April 25, 1989







MEMORANDUM

SUBJECT:  MassPower PSD -- Urban vs Rural for Background Source

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

TO:       Susan C. Kulstad, Air Modeling Contact
          Region I (ATS-2311)


     In response to your request the Model Clearinghouse has
reviewed your draft position regarding the appropriate modeling
for the subject PSD permit.  We agree that your position is a
generally accepted interpretation of current guidance.  However,
as a result of several internal discussions and follow-up
telephone calls with you, we agree that it would make sense in
this case to provide an alternative interpretation of the
guidance.

     It is our understanding from conversations with you that the
receptor or receptors of interest are located such that the
plumes from MassPower and from MMWEC would not impact the same
receptor(s) during the same hourly period.   It can be reasoned
that for those hourly periods for which the MMWEC plume impacts
the critical receptor(s), this plume would be experiencing rural
dispersion.  In contrast, for those hourly periods for which the
MassPower plume is impacting the critical receptor(s), this plume
would be experiencing urban dispersion.  While we do not believe
that it is sound technical practice to add impacts together from
rural and urban plumes, it can be rationalized that such is
really not the case here but that hourly, either rural or urban,
impacts are only summed to obtain 3- or 24-hour estimates.

     Thus, although it not a generally accepted interpretation of
the guidance, the mixed mode modeling of the urban and rural
plumes from the two plants is technically supportable and we
agree that it may be done in this simple case.  Unfortunately,
since our guidance is not set up in this way, our models also are
not capable of executing such a mixed mode operation.  Separate
model runs will need to be made for each source and some type of
post processing may be necessary to sum the impacts over 3- or
24-hour periods.  (We understand that the annual period is not a
problem.)  Although we have not fully explored this idea, it may
be possible to limit the remodeling to critical time periods/
receptors of concern.  In any event the circumstances, details,
and rationale used in the reanalysis should be fully documented.

     If you have any questions, please contact Dean Wilson at
629-5683.

cc:  D. deRoeck, SO2/PMPB (MD-15)
     S. Reinders, SRAB (MD-14)
     D. Wilson, SRAB (MD-14)