PETITIONED HEALTH CONSULTATION
OGDEN MARTIN INCINERATOR COMPANY
BRISTOL, HARTFORD COUNTY, CONNECTICUT
Screening Method of Determining Annual Averages
Regulatory screening procedures require an examination of a range of stability classes and wind
speeds to identify the "worst case" meteorological conditions. For this modeling procedure, the
wind speed and stability class combinations preselected by SCREEN were utilized1. The "worst
case" condition is the combination of wind speed and stability that results in the maximum ground
level concentrations. This value reflects the maximum hourly concentration expected at each
receptor. The maximum hourly value is then factored to provide concentrations over longer
averaging periods. The U.S. EPA's guidance for screening modeling procedures1 does not
provide a factor for converting 1-hour to annual average concentrations, but there have been
some values recommended as a result of extensive studies. Air/Superfund 2 suggests a 1-hr to
annual concentration conversion factor of 0.025 for point sources. The U.S. EPA recommends a
maximum 1-hour to maximum annual conversion factor of 0.08 for both point and area sources
within its procedures for assessing risks 3. This value is supported by recent studies 4, researching
the dispersion from point sources that undergo initial mixing, which have reported values within
the range of 0.06 - 0.10. A conversion factor of 0.10 was used in this study - being on the upper
end of the range of the values published, it produces the most conservative results.
Refined modeling is used when the ambient air concentrations predicted using a screening method fail to meet the requirements. In this case the requirements were cursory health thresholds.
The Industrial Source Complex Short Term (ISCST) model was used as the refined modeling tool. Although ISCST is another screening tool, it includes more complex algorithms than the SCREEN model. The ISCST uses three dimensions instead of two. This allows us to evaluate the effect of the variability in the wind direction. Put another way, it allows us to account for the times when the wind is blowing away from, askew to, and toward the receptor of concern.
ISCST allows the modeler to evaluate the ambient concentrations that occur for as many as 8,784 different weather reports. This allows us to predict the average concentrations for a year of 1-hour weather reports. This is a more precise method of estimating exposures. Another advantage to having the ability to evaluate concentrations three-dimensionally is that we can get an illustration of the areas of concern to determine if the area of maximum concentration is in an area where people can be exposed.
