INB,  in the figure represent the  values of  pH
at which the acid- or base-catalyzed processes begin to make  significant  /
contributions to the hydrolysis rate constant, K,..  If these  transition
points are between the typical aquatic environmental pH  range of  5-8, acid
or base catalysis must be considered in predicting rates of hydrolysis.
                                    4-50

-------
         log k
               o
                      log ky = log kH — pH
              log kT = log kQH KW + pH
 \  log kT = log kQ  /'
 K.  X.               ' I
                                 AN
                 S  'NB
                                             'AB
         PH
SOURCE: Harris (1982)
CAMP DRESSER & McKEE INC.
                FIGURE 4-4
  pH DEPENDENCE OF Kj FOR HYDROLYSIS BY
ACID-, WATER-, AND BASE-PROMOTED PROCESSES

-------
Current Understanding and Gaps

The complex aspects of the mechanisms of hydrolysis  has  received
considerable attention.  For the purposes of this project the major
emphasis is to identify those classes of compounds that  are  likely to  be
found in underground tanks that may undergo significant  hydrolysis.

Compounds Resistant to Hydrolysis.   Involving the reaction of a chemical
with water, hydrolysis of compounds primarily takes  place i'n the  saturated
zone of the soil.  In this zone, a  compound's potential  to react  with  water
is increased.

Some organic compounds are generally resistant to hydrolysis.  Table 4-7
lists organic functional  groups that are relatively  resistant  to
hydrolysis.  Functional groups that are potentially  susceptible to
hydrolysis are listed in  Table 4-8.  The multi-functional  groups  of the
categories listed in Table 4-7 may  be hydrol ytically reactive  if  they
contain a hydrolyzable functional group in  addition  to the organic groups
in the table, i.e., alcohols, acids (Harris,  1982).   Examples  of  the range
of hydrolysis half-lives  for various types  of organic compounds that are
susceptible to hydrolysis are illustrated in  Figure  4-5.

Comparison of the functional  groups in Tables 4-7 and 4-8 to the  petroleum
products and hazardous substances in Section  5.0  indicates that petroleum
products are primarily comprised of organic compounds that are generally
resistant to hydrolysis.   Based on  this comparison,  it will  be assumed that
hydrolysis is not a major fate of petroleum products.

The determination of which hazardous substances hydrol yze  is  complex and
chemical specific.  The classification of a specific  chemical within a
functional  group that is  susceptible to hydrolysis is dependent on factors
such as molecular structure and electronegativity of  the  compound  and the
other compounds interacting in the  hydrolysis  reaction.   For some  hazardous
substances hydrolysis has been identified to  be a major  process for the
substance's removal in the subsurface.   These hazardous  substances are
listed in Table 4-9'.  The hazardous substances were  identified from
tabulated data from the following sources:
                              4-52

-------
                                 TABLE 4-7

                TYPES OF ORGANIC FUNCTIONAL GROUPS THAT ARE
                     GENERALLY RESISTANT TO HYDROLYSIS
Alkanes

Alkenes

Alkynes

Benzenes/biphenyl s

Polycyclic aromatic hydrocarbons

Heterocyclic polycyclic
  aromatic hydrocarbons


Halogenated aromatics/PCBs

Dieldrin/aldrin and related
  halogenated hydrocarbon pesticides
Aromatic nitro compounds

Aromatic amines

Alcohols

Phenols

Gl ycols

Ethers

Aide! ydes

Ketones

Carboxylic acids

Sulfonic acids
Source:  Harris (1982)
                                    4-53

-------
                                 TABLE 4-8
                TYPES OF ORGANIC FUNCTIONAL GROUPS THAT ARE
                   POTENTIALLY SUSCEPTIBLE TO HYDROLYSIS
Alkyl halides
Amides
Amines
Carbamates
Carboxylic acid esters
Epoxides
Nitriles
Phosphonic acid esters
Phosphoric acid esters
Sulfonic acid esters
Sulfuric acid esters
Source:  Harris (1982)
                                    4-54

-------
Alkyl
Hal ides
n = t3
ANyl and
Benzyl Halides
n=9
Polyhalo
Methanes
n= 10
E pox ides
n= 14
Aliphatic
Acid Esters
n = 18
Aromatic
Acid Esters
n=2l
Amides
n « 12
Carbamates
n = 18
Phosphonic Acid Esters,
Oialkylphosphonates
n = 6
Phosphoric Acid,
Thiophosphoric Acid
Esters n = 6
Phosphoric Acid Halides,
Dialkylphosphonohatidates,
Oialkylphosphorohalides
n« 13
Pesticides and Misc.
Compounds
n " 13



















t—































	 [>•—






-



0 •









— 1








X = F







































'















X =CI


               Key:

               • Average
               [> Median
               n No. of Compounds Represented
                               2.2x10* 2.2x10" 2.2x10'  2.2 yi
                                yr    yr   yr
                    8 days  1.9hr I.ISmin 0.69s

                  Half-Life
  SOURCE: Harris (1982)
CAMP DRESSER & McKEE INC.
                   FIGURE 4-5
 EXAMPLES OF THE  RANGE OF HYDROLYSIS
HALF-LIVES FOR VARIOUS TYPES OF ORGANIC
 COMPOUNDS IN WATER AT pH 7 AND 25°C

         4-55

-------
                                 TABLE 4-9

                 HAZARDOUS SUBSTANCES FOR WHICH HYDROLYSIS
                      IS A SIGNIFICANT FATE MECHANISM
From RCRA Cost Analysis Method

    Chloroform
    1,2 - Dichloroethane
    1,3 - Dichloropropene
    1,3 - Dinitrobenzene
    Ethylene oxide
    Methyl chloride
    Methyl methacrylate
    Parathion
    Phthalic anhydride
    Vinyl chloride
    Cyanide (hydrogen cyanide)
                            9
From 129 Priority Pollutants"

Pesticides, PCBs, and Related Compounds
    Endosulfan and Endosulfan Sulfate Heptachlor

Halogenated Aliphatic Hydrocarbons
    Hexachlorocyclopentadiene

Halogenated Ethers
    Bis(chloromethyl)ether
                   3
From RQ Adjustments

    Acetic anhydride
    Acetyl chloride
    Ethyl, acetate
    Malathion
    Mevinphos
    Phorate
Source:   1.  ICF (1984)
         2.  Versar (1979)
         3.  Environmental  Monitoring & Services  (1985)
                                    4-56

-------
    •  RCRA Cost-Analysis Model  (ICF, 1984) - predominant fate processes of
       hazardous substances were identified,
    •  Water-Related Environmental  Fate of 129 Priority Pollutants
       (Versar, 1979), and
    •  Technical Background Document to Support Rulemaking Pursuant  to
       CERCLA Section 102 (Environmental  Monitoring & Services, 1985)  -- RQ
       requirements of a hazardous  substance were increased if a chemical
       possessed a predominant fate process which influenced the
       elimination of the chemical  in the environment.
4.6.3   OXIDATION/REDUCTION

Overview

Oxidation/reduction reactions may be biotic or abiotic.   Biotic
oxidation/reduction (biodegradation) is discussed in Section  4.2.
Oxidation is the process in which an atom losses  electrons.   Reduction  is
the process in which an atom accepts electrons.  If an atom  losses  an
electron, another atom must gain an  electron,  for this reason
oxidation/reduction reactions (redox reactions) are always paired.

Each ion-electron equation includes  both the oxidized form and  the  reduced
form of the same species.

                Cu2+    +    Zn     	>    Cu    +    Zn2+
              oxidized     reduced        reduced    oxidized

Therefore, a complete redox reaction contains  at  least two oxidants  •
(electron acceptors) and two reductants (electron donors).  A redox
reaction can occur only if the two oxidants differ in oxidizing strengths.
The tendency of a substance to donate or accept electrons in  abiotic
reactions may be given by two parameters:   electrode potential  and  redox
potential.
                                   4-57

-------
Environmental Factors

Two environmental factors influence the potential  for redox reactions to
take place in the subsurface:  oxygen availability, and pH.

Oxygen Availability.  The role of soil  in oxidation-reduction reactions is
to provide electron acceptors for the oxidation of organic compounds.
Oxygen is the strongest common electron acceptor in aerobic conditions and
generally yields less toxic products and the most energy from oxidation.
Oxygen is desirable as an electron-acceptor for these reasons.  When oxygen
is unavailable (anaerobic conditions) the prominent electron acceptors in
soils are:  Fe (III), N03", Mn2+, S042", N20,  and  H.   The decreasing order
of utilization of principle electron acceptors in  soils is 0?
                                     2+              ?+
disappearance, NO-j- disappearance, M    formation, Fe   formation,  HS~
formation, H~ formation, and CH.  formation (Bohn et.  al., 1979).   Secondary
electron acceptors yield less energy and products  may be often more toxic.
For example, hydrogen sulfide is  more toxic than sulfate, and ammonia and
nitrite are more toxic than nitrate.

pH.  The pH of the subsurface influences the availability of electrons,
i.e., redox potential.  In the pH range of 6-8, the lower the pH  (more
acidic soil), the greater the redox potential.  Low pH is characteristic of
compounds "electron rich" or reduced.  These compounds in soil 'may readily
provide electron acceptors for redox reactions.

Current Understanding and Gaps

Only a relatively narrow portion  of the total  range of electrode  and redox
potentials are available in soils (Bohn et. al., 1979).   Therefore,
oxidation/reduction can only be important when potent  oxidizing or  reducing
agents are present.  Such agents  do not exist  in natural  soil  groundwater
systems.  In natural  systems, near-surface soils and  soil  waters  will
contain oxygen, a very mild oxidizing agent,  and in deeper soils  such  mild
reducing agents as H^ and CH. may be present 'in small  amounts.  Direct
oxidation or reduction by such agents is unlikely  to  be a significant  fate
of substances leaking from USTs except  for the most reactive compounds.
                                    4-58

-------
4.7  SUMMARY OF FATE MECHANISMS

Solubility

Contaminants can go into solution when:

    •  Rainwater percolates through the unsaturated and capillary zones
       bringing dissolved contaminant with- it, possibly including those
       that were in a vapor phase.
    •  Nonaqueous phase liquids dissolve from a spreading mound on the
       water table.
    •  Contaminant dissolves from the bottom of an "oil pancake" in the
       caillary zone.

As a general rule, the lighter the petroleum product, the greater its
content of soluble components.  Gasoline, one of the most soluble petroleum
products has a solubility in water ranging from 20 to 80 mg/1.   Certain
hydrocarbon components of petroleum products and additives have a
relatively high solubility, and thus, high concentrations of these
chemicals may be found than those reported for bulk petroleum products.

Oils typically form an oil  pancake atop the water table.  Contaminants
dissolve and enter the groundwater at the interface of the oil  pancake and
the water.  The potential solution of contaminants which occurs in this
interface is proportional to the area of interface (a thinner,  more widely
spread pancake offers more  area of interface and so potentially.a faster
rate of solution).  Lab experiments have shown that at velocities
typically found in sand and gravel aquifers, saturation concentration
is typically reached in about 10 centimeters.

A diffusion corona of dissolved substance forms in the saturated zone below
the oil pancake.  There may also develop a transition zone between the oil
pancake and the diffusion zone, consisting of oil emulsions, increasing the
area of contact between water and oil and so increasing solubility.
                                   4-59

-------
Irrespective of the density of the chemical, its rate of solution  in
groundwater will depend on its solubility in water, area of contact, and
groundwater flow rate.  In an inverse sense, these parameters determine how
long the nonaqueous phase liquid remains in the subsurface.  As each of
these factors increases, the rate of solution increases.  Note that  if
groundwater movement is very slow, molecular diffusion may limit the rate
of solution.

Vaporization

Vapors can be released by contaminants in soil  or water.  Vaporization
occurs from open bodies of water, such as lakes, from subsurface soils, and
from dissolved substances in groundwater, in that order of importance.

The tendency of a chemical of limited solubility to evaporate is described
in Henry's Law constant, which expresses the driving force for transfer of
solute from aqueous to gaseous phase as the quotient of that chemical's
vapor pressure and solubility.  The higher the  Henry's Law coefficient for
a given substance, the greater its tendency to  evaporate.   The constant is
temperature-dependent; evaporation increases with temperature.

Vaporization can also be important for persistent chemicals.  DDT  and
polynuclear aromatics, for example, tend to be  strongly adsorbed in  the
soil and are resistant to degradation there.  As their transport downward
is negligible, their vaporization — however slow -- may be the only
pathway for their gradual movement out of the soil.

Adsoprtion

Adsorption of organic chemicals is most significant on organic matter or
clay minerals.  When adsorption is due to organic matter in the aquifer,
the tendency for a chemical to adsorb is expressed in terms of the K    the
                                                                    U \* '
soil adsorption coefficient.
                                  4-60

-------
Where organic carbon content in an aquifer is low (less than 0.1%), certain
mineral solids such as clay minerals may exert a substantial adsorption
effect.  The relative importance of adsorption on clay minerals, or the
organic fraction, may be very uncertain and can only be resolved by
laboratory experiments with the chemicals and soils in question.

Adsorption plays two significant roles in the fate of contaminants:

    o  It can retard the forward progress of a contaminant plume.
    o  It can act as a residual source of contamination, extending the
       period of contamination.
Biotic Processes

Some contaminants can be degraded by microbial activity;  both liquid and
gaseous compounds can be biodegraded.  Degradation is most prevalent in the
unsaturated zone where conditions are more favorable for microbial
utilization of a contaminant.

There are three modes of degradation:

    •  Aerobic degradation:  This is the predominant mode of degradation,
       taking place in the unsaturated zone of the subsurface environment
       and in the saturated zone when dissolved oxygen is available.
    •  Anaerobic degradation:  Some substances, such as
       tetrachloroethylene, can be biologically broken down only in an
       anaerobic environment.  Anaerobic conditions may result from aerobic
       conditions in which oxygen levels are reduced by microbial depletion
       of non-replaceable oxygen.
    •  Cometabo1 ism:  Contaminants that tend to be resistant to
       biodegradation can be attacked more rapidly when found in the
       subsurface with other contaminants.

The breakdown of a chemical in the natural environment will depend  on the
nature of the chemical itself, on the types of microorganisms present, and
on environmental factors.
                                  4-61

-------
Degradation of contaminant constituents is microorganism-specific and
site-specific.  In general, larger molecules and/or branch-chained
chemicals are less degradable than small molecules and/or straight-chained
chemicals.  Among the hydrocarbons, the normal paraffins are the most
susceptible to degradation, and the aromatics the least susceptible.
Encompassing a broad range of chemical properties, hazardous substances are
difficult to "label" as degradable.  However, some hazardous substances
have been widely recognized as substances that are readily degraded in
various environments.

The rate at which a compound is degraded is dependent on the structure of
the compound and the metabolic capacities of the microbes in the ecosystem
receiving the compound.  The rate of degradation is expressed in three
equations representing degradation by commetabolism and degradation in a
non-limiting and limiting environment.  No general rules may be proposed
with regard to contaminant elimination in the subsurface because of the
diversity in soils, environmental conditions, and the quality and quantity
of contaminant.

Abiotic Chemical Transformation Processes

Two types of chemical processes can occur in the subsurface:

    •  Oxidation  and  Reduction
    0  Hydrolysis.

Oxidation and reduction is the process in which an atom losses an electron
and an atom gains an electron, respectively.  Oxidation/reduction reactions
are always paired.  The tendency of a substance to donate or accept
electrons in a biotic reaction may be given by two parameters:  electrode
potential and redox potential.  Only a relatively narrow portion of the
range of electrode and redox potentials are available in soils.  Therefore,
oxidation/reduction can only be important when potent oxidizing or reducing
agents are present.  Direct oxidation or reduction by such agents is
unlikely to be a significant fate of substances leaking from USTs.except
for the most reactive compounds.
                                4-62

-------
The importance of hydrolysis from an environmental  fate point of view is
that the resulting product is usually more susceptible to further attack by
processes of biodegradation, and the presence of the hydroxyl group makes
the chemical more water-soluble.

Organic chemicals can be usefully divided into functional  groups that
represent their susceptibility to hydrolysis.  Hydrocarbons, as a class,
are generally resistant to hydrolysis.  For some hazardous substances,
hydrolysis has been identified to be a predominant  process for the
substance removal.

The kinetics of hydrolysis are influenced by environmental factors.  The
rate of disappearance of a chemical  is directly proportional to the
concentration of the compound.  The  hydrolysis half-life is independent  of
the concentration.
                                  4-63

-------

-------
            5.0  PROPERTIES OF REGULATED SUBSTANCES IN USTs
5.1  OVERVIEW


Section 5.0 addresses the physical  and chemical  nature of regulated

substances stored in underground tanks.  The section is organized as

follows:
    •  Section 5.2 provides information on "bulk" properties of
       petroleum products, as well  as on the specific chemical
       constituents present in the  bulk products.

    •  Section 5.3 concerns UST-regulated hazardous substances, and
       uses physical, chemical, and toxicological data to group the
       substances according to their likely behavior after leaking
       from USTs.

    •  Section 5.4 develops salient points-of-comparison between
       petroleum products (gasoline and fuel oil) and hazardous
       substances, as well as information on the specific constituents
       present in the bulk products.

    •  -Section 5.5 summarizes the preceding work.
A major contribution of this effort is the development of computerized
databases on the properties of substances regulated under the UST

program.  Three such data bases were developed and are contained in
their entirety in three appendices as follows:
    •  Appendix A - Properties of CERCLA-regulated hazardous
       substances stored in USTs,

    •  Appendix B - Properties of petroleum products stored in USTs,
       and

    •  Appendix C - Properties of hydrocarbons known to be
       constituents of gasoline and fuel  oil.
                                  5-1

-------
5.2 PROPERTIES OF PETROLEUM PRODUCTS

5.2.1  OVERVIEW

This section of the report addresses petroleum products and their basic
properties.  The following topics are presented, in order:

    •  bulk product properties relevant to UST leak incidents,
    •  grouping of petroleum products with respect to these properties, and
    •  discussion of the chemical composition of gasoline and fuel  oil.

5.2.2  BULK PROPERTIES DATABASE

Physical  and chemical  property information was collected for the four
groups of petroleum products addressed in this study - power fuels, heating
oils, solvents, and lubricants (Section 2.0 described the selection of
these products).  In Table 5-1 these groups are broken down into commercial
grade classifications, established by the American Society of Testing
Materials (ASTM), used in industry for both retail and wholesale commerce.
Information relevant to the mobility, toxicity, or human exposure
characteristics of a product was collected for the products listed  on Table
5-1.  Appendix B presents a complete summary of petroleum product data.

Two primary sources of information were identified for petroleum property
data.  First, the 1984 Annual  Book of ASTM standards for Petroleum  Products
and Lubricants (ASTM,  1984) details product specifications for the  commer-
cial grade petroleum products.  For the most part, property values  are
specified as a minimum/maximum range rather than absolute value, and
therefore the ASTM data are of a general  nature.  This reflects  the fact
that the  end uses which characterize the petroleum products can  be
satisfied by a relatively broad range of pure substances and mixtures.
Second, the National Institute for Petroleum and Energy Research (NIPER),
in work performed for  the American Petroleum Institute (API), compiles
annual (or semi-annual) product data from a nationwide sampling  program for
gasoline  (winter and summer),  jet fuel, heating oils, and diesel fuel
(Shelton  and Dickson,  1985).  These data, unlike the ASTM specifications,
characterize the "real" universe of products as used in commerce.

                                    5-2

-------
                                 TABLE 5-1

           COMMERCIAL GRADE CLASSIFICATION  OF PETROLEUM PRODUCTS
I.    POWER FUELS  - CIVIL USE

     A.  Aviation  gasoline and additives
        Grade 80
        Grade 100
        Grade 100LL (Low Lead)

     B.  Jet fuel  and additives
        Jet A (kerosene type)
        Jet A-l  (kerosene type)
        Jet B (wide cut or naphtha)

     C.  Automotive (motor gasoline)  and additives
        Leaded
        Unleaded

     D.  Diesel  fuel  oil  and additives
        No. 1-D,  2-D and 4-D

     E.  Gas turbine fuel  oils
        No. 0-GT, 1-GT, 2-GT, 3-GT,  4-GT

II.   HEATING AND  ILLUMINATING OILS

     A.  Fuel  Oils
        No. 1, 2, 4, 5  and 6

     B.  Kerosene

III.  SOLVENTS

     A.  Petroleum Spirits, Type  2,  3 and 4  and
        commercial hexane

     B.  Mineral Spirits or Stoddard  Solvent (Type  1,
        Petroleum Spirit)

     C.  High flash aromatic naphthas,  Type  I  and  II

     D.  VM&P Naphthas - moderately  volatile hydrocarbon
        solvents, Type  I, II, III

     E.  Petroleum Extender Oils
        Types 101, 102, 103 and  104
ASTM
Designation
        D910
        D1655




        D439



        D975


        D2880




        D396


        D3699
        D235,
        D1836

        D235
        03734

        D3735


        D2226
1
 Source:  ASTM (1984)
                                  5-3

-------
Other information sources used to compile the petroleum product data base
included textbooks in petroleum refining engineering.  Information on
petroleum products meeting Great Britain's product specifications was also
used in this work when necessary to augment US-based data and to fill data
gaps.

Certain information was unavailable for inclusion in our database.
Generally, information was available for performance-related properties
specific to particular products.  Few information sources were identified
that contained data specifically developed for the evaluation of the bulk
product properties relevant to subsurface fate or transport.  Although
petroleum products are routinely tested to demonstrate compliance with
product performance standards or specifications, most of the ASTM-specified
tests are not directly related to the mobility, toxicity, or human exposure
properties of the products.

After a review of this available information, five properties -- specific
gravity, kinematic viscosity, vapor pressure, percent composition, and
distillation temperatures, -- were chosen for inclusion in the database
because they relate to the fate and transport of the product, and/or
provide a basis for distinguishing between products.

Each of these five properties is discussed in more detail below:

    •  Specific gravity (s.g.) -- The dimensionless ratio of the density of
       a petroleum product with respect to the density of water.  Specific
       gravity is a dimensionless parameter (no units are used).  The
       petroleum industry typically reports specific gravity in degrees API
       (°API 60/60) measured at 60°F for both water and product.  °API can
       be converted to specific gravity (with respect to water) by:
                        s.g. = .(°API + 131.5)/141.5
       The specific gravity of water equals 1.0.  Petroleum products,
       except in some rare cases, have a specific gravity less than  1.0,
       typically between 0.6 and 0.9.  Thus, most petroleum products will
       float on groundwater.  Density and specific gravity have the  same
       numerical values at 4°C where the density of water exactly equals
       1.0.
                                  5-4

-------
•  Kinematic viscosity -- Calculated by dividing the viscosity by the
   density is a measure of a product's resistance to gravity flow, the
   pressure head of the fluid being directly proportional to the
   density.  The time of flow of a fixed volume of fluid is directly
   proportional to its kinematic viscosity.

                               UNITS

            c.g.s. system      1 cm2/sec = l.Stoke (ST)
              S.I. system      1 m /sec = 10  Stoke
                            p
   The centistoke (cST = 10   Stokes) is the dimension most often used
   in reporting  the kinematic viscosity of petroleum products by ASTM.
   Kinematic viscosity is a useful  parameter for comparing the relative
   rate of movement of products through the unsaturated (vadose) zone.
   The lower the kinematic viscosity, the faster a product will  migrate
   through soil driven by gravity forces.  Petroleum products are
   grouped by kinematic viscosity in Section 5.2.3.

•  Vapor pressure — The (gas phase) pressure that a material will
   develop within a closed container.  ASTM specifications require the
   Reid Vapor Pressure Test be employed for gasoline and other volatile
   non-viscous products.  The tests are conducted at 100°F using a
   special sampling and analysis procedure; the "true" vapor pressure
   is higher than the Reid vapor pressure by about 5 to 9 percent but
   this relationship varies widely (Nelson, 1969).

                         UNITS OF PRESSURE

         1 Kilopascals (kPa) = 7.5 mm Hg. = 0.01 atmosphere

   High vapor pressure products, such as gasoline and jet fuels, have
   the potential for creating vapor phase problems in other subsurface
   structures near leaking tanks.  On the other hand, when products
   with low vapor pressure leak, vapor-related impacts can usually be
   expected to be of less concern.   It should be noted, however, that
   the concentration threshold significant for fire/explosion is
   usually in the "percent range" where chronic inhalation toxicity  may
   be important for concentrations  measured in parts per million.  In
   Section 5.2.3 the relative vapor pressure of specific products is
   discussed.

•  Percent composition -- Petroleum products are mixtures of pure
   hydrocarbons (molecules consisting only of carbon (C) and hydrogen
   (H)) and, are divided into four  generic types of hydrocarbon
   molecules, as shown on Figure 5-1 and discussed below.

      %P - Paraffins (alkanes) -- Straight (n or normal) or
           branched-chain single-bonded (saturated) carbon chains.

