UNITED STATES ENVIRONMENTAL PRO~ISa<9"KBHHBES

                          WASHINGTON. DC. 20460
                                          SAB-EEC-88-030


       1QRR                                                  OFFICE OF
       J-.7OO                                               THE ADMINISTRATOR
Honorable Lee M. Thomas
Administrator
U.S. Environmental Protection Agency
401 M Street, S.w.
Washington, D.C.  20460

Dear Mr. Thomas:

     The Science Advisory Board has reviewed  the Unsaturated Zone
Code (FECTUZ) for the Office of Solid Waste's Fate and Transport
Model.   The Office of Solid Waste  sought to develop a code which
would account  for the attenuation of  chemical  constituents in
the  unsaturated zone and  have potential  uses  in  the  Toxicity
Characteristic  and  Concentration-Based  Listing rulemakings.
FECTUZ is a model package consisting of an analytical code and a
finite element  numerical code;  each  code  contains a flow module
and a transport module.   In  requesting the review,  the Director
of the  Characterization  and  Assessment Division  asked the Board
to  address three issues:  the dimensionality  of  FECTUZ,  the
appropriateness of the assumptions underlying the code,  and code
implementation.

     The  Unsaturated  Zone  Code   Subcommittee of  the  Science
Advisory Board's Environmental Engineering  Committee reviewed the
documentation for the code at open meetings December "10, 1987  in
Denver and January 19,  1988 in Washington.

     The  Subcommittee  concluded that the  dimensionality  of the
code  (one-dimensional  transport  in  the vertical  dimension)   is
probably adequate for situations  in  which  the porous medium can
be  considered  relatively  homogeneous,  without   substantial
stratification and that the one-dimensional limitation may not  be
a  serious  one  from  the  standpoint of  asymptotic,  steady state
analysis of groundwater protection, since the primary effect of
lateral spreading would be to retard downward mobility and hence
to  increase  the  time  available  for  transformation of  the
contaminant,  thus reducing  the amount  that reaches  the water
table.

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   •" The Subconunitte'e believes that there are no serious problems
associated with treating  the  fluid as incompressible,  isothermal,
and homogeneous,.  The acceptability  of  all  the other  assumptions
hinges  on the  application  of the  model.    Certain  classes  of
phenomena are not well enough understood to be incorporated  into
a  management model  of the kind reviewed here.   Because of  the
importance of these  phenomena  to site-specific applications,
there is  an  urgent  need  for scientific research to  clarify  the
scientific principles underlying  these  phenomena.   Such research
would have benefits beyond this  model.   The FECTUZ  model package
is  also  subject to  limitations  imposed  by  its simplifying
assumptions  and the  scarcity of  data  necessary for  parameter
estimation.   Both  versions  of the  model  are incomplete in  the
sense that several potentially important governing  processes  are
neglected  altogether.     The   Subcommittee  believes  these
limitations are not so debilitating as to preclude its employment
for generalized regulatory development applications, but believes
that  the inability  to take  into  account  several potentially
relevant  processes  casts serious doubt  on the  advisability  of
site-specific applications.    It  is  recommended that the  Agency
mount  directed and  continuing  efforts   (a)  to  improve   the
knowledge  base  concerning  relevant  processes  which  have  been
neglected on grounds of inadequate  understanding,and  (b)  to  seek
expert  consensus  in  these  areas,  especially biotransformation,
immiscible transport,  and fracture transport.

     The  Subcommittee  finds that  for  the intended  tasks  in
regulatory development, the  composite model  consisting of FECTUZ-
A  and EPA-SMOD  is acceptable from the  standpoint  of simplicity
and computational  ease.   Where Monte  Carlo replication of  the
model is planned, the overall uncertainty of the transport model
should be  addressed  because  the  Monte  Carlo  methodology is  not
able to account for uncertainties  arising from incompleteness  or
deficiencies in the underlying model.  However, for site-specific
decisions where the  accuracy  and completeness  of representation
of  transport  and transformation  processes  is  of paramount
importance,  the  FECTUZ  model  seems bound to  be inadequate,
especially  the analytical  version  owing  to   its   implicit
simplifications and its  inability to take  into  account temporal
variations,  site-specific  conditions and  heterogeneity.   The
Office of Solid Waste should take special care to warn  potential
users against site-specific applications of  the  composite model,
because there is a  substantial  danger  that such usage  could  be
misleading and detrimental to the protection of groundwater
quality.

     Additionally,   the  Subcommittee  has  expressed   concerns
and. made  suggestions  relating to how the  Agency establishes  the
need for the development of a new model, how existing models  are
evaluated, and  on the  existence  and use  of adequate  in-house
capability for evaluating issues  related  to  transport model
development and application.

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     The  attached report  contains  more detailed  responses  to
these issues.
     We are pleased to have had  the  opportunity  to  be of service
to  the  Agency  and  look  forward to  a written  response to  our
report.

