

Please summarize your responses for this interesting question for the net.
Thank You.

Shawn Huston

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From: [email-masked] (Dennis J. Gerson, Ph.D.)

Bill, all the codes you mentioned are used in groundwater or underground
reservoir modeling.  The codes do have chemical knowledge thru group
function activity coefficients and fugacities (ala Aspen).  The major
users of these codes are the oil companies and environmental remediation
companies.  The codes all run on SGI with OpenGL, IBM RISC with OpenGL
they may support Uniras Unigraphics, thus SUN, DEC, HP and IBM.

Regards, Dennis Gerson

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From: [email-masked] (Tom Slee)

Hi Bill,

I don't know any of the codes you mention.  However, my impression is
that most environmental models work on a cell basis.  You set up your
system as a set of cells: one cell may be a lake, one the atmosphere,
one the ground nearby, one the fish within the water, etc.  (I don't 
know how coarse or fine these
cells can be now).  Then you set up differential equations describing
the flow of materials of interest from one cell to another (or to
nowhere in the case of degradation).  These equations may be transport
equations, or may represent chemical processes: generally you will have
both physical and chemical processes affecting the distribution.  Then 
you solve your system of equations numerically.

There are clearly lots of approximations in this kind of model: it is
implicit that there is perfect mixing within each cell, and the
equations dramatically simplify the factors in determining the
distribution.  There are also, however, lots of empirical numbers 
that need to be known and typically are not.  The rate of mixing of one
"cell" with another must be very dependent on site, and many other
rates are not well known, so in the end you have a very empirical
model. 

This about sums up my total knowledge of the subject, so don't ask any
questions!

		Tom