      %0 - Olefins (alkenes) -- Straight or branched-chain carbon
           chains with at least one double bond.
                              5-5

-------
                                                COMMON HYDROCARBONS
                                                IN PETROLEUM PRODUCTS
                                      ALIPHATICS
                AROMATICS
l~n
I
ON



4
PARAFFINS
(ALKANES)
1
|
STRAIGHT BRANCHED
CHAIN
C-C-C-C
C-C-C-C
-c c
Isopentane
Pentane


or
1-Methylbutane
OLEFINS
(ALKENES)
I
1 J
STRAIGHT ^
CHAIN BRAN(
C = C-C-C-C C=C




                                                                   MONO
                                                                  BENZENE

                              NAPHTHAENO
                   NAPHTHALENE      INDANE
                                     NAPHTHENES
                                    (CYCLOALKANES)

                                         1
CYCLIC
C
cAc
c' — Ic
Cyclopentane
^ BRANCHED
C CYCLIC
CL_J r*
\^
2-Methylcyclopentane
      CAMP DRESSER & McKEE INC.
        FIGURE 5-1
COMMON HYDROCARBONS
             IN
  PETROLEUM PRODUCTS

-------
          %N - Naphthenes (cycloalkanes) -- Single bonded carbon chains
               allied in ring form;  may have branched chains.

          %A - Aromatics -- Those molecules with one or more benzene rings;
               may have branched chains.  Polynuclear aromatic hydrocarbons
               are often abbreviated as PAH's,  when discussed as a group.

       The percent composition of petroleum products directly affects the
       various performance properties of power  fuels and heating oils,
       solvents, and lubricates.  The differences in properties, both
       physical and chemical, among  these four  hydrocarbon types, (and
       among structural variations within a type) characterize individual
       petroleum products.

       Also, the typical composition can give a general  indication of
       relative toxicity of the product.  Of the four groups, the aromatics
       are known to be the most toxic to humans with respect to both
       inhalation and ingestion effects.  In Section 5.2.3, this concept is
       developed in detail for specific products, and in Section 5.2.4,
       percent composition is discussed with respect to specific
       constituents of products.  Limited data  were available for this
       property.

    •  Distillation temperatures (volume percent evaporated) -- A
       standardized test involving distillation over a programmed
       temperature span to establish the range  of boiling point of the
       product mixture.  The ASTM distillation  tests vary between products
       and are not indicative of the actual  boiling range of the
       hydrocarbon constituents of the product.  Little fractionation
       occurs in these distillation  tests, particularly for oil  and heavier
       products, and the hydrocarbons do not distill one-by-one in the
       order of their boiling points, but as successively higher boiling
       mixtures (Nelson, 1969).  The initial boiling point, end point, and
       intermediate temperatures have little fundamental  significance, but
       have been found useful in showing relative behavior corresponding to.
       points from other similar ASTM distillation tests (Nelson, 1969).
       The data can give an indication of the relative boiling points (and,
       indirectly, molecular weight  distribution) of product constituents,
       e.g., the fact that gasoline  has a lower boiling range than kerosene
       implies lower molecular weight constituents, typically lower
       viscosity, higher vapor pressure, etc. of gasoline (Section 5.2.3).


The database for the above-listed parameters, for each petroleum product in

bulk form, is included in Appendix B.  The data collected can be used with

some careful qualitative assumptions, to group  the petroleum products with

respect to their transport, fate, and subsequent potential  for human

exposure as a result of a leaking UST incident.
                                  5-7

-------
5.2.3  GROUPINGS BY BULK PROPERTIES

Petroleum products can be divided into groups based on to their fate and
transport properties.  For an overall  picture of the range of properties of
petroleum products stored in USTs, Figure 5-2 provides distillation curves
for a variety of products.  The data used to develop this figure are from
Sheldon and Dickson (1985) and represent the average distillation
temperatures observed in a nationwide sampling study.  As stated earlier,
boiling range is a general indicator of other physical and chemical
properties including viscosity, density, and vapor pressure.

Figure 5-2 illustrates the lower distillation temperatures of motor gaso-
line, the higher distillation range of fuel  oils, and the relatively narrow
distillation range of aviation turbine fuel.  It also shows the similar
distillation temperatures of No. 2 Fuel  Oil  and No. 2D Diesel Fuel.  This
is indicative of the fact that a number of oil  products with different end
uses (and specifications), have very similar distillation ranges.

Petroleum products may be divided into groups of similar potential  for
mobility and toxicity in the environment using available data for kinematic
viscosity, vapor pressure, and percent aromatic composition.  Because
property data are not available for all  products, however, some qualitative
judgment was necessary to generate the groupings.  Although other
properties (e.g., solubility and toxicity information) could also be used
to group the products, insufficient data  preclude the use of these other
relevant properties.

Kinematic Viscosity

The kinematic viscosity of a petroleum product  affects the rate at  which a
product will leak from a tank and the velocity  with which the product will
move through the vadose zone.  Simply stated, the lower a product's
viscosity, the faster it will move through the  vadose zone away from the
tank.  Table 5-2 summarizes available kinematic viscosity data from
Appendix B.  Although the data are from various sources, they can be
assumed to be, for our purposes, comparable.  The kinematic viscosity
values range from less than than 1 cST for gasoline to a (maximum)  638 cST
for No. 4GT gas turbine fuel oil.
                                   5-8

-------
                400
                300
            o
            o
            LU
            o:
            DC
            LJJ
            Q.
            2
            111
•200
                100
                    DIESEL FUEL OIL 2-D
                          FUEL OIL #2
                              I
                             20
                                            MOTOR GASOLINE
                                            WINTER UNLEADED
                                            OCTANE < 90
                        I
                       40
 \

60
I

80
100
                                   PERCENT EVAPORATED
SOURCE: Adapted From NIPER (1984)
  CAMP DRESSER & McKEE INC.
                                  FIGURE 5-2
                     DISTILLATION CURVES  OF COMMON
                           PETROLEUM PRODUCTS
                                     5-9

-------
                                 TABLE 5-2

                 KINEMATIC VISCOSITY OF PETROLEUM PRODUCTS

Product	Kinematic  Viscosity    (cST @38°C or 40°C)
	Minimum    Maximum	Typical	

AUTOMOTIVE GASOLINE            0.5         0.65<1            NA2

FUEL OIL No. 1                 1.4         2.2                 1.65
         No. 2                 2.0         3.6                 2.97
         No. 4-light           2.0         5.8                NA
         No. 4-heavy           5.8        26.4                NA
         No. 5-light         >26.4        65                  NA
         No. 5-heavy         >65         194                  NA

DIESEL FUEL OIL
         No. ID                1.3         2.4                 1.64
         No. 2D                1.9         4.1                 1.97
         No. 4D                5.5        24.0                NA

GAS TURBINE FUEL OIL
         No. 1-GT              1.3         2.4                NA
         No. 2-GT              1.9         4.1                NA
         No. 3-GT              5.5       638                  NA
         No. 4-GT              5.5       638                  NA

KEROSENE                       1.0         1.9
 Data is in centistokes (IcST = 1 cm/sec).  While kinematic viscosity is
dependent on temperature, no significant kinematic viscosity change is to
be expected between 38°C and 40 C.
2
 Not available.
                                  5-10

-------
Although data are not available for all  products, viscosity typically in-
creases with boiling point (and molecular weight) for pure hydrocarbons.
Consequently, the boiling range of the products, provide a good indication
of relative kinematic viscosity, and for the purposes of this study, where
some data were unavailable, products with similar boiling ranges were
grouped together.  Two products — petroleum extender oils and lubricating
oils -- could not be grouped because kinematic viscosity will vary among
the four groups used below depending on a specific industrial use.

Figure 5-3 identifies four kinematic viscosity groupings of petroleum
products.  Products in the lowest kinematic viscosity category, Group I,
include gasolines and solvents.  These -products will  move at the fastest
rate away from a tank leak.  Groups II and III also can be expected to move
by gravity through porous soils (but relatively slowly compared to those in
Group I).  The heavy oils in Group IV will most likely remain localized in
the tank area.

Kinematic viscosity is a temperature-dependent property.  Fuel products
with high kinematic viscosity need to be heated if fuel is to move freely.
For free flow in the storage tank, the viscosity must not exceed 30 cST (25
poises at a fuel density = 0.8533 (Hobson, 1984)).  Group IV products most
likely will be preheated in the tank prior to removal.  With respect to
mobility in the subsurface, Group IV (and at lower temperatures Group III
products) will most likely be immobile after a leak.

Vapor Pressure

Vapor pressure describes the potential for product loss through the gas
phase, i.e., its tendency to evaporate.   The higher the vapor pressure, the
faster or greater the rate of vapor formation.  Specifically, the vapor
pressure scales the peak concentration of a substance over the liquid and,
thus, the diffusion rate at which vapors enter the soil pore space.
                                 5-11

-------
                GROUP
       GROUP II
         GROUP III
         GROUP IV
I
I—1
ho
       AUTOMOTIVE GASOLINE
       AVIATION GASOLINE
       VM&P NAPHTHAS  (ALL TYPES)
       AROMATIC NAPHTHAS (TYPE I & II)
       GAS TURBINE FUEL OIL #0-GT
       PETROLEUM SPIRITS (ALL TYPES)
JET FUEL A, A-1, B
KEROSENE
FUEL OIL #1
DIESEL FUEL #1D
GAS TURBINE FUEL OIL #1-GT
FUEL OIL #2, #4
DIESEL FUEL #2D #4D
GAS TURBINE FUEL OIL #2-GT
FUEL OIL #5, #6
GAS TURBINE FUEL OIL 03-GT, #4-GT
LUBRICATING OILS
                      LOW KINEMATIC VISCOSITY
                         (FASTEST MOVERS)
                                             HIGH KINEMATIC VISCOSITY
                                                (SLOWEST MOVERS)
                       HIGH VAPOR PRESSURE
                    (GREATEST VAPOR RELEASED)
                                               LOW VAPOR PRESSURE
                                              (LEAST VAPOR RELEASED)
      CAMP DRESSER & McKEE INC.
                                  FIGURE 5-3
                   PETROLEUM PRODUCTS GROUPED  BY
                            KINEMATIC VISCOSITY

-------
As mentioned previously, the true vapor pressure is not generally measured
for mixtures such as petroleum products.  The petroleum industry uses the
Reid vapor pressure test and other methods.   The Reid test is only required
for the most volatile products such as aviation gasoline and motor
gasoline.  Loss of product due to vapor release is  high for these products
as indicated by their relatively high vapor  pressure.

Lower molecular weight hydrocarbons, i.e., those with lower boiling points
and viscosities will have higher vapor pressures.  It follows that the
higher-boiling range, more-viscous products  will have lower vapor
pressures.  Thus, the groups developed for kinematic viscosity,  shown in
Figure 5-3, may also be viewed as vapor pressure groups.

Percent Composition

No measure of toxicity has been specifically developed for many  of the bulk
petroleum products of interest here.  Some efforts  have been made toward a
toxicological understanding or quantification of the significance of
automotive gasoline (ICF, 1985), but no similar or  related data  were found
for other products.  In the absence of aquatic or mammalian toxicity data
for the products, an approximation of relative toxicity of most  products is
illustrated here based on hydrocarbon percent composition.

Four major types of hydrocarbons are found in petroleum products as discus-
sed previously and illustrated in Figure 5-1.  Of these four types, the
aromatic carbon compounds are generally known to be the most toxic to
human beings exposed through ingestion or inhalation.

For the purpose of ranking the relative toxicity of petroleum products, the
percent aromatic composition was chosen as a useful  parameter.  The
assumption is that the higher aromatic compounds in a petroleum  product,
result in relatively greater toxicity of the product.  The lack  of other
relevant data leaves the percent aromatic content as the only readily
available indicator.
                                 5-13

-------
For some of the highly mobile (low kinematic viscosity and high vapor
pressure) products shown in Group I and II in Figure 5-3, Table 5-3 lists
the percent aromatic content typically found in the bulk petroleum
products.  Thus, the products listed in Table 5-3 are all highly mobile and
may be considered to vary in relative toxicity as indicated by percent
aromatic composition.

5.2.4  CONSTITUENTS OF PETROLEUM PRODUCTS

To understand the environmental  behavior of petroleum products leaked from
USTs, a conceptual understanding of the chemical  nature of petroleum is
necessary.  Petroleum products are mixtures of hydrocarbon compounds with a
broad range of physical, chemical, and toxicological  properties.  In the
previous section, four groups of hydrocarbons were defined as being the
primary constituents of petroleums: paraffins, olefins, naphthenes, and
aromatics.  Figure 5-1 illustrated the chemical  structure of these four
groups.  It is the difference in properties, both physical and chemical,
among these hydrocarbon types that characterizes  individual petroleum
products and their respective environmental behavior.  The members of each
group act similarly with respect to environmental behavior, but to
different degrees.

This section addresses petroleum products with respect to their hydrocarbon
constituents.  By looking at the products' individual constituents, we can
illustrate the division of products into environmental  compartments.  Any
petroleum product acts as a mixture or "bulk" product only in the
unsaturated zone of the subsurface (See Section 3.0).  When, however, the
product "evaporates" or "dissolves," it is not the bulk product which is
acting, but rather the independent constituents behaving in many ways that
reflect their unique physical and chemical properties.

Development Of The Data Base

A computerized database of the constituents of motor gasoline and other
petroleum products, including their important physical  and chemical
properties, was developed for this study.  This database is presented in
Appendix C.
                                 5-14

-------
                                 TABLE 5-3
                     PERCENT AROMATIC CARBON COMPOUNDS
                              (Weight Percent)
                                                      1
    Petroleum Product
Naphthas - High Flash Aromatic
    Types I and II
VM and P Naphthas
    Types I and II
Motor Gasoline
Aviation Gasoline
Jet Fuel (A, A-l and B)
Diesel Fuel Oil
Percent Aromatic
     90-minimum




     33 maximum


     20-30 typical


     10 typical


     20 maximum


     20-40 typical
1
 Data sources are cited in Appendix B.
                                 5-15

-------
Little information is readily available concerning the specific composition
of most products, with the exception of gasoline and  No. 2 fuel oil.  Five
sources of information were identified which detailed gasoline and/or No. 2
fuel oil composition.  Appendix C lists these constituents and the
references from which the data were assembled.

The following physical and chemical  properties are included in the
database:

    •  Chemical formula
    •  Molecular weight
    •  Hydrocarbon class
    •  Density
    •  Boiling point
    •  Melting point
    •  Solubility in water
    •  Vapor pressure

The chemical constituents are grouped into the four main hydrocarbon
groups plus six other chemical groups, also known to be present in petrol-
eum products to a lesser extent.  These six groups are: aromatic amines,
alcohols, carboxylic acids, conjugated dienes, and phenols and cresols.
The chemicals in these groups may contain nitrogen or oxygen as well as
carbon and hydrogen.  Other chemical groups may be present in a specific
petroleum product depending on the source of the crude oil and the refining
process.  Additives to petroleum products could include many types of
chemicals (Section 2.2).

Initially, when developing this database, the objective was to compile con-
stituent information not only on gasoline and No. 2 fuel oil, but for other
products as well.  Although many constituents listed in Appendix C may be
present in other petroleum products, it must be understood that the heavier
oils will contain higher molecular weight hydrocarbons beyond those
included i'n our database.  Many hydrocarbons found in petroleum products

                                 5-16

-------
are not listed in Appendix C.  Also, the analyses used to characterize
samples summarized, but undoubtedly did not identify every constituent
present.  Instead the analysis indicated targeted specific "typical
compounds.  In general, it may be safe to assume that almost any stable
hydrocarbon within a specific product's boiling range may be present in
that product.

Uses of The Hydrocarbon Database

The database may be used to illustrate the nature of petroleum products
related to environmental behavior as discussed in the following paragraphs.

The solubility in water of a chemical  is related to the potential  for
groundwater related impacts of UST leaks.  Some hydrocarbon groups are
found to be more soluble than others.   The solubility data available in the
hydrocarbon database is plotted as a cumulative distribution function in
Figure 5-4. The highly soluble groups  include  the phenolics, alcohols, and
carboxylic acids followed by the aromatics.  Very insoluble compounds
include, typically, the paraffins and  the olefins.  Most gasoline
constituents have a solubility less than
100 mg/1 in water as evidenced by the  shape of the curve in Figure 5-4.

Although the alcohols and phenolic compounds are highly soluble, they are
not among the most prevalent chemical  groups in petroleum products.

Figure 5-5 summarizes percent hydrocarbon composition of an API reference
sample of unleaded motor gasoline (Reference Fuel  PS-6) used in an
inhalation study with laboratory animals sponsored by the American
Petroleum Institute (Domask, 1983). A total of 151 compounds were
identified in the PS-6 fuel sample.  Only 42 compounds, however, made up
about 75 percent of the sample volume.  These  42 compounds are the major
constituents shown in Figure 5-5.  Table 5-4 lists these 42 main compounds
by chemical group and carbon number, with their respective percent
composition.  The table also lists the solubility of the various
constituents.
                                 5-17

-------




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.0001 0.01 1 100 10000 1000000
SOLUBILITY (mg/l)


FIGURE 5-4
SOLUBILITY DATA DISTRIBUTION OF HYDROCARBON DATABASE

-------
                          100
               lit
               z

               K
                _
                O
                    100
                        API/EPA REFERENCE FUEL PS-6
90  -



80



70



60



50



40



30



20



10  -
    LEGEND           0


F\| TOTAL %

I771 WAIN  CONSTITUENTS %
                               74.2
                            TOTAL
                                       57.95
                                                     26.08
                                                          20.84
                                                                   8.96
                                                                        3.29
                    PARAFINS      AROMATICS      OLEFINS

                             HYDROCARBON TYPE
                                                                                 4.68
                                                                                     1.89
NAPHTHENES
CAMP DRESSER & McKEE INC.
                                          FIGURE 5-5
                            PERCENT COMPOSITION OF GASOLINE

-------
                                TABLE 5-4
                   Percent Composition and Solubility
                  of the Main Constituents of Gasoline
Hydrocarbon
   Type

n-paraffins
C3-C10
isoparaffins
C4-C13
% Volume
in Fuel

 11.40
 46.55
Cycloparaffins
(naphthenes)
C5-C13
mono-olefins
C2-C12
Major Constituents

C4 n-butane
C5 n-pentane
C5 n-hexane
% Accounted
for by Major
Constituents

    10.19
C4 isobutane
C5 isopentane
C6 2-methylpentane
   3-methylpentane
   2,3-dimethyl butane
C7 2-methylhexane
   3-methylhexane
   2,3-dimethylpentane
   2,4-dimethylpentane
C8 2,2,4-trimethylpentane
   2,3,4-trimethylpentane
   2,3,3-trimethyl pentane
   2,2,3-trimethylpentane
   2,2,5-trimethylpentane
C9 2-methyloctane
   3-methyloctane
   4-methyloctane	
  4.68    C5 cyclopentane
          C6 methylcyclopentane
          C7 methylcyclohexane
             1,cis,3-dimethyl-
               cyclopentane
             1,trans,3-dimethyl-
          	cyclopentane
  8.96    C3 propylene
          C4 trans butene-2
             cis butene-2
          C5 pentene-1
             trans pentene-2
             cis pentene-2
          C6 2-methylpentene-1
             2-methylpentene-2
    10.19

     1.14
    10.26
     8.81
     4.54



    11.75




     1.51


    3OT

     0.15
     0.97
     0.77




     T78?

     0.03
     0.75

     1.22


     1.26
Solubility
  (mg/1)

   6-1.4
   38.5
    9.5
   48.9

   13.8
   12.8
   22.5
                                                                          1
                                                                      54
                                                                      64
                                                                      25
                                                                    4.06
                                                                      ,14
                                                                      ,36
                                                                      ,59
                                                                    2.59

                                                                    1.42
                                                                    1.42
                                                                    0.0115
                                         160.

                                          14.

                                           7.07

                                           7.07
                                         430
                                         430
                                         148
                                         203
                                         203
                                          78
                                 5-20

-------
Hydrocarbon
   Type

Aromatics
 Alkybenzenes
 C6-C12
              % Volume
              in Fuel
               26.08
 Indans/
  tetralins
  C9-C13

 Napthalenes
 C10-C12
                1.54
                0.74
                                   TABLE 5-4
                                  (Continued)
Major Constituents
% Accounted
for by Major
Constituents
C6 benzene
C7 toluene
C8 ethylbenzene
   0-xylene
   M-xylene
   P-xylene
C9 1-methyl, 3-ethylbenzene
   1-methyl, 4-ethylbenzene
   1,2,4-trimethyl benzene
     1,
     3,
     9,
69
99
83
                                                      5.33
                                                     20.84
        Solubility
          (mg/L)
1780.
 515.
 152.
 175.
 162.
 198.
  40.
  40.
  57.
                                                                          1
TOTAL
              100
                            74.2
1
All data reported at
                      20°C.
                                  5-21

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As evidenced by Table 5-4, those chemicals of relatively high solubility
found in a large amount in gasoline are the aromatic compounds in the Cg to
Cg carbon range.  These include benzene, toluene, and xylene.

The aromatic compounds are known to be the most toxic of the gasoline
components and also appear to be somewhat soluble.  The main constituents
of gasoline are further discussed in Section 5.4 with respect to their
hazardous nature, solubility, ignitability, and vapor pressure in
comparison to regulated hazardous substances.

5.3  HAZARDOUS SUBSTANCES IN USTs

5.3.1  OVERVIEW

Not all  hazardous substances regulated by the UST program are likely to be
stored in USTs as pure products.  (A list of those substances known or
likely to be stored in USTs was developed for this study as  described in
Section 2.0).  A computerized chemical and physical  property database for
these substances was created to investigate the diversity of UST substances
and to assess their potential for causing adverse impacts when leaked in
the subsurface.  Also, in Section 5.4 the database is used to compare the
constituents of petroleum products with hazardous substances.

5.3.2  DEVELOPMENT OF THE DATABASE

Physical and chemical property data were collected for the 477 hazardous
substances identified as being known, or likely, to be stored in USTs.  The
database can be found in its entirety in Appendix A; it was  developed so
that the following kinds of work could be undertaken with the aid of
computer-based information:

    •  Grouping regulated substances based on their anticipated vapor and/
       or liquid fate and transport properties, as well as on their poten-
       tial to impact on human health.
    •  Assessing technical similarities and differences between petroleum
       and hazardous substances.
                                 5-22

-------
Properties of Interest

In order to simplify data collection, an initial screening was made of
relevant physical and chemical properties.  A qualitative evaluation of the
relevance of different properties plus an assessment of data availability
with respect to these properties was performed.  The objective was to
collect data on those properties which can be used to group as many
chemicals as possible according to their relative potential for causing
environmental impacts.

Those properties which identify or physically describe a chemical  include,
Chemical Abstract Service Registry Number (CASRN), chemical formula,
molecular weight, density, boiling point and melting point.  To determine
physical state (solid, liquid, or gas) the boiling point and melting point
must be known.  Chemical  formula and molecular weight are important to
identify chemical groups  and to make comparisons to similar chemicals.
These important properties are usually available and are included  in the
database.

Many physical and chemical properties have been identified in the
literature as important in environmental behavior (Lyman et.al.,  1981).
Properties related to subsurface fate and transport mechanisms include, but
are not limited to those  listed in Table 5-5.

Of the properties listed  in Table 5-5, the vapor pressure and water sol-
ubility may be the most important and widely studied parameters with res-
pect to  leaking UST impacts.  Vapor pressure is a property which  our
research showed to be available for a large number of substances.   It is a
useful parameter to scale the production and transport rate of hazardous
vapors following a chemical leak.  Likewise, water solubility data can
indicate the potential for a chemical to dissolve and migrate in
groundwater or percolating rainwater.
                                 5-23

-------
                             TABLE 5-5

           PROPERTIES RELATED TO ENVIRONMENTAL BEHAVIOR'

Solubility in water and other solvents
Vapor Pressure
Octanol/Water Partitioning Coefficient
Adsorption Coefficient for Soils and Sediments
Rate of Biodegradation
Dynamic and Kinematic Viscosity
Rate of Hydrolysis
Volatilization from Water (Henry's Law Coefficient)
Volatilization from Soil
Diffusion Coefficients in Air and Water
Heat of Vaporization
Vapor Density
Flash Points
Explosive Limits - Lower and Upper
*Not listed in order of importance.
                             5-24

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Many other properties on TabT'e 5-5 are considered very important to fate
and/or transport mechanisms, but data on these properties are limited
compared to the data available on vapor pressure and solubility.  With
respect to vapor phase impacts, for example, diffusion coefficients,
volatilization from water and soil, vapor density, flash point, lower
explosive limit, and heat of vaporization are all considered relatively
important.  Diffusivity and vapor pressure are the primary factors
controlling vapor transport, but diffusion coefficients are available only
for the more common chemicals.  Volatilization rates are measured by
several different methods; volatilization rates with respect to substances
in solution with water may be estimated using Henry's Law coefficients.
The "lower explosive limit" of a substance indicates the gas phase
concentration where an explosion may occur.  Flash point, another common
measure of flammability, is a measure related to the temperature at which
vapors will ignite.