                            Sincerely,
                            Norton Nelson, Chairman
                            Executive Committee
                            Science Advisory Board
                            Raymond Loehr, Chairman
                            Environmental Engineering Committee
                            Science Advisory Board
                            Paul Roberts, Chairman
                            Unsaturated Zone Code Subcommittee
                            Environmental Engineering Committee
cc: W. Porter
    S. Lowrance
    J. Denit
    S. Weil
    M. Strauss
    Z. Saleem

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                        SAB-EEC-88-030
             REPORT OF THE
  UNSATURATED ZONE CODE SUBCOMMITTEE
  ENVIRONMENTAL ENGINEERING COMMITTEE
        SCIENCE ADVISORY BOARD
 U.S. ENVIRONMENTAL PROTECTION AGENCY
        ON THEIR REVIEW OF THE
        OFFICE OF SOLID WASTE'S
UNSATURATED ZONE CODE FOR THE OSW FATE
                  AND
            TRANSPORT MODEL
              June, 1988

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                        TABLE OF CONTENTS


Background	1
   Procedural	1
   Model Structure	2
   Intended Uses of the Model	2

Responses to Questions Posed by the
Office of Solid Waste	  3
   Dimensionality of the FECTUZ Code	3
   Appropriateness of FECTUZ Assumptions 	  3
   Compatability with Comprehensive Transport Model	7
   Validation.	9
   Uncertainty Analysis with FECTUZ	9

Additional Issues. .  	 10

References	11


Appendix:  Request for Review

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              U.S. Environmental Protection Agency
                     Science Advisory Board
               Environmental Engineering Committee
               Unsaturated Zone Code Subcommittee


Chairman,  Dr.  Paul  Roberts,  Department  of  Civil  Engineering,
Stanford University,  Stanford,  CA  94305-4020

Dr. Keros Cartwright, Illinois State Geological Survey, 615 East
Peabody, Champaign, IL  61820

Dr. Raymond  C.  Loehr,  Civil Engineering  Department,  8.614 ECJ
Hall,  University of Texas, Austin,  TX  78712

Dr. Mitchell  Small,  Department  of  Civil  Engineering, Carnegie-
Mellon University, Schenley Park, Pittsburgh,  PA   15213

Dr. Calvin Herb  Ward,  Chairman Department  Environmental Science
and Engineering,  Rice  University,  P.O.  Box  1892,  Houston,  TX
77251

Dr. M.  Th. van  Genuchten,  U.S. Salinity Laboratory,  4500
Glenwood Drive, Riverside, CA  92501

 Executive Secretary

Mrs.  Kathleen  Conway,  Science Advisory  Board, A-101F,  U.S.
Environmental Protection Agency,  Washington, D.C.   20460

Staff Secretary

Mrs.  Marie  Miller,  Science  Advisory  Board,   A-101-F,  U.S.
Environmental Protection Agency,  Washington, D.C.  20460

Director. Science Advisory Baord

Dr. Donald G. Barnes, Acting Director,  Science Advisory Board, A-101,
U.S. Environmental Protection Agency,  Washington,  D.C.  20460
Date:  June 1988

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                             NOTICE
     This report has  been written as part  of  the activities of
the Science Advisory  Board,  a public group providing  extramural
advice on scientific information and advice to the Administrator
and other officials of the Environmental Protection Agency.  The
Board is  structured to provide a  balanced  expert assessment of
scientific matters  related to  problems  facing  the Agency.   This
report has  not been  reviewed for  approval by  the  Agency, and
hence, the contents of this  report do not  necessarily  represent
the views and policies of the Environmental  Protection  Agency,
nor   does  mention  of trade   names  or  commercial  products
constitute endorsement or  recommendation for use.

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                           BACKGROUND

Procedural

     On October 22, 1987 the  Office of  Solid  Waste (OSW)  made a
presentation to  the Environmental Engineering Committee  on its
unsaturated zone  code  (FECTUZ)  for the Office of Solid Waste's
Fate and  Transport  Model.   In developing FECTUZ,  the  Office of
Solid Waste sought  to create  a code which would  account for the
attenuation of chemical  constituents in  the unsaturated  zone and
have  potential  uses  in  the   Toxicity  Characteristic  and
Concentration-Based Listing rulemakings.   The  Committee  formed the
Unsaturated Zone  Subcommittee to  conduct  the review  of FECTUZ.
The  review  was  formally  requested  in  an  October   16,  1987
memorandum from  the Director  of  OSW's Characterization and
Assessment Division  (Appendix).    The  memorandum  asked the Board
to  address three issues:  the  dimensionality  of FECTUZ,  the
appropriateness of the assumptions underlying  the code,  and code
implementation.

     The  Subcommittee held open  meetings December 10,  1987 in
Denver,  Colorado  and January  19,  1988 in Washington, D.C.   The
Environmental Engineering Committee heard the preliminary report
of the Subcommittee January 20,  1988 and approved this  report on
April 15,  1988.

Model Structure

     To summarize briefly,  FECTUZ  is a model package that has
been developed in two versions:  an analytical code and  a finite
element numerical code.   For clarity we  will refer to the two
versions  as FECTUZ-A  (analytical)  and  FECTUZ-N  (numerical),
respectively.    FECTUZ-A  and FECTUZ-N  each consists of  two
modules: flow and transport.   An acronym,  FECTUZ  stands  for Finite
Element Code  for  Transport in the  Unsaturated  Zone.   In fact,
this acronym is a misnomer  if applied to  the analytical version,
as  it  is  not  a  finite  element   code.    This  inconsistency was
confusing  to  the Subcommittee,  is  likely to mislead  users,  and
therefore should be corrected  by renaming  the  analytical code.

     In developing the model, OSW and their consultant  placed  a
high priority on two objectives: 1) computational  efficiency, and
2)  ability to  deal with a wide  range  of soil  characteristics,
including  highly  nonlinear relationships between soil  moisture
and permeability.   The  justification  for  these  priorities is to
permit use  of  the model in Monte Carlo evaluations of expected
concentration levels at water supply  wells  under a wide variety
of soil conditions.

     The  flow  module  is  based  on  the  assumption  of  one-
dimensional,  isothermal  flow of  an  incompressible,  homogeneous
fluid according  to  Darcy's Law.    Vapor flow, transport through

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secondary porosity  (fractures  and  fissures),  and  migration  of
immiscible liquids are neglected.  A flux-type boundary condition
is imposed at the top of the  unsaturated  zone.

