June 13, 1994



MEMORANDUM

SUBJECT:  Good Engineering Practice (GEP) Stack Height Credit for
          Montana Sulfur Company Sulfur Recovery Unit

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

TO:       Kevin Golden, Regional Meteorologist
          Assessment, Modeling and Emissions Section, Region VIII

          Larry Svoboda, Chief
          Assessment, Modeling and Emissions Section, Region VIII


     In response to your request, the Model Clearinghouse has
reviewed the basis for your intention to disapprove a GEP stack
height analysis submitted by Montana Sulfur and Chemical Company
for a 100 meter stack recently constructed in Billings, MT. 
Based on discussions within the Office of Air Quality Planning
and Standards, we concur with your intent to disapprove the
analysis.  Also, we agree with your assessment that only through
a valid fluid modeling demonstration could a stack taller than
the de minimis 65 meters be credited toward GEP stack height.

     As you noted in your May 11, 1994 memorandum, the Technical
Support Document for the Stack Height Regulations (EPA 450/4-80-
023R) indicates that as the building height to width ratio
becomes larger the increase in maximum ground-level
concentrations due to the building is reduced.  Also, page 7 of
the Technical Support Document further states:  "The area of
influence becomes diminishingly small as the height to width
ratio of the structure increases.  Thus structures such as stacks
and radio or TV transmission towers should not be considered in
GEP stack height determinations."  Therefore, your recommendation
seems appropriate that a fluid modeling study be conducted to
demonstrate the need for a stack height greater than the de
minimis 65 meters.

     If you have any further questions or comments, please
contact me at (919) 541-5562 or Dennis Doll at (919) 541-5693.

cc:  G. Blais
     D. Doll
     J. Irwin