Drinking water impacts occur primarily when dissolved chemicals are pumped
out of wells and/or when discharges to surface water occur.  Many of the
properties of Table 5*-5 are related to primary fate mechanisms which
inhibit transport of a chemical: rate of biodegradation, adsorption
coefficients for soils and sediments, and rate of hydrolysis.  Due to the
lack of good data on the mechanisms, however, only the octanol/water
partitioning coefficient was included in the database.  Data are available
for the other parameters mentioned above to a lesser extent than for the
primary properties discussed above.  The octanol/water coefficient was
selected because of the general availability of data for priority
pollutants and other chemicals and the high reliability/reproducibility of
data.  The other fate mechanisms above (except hydrolysis) are tested in a
variety of ways, and data are not always comparable under different test
conditions.

Another important set of relevant properties comprises those which
determine the toxicity of a chemical.  Various measures of toxicity have
been established in the fields of industrial hygien.e and safety, cancer
research, and wildlife and fisheries conservation.  The toxicity scales
established are not necessarily equivalent to each other, although each

                                5-25

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scale may measure potential danger to human health and/or the environment,
The hazardous substances regulated under the LIST program are those in the
CERCLA list.  Much effort has been made by EPA in assessing the relative
toxicity of these chemicals for Reportable Quantity (RQ) rulemaking
pursuant to CERCLA Section 102.

The Reportable Quantity Rulemaking Technical Background Document
(Environmental Monitoring and Services, Inc., 1985), serves as a
comprehensive database and specifies currently available toxicological  and
other data pertaining to hazardous substances.  The RQ methodology assigns
the letters X, A, B, C, and D to recognize a reportable quantity of 1,  10,
100, 1000 and 5000 pounds, respectively.  A 1-pound (X) chemical presents
the highest level of hazard.  The RQ is the minimum amount released to  the
environment that must be reported to the Agency.  The RQ assignment is
based not only on toxicity, but also on ignitabi lity, reactivity with
water, self-reactivity, biodegradation, hydrolysis and photolysis.  Each of
these categories was also assigned an X, A, B, C, or D level  as will  be
discussed further in Section 5.3.3.

In summary, the following properties are included in the database given in
Appendix A:
       Chemical formula
       Boiling point
       Density
       Vapor pressure
       Percent lower explosive limit
Molecular weight
Melting point
Solubility in water
Vapor density
Octanol/water parti-
                                                  tioning coefficient
    •  RQ (for toxicity)                       •  RQ (for ignitability)

Data Management System

The data collected were organized using the database management program
"SYMPHONY" marketed by Lotus Development, Inc.  While fundamentally  being a
spreadsheet application program, data entry and editing is done on a "form"
which transmits information .directly into the spreadsheet on a
one-chemical-at-a-time basis.  The SYMPHONY program has applications which
allow graphing and sorting of the data.

                                 5-26

-------
 Data  Sources

 Six primary  sources  were  used  to  collect  chemical  and  physical  property
 data.   They  are  listed  in Appendix  A and  were  used in  the  following  manner.

 The data  collection  effort  began  with  the CRC  and  Merck  Index  as  primary
 sources.   Basic  information such  as formula, molecular weight,  specific
 gravity,  melting point  and  boiling  point  were  sought for each  chemical.
 Also,  solubility in  water and  other solvents was noted.  Not all  chemicals
 were  found in  the CRC or  Merck  Index.   Environmental properties such as
 octanol/water  partitioning  coefficient were not available  in these primary
 sources.   The  first  phase of data collection ended by  a  search  of the
 Handbook  of  Environmental  Data  on Organic Chemicals to identify basic
 property  data  for chemicals not found  in  the primary sources.

 The second phase of  data  collection focused on environmental and  vapor
 phase  property data.  It  began  by utilizing the Hazardline Computer  access
 database  developed by the Occupational  Health  Services (a  private or-
 ganization located in Seacacus, New Jersey).   About 300  chemicals for which
 limited information  had been collected in the  first phase  were  queried
•through the  Hazardline  system.  The Hazardline database  not only
 supplemented data collected in  the  initial phase,  but  was  used  for data
 verification  (discussed below).  The NFPA guide to Hazardous Materials was
 used  to collect  data on the lower explosive limit  of flammable  chemicals.

 The Handbook of  Environmental  Data  for Organic Chemicals was utilized to
 supplement environmental  property data and vapor phase property data
 collected from Hazardline.   The water  related  environmental fate  of  129
 Priority  Pollutants, data available through EPA, was used  to verify
 information  regarding these relatively well-studied chemicals.
                                  5-27

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Data Verification

Standard procedures for data verification were to check data input to the
computer on the previous day, against the computer-stored data and hand-
written data worksheets.  All identified errors were corrected and any data
for which questions arose were marked and revised as necessary.

Database Limitations

Readily accessible and relevant data sources have been referenced in this
effort.  Of the 477 substances investigated, limited or no data were found
for about 7 percent.  Most of these chemicals are either phenoxy herbicides
or complicated nitroso compounds and sulfonates.  These chemicals are not
included in the following discussions of the database.  Also, a few of the
477 chemicals in the database are cited more than once such as sulfuric
acid which appears under two CASK numbers.  In addition, several  chemicals
addressed are simple mixtures (e.g., dichloropropane and dichloropropene
mixture) or general chemicals (e.g., cresol(s)) for which the components of
the mixture are known and are themselves included among the 477 (e.g.,
ortho-, meta-, and para-cresol).  In all data presentations in the text the
"population" of substances being addressed is defined.

5.3.3  USES OF THE DATABASE

The universe of hazardous substances stored  in underground tanks  includes a
diverse group of chemicals with a broad range of physical and chemical
properties.  The development of the database, described in the previous
section, allows the extraction of information pertinent to the UST program.
Information is presented addressing three major topic areas:

    •  the nature and diversity of UST substances,
    •  grouping of hazardous substances in relation to potential  environ-
       mental impact,
    •  a comparison of hazardous substances  with petroleum products
       (Section 5.4).
                                 5-28

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Diversity of Hazardous Substances


To appreciate the diversity among the various substances regulated under

the UST program, the database was utilized to divide the total  group of

substances into subgroups based on various physical  and chemical  features.

Of interest specifically were the following:  the phase normally found

(i.e., solid, liquid), the broad chemical  categories in which the various

substances fall, the solubility of the chemicals in  various solvents, and
the liquid and vapor density of chemicals.


    •  Physical states — As indicated on  Figure 5-6,  the hazardous
       substances are found at normal conditions to  be 5% in the vapor
       phase, 42% in the liquid phase, and 53% as solids.  Data collected
       for melting point and boiling point were used to assign  the
       reference state.  In the absence of data, the phase was  assigned
       based on information in the cited references  (Section 5.3.1).

       Normal conditions in this study have been assumed to be  15.6  °C
       (60 F) and one atmosphere pressure  (760 torr).   Most other data
       included in the database is referenced at standard temperatures of
       20 or 25 C.  But because the UST regulation defines liquid petroleum
       products as inclusive if liquids at 15.6°C,  rather than  the more
       conventional  metric use of 20 or 25°C, this lower temperature
       standard was  selected.

       The fact that the majority (53%) of the substances in the  database
       are solids shows the importance of  industrial  solvents used in
       storing hazardous substances.  Many solvents  are also hazardous and
       fall  in the (42%) liquid category.   As for the  solids, gases  may be
       stored either dissolved in a solvent.or stored  under pressure in
       liquid form.

    •  General chemical composition -- As  shown in Figure 5-7,  the database
       indicates that roughly 20% of the substances  of interest are
       inorganic compounds; the remaining  80% are organic.  Most  hazardous
       man-made chemicals, e.g., pesticides and herbicides, are organic.
       The hazardous characteristic of the majority  of the inorganic
       compounds arise from the cation (metallic) species.

       The organic and inorganic chemicals can be divided into  basic types
       of chemicals  which are associated by chemical  structure.   These
       classes of chemicals behave differently and have different  degrees
       of environmental impacts.  Although generalizations can  be made
       about classes of chemicals, variation  of properties within  a  group
       can be quite  large.  Table 5-6 lists some class names commonly
       associated with chemicals found to  be  hazardous substances.
                                5-29

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                             GAS
                             5%
                        PHYSICAL STATE
                        AT 15.6°C (60°F) AND 1 ATM.
CAMP DRESSER & McKEE INC.
             FIGURE 5-6
PHYSICAL STATE DISTRIBUTION OF UST
      HAZARDOUS SUBSTANCES
                             5-30

-------
CAMP DRESSER & McKEE INC.
           FIGURE 5-7
PERCENT INORGANIC AND ORGANIC
  UST HAZARDOUS SUBSTANCES
                            5-31

-------
                 TABLE 5-6

CHEMICAL CLASSES OF uST HAZARDOUS SUBSTANCES
      Alcohols and Glycols
      Aldehydes
      Aliphatics
      Alkyl  Amines
      Alkyl  Hal ides
      Alkylene Oxides
      Amides, Anilides, Imides
      Aromatics
      Aryl  Amines
      Azo Compounds
      Chromates
      Cresols
      Cyanates
      Cyanides and Nitriles
      Epoxides
      Esters
      Ethers
      Halogenated Aliphatics
      Halogenated Aromatics
      Halogens
      Heavy Metals
      Hydrazines and Hydrazides
      Inorganic Hal ides
      Ketones
      Nitrates, Nitrites
      Nitro Compounds
      Olefins   •
      Organic Acids
      Organic Ammonium Compounds
      Organics
      Organo Metal lies
      Organophosphates
      Oxides
      Phenols and Creosols
      Phosphates and Phosphonates
      Phosphorous and Compounds
      Sulfates
      Sulfides
      Sulfites
      Sulfones, Sulfoxides, Sulfonates
      Ureas
                   5-32

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 .;  Many chemicals in the database fall  into more than one chemical
   class such as diethylarsine, an organic chemical associated with1 .a
   toxic element.  Also, some chemicals in the database are very
   complicated, and do not fit easily into simple chemical classes.
   Because of the complications which arise in assigning specific
   chemical classes to each chemical, the database was not used to
   generate data of this nature.

•  Solubility of solids — About 53% of the hazardous substances are
   found as solids as shown on Figure 5-6.  An important aspect of the
   fate and transport of these chemicals is the type of solvent in
   which they are stored.  Most often,  these solids will be transported
   through the environment with the solvent until precipitation of the
   solid, vaporization of the solvent,  or dissolution into water.  To
   address this issue, the solubility of chemicals in four solvents was
   noted.  The bar chart shown on Figure 5-8 illustrates the number of
   chemicals "highly" soluble in water, ether, ethyl  alcohol and
   benzene.  As expected, some substances are soluble in more than one
   solvent -- especially alcohol and water.  Also, solubility data were
   not available for many chemicals in  solvents other than water.
   Highly soluble was defined as solubility greater than 10,000 mg/1  or
   1 percent.  Solubilities reported as "very soluble", "freely
   soluble," and "miscible" were included in this bar chart.

   Hundreds of industrial solvents are  used in the United States.  Most
   of them are organic chemicals similar to benzene or ether and will
   readily dissolve solid organic chemicals.  Inorganics tend to be
   solubilized most easily by water.  Solvents may serve as an
   indicator of the presence of other hazardous substances which
   originally had been associated with  the solvent -- in the soil
   environment.

•  Density data — The distribution based on this parameter is
   presented in Figure 5-9.  Density data were found only for 318
   hazardous substances.  The results show 29% of liquid chemicals and
   46% of solids are heavier than water which has a density of 1.0.

   Groundwater contamination occurs not only by dissolved substances
   but also by those which are insoluble or slightly soluble and found
   floating on the water surface or sinking to the aquifer bottom (see
   Section 3.0).  These chemicals will  not travel with the groundwater.
   Some insoluble chemicals that are less viscous and dense than water
   may move along the groundwater surface at a rate greater than water.

•  Vapor density -- Figure 5-10 shows the distribution of available
   vapor density data (119 of 477) for  substances in the database,
   which is related to gravitational effects of vapor movement.  Many
   chemicals produce vapors heavier than air (density = 1.0) and will
   tend to sink in the subsurface or remain near to the groundwater
   surface.
                           5-33

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                                      FIGURE 5-8

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                                      FIGURE 5-9
                  DENSITY DATA DISTRIBUTION OF LIQUID CHEMICALS

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CAMP DRESSER & McKEE INC. FIGURE 5-10
VAPOR DENSITY DATA DISTRIBUTION OF LIQUID CHEMICALS

-------
The above illustrations of data show that chemicals of many types are found
in USTs, many of which will cause potential  adverse environmental impacts

when leaked in certain environments.  All of the chemicals are hazardous
whether due to their toxicity or highly reactive nature.  The following
section presents groups of chemicals which may have similar impacts on
humans and the environment.

5.3.4  GROUPS OF HAZARDOUS SUBSTANCES



While the information provided above indicates the range and diversity of
substances regulated under the UST program,  it does not directly relate to

potential  leaking UST impacts on human health and the environment.   To

provide useful and directly relevant information, presented below are
database analyses focused on  potential impacts.


To group the substances, for  illustration, a "cause and effect"  approach
has been used.  The methodology relies on two-parameter "sortings"  of the
database to assess potential  impact as shown on  Figure 5-11.  There are two

contamination pathways of concern:  groundwater  and vapor migration leading
to human exposure.  Figure 5-11 cites the basic  concepts utilized in

deciding what chemical, physical, and/or toxicological parameters might be
combined to yield useful information.


    •  Solubility and toxicity (Sort One) -- As  discussed in Sections 3.0
       and 4.0, the solubility of a contaminant  reflects its likelihood to
       become dissolved in the saturated zone of the subsurface  or to be
       solubilized by percolating rainwater.  Toxicity measures  potential
       impact on human health.  Thus the combination of these parameters
       yields information on  which of the regulated substances may  have
       relatively greater or  lesser potential to affect human health
       through ingestion or skin exposure to contaminated groundwater.

    •  Vapor pressure and toxicity (Sort 2)  -- Vapor pressure is a  measure
       of the extent to which a substance will release mass as a vapor.
       When combined with the toxicity parameter, vapor pressure and
       toxicity together represent a measure of  the extent that  significant
       vapor phase exposure might cause human health impacts.

    •  Ignitability and vapor pressure (Sort 3)  -- Ignitability  as  used
       here, represents a measure of how likely  a sustance is to cause fire
       or explosion.  Combining ignitability with vapor pressure yields
       insight regarding whether particular  substances might be  considered
       to pose high risk of fire or explosion.

                                 5-37

-------
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  IMPACT SCENARIO
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                                  AND SUBSEQUENT HUMAN
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HUMAN EXPOSURE TO
   TOXIC VAPORS
   EXPLOSION OR
        FIRE
PARAMETERS SELECTED & REASON
                            • Solubility in water—related to pollutant
                              concentration in the saturated zone.
                            • Toxicity —related to adverse health
                              impacts.
• Vapor Pressure—related to vapor
  phase concentration.
• Toxicity—related  to adverse health
  impacts.
  Ignitability—related to ignition of
  concentrated vapors.
  Vapor Pressure—related to vapor
  phase concentration.
     CAMP DRESSER & McKEE INC.
                           FIGURE 5-11
                   GROUPING METHODOLOGY

-------
In order to perform the three, dual-parameter sortings specified above, it
was first necessary to decide how to assign meaningful ranges of the
parameters which denote high (H), medium (M), or low (L) values of
solubility, toxicity, vapor pressure, and flammability.

Hazardous substances exist as solids, liquids, or gases at normal
conditions.  Solids obviously are not stored in tanks, and will be first
dissolved in an appropriate solvent.  Thus, phase is important in  assessing
the relative impact of a chemical.  Solids were therefore not included in
the analysis concerning vapor impacts.  The 19 gases in the database are
also not included in the dual-parameter sortings, but they are evaluated
with respect to the same parameters  in this section.  Two chemicals which
are found as gases are known to be always marketed and stored in solution:
formaldehyde and hydrochloric acid.   These two chemicals have been assigned
to be liquid solutions in this analysis.

Some chemicals included in the database are highly reactive with water,
often immediately producing toxic gases.  Other substances will chemically
transform (oxidation, hydrolysis) immediately upon exposure to ambient
conditions.  Of these chemicals, those with the potential for catastrophic
effects were also removed from the analysis and evaluated individually.

The following addresses the four parameters used to  perform the sortings.

Toxicity

As discussed in Section 5.3.1, data  used for toxicity are from the CERCLA
Reportable Quantity (RQ) Background  Document (Environmental  Monitoring &
Services, Inc., 1985).  In that study each chemical  was evaluated  based on
available data for aquatic, mammalian, carcinogenic  and chronic toxicity.
Final rulemaking established RQs for 340 of the 698  CERCLA substances;
those for which it was felt sufficient data/information existed (USEPA,
1985a).  After further efforts, RQs  for 105 more chemicals were established
and reported as a Notice of Proposed Rulemaking (NPRM).  The remaining 253
chemicals are being further assessed for carcinogenic and/or chronic
                                 5-39

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toxicity.  These 253 chemicals are "assigned RQs" equal to either the RQ
assigned for the Clean Water Act (CWA 311) Reportable Quantity Rules, or an
RQ equal to 1 pound (X) - the lowest RQ.

Figure 5-12 summarizes the RQ-toxicity assignments made for the chemicals
in the database.  The letter code X, A,  B, C and D are used for 1, 10, 100,
1000, and 5000 pound RQ's, respectively.  So called "statutory chemicals"
are those for which explicit toxicity data are insufficient and which are,
therefore, subject to change.  These are identified by the upper portions
of the bars on Figure 5-12.  Many of the statutory chemicals are human
carcinogens and will most likely remain  in the X, A, and B categories (and
are herein assumed to be relatively high in toxicity).

The strength of using the RQ toxicity information lies in the fact that  all
477 chemicals of issue in this study can be assigned a relative toxicity
value.  Thus, for illustrative purposes  and completeness, the RQ data are
used to group toxicity information.

Solubility

As shown on Figure 5-13, numerical  data  available in the database for water
solubility were plotted on a logarithmic scale as a cumulative distribution
function.  Figure 5-13 indicates where the high, medium and low
classifications were set.  Of the 477 substances in the database,
solubility data existed for 374 substances.  For about 40% of the
chemicals, only written descriptions of  solubility were available, i.e.,
"soluble," "slightly soluble," etc.  To  assign high, medium and low
solubility for written descriptions, a review of suggested definitions for
solubility was made as shown in Table 5-7.  Note the 1000 - 10,000 mg/1
category may be described as "highly soluble" or "slightly soluble."

With all of this in mind, relative high, medium, and low solubility groups
were assigned as shown on Figure 5-13.  Figure 5-14 shows the number  of
substances in each group; the 374 substances shown represent about 80% of
the total population of 477.
                                 5-40

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                                     FIGURE 5-12

                           TOXICITY GROUP DISTRIBUTION

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                           FIGURE 5-13

SOLUBILITY DATA DISTRIBUTION OF HAZARDOUS SUBSTANCE DATABASE

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                                 TABLE 5-7

                                DEFINITIONS
                            SOLUBILITY IN WATER
                                   (mg/1)
Solubility Class
Verschuessen (1977)
Bennett (1974)
Extremely Soluble
Freely Soluble
 > 10,000
                                > 10,000
Highly Soluble
Slightly Soluble
 1,000-10,000
                                1,000-10,000
Moderately Soluble         200-1,000
Very Slighly Soluble
                                100-1,000
Slighly Soluble            20-200
Practically Insoluble      < 20
                                < 100
                                5-43

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CAMP DRESSER & McKEE INC.
                                     FIGURE 5-14
            HIGH, MEDIUM AND LOW SOLUBILITY GROUP DISTRIBUTION

-------
Those substances which decompose in water (11 total) are not included in
this analysis.  Also, numeric data not reported between 15°C and 25°C were
individually placed in the appropriate high, medium, or low group.

Vapor Pressure

A cummulative distribution function for vapor pressure data (for liquid
substances) was developed and is shown in Figure 5-15.  High, medium, and
low categories as indicated on this figure are:

       High (H) range            > 100 torr
       Medium (M) range         10-100 torr
       Low (L) range             > 10 torr

       Note:  760 torr = 1 atmosphere

Vapor pressure were available for 117 of the 188 liquid chemicals  in the
database.  However, based on chemical structure, 34 additional  chemicals
were assigned, "H", "M" or "L" values.

Figure 5-16 indicates the number of substances falling in  the H, M  and L
groupings.  The largest fraction has low vapor pressure and therefore can
be considered not to pose significant vapor problems.

Ignitability

In assigning RQs to the CERCLA substances, EPA reviewed ignitability as a
special characteristic.  Relying primarily on these data,  the substances
are grouped in terms of high, medium, and low potential for
ignition/explosion.  The X category of RQ assignment was not used by EPA
with respect to ignitability.  The A, B, C and D assignments as defined by
EPA are shown on Table 5-8 and the results for 111  chemicals presented on
Figure 5-17.  Data for 77 liquid chemicals were not available.   As  stated
previously, solids were not assessed for explosion/fire impact  and  gases
were evaluated separately.
                                  5-45

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VAPOR PRESSURE DATA DISTRIBUTION OF HAZARDOUS SUBSTANCE DATABASE

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     HIGH, MEDIUM AND LOW VAPOR PRESSURE GROUP DISTRIBUTION

-------
                                 TABLE 5-8


          IGNITABILITY SCALES FOR REPORTABLE QUANTITY ADJUSTMENTS
Category
D
C
B
A
X
Ingitability
(Fire)'
FP(cc) 100-140°F
(37.8-60°C)
FP < 100°F (37.8°C)
BP _> 100°F (37.8°C)
FP(cc) < 100°F (37.8°C)
BP < 100°F (37.8°C)
Pyrophoric or
self-ingitable
(Not used)
RQ (Pounds)
5000
1000
100
10
1
Notes:   FP(cc) = Flash point, closed cup
        BP     = Boiling Point
Source:  Environmental Monitoring and Services Inc. (1985)
                                 5-48

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CAMP DRESSER & McKEE INC.
                                   FIGURE 5-17


            HIGH, MEDIUM AND LOW IGNITABILITY GROUP DISTRIBUTION

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With respect to using the RQ data in this report, the following points
should be kept in mind:  class A chemicals are pyrophoric and if large
quantities of these materials are released into the environment,
catastrophic results are to be expected.  Six class A chemicals have been
removed from the dual parameter sorts as discussed below.  The ignitability
of a chemical can be related, in general terms, to the flash point and
boiling point of a substance.  The RQ measure of ignitability is associated
(in the dual-parameter sorts) with vapor pressure, though these are not
totally independent variables.

Reactivity

Some substances in the database are very highly reactive with water.
Others are unstable at normal conditions and may polymerize or
spontaneously burn.  The RQ assessment work (Environmental  Monitoring &
Services Inc., 1985) reviewed water-reaction and self-reaction (stability)
as a separate chemical  feature.  Highly reactive chemicals, with potential
catastrophic impact, have been removed from the dual  parameter sorts and
are listed in Table 5-9.  More chemicals than those listed  in Table 5-9 may
be highly reactive.  Further investigation is necessary in  this regard,
particularly with respect to toxic gas production at  a water interface.

Self-reactive compounds, which may polymerize without stabilizers,  are
listed on Table 5-10.  These are a .special group of chemicals, which may
easily transform and solidify due to polymerization when leaked from a
tank.

Results of Gas Phase Analysis

Hazardous substances found as gases under standard conditions are treated
as special cases herein.  Table 5-11 summarizes relevant data for the 19
gases in the database.  Some of the gases are highly  soluble in water, such
as ammonia and trimethylamine.  Most of the gases are in the highly
flammable "B" group for ignitability.
                                 5-50

-------
                                 TABLE  5-9
               PYROPHORIC,  EXPLOSIVE  AND  HIGH  WATER  REACTIVE
                            HAZARDOUS SUBSTANCES
CHEMICAL NAME
Aluminum phosphide
Ammonium bi fluoride
Ammonium sulfide
Calcium carbide
Calcium hypochlorite
Chromic acid
Diamine
Di ethyl arsine
Dimethyl hydrazine
Furandione, 2-
Hydrocyanic acid
Methyl hydrazine
Methyl isocyanate
Nitroglycerine
Phosphorus pentasulfide
Potassium cyanide
Sodium
Sodium cyanide
Zinc cyanide
Zinc phosphide
REACTIVITY
CLASS
B
B
B
A
A
A
A
A
A
A
A
A
-
A
B
A
A
A
A
B
                                                     REASON  FOR
                                               EXCLUSION  FROM  ANALYSIS
                                               PH3: Water Reaction
                                               HF:  Water Reaction
                                               H2S:   Water Reaction
                                               Inflames  (solid):  Water
                                               Reaction
                                               Strong oxidizer (solid):
                                               Causes fires
                                               Strong oxidizer (solid)
                                               Spontaneously ignites
                                               Pyrophoric
                                               Pyrophoric
                                               Reacts with water
                                               CN":   Water Reaction
                                               Spontaneous ignition
                                               Water  Reactive
                                               Explosive
                                               H2$: Water Reaction
                                               CN":   Water Reaction
                                               Inflames:   Water Reaction
                                               CN~:   Water Reaction
                                               CN":   Water Reaction
                                               PH:   Water Reaction
Class:
A = Inflames, pyrophoric
B = Extreme reaction
Source:  Environmental  Monitoring & Services  Inc.  (1985)
                                 5-51

-------
                                 TABLE 5-10

                     CHEMICALS REQUIRING STABILIZATION

    Chemical                    Reactivity Class

    Aziridine                        B

    Benzoylchloride                  B

    Ethanal                          B

    Propylene oxide                  B

    Styrene	    C_


B - High self reaction; may polymerize; requires stabilizer

C - moderate; contamination may cause polymerization; no inhibitor
    required.


