February 8, 1989







MEMORANDUM

SUBJECT:  Yates Power Plant GEP SIP

FROM:     Dean A. Wilson, Meteorologist
          Source Receptor Analysis Branch (MD-14)

TO:       Kevin D. McGee, Meteorologist
          Stationary Source Planning Section 
          Region IV (4APT-APB)


     In response to your request, the Model Clearinghouse has
reviewed your proposed response to the State of Georgia regarding
the appropriate modeling procedures for the Yates Power Plant GEP
SIP.  We do not have any significant problems with your position
on the issues that are raised by the State.  To be more respon-
sive to the State's questions, however, we suggest the addition
of the following Regional Response to Paragraphs 2 and 5:

     The statement on Page 2-19 of the ISC users manual is
     meant to caution the user that the receptor above stack
     height may need to be analyzed with a complex-terrain
     model and that a determination should be made whether
     the complex-terrain model estimates or the ISC esti-
     mates are more appropriate.  The "Guideline on Air
     Quality Models (Revised)" provides the current
     rationale for making that determination.  The fact that
     the concentration estimates with ISCST in transitional
     terrain can be several times higher than those obtained
     from RTDM is one reason why the guidance is written the
     way it is.  We are aware that if the receptor is well
     below plume height, but still above stack height, RTDM
     may calculate  a very low concentration.  Such an es-
     timate may be unrealistically low compared to reality.
     Since we lack the data needed to determine what the
     best estimate should be, the Guideline says that the
     estimate may be as high as that concentration which
     would occur at stack height, for the same downwind
     distance from the source.

      With regard to your proposed response to Paragraph 6 of the
State's letter, we suggest the appropriate answer is:

      To date no court has ruled on this issue.
tending that because of wind speed considerations the plume
cannot arrive at the receptor within the averaging time of the
standards, the Model Clearinghouse has provided some guidance to
Region IV in a December 20, 1983, memorandum (attached) involving
a Florida issue.

     If you have any further questions, please contact me.

Attachment

cc:  D. Grano, SDPMPB (MD-15)   
     S. Reinders, SRAB (MD-14