     Likewise, solute transport  is assumed to be one-dimensional,
incorporating advection and Fickian  dispersion, i.e. a diffusion-
like model  of solute  front  spreading.   Sorption  is  assumed  to
follow  a  linear equilibrium relation.  First-order
transformation is  incorporated,  but at  present  only hydrolysis
is considered in the selection of the rate coefficient,  owing to
a  perceived  lack of  knowledge  concerning  biotransformation
processes.  The documents provided are ambiguous about the nature
of  the  transformation  submodel,  as they refer  to "first-order
decay",   implying  that  biotransformation and  chemical reactions
other than hydrolysis are included.   These other reactions can be
incorporated within the current structure of the model,  but they
are not included in the current  implementation.

     The model is capable of performing either transient  (FECTUZ-
N) or steady-state  (FECTUZ-A)  simulations  of  flow and transport.
However,  in  the  proclaimed uses  for regulatory  development,
namely Toxicity Characteristics  and  Concentration-Based Listings,
it apparently will  be used only in the  steady-state mode,  i.e.
a  constant  water  flux is  assumed.  The  procedure  consists   of
calculating the steady-state concentration  at  a monitoring well
ouside the landfill site,  with  the distance  to the  monitoring well
treated  as  a  distributed parameter  estimated  from  OSW's  data
base.

Intended Uses of the Model

     The questions posed to the Science Advisory Board cannot be
addressed properly without a clear  idea  of  the prospective uses
of  the  model.   In. delineating  the  intended  applications,  OSW
informs  us  that  they  envision  using FECTUZ  in  the immediate
future mainly as a tool in developing regulations for disposal to
municipal landfills:  i) for use  by the regulated community  to
assess  whether  a waste  is hazardous (Toxicity Characteristic);
and  ii)  for use  by OSW  to  reassess previous listings,  to set
concentration criteria, and  to  delist substances  if appropriate
(Concentration-Based  Listing).  It  is implicit  that,   in  these
applications, FECTUZ  will  be  used in combination with  the
saturated zone  groundwater flow and  transport  model,  EPA-SMOD,
which is  a two-dimensional, steady-state model.  A  previous
version  of  the   model  was  reviewed  by  the   Environmental
Engineering Committee  of  the  SAB.    As  such,  the steady-state
version of  FECTUZ  (what we refer to  above  as  FECTUZ-A)  will  te
used.

     Concerning other  uses of the FECTUZ model,  OSW states that
it "does not have any  other  specific uses planned  for the FECT'JZ
Code at this time.  However,  two site-specific uses are possible.
These are use in decisions on ACLs  (alternate  concentration
limits)  and in Clean Closure decisions."   It is  implicit  that,  :r.
these site-specification applications, the intended use  of FECTV:

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will require the more  sophisticated  dynamic  mode (what we refer
to  above  as FECTUZ-N)   in  conjunction with  a  dynamic numerical
simulation model for  the saturated  zone.   The use  of the more
detailed  models is  intended  to  allow  incorporation of site-
specific soil and geological properties  in  these  applications.

     If EPA  changes its  assumption  to  include  a liner  in its
assumed landfill,  the  liner effect  should  be  incorporated  in
FECTUZ, as it will  control flow.

     RESPONSES TO QUESTIONS POSED  BY  OFFICE OF  SOLID  WASTE


Dimensionality of FECTUZ Code

     The dimensionality of the code (one-dimensional  transport in
the vertical  dimension)  is probably  adequate  for situations in
which the porous medium can be considered relatively  homogeneous,
without substantial stratification.   However,  in soil layers or
zones with  low permeability relative to the surrounding  media,
considerable lateral spreading may occur,  particularly if  free-
phase hydrocarbons are present (Mull, 1971; Schwille,  1984).  The
one-dimensional limitation may  not be very  serious from the
standpoint of an asymptotic, steady  state  analysis of  groundwater
protection,  since  the  primary effect of lateral  spreading would
be  to  retard downward mobility and  hence to  increase the time
available for transformation of the contaminant,  thus reducing
the amount  that reaches  the water  table.    On  the  other hand,
this same phenomenon  could  lead   to circumstances  in which the
unsaturated zone accumulates  contamination to  a  greater  degree
prior to  detection in  groundwater monitoring wells,  compared to
the case without stratification.

Appropriateness of FECTUZ Assumptions

     The Subcommittee believes that there are no  serious  problems
associated with treating the fluid as incompressible,  isothermal,
and homogeneous.   The  reliance on Darcy's Law  is a  widely used
expedient that is  justifiable   at the  level  of  regulatory
development  applications,  but  may  be  unsatisfactory  in site-
specific applications if transport through secondary porosity or
migration of immiscible liquids plays a  role.   These  factors are
discussed below under the category of processes neglected  in the
model.

     Neglecting hysteresis  effects  (physical  effects  which lag
their  cause)  in  the  relative permeability  relationships  is
permissible  under   asymptotic   (long-term,    steady   state)
conditions.   However,  the omission of the differences  in relative
permeability relations between   imbibition  (saturation with
liquid)  and drainage will result in  a gross misrepresentation of
transport under nonsteady conditions, especially  for  soils  having
strongly nonlinear characteristics.   Hence,  this  simplification

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will hinder accurate  simulation of nonsteady-flow situations  with
FECTUZ-N,  even though the numerical model is otherwise capable  of
handling nonsteady inputs.