Source:  Environmental  Monitoring & Services Inc. (1985)
                                 5-52

-------
                               TABLE  5-11

           GAS PHASE TOXICITY,  IGNITABILITY,  SOLUBILITY DATA

                    REPORTABLE  QUANTITY INFORMATION
RQ-TOX
CHEMICAL NAME (XABCD)
Ammonia
Carbonyl chloride
Chlorine
Chlorine cyanide
Chloroethane
Cyanogen
Dichlorodi fluoromethane
Dimethyl ami ne
Ethyl ene oxide
Flourine
Hydrogen phosphide
Hydrogen sulfide
Methanethiol
Methyl bromide
Methylchloride '
Monomethylamine
Nitric oxide
Trimethylamine
Vinylchloride
A
A
A
A
D
C
D
C
X
C
B
B
B
C
C
C
C
C
X
RQ-IGN
(ABCD)
B



B
B

B
B

B
B
B

B
B
A
B
B
RQ-STATUS
(FPS)1
P
F
F
F
P
F
F
P
S
F
F
P
F
F
P
F
F
P
S
RELATIVE SOLUBILITY
GROUPING
High-
Medium
Medium
High
Medium
Medium
Low
High
High

Low
Medium
High
Medium
Medium
High

High
Low
P = Proposed
S = Statutory
                               5-53

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Toxic and explosive gases are a special  case of hazardous substances.  All
should be considered to have a high potential for causing vapor incidents.


Results of Dual  Parameter Sorts


Having established the relative solubility, vapor pressure, ignitability
and toxicity categories for as many of the substances in the database as
possible, the procedure in performing the dual-parameter sorts was
straightforward.  For example, with respect to solubility and toxicity, a
chemical assigned both high solubility and high toxicity combined to form a
substance with a high potential for groundwater contamination.  The sorting
procedure indicates which regulated substances  have this HIGH-HIGH
characteristic;  these clearly have greater potential for adverse impacts
than those with  LOW-LOW characteristics.  In between, there are many
substances which exhibit either HIGH/LOW or LOW/HIGH or MEDIUM values of
the dual parameters.  Similar concepts apply to each of the dual parameter
sorts performed  here.


Figure 5-18 through 5-24 present the results of the three dual parameter
sorting procedures.  (Note that ordinate scales vary from figure to
figure).  For all results presented below, complete lists of chemicals
falling in each  group can be found in Appendix D.
    t  Figures 5-18, 5-19, and 5-20 correspond to Sort One (toxicity and
       solubility) and show the distribution of toxicity (RQ assignments)
       for highly soluble, medium soluble,  and low soluble substances,
       respectively.  Note that the figures differentiate between
       substances for which adequate toxicity data exists (those with
       final/proposed RQs) and those assigned RQs based on statutory
       directives (insufficient data).

    •  Figures 5-21, 5-22, and 5-23 correspond to Sort Two (toxicity and
       vapor pressure) and again show the toxicity RQ assignments for
       substances with high, medium, and low vapor pressures,  respectively.
       Distinctions between whether RQs are final/proposed final or
       statutory are shown in these figures.

    •  Figure 5-24 corresponds to Sort  Three (ignitability and vapor
       pressure) and shows the low, medium, and high  vapor pressure
       substances in their appropriate  ignitability class.  Ignitability
       RQs are all based on actual  data so  no differentiation  between
       assignments is shown here as on  earlier figures.

                                 5-54

-------
01
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     STATUTORY


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                           RQ - TOXICITY ASSIGNMENT
                                                                                 ////
     CAMP DRESSER & McKEE INC.
                                     FIGURE 5-18

                 TOXICITY — SOLUBILITY SORT: HIGH SOLUBILITY RESULTS

-------
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  RQ - TOXICITY ASSIGNMENT
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               |T1 FINAL/PROPOSED RQ
CAMP DRESSER & McKEE INC.
                                   FIGURE 5-19

            TOXICITY - SOLUBILITY SORT: MEDIUM SOLUBILITY RESULTS

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                     RQ - TOXICITY ASSIGNMENT
                                                                                     15
     CAMP DRESSER & McKEE INC.
                                   FIGURE 5-20

            TOXICITY — SOLUBILITY SORT: LOW SOLUBILITY RESULTS

-------
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21
20
19
18
17
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 9
 8
 7
 6
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 4
 3
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                         TOXICITY ASSIGNMENT
      CAMP DRESSER & McKEE INC.
                            FIGURE 5-21
  TOXICITY — VAPOR PRESSURE SORT: HIGH VAPOR PRESSURE RESULTS

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                     TOXICITY (RQ ASSIGNMENT)
     CAMP DRESSER & McKEE INC.
                               FIGURE 5-22

   TOXICITY — VAPOR PRESSURE SORT:  MEDIUM VAPOR PRESSURE RESULTS

-------
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                                                 RQ - TOXICITY ASSIGNMENT
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       TOXICITY — VAPOR PRESSURE SORT: LOW VAPOR PRESSURE RESULTS

-------
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               [77! MEDIUM V.P,

               |  /\ LOW V.P.
                                         12
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                  RQ-IGNITABILITY ASSIGNMENT
CAMP DRESSER & McKEE INC.
                       FIGURE 5-24
     IGNITABILITY — VAPOR PRESSURE SORT: HIGH, MEDIUM
          & LOW VAPOR PRESSURE RESULTS

-------
Some salient observations on the nature and degree of impacts associated
with UST-regulated hazardous substances follow from these results.
Appendix D provides lists — by name -- of which substances fall  into which
categories.  More generally, however, the results suggest that it is
possible to group a high number of substances based on various attributes
related to fate and transport as well as to impact.

For example, the highly soluble substances in toxic groups X, A,  and B
(Figure 5-18) clearly pose a greater threat of groundwater contamination
(and, potentially to human health) than do the low solubility substances in
toxicity groups C or D (Figure 5-20).  Similarly, the substances  with high
vapor pressure and in toxic groups X, A, or B (Figure 5-21) pose  greater
threats of vapor-related health impacts than the low vapor pressure
substances in toxic groups C and D (Figure 5-23).

The degree of fire/explosion potential is self-evident from Figure 5-24;
the high vapor pressure substances in the B ignitability class are
generally more pisky than the low vapor pressure substances in the C
ignitability class.  The results reflect the fact that vapor pressure and
ignitability are not completely independent variables.  Future research on
vapor phase migration in the subsurface may support a different approach to
grouping the substances by fire/explosion potential.

5.4 COMPARISON OF HAZARDOUS SUBSTANCES AND PETROLEUM PRODUCTS

RCRA subtitle I explicity allows EPA to establish different regulatory
requirements for petroleum products and hazardous substances.  Recognizing
this, the following discussion presents a comparison of petroleum product
constituents with those substances in the hazardous substances database.
The discussion is limited by what is explicity known about the specific
chemical composition of petroleum products.

In simplified form, Figure 5-25 illustrates the general  make-up of
petroleum products.  While all petroleum products do not necessarily have
every component shown on Figure 5-25, the majority of widely used products
                                 5-62

-------
                                       PETROLEUM PRODUCTS
Ul
I
ON
OJ
                         ADDITIVES
                                         PURE HYDROCARBONS
                                   AROMATICS
       PARAFFINS
                                         NAPHTHENES
                          OTHER
HYDROCARBON
 DERIVATIVES
               OLEFINS
     CAMP DRESSER & McKEE INC.
                FIGURE 5-25
GENERAL MAKE-UP OF PETROLEUM PRODUCTS

-------
do.  The hazardous nature of petroleum products is due primarily to the
aromatic fraction of pure hydrocarbons, and secondarily, to additives and
trace contaminants.

5.4.1  GASOLINE

Gasoline is the most ubiquitous petroleum product stored in underground
tanks.  Also, of all petroleum products, gasoline is the most widely
studied.  Motor gasoline is considered one of the most mobile petroleum
products with high potential for vapor and aqueous phase contamination
(Section 5.2) and is grouped in the lowest kinematic viscosity group (Group
I) in Figure 5-3.  Available information on gasoline composition (Section
5.2.3) allows a direct comparison of gasoline components to hazardous
substances.

Figure 5-5, given in Section 5.2.3, illustrates the hydrocarbon composition
of a reference sample of gasoline (Reference Fuel PS-6), used in an
inhalation study with laboratory animals sponsored by the American  Petroleum
Institute (Domask, 1983).  As discussed previously, 42 of 151 identified
compounds accounted for about 75 percent of the gasoline volume analyzed.
Of these 42 main consitiuents, 5 are hazardous substances regulated under
CERCLA, and are found in the following volume percent concentrations:
         Chemical
         Benzene
         Toluene
         Xylene and Ethyl  benzene
         TOTAL
Percent of Gasoline (by Volume)
           1.69
           3.99
           9.83
          15.51
Therefore, approximately 16 percent of the composition of gasoline may be
considered to be made up of regulated hazardous substances.  The substances
listed above are aromatic hydrocarbons.  Some of the remaining aromatic
hydrocarbons (roughly 10% based on Figure 5-5) may be toxic as well.
Additional gasoline constituents listed in the petroleum hydrocarbon
database (Appendix C)  which are regulated hazardous substances include,
cyclohexane, naphthalene, and phenol.  All the above listed substances may
produce toxic and explosive vapors as well as contaminate water supplies.
                               5-64

-------
Some other constituents of gasoline may be hazardous to humans and/or the
environment but, if they are not regulated under the LIST program, they are
not cited here.

In another study by Maynard and Sanders (1969), toluene was found to be the
constituent with the highest percent concentration (by mass, not volume)
gasoline at 12.20 percent.  Percent concentrations of specific constituents
vary from sample to sample, but for the purposes of this discussion, it is
fair to assume that the aromatic fraction in gasoline will  be on the order
of 25 to 35 percent.  Moreover, Morris (1985) states that as a result of the
restriction of tetraethyl  lead as an anti-knock additive of gasoline, the
aromatic hydrocarbon content of gasoline has increased and can be expected
to approach 35 percent for unleaded gasoline instead of the current average
of between 28 and 31 percent for both leaded and unleaded product.  This is
noteworthy because aromatics pose the greatest potential for impact on human
health and the environment.

The hazardous gasoline components listed above are included in the dual
parameter sorts/groupings  of the database presented in the previous section.
The resulting groups of these chemicals are for the most part in the
mid-range of potential impact as summarized on Table 5-12.

5.4.2 . FUEL OIL

Like gasoline, fuel oil is extensively used in the U.S. and is produced in
great quantity.  The composition of fuel oil includes a wide range of
hydrocarbons and trace contaminants because it is a less refined distillate
of crude oil with much less stringent specification than motor or aviation
fuels.

The data sources for the composition of No. 2 fuel  oil  used in this study
did not provide information on constituents as comprehensively as the data
sources used for gasoline.  Thus, the hazardous substance database was used
to assemble/compare properties based on the chemical  names  of identified
fuel oil constituents.  The following chemicals were found  to be hazardous
constituents of No. 2 fuel oil:
                                 5-65

-------
                                 TABLE 5-12

                   HAZARDOUS CONSTITUENTS OF GASOLINE AND

                               NO. 2 FUEL OIL

                            PARAMETRIC COMPARISON
                     Toxicity
   Relative Parameters1
                Vapor
Solubility    Pressure
GASOLINE AND FUEL
OIL CONSTITUENTS
#2 FUEL OIL
CONSTITUENTS (ONLY):
Ignitibi1ity
Benzene
Cyclohexane
Ethyl benzene
Naphthalene
Phenol
Toluene
Xylene (0-.M-.P-)
B(s)1
C
C
B
B
C
C
M
L
M
no data
H
M
M
M
M
L
!_**
!_**
M
M
C
C
no data
D
D
C
C
Anthracene
Benzole Acid
Cresol (o-, p-)
Phenanthrene
Quinoline
Methanol
D
D
B
D
C
D
L
M
L
L'
H
H
!_**
!_**
[_**
[_**
L
H
D
D
D
D
D
C
1See Section 5.3.3, (s): Statutory

**Solid at normal conditions but may be dissolved in liquid fuel
                                5-66

-------
         Anthracene                         Phenanthrene
         Benzole Acid                          Quinoline
         Benzene                                Methanol
         Cresol  (o-,m-, & p-)                    Toluene
         Ethyl benzene                             Xylene
         Naphthalene

Of these chemicals, six were also found to be hazardous constituents of
gasoline as discussed above.  Table 5-12 cites these substances.

Similar to gasoline, these substances fall in the mid-range of the
parametric groupings based on relative impact.  Constituents include
polynuclear aromatic hydrocarbons (PAH's)  which are important carcinogens.
Typically these  environmentally significant substances are found in
groundwater contaminated by oils.

5.4.3  ADDITIVES AND TRACE COMPONENTS
                                                *
In addition to  examining gasoline and fuel oil from the point of view of the
basic hydrocarbon composition, additives and trace contaminants can be
present in gasoline and other petroleum products in significant
concentrations.

Because most additives are complex mixtures and are used in small  quantities
and/or their chemical specific formulation is proprietary information, very
few specific additives have been identified as regulated hazardous
substances (Section 2.0).  The following,  however are known to be hazardous
substances and  are used widely in petroleum product property modification:

    •  Ethylene  dibromide
    •  Ethylene  dichloride
    •  Methanol
    •  Tetraethyl  lead
    •  Dimethyl amine
Of the above, methanol is probably the most widely used as as a deicer and
in "gasohol."
                                 5-67

-------
Trace contaminants include heavy metals often found in heavier fuel  oils and
lubricating oils, as well  as hydrocarbon-derived chemicals such as the
halogenated aromatics (e.g. dichlorobenzene).  A sampling study of New
Jersey refineries and fuel oil  bulk storage terminals showed that only trace
amounts of heavy metals are found in No. 2 fuel  oil, but that higher, though
not necessarily significant, amounts are found in No. 4 and No. 6 fuel oil.
(Turgeon, 1985; GCA, 1983), as  shown on Table 5-13.

5.4.4  PETROLEUM SOLVENTS

The principal  finished petroleum products used as solvents (covered  under
ASTM specification) are mineral  and/or petroleum spirits, Varnish Maker's
and Painter's  (VM&P) naphtha, and high flash aromatic naphthas.  Typical
physical  properties of these products are included in Appendix B.  Based on
their properties, these solvents fall in Group I in Figure 5-3; like
gasoline, solvents are considered to have high potential for vapor and
aqueous phase  impact on the environment.  The aromatic portion of these
products  most  likely includes benzene, xylene, toluene, and other alkylated
aromatics known to be hazardous.  (This can be inferred because the  boiling
points of these chemicals  falls  in the range specified for the solvents).

Petroleum solvents are widely used in industry;  however there are many other
organic industrial solvents on  the market.  Many of these industrial
solvents  are on the list of hazardous substances (about 50 hazardous
substances are easily identified as solvents).  Table 5-14 summarizes
chemical  types and uses of organic industrial  solvents and differentiates
between those  categorized  as "petroleum products."  Examples are given of
various industrial solvents included in the database.  Some organic  solvents
may be manufactured by the processing (e.g., chlorination) of hydrocarbon
solvents.

The importance of solvents is three-fold:

    •  A significant number of organic industrial solvents may be considered
       as hazardous petroleum products and/or as hazardous substances,
                                 5-68

-------
                                  TABLE 5-13

                 AVERAGE HEAVY METAL COMPOSITION OF  FUEL  OIL

                                   (mg/1)


Fuel  Type   # Samples      As     Cd_     £r    _H£      Ni_    Ste      R>      _Zn_

No. 2         8            2.6    <1   <1.2   <0.05   <1.5  2.7      1.3

No. 4         12           2.1    <1    0.58  <0.05   11.8  1.1      3.3   10.3
No. 6         18           2.75   <1    0.9   <0.05   28.4  4.3      5.2    1.1
Source: 6CA (1983)
                                 5-69

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                                 Table 5-14

                    CHEMICAL GROUPS OF PETROLEUM SOLVENTS

                        AND OTHER INDUSTRIAL SOLVENTS
Major Chemical Class

Petroleum Solvents

   Paraffinic




   Aromatic



   Naphthenic




Other Industrial  Solvents

   Alcohols




   Esters


   Ethers and glycolethers




   Halogenated Hydrocarbons


   Ketones
Applications
Example of Solvent
or Hazardous
Substance List
adhesives, coatings,
drycleaning, inks,
metal decreasing,
Pharmaceuticals

adhesives, coatings,
inks, fuel additives,
Pharmaceuticals

adhesives, coatings,
Pharmaceuticals,
metal degreasing
    NAJ
    benzene
    methylcyclohexane
adhesives, coatings,    methanol
inks, fuel additives,
Pharmaceuticals,
photographic film

adhesives, coatings,    amylacetate
inks, Pharmaceuticals
coatings, inks, fuel
additives, pharma-
ceuticals, lubricating
oil refining
    tetrahydrofuran
metal degreasing, fuel  trichloroethylene
additives dry cleaning

adhesives, coatings,    acetone
textile manufacturing,
lubricating oil
refining, inks,
photographic film,
Pharmaceuticals
 No paraffinic solvents  are on  the hazardous  substance  list;  however,  these
can be very volatile.
Source:  Adapted from Standen  (1969)
                                         5-70

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    •  Solvents are used in large quantities and are often stored in USTs,
    •  Solvents are used to dissolve solid chemicals or to dilute liquid
       chemicals such that they are in a usable form ;  The solute, or
       dissolved substance, may be a hazardous substance.
Groundwater contamination by solvents is relatively common compared to
contamination by other substances.  Solvents are highly mobile, often quite
volatile, widely used, chemicals.

5.4.5  KEY FINDINGS

Motor gasoline, fuel  oil, and petroleum solvents all  have hazardous
constituents as shown in the preceding discussion.   Primarily, the aromatic
hydrocarbons are the  hazardous constituents present to  the greatest extent
in these products, and the conclusion can be made that  the aromatics pose
the greatest threat from UST-related incidents.  Other  components of concern
in petroleum products include heavy metals and hydrocarbon derivatives,  but
these components are  typically found in the part per million  range, whereas,
aromatics are found in the parts per hundred (percent)  range.

The dual parameter sorts discussed in the previous  section lend themselves
very well to a comparison of hazardous substances and petroleum products.
As presented above the hazardous components of gasoline and fuel  oil fall
primarily in the mid-range groups for potential to  impact drinking water
aquifers, (toxicity-solubility) and the potential to expose people to toxic
vapor (toxicity-vapor pressure).  Ignitability-vapor pressure   data also
fall in the mid-range group, however the high ignitability group  is reserved
for highly explosive  chemicals, with immediate catastrophic effect.  A
mid-range potential for adverse impact, based solely on this  admittedly
simplified sorting procedure, obviously does not preclude the  occurrence of
problems, it indicates that some hazardous substances have greater potential
for adverse impact than gasoline and other petroleum products.
                                 5-71

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5.5  SUMMARY

The properties of regulated substances in USTs have been identified and
three databases were developed:  1. properties of petroleum products stored
in underground tanks;  2. properties of hydrocarbons known to be
constituents of gasoline and fuel  oil; and, 3. the properties of
CERCLA-regulated hazardous substances.

Petroleum products were examined with respect to five parameters judged to
be relevant to subsurface fate and transport  behavior.   The properties
included:

    •   specific gravity,
    •   kinematic viscosity,
    0   vapor pressure, and
    •   percent composition, of its constituents.

Petroleum products, except in rare instances, have a specific gravity less
than 1.0 and will float on groundwater.

Kinematic viscosity is related to transport by gravity  in the vadose
(unsaturated) zone.  The lower the kinematic  viscosity  the faster a product
will migrate through soil (driven by gravity  forces).

High vapor pressure products, such as gasoline and jet  fuels, have greater
tendency to release vapor phase contaminant than do substances with low
vapor pressures.

Relying on three parameters cited  above,  petroleum products were "grouped"
with respect to movement through the subsurface.  Group I, or high vapor
pressure, low viscosity products,  included motor and aviation gasoline,
solvents and No. 0-GT gas turbine fuel oil.
                                 5-72

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Because little toxicity data has been explicitly developed for most bulk
petroleum products, the percent aromatic carbon composition was used here to
illustrate relative toxicity of petroleum products.  Petroleum solvents,
automotive and aviation gasoline were found to have a high aromatic carbon
content and to be highly mobile (as judged by vapor pressure and kinematic
viscosity).

To appreciate the diversity among the various regulated substances in the
UST program, the database was used to generate information regarding phase,
chemical  classes, solubility, and liquid (and vapor) density of chemicals.

The hazardous substances are found (at standard conditions) to be 5 percent
gases, 42 percent liquids and 53 percent solids.  Roughly 20 percent of the
substances of interest are inorganic compounds; the remaining 80 percent are
organic.   Density data revealed that 29 percent of liquid chemicals and 46
precent of solids are heavier than water.  Groundwater contamination occurs
not only  by dissolved substances but also by those which are insoluble or
slightly  soluble, and which float on the water surface or sink to the
aquifer bottom.

The hazardous substances were grouped by using a "cause and effect"
approach.  Two-parameter "sortings" of the database were used to assess
potential impact.  Three impact scenarios were related to specific
parameters collected in the database, as follows:

    t  Groundwater contamination: solubility-toxicity data
    •  Human exposure to toxic vapors: vapor pressure-toxicity data
    •  Explosion/Fire:  Ignitibility-vapor pressure data
After assigning ranges which denote high, medium or low values of these
parameters, and after removing highly reactive chemicals from the analysis,
the sorts were performed resulting in lists of substances considered to have
"similar  potential" for UST-related impact.
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The results indicate that 32 chemicals with toxicity data (final or proposed
RQ) fall  in the high toxicity/high solubility group and are considered to
have very high groundwater impact potential; only 5 chemicals may be
considered to have very high potential for human health impact due to
inhalation of vapors (high vapor pressure high toxicity); and 9 substances
are found in the highest ignitibility high vapor pressure group.

A comparison was made of hazardous substances in the database with the
constituents of gasoline, fuel  oil, and petroleum solvents.  By comparing
the hazardous chemical  lists resulting from the sorts to the petroleum
product constituents some valuable insight is achieved.

The hazardous components of gasoline and fuel oil  fall  primarily in the
"mid-range" groups for potential to impact drinking water aquifers, the
potential to expose people to toxic vapor; and the potential for
fire/explosion.  This mid-range finding reinforces what is felt intuitively:
certain hazardous substances have greater potential  for greater adverse
impact than the constituents of gasoline and other petroleum products.  This
does' not  mean, however, that petroleum products do not  present environmental
problems  when released in the subsurface.

The hazardous components of gasoline include benzene, toluene, and xylene
which are fairly soluble, toxic and produce explosive/toxic vapors.  These
chemicals make useful indicators of gasoline spills and the degree and
extent of contamination.  With respect to oil spills, the above chemicals as
well as the polynuclear aromatic hydrocarbons (PAHs) are useful indicators.

The organic industrial  solvents are important for they  may be considered as
hazardous petroleum products and/or as hazardous substances.  Solvents are
used in large quantities and are often found in underground tanks.  These
solvents  are used to dissolve solid chemicals or dilute liquid chemicals
such that they are in a useable form, and the solute, or dissolved
substance, may be a hazardous substance.  Groundwater contamination by
solvents  is relatively common compared to contamination by other substances.
Solvents  are highly mobile, often quite volatile,  widely used chemicals.
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                         6.0 ENVIRONMENTAL SETTINGS
6.1  OVERVIEW

When a leaking UST incident occurs, two types of environmental  factors are
of concern:  natural  and non-natural.  The natural  environmental  factors
determine how vulnerable the site is to contamination; climate  and local
hydrogeology are two important examples of such factors.  Non-natural
environmental factors exacerbate the site's vulnerability to contamination.
As such, non-natural  factors can increase the hazards resulting from
leaking USTs.

To completely characterize an UST incident, it is necessary to  integrate
these two types of environmental factors.  The potential hazards  resulting
from leaking UST incidents vary from incident to incident based on
site-specific environmental factors and hence are highly dependent on
site-specific conditions.  The objective of this section is  to  describe  the
current understanding of the environmental  factors  that influence the  fate
and transport of a contaminant from a leaking UST.

The environmental  factors for liquids and vapors are presented  in 6.2  and
6.3, respectively.  In these sections,  the natural  environmental  factors
are grouped into two categorires:  climate and hydrogeology. Cultural and
physical variables reflect the non-natural  environmental factors  that
influence the potential hazards posed by leaking UST incidents.

