Re: Radioactivity in soil

[lfiske@shepmill.com (Larry Fiske)]

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
>
>