     The formulation  of the dispersion term  according to  Fick's
Law is a representation that is commonly practiced  in transport
modeling,   and  is  acceptable  in  most   situations.      This
representation is certainly adequate for simulations  at the level
of regulatory development.   Substantial deviations  from  Fickian
behavior  must  be anticipated in  strongly heterogeneous  or
stratified media,  however,  and should  be  taken into account  in
site-specific applications  under  such  circumstances. Even  more
serious inadequacies  will arise if transport  in secondary  porosity
is significant,  as discussed below.

     The treatment  of  sorption  as conforming to  the  linear
equlibrium  assumption  is  also adequate  for  dealing with  most
situations  of  trace  organic  contaminant  transport  in   a
homogeneous  medium.   However,  in   concentrated  leachates,
cosolvent effects  and facilitated transport  may be  significant.
Cosolvent effects occur  when chemicals are more readily dissolved
in the fluid phase due to the presence  of  other chemicals  in the
contaminant stream.  Facilitated transport occurs when contaminant
mobility is  increased due  to interactions with other dissolved
organic substances such  as naturally occuring humic acids.
These   processes  tend  to   increase  the   mobility  of  the
contaminants  through  the  unsaturated  zone  and along the ground
water  flow path.  The  linear equilibrium  assumption  is usually
inadequate for  contaminants such  as  trace  metals, for which
selective  cation exchange  is  the  dominant  sorption mechanism.
Moreover,  in  fractured or  strongly  stratified media,  it  is
questionable whether  equilibrium will be obtained for contaminants
of any kind, as diffusional limitations govern the exchange between
the mobile fluid  and the  matrix or  immobile zones.   Ignoring
these  complexities  is  justifiable in regulatory  development
applications, but certainly not in site-specific applications.

     Solute  transformation  according to  a  first-order  rate
expression  likewise can  be  justified   as  an  expedient  in
simulations  supporting  regulatory development.  .The first-order
rate formulation is an acceptable approximation for many  chemical
reactions, and in some instances for biotransformation processes.
On the other hand, the transformation submodel and its associated
data  base  are  inadequate  in some respects,  and  the  FECTUZ
documentation does not  clearly recognize those  inadequacies.   At
present, only a  single transformation  mechanism is incorporated:
chemical hydrolysis.   Although  hydrolysis  rates of many  compounds
are  strongly  dependent on  pH,  the  model" is not  capable  of
predicting the  pH  in  the leachate  plume,  relying instead on
estimates of the pH of native groundwater.   The data-base  for  this
estimate  currently relies  on  sampling data  from  saturated  zor.e
measurements.  This is inappropriate for use in an unsaturated zor.e
model.  However,   even  if  native  unsaturated zone  data  were

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collected,  this  would  not  account for the effect of the leachate
on the pore-water pH.  This deficiency could lead to inaccuracies
of several orders of magnitude in estimating hydrolysis rate
constants.   Further, the analysis could be  seriously biased in the
direction of overestimating transformation  rates  for compounds for
which base hydrolysis  is the dominant mechanism,  if the majority
of leachate plumes are acidic.

     Similarly,  biotransformation could be  included as  a simple
first-order rate process.   However,  consensus is lacking for
generalized prediction of transformation rate  constants, as these
depend strongly on conditions such  as  organism  adaptations  and
concentrations, pH,  and  the presence or absence of  electron
acceptors (oxygen under aerobic conditions), toxicants, essential
nutrients,   etc.  which  are   site-specific.    Site-specific
applications of the  FECTUZ model  package  can lead to  over-
estimates    of   solute    transport    since   site-specific
biotransformation   analyses generally  result in biodegradation
being  a  primary process  influencing chemical  fate.    Hence,
estimates of  chemical transport  made without considerations of
biotransformation are  almost always so overly  conservative as to
affect regulatory decisions.   Generalized chemical transport
predictions will  necessarily  suffer due  to  lack  of generally
applicable biotransformation  rate  constants;  however,  site-
specific analyses should  include all  of  the  fate processes  for
which specific data can be reasonably obtained.

     Implementation   of   the   full  range   of  transformation
possibilities,  including  the  uncertainties  in  conditions  that
influence  the  rate constants,  would  magnify enormously  the
uncertainty spectrum of predicted  outcomes  in  Monte  Carlo
simulation.

     The  transformation  submodel  considers  neither  chain
reactions  nor  the formation  of hazardous daughter products.
Including transformation reactions without  considering the possible
formation of hazardous intermediate products can  bias the results
of  simulations  toward  underestimating  the risk of  serious
contamination  of  groundwater   resources.   In   regulatory
uses,  including both toxicity  characteristic  assessment  and
concentration-based  listings,  it would be  dangerous  to  use  the
transformation submodel  without  considering byproduct formation
where  relevant.  It  is  recommended that  the transformation
submodel be decoupled in  the  analysis of  any  compound  known  to
form  hazardous intermediate products,  unless  the byproduct
formation is incorporated explicity.

     Important    mechanisms  of  contaminant  transport  or
transformation that  are  neglected by the FECTUZ-A model include
vapor  phase transport  (volatilization),  abiotic   tranformation
other than hydrolysis,  biotransformation, free-phase  flow of
organic liquids, and  fracture  flow.   Further,  FECTUZ  does  not
permit nonlinear equilibrium relations  for sorbing solutes,  nor
does it  allow for competition  or similar  interactions  that  may
arise  in multisolute systems,  especially cosolvent  effects.

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Neglecting such potentially  important mechanisms raises questions
as to  whether  the model can  provide  accurate estimates of  the
central tendency,  whether the  overall effect of the omissions  may
tend to  underestimate  the  threat  to  groundwater  quality,   and
whether   the  range  of  uncertainty  may   be   substantially
underestimated.