For regulatory purposes, it is useful to establish  generic environmental
settings to evaluate the potential  consequences of  leaking UST  incidents.
These generic environmental settings are not intended to describe all
variations encountered at individual sites, but rather to describe types of
settings and the types of problems  each can present.  To address  this
regulatory need, a study was made of some existing  methodologies  used  to
develop environmental settings were investigated to determine their general
applicability to evaluating potential hazards resulting from leaking UST
incidents.  These methodologies are discussed in 6.4.
                                    6-1

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6.2  ENVIRONMENTAL FACTORS AFFECTING LIQUID FATE AND TRANSPORT

6.2.1  NATURAL ENVIRONMENTAL FACTORS

The environmental  factors that influence the potential  hazards posed by
liquids leaking from USTs were selected from the analysis of the fate and
transport mechanisms presented in Sections 3.0 and 4.0.  As  these
environmental factors are addressed in the discussion of each fate and
transport process  in Sections 3.0 and 4.0, this section presents a summary
overview of these  factors.

Climate

The important climatic environmental factors are net recharge and
temperature.  With respect to the potential hazards posed by leaking UST
site(s) within an  ecosystem, climate is unlikely to vary significantly
(Kendeigh, 1974).   However, within the ecosystem, climatic factors may vary
seasonally.  For example, during rainy seasons when there is high net
recharge, there can be increased movement of liquid contaminants in the
unsaturated zone.

Net Recharge.  The primary source of recharge to groundwater is
precipitation, which infiltrates the surface soil and percolates through
the subsurface.  Not all  precipitation infiltrates and  percolates through
soil.  Precipitation may "pool" in surface depressions, evaporate, or
transpire, reducing the amount of precipitation that recharges the
groundwater.  It is the amount of precipitation that percolates  through the
soil that is important, and not the amount of precipitation.  The amount of
water per unit area of land which penetrates the ground surface  and reaches
the water table is net recharge.  Some examples of how  net recharge
influences both the transport and fate of a contaminant are  discussed
below.

Recharge water is  the principal  vehicle for leaching and  transporting
liquid contaminants to the water table.  Percolating water may dissolve
soluble components of a contaminant retained in the soil  and transport  the
                                    6-2

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dissolved components through the soil, possibly reaching the water table.
In the saturated zone, the quantity of water available for dispersion  and
dilution is also controlled by net  recharge.  In areas where an aquifer is
unconfined, recharge to the aquifer usually occurs more readily and the
pollution potential is generally greater than for confined aquifers (Alien
et.al., 1985).

With respect to leaking USTs, the pores (or voids) of the unsaturated  zone
matrix are typically filled with air, water, and the leaked contaminant.
As recharge water percolates through the soil,  the water fills  some of the
pore space, displacing the air.  The amount of  moisture in the  soil
influences the fate of a contaminant.  For example, the influence  of
recharge water on biodegradation can be summarized as follows:  as  recharge
water replaces air in soil pore spaces, the amount of oxygen available for
biotic degradation is reduced.  The amount of available oxygen  influences
the type and rate of biodegradative reactions.

Temperature.  Temperature at the site can affect both transport and fate
mechanisms in the subsurface, but it has the more important  effect  on  fate
mechanisms.  For example, since adsorption is an exothermic  process,
adsorption may decrease with increasing temperatures.   Biodegradation,
however, generally increases with increasing temperatures (Scow, 1982).
These effects primarily manifest themselves in  the unsaturated  zone,
because at a given site the temperature of. groundwater remains  fairly
constant.

Hydrogeology

The hydrogeologic factors that are  relevant to  evaluating the potential
hazards from a leaking UST are:

    •  Unsaturated zone media;
    •  Depth to groundwater; and
    0  Saturated zone characteristics.
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Unsaturated Zone Media.  The nature of the soil  media  of the  unsaturated
zone is one of the most important environmental  factors  in the assessment
of potential hazards from a leaking UST incident.   The composition  and
characteristics of the soil -- such as the particle size and  porosity and
the relative percentage of organic and inorganic matter  -- determine  the
attenuation characteristics at a site.  The media  also control  the  path
length and routing, affecting the time available for attenuation  and  the
quantity of contaminant that can be transported.

The ability of a soil  to transport contaminants  is  the soil's  transport
capacity (Wood, 1984).  The greater the transport  capacity, the greater the
migration of contaminants.  Variables  of the soil  media  that  affect  its
transport capacity include porosity, heterogeneity, and  permeability.

Porosity and permeability affect the ability of  a  soil media  to transport  a
liquid, with permeability as the most  important  variable in its influence
on transport capacity.  The lower the  permeability of  the soil, the more
slowly a liquid will be transported.  The influence of media  properties
such as distribution of grain sizes, the sphericity and  roundness of  the
grains, and the nature of their packing are reflected  in the  permeability
of a soil (Freeze and  Cherry, 1979).  Porosity is  the  ratio of  the  volume
of voids (open pores spaces) in the soil  to the  total  volume  of the soil;
it determines the amount of liquid that can be stored  in soil.  Generally,
within deposits of well-sorted sand or in fractured rock formations,  high
porosities are associated with high permeabilities. This relationship,
however, does not hold for all soil types.  For  example, clay-rich  soils
usually have high porosities and low permeabilities because of  many
factors, including the distribution of grains  and  nature of packing
(Freeze and Cherry, 1979).

In the subsurface, variations in soil  types may  be  present, e.g., clay
lenses or stratification of soil layers,  termed  heterogeneities.  The
presence of heterogeneities influences contaminant  transport patterns in
the subsurface.  To illustrate the effect of heterogeneities on the
transport pattern, a schematic of transport patterns for homogeneous  and
                                    6-4

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heterogeneous soil media are shown in Figure 6-1.  If it were not for the
effects of heterogeneities in natural geologic materials, the problems of
prediction and detection of contaminant behavior would be greatly
simplified (Freeze and Cherry, 1979).

Biodegradation, adsorption, chemical  transformations, solubility and
vaporization are affected by the soil media.  The composition and
characteristics of a soil media that  are important to the fate mechanisms
are:  pH, the percent of organic matter, and particle size.  The degree to
which these fate mechanisms are influenced by these factors varies.   For
example, solubility is primarily influenced by the pH and organic matter of
the soil media.  Adsorption of organic chemicals is primarily a function of
the fraction of organic matter.

Depth to Groundwater.  The depth to groundwater is important because it
determines the depth of unsaturated zone media which a contaminant must
travel before reaching the water table.  In general, as  the depth to water
increases there is a greater chance for the attenuation  of a contaminant to
occur because deeper water levels result in longer travel  times.   For
example, biodegradation of a contaminant may be increased because of
prolonged exposure to microbial  populations in the saturated zone.

Saturated Zone Characteristics.   Characteristics of the  saturated zone that
are important to assessing the potential  hazards from leaking UST incidents
are the saturated zone media, hydraulic gradient, and hydraulic
conductivity.

Saturated zone media refers to the consolidated or unconsolidated soil
particles whose pores are filled with water.  Media in the saturated  zone
is termed "aquifer media" when its permeability is sufficient for it  to
transmit significant quantities  of water for use (Freeze and Cherry,  1979).
The saturated zone media affects the  rate and path length  over  which  a
contaminant is transported. The  path  length determines the amount of  time
available for attenuation processes to occur such as adsorption,
dispersion, and degradation.  In rock media, the path which a contaminant
follows is strongly influenced by fracturing and/or solution channels.
                                    6-5

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                                   LAND SURFACE

HIGHLY PERMEABLE
HOMOGENEOUS SOIL
                             LESS PERMEABLE
                            HOMOGENEOUS SOIL
 STRATIFIED SOIL WITH
VARYING PERMEABILITY
SOURCE: CONCAWE (1979)
CAMP DRESSER & McKEE INC.
                                       FIGURE 6-1
                            CONTAMINANT TRANSPORT THROUGH
                        HOMOGENEOUS AND HETEROGENEOUS SOILS
                                    6-6

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The hydraulic gradient and conductivity of the saturated zone influence  the
rate at which groundwater flows.  The travel  time of a contaminant  is
determined by the quotient of the hydraulic conductivity and  effective
porosity (the fraction of the porosity available for flow).   Thus,  the
higher the hydraulic conductivity of a medium, the faster a  contaminant
will travel.  The hydraulic conductivity of a media is a function,  not only
of the porous medium, but also of the fluid (Freeze and Cherry,  1979).   If
the fluid has a density or viscosity significantly different  than water,
the intrinsic permeability of the media must  be used along with  these fluid
properties to determine the flow rate of a contaminant (Section  3.2
discusses this concept in detail).  Transport of a contaminant  in the
saturated zone is described by Darcy's Law and dispersion (Section  3.5.2).

The hydraulic gradient is the absolute value  of the change in groundwater
elevation per unit of length.  The direction  which a contaminant will be
transported away from the point at which it entered the saturated zone is
influenced by the hydraulic gradient.  High hydraulic gradients  and
conductivities increase transport away from the source of entry  and reduce
the attenuation time, thus increasing the potential  hazards  resulting from
a leaking LIST incident.

6.2.2  NON-NATURAL ENVIRONMENTAL FACTORS

Non-natural  factors (cultural  and physical  variables)  are also important
for the assessment of the significance of an  UST leak.   For example, a
leaking UST may have a low hazard potential because  of  natural factors, but
because it is located near a highly populated area,  it  may be of greater
concern than a leaking UST located in a remote area  where natural
environmental factors would otherwise indicate high  hazard potential.  The
cultural  and physical variables are therefore more highly site-specific
than natural factors and thus their effects are not  as  easy to
characterize.  The cultural  and physical  variables that  are important to
leaking UST incidents include:
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    •  Population density near the leak site;
    •  Horizontal distance to the point of concern;  and
    •  Quantity and rate of the release.

Population Density

Population density refers to the number of individuals who  would  be  exposed
to a contamination incident.  As long as the leaking contaminant  remains  in
the unsaturated zone, the population  exposed may be  limited because  of  the
localized nature of liquid contaminant from a  leaking UST (vapors, however,
may travel significant distances in the unsaturated  zone  (Section 6.3)).
When contaminants reach the saturated zone and contaminants enter the
groundwater, a much larger population may be exposed to risk.

Horizontal Distance

Horizontal distance represents the distance between  an UST  and  the point  of
concern.  For liquids leaking from USTs, the point of interest  is generally
a well which supplies drinking water.  Obviously, the greater the
horizontal distance, the lower the potential for the water  supply to be
contaminated.  However, the hydraulic gradient and conductivity of the
saturated zone may influence the significance  of the horizontal distance.
For example, an aquifer having high hydraulic  conductivity  and  gradient
will transport a contaminant further  and faster than an aquifer with low
hydraulic conductivity and gradient.

Quantity and Rate of Release

The quantity and rate of release affect the distribution  of a contaminant
in the subsurface.  As discussed in Section 3.1, insidious  spills are
characteristic of UST leaks.  Insidious spills may be either continuous or
intermittent.  Generally, UST leaks are continuous.   Intermittent leaks,
however, can be associated with overfilling of an UST.  In  general, a
continuous leak will have a greater potential  hazard than intermittent
leaks.  Factors that influence the rate of  a continuous leak are

                                   6-8

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controlled by the size and location of the tank's point of leakage.   The
location of the tank's leakage point is important because the volume of
contaminant available for leakage varies with the fluctuating level  of the
tank contents.  If the leakage point is located near the top of the  tank,
the contaminant can leak only when the tank is full.

In evaluating the significance of potential hazards posed by a leaking LIST,
the quantity of contaminant released affects the method, and often degree,
of groundwater contamination.  If the volume of the release exceeds  the
retentive capacity of the soil,  the leaked contaminant  will  reach the water
table, and groundwater contamination may result.

6.3 ENVIRONMENTAL FACTORS AFFECTING VAPOR FATE AND TRANSPORT

The generation of vapors from substances leaking from USTs is a common
occurrence.  The vapor concentration is determined by the physical and
chemical properties of the substance (especial ly the vapor pressure) with
the greatest vapor release associated with more volatile substances.  Vapor
flows are principally generated  from liquid substances  as they migrate
downward through the unsaturated zone, and from the exposed surface  of
immiscible substances floating on the water table.   Vapors can also
originate from a plume containing dissolved volatile contaminants, though  •
to a much lesser extent.

The underground release of a substance with a high  vapor pressure does not
by itself constitute a problem,  since the surrounding environment must be
vulnerable also.  The following  section identifies  conditions that render
the environment vulnerable to vapor contaminants.  Many of these factors,
either individually or in combination, create paths  of  lowered resistance
to vapor movement and in regions where accumulation occurs,  create
conditions favorable to vapor movement.  Other factors  are important
because they inhibit vapor movement, particularly the restriction of loss
to the atmosphere.  When venting to the atmosphere  is  reduced, an increase
in vapor concentration can result in increased lateral  infiltration  into
subsurfce structures.  The principal  environmental  factors relevant  to
vapor transport were introduced  in the analysis presented in Section 3.6.
In this summary overview, as with the liquid phase  discussion, these
factors are divided into natural  and non-natural  environmental  factors.
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A major basis for this summary is a recent study conducted  by COM  (1985)  -
aimed at summarizing the vapor phase problems from UST leak incidents.
This qualitative assessment was based on information  collected on  incidents
involving the subsurface movement and infiltration of vapors, i.e.,  vapor
incidents.

Investigation by COM (1985) of the vapor incident  problem has identified  a
number of variables that are suspected of influencing the probability and
degree of vapor incidents.   A vapor incident  is  most  likely to occur  when
any of the following site-specific conditions are  met:

    •  A substantial quantity of a substance  with  relatively high  vapor
       pressure, low heat of vaporization, low solubility in water, and
       specific gravity less than that of water
    •  A favorable environment with restriction  of escape from the ground
       surface
    t  A susceptible underground structure
    •  Points/sources of entry into the structure

The following discussion describes the environmental  characteristics
associated with vapor incidents.

6.3.1  NATURAL ENVIRONMENTAL FACTORS

Climate

Climatic environmental  factors that are important  to  vapor  phase fate and
transport processes are:

    •  Precipitation
    0  Groundfrost
    •  Barometric Pressure
    •  Wind
    •  Temperature
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Precipitation.  Precipitation is important to vapor transport because heavy
rain tends to saturate the upper portion of the unsaturated zone.  As
precipitation enters the ground surface, the air that normally occupies
much of the surficial  soil pore space is displaced with water.  This  may
result in a buildup of pressure in the soil  gas and a counterflow that
vents soil gas vertically to the surface.  If the precipitation rate  is
high and the permeability of the soil is low, the resulting "saturated
layer" can act as an impermeable upper boundary by eliminating the
available open pore space, thereby impeding upward movement of vapors.  The
end result is that instead of being dispersed to the atmosphere,  vapors
move laterally, increasing the possibility that they might  enter  an
underground structure.  Vapors move toward underground structures because,
like the atmosphere, such structures present relatively low concentration
zones, or "sinks," towards which the concentration gradient (driving  force
for diffusion) is established.  The saturation of the upper part  of the
unsaturated zone, though temporary, will persist until  the  moisture
saturating this zone evaporates, transpires, and/or percolates through the
subsurface.

Precipitation also plays a role in the influence of groundwater on vapor
transport.  Net recharge, which is the precipitation that  reaches the water
table, is the primary source of recharge to groundwater.  As  such,
fluctuations in the elevation of the water table result due to seasonal
changes in precipitation and thus net recharge.  Because the  saturated zone
represents the lower boundary to vapor movement, an increase  in the
groundwater level will  force any free product floating  on the  surface and
the associated vapors  upward.  In effect, this shortens the distance  to  the
atmosphere and/or to a susceptible area and provides a  pressure driving
force or gradient in that direction.

As water percolates through the unsaturated  zone,  it may dissolve vapors
which were generated from a leaking substance.  These dissolved vapors can
be a significant source of groundwater contamination.   The  resolution of
vapors can be responsible for much of the groundwater contamination found
up the groundwater gradient from the spill  site.  On the other hand,  once
in solution, vapor contaminants may be susceptible to fate  processes  such
as biodegradation.
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Groundfrost.  Like precipitation, the existence of groundfrost results in
the formation of a relatively impermeable zone at the ground surface.   In
this case, moisture present in the upper level  of the soil  freezes,  thereby
essentially "plugging" the open pore space and restricting vapor transport
through this zone and preventing the release of vapors to the atmosphere.
The restricted upward vapor movement again leads to greater lateral
movement.  Unlike precipitation, however, which generally results in only
temporary saturation of the ground surface,  groundfrost generally exists
for an extended period.  This presents the potential  for sustained lateral
vapor migration, and in turn a greater potential for vapor infiltration
into an underground structure.

Barometric Pressure.  Fluctuating barometeric pressure creates a pumping or
"breathing" effect within the unsaturated zone.  Changes in barometric
pressure are transmitted into the subsurface.  The transmission of air
pressure through the soil results in alternating compression and expansion
of the soil air, which in turn creates a pumping effect.  Vapors present in
the soil pores are entrained in the resulting flow of air and are more
readily dispersed to the atmosphere.  Because the depth of  penetration is
limited, this process is most important where the depth to  the
contamination area is much less than the full  depth of the  unsaturated
zone.

Wind.  Wind also plays a role in subsurface  vapor phase fate and transport.
Windy conditions increase the dispersion rate of vapors into the
atmosphere.  The distinct concentration gradient across the boundary
between the vapor-laden soil and the relatively concentration-free
atmosphere is thereby maintained.  Research  by Fukuda (1955) and others
also suggests that wind eddies create a slight  fluctuation  (high frequency,
low amplitude) of atmospheric pressure over  the soil  surface.   As  with a
change in barometric pressure, air pressure  transmitted into the soil
results in fluctuating vertical  air movement through  the soil.   The
movement of soil air entrains vapors present in the soil  pore spaces,  thus
increasing the effect of advection on vapor  phase transport.  However,
investigation by Fukuda (1955) has shown that the amount of vapor
transported from the soil as a result of windy conditions  is often small
compared to the effects of other processes.
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Temperature.  Increases in temperature result in higher rates of
vaporization of a leaked contaminant.  Changes in temperature also affect
several of the vapor transport mechanisms, such as diffusion.  Section 3.6
addresses this topic in more detail.

Figure 6-2 illustrates the climatic effects on subsurface vapor transport.
Figure 6-2 (A) shows the influence of precipitation or groundfrost on
vertical  and lateral migration of vapors.  Figure 6-2 (B) depicts the
entrainment of subsurface vapors by soil  air movement created by
fluctuations in atmospheric pressure.  As vapors reach the soil-air
interface, they are readily dispersed by the wind.

Hydrogeology

The hydrogeologic factors that are important to vapor phase fate and
transport are:

    •  Unsaturated zone media
    •  Groundwater characteristics

Unsaturated Zone Media.  The nature of the soil  media of the unsaturated
zone is one of the most significant environmental  factors in contro.lling
vapor movement.  The most important soil  parameters are:

    •  Porosity
    •  Type and extent
    0  Moisture content
    •  Adsorption capacity

The porosity of a soil is the percentage  of the total  volume of the
material  that is occupied by open pores.   Generally, the pore spaces  are
interconnected, permitting the movement  and storage of liquids  and/or
gases.  In the unsaturated zone the pore  spaces are filled with water and
air.  The open, air-filled pore spaces provide pathways  along which vapor
movement  occurs.  Vapor transport is governed largely by diffusion and
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       GROUNDFROST OR SATURATED GROUNDSURFACE
                                                        PRODUCT VAPORS
                                         FREE FLOATING PRODUCT
                                      (A)
            INFLUENCE OF GROUNDFROST AND PRECIPITATION ON VAPOR MOVEMENT
       WIND
     PRODUCT VAPORS
       ENTRAINED IN
    SOIL AIR MOVEMENT
        SOIL AIR MOVEMENT
     ENHANCED BY FLUCTUATION
       x IN ATMOSPHERIC
      /    PRESSURE
                      GROUNDSURFACE
                                                  FREE FLOATING PRODUCT
                                       (B)
             INFLUENCE OF BAROMETRIC PRESSURE AND WIND ON VAPOR MOVEMENT
CAMP DRESSER & McKEE INC.
              FIGURE 6-2
CLIMATIC EFFECTS ON SUBSURFACE
         VAPOR TRANSPORT
                                      C-14

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advection.  Soils with a greater porosity are therefore more conducive to
vapor transport.

The type and extent of soil  also affects vapor movement.  Coarse-grained
soils such as gravel and sand generally have large porous  spaces  and
therefore are very permeable* allowing relatively easy vapor transport by
advection.  Fine-grained soils such as silts and clay have exceedingly
small pore sizes and therefore are often quite impermeable.   In the
subsurface, variations in soil types are common.  The extent of a permeable
soil type will  influence the degree of vapor movement.   The  presence  of an
impermeable zone such as a clay lens may restrict vapor migration almost
completely.  On the other hand, the availability of loosely packed soil and
bedrock fractures or other channels provide extremely permeable pathways.

Because vapor transport in the unsaturated zone occurs  through the
interconnected soil pore spaces, the moisture content of the soil  has  an
important influence.  In a soil  with a high moisture  content, a large
percentage of the pore space is occupied by water.   Thus^  the available
air-filled pore spaces through which vapors migrate are limited,
restricting the extent of vapor transport.  In the areas where the entire
pore space becomes saturated with water, no vapor movement occurs.  This
condition periodically arises after a heavy period of precipitation as  was
described under climatic environmental  factors.  Increases in the soil
moisture content can result  in the resolution of vapors or in an  increase
in the vaporization of a contaminant that  was weakly  adsorbed to  the soil
(Thomas, 1982).

As vapors move through the soil  there is a tendency for contaminant
molecules to adhere to the soil  particles  (adsorption). The adsorptive
capacity of the soil is, therefore, important in assessing the extent  of
vapor movement that is restricted by adsorption.  Although adsorption  slows
down the rate of progress of the vapor front, it does not  otherwise alter
its form or peak concentration.  With the  passing of  a  vapor plume and
reduction in vapor concentration, the vapor molecules are  released from the
soil particles again.
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Groundwater Characteristics.  Groundwater characteristics in the vicinity
of the contaminated area also influence vapor phase transport.  Groundwater
is particularly important because:

    •  it transports liquid contaminants away from the original  spill  area
       thereby increasing the zone of liquid contamination from which
       vapors are generated;
    •  immiscible substances with a density less than water float on its
       upper surface, the water table, increasing the potential  for
       vaporization of a leaked contaminant;
    •  fluctuations of the water table result in a "pumping of vapors  and
       soil gas";  and
    0  fluctuations of the water table result in a greater vertical  spread
       of a floating substance increasing the potential  for vaporization.

6.3.2  NON-NATURAL ENVIRONMENTAL FACTORS

Non-natural factors (cultural  and physical  variables) can exacerbate the
potential hazard from vapors.   The non-natural  environmental  factors
important to vapor phase release from leaking LIST incidents include:

    •  Man-made conditions;
    •  Population density;
    •  Proximity to susceptible structure;  and
    •  Quantity released.

Man-made Conditions.  Man-made conditions have  been observed  to  greatly
influence the movement of subsurface vapors.  The construction of
underground structures such as sewers and utility conduits provide  pathways
of reduced resistance to diffusion through  which vapors  can be transported
considerable distances.  In addition, the presence of underground
structures such as basements act as "target" areas to which vapors  can
infiltrate.  The construction  of these structures also disturbs  the
adjacent soil media, generally rendering the soil surrounding the structure
more permeable and therefore more conducive to  vapor transport.   The
importance of these conditions is evident in that a number of vapor
incidents have involved man-made conditions (COM, 1985).
                                    6-16

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Like groundfrost or precipitation,  paved  surfaces  restrict  the  upward
movement of vapors.  The increase in contaminant concentration  in the
subsurface because of reduced  atmospheric venting  can  lead  to an increase
in the lateral  diffusive flux.  Under these  circumstances,  there is  often
an increased vapor infiltration  of  subsurface  structures.

Population Density.  Population  density defines the  number  of individuals
who could be exposed to vapor  concentration  following  a  leaking UST
incident.  This factor does  not  directly  influence any vapor fate and
transport processes, but it  is extremely  important to  scaling the
significance of the potential  hazards from vapors.  Commercial  and
residential areas tend to be susceptible  to  vapor  infiltration  and
subsequent detection because of  high population density  coupled with the
large number of USTs (especially for petroleum products) and underground
strutures in these areas.  Industrial  locations maybe somewhat isolated
from residential  and commercial  areas,  and because vapor impacts generally
occur in relatively close proximity to  the leak or fill  product, there is
considerably less potential  for  vapor exposure or  detection in  industrial
settings.

Proximity to a  Susceptible Structure.   The vertical  and  lateral distance to
a susceptible structure is of  interest  because vapor transport  generally
has a localized effect.  Vapor movement is principally restricted to the
area in close proximity to the actual  leak or  resulting  free product,
unless a conveyance mechanism  such  as a gravel lens  or other favorable
subsurface stratum or man-made conduit  exists. Structures  in close
proximity to the leak are therefore usually  more vulnerable to  vapor
infiltration.

