Re: 2X Background Reference

[scherer@uiuc.edu (David Scherer)]

>>Lesson learned. Specify background, in particular, soils with 
>>concentrations below the upper 95.5% confidence limit (2*sigma)
>>will be considered at background.
>
>What I take that to mean is, 'soils for which a screening measurement below
>the upper 95.5% confidence limit (2*sigma) is obtained will be considered at
>background.'
>
The intent of this kind of cleanup standard is to assure that no additional
radioactivity remains beyond what was present before operations began.  This
is not because natural activity is "good" while added activity is "bad" as
some have suggested in the past.  Instead it is because the site users are
only responsible for the amount they added.

As Luke and Albert have pointed out, background concentrations in some
location obey some frequency distribution.  Using the mean as an absolute
limit is not reasonable for the reasons already stated.  Likewise, using a
2-sigma limit may not achieve the goal either, because the site may still
have a higher mean concentration than the surrounding area.  In fact, I do
not believe that a standard for each shovelfull of soil is the appropriate
approach.

Instead compliance should be demonstrated if the mean and sigma of the final
site should be no greater than those of the "background" distribution.  Of
course this adds some work for the contractors since they would have to do
some planning and sampling as they select fill, but it meets the goal.  I
believe that this approach was used for the West Chicago site here in
Illinois.  Since remediation appears to be a growth industry, I would
recommend that the HPS take the lead in advocating this type of standard.

Dave Scherer
scherer@uiuc.edu