     For  organic  contaminants,  the  first three  mechanisms   —
vapor    phase    transport,    abiotic    transformation,    and
biotransformation  --  serve  mainly  to  reduce  the  amount  of
contaminant which would  otherwise  reach  the water  table.   Hence,
neglecting these processes  in  the  FECTUZ model does  not increase
the risk of protecting groundwater resources  inadequately if  the
model  is  used  as a  basis  for regulatiory decisions.   However,
transport in  fractures  or  macropores,  free-phase  flow   of
immiscible organic  liquids,  and  cosolvent  effects' would  allow
contaminants to migrate more rapidly than would be  predicted by
the model, and  thereby reduce  the  contaminant's residence time in
the  unsaturated  zone,  thus  reducing  the  opportunity  for
transformation.

     Transport in  fractures  is  a  phenomenon  that  has only
recently attracted attention  as  an  important mechanism  of
contaminant migration.    In  a  recent review,   Jury  (1987)  states
that "in any natural setting,  interaggregate  structural features
such as  soil  macropores,  cracks,  plant  root holes,   or animal
burrows   will   be   interspersed  with  the  bulk  structural
characteristics"  and that  "these geometric   features,  although
very  small  in  volume,  can  have  a  significant  effect  on  the
transport of chemicals and particularly  on those which are highly
absorbed."  In general,  it can be expected that,  in systems that
possess  substantial  secondary porosity  in the form  of fractures
or  macropores,   transport  will  be  characterized   by  early
breakthrough and extended tailing  of contaminants  (van Genuchten,
1985).  Fracture  transport  is  of  special  significance  in  media  of
low  permeability  such  as  clays,  which furthermore  are prone  to
fracturing owing to their tendency to swell or shrink in response
to changing chemical conditions  or moisture content.   Hence,  this
mechanism  is likely  to  be  of  great importance precisely  in  the
kind of media that we tend to rely on to isolate waste disposal
sites.   Despite this recognition of  the  potential significance of
fracture  transport,  there  is  a dearth of  conclusive confirmatory
evidence,  because  of the extreme difficulty of  conducting  the
necessary  investigations, which  would  need  to be  conducted at
field  scale.   At  present, various modeling approaches  are being
explored to deal  with  this  problem,   including  deterministic
models based on  advection  in  fractures with  diffusion into  the
matrix  and  stochastic models  that  rely  on  probabilstic
representation  of  transport,  but  experimental evidence suitable
for  model  verification  is  meager.  A  recent review by
van Genuchten and Jury  (1987)  summarizes the promise and limitations
of modeling approaches, concluding that rigorous, structure-
oriented  models  are presently too  complex  for  routine use in
management.  In summary, our  inability  to formulate an adequate

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model  for  this transport  mechanism is  a severe  hindrance  to
modeling transport  at  sites where  secondary  porosity  exists in

substantial   measure.      The   importance   for   transport
notwithstanding,  it is understandable that fracture flow has not
been corporated into FECTUZ.

     The  migration  of  free-phase  organic  liquids  in  the
subsurface  has been incorporated in  several  models (Corapcioglu
and Baehr,  1987; Hochmuth  and  Sunada,  1986;  Abriola and Finder,
1985;  Faust,  1985).   These models  could  be  used in conjunction
with or in place  of FECTUZ if the presence of  free-phase organics
is suspected.  Leakage of free-phase organics  into the  subsurface
might occur if the  low-permeability clay liners used at landfill
sites  are  attacked  by  the waste.   Brutsaert  (1987) showed  that
some solvents can cause  these  clay  liners to shrink and develop
cracks that  could allow  breakthrough  of  the  solvents  and
associated  contaminents.    Another   aspect of  contamination by
free-phase  organics that cannot  be modeled  with FECTUZ  is the
long-term contamination  of  groundwater  caused by dissolution of
residual hyrocarbons trapped in the  capillary fringe (Pfannkuch,
1983) .

     In  summary,   the   FECTUZ  model  package  is  subject to
limitations imposed by  its simplifying  assumptions, our lack of
understanding  of  important phenomena,  and the  scarcity  of data
necessary for parameter estimation.   Both versions of the model are
incomplete in the sense that  several  potentially  important
governing  processes are  neglected  altogether.    The  Subcommittee
believes that these limitations  are not  so  debilitating  as to
preclude  its employment for generalized regulatory  development
applications, but  believes  that  the  inability  to  take  into
account several potentially relevant processes casts serious  doubt
on the advisability of site-specific applications.

Compatability With Comprehensive Transport Model

     This  question  is  conceptually  difficult to answer,   because
the  scope  of  modeling  objectives  is  subject  to  change.    The
Office of Solid Waste should recognize that the  requirements  for
a model to  be used  in  regulatory development are different  frcr.
those  for  other  more  site-specific applications for  which the
unsaturated zone  transport code is intended to be used  as a tool.
Further, the Subcommittee has been informed by OSW that the existing
model for groundwater transport (EPA-SMOD)  is  an  analytical code
that is compatible  only with  the analytical  version of FECTUZ.
Hence  the  comprehensive  transport model  is  subject to the  same
limitations as FECTUZ-A, namely  it  is  suitable  only for steady-
state  simulations  of transport  in  a  homogeneous  medium.   The
Subcommittee is hindered from answering  the question of compatabil;ty
in detail, because  OSW has  not provided updated  documentation on
the current version of EPA-SMOD.

