File: ABSTRACT.TXT


                       EXAMSII Model System Abstract


               Center for Exposure Assessment Modeling (CEAM)
     National Exposure Research Laboratory - Ecosystems Research Division
                  Office of Research and Development (ORD)
               U.S. Environmental Protection Agency (U.S. EPA)
                          960 College Station Road
                         Athens, Georgia 30605-2700

                                706/355-8400



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                                  Summary

EXAMS is an interactive computer-based system for specifying and storing the
properties of chemicals and ecosystems, modifying them using simple commands,
and conducting rapid evaluations and error analyses of the probable aquatic
fate of synthetic organic chemicals.  EXAMS constructs simulation models by
combining the loadings, transport, and transformations of a chemical into a
set of differential equations, using the law of conservation of mass as an
accounting principle.  This is accomplished by computing the total mass of
chemical entering and leaving each section of a body of water as the
algebraic sum of external loadings, transport processes that distribute
chemicals through the system and export them across its external boundaries,
and transformation processes that convert chemicals to daughter products.
The differential equations are then assembled and solved to give a picture of
the behavior of chemicals in an aquatic ecosystem.  The program produces
output tables and simple graphics describing chemical
  
      o     exposure: the expected environmental concentrations (EECs)        
            resulting from a particular pattern of chemical loadings,  
  
      o     fate: the distribution of the chemical in the system and the
            fraction of the loadings consumed by each transport and
            transformation process, and  
   
      o     persistence: the time required for purification of the system
            (via export/transformation processes) should the chemical
            loadings cease. 
 
EXAMS includes separate mathematical models of the kinetics of the physical,
chemical, and biological processes governing transport and transformations of
chemicals.  This set of unit process equations is the central core of EXAMS.
EXAMS' "second-order" or "system-independent" approach makes it possible to
study the fundamental chemistry of materials in the laboratory and then,
based on independent studies of the levels of driving forces in aquatic
systems, evaluate the potential fate of materials in systems that have not
yet been exposed (Baughman and Burns 1980).  EXAMS treats ionization, and
partitioning of the compound with sediments and biota, as thermodynamic
properties or purely local equilibria peculiar to each segment of the
environmental model--as opposed to a treatment as system-wide "global"
equilibria.  In this way, EXAMS allows for the impact of spatial variation in
sediment properties, pH, etc. on chemical reactivity.  EXAMS computes the
behavior of trivalent organic acids, bases, and ampholytes; each ionic
species can have its own distinctive pattern of sorption and complexation
with naturally occurring particulates and dissolved organic matter.  Reaction
pathways can be entered for the production of transformation products, whose
further transport and transformations are then also simulated by EXAMS.

EXAMS computes the kinetics of transformations by direct photolysis,
hydrolysis, biolysis, and oxidation reactions. The input chemical data for
hydrolytic, biolytic, and oxidative reactions can be entered either as single
valued, second-order rate constants, or as pairs of values defining the rate
constant as an Arrhenius function of the temperature in each segment of the
water body.  EXAMS has been designed to accept standard water-quality
parameters and system characteristics that are commonly measured by
limnologists throughout the world, and chemical datasets conventionally
measured or required by United States Environmental Protection Agency
regulatory procedures.

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