Quantity Released.  The volume of product released is  important in
characterizing  the potential scope  of the area that  could be affected.
Large spills can create a greater sphere  of  influence  than  small spills.
In addition, large liquid spills  may tend to migrate on  the groundwater
table posing the potential for vapor incidents at  some considerable
distance from the leak.  The size of the  leak  will also
                                    6-17

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influence the duration of the vapor incident.  Substantial  spills may cause
vapor problems well after most of the free product has been recovered.

6.4  ENVIRONMENTAL SETTING METHODOLOGIES

To address a regulatory need for generic environmental settings,  some of
the methodologies used to describe generic settings were studied.  The
methodologies were evaluated for fulfilling the requirement of representing
the environmental, physical, and cultural  factors determined important  to
potential hazards posed by leaking UST incidents  in Sections 6.2  and  6.3.
The methodologies to be discussed were utilized in the following:

    •  RCRA Risk and Cost Analysis Model  (W-E-T);
    •  Liner Location Risk and Cost Analysis Model; and
    •  DRASTIC

The discussion is broken down into an overview of each methodology and  its
applicability to the UST program.

6.4.1  RCRA RISK AND COST ANALYSIS MODEL (W-E-T)

The RCRA Risk and Cost Analysis Model (ICF, 1984) determines the  risks  and
costs of management methods for hazardous  wastes.  Hazardous waste
management practices are represented by W-E-T cells.  These W-E-T cells  are
combinations of Waste streams, an Environment, and Technologies.   For
example, the combination of landfilling (T), spent stripping and  clean  bath
solutions from electroplating operations  (W), in  a high population density
area underlain by a productive aquifer (E) comprises a W-E-T cell  in  the
model.  The methodology for defining environmental  settings in  a  W-E-T  cell
is the item of interest here.

Methodology

The environmental  settings of a W-E-T cell  are defined in terms of three
factors:  population density, surface water environment,  and groundwater.
                                    6-18

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These factors were assumed to be the most significant ones  for evaluating
the potential for human and ecosystem exposure to hazardous chemicals.

Population density is defined in three ranges varying by one order of
magnitude each:
                           2
    •  High - 250 people/km  and over
    •  Medium - 25-249 people/km2
                                    2
    •  Low - fewer than 25 people/km

The surface water environment is classified by two schemes.  Human health
risk classification is  based on surface water assimilative capacity.   This
classification is divided into two categories: low (e.g., stream flows  of
  co                                         73
10  m /day), and high (e.g., stream flows of 10  m /day and lakes greater
than 3x10   m ).  "Ecorisk" classification  is based on  ecosystem type,
using seven groups, such as large rivers, small  streams,  and sea coasts.

Two types of groundwater systems were defined:  areas underlain  by
productive groundwater formations, and all  other areas.   These two types
were assigned primarily on the basis of hydrodynamic characteristics  of the
water-bearing formations.  Productive formations are those  generally
capable of yielding 50 gpm of water and containing less  than 2,000 ppm  of
dissolved solids.
Combining the various environmental  factors,  a  total  of  12  standard
environmental settings is available  for evaluating  human health  risks  as
shown on Table 6.1.  Each three-digit  zip  code  region was assigned one of
these settings.
                                    6-19

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                                 TABLE 6-1
           STANDARD ENVIRONMENTS FOR HUMAN HEALTH RISK EVALUATION
Population
Density
High
High
High
High
Medium
Medium
Medium
Medium
Low
Low
Low
Low

Assimilative
Capacity
Low
Low
High
High
Low
Low
High
High
Low
Low
High
High

Groundwater
System
Productive
Non-Productive
Productive
Non-Productive
Productive
Non-Productive
Productive
Non-Producti ve
Productive
Non-Productive
Productive
Non-Productive

Surface Water
Number of
Zip-Code Areas
10
22
25
19
135
141
49
17
39
94
6
2
Total 559
Source:   ICF (1984)
                                    6-20

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Applicability to Leaking USTs

The W-E-T model methodology does not fulfill  the requirements  for the
development of environmental  settings for leaking USTs for two primary
reasons:  1. the settings do not include the  important environmental
factors in leaking UST incidents described in Sections 6.2 and 6.3;   and  2.
the settings are distributed geographically over large areas.

To properly assess the consequences of leaking UST incidents utilizing
environmental  settings, the influence of the  environmental  factors  in
Sections 6.2 and 6.3 must be reflected in the generic settings.   The  RCRA
model  of environmental  settings  uses two of the environmental  factors from
these sections—population exposed and groundwater characteristics.   It
does not include the influence of fate mechanisms on  leaking chemicals.

As stated in the introduction to this section, leaking UST incidents  are
site-specific.  The distribution of generic settings  by three-digit  zip
code characterizes areas too large for the evaluation of hazards  resulting
from leaking USTs.  For example, the three-digit zip  code distribution for
the state of Florida is shown in Figure 6-3;  an environmental  setting
representing the three-digit  zip code 326, Gainesville,  would  include two
types of aquifer media: coastal  plain sand underlain  by carbonate rocks,
and carbonate rocks.  These, aquifer medias may not  have  the same  influence
on the potential hazards from a  leaking UST incident, but would be
characterized in the same environmental  setting utilizing three-digit zip
codes.  The assignment of generic settings by four-or five-digit  zip  codes
might be adequate for the requirements of leaking UST environmental
settings.  If, for example, five-digit zip codes were used to  represent an
environmental  setting,  the area  of Gainesville would  be  divided into  eight
zip-code areas.

6.4.2  LINER LOCATION RISK AND COST ANALYSIS  MODEL

The Liner Location Risk and Cost Analysis Model (EPA, 1985b) estimates:
1. the chronic risk to human  health from land disposal of hazardous wastes
                                    6-21

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                                                                                 MIAMI
                                                                               330*
                                                                                  • 331
                                                                               MILITARY
                                                                                340
        LEGEND
         •  Serves as associate post offices within that 3-digit ZIP Code Area
         •  Is a city which has been assigned its own 3-digit ZIP Code but
            which is not a sectional center.
SOURCE: U.S. Postal Service — Retail Operations Division (1981)
 CAMP DRESSER & McKEE INC.
         FIGURE 6-3
THREE-DIGIT ZIP CODE MAP
           FLORIDA
                                            G-22

-------
under different scenarios of technology, location,  and waste stream;  and
2. the cost of different designs and sizes of landfills and surface
impoundments.  The locations of land disposal  sites are characterized by
environmental settings.

Methodology

The environmental  settings for the Liner Location  Risk and Cost Analysis Model
characterize the saturated zone at the location of the land disposal  site.
The model  uses the generic well  distance,  mobility class of a chemical,  and
flow field combinations that most closely resemble  conditions at a site.

These nine generic flow fields are shown in Figure  6-4.

Three generic well distances were used:   60, 600,  and 1,500 meters.  These
distances  were determined from the review  of distances to wells
downgradient of existing facilities,. the density of wells in areas where
private wells are  used, and the predictive capabilities  of the model.

The contaminants of concern were grouped into four  broad mobility classes
based on their retardation value.  The model  uses  the median retardation
value for each class as the value describing, the mobility of contaminants
in the generic flow field scenarios.   These classes are  presented  in  Table
6-2.  The  model generally ignores chemical  interactions  which could serve
to increase mobility for contaminants except those  in Class 4.

The development of the generic flow field  was  based on a three-step
process:

    1.  determination of representative  ranges for  parameters describing
        aquifer configurations, aquifer  sizes, and  groundwater flows;
    2.  construction of eight generic hydrogeologic settings using aquifer
        configuration data ranges; and
    3.  matching these eight settings with  representative aquifer  size and
        groundwater flow data to create  the nine generic flow fields.
                                    6-23

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                                              B
                                            C .
                 30M
                     1 M/Y
                             01 M/Y
                                    30M
                   10 M/Y
                                                 1 M/Y
                                  30M
             100 M/Y •
                                              IOM/Y
                            30M
1000 M/Y-
                                       100 M/Y
               30M
                                                                           10,000 M/Y
                         1000 M/Y
o
i
                 15M
                 30M
                     0.05 M/Y
                             0.5 M/Y
100M/Y-*-
                                2 M/Y
                    30M
                    60M
                        100 M/Y -»-
                                  10 M/Y
10 M/Y-*-
                                                       0.1 M/Y
                             EXPLANATION

                                   WATER TABLE BOUNDARY
               OUTFLOW .
               BOUNDARY
                        30M
       10 M/Y-
                                     1 M/Y
                         INFLOW
                         BOUNDARY
                             NO-FLOW
                             BOUNDARY
                                                                            H
                                                                 15M
                    15M
                        10 M/Y -*-
                                  .1 M/Y
                                                100 M/Y •*-
    I
   1 M/Y
                                           30M
30M
                                                                                         30M
10 M/Y «

nn1^ ««/v"^—

mn M/Y «


0.5 M/Y

I
0.5 M/Y

1 M/Y
                     AVERAGE LINEAR GROUNDWATER VELOCITY VECTORS
                     (METERS/YEAR) THROUGH EACH LAYER OF SATURATED
                     MATERIAL WITH CONSTANT THICKNESS (METERS).


                     PI INDICATE LAYER IS A NON-AQUIFER.
      SOURCE: U.S. EPA (1985b)
      CAMP DRESSER & McKEE INC.
                                                   FIGURE 6-4
                                    LINER-NINE GENERIC GROUNDWATER
                                                  FLOW FIELDS

-------
                                 TABLE 6-2





                           FOUR MOBILITY CLASSES





       Class       Retardation Range              Retardation



       1            _< 10                            1.3



       2           10 and <_ 100                      32



       3           >100 and <_ 10,000                360



       4           > 10,000                       160,000
Source:  EPA (1985b)
                                    6-25

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The nine generic flow fields, three exposure distances, and four mobility
classes can he combined to describe 108 scenarios of contaminant transport.

Applicability to Leaking USTs

The methodology util ized in the .Liner Model  represents only the saturated
zone, so, the methodology is not applicable to the development of generic
settings for leaking UST incidents that affect two primary zones of the
subsurface, the unsaturated and saturated zones and may involve vapor phase
concern.  (In fact, in some leaking UST incidents, only the unsaturated
zone will be affected).  The methodology may be useful, however, to the
development of environmental  settings for leaking UST incidents for the
saturated zone portion of the generic settings.

6.4.3  DRASTIC

DRASTIC (Alien et.  al., 1985) is a standardized system for evaluating
groundwater pollution potential using hydrogeologic settings  developed
by the National  Water Well  Association (NWWA).  The system has two  major
portions: the designation of mappable units, termed hydrogeologic settings;
and the application of a scheme for relative ranking of hydrogeologic
parameters which helps the user evaluate the relative groundwater pollution
potential of any hydrogeologic setting.

Methodology

The hydrogeologic settings  developed were based on the  15  groundwater
regions described by Heath  (1984) as shown in Figure 6-5.   Because
pollution potential cannot  be determined on  a regional  scale,  smaller
"hydrogeologic settings" were developed within these regions.   A typical
hydrogeologic setting is presented in Figure 6-6.

Evaluation of the relative  groundwater contamination potential  of any
hydrogeologic setting was determined to be controlled by the  following
(most important) mappable factors: . jDepth to water, Recharge  (Net),  Aquifer
Media, Soil  media,  Topography, Jjnpact of the vadose zone and  Conductivity

                                    6-26

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                     2. Alluvial Basins
                       i- »•——__
                            ft.
                                                                                               Glaciated
                                                                                               Central
                                                                                                Region
                                                                                           '6. Nonglaciated
                                                                                               Central
                                                                                               Region
                                      j>\        ur\   «/  f.
                                      \\6- Nonglaciated Central "% y

                                      =r*^r	|Re9io"~ \
                                                                                I
                                                                               800 kilometers
SOURCE: Heath (1984)
 CAMP DRESSER & McKEE INC.
                      FIGURE 6-5
GROUNDWATER REGIONS OF THE UNITED STATES

-------
           Northeast and Superior Uplands

           (9Ga)  River Alluvium With Ovarbank

           This hydrogeologic setting is characterized by low
           topography and thin to moderately thick deposits of
           alluvium along parts of river valleys. The alluvium is
           underlain by  fractured  bedrock of  sedimentary,
           metamorphic,  or igneous origin. Water is obtained
           from sand and gravel  layers which are interbedded
           with finer-grained alluvial deposits. The flood plain is
           covered by varying thicknesses of fine-grained  silt
           and clay, called overbank  deposits. The  overbank
           thickness is  usually greater along major streams (as
           much as 40 feet) and  thinner along minor streams.
           Precipitation is abundant, but recharge is somewhat
           reduced because of the silty overbank deposits and
           subsequent  clayey loam soils which typically cover
           the surface. Water levels are typically moderately
           shallow and may be hydraulically connected to the
           stream or river. The alluvium may serve as a signifi-
           cant source of water  and is also usually in direct
           hydraulic connection with the underlying bedrock.
SOURCE: Alleret. al. (1985)
           Setting 9 Q« Rivar Alluvium With Ovarbank
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range

15-30
7-10
Sand and Gravel
Clay Loam
0-2%

Silt/Clay

1000-2000

Weight

5
4
3
2
1

5
•
3
General
Rating

7
8
8
3
10

1

8

Number

35
32
24
6
10

5

24
                                       DRASTIC Index  136
           Setting 9 Oa River Alluvium With Ovarbank
                                          Agricultural
              Feature
                           Range
Weight  Rating  Number
                                                                                       Depth to Water
                                                                                        Table
                                                                                       Net Recharge
                                                                                       Aquifer Media
                                                                                       Soil Media
                                                                                       Topography
                                                                                       Impact Vadose
                                                                                        Zone
                                                                                       Hydraulic
                                                                                        Conductivity
                            15-30
                            7-10
                        Sand and Gravel
                          Clay Loam
                            0-2%

                          Silt/Clay

                          1000-2000
  5
  4
  3
  5
  3
 7
 8
 8
 3
10
35
32
24
15
30
                                                     16
                                                                                                                   Agricultural
                                                                                                                   DRASTIC Index
                                                                                                                                156
 CAMP DRESSER & McKEE INC.
                  FIGURE  6-6
DRASTIC HYDROGEOLOGIC  SETTING

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(Hydraulic) of the aquifer.  These factors form the acronym, DRASTIC.  The
numerical ranking system used to assess groundwater contamination potential
in hydrogeologic settings is calculated using the DRASTIC factors.  The
system contains three parts:  weights, ranges, and ratings.  Each DRASTIC
factor was evaluated with respect to the others and assigned a relative
weight ranging from 1 to 5.  These weights are shown in Table 6-3; the most
significant factors having a weight of 5 and the least significant a weight
of 1.  These weights are constant in the application of DRASTIC.

For each DRASTIC factor, there have been defined either ranges or media
types that have an impact on pollution potential.  These ranges have been
evaluated with respect to each other within each DRASTIC factor to
determine the relative significance to pollution potential, and have been
assigned a rating varying between 1 and 10  (the higher the rating, the
greater the pollution potential).  For example, the ranges and ratings for
depth to water are shown in Table 6-4.

The numerical  value of pollution potential  for any hydrogeologic setting is
calculated by using an additive model.  The equation for determining the
pollution potential  is:

       DRDW + RRRW + ARAW + SRSW + TRTW + IRIN + CRCW = DRASTIC index

where:

       R = Rating
       W = Weight

The higher the DRASTIC index, the greater the groundwater pollution
potential.  From the DRASTIC index it is possible to identify and compare
the relative susceptibility of different areas to groundwater
contamination.
                                    6-29

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                                 TABLE 6-3


                   ASSIGNED WEIGHTS FOR DRASTIC FEATURES
    Feature	Weight
Depth to Water Table                     5

Net Recharge                             4

Aquifer Media                            3

Soil Media                               2

Topography                               1

Impact of the Vadose Zone                5

Hydraulic Conductivity                   3
 of the Aquifer
Source:  Alien et.al. (1985)
                                    6-30

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                                 TABLE 6-4

                   RANGES AND RATINGS FOR DEPTH TO WATER
                               Depth  to Water
                                   (feet)
Range
0-5
5-15
15-30
30-50
50-75
75-100
100+
Rating
10
9
7
5
3
2
1
Source:   Alleret.al.  (1985)
                                    6-31

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Applicability to Leaking USTs

The feasibility of applying DRASTIC to petroleum USTs has been studied in
detail by the National Water Well Association (NWWA).  A revised version of
DRASTIC called UST DRASTIC/IMPACT has been developed (NWWA, No Date).  In
LIST DRASTIC, the weights and ratings of the DRASTIC factors have been
revised to reflect groundwater pollution potential  for any hydrogeologic
setting as related to petroleum USTs.  The UST DRASTIC weights are shown in
Table 6-5.   A hydrogeologic setting with a UST DRASTIC value of 140 or
above has been defined to have a potentially high vulnerability to
pollution from leaking petroleum USTs.

IMPACT incorporates site-specific factors into the  UST DRASTIC/IMPACT
system.  The important site-specific factors were determined to be:
inclination of the water table, Measured horizontal  distance, population
exposed, Application rate,  Concentration, and Trapping/migration potential
of non-aqueous liquid wastes; forming the acronym,  IMPACT.   The numerical
ranking system is similar to DRASTIC although the factors were assigned
equal  weights.  Table 6-6 shows the ranges and rating for each of the
IMPACT factors.   The numerical  IMPACT index is determined by the following
equation:

                            IMP x ACT = IMPACT
                             3     3

The combined UST DRASTIC/IMPACT value that is assigned  to a specific petroleum
UST site is calculated by multipl ying the DRASTIC index by  the IMPACT
index.  This resultant value is capable of "changing"  one of the most
harmless UST sites (as calculated by UST DRASTIC only)  into an area  that
might warrant concern because of, for example, its  relative value to the
local  community and other factors.
                                    6-32

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                                 TABLE 6-5





                 ASSIGNED WEIGHTS FOR UST DRASTIC FEATURES
Feature
Depth to Water Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact of the Vadose Zone
Hydraulic Conductivity of the Aquifer
UST
DRASTIC
5
3
3
4
1
5
2
DRASTIC
5
4
3
2
1
5
3
Source:   Alien et.al.  (1985)  and  NWWA (No  Date)
                                    6-33

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                                 TABLE 6-6

                   RANGES AND RATINGS FOR IMPACT FACTORS
    Factor
Range
Rating
Inclination of Water Table
0.1 ft/ft
0.01 ft/ft
.001 ft/ft
1.5
1.25
1.0
Measured Horizontal Distance
Population Exposed
Application Rate
Concentration
Trapping/Migration Potential
< 1000 ft           1.5
1000-3000 ft        1.25
> 3000 ft           1.0

> 100 people        3.0
10-100 people       2
> 10 people         1.0

Continuous          1.5
Intermittent        1.25
Slug                1.0

High                1.5
Medium              1.25
Low                 1.0

High                1.5
Moderate            1.25
None                1.0
Source:  NWWA (No Date)
                                    6-34

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The UST DRASTIC/IMPACT approach is of interest because it focuses on the
environmental factors that are recognized to be important in assessing the
hazard of leaking petroleum USTs.  It must be recognized, however, that the
individual numerical ratings are rather arbitrary and are not specifically
related to the physical, chemical, and biological  characteristics of a
site/contaminant that determine the actual hazard.  Also, it needs to be
recognized that, because of the handbook nature of the UST DRASTIC/IMPACT
approach, the method is susceptible to misuse by individuals who lack the
professional  expertise to judge the nature of the  setting and the relative
importance of various factors.  For these reasons, the method should be
subjected to  independent field testing before it can be considered as a
reliable tool for generic evaluation of the hazards of leaking petroleum
USTs.  Such testing could be done by using information from several
specific sites in different environments where leaks have occurred,  and
having several individuals independently evaluate  the site characteristics
within the UST DRASTIC/IMPACT framework.  (These individuals would not have
access to detailed information on the contamination situation at the
sites.)  The  UST DRASTIC/IMPACT numerical  ratings  could then be compared
with the actual  contamination consequences at the  specific sites' in  order
to assess the reliability of the generic approach.

6.5  SUMMARY

In this section, the environmental  factors which influence the fate  and
transport of  substances leaking from USTs were identified.  Two types of
environmental factors were presented:  natural  and non-natural.  The
natural environmental factors determine how vulnerable a site is to
contamination, while the non-natural  factors exacerbate a site's
vulnerability to contamination.  As such, non-natural  factors can increase
the hazards resulting from leaking contaminants from USTs.

The environmental factors important in the assessment of the potential
hazards resulting from leaking USTs are listed for liquids and vapors in
Tables 6-7 and 6-8, respectively.  The ultimate significance of these
factors with  respect to leaking USTs  varies from site to site, and also
depends on the properties and quantity of the leaked contaminant. For some

                                    6-35

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                          TABLE 6-7
               ENVIRONMENTAL FACTORS AFFECTING
                  LIQUID FATE AND TRANSPORT

NATURAL FACTORS

•  Climate
      Net 'Recharge
      Temperature

•  Hydrogeology
      Unsaturated Zone Media
      Depth to Groundwater
      Saturated Zone Charateristics

NON-NATURAL FACTORS

•  Population Density
•  Horizontal Distance to Point of Concern
•  Quantity and Rate of Release
                             6-36

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                          TABLE 6-8

                    ENVIRONMENTAL FACTORS
              AFFECTING VAPOR FATE AND TRANSPORT

NATURAL FACTORS

•  Climate
      Precipitation
      Groundfrost
      Barometric Pressure
      Wind
      Temperature

•  Hydrogeology
      Unsaturated Zone Media
      Groundwater Characteristics

NON-NATURAL FACTORS

•  Man-Made Conditions
•  Population Density
•  Proximity To A Susceptible Structure
•  Quantity Released
                             6-37

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of the natural  environmental  factors,  such  as  climatic  variables  or  the  use
of groundwater for drinking wat£r supplies, "it may be possible to assess
the significant variations on a nationwide  geographic basis.   For other
environmental  factors, particularly the non-natural  factors,  it is possible
to assess the  influence of these factors only  on  a site-specific  basis.
For example, in determining the effect of vapor contaminants,  a knowledge
of the specific micro-stratigraphy in  the area of a  leak  and  the  presence
of receptor structures is very important.

It is also important to recognize the  effects  that an environmental  factor
has on fate and transport mechanisms can be in similar  or conflicting
directions.  Some examples of these effects are:

    •  Areas with high net recharge increase the  potential  for a
       contaminant to enter solution but can decrease the importance of
       biodegradation and adsorption mechanisms.
    t  High permeability soil media general ly  increases the migration  of
       liquids and vapors.
    •  Increases in temperature in the subsurface generally increase
       vaporization and solubility of  a contaminant,  but  decrease
       adsorption.
    •  Adsorption of a contaminant increases in soils with  a  high organic
       content.
    •  Dissolved contaminants will  migrate  further and  faster  in  an  aquifer
       with high hydraulic conductivity and gradients.
    •  In areas with a shallow water table, the potential  for  groundwater
       contamination is high.
    t  In northern climates where the  upper soil  zone freezes  in  the
       winter,  the reduction in venting of  vapor  contaminants  to  the
       atmosphere can result in increased lateral  movement  of  vapors,  which
       in turn creates the potential for greater  vapor  infiltration  into
       subsurface structures.

These examples illustrate the virtual  impossibility of determining the
potential hazards of leaking USTs, except on a site-specific  basis.
However,  since within a regulatory framework the  development  of generic
environmental  settings is useful  to evaluate the  potential  consequences of
leaking UST incidents, three existing  methodologies were  examined to
                                    6-38

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determine their applicability.  The methodologies are used in the
following:

    •  RCRA Risk and Cost Analysis Model  (W-E-T)
    •  Liner Location Risk and Cost Analysis Model
    •  DRASTIC and its revised version UST DRASTIC/IMPACT

The methodology for the RCRA model was determined to be unsatisfactory for
evaluating the potential  hazards from leaking USTs  because it does  not
consider fate mechanisms, and the scale of application is too large
(three-digit zip code regions).

Because the methodology used to develop generic environmental  settings in
the Liner model only characterized the saturated zone, use of this
methodology in assessing  the potential hazards from leaking USTs  is
relevant only if the hazard of concern is contamination of groundwater
resources.  While this methodology is not presented in a form which can be
used to evaluate the chronic risk to human health from drinking  groundwater
contaminated with substances leaking from USTs, it  may be possible  to
modify the methodology to do this.

When assessing which environmental factors should be included in  DRASTIC (a
numerical system for the  ranking of groundwater pollution potential),  the
NWWA selected factors that required information which  is available  from a
variety of sources.  As such, DRASTIC is  a simple and  easy-to-use approach
which when properly applied may be a useful  planning or screening tool.
The NWWA assessed the feasibility of applying DRASTIC  to petroleum  UST
related problems; consequently, several  of the weights and ratings  were
revised.  The need to address site-specific factors was also recognized;
these are included in IMPACT.  The revised DRASTIC  and IMPACT make  up  the
proposed UST DRASTIC/IMPACT system.  It appears that this system  could be  a
useful  tool for comparative evaluation of environmental  settings  for
hazards posed by leaking  UST incidents for two reasons:   1.   the
methodology includes many of the important liquid and  vapor  environmental
factors determined relevant to leaking UST occurrences,  and  2. the  scale at.
                                    6-39

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which DRASTIC develops an environmental  setting is appropriate for LIST
incidents.