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     In order to derive any benefit  from the non-steady state
capabilities of the FECTUZ  model,  the Agency would need to couple
it  with an  appropriate peer-reviewed  dynamic  saturated  zone


model.   Full  use  of  non-steady state  capabilities  will  be
particularly important if EPA modifies its  assumption  of  an
infinite contaminant source.

     For the intended tasks  in regulatory  development,  the
composite model consisting  of  FECTUZ-A and EPA-SMOD is acceptable
from the  standpoint of  its simplicity and  computational  ease.
Indeed, for the kinds of tasks contemplated in  regulatory develop-
ment applications,   an  even simpler,  steady-state model  relying
on analytical solutions, such  as  the RITZ Model  described  at  the
December  1987 meeting  of  the Subcommittee,  would probably be  a
suitable  compromise between  adequacy  of representation  and
computational efficiency.  If the situation is to be simplified
by assuming steady, one-dimensional  flow and considering only the
asymptotic condition at  very  long times,  it is  not  necessary to
consider dispersion in  the  unsaturated zone, and the nonlinearity
of the soil's permeability  relation  is of minor importance.

     Hence,  if  the  transport   in the  unsaturated  zone  is
restricted to a one-dimensional,   steady-state situation,  some of
the complexities of  FECTUZ  could  be avoided in  the  interest  of
computational efficiency.  Rather, in such an asymptotic analysis,
predicting the extent of transformation is the crux of the matter,
because in the long-term steady state,  all of the solute that is
not transformed ultimately will reach the water table.  Assuming
a first-order reaction,  the extent  of  transformation depends  on
the product,  kt,  of the transformation rate  constant times the
solute's residence time.   The transformation rate  constant  k
depends on  the  reactivity  of  the solute as well  as  on chemical
conditions.   The  solute residence time is  influenced mainly  by
advection and sorption.  Dispersion  influences  the residence time
distribution but not the average  residence time, and might   be
safely  neglected   as  a  factor  of  secondary    importance   in  a
steady-state analysis  of the  kind contemplated  by OSW  for
regulatory development  purposes,  in  the  sense  that its influence
is  much smaller than the  potential impact  of transformation
processes.

     For  site-specific  decisions  where  the  accuracy  and
completeness  of  representation of  transport  and transformation
processes  is of paramount importance,  the  FECTUZ  model  seems
bound to be  inadequate,   especially the  analytical   version,
FECTUZ-A, owing to its implicit simplifications and its inability
to  take into   account    temporal   variations,.  site-specific
conditions and heterogeneity.   The Subcommittee feels   strongly
that  OSW   should  take special  care  to  warn  potential  users
against site-specific  applications  of  the   composite    model.
                           8

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There is  a substantial danger    that such    usage  could   be
misleading and  detrimental  to  the protection   of   groundwater
quality.

Validation

     The  flow module  of  the  FECTUZ  model  package has  been
validated for  extreme  conditions  of  soil   characteristic
nonlinearity,  and can be considered to represent  state-of-the-art
modeling  capability.  However,  the  Subcommittee has  seen no
evidence that  the  combined  flow  and transport model, including
the  transformation module,  has been  confirmed  by comparing to
observations  in  a  real system.   Neither  has the  comprehensive
model, combining transport in the  unsaturated  and saturated
zones, been validated  by  comparing  to observed  field behavior.
Given  this lack of  validation,  OSW  should  be cautious in
authorizing general use  of these models  for any  purpose.   The
Agency should  initiate  a concerted effort to gather  or  generate
the  needed field  data to  allow validation of the proposed
groundwater models.

Uncertainty Analysis with FECTUZ

     The FECTUZ model is to be applied in conjunction with  EPA-
SMOD  to determine the degree of uncertainty associated with
Toxicity Characteristic and deListing decisions.   The intent of
the  uncertainty analysis in  the rulemaking procedure  is to
identify the fraction of potential land disposal  sites across the
United States  where a given  leachate concentration will result in
exceedances of a health-based standard.  So  long as this fraction
is low, eg.,  below 5  or 1  percent  (the  actual  value used  is a
policy decision),  then the allowable leachate concentration is
considered to  be protective for hazardous waste  classification.
Monte  Carlo  replication of  the model  is  thus  intended to
characterize  the  range  and   distribution  of  soil  transport
conditions encountered  throughout the United States.

     A particular problem with  the Monte Carlo  methodology
described above is that it  is limited to evaluations of parameter
uncertainty:  it  is not able to account for  uncertainties which
arise due  to   incompleteness or  deficiencies in.  the  underlying
transport models such as those  described previously  for the
FECTUZ model.   Unfortunately, this is  the  case for virtually all
model uncertainty analyses performed today,  as effective methods
for  representing  and evaluating  the  uncertainty in  model
structure are not  currently available.   Implicit  in the use of
parameter uncertainty techniques  is the assumption that the
variations in parameter values  used in the  Monte  Carlo  analysis
somehow capture  the  overall uncertainty  in.  the  model  itself.
This  may  not   always  be the  case,  depending on  the potential
impact and importance  of  omitted  or misrepresented processes in
the model.

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     The  overall  uncertainty  of the  transport  model  should
be considered.  First,  there needs to be a more detailed discussion
of omitted processes and their  potential  impact  in the EPA
reports.   Second,  it  should be recognized  that  in  the  current
application,  the  Monte  Carlo  distributions are  formulated  to
represent only the spatial variability of parameters  across the
United States.   The distributions do  not  reflect  uncertainties,
either in the parameters themselves  or in the overall  output  of
the  model.  To obtain a reasonable,  conservative  level  of
regulation,  it may thus be necessary  to  superimpose overall
judgments of model  uncertainty  on  the  model  results.   For
example, the distribution of unsaturated zone travel times may  be
modified  to  account  for the occurrence of  direct macropore  or
fracture transport.   The consideration  of model uncertainty
is certainly a  new and difficult area, however,  some initial  steps
incorporating  basic  engineering  judgments  are  appropriate  if
uncertainty analysis  techniques are  to  be used and  trusted for
regulatory development.

