Re: Radioactivity in soil -Reply

[Craig D Brown <CBROWN@envc.sandia.gov>]

Larry,

I agree with your position as to some of the trying aspects of
meeting the 15 mrem/yr cleanup limit.  If you are working in
the DOE world under a RCRA  permit you will also find that
DOE risk assessment and EPA risk assessment don't
mean the same thing.  Add your home state into the
equation and it starts to get even more complicated. 

Anyway, I'm not sure how the approach is viewed by the
NRC but using the mean of your background would seem to
back you into a corner.  We have been calculating 95th
percentile or upper threshold limits on our background
populations (mostly for EPA).  This seems to be somewhat
more reasonable and involves fitting the population to a
normal or lognormal distribution, if possible.  If the
population does not fit either population you can use
non-parametric approaches or even head into more exotic
distributions.  There are several statistical tests that can be
run to see which distribution your population best fits.

Once you have your background population distribution you
may find that your "hottest" background sample may be
less than your 95th percentile.  This is highly dependent on
the type of distribution, or if non-parametric approaches are
used, and the number of samples that are taken for
background.  There are also outlier tests that you can run on
your background samples to see if they actually belong to
the background distribution.  I question these tests when
used on an "unknown" population (and have seen them
include lots of contamination in the background distribution)
but if you are pretty confident that your background samples
are most likely non-contaminated (i.e., professional
judgement) I think you are in pretty good shape.

Craig D. Brown
cbrown@envc.sandia.gov

>>> Larry Fiske <lfiske@shepmill.com> 09/10/96 11:24am
>>>
Chris:

With regard to detection of 1 pCi/g, based on my field
experience, I believe
that you are correct in you assessment of the delectability
of a 1 pCi/g
change in Ra-226 concentrations using NaI(Tl) detectors. 
However, the
problem with the pending 15 mrem/y requirements is that 1
pCi/g is often in
the "noise" range for background concentrations of naturally
occurring
radionuclides.  That is, within a given well defined soil type,
it is not
uncommon to see background radionuclide concentrations
vary from sample to
sample by as much a 0.5 to 1+ pCi/g.  This is usually
addressed by saying
"take more samples and characterize the population
statistically."

Statistical analyses of a background sample population has
a great deal of
utility, however there are problems with this as well.  Using
the
statistical sample adequacy equation, as given in
NUREG-5849, it is possible
to determine how many samples are required to calculate
the mean of a given
population.  But, even so, there is an inherent variability,
both in the
calculated mean, and in the soil.

Given these sources of significant variability (relative to 1
pCi/g) there
are many of us in the environmental cleanup realm who are
very concerned
about the proposed 15 mrem standards as they apply to
naturally Occurring
isotopes.

As an example, consider the following site specific data
from a uranium
tailing site:

20 background samples were collected from non-affected
areas.
Sample concentrations ranged from 0.5 to 1.8 pCi/g.
Sample adequacy analyses determined that 20 sample were
statistically
sufficient to calculate the mean.
The mean was calculated to be 0.99 pCi/g.

Given a limit of 1 pCi/g above the mean of  0.99 pCi/g we are
now trying to
distinguish a difference in count rate resulting from a
difference in
concentration of only 0.19 pCi/g (1 + 0.99 - 1.8) for at least
one
background location which is statistically representative of
1/20th of the
sight as a whole.  A 0.19 pCi/g distinction between clean
and dirty is
difficult whether you are using gamma surveys or laboratory
analyses.

OK.  That my soapbox speech.

Please feel free to contact me directly if anyone would like
to pursue this
discussion.

Oh, I almost forgot NUREG-1500 is the NRC working draft
guide on release
criteria that provides soil concentrations for 3 mrem/y and 15
mrem/y for
various scenarios.

Lawrence Fiske
lfiske@shepmill.com
(970)206-4232

These are my opinions not necessarily those of my
employer, YaDa, YaDa, YaDa.


At 08:45 PM 9/9/96 -0500, you wrote:
>>that the national average (U.S.) of Ra226 in soil is just
about 1 pCi/gm, at
>>least that's my recollection of a paper of Fred Haywood's
ca. 15-20 years
>>ago. If the background were 1-2 pCi/gm, and Ra226 were
the only radionuclide
>>of concern, I'd think that 1pCi/gm above that would be
detectable with, say,
>>a 2x2"NaI probe. I hasten to CMA by stating that it's been
many years since
>>I've done environmental work, and my remembrance of
likely countrates maybe
>>off the mark.
>>
>>Chris Alston
>
>Actually, you've got it about right.  The mean natural
Ra-226 in soil in
>Ontario (where I am) is around 0.025 Bq/g (or 0.7 pCi/g),
and I expect the
>US national average is slightly higher.
>
>A semi-infinite source of Ra-226 in soil results in about 40
cps per pCi/g
>in a 2x2" NaI scintillometer.  A typical background (of
about 5 uR/h)
>results in approximately 100 cps in the same instrument. 
An increase of 1
>pCi/g (ie. 100 to 140 cps) would easily be detected with
even a one second
>count.  Even using a 1x1" NaI probe you would see an
increase from about 25
>cps to about 35 cps, also easily detectable in a one
second count.
>
>Chris Clement
>Low-Level Radioactive Waste Management Office
>Atomic Energy of Canada Limited
>clementc@crl.aecl.ca
>clement@vaxxine.com
>
>