Because the LIST DRASTIC/IMPACT system appears to be the most  applicable of
the available methods to evaulate the groundwater pollution potential  from
a leaking UST site, some notes on the use of the system are presented  here.
One possible use of this system would be to rank leaking UST sites for
prioritizing corrective action and/or enforcement action.   This  system,
however, has not been developed to evaluate the possible hazards of two
nearby sites, e.g., two service stations on opposite corners  of  an
intersection.  Also, as discussed in Section 6.4.3, because of the handbook
nature of the UST DRASTIC/IMPACT approach, the method is susceptible to
misuse by individuals who lack the professional  expertise  to  judge the
nature of the setting and the importance of various factors.   A  testing
system to compare the numerical  ratings  with actual leaking UST  sites  is
proposed in this section.
                                    6-40

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        7.0  APPLICABILITY OF TECHNICAL FINDINGS TO THE UST PROGRAM
7.1  INTRODUCTION

In the preceding four sections of this report, we have presented a compre-
hensive description of the current understanding of the fate and transport
of substances leaking from USTs.  The objective of these preceding sections
was to incorporate all of the relevant technical and scientific information
into a form and format useful  for UST program activities.  This has been
accomplished by organizing the preceding text as follows:

    •  A description of the transport and fate of regulated substances  in
       the subsurface environment (Sections 3.0 and 4.0);
    t  The chemical and physical properties of petroleum products  and
       hazardous substances as they relate to fate and transport processes
       (Section 5.0); and
    •  The environmental  context (or setting) and its relationship to fate
     .  and transport phenomena  (Section 6.0).

In order to place these scientific/technical  efforts in proper perspective,
it is useful to consider the chain-of-events  that relates the source of a
contaminant to its potential  environmental effect.  The diagram in Figure
7-1 illustrates this chain-of-events, and relates each step in the sequence
to studies of underground storage tanks.  For the most part, the work
reported here concerns the portions of Figure 7-1 shown within the dashed
lines, although relevant  information regarding the source of releases and
causation of impacts is incorporated in the report as appropriate.

Figure 7-1 suggests that  there are various points of intervention  in the
process of moving from the source or release  of a contaminant to its effect
on the environment.  The  Agency — the UST Program Office -- has the
responsibility to decide where and how the regulatory process intervenes.
Making such choices in an informed manner necessitates an understanding of
the science presented previously.  However, because the number and
                                    7-1

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      GENERIC DESCRIPTION
              UST • RELATED DETAILS
                  SOURCE
      r
           TRANSPORT
                     EMITS
                POLLUTANT(S,
                     TRANSMITTED
                  MEDIA
                      TOWARDS
                               	1
                RECEPTOR(S)
        FATE
                     CAUSING    -
                  IMPACT(S)
                                                       * TANKS
                      • LEAKS
                      • RUPTURES
                      • IMPROPER OPERATIONS
                                                         PETROLEUM PRODUCTS
                                                         HAZARDOUS SUBSTANCES
                      • FREE PRODUCT
                      • SOLUTIONS
                      • VAPORS
                      • SOIL
                      • AIR
                      • WATER
                      • MAN-MADE STRUCTURES
                                                       • SOURCES OF
                                                         DRINKING WATER
                                                       • HABITABLE SPACES
                       HUMANS
                       SENSITIVE ECOSYSTEMS
                                                         CHRONIC EXPOSURE
                                                         ACUTE EXPOSURE
                        MORTALITY
                        MORBIDITY
                        PROPERTY DAMAGE
CAMP DRESSER & McKEE INC.
               FIGURE 7-1
   CHAIN OF EVENTS LINKING UST
INCIDENTS WITH POTENTIAL IMPACTS
                                      7-2

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complexity of relevant scientific topics is large, a summary is presented.
here to place technical matters in the context of the UST program and,
therefore, to help guide and focus future work assignment efforts.  The
model of the "cause-effect" process given in Figure 7-1 provides a
reference point that helps keep various topics in perspective to the total
picture.

The Agency has available many possible activities that might be used to
prevent and remedy the effect of leaking USTs.  Some activities, such as
development of regulations, are the result of congressional  requirements.
Others -- for example, priority-setting or outreach programs -- are dis-
cretionary within the Agency as to extent and focus.  A list of the kinds
of activities the UST program may eventually utilize is found in Table 7-1.
This list was developed  -- by COM and only for the purpose  of illustration
-- based upon our understanding of recent Agency experiences.  Scientific
and technical information is, of course, relevant to many of these
activities.

In an attempt to identify the most likely and relevant of the possible   ;_
Agency endeavors, the early stages of this work assignment included
contact/conversation with numerous personnel  involved in UST matters --
both within and outside the EPA.  As a result of these discussions, we have
presented and organized  our findings in this section as follows:

    •  Section 7.2: Problem Definition — Attempts to put technical
       information in the context of helping to consistently and correctly
       understand the overall problem of leaking USTs.
    •  Section 7.3: Regulation/Guidance Development -- Suggests how and why
       technical and scientific matters can relate to the development of
       priorities for technical standards, corrective action, and  reporting
       requirements.
    •  Section 7.4: Compliance and Enforcement -- Focuses on how the EPA
       may use scientific/technical findings to implement and target its
       activities; i.e., to set priorities.
    •  Section 7.5: Information Transfer -- Discusses how the need for
       research and development, demonstration projects, training  and
       technical assistance, and outreach programs may be tailored and
       adjusted to achieve the greatest environmental  benefit based on
       scientific and technical information regarding leaks  from USTs.

                                    7-3

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                   TABLE 7-1



           TYPICAL PROGRAM ACTIVITIES








0    PROBLEM DEFINITION AND ASSESSMENT



•    REGULATION AND GUIDANCE DEVELOPMENT



•    PRIORITY SETTING AND CONFLICT RESOLUTION



•    BUDGETING AND PROGRAM EVALUATION



•    DATA COLLECTION AND MANAGEMENT



•    STATE AND LOCAL PROGRAM DEVELOPMENT



•    COMPLIANCE MONITORING AND ENFORCEMENT



•    TRAINING AND TECHNICAL ASSISTANCE



•    RESEARCH, DEVELOPMENT, AND DEMONSTRATION PROJECTS



•    COMMUNICATIONS AND OUTREACH
                      7-4

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7.2  PROBLEM DEFINITION AND ASSESSMENT


Since the enactment of subtitle I in the RCRA amendments of 1984, EPA has

been moving ahead to develop and implement a comprehensive regulatory
program.  While this process continues, it is useful to extract from the

technical information presented earlier in this report those items that

provide a similar and technically sound view of the problem at hand.  In

the text to follow, we attempt to establish a foundation for this similar-
ity of viewpoint by providing a synopsis of key facts and highlighting

those topics which warrant careful  attention to technical and scientific
matters.


7.2.1  PROPERTIES OF REGULATED SUBSTANCES


Subtitle I draws a distinction between petroleum products and other

hazardous substances.  It is worthwhile to point out certain facts
concerning these regulated substances.


    •  Petroleum products, as a category, are generally not distinguishable
       from the list of regulated hazardous substances.  Gasoline, for
       example, is known to contain — as pure hydrocarbons, additives,
       and/or trace contaminants -- over 15% of substances which appear as
       regulatable hazardous substances (i.e., are on the CERCLA list).
       Moreover, of these substances, a large fraction are aromatic
       hydrocarbons and therefore,  are likely to be relatively toxic and
       move readily in the environment (Section 5.4).

    •  The bulk properties of petroleum products are such that various
       sub-groupings related to the potential for movement in the environ-
       ment can be assembled.  In general, the lighter fractions of petrol-
       eum products with low kinematic viscosity and high vapor pressure
       can be expected to be more mobile than the heavier (tar-like)
       petroleum products such as No. 6 fuel  oil (Figure 5-3 in Section
       5.2.3).

    •  The hazardous substances subject to UST program regulation span a
       range of physical, chemical, and toxicological properties.  As with
       petroleum products, the entire list of hazardous substances may be
       divided into subsets that represent varying levels of potential for
       movement and/or impact on the environment.  (Figures 5-18 through
       5-23 in Section 5.3.2).
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7.2.2  MANIFESTATION OF PROBLEMS


The cause-effect model  cited in Figure 7-1 identifies various receptors and

impacts that are relevant to the regulation of substances in USTs.  Some of
the technical  and scientific facts presented in the previous sections are
relevant to a thorough  understanding of how releases of regulated

substances may cause actual  problems.  The following findings emerge:


    •  The contamination of groundwater aquifers is one possible result of
       a leaking UST incident.  Where such aquifers are used directly for
       water supply or  communicate with surface water bodies used for
       potable water, the potential  exists for human exposure.

    •  Depending on a combination of factors, including the hydrogeological
       conditions at a  site and the  chemical and physical  properties of a
       substance released from an UST, not all releases have the potential
       to reach the saturated zone.  (Sections 3.3, 3.4, and 3.5).

    0  Scaled by the vapor pressure  of a substance released into the
       subsurface environment, there is the potential for vapor phase
       contaminants to  migrate and impact human beings and/or the
       environment.  Hazards resulting from vapor phase migration can be
       acute or chronic.  For example, migration can lead to an increase in
       the vapor phase  concentration of certain contaminants such that
       there exists a high potential for explosion.  On the other hand,
       even low concentration levels of a vapor phase contaminant can enter
       habitable space  undetected such that chronic human exposure (and
       resulting health "impact) is possible.  (Section 3.6).

    •  Due to the underlying physics of the fate and transport process,
       particularly"in  altered settings where most petroleum product tanks
       are located, vapor phase movement can occur much more rapidly than
       liquid contaminant transport  in solution in the saturated zone.
       Movement of vapor phase contaminant in altered settings (conduits)
       can be as high as "tens of feet per hour" as opposed to contaminants
       in solution which move at the rate of the ambient groundwater, on
       the order of "feet per day."

    •  Vapor release can be increased by corrective action measures
       involving excavation.  This is because, as soil is removed, the
       distance to the  atmosphere (to near-zero concentration) is decreased
       and therefore the gradient-driven diffusion process is accelerated.

    •  As an immiscible substance moves through the subsurface, the soil  it
       encounters becomes contaminated.  The amount of residual con-
       tamination that  remains in the soil matrix is a function of the
       physical and chemical, properties of both the contaminant and the
       soil media. Contaminated soil may produce a slow (aperiodic or.:
       chronic) release of contamination over a long period of time, the
       residual contaminant in the soil may remain in place almost
       indefinitely, or be biodegraded and/or transformed over time (see
       Section 3.0 and  4.0).

                                  7-6

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7.2.3  RELEASE SETTING

When Congress enacted RCRA subtitle I, -it explicitly allowed fcff the

implementation of the UST regulatory program to reflect differences in

hydrogeological  settings.  As reported in Section 6.0, the setting in which

a release of contaminant occurs can greatly influence whether and how

problems may result.  Some observations are as follows:


    •  Hydrogeological features such as depth-to-groundwater, soil  type,
       the localized slope of the water table, etc., all  play a role in the
       transport of regulated substances in the subsurface environment.
       However,  only in site-specific terms is it possible to combine the
       chemical  and physical  properties of a contaminant  with the
       hydrogeological properties of a site to quantify information
       concerning environmental or human health impacts.   (Section 6.2).

    •  In addition to the transport-related aspects of the hydrogeological
       setting,  many issues related to the fate of particular chemicals
       depend on the setting  in which they are released.   For example,
       soils that have a high carbon content can be expected to adsorb
       contaminants, "delaying" their dispersion into the environment.  The
       properties of certain  regulated substances, in combination with
       certain soils, may limit the severity or extent of present or future
       impacts from a release, as discussed in parts of Sections 3.0, 4.0,
       and 6.0.

    •  Because most USTs are  located in or near areas of  human activity,
       human influence on the overall impact chain-of-events (Figure 7-1)
       must be taken into account.  Obviously, whether or not an aquifer  is
       "vulnerable" to contamination is linked to whether or not it is (or
       may be) used as a source of drinking water, or recharges surface
       waters used for drinking water.  Also human-related is the presence
       of disturbed soil or utility conduits (e.g., sewer lines, electric
       and telephone lines,)  which can provide for the rapid and widespread
       migration of subsurface contaminants, particularly in the vapor
       phase.

7.3  REGULATION/GUIDANCE DEVELOPMENT


The foregoing section highlights the technical/scientific investigations

undertaken in this work assignment.  The objective has been  to ensure that

the full complexity and scope of the problem is understood.   Here,  the

objective is to provide the details of our technical and  scientific

findings that may prove illustrative to the Agency'.s development of

regulations and/or .guidance.   As such, technical  findings are applied to

the three areas  in which the  Agency is currently pursuing regulatory and
                                    7-7

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guidance development:   technical  standards, corrective action, and

notification/reporting requirements.  Each of these areas is discussed

separately in the material  that follows.


7.3.1  TECHNICAL STANDARDS


Technical  standards -- as part of  the UST program — could specify per-

formance-related details of tank design, installation, monitoring, and/or

construction.  To consider  levels  of performance in various settings may

require the use of the types of scientific information presented in the

earlier sections of this report.  Thus,  in an effort to support the

Agency's development of technical  standards, the following observations are

offered:
    •  With reference to Figure 7-1 provided earlier, a range of receptor
       effects is possible as a result of an UST incident:

       -  If one is concerned with contamination of groundwater resources
          (i.e., pollution of the saturated zone),  toxicity and solubility
          characteristics of the leaked substance are of major concern.  A
          screening of the regulated hazardous substances based on  these
          two parameters is provided in Section 5.3.3.  About 32 hazardous
          substances -- when sorted according to high toxicity and  high
          solubility criteria -- are (or may be) considered to have a very
          high potential for groundwater contamination out  of the 374
          liquids and solids with available data.

          If one is concerned about potential vapor phase impacts of
          human exposure to a contaminant, toxicity as well as vapor
          pressure properties are significant.  Liquid substances were
          screened with respect to these two parameters to  illustrate the
          concept.  The results, as reported in Section 5.3.2, indicate
          that only 5 substances for which final RQ data exist and  another
          2 which await final RQs would be considered high  risk from this
          dual-parameter standpoint.

          If one is concerned about the potential for explosive hazards
          from release of regulated substance, the  vapor pressure and
          ignitability parameters are significant.   Again,  the screening in
          Section 5.3 indicates that only 9 substances can  be considered
          high risk from this perspective.

       -  There are no chemicals that appear as "high risk" from all  three
          sorts.  On the other hand, if one adds the screening results from
          the above dual-parameter sorts, excluding duplication, the
          results indicate that roughly 50 substances may be considered --
          for one reason or another -- high risk.  In addition, 19  vapor

                                    7-8

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      phase substances and 20 highly reactive substances must also be
      considered high risk.

   -  If one is concerned about the fate and transport of petroleum
      products in their bulk form, then two parameters are of
      significance.  The kinematic viscosity affects the rate of
      movement of the substance, and the vapor pressure indicates the
      potential for the substance to have an associated vapor phase.
      Starting with 17 individually identified petroleum products as
      listed in Figure 5-3, the results of sorting petroleum products
      according to kinematic viscosity and vapor pressure indicate that
      6 of these products would be considered high risk when viewed in
      terms of the potential to contaminate groundwater and/or the
      potential to release vapor to the environment.  On the other
      hand, the 3 petroleum products shown in Group IV (Figure 5-3) may
      be regarded as largely immobile.

•  The media in which pollutants move has direct bearing on the impacts
   that will or may be manifest.

   -  When considering the potential for human exposure to a
      contaminant, particularly in the vapor phase, the presence or
      absence of man-made structures is an important feature of the
      environmental setting.  Most important is the presence of sub-
      surface structures (basements, for example)  where gas-phase
      contaminants can collect and, potentially, lead to toxic exposure
      or explosion/fire hazards.  The existence of conduits (e.g.,
   i   sewer lines) to act .as channels for the migration of vapors (and
      in some cases liquids) is obviously a site-specific feature of an
      UST incident.  However, because such conduits often lead to
      basements and/or can cause migration well away from the area in
      which the tank is located, the presence or absence of conduits in
      the tank's vicinity may prove to be useful in establishing
      regulations and standards.  A distinction between tank standards
      in "developed" and "undeveloped" settings, for example, might be
      a worthwhile differentiation.

   -  The natural hydrologic features at the site  of an UST leak have
      an obvious role in the extent of migration (and consequent
      environmental impacts).  Systems for evaluating environmental
      parameters, such as the UST DRASTIC/IMPACT model, have been
      discussed in Section 6.0.  From a technical  standards viewpoint,
      such evaluation systems offer the theoretical potential  to
      incorporate environmental settings in the regulatory process.
      However, application of these systems is obviously site-specific.
      In addition, these parameter-based systems are inherently
      somewhat subjective and may require expertise to be used
      properly.

   -  Regulations and standards should be directed toward avoidance of
      the aquifer contamination that results when  a toxic contaminant
      enters an environment that permits access to groundwater.  One
      may envision a system of technical standards wherein the required
      level of performance is tailored to whether  or not the underlying

                                7-9

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          aquifer is (or could be) used as a source of potable drinking ••
          water, interconnects with areas of valuable/unique environmental
          habitat, or is used for agricultural  purposes.  The Agency's
          groundwater protection strategy may prove useful in this regard.
          Such broad-based attributes of a setting (as contrasted with
          DRASTIC-type parameters) would be easier to determine than
          site-specific hydrogeological parameters.

       -  As is clear from the above discussion, environmental settings
          cannot be separated from the chemical  and physical  properties of
          the substances at issue.  To illustrate the point,  as the
          discussion in Section 4.0 indicates,  some soils, when combined
          with some regulated substances, yield  a situation wherein
          biotransformation can result in the formation of innocuous
          products.  In contrast, certain hydrogeological  characteristics
          may combine with physical/chemical  properties to result in by-
          products more toxic than the original  contaminant,  or situations
          of rapid and extensive spread of contaminant plumes.

7.3.2  CORRECTIVE ACTION


While the scientific/technical information developed in this  report has
potential utility with respect to tank technical standards, there is,

perhaps, an even greater utility with respect to corrective action.  The
following discussion illustrates the usefulness  of the scientific and

technical information presented in Sections 3.0  through 6.0.  with respect
to several distinct topics relevant to corrective action:


    •  Extent of Contamination

       -  Contamination from an UST incident  has the potential to occur in
          both the unsaturated and saturated  zones.

       -  Given the location of USTs, virtually  all leaks  result  in
          contaminants entering the unsaturated  zone; these then  have  the
          potential to exhibit vapor phase movement.

       -  Some contaminants (e.g., Group IV in  Figure 5-3) may remain
          "trapped" in the unsaturated zone.   These contaminants  can remain
          bound up in the soil matrix substantially immobilized,  migrate
          away from the leak as a vapor, or be  solubilized in percolating
          rainwater.  Other contaminants may  be  biodegraded and/or
          transformed into new products.

       -  Some contaminants reach the saturated  zone and produce  a "plume"
          of contaminant in solution with (or as part of).the overall
  ••       groundwater flow field.  Depending  on  its relative  solubility,
          the proportions of the spill and the  groundwater flow,  the
          contaminant may float as a lens on  top of the water table,
          dissolve in the groundwater flow, or  sink to the impermeable
          surfaces which confine the aquifer.

                                  7-10

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•  Limited Actions

   -  One key technical  finding of this work assignment, is the
      realization that under some circumstances, regulated products do
      not and will not migrate very far.  If these same substances are
      not very toxic or in other ways threatening, detailed study
      and/or sophisticated corrective action may not be warranted.
      Information and findings presented earlier (e.g., sorting results
      and settling information) may be useful  in deciding when and
      where "limited" corrective actions may be justified.

•  Clean Up Level

   -  Among the difficult issues that have been raised in all  of EPA's
      corrective action programs is "how clean is clean."  In
      establishing this standard for LIST regulations, the physics and
      chemistry involved in the fate and transport processes must be
      taken into account.  The technical decision as to where, when,
      and how to monitor for the presence of contaminants is influenced
      by numerous and complex (sometimes uncertain) scientific
      principles.  For some chemicals in some settings the probability
      of large-scale vertical migration is minimal, negating the need
      to monitor in the saturated zone.  Vapor phase monitoring in the
      vadose (unsaturated) zone may be appropriate only for certain
      chemicals with high vapor pressure.  The fact that for UST
      situations, solid hazardous substances are most often stored in
      solution (in particular solvents) suggests that monitoring for
      the presence of the solvent may be an appropriate indicator of
      the presence of the chemical of interest.  Thus, scientific
      insight is useful  in deciding on how monitoring can play a role
      in the overall UST program.

•  Effectiveness of Actions

   -  The "no action" alternative for corrective action might  be
      appropriate in some situations.  Obviously, the source of the
      release should be remedied in all instances.  However, when the
      amount or nature of the released contaminant(s) in conjunction
      with the environmental setting appears to present no acute threat
      to human health and the environment (and substantial dilution,
      biodegradation and/or chemical  transformation is likely), the no
      action alternative may be appropriate.

   -  When large quantities of contaminants are released, large
      portions of the unsaturated zone are often contaminated.  Again,
      as a function of the long-term fate possibilities  and immediate
      exposure potential at a given site, soil removal activity may be
      warranted.  In such instances 1. the remediation work is usually
      costly, and 2. if free product is present on the surface of the
      groundwater table during- excavation, the rate of release of
      vapor-phase contaminant will be accelerated by the soil  removal
      process itself.  The latter effect could result in local acute
      air quality impacts where, before soil removal, slow release of
      the subsurface contaminant may have been negligible.
                                7-11

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       -  Where extensive saturated zone contamination occurs and the
          aquifer is the source of present or future water supply or
          agricultural use, there may be no alternative to extraction of
          groundwater and subsequent treatment.  This is an expensive
          proposition.  Bioreclamation and other novel methods of in-situ
          remediation hold some promise, but they are relatively
          experimental and may .engender public fear of "biological
          meddling."

    •  Reponsible Party Designation

       -  The majority of discovered LIST incidents occur in developed as
          opposed to undeveloped settings.  This reflects both the  large
          numbers of tanks located in developed areas as well as the
          presence of receptors to "sense" that a leak has occurred.
          However, this fact may complicate the process of identifying the
          particular tank responsible for a given leak.  For motor  fuels in
          particular, it can be difficult to differentiate one gasoline
          from another and there are often many gasoline stations in an
          area where a leak has occurred.  Man-made utility conduits (which
          provide channels for rapid and broad migration of contaminants)
          also complicate the process of identifying and locating a
          particular leaking tank.
7.3.3  NOTIFICATION AND REPORTING


There are three areas of information exchange applicable to the LIST program
where the scientific and technical  information developed in this document

may prove pertinent.  Understanding the science that underlies the fate and

transport of regulated substances gives an indication of what information
or data are pertinent to collect and/or to analyze.  The three information-

related topics include: 1. the notification requirements for all owners and
operators of USTs, 2. requirements  associated with the reporting of a
release, and 3. the reporting of corrective actions taken.


The notification program currently  underway requires owners of regulated

USTs to inform the state wherein they are located of certain features

regarding the tank and its contents.  This effort is clearly worthwhile for
the Agency, as well as the states,  to define the universe of tanks subject
to regulation.  The manner in which these data are organized and used can

be directly tied to the kinds of "screening options," if any, the Agency
selects for technical standards and corrective action.  At present, the
                                    7-12

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level of information forthcoming in the first round of notification
compliance may not provide all of the information necessary for very
sophisticated sorting.  Nonetheless, a great deal of useful information can
be obtained by combining the expected information with knowledge of
physical and chemical properties of constituents to obtain an estimate on
the extent and severity of potential problems.

With respect to the reporting of a release and the corrective actions
taken, there is a direct link between the reporting requirements and
whether or not a policy of limited response — in some cases -- is adopted.
Information to be reported at the time of discovery of a leak can be
significant in terms of prioritization and targeting of EPA and/or state
agency resources towards corrective actions at that site.  The information
reported and the level of detail required can be used in specifying the
extent of corrective action requirements in site-specific situations.
Certainly, adequate information regarding the magnitude and type of
contaminant released, as well as the natural and man-made environment in
which the release occurred, will be useful.  As already discussed, in some
instances this* infjprmation would lead to a conclusion that no short- or
long-term problem would exist, and therefore, corrective action need not be
rigorous nor timely.  In other instances just the opposite would be true.

Reporting requirements associated with the completion of corrective action
would obviously relate to the "how clean is clean" issue.  Here again, an
understanding of the physics and chemistry that underlie the fate and
transport processes would be necessary in establishing that a given
standard of cleanup has been achieved.  This information would also relate
to where, when, and how monitoring needs to be done to demonstrate
regulatory requirements have been satisfied.