                        ADDITIONAL ISSUES

     The  Subcommittee  is concerned  about  several other  issues
related  to the  development and  use  of FECTUZ that arose  in the
course of its deliberations:

     1.  How does OSW  proceed in establishing the need for the
         development of a new model?  How are existing models
         evaluated, including those recently developed within
         EPA and at other Federal agencies such as USGS and ARS?

     2.  Does OSW have adequate  in-house capability for evaluating
         issues  related  to transport model  development and
         application,  and does OSW capitalize on the expertise
         available elsewhere within EPA?

     These and other  issues related to the use of  models  by the
EPA  have  been  of  concern to  the  Environmental Engineering
Committee  during  reviews of other  programs within  the  Agency.
The problems appear to be common to  many of the modeling studies
we have  reviewed.   The  EEC  is in  the  process of developing  a
generic  resolution on the proper selection and use of  models  by
the Agency, where these issues are addressed.
                           10

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                           REFERENCES

1.  Abriola and Finder,  "A Multiphase Approach to the Modeling of
    Porous   Media   Contamination  by  Organic   Compounds,    I.
    Equation Development,  and II.  Numerical Simulation",  Water
    Resources Research,  21(1),  11-18 and 19-26 (1985).

2.  Brutsaert,  W.F.,  "Suitability of  Marine  Clay  as  Hazardous
    Waste Site Liners", J.  of Environ. Eng.,  113(5),  1141-1148
    (1987) .


3.  Corapcioglu,  M.Y.,   and  A.L.   Baehr,  "A  Compositional
    Multiphase Model for Groundwater by Petroleum Products,  I.
    Theoretical Considerations, and II.  Numerical Solution",
    Water  Resources  Research,   23(1),  191-200 and 201-213
    (1987).

4.  Faust, C.R.,  "Transport of Immiscible fluids within and
    Below the Unsaturated Zone:  A Numerical Model",  Water
    Resources Research,  21(4),  587-596 - 1985.

5.  Hochmuth, D.P.,  and O.K.  Sunada,. "Ground-Water Model of  Two-
    Phase Immiscible Flow in Coarse Material", Ground Water,
    23 (5), 617-626  (1985).


6.  Jury, W.A., "Chemical  Movement Through Soil", Chapter 6  in
    "Vadose Zone Modeling of Organic Pollutants", S.C.  Hern  and
    S.M.  Melancon, editors,  Lewis Publishers, 1987.

7.  Mull, R., "The Migration of Oil Products in the Subsoil  with
    Regard to Ground-Water Pollution by Oil, Paper HA-7A in
    Advances in Water Pollution Research,  Proceedings of the Fifth
    International Conference of the Intrnational Association for
    Water Pollution Research, 1971.


8.  Pfannkuch, H.O., "Hydrocarbon Spills,  Their Retention in the
    Subsurface, and Propagation into Shallow Aquifers",  Office of
    Water Research  and  Technology,  April 1983,  available through
    NTIS as PBB3-196477.

9.  Schwille, F., "Migration of Organic Fluids Immiscible with
    Water in the Unsaturated Zone",  in Pollutants in Porous  Media  •
    The Unsaturated Zone Between Soil Surface and Groundwater,  B.
    Yaron,  G.  Dagan,  and J.  Goldschmid,  eds.,  Springer  Verlag,
    1984.
                           11

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10.  van Genuchten,  M.  Th.,  "Solute Transport in Structured  Soils",
    in Proceedings  of the 5th Annual AGU (Front Range Branch)
    Hydrology Days and  14th  Annual Groundwater Conference,  H.J.
    Morel-Seytoux and D.O.  Doehring, editors,  Hydrology Days
    Publications, Fort Collins,  CO, 1985.

     In addition, the following two Proceedings volumes provide a
good overview of recently gained  insights into  transport in the
unsaturated zone.

Hern, S.C.,  and S.M.  Melancon,  editors,  Vadose Zone  Modeling  of
Organic Pollutants, Lewis Publishers, 1987.

Yaron, B.,  G.  Dagan,  and  J.  Goldschmid,  editors, Pollutants  in
Porous Media: The Unsaturated Zone Between  Soil  Surface  and
Groundwater, Springer Verlag, 1984.
                           12

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 •••?
>SR
\«***
          UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

V                    WASHINGTON, D.C. 20460
                                OCT  I 6 1987
                                                          OFFICE OF
                                                  SOLID WASTE AND EMERGENCY RESPONSE
  MEMORANDUM
  SUBJECT:   Science Advisory Board Review of the Unsaturated 7one
            Code  for the OSW Fate and Transport Model

  FROM:      Sylvia K. Lowrance, Qirector^/X-A-  r-- j-&*	
            Characterization and Assessment Division (WH-562B)

  TO:        Or. Terry F. Yosie, Director
            Science Advisory Board (A-101)


      The  purpose of this  memo is to transmit  for your review a code
  for the simulation of flow and transport of chemical constituents
  through an unsaturated zone.  The  code accounts  for the attenuation
  of chemical constituents  in the unsaturated zone and has ootential
  uses in the Toxicity Characteristic (TC) and  the Concentration-
  Based Listing rulemakings.  The code was developed to fully explore
  the comments received on  OSW's TC  proposal  (SI FP 21648, June 13,
  1986).  As we have indicated to you earlier,  the TC proposal would
  modify the existing toxicity characteristic by:  (1) introducing  a
  new leach  procedure -- Toxicity Characteristic Leaching Procedure
  (TCLP); (2) adding 38 organic compounds to the list of TC consti-
  tuents; and (3) using a ground-water fate and transport model to
  establish  dilution/attenuation factors for the specific organic
  compounds.  The ground-water fate  and transport  model used in
  the TC proposal was reviewed by SAB as part of the land disposal
  restrictions ground-water screening procedure in 1985.

