December 7, 1993



MEMORANDUM

SUBJECT:  Revised Technical Comparison Document--Phelps Dodge    

FROM:     Joseph A. Tikvart, Chief
          Source Receptor Analysis Branch (MD-14)

TO:       Gerald Fontenot, Chief
          Air Programs Branch, Region VI (6A-AP)


     As follow up to my May 19, 1993 memorandum to you, and in response to
Quang Nguyen's verbal request, we have reviewed the Technical Comparison
Document (TCD), as revised on September 17, 1993.  The cover letter to that
document indicates that the revision is intended to respond to the comments
we raised in our May 19 memorandum.  We agree that they have adequately
responded to our concerns.  However, they have also made several additional
changes to the model comparison protocol; we have one comment regarding
those changes.  We are also passing along a comment from the Air Quality
Management Division regarding the claimed confidentiality of the subject
document.

     Recall that in their original TCD the Company essentially proposed
that the performance of their proposed model, Mesoscale Puff Dispersion
Model (MPDM), should be compared to that of a reference model, Rough
Terrain Dispersion Model (RTDM)/Industrial Source Complex Short Term
(ISCST2).  The model that showed the best performance would then be used in
the State Implementation Plan (SIP) demonstration.  They did indicate at
that time that if the revisions to the Guideline on Air Quality Models were
promulgated, then the reference model would become Complex Terrain
Dispersion Model Plus (CTDMPLUS)/ISCST2, as recommended in the Guideline. 
We had no problem with that position.

     Indeed, the revisions to the Guideline were promulgated in July 1993. 
Pursuant to that event, as part of the September 17 revisions to the TCD,
the Company now proposes to use CTDMPLUS/ISCST2 as the reference model. 
However, they also propose to retain the RTDM/ISCST2 for comparison.  In
addition they propose a second version of CTDMPLUS/ISCST2 and several
different procedures for applying their MPDM model as contenders in the
performance evaluation.  We believe that this is not appropriate as it
really amounts to using the performance evaluation as a diagnostic tool
rather than purely a test.  We believe that RTDM/ISCST2 as well as the
second version of CTDMPLUS/ISCST2 should be dropped from contention since
neither represent the Environmental Protection Agency's (EPA's) recommended
model for this situation.  The Company should then choose how they believe
MPDM should be run in the regulatory mode and only enter that procedure
into the competition.  In this way we are back to having one reference
model competing against one proposed model, which we believe to be
appropriate.  If the Company wants to use another data set (or a subset of
the data set being collected in conjunction with the performance
evaluation) for diagnostic purposes in order to decide on how they want to
run MPDM, we would be willing to entertain such a proposal.

     Regarding the confidentiality question, we believe that it would be
inappropriate to allow the Company to deny access to the TCD during the
public comment period.  The SIP revision process has always been an open
one.  While our regulations do not specifically address this issue to the
extent that the Prevention of Significant Deterioration (PSD) regulations
do, we believe it would be a significant departure from the policy of
providing open and ready access to SIP documentation.  Such a departure
could set an undesirable precedent which could affect future regulatory
actions.  We have not as yet received any input from the Office of General
Counsel (OGC); however, we believe that EPA's position on this issue should
be that all model documentation including the TCD be made available during
the public comment period, assuming the MPDM is used in the SIP revision. 
We recommend that you seek legal advice from your Regional counsel on this
matter and we will continue to seek OGC's opinion.  We will inform you if
OGC's opinion differs from our own.

     If you have any questions please contact Dean Wilson at 919-541-5683. 

cc:  G. Blais
     P. Embrey
     J. Paisie