In considering options for the development of corrective action require-
ments, it may be worthwhile for the Agency to retain some flexibility to
respond to leaks.  That is, it is conceivable for the Agency to take the
position that corrective action requirements be tailored towards the
potential for migration from or exposure at a given site.  Thus, if cer-
tified hydrogeologists or engineers analyze and investigate a particular

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situation and can demonstrate that minimal impacts are likely, then re-
quirements for rigorous (and costly) corrective action might be diminished.
An analogy to the Agency's implementation of Section 301(h) of the Clean
Water Act might be drawn.

7.4  COMPLIANCE AND ENFORCEMENT

The discussion above has focused on how and why technical and scientific
information might prove relevant to technical standards, corrective action,
and various reporting requirements.  All of these necessary LIST program
activities are explicitly required under the provisions of subtitle I.  In
the area of compliance and enforcement, however, the Agency may have
somewhat more latitude in organizing and implementing the LIST program.
This is especially important in light of the fact that the regulated
community consists of very large numbers of small scale enterprises.

Scientific and technical findings of the sort presented herein can be
useful  in helping EPA (and the states) to allocate and target limited
resources towards the greatest environmental benefits or safeguards.  For
example, compliance and enforcement of technical standards attempts to
prevent the release of contaminants into the environment.  Because the
chemical and physical properties of regulated substances combined with
their environmental settings cause certain situations to be more
significant than others, these release prevention efforts ought to target
such situations as high-priority.  Information regarding the location and
contents of USTs can be used to identify either geographic areas and/or
particular industrial or commercial sectors which seem to exhibit high risk
potential.

As opposed to technical standards and their objective of problem "preven-
tion," corrective action may be thought of as focusing on "remediation."
Again, scientific and technical information can be most fruitfully applied
in directing limited resources towards the "high risk" problem areas.  In
deciding where corrective action reporting requirements (both pre-action
and post-action) must be most closely monitored with respect to compliance
and enforcement, for example, the Agency's efforts might be guided by a

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combination of the physical and chemical properties of the contaminant
combined with information regarding the setting wherein the UST incident
occurs.  In implementing and enforcing corrective action measures at a
given site, necessary levels of cleanup can, of course, also be guided by
the risk potential inherent in the situation.

In a sense, the Agency's ability to target compliance and enforcement
activities towards sites or situations with high-risk characteristics might
conceivably be more effective in achieving the objectives of subtitle I
(i.e., reducing the risks of public health or environmental damage from
leaking underground tanks) than simply implementation of regulatory and
guidance documents as directed by Congress.  Programs designed to "reach
out" to high-risk portions of the regulated community, as will be discussed
in Section 7.5 below, can demonstrate why and how compliance with technical
standards, for example, is in the best interest of the tank owners.
Scientific and technical information can be used to identify these
potential  high-risk groups.

7.5  INFORMATION TRANSFER

In the remaining portions of this section, we offer some suggestions and
insights regarding research and development activities, demonstration
projects,  training and technical assistance activities, and outreach
programs.   In all cases the objective will be to point out how and why
scientific and technical findings developed here have a place in such
efforts.

7.5.1  RESEARCH AND DEVELOPMENT

As evidenced in earlier discussions in Sections 3.0 through 6.0, a great
deal of research and development activity has been already (and continues
to be) performed in relevant topic areas.  The impetus for much of this
work goes  beyond simply the UST program and encompasses related regulatory
programs under TSCA, CERCLA, RCRA, Clean Water Act, Safe Drinking Water
Act, etc.   The work itself is being performed at universities, by private
enterprise, and by EPA's own ORD organization.

                                    7-15

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Although the LIST program cannot directly influence research activities at

all relevant facilities, many of these on-going research efforts will be

useful in current UST efforts.  The urgency of need for various research

activities must reflect the kinds of regulatory options the Agency selects.

With reference to Figure 7-1, for example, if technical standards are

promulgated (and complied with) such that the possibility of future

releases is reduced, then perhaps research efforts aimed at pollutant
migration may not be a high priority.


Based on our investigations of the current state of science regarding

UST-related phenomena, and noting that UST releases have already occurred

and are likely to continue to occur for some time into the future, we
present here a list of some of the topic areas we believe to merit

consideration for future research and development.


    •  Data Regarding Regulated Substances -- As indicated in Section 5.0,
       it is possible to group regulated substances into subsets based on
       physical, chemical, and toxicological properties.  Unfortunately, a
       complete set of the requisite data regarding each regulated sub-
       stance is not available.  Worthwhile research could, for example,
       concentrate on developing toxicological parameters for chronic and
       acute exposure via water borne and, perhaps, vapor phase pathways
       for all regulated substances.  Viscosity is another parameter that
     .  is currently quantified for only a small portion of the substances
       at issue in the UST program yet is a property of considerable
       significance to subsurface transport rate (Section 5.0).

    •  Vapor Migration Modeling -- As pointed out previously, vapor phase
       contamination resulting from UST releases is of demonstratable and
       serious concern in some settings.  To date, the processes governing
       release and migration of vapor phase contaminants in the subsurface
       environment have not been thoroughly investigated nor quantified.
       The development of an analytical model aimed at evaluating vapor
       phase release and migration would be a useful  addition to currently
       available techniques.  Such a model would enable the Agency (and/or
       the states) to quickly judge the extent of vapor phase migration and
       subsequently draw judgements as to whether or not a potentially
       serious impact is likely.

    •  Unsaturated Zone and Multi-Phase Transport -- As Section 3.0
       indicates, details of fate and transport in the unsaturated zone as
       well as fate and transport of multi-phase contaminants are presently
       not well understood.  Because such phenomena may have serious
       environmental consequences, however, the need for research in these
       areas is important.  The Ground Water Research Review Committee of

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       EPA's Science Advisory Board (USEPA, 1985c) makes the following
       point: "The Committee recommends that EPA increase research in the
       basic process that govern the transport and fate of contaminants in
       groundwater ..."

    •  Make-Up of the Regulated Community -- Data collection and management
       activities aimed at fully understanding the nature and extent of the
       LIST problem is crucial to effective program development.  Through
       the notification program currently underway, as well  as through the
       reporting activities associated with corrective action, the Agency
       has the opportunity to collect useful information that will help
       identify and characterize the regulated community in  detail.
       Unfortunately, the states are thus far only charged with collecting
       required information, although EPA has developed a basic system
       available to the states to organize/manage this data  and work
       continues on improving the package.  The processing and summarizing
       of these data on a nationwide basis would be helpful  in the future
       development of the program in conjunction with  the information
       presented in this report on, for example, the solubility or mobility
       of some chemicals.
7.5.2  TRAINING AND TECHNICAL ASSISTANCE


One of the fundamental  precepts of the UST program is that implementation

will be accomplished largely at the state level.  With this in mind,  it

seems reasonable to expect that the kinds of technical and scientific

information presented here in Sections 2.0 through 6.0 could be packaged

for distribution in the form of a guidance documents.  This would assist

state personnel as well as EPA regional  personnel.  In addition, such an

exposition of technical information,'properly focused, would be useful  to

local levels of government as well as  to portions of the regulated

community.  Also, depending on the form and substance of technical

standards and corrective action regulations, background documents

explaining why or how various groupings  or sub-groupings of substances

and/or settings are to  be regulated differently would be necessary.

Guidance or background  documents such  as these may serve as the outline for

training workshops to be conducted by  the Agency.


In addition to an overview of the basic  science involved in regulating

USTs, more explicit guidance may be necessary in the implementation of

state .(and perhaps local) regulatory programs.  Perhaps one of the most

valuable contributions  the Agency might  make towards effective


                                    7-17

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implementation would be the development of "models" or protocols for
actions to be taken when one discovers an actual UST incident.  Technical
assistance in the areas of prioritizing situations and guiding corrective
action will be necessary.  For CERCLA, for example, these technical
assistance efforts take the form of computer-aided systems to provide
guidance on the complex task of establishing appropriate corrective action
in a site-specific situation.  In the jargon of today, such an approach may
be considered an "expert system."  On the other hand, as a first step, the
same objective can be achieved with more traditional  guidance checklists or
manuals.  There are advantages and disadvantages to both approaches.

7.5.3  DEMONSTRATION PROJECTS

Because there is only limited documented experience to date, the
effectiveness of various corrective actions for remedying UST-related
problems remains an area of uncertainty.  In order to better understand
this issue, there is a need for demonstration projects.  Not only would
such demonstration projects help to determine the technical methods best
suited to particular situations, they would help to quantify the real costs
involved in various types of remedial  activities.  Moreover, demonstration
projects may prove useful in dispelling false notions of clean-up related
risks such as uncovered in the Provincetown situation.  As briefly
described in Section 1.2, when Provincetown, Massachusetts sought to remedy
groundwater contamination caused by an underground storage tank leak, a
system of groundwater extraction wells and subsequent treatment was the
clean-up technique implemented at a cost of over $3 million.  However,
in-situ biodegradation may have been a less costly and equally effective
remedy.  The public's perception was that the biodegradation option
involved "risks" due to the use of microorganisms, although the
microorganisms involved are naturally found in the environment.
Demonstration projects may be able to provide objective data and
consequently lead to more cost-effective corrective action options.
                                    7-18

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7.5.4  OUTREACH PROGRAMS

The potential utility of outreach programs was briefly touched upon in the
previous discussion of compliance and enforcement.  We return to the topic
now because it represents an area of activity that can prove to be useful
for the Agency to achieve LIST program objectives.  Assuming that the data
collection efforts of the Agency, combined with the kinds of scientific and
technical information provided herein, can be used to target high risk UST
populations (by geography, industrial sector, etc.), programs can be
designed to point out to these high-risk groups the advantages of complying
with both technical standards and corrective action requirements set by the
Agency.

To illustrate the point, assume high-risk populations can be defined, as
1. all  tanks over sole source aquifers;  2. all tanks containing specific
chemicals regardless of location, etc.  Once high risk groups are
identified, programs can be developed to commmunicate to these audiences.
Issues such as third party liability, responsible party designation, etc.,
can be explained to demonstrate why and  how it is in their best interest to
meet (or exceed) regulatory requirements.

Even if the Agency itself does not wish  to target high-risk audiences based
on the sorts of scientific/technical principles illustrated herein, those
who own or buy tanks might find it useful to understand the scientific and
technical issues involved to appreciate  the risks they face from their
ownership and operation of USTs.  This risk management information might be
distributed by the Agency through tank manufacturers, local fire marshals,
and others.  The result might be that owners of tanks with extremely
dangerous substances or in sensitive settings understand and appreciate
their situation, and take prudent precautionary measures in the
installation and operation of tanks.  One possible extension of this con-
cept would be to use the insurance industry as a vehicle to help achieve
compliance with UST program objectives.   That is, it is possible to
envision that the insurance industry might set premiums as a function of
risk categories developed through the scientific and technical  groupings
suggested here.  Should this occur, the  insurance premium rate structure
may help achieve satisfaction of UST program objectives.
                                    7-19

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7.6  SUMMARY


Several preliminary suggestions are offered as to how scientific and

technical information — as presented in this report -- can be used in the

development and implementation of the UST regulatory program.  Obviously,

precise recommendations must follow from the choices the Agency makes in

developing and implementing the regulatory program.  In turn, although

scientific and technical information is pertinent, these Agency choices may
reflect, in large measure information regarding how many tanks, containing

what substances, are distributed throughout the United States.


Although this report presents findings of only the first phase of the work

assignment, there are several ways in which the findings may have immediate

and important use.  These are summarized below:


    •  Sections 2.0 through 6.0 of this report assemble, in one document,
       up-to-date information on the relevant scientific topics to the UST
       program.  This information can prove quite timely as the program
       begins to build its regulatory framework.  Also, the information can
       be "re-packaged" to be distributed to state and local government
       agencies, industrial organizations, etc., that are faced with
       real-world UST incidents.

    •  The three databases developed under this work assignment thus far
       (Volume II) can be of value in an immediate way, as a readily
       available reference for properties of UST-regulated substances.
       Beyond the general value of such a reference, the data provided in
       these computerized information systems can help to prioritize
       program efforts.

    •  From a global regulatory perspective (i.e., without focusing on
       individual incidents for which specific chemical and setting
       information is known), the technical information offered in this
       report generally indicates the following:

       -  From the point of view of environmental  settings, the use (or
          potential use) of the underlying aquifer, the presence or absence
          of man-made conduits, the proximity to population, etc. are the
          important features relevant to UST incidents.  However, specific
          hydrogeological details cannot be easily incorporated in the
          regulatory program without reference to specific and very
          localized situations.

       -  As far as the contents of UST-regulated tanks are concerned, it
          may be possible to state certain findings more defensibly.in the
          "negative" as opposed to the "positive."  For example, orife can


                                    7-20

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   state with a high degree of certainty that some petroleum
 .  products are unlikely to cause problems because they are largely
   immobile if released in the subsurface environment.
   Unfortunately, issues such as acceptable levels of toxicity and
   exposure make positive statements about "very hazardous"
   chemicals a more subjective matter.  Also, information on the
   distribution of tanks containing various substances may be
   important.to decide whether and where to draw "hazard" standards.

On a site-specific basis, with information regarding tank contents
and localized hydrogeologic features in hand, findings of this
report can be used to prioritize actions in the areas of compliance
monitoring, corrective action requirements, and enforcement.  For
example, state level personnel charged with implementing UST
programs may do the following:

-  Having received notification of a leak and some degree of infor-
   mation regarding the site and the contents of the tank, ranking
   systems such as UST DRASTIC/IMPACT can be useful  in prioritizing
   (ranking) where to invest what levels of available resources.

-  Use the databases developed herein to assess the degree of hazard
   associated with an UST "incident.  In particular, if such
   databases were to be merged with "knowledge-based" systems for
   guiding UST incident response activity, state and local level
   personnel may be better able to make proper use of resources  and
   take actions scaled to the level of hazard posed in a particular
   situation.

Finally, the technical and scientific findings offered earlier
indicate generally where and how research activities may be useful
to the UST program -- for example, to study the movement of
contaminants in the unsaturated zone, fate mechanisms in general,
and vapor-phase movement in general.
                             7-21

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                              8.0  REFERENCES
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Agar, J.G., King. R.D. and O'Connor, M.J. 1984.  Practical  Experience  in
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Alien, L., Bennett, T., Lehr, J.H.  and  Petty, R.J.  1985. DRASTIC:  A
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Alliance of American Insurers.  1981.  Handbook of Organic  Industrial
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Bear, J.  1972.  Dynamics of Fluids in Porous Media.  New York: American
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Carsel, R.F., Mulkey, L.A., Lorber, M.N. and  Baskin, L.B.  No Date. Draft


                                    8-2

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   Report.  The Pesticide Root Zone Model  (PRZM):  A Procedure for
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                                    8-4

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                                    8-5

-------
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                                    8-6

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-------
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                                    8-8

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                                    8-9

-------
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                                    8-11

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                                    8-12

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               HOW  DOES  THE  UST  MODEL WORK.?
SOME KEY QUESTIONS PERTAINING TO REGULATING L'ST'S


               HOW  DOES  THE  UST  MO DE L_. WORK?   ...--.







     THE OBJECTIVE  AND BASIC  METHOD OF  THE UST MODEL




     Difficulties in  tank/piping/detection analyses




Breaking the analysis into  subsystems and component parts




      The role of randomized  outcomes and repetition






          . THE COMPONENT PARTS  OF THE UST MODEL






 Environmental Setting,  Tank-Pipe System, Tank Monitoring




                          Failure




                          Re lease




                         Detection




                         Transport




                     Repair/Replacement




            Corrective Action/  Remedial Action
                   WHY  USE'THE.-UST ;MODEL?
                ROLE  OF  THE MODEL ..IN. THE' RIA
                  SOME  PRELIMINARY R'ESULTS '

-------
                 HOW DOES THE UST MODEL  WORK?
               THE OBJECTIVE  AND BASIC  METHOD
5 = = = = = = = = = = = = = — = = = = = = — =S = .= = = == = = = = = = = = = = = S=S = = = = S = = = -= = = = = =: = = = =:
  PROBLEM FACED BY ANALYSTS OF TANK/PIP I NG/DETECT10N  SYSTEM

                  Complexity of Che systems
                   Unexpected interactions
                       Unpredictability

   APPROACH USED BY UST SIMULATION TO  OVERCOME  THIS  PROBLEM

 Build  a  simulated system realistic enough  to mimic
 real  systems, but simple enough to understand  and  use

 To  build a simulated system that acts like  a typical  real
 system,  the model uses:

     Data on the performance of individual  elements
     Engineering judgment on their interaction
     Randomizing factors to simulate the  effects  of  chance

 USE  OF  REPEATED TRIALS TO INCREASE CONFIDENCE  IN  THE  RESULTS

                  WAYS TO USE THE SIMULATION

           INCREMENTAL IMPROVEMENTS IN THE  ANALYSIS
             THE COMPONENT PARTS OF THE  UST  MODEL

-------
                  HOW DOES THE UST MODEL WORK?
                THE OBJECTIVE AND BASIC METHOD
:== = = = = = = = = = = = = =: = = = = =: = = = = = = ==^=2 = = = = — = = = = = = = = =: = = = = = = = = === = = = = = :

  PROBLEM  FACED  BY ANALYSTS OF TANK/PIPING/DETECT ION SYSTEM

  APPROACH  USED  BY UST SIMULATION TO OVERCOME THIS PROBLEM

 USE  OF  REPEATED TRIALS TO INCREASE CONFIDENCE IN THE RESULTS

  On  any  one  trial, the simulated tank system could cause
  a  little  or  a  lot of trouble

  But  by  repeating the trial many times the combination of
  events  that  would be seen in the real world is approximated

  We  can  then  see  not only the average performance but how
  often  particularly bad or good outcomes will occur

                   WAYS TO USE THE SIMULATION

 We  can  also  compare different tank and detection options in
 virtually  identical circumstances to see which performs
 better  according  to our criteria

 We  can  compare  the outcomes generated by the program to
 data  from  the  real world

 STRUCTURE  OF  ANALYSIS FACILITATES.INCREMENTAL IMPROVEMENTS

 Those  interested  in assessing the model need not have
 expertise  in  every aspect of tank systems to make useful
 contributions.
              THE COMPONENT PARTS OF THE UST MODEL

-------
                                           - 13 -


                                    HOW  THE' MODEL WORKS
                              (Tank  by  Tank, Month by Month)
         no
                         MODEL  INPUTS
                       SYSTEM  FAILURES
•f Release ? J
                              yes
                               I
                     RELEASE CHARACTERISTICS
                TRANSPORT  /  PLUME  CHARACTERISTICS
                         LEAK  DETECTION
G
                            Release
                           Detected
                                                      •ye s-
                                          REPAIRS,

                                        REPLACEMENTS,

                                             &

                                        CORRECTIVE

                                          ACTION
                              -. no

-------
                  -  19 -
ANALYTICAL PROBLEMS AND THE MODEL  APPROACH
            HOW THE MODEL WORKS
               MODEL  INPUTS

      Tank  (System)  Characteristics:

              Tank Material
             Piping Material
     Primary or Secondary  Containment

          Failure Probabilities:

         Structural Deficiencies
                Rup t ures
                Corrosion

      Detection and Monitoring  Methods;

         Type of Equipment  Installed
      Frequency of Testing  or Monitoring
      Sensitivity of  Detection  Threshold

         Hydrogeological Setting:

           Soil Characteristics
             Depth to Aquifer
           Ground Water Velocity
              SYSTEM  FAILURES
         RELEASE  CHARACTERISTICS
     TRANSPORT  /  PLUME  CHARACTERISTICS
              LEAK  DETECTION
 REPAIRS,  REPLACEMENTS  &  CORRECTIVE ACTION

-------
                   - 20 -
            HOW THE MODEL WORKS
MODEL INPUTS


             SYSTEM FAILURES

   Using the model inputs and a routine that
   simulates random events, the model:

        Calculates Time of Failure

  Determines Type and Location of Failure

         Tank or Pipe Deficiency
               Tank Rupture
  Tank or Pipe Corrosion -- Initial Hole Size
          RELEASE CHARACTERISTICS
     TRANSPORT / PLUME CHARACTERISTICS
             LEAK DETECTION
REPAIRS, REPLACEMENTS, & CORRECTIVE  ACTION

-------
                 HOW THE 'MODEL  WORKS
                     MODEL  INPUTS
                    SYSTEM  FAILURES
                RELEASE CHARACTERISTICS
         TRANSPORT / PLUME CHARACTERISTICS

Using inputs from RELEASE CHARACTERISTICS  module on
release volume and release duration:

Calculates the Volume of Product  in  Unsaturated Zone

         Calculates Area of  Floating  Plume

         Floating Plume Areas  are  Linked  to
            ground water transport model
                 LEAK DETECTION
    REPAIRS, REPLACEMENTS  &  CORRECTIVE  ACTION

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                         - 23 -
                   HOW THE MODEL  WORKS
                      MODEL  INPUTS
                    SYSTEM FAILURES
                RELEASE CHARACTERISTICS
           TRANSPORT / PLUME CHARACTERISTICS'
                     LEAK DETECTION

Using Inputs from RELEASE and TRANSPORT  /  PLUME  modules
concerning leak rates, leak durations, release  volumes,
and transport, determines if release  is  detected:

Is Total Volume Released > Volume  Threshold  of  Detection
   Method?
Is the Total Concentration of Product  in Water  >
   Concentration (PPM) Threshold of  Detection Method?
Is the Total Concentration of Product  in Vapors  >
   Concentration Threshold of Detection  Method?

 If Release is NOT Detected, Continue  Transport  Routine

  If Release IS Detected, Calculate  Time to  Detection
       REPAIRS, REPLACEMENTS  &  CORRECTIVE  ACTION

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                  HOW THE MODEL WORKS
MODEL INPUTS


SYSTEM FAILURES


RELEASE CHARACTERISTICS


TRANSPORT / PLUME CHARACTERISTICS


LEAK DETECTION


      REPAIRS, REPLACEMENTS & CORRECTIVE ACTION

   Using inputs from previous modules on type and
   location of failure, volume of release  in
   unsaturated zone, and area of floating  pluae:

       Calculates Cost of Any Repairs to System

  Calculates Cost of Tank Excavation and Retirement

        Calculates Cost of Replacing New Tanks

Calculates Area to be Excavated  for Corrective Action

         Calculates Cost of Corrective Action

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                            - 25 -
                    WHY USE THE UST MODEL?
       SOME KEY QUESTIONS PERTAINING TO REGULATING USTS

                 HOW DOES THE UST MODEL WORK?
                    WHY USE THE UST MODEL?
        The model addresses many of the key questions.

          What Is the extent of the current problem?

   What are the most cost-effective ways to reduce damages?

How will better leak detection reduce corrective action costs?

    Need a tool to aid in understanding the entire system.

                 Tank performance / failures

                   Release Characteristics

                          Transport

                      Corrective Action

                            Costs

      The interactions among these elements are complex

                 UST information is limited.

For example, can never actually know how many plumes  from USTs

                  Can only simulate results

            Model can handle probabilistic events

Model allows sensitivity analysis, allowing us  to  understand
 the effects of uncertainty in key parameters

  We have no real world experience with many policy options
    of interest.

  Cannot investigate actual instances of use of most  options

               Again, results must be simulated

                               I

               ROLE OF THE UST MODEL IN THE RIA
                     PRELIMINARY RESULTS

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                      -  26  -
           ROLE OF UST MODEL  IN  THE  RIA
SOME KEY QUESTIONS PERTAINING  TO  REGULATING USTS
          HOW DOES THE UST  MODEL  WORK?
              WHY USE THE  UST  MODEL?
         ROLE OF THE UST MODEL  IN  THE RIA
           MODEL'S PRIMARY  INPUTS  TO  RIA:
                       Costs
                   Effect iveness

    MODEL RESULTS ARE  INPUTS  TO  OTHER ANALYSES:
                  Economic  Impacts
                   Implement at ion
                 Risks, Benefits

                OTHER  INPUTS  TO  RIA:
              Versar Damage  Case  Study
  OTS National Survey  of Underground  Storage Tanks
                   Other Studies
           SOME PRELIMINARY  RESULTS

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                 SOME  PRELIMINARY RESULTS
   SOME KEY QUESTIONS  PERTAINING TO REGULATING  USTS
             HOW  DOES  THE  UST MODEL WORK?
                 WHY  USE  THE  UST MODEL?
             ROLE  OF  THE  MODEL IN THE RIA
                SOME  PRELIMINARY RESULTS
A LARGE NUMBER  OF  BARE  STEEL TANKS ARE PROBABLY  LEAKING
       BIG  IMPROVEMENTS  WITH CORROSION PROTECTION:
                      Leak  Prevent ion
                       Release Volume
                      Number of Plumes
                            Risk
              IMPROVEMENTS  WITH LEAK DETECTION:
                  Minimize  Number of Leaks
                      Minimize Damages
       BASELINE  HEALTH  EFFECTS ARE NOT ALARMING:
     One cancer  case  per  one million tanks per  year
          IMPORTANCE  0,F  HYDROGEOLOGICAL SETTING:
                      Number of Plumes
                            Risk

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