      In the TC  proposal,  the ground-water model  did not incoroorate
  an unsaturated  zone.  The model assumed that  the bottom of the
  landfill was directly in  contact with the wat^r  table.  Conseouentlv,
  any attenuation of chemicals in the unsaturated  zone was  ignored
  in the model.   OSW made this assumption at that  time because of
  a lack of  adequate data to characterize the unsaturated zone for
  the implementation of the model on a generic  'oasis.  Because of
  this, OSW  opted to be conservatively protective  of human  health
  and the environment.

      The Agency received  numerous  comments  on the  absence of
  unsaturated zone in the model.  The commenters stated that  the
  assumption was  unrealistic and landfills  are  not directly connected

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to the saturated zone.  In addition, since the proposal date,
the Agency has obtained data, as part of OSW's Solid Waste Landfill
Survey, on the distribution of unsaturated zones at the municipal
landfill sites in the U.S.  These data indicate that at least  95
percent of these sites have unsaturated zones.  Therefore,  in
order to fully explore the comments on the TC proposal, OSW has
developed an unsaturated zone code (FECTUZ).

     FECTUZ is a finite-element code for the  simulation of water
flow and solute transport in the unsaturated  zone (variably
saturated porous media).  The code allows for a wide range of
nonlinear flow conditions and handles various transport processes,
which are considered in the saturated zone code, including hydro-
dynamic dispersion,  advection,  sorption, and  first order decay.
The code allows for consideration of heterogeneities in the
vertical dimension (layering) in the unsaturated zone.

SPECIFIC AREAS FOR SAB REVIEW

     We are interested in SAB's evaluation of the FECTUZ code  and
have identified the following specific areas  for Board's review: •

     1)  DIMENSIONALITY OF FECTUZ CODE

     FECTUZ code is  one-dimensional in the vertical dimension  for
the following reasons:

     (a)   The flow in the unsaturated zone for all practical purposes
          can be assumed to be in the vertical direction; and

     (b)   OSW needs a code which can handle nonlinearites in the
          unsaturated, zone and at the same time is computationally
          efficient because of the need to perform Monte Carlo
          analyses.   Therefore, FECTUZ was developed as a one-
          dimensional numerical code (unlike the saturated
          zone code,  which is an analytical code).  Two- or
          three-dimensional codes will not satisfy this criteria.

     (c)   In addition, data to characterize the unsaturated zone
          for modeling in two or three dimensions are  not available
          at this time.

      Is the one-dimensional code (FECTUZ) an appropriate tool
for the intended use under the circumstances characterized by the
limitations of available data?

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     2)   ASSUMPTIONS UNDERLYIMG THE CODE

     FECTUZ code was developed based on several simplifying
assumptions to represent the physical system to be modeled.   The
assumptions include:

     (a)  Flow of the fluid phase is considered isothermal and
          governed by Darcy's law.

     (b)  The fluid considered is slightly compressible and
          homogeneous.

     (c)  Hysteresis effects in the constitutive relationships of
          relative permeability versus water saturation,  and *ater
          saturation versus capillary pressure head,  are assumed  to
          be negligible.

     (d)  Diffusive/dispersive transport in the porous media is
          governed by Pick's law.  The hydrodynamic dispersion
          coefficient is defined as the sum of the coefficients of
          mechanical dispersion and molecular diffusion.

     (e)  Adsorption and decay of the solute may be described by
          a linear equilibrium isotherm and a first order decay
          constant.

     (f)  The code handles only single phase Flow (water) and
          ignores the flow of second phase  (air).

     (g)  The code does not consider chain  reactions.  The porous
          media is considered as single-porosity soil media
          (non-fractured media).  The kinematic sorption effects
          are not considered.

     Are the assumptions made in the development of the code
appropriate, considering its intended use and the limitations of
the available data?
     3)   CODE IMPLEMENTATION

     If we decided to incorporate an unsaturahsd zone  into the
ground water model, PFCTUZ is to be implemented as an  integral
component of the ground water flow and transport code.  The
implementation of the code is described in the attachment.

     We would appreciate your review of the procedure  for
incorporation of FECTUZ in the ground-water f.ow and transport
model, and any specific suggestions the Board  .nay  have for
improvements considering the limited data available.

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     Thank you for your help on this project.   The code alona with
the necessary support materials are attached for your consideration
Please contact me (382-4637), or Dr. Zubair Saleem (382-4767),  if
we can be any assistance during the review process.

Attachment

cc: Matt Straus
    Stephen R. Weil
    R. Scarberry
    Zubair Saleem

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                       ATTACHMENTS





1.   Finite Element Code for Simulating One-Dimensional



      Flow and Solute Transport in the Unsaturated Zone.





2.   Methodology for Simulating Flow and Transport in the



     Unsaturated Zone.





3.   Developing Joint Probability Distributions of Soil-Water



     Retention Characteristics — Robert F.  Carsel and



     Rudolf S. Parrish.   Paper under publication in



     Water Resources Research.

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