January 11, 1990 
 
 
 
 
Mr. Bill Bridwell 
Chief Engineer, Environmental Division 
Ponca City Refinery 
Conoco Inc. 
Post Office Box 1267 
Ponca City, Oklahoma  74603 
 
Dear Mr. Bridwell: 
 
       Thank you for taking the time to discuss Conoco's proposed sulfur
recovery unit at the Ponca City refinery.  As a result of the material you
provided to the Environmental Protection Agency (EPA) and our conference
call on December 18, 1989, we have gained a more complete understanding of
the air quality issues at Conoco's Ponca City refinery.  The remaining
issue and the focus of our December 18 discussion was the adequacy of the
demonstration of ambient equivalence.  However, as described below, we have
determined that the demonstration does not yet meet the emission trading
policy requirements, and we do not believe an exception to the policy is
warranted. 
 
       The emission trading policy clearly requires a Level III analysis,
where a Level II analysis predicts one or more exceedances of the Level II
significance values.  Such exceedances have been found at the Ponca City
refinery.  While "other techniques," as discussed in the conference call
and contained in the February 17, 1983 policy memorandum, may be used to
determine ambient equivalence, this does not eliminate the requirement for
a Level III analysis once an exceedance of the significance levels is
identified. 
 
       You expressed concern about the time and expense involved in Level
III modeling and asked if EPA could use discretion to exempt the plant from
a Level III analysis.  The emission trading policy provides that EPA may
decide on a case-by-case basis to allow a geographically-limited Level III
analysis.  Therefore, we believe that it will be possible to keep the cost
of modeling to within reasonable bounds by using sound judgment in
determining which sources should be included in the analysis.  While some
flexibility is provided by this provision, it still requires Level III, or
full dispersion modeling.  The policy does not envision the use of other
techniques in lieu of Level III modeling. 

        In pursuing the analysis, we strongly encourage Conoco to consult
with the EPA Region 6 Office.  As indicated in the conference call, there
are several issues which will require further discussion.  Specifically,
the analysis needs to include receptors in all relevant areas of ambient
air and in areas where historical violations have been documented.   
 
       Finally, as the Region 6 staff stated in the conference call, final
approval of State implementation plan (SIP) revisions must be published in
the Federal Register.  Although the process of review and approval can be
expedited, it cannot be completed so quickly as to coincide with the
desired startup of the sulfur recovery unit in January 1990.  The more
closely the analysis comports with the emission trading policy, however,
the more likely EPA can quickly review and act on the SIP revision.   
 
       I appreciate the effort you took to present these issues and the
opportunity to discuss them with you.  While I recognize and encourage the
plantwide, net reduction in emissions from this project, I remain concerned
that localized concentrations of sulfur dioxide need to be protective of
public health.  I further urge you to work with our Region 6 staff, who are
prepared to assist you.  If you have any questions regarding this letter,
please call me at (919) 541-0877. 
 
                                Sincerely, 
 
 
 
                                Eric O. Ginsburg 
                                     Chief 
                         Sulfur Dioxide Programs Section 
 
cc:    Robert Bauman, OAQPS 
       Tom Diggs, Region 6 
       John Draft, Oklahoma Air Quality Service 
       Doug Grano, OAQPS 
       Brad Raffles, Conoco 
       Laurel Schultz, OAQPS 
       Steven Wellner, OGC  
       Dean Wilson, OAQPS